poss polymers: physical properties and biomaterials ... · the physical properties exhibited by...

Journal of Macromolecular Science R©, Part C: Polymer Reviews, 49:25–63, 2009Copyright © Taylor & Francis Group, LLCISSN: 1558-3724 print / 1558-3716 onlineDOI: 10.1080/15583720802656237

POSS Polymers: Physical Propertiesand Biomaterials Applications

JIAN WU AND PATRICK T. MATHER

Syracuse Biomaterials Institute and Department of Biomedical and ChemicalEngineering, Syracuse University, Syracuse, NY 13244

Research into polymers incorporating polyhedral oligomeric silsesquioxane (POSS) hasintensified during the past several years, revealing new fundamental polymer physics,new synthetic routes, and unexpected applications. The present review article criti-cally examines the recent scientific literature on POSS polymers with an emphasis onstructure-property relationships. We conclude that it is an exciting time to work on suchmaterials and we expect the field to continue to grow in the foreseeable future.

Keywords POSS, hybrid materials, rheology, microstructure, biomaterials

1. Introduction

1.1. Polymeric Nanocomposites

To reinforce polymers, it is common to physically disperse in the polymeric host inorganicfillers chosen from a variety of different shapes, such as fibers or whiskers, platelets, orspheres. This ubiquitous approach attempts to combine acceptable processibility typicalof thermoplastic polymers with desired characteristics from the inorganic filler, such ashigh modulus, high oxidation resistance, or high use temperature. Ideally, the resultantproperties will represent not only the volumetric averaging of contributions from indi-vidual components, but also the synergic effects of the components included. In suchcases, the improvement of physical properties can often be found at relatively lower fillerinclusion.1−4

As the filler is decreased to smaller than 100 nm, the resulting composites, termednanocomposites, may achieve dramatic improvements in such physical properties as gasbarrier, thermal stability, elastic modulus, and ultimate mechanical properties.5−9 Suchsubstantial enhancement can be attributed mainly to the filler particle surface propertiesand interfacial interactions that become increasingly important with decreasing particlesize. Nanofillers exist in various shapes, such as spherical (metallic particles10−12 andsemi-conductive particles10−12), layered (layered silicate13−15), and fibrous (nanofibers16,17

and carbon nanotubes15,18,19). Applications ranging from automotive components to foodpackaging and to biomaterials have been pursued.

Received November 29, 2008; Accepted November 30, 2008.Address correspondence to Patrick T. Mather, Syracuse Biomaterials Institute, Biomedical

and Chemical Engineering, 121 Link Hall, Syracuse University, Syracuse, NY 13244, E-mail:[email protected]

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Downloaded By: [Mather, Patrick] At: 22:18 28 January 2009

26 J. Wu and P. T. Mather

The physical properties exhibited by polymeric nanocomposites are determined by thequality and the nature of dispersion of the nano-fillers in the polymer matrix. However, itis known that the surface energy substantially increases with the decrease of particle size.Consequently, nanoparticles tend to aggregate in order to reduce the total surface energy,creating a pervasive manufacturing challenge. To ameliorate this problem, nanoparticlesare often grafted or otherwise modified with organic groups (commonly alkyl ammoniumsurfactants) similar or compatible with the polymer matrix, followed by melt-mixing orin-situ polymerization. The resulting materials feature microstructure and properties thatare quite sensitive to processing conditions. The complexities mentioned concerning “top-down” nanocomposites processed by dispersion, coupled with the fact that the size scale offillers approaches the molecular scale, present the need for a synthetic approach utilizingnanoscale monomers that would naturally disperse and feature covalent incorporation.Polyhedral oligosilsesquioxane, so-called “POSS,” is just such a nanoscale monomer andis the focus on the remainder of this review.

1.2. What is POSS?

Polyhedral oligosilsesquioxane (POSS) is one of many kinds of silsesquioxane molecules.The term silsesquioxane refers to the molecules, whose chemical structure follows the basiccomposition of RnSinO1.5n, for example Me8Si8O12. Here, the R-group, also called the ver-tex group for polyhedral molecules, may be hydrogen, alkyl, alkylene, aryl arylene, amongothers. Such silsesquioxanes can form oligomeric organosilsesquioxanes (CH3SiO1.5)n

through chemical reactions and the chemical structures of the derivative silsesquioxanesare quite versatile and the interested readers are referred to the review by Barney et al.,20

which focuses on the preparation, structures, and applications of such silsesquioxanes. Themolecular architecture of silsesquioxanes can be classified into two categories: (a) non-caged structure and (b) caged structure, each shown in Scheme 1(a) and Scheme 1(b). Asshown in Scheme 1(a), the non-caged silsesquioxane molecules can be further classifiedinto: (a) random structure; (b) ladder structure, and (c) partial-cage structure.

Scheme 1. Chemical structures of silsesquioxanes. (a) non-caged silsesquioxanes: (i) random, (ii)ladder; (iii) partial caged structures, and (b) caged silsesquioxanes: (i) T8, (ii) T10, (iii) T12 structures.


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Cage-like silsesquioxanes are usually called polyhedral oligosilsesquioxanes or Poly-hedral Oligomeric Silsesquioxanes, abbreviated as POSS. This class of well-defined, highlysymmetric molecules usually features a nanoscopic size, approximately 1.5 nm in diameterwhen the vertex (R) groups are included. They can be loosely regarded as the smallestpossible silica particles.21 POSS molecules with a T8 cubic inorganic core composed ofsilicon-oxygen (R8Si8O12 or R′

1R7Si8O12) are the most prevalent system studied, althoughthe Q8 structure (R8Si8O20) has also been given significant attention. Here, “T” and “Q”refer to conventional nomenclature from silicon nuclear magnetic resonance (NMR) lit-erature, with T and Q referring to silicon atoms bonded to three (3) or four (4) oxygenatoms, respectively.22 The hybrid organic-inorganic framework renders POSS thermallyand chemically robust, so much so that one of the promising applications of POSS-basedpolymers is for use in the highly oxidizing environment of orbiting space vehicles.23,24 Thisaspect will be discussed later, in Section 3.3 on Surface and Interfacial Phenomena. In asimilar fashion to atomic oxygen resistance, POSS can improve the oxidative stability andflame retardance of polymers in terrestrial applications.25−27

1.3. POSS-Based Polymeric Nanocomposites

From the microscopic viewpoint, the characteristic nanoscopic size of the POSS molecule(1.5 nm) is comparable to the dimensions of polymeric segments or “blobs”28 in thecondensed phase (molten or solid), yet nearly double typical intermolecular spacing. Un-doubtedly, the incorporation of POSS moieties into linear polymer chains and/or polymernetworks will modify the local molecular interactions, local molecular topology, and theresulting polymer chain and segment mobility. These microscopic modifications are mani-fested in the macroscopic physical properties and performance, such as modulus, strength,glass transition temperature, thermal stability, and dimensional stability. Considering di-mensionality, nano-fillers can be classified as one-, two-, or three-dimensional (1D, 2D,or 3D), depending on their geometrical symmetry. Layered nano-clays29−31 are alumi-nosilicate sheets that extend in two dimensions and featuring one dimension, the layernormal, n for interaction with a polymer host. In this sense such fillers are consideredone-dimensional. In contrast, single and multi-walled nanotubes, carbon nanofibers, andmolecular rigid rods32 extend in one dimension and interact with a polymer host in twodimensions of the surrounding space (r and θ of cylindrical coordinates), and thus areconsidered 2D. Finally, POSS is like other highly symmetric molecules, including den-drimers, by being roughly spherical and interacting with the polymer host in the threedimensions of the surrounding space (r , θ , and ϕ of spherical coordinates). As we willshow, POSS moieties can aggregate or crystallize into supramolecular objects of lowersymmetry and then interact with the polymer host in a geometrically distinct way. Whilenon-reactive POSS is often studied as a filler capable of molecular level dispersion due tothe small size, we feel that the more powerful implementation of POSS in nanocompositesis through copolymerization with the POSS monomer. In this bottom-up approach, gooddispersion is assured through covalent attachment to the host polymer, while nanocompositereinforcement occurs by self-assembly—aggregation or crystallization—that can be tunedarchitecturally and compositionally. This self-assembly impacts the resulting macroscopicphysical properties in a rational manner.

Unlike silica or silicones, each POSS molecule is tethered to eight organic groups sur-rounding its cage and bonded to the silicon vertices, these groups varying in composition toinclude methyl, isobutyl, cyclopentyl, cyclohexyl, phenyl, aniline, among others. Positioned



at the cage vertex, all but the smallest of tethers collectively form a voluminous shell—asmuch as 80% of the POSS volume33—around the Si8O12 core that mediates the interactionsbetween POSS moieties and polymer matrix. Thus, POSS molecules embody a truly hybridinorganic core/organic shell architecture that is naturally compatible with organic hosts,such as polymers and natural biomaterials. Furthermore, one or more corner groups canbe substituted by a functional group through conventional organic conversions.34−37 Theseversatile functional groups, such as methacrylate, acrylate, styrene, norbornene, amine,epoxy, alcohol, and phenol, provide the possibility to incorporate POSS into a polymerchain or network through polymerization or grafting. In this manner, a large diversity ofPOSS-polymer architectures is possible.

Early POSS research focused on random tethering of POSS along the polymer chainthrough free radical chain-growth polymerization38−40 and step-growth polyadditions ofcopolymers.41 However, as shown in Scheme 2, polymerizations and reactions with POSScan be designed that architecturally locate the POSS moiety at a single end of a polymerchain to form a hemi-telechelic POSS polymer, at both ends to yield POSS telechelicmolecules, and tethered to a single block of block copolymers or multiblock polyurethanes.Examples and their references will be given later in this review.

Scheme 2. Molecular architectures of polymer chain with POSS incorporation. (a) random; (b)random block; (c) tri-block; (d) di-block; (e) end-capped telechelics; (f) end-capped hemi-telechelicsand (g) centered telechelics. The straight line and solid circles represent polymer chain and POSSmolecules, respectively.

Regardless of the preparation approaches, the key factor in determining physical prop-erties is the dispersion and self-assembly of POSS moieties in polymer host. This behaviordepends on the thermodynamic interaction between POSS and the polymer matrix, par-ticularly the constituent polymer segments. If the interaction is favorable or mutually


POSS Polymers 29

unfavorable relative to POSS-POSS interactions, POSS moieties will disperse well; other-wise POSS will aggregate. Unlike a filled system or blend, however, POSS aggregation willbe limited due to covalent attachment to the polymer backbone that prevents aggregationbeyond a scale of about one radius of gyration. Naturally, the resulting physical proper-ties will vary with the POSS dispersion (or the aggregation) level. It is critical, therefore,to understand the nanostructure-property-processing relationship for given systems if oneis to successfully target properties per intended application. The significant influence ofincorporating POSS moieties on the resulting properties of POSS-related polymeric ma-terials stimulates scientists to also find a rational way to explain the correlation betweennanoscopic microstructures and macroscopic properties in these polymeric nanocompositesemploying modeling and simulation.33,42−46

The first successful synthesis of a well-defined POSS structure was reported byScott47 in 1946, which was primarily used for electrical insulation at high temper-ature. After a couple decades of inactivity, research on POSS was reinvigorated inthe 1990s, spawned by the discovery of a method to prepare polymerizable POSS.At the Air Force Research Lab at Edwards Air Force Base, CA, (then the USAir Force Phillips Laboratory) Lichtenhan and Haddad et al. successfully synthesizeda series of linear random copolymers incorporated by POSS molecules. The sys-tems include styryl-POSS,38 methacrylate-POSS,40 norbornyl-POSS,48 siloxane-POSScopolymers41 (Scheme 3). While research on POSS-based polymers has continued inearnest at the Air Force Research Lab, the establishment of Hybrid Plastics, Inc. now head-quartered in Hattiesburg, MS, has led to commercial availability of POSS monomers andpolymers, enabling much fascinating POSS-related materials research49 across academic,industrial, and government laboratories. A number of research groups devote themselves tothe studies of POSS-related polymeric nanocomposites, varying from synthesis to materialscharacterization, attempting to understand the relationships among processing, structure,and property. The field of POSS polymer research is expanding rapidly; indeed, the numberof publications pertaining to POSS research has increased 10-fold during the past decade,following an exponential growth in time. In response to this high level of research activity,review articles in the area have appeared every two or three years. These review articleshave introduced the latest developments with varying emphasis ranging from the synthe-sis of POSS polymer materials to the formation of POSS nanocomposites and associatedenhanced properties.21,50−52

As indicated previously, the prevailing methodology to incorporate POSS groups intoa polymeric system is to copolymerize the POSS macromer, bearing one or two polymeriz-able groups and the remaining inert vertex groups, with a suitable host comonomer to obtainthe desired organic-inorganic hybrid polymeric nanocomposites. It has been observed thatpolymerization reactivity for POSS monomers is high and that even homopolymers of thePOSS monomer are possible, so long as a spacer molecule between the POSS moiety andthe polymerizable group of sufficient length is utilized. Thus far, copolymers achievedand studied by this methodology include poly(styrene),53−56 poly(methacrylate),57−60

epoxies,61−65 polyurethanes,66−68 polyimide,69−71 polyolefin,72−74 poly(siloxane)75−78 andpolycarbonate.79,80 When compared to the top-down approach of filler-based composites re-quiring dispersion, such as silica, POSS copolymers have been observed to yield nanocom-posites with excellent properties, such as higher glass transition and usage temperature,increased thermal stability and oxygen permeability, enhanced mechanical properties andlowered dielectric constant, while largely preserving processability.

The rest of our review is divided into two parts. First, we will discuss the latest ad-vances in the studies of morphology and rheological behavior of polymeric nanocomposites



Scheme 3. Schematic drawing of chemical structures for POSS-based polymeric nanocomposites:(a) norbornyl-POSS copolymers; (b) siloxane-POSS copolymers; (c) methacrylate-POSS copolymersand (d) styryl-POSS copolymers.

incorporating POSS, along with studies on the morphology and self-assembly behavior ofamphiphilic POSS nanocomposites at the surface and the interfaces. Following that, weturn to a focused examination of applications of POSS nanocomposites. Indeed, manyapplication areas have emerged for POSS-based materials, including biomaterials,81−83 di-electric materials,69,84−87 organic light emitting diode devices,88−91 lithography resists,92−95

catalyst,96−98 and fuel cell and battery membranes.99−101 For our review we will turn ourattention to biomaterials.

2. Microstructure and Rheological Behavior of POSS-Based PolymericNanocomposites

2.1. Well-Defined Molecular Architectures

2.1.1. Amphiphilic POSS Telechelics. As previously discussed, POSS moieties incorpo-rated into polymers feature eight vertex (corner) groups, generally with one (1) tetheringPOSS to the polymer backbone and the other seven (7) being identical in composition, ow-ing to the preparation procedure from a single trichlorosilane (RSiCl3) or tri-alkoxysilane(such as RSi(OCH3)). In nearly all cases, these vertex groups on the silicon-oxygen cage


POSS Polymers 31

are linear- or cyclo-aliphatic in nature. This renders the POSS cage quite hydrophobic innature (unlike silica) and further provides the potential to create amphiphilic molecules(and accompanying structures) when combined covalently with hydrophilic chains. As forany amphiphilic system, self-assembled microstructures can be expected in both the bulkand in solvent, especially if a solvent selective for one of the components (hydrophilic orhydrophobic) is utilized. In such a selective solvent, amphiphilic molecules can displaysupermolecular aggregates with shapes ranging from spheres, to rods, to lamellae.

Specifically considering POSS-based amphiphilic polymers, one architectural strategyis to fix POSS molecules at the end(s) of a hydrophilic chain, yielding either a dumbbell(telechelic) or tadpole polymeric architecture. Natural choices for the hydrophilic chain usedin such structures are poly(ethylene glycol) (PEG) and poly(ethylene oxide) (PEO), respec-tively, due to their commercial availability, nonionic simplicity, and (not insignificantly)mutual solubility with POSS in such reaction media as tetrahydrofuran (THF). Knischkaand co-workers102 synthesized hemi-telechelic POSS-PEO molecules by hydrosilylationof H8Si8O12 to allyl-functional poly(ethylene oxide) and subsequent conversion of theremaining seven vertices to ethyl groups. In solution, the hemi-telechelic molecules self-assembled into micellar and vesicular structures. The researchers further cross-linked thePOSS moieties at high pH to form the silica shells of the spherical vesicles. EmployingTEM and AFM microscopy, the authors observed that bimodal aggregations form in waterwhen the pH value was elevated up to 9.3, one featuring a micellar-type structure 10 nm indiameter, the other featuring a vesicular-type aggregate 60 nm in diameter.

Kim and Mather103 synthesized well-defined POSS-PEO-POSS telechelics by endcap-ping PEG molecules with two equivalents of a cyclohexyl-POSS monoisocyanate, yieldingthe structure depicted schematically in Scheme 2(e) and Fig. 1. This resulted in a dumb-bell configuration and, quite surprisingly, led to physical behavior akin to triblock ABAcopolymers. As shown in Fig. 1, WAXD observations provided evidence that the covalentlylinked POSS end-caps and PEG bridges separately formed their individual characteristiccrystalline phases, driven by thermodynamic incompatibility. Based on this data, combinedwith thermal analysis and hot-stage polarizing optical microscopy, the authors proposed asequence of microstructures on heating shown schematically in Scheme 4, and investigatedthe relationship of microstructures and rheological behavior through these transitions.104

Scheme 4. A proposed microstructure evolution of POSS telechelic, i.e. Tel10k, with the temperatureincrease. Reprinted from Kim, B. S.; Mather, P. T., Macromolecules 2006, 39, 9253–9260 (reference104) with permission of American Chemical Society.



Figure 1. Wide angle X-ray diffraction (WAXD) patterns of the as-synthesized amphiphilic POSSend-capped PEG telechelics: (a) PEG8k, (b) Tel10k, (c) Tel8k, (d) Tel3.4k, (e) Tel2k, (f) Tel1kand (g) POSS macromers, “Tel” is the abbreviation of telechelic and the number stands for themolecular weight of PEG bridge. Reprinted from Kim, B. S., Mather, P. T., Macromolecules 2006,39, 9253–9260 (reference 104) with permission of American Chemical Society.

Within the range of the explored temperature (50◦C < T < 180◦C), the rheological be-havior of the POSS telechelics did not obey the Time-Temperature-Superposition (TTS)principle, suggesting a temperature-dependent morphology. Above the PEG melting point,the persistence of a POSS nanocrystalline phase, acting as physical crosslinking sites, pre-served solid-like rheological characters of the telechelics. In particular, G′ was observedto be higher than G′′ between the PEG-phase and POSS-phase melting points. Crossingthe POSS melting point, the telechelics showed a solid-liquid transition, manifested as aprecipitous drop in both G′ and G′′ and resembling the order-disorder transition of blockcopolymers.

The amphiphilic nature of POSS-PEG telechelics also underlies the sensitivity to sol-vent polarity and the possible self-association behavior in given solutions. For example,they were found insoluble in both water and hexane, which are the selective solvents good


POSS Polymers 33

for PEG and POSS, respectively. Kim and Mather105 further investigated the influenceof polymer concentration and solvent polarity on viscometric properties of POSS-PEGtelechelics in order to reveal the molecular association of POSS-PEG telechelics in asalt-free solution. Like pure PEG, POSS-PEG telechelics in THF showed the linear depen-dence of reduced viscosity on concentration, indicating a lack of self-association behavior.However, in mixed water/THF solutions, adding more water (increasing solvent polarity)led to a dramatic upturn of reduced viscosity in the very dilute regime, a polyelectrolyte-likeeffect, which was interpreted to indicate the formation of micelles induced by POSS-POSSassociations. This hydrophobic-hydrophobic association behavior of POSS-PEG telechelicswas found to vary with the POSS content and the length of PEG blocks, as well as solventpolarity. Water, a good solvent for PEG and selective non-solvent for POSS, was found totune the intra- and intermolecular interactions of POSS-capped POSS-PEG telechelics(via a hydrophobic interaction) and the more water that was added, the more significant theassociation as reflected in the reduced viscosity.

Recently, Lee et al.106 reported on the synthesis of isobutyl-POSS-centered poly(ε-caprolactone) (PCL) polyol telechelics with varying PCL molecular weight, achievingthe structure shown simply in Scheme 2(g). By end-capping the POSS-PCL telechelicpolyols with acrylate groups and photocuring with the stoichiometric addition of a tetrathiolcross-linker, a family of POSS-PCL networks was achieved. Like the case of POSS-PEO-POSS telechelics, this system showed evidence for microphase separation and complexcrystallization competition between PCL and POSS. It was found that only those PCLnetworks with high POSS loading (>34 wt%) led to POSS crystalline domains as indicatedby differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD).Dynamic mechanical analysis showed two distinct rubbery plateaus in the sample with42 wt% POSS loading, one between the glass transition temperature of amorphous PCLand the prominent POSS melting point, the other appearing above the POSS melting pointand afforded by the covalent crosslinks. For samples with high POSS loading, and thusfeaturing POSS crystallinity, an excellent one-way shape memory response (shape fixingand recovery) was observed. This indicated that POSS crystallization on cooling of strainedsamples was capable of fixing a large elastic strain and thus percolated space to form amechanically robust network capable of resisting the elastic strain energy of the covalent,PCL network.

2.1.2. Block Copolymers Incorporating POSS. Haddad and coworkers107 reported on thesynthesis of AB block copolymers, poly(norbornene-POSS)-b-poly(norbornene) preparedusing ring-opening metathesis polymerization. As shown in Fig. 2, the TEM images ofthin sections revealed that such diblock copolymers featured prominent micro-phase sep-aration when the POSS content was above 10 wt%. The distinct dark POSS-rich phase,due to its higher electron density, featured the strong ordering as cylinders. However,the long-range ordering of cylindrical POSS-rich micro-domain was poor, perhaps dueto polydispersity or contamination with the PN homopolymer. In other work on POSSblock copolymers, Pyun and co-workers108 successfully synthesized well-defined ABAtriblock copolymers, poly(MA-POSS)-b-poly(n-butyl acrylate)(pBA)-b-poly(MA-POSS),employing atom transfer radical polymerization (ATRP). Transmission electron microscopy(TEM) images obtained with selective staining of POSS revealed that microphase sepa-ration did not occur at lower POSS loadings, where the degree of polymerization (DP)followed poly(MA-POSS)6/pBA481/poly(MA-POSS)6. At a higher molar ratio (poly(MA-POSS)10/pBA201/poly(MA-POSS)10), strong micro-phase separation was observed but witha morphology inverse that expected: pBA cylinders were periodically distributed in a



Figure 2. Transmission electron microscopy (TEM) images of a series of poly(norbornene)-b-poly(norbornene-POSS) di-block copolymers varying with CyPOSS(upper) and CpPOSS(below)derivatives. The POSS loading is 10, 30, 60 wt% from left to right. Reprinted from Haddad, T. S.,Mather, P. T., Jeon, H. G., Chun, S. B., Phillips, S. Materials Research Society Symposium Proceedings2000, 628, CC2.6.1–CC2.6.7 (Reference 107) with permission of Materials Research Society.

continuous poly(MA-POSS) (as shown in Fig. 3 (left)) despite it being the larger weightfraction component. Above both glass transition temperatures of the two phases, the rhe-ological behavior showed a lack of fluidity at even the highest temperature and lowestfrequency as shown in Fig. 3 (right). The slope near 1/2 at lower frequency, observed forboth log G′ & G′′ vs log reduced frequency, was consistent with other rheological observa-tions for ordered block copolymers. This non-terminal rheological behavior was attributedto elasticity derived from the micro-phase separated morphology of a strongly segregatedsystem.

It was reported in the same paper108 that longer POSS lengths were not possibleusing ATRP and the POSS monomer utilized. More specifically, the molecular weight ofthe poly(MA-POSS) homopolymer synthesized by ATRP was limited to just 12 kg/mol(Mw/Mn = 1.8) or approximately 12 repeated units per chain, and this could be due tosteric crowding during the polymerization of the homopolymer of POSS-methacrylate inthat block, noting that a propyl spacer separated the methacryl group from the POSS moiety.Indeed, this may be the reason why very few reports exist pertaining to POSS-based block


POSS Polymers 35

Figure 3. The microstructures and rheological behavior of tri-block copolymer: p(MA-POSS)10-b-p(BA)210-b-p(MA-POSS)10: (left) Transmission electron microscopy (TEM) images of (a) lowmagnification images with a overall morphological feature, (b)–(c) high magnification micrographsshowing well-defined bright pBA cylindrical phase regularly dispersed in the dark pMA-POSS con-tinuous phase. The local hexagonal packing of the cylinders was confirmed by (d) Fourier transform ofselected area from micrograph, (right) Master curves of dynamic oscillatory shear storage modulus(G′)and loss modulus(G′′) for the tri-block copolymer p(MA-POSS)10-b-p(BA)210-b-p(MA-POSS)10.The reference temperature was 80◦C. Reprinted from Pyun, J., Matyjaszewski, K., Wu, J., Kim,G.-M., Chun, S. B., Mather, P. T., Polymer 2003, 44, 2739–2750 (Reference 108) with permission ofElsevier Ltd.

copolymers containing poly(POSS) block(s). More broadly, it remains a distinct challengeto achieve POSS homopolymers with molecular weight high enough to achieve an entangledsystem.

2.2. Less–Defined Molecular Architectures

2.2.1. Polymer Blends Incorporating Molecular POSS. Most POSS studies have focusedon POSS-based polymeric nanocomposites prepared in one of two ways: physical mixingof free (monomeric) POSS or random copolymerization. In the former case, POSS moietiescan be dispersed in the polymer matrix at a level mediated by weak (van der Waals) or strong(hydrogen-bonding) interactions. In the latter case, POSS can be incorporated covalently asa pendant group along a polymer chain through homo- or copolymerization. A fundamentaldifference of these two approaches is that physical blending may lead to macroscopic phaseseparation between POSS and the polymeric host, if driven to do so thermodynamically,while this cannot occur for POSS copolymers due to covalent attachment. Practicallyspeaking, physical mixing is an easier way to prepare POSS nanocomposites and, naturally,has been adopted by many research groups.

As one prominent example, Fu and Hsiao109 studied the nanocomposites of ethyl-propylene copolymer and octamethyl-substituted POSS (methyl-POSS) prepared by meltmixing. An analysis of the microstructure using WAXD revealed patterns revealed evidencefor a crystalline POSS phase with distinct crystalline peaks appearing virtually unmodified



from the pure methyl-POSS. The authors estimated the average size of methyl-POSScrystalline domains dispersed in ethyl-propylene copolymer to be 50 nm by analysis of thediffraction peak widths. The researchers further investigated the melt rheology of ethylene-propylene polymeric nanocomposites with methyl-POSS and compared this behavior withoctaisobutyl-POSS (iBu-POSS) and observed, for both cases, alteration of the low frequency(long time) behavior transition from liquid–like to solid–like, indicating a loose elasticnetwork of crystalline POSS particles. This was further substantiated by observation of afinite yield stress that increased with increased POSS loading.

In a similar vein, Joshi et al.110 investigated the relationship between morphology andrheology of methyl-POSS/HDPE blended nanocomposites. In contrast with the work ofFu and Hsiao,109 it was found that methyl-POSS can be dispersed in HDPE at the molec-ular level, for very low loading levels, lower than 1.0 wt%. Rheological characterizationshowed that POSS particles reduced the complex viscosity magnitude when the POSScontent was in the range 0.25 to 0.5 wt%. Above a POSS loading of 1.0 wt%, POSSaggregation in the form of nanocrystals was witnessed. Consequently, the complex vis-cosity increased with POSS loading and solid-like behavior emerged at low frequenciesfor POSS loading >5.0 wt%, a finding attributed to the aggregation of nanocrystallinePOSS domains and the formation of a physical network of such domains. Cole-Cole(η′ vs η′′) and van Gurp-Palmen (δ vs log|G*|) plots111 allowed clear visualization ofthese phenomena, especially the transition to solid-like behavior above a POSS loadingof 1 wt%.

Zhou et al.112 examined the impact of reactive blending in octavinyl-POSS/isotacticpolypropylene (iPP) blends on rheological behavior. The baseline iPP/POSS compositesprepared by physical blending exhibited rheological behavior dependent on the miscibilitybetween POSS and iPP and the resulting morphological transition. Specifically, miscibleblends (POSS ≤ 2 wt%) featured lower modulus and viscosity compared with pure iPP.When the POSS loading was increased to greater than 2 wt%, the modulus and viscositywere found to increase with POSS loading and thermorheological simplicity was lost,as evidenced by non-collapsed Han (log G′ vs log G′′),113,114 Cole-Cole (η′ vs η′′) andvan Gurp-Palmen (δ vs log|G*|) plots. It was concluded that molecularly dispersed POSSplasticized the iPP, while aggregated POSS domains lead to fluid-solid transition at highertemperature due to their interactions, consistent with other findings reported above. With theaid of the thermal radical generator, dicumyl peroxide (DCP), octavinyl-POSS moleculescould be reacted with the host iPP chains. As a consequence, both G′ and G′′ significantlyincreased, more so at higher POSS contents. The frequency dependencies of G′ and G′′

monotonically decreased (flattened) with increasing POSS loading. The Han plot of thereactively blended composites deviated significantly from the expected form for entangledlinear polymers, even at a quite low POSS content (0.5 wt%). These observations wereascribed to the grafted POSS’s strong anchoring effect that retarded the polymer chainrelaxation at a low POSS content (≤0.5 wt%), and the formation of a POSS-iPP networkat a higher POSS loadings (>0.5 wt%).

Studying blends of POSS and poly(methyl methacrylate) prepared by physical mixing,Kopesky and coworkers58 found that both isobutyl-POSS and cyclohexyl-POSS can crystal-lize, even for loadings as low as 1 vol%. (It is noted that in POSS systems, the vol% and thewt% should be approximately the same, volumetric equation-of-state measurements (PVT)have not been conducted to prove this.) Above 1 vol%, untethered POSS crystallizationwithin the PMMA matrix was significant enough to increase both the shear viscosity andplateau modulus, while below this level untethered POSS dispersed at a nanoscopic leveland decreased the zero-shear viscosity. In order to study amorphous yet untethered POSS in


POSS Polymers 37

PMMA, Kopesky and coworkers115 selected two POSS alternatives that are liquid at roomtemperature: octamethacryl-propyl-POSS (MA-POSS) and hydrogenated octamethacryl-propyl-POSS (hMA-POSS). For loadings below 10 vol%, WAXD analyses revealed only asingle amorphous halo centered at d =6.27Å, which was attributed to interchain spacingsfor PMMA. For both such systems, POSS lowered the viscosity and glass transition of theblends monotonically with POSS loading. Time-Temperature-Superposition was success-fully employed for all blends and, surprisingly, the Williams-Landel-Ferry (WLF) analysisindicated that the decreases in Tg and viscosity observed could not be ascribed to an increasein free volume with increasing POSS content. This argument was made on the basis of thefree volume at Tg being virtually the same for all blends studied. For volume fractions ofPOSS greater than 20 vol%, POSS was found to aggregate in the PMMA matrix and therheological behavior similar to the POSS-filled system was recovered.

POSS moieties physically blended into a polymer matrix tend to aggregate and crys-tallize as nanoscale particles and only disperse molecularly at very low loadings and withfavorable POSS-matrix interactions. The resulting rheological behavior is sensitive to thisdispersion level. In the miscible systems, POSS incorporation decreases the zero shear vis-cosity and modulus.115 Conversely, POSS crystallized at the nanoscale not only increasesthe rubbery plateau moduli, but also engenders a secondary rubbery plateau and induces thefluid-to-solid transition. This rheological effect is attributed to strong interactions betweenPOSS aggregated domains.58

2.2.2. Random POSS Copolymers. Besides physical mixing, POSS moieties can be intro-duced into a polymeric matrix through copolymerization. For such systems, one may ask:(1) How does the polymer chain configuration influence the tethered POSS dispersion and(2) How do the tethered POSS cages impact the resulting rheological behavior? Driven toanswer these two questions, Romo-Uribe et al.116 studied poly(4-methylstyrene) copoly-mers incorporating POSS. The polymers were largely unentangled, due to synthesis limi-tations at that time, with degrees of polymerization ranging from 150 to 400. The WAXDpatterns of the poly(4-methylstyrene) copolymers tethered by cyclopentyl (Cp) and cyclo-hexyl (Cy)-POSS moieties showed two distinct amorphous halos without any crystallinepeaks related to POSS crystals evident for POSS weight fractions is up to 8 mol%. Thetwo amorphous peaks were indicated to represent the polymer chain and side chain/groupcorrelations, respectively. When POSS content increased above 16 mol%, the X-ray diffrac-tion peaks in the POSS region sharpened, particularly for CpPOSS, and this was attributedto limited POSS aggregation. By comparison with the results on filled systems,58,115 it isclear that molecular-level dispersion is facilitated by copolymerization. Rheologically, theunentangled poly(4-methylstyrene) homopolymer showed terminal (G′ ∝ ω2) and Rousezones (G′ ∝ ω1/2) without an intervening rubbery plateau. Copolymerization of CyPOSSat a modest level (4 mol%; 27 wt%) did not significantly alter the rheological behaviorrelative to the homopolymer, but increasing the loading level to 8 mol% (42 wt%) resultedin the replacement of the terminal zone with a rubbery plateau with G′ ∼ 103 Pa. At astill higher CyPOSS loading of 17 mol% (64 wt%), a very broad rubbery plateau spanning4-decades in frequency was observed. Similar observations were made for the CpPOSScopolymers, though with quantitative differences. The authors suggested the mechanism of“sticky reptation”117 originally conceived for hydrogen-bonded polymer melts, hypothe-sizing attractive interactions between POSS groups. Because of the limited polymerizationdegree, the determination of an effect of POSS on the rubbery plateau was not possible.

Continuing this line of research, Wu et al.118 reported the rheological properties ofhigher molecular weight and entangled random polystyrene-isobutyl-POSS copolymers. As



Figure 4. The microstructures characterization of iBuPOSS-PS random copolymers: (I) TEM imagesof the copolymers with (a) 6 and (b) 30 wt% iBuPOSS. The dark iBuPOSS particles disperse in brightPS matrix almost with the size of a single POSS size, 1.5∼3 nm. (II) WAXS patterns of as-cast filmsof the copolymers with (i) 0, (ii) 6, (iii) 15, (iv) 30, (v) 50 wt% iBuPOSS and (vi) styryl iBuPOSSmacromer. The copolymers are devoid of any crystalline features of styryl iBuPOSS macromer.Reprinted from Wu, J., Haddad, T. S., Kim, G. M., Mather, P. T., Macromolecules 2007, 40, 544–554(Reference 118) with permission of American Chemical Society.

shown in Fig. 4, they found with WAXS and TEM that copolymerized iBu-POSS segmentsdispersed in the PS matrix at a molecular level for loadings up to 50 wt%. The copolymerswere found to be thermorheologically simple, obeying time-temperature superposition(TTS) up to 50 wt% loading, indicating single phase, amorphous polymers far from anytransformation event. Moreover all of the copolymers exhibited a terminal zone, a rubberyplateau, and a transition zone with increasing frequency, though the rubbery plateau wasill-defined owing to polydispersity. The temperature-dependent shift factors, aT(T) werewell described by the WLF equation and analysis thereof revealed, interestingly, that thefractional free volume at Tg increased with the iBuPOSS loading, while the correspondingfree volume expansivity decreased. In other words, the copolymers became increasinglyless temperature-sensitive with increased POSS loading. Moreover, iBuPOSS incorporationdramatically decreased the rubbery plateau modulus in proportion to the copolymerizationlevel, indicating a strong dilation effect of iBuPOSS on the entanglement density. Theauthors suggested that this dilation effect originated from the topology alteration to the hostPS chain, with pendant POSS moieties acting as small, spherical branches as depicted inFig. 5.

In the context of the tube model of polymer dynamics, a length scale known as the“packing length” emerges:118−121

lp =(

3.67 × 103 RT

G0N

)1/3

(1)

where R(J·mol−1·K−1) is the universal gas constant, T(K) is absolute temperature, G0N (Pa)

is the rubbery plateau modulus and the unit of constant, 3.67 × 103, is Å3·m−3·mol−1.The packing length, lp(Å), is the occupied volume of a chain divided by the mean squareend-to-end distance, a scale proportional to the tube diameter, dt (dt = 19lp).121 Based ontheir observations of monotonic decrease in G0

Nwith increasing POSS copolymerization


POSS Polymers 39

Figure 5. Schematic drawing of the influence of the pendant POSS group on PS chain topology, theresultant tube dimension, and the resultant rubbery plateau modulus as a function of iBuPOSS weightfraction. Reprinted from Wu, J., Haddad, T. S., Kim, G. M., Mather, P. T., Macromolecules 2007, 40,544–554 (Reference 118) with permission of American Chemical Society.

level, Wu et al.118 reported that the packing length increased monotonically; i.e., POSScopolymerization dilates the tube diameter. The same researchers have undertaken furtherresearch on the role of the vertex group composition in determining viscoelastic propertiesof the polystyrene system, including the comparison of isobutyl (iBu), cyclohexyl (Cy),and cyclopentyl (Cp) POSS vertex groups, revealing significant differences.122

So far, we have summarized reports on the linear viscoelastic behavior of both physi-cally mixed and copolymerized POSS nanocomposites, revealing distinct differences. Thecombined system, physically mixed and copolymerized, was studied by Kopesky et al.58

who investigated the rheological behavior of three categories of POSS-based polymericnanocomposites: POSS-poly(methyl methacrylate) (PMMA) random copolymers, blendsof POSS and neat PMMA, and blends of POSS and POSS-PMMA random copolymers.Like POSS-PS random copolymers,118 the storage modulus master curves of POSS-PMMArandom copolymers were shifted strikingly downward relative to the pure PMMA ho-mopolymer, indicating a lowering of the plateau modulus (and entanglement density)of POSS-PMMA copolymers. At the same POSS loading level, the copolymer incor-porating iBuPOSS showed a lower plateau modulus and higher entanglement molecularweight than the CyPOSS counterpart. In contrast, the linear viscoelastic behavior of the



Figure 6. Time-temperature-superposition (TTS) master curves of dynamic oscillatory shear (a)storage modulus (G′) and (b) loss modulus (G′′) for blends of 25 wt% iBuPOSS inclusion andiBuPOSS-PMMA random copolymers with varying 0 and 30 vol% iBuPOSS. The reference tem-perature was 80◦C. Reprinted from Kopesky, E. T., Haddad, T. S., Cohen, R. E., McKinley, G.H., Macromolecules 2004, 37, 8992–9004 (Reference 58) with permission of American ChemicalSociety.

POSS-PMMA copolymer incorporating untethered octa-isobutylPOSS(octa-iBuPOSS), asshown in Fig. 6, featured storage and loss moduli that each increased monotonically withocta-iBuPOSS content. In addition, the storage modulus featured a noticeable change inthe terminal slope for samples incorporating 30 vol% octa-iBuPOSS. In contrast to thetube dilation effect for POSS-copolymers,118 this system featuring POSS nano-crystalliteswas well modeled (at least with respect to the plateau modulus) using the Guth-Smallloodequation for filled systems:123

G0N (ϕ) = G0

N (0) [1 + 2.5ϕ + 14.1ϕ2] (2)

where G0N (φ) is the plateau modulus of pure PMMA and φ is the POSS nano-crystallite

volume fraction. Actually, at a higher POSS loading level, the plateau modulus of thePOSS filled PMMA remained constant and then monotonically increased at high POSSloading (greater than 5 vol%). In particular, the enhancement of CpPOSS is much largerthan that of iBuPOSS due to the interaction between that R-group and polymer matrix. Incontrast, the plateau modulus of iBuPOSS-filled copolymer featured a simple upturn trendfor all POSS loading and the data obeyed the prediction of Guth-Smalllood equation verywell. Meanwhile, the authors considered that if POSS nanocrystallites can be regarded ashard sphere fillers, the shear viscosity of POSS filled homopolymer and copolymer shouldsatisfy the prediction of Einstein-Batchelor equation:124

η0 (φ) = η0 (0) [1 + 2.5φ + 6.2φ2] (3)

As this equation predicts, the zero shear viscosity of POSS-filled polymer blends should in-crease monotonically with POSS loading. Actually, the zero shear viscosity of POSS-filledPMMA homopolymer negatively diverged from the prediction of Eq. (3). This observationcan be attributed to molecularly dispersed POSS inclusions that introduced additional freevolume and associated plasticization, resulting in an increase of polymer chain mobilityand a decrease in viscosity. However, while POSS-filled PMMA homopolymer negativelydeviated from Eq. (3), the iBuPOSS-filled copolymers did exhibit a monotonic increase inzero shear viscosity. This increase was attributed to a strong thermodynamic interaction


POSS Polymers 41

Figure 7. TEM micrographs of (a) pure polyurethane-urea (PUU) and (b) PUU-based compositesincorporating with 10 wt% POSS. The dark hard segment particles disperse in the bright soft segmentmatrix. The POSS incorporation makes PUU composites feature finer morphology. Reprinted fromMadbouly, S. A., Otaigbe, J. U., Nanda, A. K., Wicks, D. A., Macromolecules 2007, 40, 4982–4991(Reference 125) with permission of American Chemical Society.

between untethered POSS and tethered POSS. The plasticizer effect played a minor role inthe zero shear viscosity of POSS-filled POSS-PMMA random copolymers.

Other researchers have reported on the impact of POSS on the formation of nanocrys-talline domains in a polymeric host. Recently, Madbouly et al.125 found that the incorpo-ration of diamino-POSS within the hard segments of multiblock polyurethane-urea (PUU)led to a finer nanostructure between hard and soft segments of PUU. As shown in Fig.7, TEM observations revealed that the PUU with 10 wt% POSS exhibited a much finernanoscale micro-phase separation than pure PUU. The authors explained that the largesurface area of POSS nanoparticles created a large interaction zone with the PUU segmentsand concomitant higher affinity between PUU hard and soft segments. Accordingly, 10wt% POSS incorporation made the microphase separation temperature (Tmps) shift upwardfrom 140◦C, for pure PUU, to 160◦C. Meanwhile, the rheological behavior of PUU wasalso significantly changed by the incorporation of diamino-POSS, particularly a signifi-cant increase in melt viscosity and zero shear viscosity with POSS incorporation. Figure 8showed that TTS applied only temperatures below Tmps, above which G′ showed a strongdependence on temperature. Meanwhile, the incorporation of POSS dramatically increasedthe viscosity and flow activation energy of PUU-POSS nanocomposites.

In a quite distinct approach, Lee et al.126 synthesized aluminum (Al)-containing POSS-grafted poly(styrene-vinyl diphenylphosphine oxide) random copolymers. Different fromPOSS cages covalently pendant to the polymer chain, Al-containing POSS are attached toP O by a coordination bond. This enabled the authors to investigate the dependence ofrheological behavior of the random copolymers purely on POSS loading level, where thestudied polymers had the same polymerization degree and molecular weight distribution.Similar to other copolymers with covalently bonded POSS cages, the relaxation time ofthese copolymers increased with POSS loading and the time-temperature-superpositionprinciple was obeyed. Interestingly, the POSS-coordinated polymers also obeyed a time-composition superposition principle as shown in Fig. 9, indicating that an increase of POSSloading at a fixed temperature simply shifts the distribution of relaxation times but doesnot alter the shape of the distribution. This important result suggested that other POSS



Figure 8. Time-temperature-superposition (TTS) master curves of: (a) pure polyurethane-urea(PUU) and (b) PUU-based composites incorporating with 10 wt% POSS at reference temperatureof 120◦C. The POSS incorporation makes the microphase separation temperature (Tmps) increase,above which the rheological behaviors of PUU-based composites obey TTS principle. Reprinted fromMadbouly, S. A., Otaigbe, J. U., Nanda, A. K., Wicks, D. A., Macromolecules 2007, 40, 4982–4991(Reference 125) with permission of American Chemical Society.

copolymers should be examined in a similar manner. Nevertheless, it was observed thatthe copolymer with a high POSS content exhibited a slow gelation response, violating thetime-composition superposition, which the authors attributed to POSS-POSS interactions.

To summarize this section, the rheological behavior of POSS-based polymericnanocomposites depends strongly on the underlying microstructure, itself depending oninteractions between the POSS moieties and between POSS and the host polymer seg-ments. This leads to a sensitivity of microstructure and rheological properties to the vertex(R) groups at the corners of the POSS cage.

As can be seen in this review thus far, the rheological studies have focused exclusivelyon linear viscoelastic properties of the linear random copolymers, with no reports onnonlinear rheological phenomena such as normal stresses under steady shearing, shearstart-up, elongational flow, or capillary rheometry. However, knowledge of these rheologicalphenomena is of great practical importance for plastics processing operations. Moreover,the study of non-linear rheological behavior provides a rigorous testing platform thatcan clearly discern different models for polymer dynamics. Thus, a significant opportunityexists for researchers to begin a study of non-linear rheological phenomena for POSS-basedpolymers.

Beyond bulk rheology, the rheological behavior of polymers and other complex fluidsin confined geometries, such as at surfaces and interfaces, has been paid increasing attentionby rheologists, especially following the advent of Fuller’s interfacial stress rheometer,127 aswell as passive single particle tracking128 or active optical tweezers.129 At the same time, re-cent studies have indicated that trisilanol-POSS130−132 and POSS-PEO-POSS telechelics133

can form a stable monolayer at the air/water interface. While such interfacial structures


POSS Polymers 43

Figure 9. Isothermal rheological behaviors of poly (styrene-co-vinyl diphenylphosphine oxide)(PSP) grafted with POSS at 170◦C (The diphenylphosphine oxide (dPhPO) content is 7.4 mol%): (a)loss modulus (G′′) and complex viscosity (η*) with varying POSS grafting, (b) Time-composition-superposition(TCS) mater curves of loss modulus (G′′) and complex viscosity (η*) of PSPs withvarying POSS content: (•,◦) 0, (�, �) 10.9, (�,�) 21.7, (�, ∇) 54.5 mol% of dPhPO sites attachedwith POSS. Solid and empty symbols stand for G′′ and η*, respectively. Reprinted from Lee, A., Xiao,J., Feher, F. J. Macromolecules 2005, 38, 438–444 (Reference 126) with permission of AmericanChemical Society.

will be discussed elsewhere in Section 3.1, it should be mentioned here that a significantopportunity exists to study the interfacial rheology of such systems, heretofore largelyunexamined. Such studies could provide great insight into the dynamic nature of interfacialphenomena that underpin the properties of hybrid emulsions, blends, and composites.

3. POSS Behavior at Surfaces and Interfaces

3.1. Langmuir-Blodgett (LB) Films at the Air/Water Interface

In the last decade, three dimension (3-D) bulk properties of POSS-based polymeric mate-rials have been extensively studied and feature very wide potential applications. Equallyimportant are studies on the influence of POSS incorporation on two-dimensional (surfaceand interfacial) properties of polymeric materials, especially considering applications inthin films and coatings. Though comparatively less studied, several excellent contributionsalong these lines, particularly those from the laboratory of Prof. Esker of Virginia Tech,



bear consideration. Deng et al.130,131,134 for the first time, revealed that incompletely con-densed POSS cages, trisilanol-POSS derivatives, can form stable Langmuir-Blodgett (LB)monolayers at the air/water interface due to their amphiphilic nature. Since then, thetrisilanol-POSS has been broadly employed as a model amphiphilic nanoparticle in thestudy of LB monolayers consisting of polymer and POSS. Hottle et al.135 reported the LBfilms prepared from trisilanolisobutyl-POSS/PDMS blends. The two amphiphilic compo-nents were blended to form a homogeneous monolayer at the air/water interface, at least athigh surface dilution. With the increase of surface concentration, a transition from a mono-layer state into multilayer structures through PDMS desorption was observed. Still furthersurface compression led to the collapse of the POSS moieties into multilayer domains.Additionally, it was reported that the concentration of PDMS can change the aggregationstate of POSS particles in the multi-layers. For example, a decrease of the PDMS contentcan result in the trisilanolisobutyl-POSS forming network-like aggregation, as witnessedby Brewster-angle microscopy.

Interestingly, if the PDMS was replaced by its phosphine oxide derivative, the network-like POSS aggregation structure can be converted into uniform dispersion of the so-called“particle aggregates.”136 In order to further investigate the influence of nanoparticle am-phiphilicity, Hottle et al.137 incorporated non-amphiphilic octaisobutyl-POSS into PDMS,instead of amphiphilic trisilanolisobutyl-POSS. They found PDMS can dramatically reducethe aggregation degree of non-amphiphilic nanoparticles at the air/water interface, whilepure octaisobutyl-POSS formed aggregates at all surface concentrations. Surface pressureisotherms and Brewster angle microscopic observations indicated that the surface mor-phology of the amphiphilic polymer/non-amphiphilic octaisobutyl-POSS blends featuredquite strong composition dependence. For compositions with greater than 70 wt% POSS,the blends formed large rigid domains, though smaller than those from pure octaisobutyl-POSS films, with a surface pressure isotherm shape that deviated from that of pure PDMS.Similar to pure octaisobutyl-POSS, the blends featured a sharp upturn in surface pressurewith the decrease of the average area per monomer, while pure PDMS showed a transitionto a plateau. As for blends with POSS contents between 40 wt% and 70 wt%, it was foundthat POSS particles formed extensive networks and the corresponding isotherms werequalitatively similar to pure PDMS. Finally, for POSS contents below 30 wt%, POSS wasapparently homogeneously dispersed in the PDMS matrix (without large-scale aggregation)and no distinct difference in the surface pressure isotherm was observed in comparison tothe pure PDMS. The authors proposed that the observed 2D morphologies are relevant tounderstanding the more complex 3-D (bulk) counterparts and thus indicate an opportu-nity to study the aggregation/dispersion mechanism of nanoparticle inclusion in polymermatrices. Indeed, we feel that the LB platform presents a powerful tool for such studiesdue to comparative simplicity and facile reversibility of structure formation via surfacecompression.

In addition to the study of POSS-polymer blends as LB monolayers, the samePOSS-based amphiphilic telechelics discussed earlier103 have been studied at the air/waterinterface.133 Like amphiphilic trisilanol-POSS derivatives, the amphiphilic telechelic poly-mers consisting of two non-surface-active components (e.g. hydrophilic PEG and hy-drophobic iBu-POSS), exhibited surface activity at the air/water interface. In particular,it was reported that the surface activity depended strongly on the molecular weight ofthe PEG bridge. For a PEG ≥ 8 kg/mol, the amphiphilic telechelics did not form sta-ble monolayers, a finding attributed to the fact that, for these telechelics, the POSS endgroups were not sufficiently hydrophobic to balance the large hydrophilic PEG bridge at the


POSS Polymers 45

air/water interface. Shorter PEG bridges (1 kg/mol, for example) afforded excellent surfacestability and allowed the transfer of Langmuir-Blodgett multilayers. X-ray reflectivity ofPOSS-PEG1k-POSS multilayers, grown to >60 nm in thickness, indicated Y-type LB mul-tilayers films with double layer thickness measuring 3.52 nm. Meanwhile, observations ofthe surface pressure isotherm, Brewster angle microscopy, and X-ray reflectivity revealedthat surface compression can induce the packing structure evolution of the amphiphilictelechelics at the air/water interface, show schematically in Fig.10. The authors proposedthat the versatility in surface morphology and packing model of the inorganic-organicamphiphilic telechelics proved to be a new strategy to create nanostructured coatings.

Recently, Mitsuishi et al.138 studied LB films of unique POSS-based amphiphilicrandom copolymers on the air/water interface, which were synthesized through free rad-ical copolymerization employing amphiphilic N -dodecylacryamide and non-amphiphilicmethacryloypropyl-POSS macromers containing seven non-reactive vertex (R) groups, tri-fluoropropyl (SQF) or phenyl (SQPh). The yielded amphiphilic random copolymers canform stable monolayers at air/water interface and featured high LB film deposition capac-ity, e.g. ∼400 layers. The multilayer LB films prepared by Y-type LB technique showed awell-defined layer structure, each layer being approximately 1.7 nm in thickness. Moreover,the POSS units homogeneously dispersed in the film. The film surface was very flat andsmooth (the value of the root-mean-square is 0.4 nm at 1 µm × 1 µm). Importantly, POSSincorporation had a unique influence on the temperature dependence of the refractive indexof the obtained LB films. Upon heating, the refractive index of the LB films prepared fromthe random copolymers with phenyl POSS increased from 1.43 (200◦C) to 1.49 (270◦C)Conversely, the refractive index of those prepared from pure poly(N -dodecylacryamide)decreased from 1.38 to 1.28 when the temperature increased from 200◦C to 220◦C.The authors attributed this phenomenon to the compact POSS packing configurationin the thin film and proposed this unique optic property for optoelectronic nanodeviceapplications.

Figure 10. A schematic cartoon proposed for the packing model evolution of POSS-PEG1k-POSStelechelics LB film with the increase of surface pressure (�). The solid particles and solid lines standfor POSS cages and PEG chains, respectively. (A) a monolayer in a gas-like state (� ∼ 0 mN·m−1),(B) a liquid-expanded (LE) phase(� ∼ 1 mN·m−1), (C) a phase with brushlike conformation of PEGchains (1 < � < 5 mN·m−1), (D) a liquid-crystal (LC) film (5 < � < 30 mN·m−1) with closelypacked POSS cages and PEG loops, and (E) a film with multilayer POSS cage collapse (� > 30mN·m−1). On the right is shown the total film thickness vs. number of LB deposited film layers forPOSS-PEG1K-POSS deposited at a surface pressure of 25 mN.m−1. The slope of d = 1.76 +/- 0.09nm provides the thickness of a POSS-PEG1K-POSS monolayer. Reprinted from Lee, W., Ni, S. L.,Deng, J. J., Kim, B. S., Satija, S. K., Mather, P. T., Esker, A. R., Macromolecules 2007, 40, 682–688(Reference 133) with permission of American Chemical Society.



3.2. Anti-Dewetting Effect of POSS Moieties in Polymer Thin Film

Polymer thin films feature broad technological applications, including dielectric layers,biocompatible coating, microelectronics, among others. The particular problem in produc-ing polymer thin films is how to prevent them from dewetting and breaking up on thesubstrate.139 Adding nano-particles into the polymeric matrix has been found to be an ef-fective way to stabilize polymer thin film, since Barnes et al.140 for the first time discoveredthat the spin-coated polystyrene (PS) and polybutadiene (PB) thin films can be preventedfrom dewetting on the silicon wafer substrates by adding a small amount of fullerene (C60)nanoparticles. Furthermore, nanofillers improve the stability of polymer thin films not onlyon inorganic substrates,141,142 but on organic ones as well.143,144

With a well-defined size of approximately 1.5 nm, POSS moieties are also goodcandidates to be utilized as a stabilizer to polymer thin films. Recently, Hosaka et al.145

found the incorporation of cyclopentyl-POSS(CpPOSS) could lead to a dewetting inhibitionof PS thin film on silicon wafers. After annealing at 393 K for 20 min, the PS thin filmwas completely dewetted. However, the holes formed on the PS-based blend thin filmincorporating 10 wt%CpPOSS stopped growing and the dewetting process was inhibited.Furthermore, the dewetting area fraction of CpPOSS/PS thin film decreased with theincrease of CpPOSS incorporation. Once the CpPOSS concentration was higher than 15wt%, almost no holes formed on the substrate. The XPS analysis indicated that CpPOSSredistributed in the thin film during the annealing process. Accordingly, CpPOSS moietieswere rich in two regions: one was at the surface of the film, where the Si and O concentrationwas 6 times as high as the theoretical value of the mixture. The other was at the film-substrateinterface (the rim of the hole). The authors proposed that CpPOSS segregation at the filmsurface reduced the surface free energy and spreading coefficient, and its segregation atthe film-substrate pinned the contact line of PS film and substrate, leading to the inhibitionof dewetting. Meanwhile, they realized the POSS segregation and dispersion state were afunction of vertex (R) groups surrounding the Si-O cage, which determined the interactionbetween POSS moieties and the polymer matrix. They selected three kinds of POSSmolecules with various vertex R-groups: phenethyl (PhPOSS), fluoroalkyl (CpPOSS-Rf)and hydroxyl (CpPOSS-2OH). The chemical structures of these POSS molecules and theirinfluence on dewetting of PS thin films are shown in Fig. 11.

The presence of phenethyl group rendered PhPOSS homogeneously dispersed in thePS thin film, resulting in a decrease of glass transition and viscosity. Consequently, thefilm dewetting rate was accelerated and the corresponding rupture was enhanced. Dueto the presence of fluoroalkyl groups, CpPOSS-Rf was immiscible with PS and tendedto strongly aggregate at the surface of the PS thin film. This reduced the surface freeenergy and resulted in the retardation of film dewetting on the silicon wafer substrate.Like CpPOSS-Rf, CpPOSS-2OH was also immiscible with the PS matrix. Interestingly,CpPOSS-2OH nanoparticles not only aggregated on the film surface, but also segregatedat the film-substrate interface, which was assigned to the specific attraction between –OHgroups of CpPOSS-2OH and silicon substrate. The rough and immobilized layer at theinterface played the pinning role to inhibit the dewetting. The authors proposed that thePOSS dispersion state be one of the key parameters to control the dewetting of the polymerthin film on the given substrate.

Like any system with two components, the miscibility of POSS moieties and the poly-mer is a function of temperature and composition, which can be described by a phasediagram. Paul et al.146 studied the thin film blends of poly (tert-butyl acrylate)(PtBA)and trisilanolphenyl-POSS(TPP) and demonstrated that this blend featured lower critical


POSS Polymers 47

Figure 11. Optical microscopic images of LB thin film PS-based blends with varying POSS moieties:(a) pure PS, (b) Ph-POSS/PS, (c) CpPOSS-Rf/PS and (d) CpPOSS-2OH/PS. The molecular weight ofPS uesed is 2 kg/mol and POSS inclusion is 10 wt%. All of the thin films with ∼60 nm thickness wereannealed at 100◦C for 24 h under vacuum. The embedded bar is 300 µm. Reprinted from Hosaka, N.,Otsuka, H., Hino, M., Takahara, A., Langmuir 2008, 24, 5766–5772 (Reference 145) with permissionof American Chemical Society.

solution temperature (LCST) behavior. The critical temperature and composition were 70◦Cand 60 wt% TPP, respectively. Off the critical condition, the thin film blends with 58 wt%and 62 wt% POSS followed the nucleation and the growth mechanism of phase separation.As to the sample with 60 wt% POSS, Fast Fourier Transform (FFT) analysis of their opticalmicroscopic images revealed the thin film blends underwent spinodal decomposition at ele-vated temperatures. At the early stage, the characteristic wavevector (q) followed the scalinglaw with time (t), e.g. q ∼ tn with n = −1/3 ∼ −1/4. At the late stage, the phase separateddomains were pinned. Apparently, the dispersion and aggregation of POSS nanoparticles inthe polymer thin film could be controlled by phase behavior and phase separation, leadingto the controllable dewetting behavior on the substrate. Related significant studies are inprogress.

Paul et al.147 similarly investigated the morphological evolution of the POSS/polymerthin film bilayer, which was different from the system of the POSS/polymer blends previ-ously reviewed. The selected trisilanolphenyl-POSS (TPP) was deposited on a PS-coatedsilicon wafer employing the spin-coating technique. It was found that the morphologicalevolution of the TPP/PS bilayer did not follow nucleation-growth nor spinodal decomposi-tion. Annealing at high temperature initially made the TPP layer crack because of its internaltensile stress. Due to the fact that they were the nucleation sites, cracks were induced thedewetting and aggregation of TPP on the PS layer, which further induced dewetting ofthe lower PS layer. The two stage cracking/dewetting mechanism can be attributed to abrittle upper TPP layer that, unlike its polymer counterpart, underwent plastic deformation.Interestingly, the TPP/PS bilayer finally formed the TPP encapsulated PS droplet aftercompletely dewetting.

3.3. Photo-Oxidative Resistance of POSS

As mentioned previously, POSS cages are usually terminated by alkyl groups at eight cor-ners of the Si-O bonded cage. Relative to the stronger Si-O bonds, C-H, C-C, and evenC-Si are significantly weaker. Once POSS cages are exposed to a severe environment,including high temperature, high-energy ion beams, and oxygen plasma, only Si-O bonds



can survive, while others degrade and form volatile organic compounds. More impor-tantly, the survived Si-O bonds can further form a SiO2-like surface layer on the POSSmaterials to prevent further etching and consumption. Due to such excellent oxidationresistance, POSS-based polymeric nanocomposites become new promising materials uti-lized for photooxidatively resistance materials and the next generation of lithographytechnology.93,148

Eon et al.93,148 investigated poly(tert-butyl methacrylate) (PtBMA)-based POSS ran-dom copolymers as the photoresist components for 193 nm lithography, a process intendedfor the achievement of microelectronic circuits with a minimum critical dimension around50 nm. Employing in situ ellipsometry and in situ X-ray photoelectron spectroscopy, theauthors observed the etching and thickness loss behavior of PMMA-POSS random copoly-mers in an oxygen plasma with a voltage bias varying from 0 to − 100 V. It was found thatthe copolymer etching rate decreased exponentially with plasma exposure time and that thispassivation-type etching resistance increased with the increasing POSS content. A partialreplacement of methacrylic acid (MA) with tBMA did not change the etching rate and itstime evolution. With regard to bias voltage effect, the initial etching rate of the copolymerswith high POSS loading (>60 wt%) was almost independent of bias voltage; however, thematerials with lower POSS content (≤40 wt%) exhibited a significant increase in the initialetching rate as the negative bias voltage was increased from 0 V to − 100 V. Using FTIR(Fig. 12), the authors found that with increasing of oxygen plasma exposure time, the Si-O-Si absorption peak (asymmetric stretching mode) shifted from 1105 cm−1 to 1050 cm−1

and at the same time the Si-CH3 peak diminished. It is known that the peaks centered at1105 cm−1 and 1050 cm−1 are assigned to Si-O-Si in POSS and SiO2, respectively. Thus,

Figure 12. FT-IR spectra for 100 wt% ethyl-POSS materials varying with oxygen plasma exposuretime. The peaks of Si-O-Si in SiO2 and in POSS are centered at 1150 cm−1 and 1050 cm−1,respectively. The peak of Si-CH3 is centered at 1250 cm−1. Reprinted from Eon, D., Raballand, V.,Cartry, G., Cardinaud, C., Vourdas, N., Argitis, P., Gogolides, E., J. Vac. Sci. Technol. B 2006, 24,2678–2688 (Reference 148) with permission of American Vacuum Society.


POSS Polymers 49

the oxygen plasma oxidized the POSS materials to form a SiO2-like layer on the surface ofthe POSS-based random copolymers. XPS was further employed to analyze the change ofcontent of the various elements at the surface of the POSS-based random copolymer afteroxygen plasma exposure. The peak intensity integration revealed that after oxygen plasmaexposure, the ratio of Si to O at the surface layer was almost 1/2, and the atomic percentageof carbon (C) was minimal in the oxide layer. Thus, FTIR and XPS data indicated that theoxygen plasma oxidized the POSS-based copolymers to form a passivating silicon dioxidelayer that prevented further etching because of strikingly high bond energy of Si-O. Thismay be the mechanism by which POSS can improve the photo-oxidative resistance ofmaterials.

Another promising and related application for POSS-based polymers is their usefor space-bodies in earth orbit.23 In space, whether low earth orbit or geosynchronousorbit, severe environmental conditions prevail: the atomic oxygen and vacuum ultra-violet radiation can make the highest performance polymers,149 such as KaptonTMandTeflonTM, degrade rapidly. The reason is that the bonds of organic molecules will un-dergo scission at about 4 eV, which is lower than the energy of atomic oxygen col-lision with about 5 eV. By comparison, the bond energy of Si-O is approximately8 eV.49 Although the surface organic sections of the polymeric materials incorpo-rated by POSS will be eroded by atomic oxygen, it will result in the formation ofa SiO2 thin-layered network protecting the surface from further erosion of atomicoxygen.150−152

The formation of such a SiO2-like surface layer also changes the surface energy andhydrophilicity of the materials. For POSS-based random copolymers, the hydrocarbonbackbone and the alkyl groups surrounding POSS cage are typically hydrophobic. Afteroxygen plasma exposure, these weak bonds, including C-H, C-C, and C-Si bonds, arepreferentially oxidized, resulting in the glass-like layer at the surface. Augustine et al.153

studied the effect of plasma exposure on the surface hydrophilicity of iBuPOSS-basedPMMA random copolymers. They found plasma exposure not only made the thicknessof iBuPOSS-PMMA thin film decrease, but also significantly reduced their contact anglewith water. Moreover, the contact angle can be tuned by the ratio of oxygen to nitrogen inthe plasma gas. For the copolymer with 40 wt% POSS after 100 s exposure, the oxygenplasma resulted in a contact angle of θ ∼ 41◦, while nitrogen plasma led to a morehydrophobic surface with a contact angle of θ ∼ 57◦. The authors suggested that the surfacehydrophicility or hydrophobicity of POSS-based polymeric materials can be adjusted by theplasma condition, which is significant to control the surface chemistry and the wettabilityof biomedical devices.

4. Applications in Biomaterials

The framework of POSS, constituted by Si-O and Si-C bonds, is similar to silicone, whichis a favored option in biomaterials and first introduced into breast surgery in the 1960s dueto its inert nature and low inflammatory response. Its biocompatibility can be ascribed to thefoci of silicon-rich areas with increased surface free energy.154 Unlike carbon nanotubes,155

POSS moieties are non-toxic and cytocompatible.156,157 Additionally, it has been confirmedthat POSS cages, as nanoscale building blocks, can be incorporated into other polymers withimproved mechanical and viscoelastic properties. Thus, materials scientists are motivatedto extend POSS-based polymeric nanocomposites to tissue engineering and biomedicalapplication.



Figure 13. Optical microscopic images of Toluidine Blue (TB)-stained human umbilical vein en-dothelial cells (HUVEC) on POSS-PCU: (A) at 48 hours, (B) at 6 days. The cell confluence increaseswith culture time. After 6 days, the cell confluence is around 80∼90%. Reprinted from Kannan,R. Y., Salacinski, H. J., Sales, K. M., Butler, P. E., Seifalian, A. M., Cell Biochem. Biophys. 2006, 45,129–136 (Reference 158) with permission of Humana Press, Inc.

4.1. Cardiovascular Nanocomposites

Kannan and Seifalian et al.81−83,158,159 introduced POSS moieties into poly(carbonate-urea)urethane (POSS-PCU), and systematically studied their cytocompatibility, antithrombo-genicity, and biostability. By culturing primary human umbilical vein endothelial cells(HUVEC), they first assessed the cell viability, adhesion, and proliferation of hybrid PCUnanocomposites with 2 wt% POSS.158 The Alamar Blue assay showed that the endothelialcells were able to adhere to POSS-PCU nanocomposites within 30 minutes of contactwithout difference from the control cell culture plates. The PicoGreen assay also evidencedthat POSS-PCU nanocomposites were able to sustain good cell proliferation for up to 14days (even from low seeding density, i.e. 1.0 × 103 cells/cm2) and reach saturation by21 days. As shown in Fig. 13 and Fig. 14, light microscopy and scanning electron mi-croscopy (SEM) observations revealed that endothelial cells can reach confluence on thesurface of POSS-PCU nanocomposites with optimal endothelial cell motility. Additionally,the authors successfully improved cytocompatability and endothelialization by ammonia(NH3) gas-UV surface modification160 and incorporation of bioactive peptide (Arg-Gly-Asp, RGD),161 respectively. After 5 min irradiation of UV light of Xe2*-excimer lamp ata wavelength of 172 nm in a NH3 gas, the treated nanocomposites featured a significantly

Figure 14. Scanning electron microscopic images of adsorbed human umbilical vein endothelialcells (HUVEC) morphology on POSS-PCU at 48 hours. There are a lot of cellular filopodia to formwithout “rounded” cells and cell retraction observed. Reprinted from Kannan, R. Y., Salacinski, H. J.,Sales, K. M., Butler, P. E., Seifalian, A. M., Cell Biochem. Biophys. 2006, 45, 129–136 (Reference158) with permission of Humana Press, Inc.


POSS Polymers 51

increased cell proliferation between 3 and 8 days after HUVEC seeding as compared tothe untreated ones. Due to the incorporation of RGD, the number of endothelial progenitorcells (EPC) colonies on the bioactive nanocomposites was almost four-fold larger than theuntreated counterparts.

The antithrombogenic properties were evaluated using thromboelastography (TEG)Fibrinogen ELISA assays and Anti-factor Xa test.83 TEG analysis showed a significantdecrease in clot strength and an increase in clot lysis, although the nanocomposites didnot have a significantly lower TEG amplitude value (MA) than pure PCU and the con-trolled PS sample. Fibrinogen ELISA assays and the Anti-factor Xa test indicated thatthe nanocomposites featured a lower fibrinogen adsorption, which can be seen as the mainreason why the nanocomposites featured a significantly less platelet adsorption than both ofpure PCU and PTFE control. It was believed that this antithrombogenic effect was also theresult of the surface morphology/topology of POSS-PCU. The POSS nanocages induceda nanoscale extended surface configuration with a “mushroom/domelike” profile, whichlowered fibrinogen adsorption and improved the protein repellent activity and the repulsionof platelet in the blood case as well.

Biostability is one of the most important considerations for the selection of polymers formedical use. In vitro hydrolysis and oxidation tests were conducted to assess the degradativeresistance of POSS-PCU.159 Strength and toughness/elasticity analysis revealed that allsamples showed no significant difference in their elastic properties after a 70-day hydrolysisand oxidation test, indicating their striking biostability for all forms of in vitro degradationenvironments. The authors proposed that POSS nanocages chemically incorporated inPCU chains conferred a type of “protective” or “shielding effect” on the soft phase (CU),thereby preserving its elasticity and compliant properties from all forms of degradation,particularly in oxidation and hydrolysis. Although it is known that strongly basic162 andstrongly acidic163 conditions can cause the opening and reaction of POSS cages, the POSS-PCU backbone remained intact for all in-vitro degradation environments. After implantingthe POSS-PCU samples in healthy sheep, the authors found that POSS-PCU adsorbed andinactivated the fibrinogen on their surface, leading to inflammation inhibition. In contrast,the siloxane control sample showed significant inflammation, degradation, and fibrousencapsulation.81 Simply put, both in vitro and in vivo experiments revealed that POSSincorporation enhanced the biological stability as compared with traditional PTFE andsilicone biomaterials. It can be concluded that POSS-PCU may be an alternative materialselection in place of poly(tetrafluoroethylene) (PTFE) and poly(ethylene terephthalate)(PET, DacronTM), for the construction of both vascular prostheses and bypass grafts. Weshould note that not all POSS polyurethanes are biostable; in fact, biodegradability ispossible through the use of a different soft segment. Recently, Knight et al.164 foundpoly(lactide)(PLA)-based polyurethanes with 20.8 wt% POSS incorporation showed 60%loss in molecular weight as compared to the original one after one week incubation in aPSB buffer. This observation indicated that the biostability of POSS-based polyurethanedepended on the polyurethane soft segment.

4.2. Dental Nanocomposites

Methacrylate-based polymers have been extensively introduced as dental implant materi-als for 40 years. However, as compared with metallic and ceramic materials, the familyof methacrylate-based polymeric dental implants features several clinical weaknesses tobe overcome, such as volume shrinkage during polymerization and lack of strength and



toxicity due to the residual monomers. Recent research156,165−168 showed one promising so-lution to these problems was to chemically incorporate POSS moieties into these polymericmaterials. Gao and Culbertson168 tried three synthesis routes to incorporate methacrylate-POSS (MA-POSS) into poly(methyl methacrylate) (PMMA): (1) Copolymerization ofdental monomer (methyl methacrylate, MMA) and MA-POSS, (2) Copolymerizationof POSS-containing macromonomer and MMA, and (3) Synthesis of POSS-containingcopolymers followed up by an in situ polymerization with a dental monomer. It was foundthat the incorporation of a small amount of MA-CpPOSS can efficiently reduce the shrink-age of MA-based neat resins and increase the double bond conversion in all of the threecases. More importantly, the authors found that a proper synthesis route is critical to POSScage dispersion and mechanical properties improvement. As compared with synthesis route(1), both synthesis route (2) and (3) allowed POSS moieties to disperse better in the poly-meric composites matrix, resulting in the increase of compressive strength, flexural strength,and tensile strength over a range from 20∼50%.

Fong et al.166 evaluated the effects of MA-POSS content on the properties of 2,2′-bis-[4-(methacryloxypropoxy)-phenyl]-propane (Bis-GMA)/tri-(ethylene glycol) dimethacrylate(TEG DMA)-based dental composites. They found that the volume shrinkage of poly-merization was independent of the MA-POSS content and the double bond/monomerconversion decreased with the MA-POSS content increase. A small amount of the MA-POSS (<10 wt%) partially in place of Bis-GMA can enhance the mechanical properties,e.g. 20% increase in flexural strength and 35% increase in modulus, while a large amountof MA-POSS(≥10 wt%) led to less desirable mechanical properties, lower double bondconversion, and slower photo-polymerization. They suggested that the judicious choiceof monomer composition (POSS-MA, Bis-GMA, and TEGDMA) was essential to obtainpolymeric dental restorative composites with improved mechanical properties.

Kim el al.156 further assessed the influence of POSS incorporation on the biocompat-ibility of methacrylate-based hybrid dental composites. The metabolic and mutagenesisassay revealed that POSS-based PMMA nanocomposites, especially after immersing indistilled water for 72 h, showed the highest metabolic activity and the lowest cytolysisand mutagenicity as compared to three commercial acrylic denture base resins. In sum-mary, POSS incorporation not only overcomes the weak points of conventional commercialacrylic dental composites to a great degree, but improves their biocompatibility as well.

4.3. Cationic POSS Conjugates

The eight vertices of the POSS cage can be easily functionalized with a variety of organicgroups, such as –NH2, -SH, -OH, -COOH, among others. Most notably, cationic POSS(e.g. ammonium-POSS) has been paid more and more attention because it can form acomplex with anionic molecules with great potential application in DNA/gene delivery,drug delivery, and DNA/protein detection. Zou et al.169 were the first to assess POSS as aprobe to detect biomolecules such as DNA and proteins, using a resonance light scatteringtechnique (RLS). The authors found that the RLS intensity at a wavelength of 360 nm can bestrongly enhanced by adding DNA molecules into a cationic POSS aqueous solution. TheRLS intensity increased with the increase of DNA concentration, and reached a maximumfor a ratio of cationic-POSS: DNA = 1:2. The RLS intensity of the POSS-DNA complexwas also strongly dependent on the pH value and ionic strength. The maximum value ofthe RLS intensity appeared when [NaCl] = 4.8 × 10−3mol/l and pH = 4.5 due to thestrongest interaction between DNA and cationic POSS. Furthermore, the maximum RLS


POSS Polymers 53

intensity remained constant in 2 h at least. Due to its strikingly sensitive, convenient, rapid,and reproducible (3σ = 0.32 ng/ml) features, cationic POSS can be potentially used as theprobe to determine DNA concentration via the RLS technique as the author suggested.

McCusker et al.170 tried to probe the utility of POSS as a drug delivery agent employ-ing Octa-Ammonium-POSS-BODIPY (fluorescence label), and found that the POSS unitwas the key factor to make POSS-BODIPY conjugate disperse in the cytosol as comparedwith BODIPY control. As shown in Fig. 15, confocal microscopy images showed thatonly the POSS-BODIPY conjugate molecule can be uptaken by the cytosol of a Cos-1cell. Moreover, the uptaken POSS-BODIPY did not influence the cellular morphology. TheMTT viability assay further proved that the Cos-1 cell with POSS-BODIPY had the sameactivity level as the control, indicating that POSS featured an extremely low toxicity level.Furthermore, even dispersion in cytosol demonstrated that the POSS-BODIPY conjugateentered into a cell via diffusion, not via endocytosis. The lack of nuclear uptake demon-strated its specific localization in the cell. Apparently, these observations evidenced thatthe POSS cages can be a potential drug delivery carrier by direct conjugation with drugmolecules that were insoluble in water or exhibited a lower cellular uptake.

Cui and Zhu171,172 studied the influence of the POSS molecular shape and the POSScrystallization behavior on the mesophase/morphology of double-strand DNA (fish spermDNA)-cationic POSS imidazolium lipids in details. Systematically changing the molecular

Figure 15. Fluorescence confocal microscopy obserations on the influence of POSS on BODIPYdelivery: (a) POSS-BODIPY can be up-taken by the cytosol and (b) pure terminated BODIPY cannotenter into the cytosol without the aid of POSS. Reprinted from McCusker, C., Carroll, J. B., Rotello,V. M., Chem. Commun. 2005, 996–998 (Reference 170) with permission of The Royal Society ofChemistry.



Figure 16. Bright-field TEM microscopy images of DNA-POSS imidazolium salt complex. The darkDNA domains were packed into: (A) the inverted hexagonal and (B) the lamellar phases obtainedunder relatively fast (0◦C) and slow (130◦C) isothermal crystallization, respectively. The insert imagein (a) shows a head-on view of hexagonally packed DNA cylinders. (Reprinted from Cui, L., Chen,D. Y., Zhu, L., ACS Nano 2008, 2, 921–927 (Reference 172) with permission of American ChemicalSociety).

shape in cationic tails from rod-like (cyanobiphenyl), to discotic (triphenylene), and cu-bic (POSS cage), they achieved the morphological transitions in the DNA-cationic lipidcomplexes from a typical Smectic-A lamello-columnar phase for the DNA-rod complex,to a double lamello-columnar phase for the DNA-disk complex, and finally to an invertedhexagonal phase for the DNA-POSS cube complex. They also found that the morphol-ogy of the DNA-cationic POSS inidazolium lipid self-assembly complex also dependedon POSS crystallization behavior. Due to the competition between POSS crystallizationand negative curvature of cationic imidazolium lipids, fast (i.e. quenched from the meltto room temperature) and slow (i.e. annealing at 130◦C) isothermal POSS crystallizationinduced an inverted hexagonal phase (shown in Fig.16(a)) and lamellar phase (shown inFig.16(b)), respectively. Above the POSS melting point, the amorphous POSS only in-duced an inverted hexagonal phase due to the negative spontaneous curvature of the POSSimidazolium cationic lipid. Their research work provides a new pathway to control themesophase morphology of lipoplex, including the lamellar phase vs the inverted hexagonalphase, which is one of the most important factors for efficient gene transfection.

5. Summary

In this review article, we have attempted to summarize the diverse and significant researchon POSS-based polymers occurring during the past several years. We have learned from ourstudy of the literature that certain elements of structure-property relationships are prevalentthroughout widely varying POSS-polymer systems. In particular, the state of aggregationor crystallization of POSS moieties plays a prominent role in determining such physical


POSS Polymers 55

properties as viscosity and melt elasticity. Also, connectivity (or not) of POSS to thepolymeric host influences microstructure and physical properties in a profound manner.Variation in the POSS vertex (R) group has been found to determine the interactionsbetween the POSS moiety and the host polymer segments and this, in turn, has been foundto impact microstructure and rheology. Chemically, the POSS cage has been shown toafford a photo-oxidative stability to polymers through a passivation mechanism and thiscan be exploited in a diversity of applications from the stability in space to high-resolutionlithography. A hot topic of POSS research has emerged in the study of amphiphilic POSSmoieties and macromolecules at surfaces and interfaces, including the ability to growa precision nanostructured coating by the Langmuir-Blodgett transfer methods. Finally,researchers are actively pursuing biological and medical applications of POSS and POSS-based polymers and their studies have revealed desirable characteristics ranging fromnon-toxicity to biostability. We anticipate that research on POSS-based polymers willcontinue to grow and we look forward to exciting fundamental science as well as continueddevelopment of applications that exploit the unique properties of this hybrid, nanoscopicgroup known as POSS.

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