polyhedral oligomeric silsesquioxane(poss)-based polymers

Polyhedral Oligomeric Silsesquioxane(POSS)-Based PolymersJoseph J. Schwab1* and Joseph D. Lichtenhan2

1Hughes STX Corporation, Phillips Laboratory, Propulsion Directorate Edwards AFB, CA 93524, USA2Phillips Laboratory, Propulsion Directorate, Edwards AFB, CA 93524, USA

A diverse and entirely new class of monomer andpolymer technology based on polyhedral oligo-meric silsesquioxane (POSS) reagents has beendeveloped. POSS reagents are unique in thatthey are physically large (approx. 15 Adiameterand 1000 amu) and are composed of a robustsilicon–oxygen framework that can be easilyfunctionalized with a variety of organic sub-stituents. Appropriate functionalization of POSScages allows for their incorporation into tradi-tional thermoplastic resins without modificationof existing manufacturing processes. The incor-poration of POSS segments into linear copoly-mer systems results in increased glass transitionand decomposition temperatures, increased oxy-gen permeability and reduced flammability andheat evolution, as well as modified mechanicalproperties relative to conventional organicsystems.# 1998 John Wiley & Sons, Ltd.

Keywords: polyhedral oligomeric silsesquiox-ane; hybrid; thermoplastic; polymerReceived 17 June 1997; accepted 2 September 1997

INTRODUCTION

Since their discovery almost 70 years ago, syntheticpolymers have played an increasingly importantrole in daily life. This is especially true today, whenpolymers and plastics are finding their way into anevermore diverse array of products and applica-tions. However, along with this increased use havecome more demanding requirements, such ashigher temperatures of use and greater resistanceto oxidation. Although the polymer industry has

been able to keep abreast of these demands, throughthe use of additive, filler and blend technologies, aswell as increased empirical understanding ofpolymer microstructures, new developments tendto be incremental and the existing technology andknowledge are rapidly being pushed to their limitsby the more demanding requirements of end-users.Although the range and types of plastic andpolymer systems are quite impressive, much ofthis work has been accomplished using a relativelylimited number of basic chemical building blocks,i.e. the monomers which make up the primarystructure of the polymer system. Without excep-tion, these monomers are based on organic systemsand the pool of available building blocks hasremained almost static over the past 25 years.

A recent approach to meeting these demands hasbeen the development of new plastics that haveproperties intermediate between those of traditionalorganic systems (i.e. polymers) and those oftraditional inorganic systems (i.e. ceramics). Thisinterest, along with the concomitant research effort,has given rise to the field of hybrid materialsresearch, which endeavors to combine the proper-ties of traditional organic polymer systems (i.e.processability, toughness, cost) with the propertiesof inorganic compounds (i.e. thermal and oxidativestability) (Fig. 1). For reviews and leading refer-ences on recent progress in hybrid materials, thereader should consults Refs 1–3.

Figure 1 Schematic representation of the area of hybridmaterials.

APPLIED ORGANOMETALLIC CHEMISTRYAppl. Organometal. Chem.12, 707–713 (1998)

# 1998 John Wiley & Sons, Ltd. CCC 0268–2605/98/100707–07 $17.50

* Correspondence to: Joseph J. Schwab, Hughes STX Corporation,Phillips Laboratory, Propulsion Directorate Edwards AFB, CA93524, USA.Contract/grant sponsor: US Air Force Office of Scientific Research,Directorate of Chemistry.Contract/grant sponsor: Phillips Laboratory, Propulsion Directorate.

Using this basic idea, severalbroad classesofhybridplasticshavebeendevelopedandareshownschematicallyin Fig. 2. Examplesof theseincludethe sol–gelnetworks,which arethree-dimensionalcrosslinkedsystemshaving a dispersedinorganic,silica-like phasein a polymer matrix; preceramicpolymers,suchas the polyphosphazenes,polycar-bosilanes, polysilanes and polysiloxanes; andinorganic/organicpolymer blends.Furthermore,arevitalizationof clay-filler technologyhasoccurredwith the developmentof methodsfor exfoliatinglayered clays into sheets which then becomedispersedon a finer level thanpreviouslypossible.More recentlya diverseandentirelynewchemicaltechnologyof polyhedraloligomericsilsesquioxane(POSS)nanocompositeshasbeendeveloped.Thistechnology affords the possibility of preparingplastics that contain nanoscale reinforcements(POSSsegments)directly bound to the polymerchains.Unlike previoushybrid systems,the POSStechnologymaterialshavetheadditionaladvantageof beingemployablein thesamemannerasorganicmonomersor in the form of blendableresins.

Herewedescribesomeof ourpreliminaryeffortsto preparewell-defined, nanoreinforced,thermo-plasticpolymerichybridswith enhancedpropertiesthatcanbeunderstoodandtailoredat themolecularlevel.

RESULTS

Whereasthe majority of thework on the synthesisof hybridmaterialshasbeendirectedtowardsuseofthe sol–gel process,4 our approachhas been topreparehybridmaterialswhichcanbeincorporatedinto traditional linear, thermoplasticpolymer sys-tems. The hybrid materials developed in ourlaboratoryarebasedon POSSprecursors.5,6 POSSprecursorsare discrete, structurally well-definedcompoundscomposedof a silicon–oxygenframe-work having the generalformula (RSiO3/2)n, andarepreparedvia the hydrolysisof organotrichloro-silanesasshownin Scheme1.7,8,9

Although someof the POSSprecursorsisolatedfrom thehydrolysismixturecanbeuseddirectly inpolymerizationreactions,additionalfunctionaliza-tion is often required.In order to incorporatethePOSSreagentsinto linear polymer systemsandthusavoidtheformationof intractable,crosslinkednetworks, it is necessaryto limit the numberoffunctional groups to no more than one or two.Functionalizationof the POSSframeworkis mosteasily accomplishedby corner capping of thePOSS-trisilanolswith silanecoupling agentscon-tainingorganicgroupssuitablefor polymerizationsand/or grafting reactions (Scheme 2). Isolatedyields from the corner capping reactionsexceed

Figure 2 Schematicrepresentationsof different hybrid plastics.

# 1998JohnWiley & Sons,Ltd. Appl. Organometal.Chem.12, 707–713(1998)

708 J. J. SCHWAB AND J. D. LICHTENHAN

Scheme1 POSSproductsfrom the hydrolysisof cyclohexyltrichlorosilane,(R = c-C6H11).

Scheme2 Synthesisof polymerizablePOSSreagents(R = non-reactivefunctionality;R' = polymerizable or graftablefunctionality).

Scheme3 POSSfunctionalgroupinterconversion.

Figure 3 Someof the functionalitieswhich canbe preparedfrom POSS-trisilanolprecursors.


POSS-BASEDPOLYMERS 709

90% and are typically greaterthan 95% if careistakento excludemoisturefrom thesystem.

In caseswhere the appropriatefunctionality isnot directly available by the corner cappingsequence,subsequentfunctionalgrouptransforma-tionsof thereactivegroupon theuniquesiliconare

possible.For example,the conversionof POSS-a-olefins to POSS-epoxidesusing m-chloroperben-zoic acid (MCPBA) is shownin Scheme3.

Using thesemethods,we havebuilt up a largecatalogueof POSSreagentscontainingpolymeriz-able and/or graftable functionalities. This makes

Figure 4 Polymerarchitecturesavailablefrom POSSmonomers:(A) pendant;(B) ABmultiblock ‘bead’, (C) ABA triblock; (D) star.



possible the incorporation of POSS units intotraditional resinsystems.Sometypical functional-ities arelisted in Fig. 3.

ThesePOSSreagentshaveanumberof desirablephysicalproperties.For example,they are highlysoluble in common organic solvents such astetrahydrofuran(THF), toluene, chloroform andhexane.They havehigh thermalstabilities,10 andcanbe polymerizedusingstandardpolymerizationprotocol(i.e. radicalpolymerizations,condensationpolymerizations,ring openingpolymerizationsetc.)to providepolymerswith a varietyof architecturesasillustratedin Fig. 4.

Whereashybrid materialsproducedfrom eitherthe sol–gelprocessor from multifunctional POSSprecursors11 (i.e. with functionality> 2) arehighlycrosslinkednetwork polymers(akin to thermosetplastics)andassuchtendto bedifficult to process,our reagentscanbe usedto producethermoplasticpolymers in which the inorganic portion of thehybrid is covalentlylinked to the organicpolymerchain. It is this uniqueability to start from well-definedhybrid monomersthat makesit possibleto

control the polymerarchitectureandultimately totailor thepolymerpropertiesat themolecularlevel.

Incorporation of POSS-silanol,12 POSS-acry-late,13 POSS-styrene,14,15 POSS-norborna-diene,16,17 and POSS-bisphenolA monomers18,19

into the respectivepolysiloxanes,polyacrylates,polystyrenes, polynorbornadienes and polyur-ethaneshasbeendemonstrated.All of the POSS-hybrid polymerspreparedso far showthermoplas-tic behavior,and the POSS-urethanesexhibit fullthermoplasticelasticity. To our knowledgetheseare the first hybrid organic/inorganicpolymerstoexhibit this behavior.

Incorporationof POSSmonomersinto ordinarypolymerscanhavedramaticeffectson thecopoly-mer properties.For example, Fig. 5 shows thethermomechanical(TMA) data for a set ofcyclopentyl-POSS-methacrylate/butyl methacry-late copolymers.Copolymerscontaining10mol%POSShavea softeningpoint almost130°C higherthanthe20°C glasstransitiontemperature(Tg) foratactic poly(butyl methacrylate).The value of Tgfor atacticpoly(butyl methacrylate)is takenfrom

Figure 5 TMA data from cyclopentyl-POSS-methacrylate/butylmethacrylatecopolymers: top, 50:50copolymer;middle,20:80copolymer,bottom:10:90copolymer.(Au proportionsin mol%.)



Ref 20 which also reports a range of 13–35°C.Incorporationof 20mol%POSSresultsin a furtherincreasein the softeningpoint by approximately70°C, while a 50:50 (mol/mol) copolymerhasasofteningpoint over220°C higherthanpoly(butylmethacrylate).To date,similar resultshave beenobservedin everyPOSScopolymersystemwehavestudied.

Furthermore,as a result of their high formulaweights, incorporation of only a small molpercentageof POSS monomer is necessarytoachieveincorporationof a high weight percentageof POSS(Fig. 6).

We also find that relatively small amountsofPOSSmonomerarenecessaryto achievemanyofthese property enhancements.For example, incyclopentyl-POSS-methacrylate/methylmethacry-late copolymers we find that addition of only5 mol% (35wt%) POSSresultsin a 50°C increasein Tg. Moreover, the relationship between thepercentagePOSSincorporationand the increasein Tg appearsto be roughly linear up to thedecompositionpoint, thusdemonstratingthetailor-ability of POSS-containinghybrid polymers. Inaddition to increasedTg, we have also observedincreasedTdec, increasedresistanceto oxidation,increasedoxygenpermeability,reducedflammabil-ity, reducedheat evolution, increasedchar yieldsandenhancedmiscibility.

Althoughwe areonly now beginningto developan understandingof the effects that POSSframe-

works have on polymer microstructureand theresulting effects on macroscopicproperties,webelieve that the observedproperty enhancementsarisein part from theability of thePOSSsegmentsto dominate polymer chain motions. Comparedwith traditional organic monomers,POSSmono-mers are physically very large. For example,theareaswept out by an octomericPOSSmonomercontaining cyclohexyl substituents is approxi-mately 15A, in diameter with an inner Si–Sidiameterof 5.4A . Furthermore,POSSmonomershaveformulaweightsof greaterthan1000amu.Byway of comparison, the dimensions or crystallamellae in semicrystallinepolymers are of theorderof 10–25A � 0.1–1mm with the amorphouslayer thickness ranging from 5 to 10nm.21 Ittherefore seems reasonableto consider POSSframeworksasmacromericwith respectto polymerchaindimensions.

SUMMARY

The developmentof POSS-basedreagentshasprovided polymer and materialsscientistswith anewclassof hybridbuildingblocks.POSSreagentsare discrete,well-defined monomersthat can beincorporatedinto polymer systemsusing estab-lished polymerizationprotocol. The hybrid poly-mer systems into which POSS has been

Figure 6 Relationshipbetweenmol as percentageand weight percentageofPOSSin cyclopentyl-POSS-methacrylate/methyl methacrylatecopolymers.



incorporatedhavebeenshownto be thermoplasticin nature,andto haveenhancedthermal,mechan-ical and oxidative propertiesrelative to the non-POSS-containingpolymers.In addition,theproper-tiesof aparticularpolymersystemcanberationallytailored by controlling the percentageof POSSincorporated.

Acknowledgments We gratefully acknowledgetheAir ForceOfficeof ScientificResearch,Directorateof ChemistryandLifeSciences,and the Phillips Laboratory,PropulsionDirectorate,for financialsupport.

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polyhedral oligomeric silsesquioxane(poss)-based polymers

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