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Polymer 48 (2007) 3041e3058www.elsevier.com/locate/polymer

Formation of nanostructured epoxy networks containing polyhedraloligomeric silsesquioxane (POSS) blocks

Adam Strachota*, Paul Whelan, Jirı Krı�z, Jirı Brus, Martina Urbanova,Miroslav Slouf, Libor Matejka

Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovskeho nam. 2, CZ-162 06 Prague, Czech Republic

Received 25 September 2006; received in revised form 5 March 2007; accepted 21 March 2007

Available online 27 March 2007

Abstract

Nanostructured epoxy networks, based on DGEBA and poly(oxypropylene)diamine (Jeffamine D), containing nano-sized inorganic blocks,polyhedral oligomeric silsesquioxanes (POSS), were investigated. The POSS were incorporated in the network as crosslinks or as pendant unitsby using octa- or monoepoxy-POSS monomers, respectively, as well as diepoxides with pendant POSS. The authors focused on investigating therelationship between the network formation process and the final product properties. The reactivity of the epoxy-functional POSS monomers, thehybrid systems’ time of gelation, the gel fractions and the phase structure of the networks were determined using 1H or 13C NMR spectroscopy,chemorheology experiments, solegel analysis and transmission electron microscopy (TEM).

All the POSS epoxides tested show a reduced reactivity if compared to their respective model compounds due to sterical crowding in theneighborhood of their functional groups and due to reduced epoxy group mobility. The incorporation of pendant POSS into networks of thetype DGEBAeJeffamine Demonoepoxy-POSS hence took place only in the late reaction stage. Together with the high tendency of thesePOSS to aggregation, the kinetics favors the formation of small nano-phase-separated POSS domains, which act as physical crosslinks dueto their covalent bonds to the organic matrix. At POSS loadings higher than 70%, topological constraint by POSS leads to a strongly reducedelastic chain mobility, thus additionally strongly reinforcing the networks. The network build-up and gelation of the octaepoxy-POSSeJeffamineD system were slow compared to the reference DGEBAeJeffamine D network due to a low octaepoxy-POSS reactivity and due to its strongtendency to cyclization reactions with primary amines. The topology of the amino groups is shown to be very important. In contrast to mono-epoxy-POSS, the octaepoxy-POSS becomes dispersed as oligomeric junctions (purely chemical crosslinks) of the network in the cured product.The octaepoxide’s reinforcing effect is small and is given only by its high functionality and not by its inorganic nature. The functionality effect isreduced by the mentioned cyclizations.� 2007 Elsevier Ltd. All rights reserved.

Keywords: POSS; Epoxy network formation; Kinetics

1. Introduction

Epoxy-functional POSS (polyhedral oligomeric silses-quioxanes), whose reactivity and incorporation into epoxy net-works are discussed in this work, are nano-sized inorganicbuilding blocks (size ca. 1.5 nm). Polymer networks with in-corporated POSS can be regarded as an organic matrix with

* Corresponding author. Tel.: þ420 296 809 264; fax: þ420 296 809 410.

E-mail address: [email protected] (A. Strachota).

0032-3861/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymer.2007.03.052

a chemically bound nanofiller phase. The synthesis of POSSwas first reported in 1946 by Scott [1] (non-functional POSSwith aryl or alkyl substituents on Si) and since then, manypreparative paths to POSS derivatives have been developed,among others by Agaskar [2], Crivello and Malik [3] (hy-drido-POSS), Brown and Vogt [4], Feher et al. [5], Lichtenhanet al. [6] (monofunctionalized POSS) and by Gravel and Laine[7] (non-functional or octafunctional POSS). Syntheticallymost important are ‘‘T8-POSS’’ compounds (octamers), whichhave the general formula R8Si8O12 (see Scheme 1). Numerous

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3042 A. Strachota et al. / Polymer 48 (2007) 3041e3058

compounds of this type are nowadays commercially availableand industrial scale production of POSS has been announcedby Hybrid Plastics [8].

Incorporation of POSS into polymers affects the polymerchain mobility through POSS aggregation (author’s remark)and leads to a change in their thermomechanical properties.Modification of linear polymers by POSS attached as danglingunits on the chain is described in literature [6,9,10e13]. Rel-atively small POSS amounts (ca. 10 wt%) were observed tostrongly raise the glass transition temperature of the modifiedpolymers and to cause the appearance of a broad rubberyplateau in linear polymers, thus reinforcing the modifiedmaterials. The preparation of linear AeBeA triblock metha-crylate copolymers with POSS yielded products with interest-ing phase properties [14]. The modification of polymernetworks by incorporation of POSS into network junctionswas also investigated [3,15]. In this case the effect of POSSincorporation on the thermomechanical properties was foundto be not always unambiguously reinforcing [16e18]. Lichten-han and others [19e21] suggested that POSS aggregation isthe key factor determining modification of the polymer prop-erties. Other researchers [22,23], and also Romo-Uribe et al.[20] (cited above) proposed the inertia of the relatively heavyPOSS units or the interaction of POSS with polymer chains asexplanation for the reinforcing effect, despite some contradict-ing argumentation [24]. Topological constraint to elastic chainmovements by the large and hard POSS molecules occupyinga sizeable volume was also reported as an explanation for thereinforcing effect [25]. Numerous works report the incorpora-tion of POSS as junction into epoxy networks as octaepoxides[26e31], as octaamines [32,33] or as dangling chains [34].The synthesis of hybrid epoxy resins from the incompletelycondensed T7-POSS-triol was also reported [35]. Promisingmaterials have been obtained by incorporating POSS danglingchains or junctions into methacrylate networks [36]. Polyure-thanes [37] have also been crosslinked by POSS junctions.

In our recent papers [38,39] about POSS incorporated intoepoxy resins we showed that the tendency of suitable danglingPOSS units to aggregate (POSSePOSS interaction, see alsoScheme 3) and thus to form strong physical crosslinks playsthe primary and dominant role in the ‘‘POSS-specific’’ rein-forcement of polymers by POSS. The authors do not statethat the effect of POSS inertia or of its interaction with thepolymer chain is exactly equal to zero. The effect of topolog-ical constraint to chain movement by POSS was shown tobecome a very important factor at high POSS loadings in

O Si

O

Si

O

SiSiO

O O

OSi

O

Si

O

SiSiOO O

RR

R

RR

RR

R

Scheme 1. T8-type (octameric) POSS of the composition R8Si8O12, R¼H or

organic group.

the present work, as well as in further investigations concern-ing segment mobility [40]. Only the networks with strongnetwork-bound POSS aggregates (nano-crystallites, seeScheme 3) exhibited an increase in their rubbery modulusand a slight growth of Tg if compared to POSS-free systems.Also all the reinforced polymers reported by Xu et al. [41] in-volved aggregating POSS or percolating crystallites of thePOSS nanofiller. We showed [38,39] that the tendency of dan-gling POSS units to aggregate (the strength of POSSePOSSinteraction) was determined by the organic substituents R ontheir surface (see Schemes 1 and 3bed). In contrast to this,multi-functional POSS, which was incorporated in networkjunctions and well dispersed in the matrix, did not bring any‘‘POSS-specific’’ reinforcement stemming from its inorganicnature. The observed mild increase in the rubbery moduluswas a result of a high crosslinking density due to the highPOSS functionality and could be obtained by the use ofa highly functional purely organic multiepoxide as well. Gen-erally, octafunctional POSS derivatives like POSS,E8 are ofgreat synthetic interest, because they represent highly func-tional building blocks, which are relatively small, highlysymmetrical and whose functional groups are chemicallyequivalent. The network with junction-POSS investigatedby us showed even a lower glass transition temperature thanthe unmodified reference network.

In this paper, we study the formation process of epoxy net-work structures based on poly(oxypropylene)diamine (Jeff-amine D2000) and diglycidylether of bisphenol A (DGEBA),modified by the addition of epoxy-functionalized POSS. Weinvestigated two types of POSS epoxides: (1) dangling-chainPOSS (POSS,E1 or POSScp-DGEBA) and (2) octaepoxy(POSS,E8) compounds. The influence of the network forma-tion process on the final product properties was the focus ofour interest: the network components’ reactivity, the nano-scale phase separation in the reaction mixtures (which canbe strongly influenced by the POSS reactivity), and the gela-tion of the forming networks were studied.

In order to explain the effect of POSS reactivity on the mor-phology of the hybrid networks, we have followed the reac-tions of several POSS epoxides with dibutylamine and withJeffamine, and compared them with the analogous reactionsof DGEBA as well as of model epoxides. The progress ofPOSS,E1 incorporation (as pendant unit) into formingnetworks with DGEBA and Jeffamine D is determined byPOSS’s relative reactivity in comparison to the other epoxideco-monomer, DGEBA. Due to the mentioned strong tendencyof POSS,E1 to aggregation, different network structures wouldbe formed, if POSS,E1 was incorporated preferentially in thebeginning reaction stage, or with the same rate as DGEBA, orin the final reaction stages. The eventual presence and amountof unbound POSS and its effect on the product properties wereto be determined.

In order to elucidate the mechanism of the gelationreactions, the formation of DGEBAePOSS,E1eJeffamineD2000 (pendant POSS) and of several POSS,E8eaminenetworks (junction-POSS) was followed by chemorheologymeasurements, yielding exact time of gelation. The

3043A. Strachota et al. / Polymer 48 (2007) 3041e3058

networks based on POSS,E8, which has a specific chemicalstructure, were studied in more detail: the fraction of the gelin dependence on the network composition and stoichiometrywas studied, as well as the effect of functional group topology(e.g. primary vs. disecondary amino groups). The nano-scalemorphology of the POSS,E8eD2000 network was investi-gated by TEM in order to verify the conclusions drawn fromthe mechanistic study and from previous SAXS data.

2. Experimental part

2.1. Materials

The following POSS epoxide derivatives were obtainedfrom Hybrid Plastics: glycidyloxypropyl-heptaphenyl POSS(POSSphE1), glycidyloxypropyl-heptaisooctyl POSS (POSSoct-E1), glycidyloxypropyl-heptaisobutyl POSS (POSSiBuE1),heptacyclopentyl-POSS-DGEBA (POSScp-DGEBA) and hepta-cyclopentyl-POSS-DGEBA oligomer (POSScp-DGEBA,olig).The poly(oxypropylene) diamines: Jeffamine D2000, D400and D230 (MW¼ 2000, 430 and 230 g/mol, respectively) weredonated by Huntsman Inc., as well as the poly(oxypropylene)monoamine M600 (MW¼ 600 g/mol). The diglycidyletherof bisphenol A (DGEBA, 99.7% highly pure monomer) wasobtained from SYNPO a.s. Pardubice. 1,6-Hexanediamine(HDA), n-hexylamine (HA) and N,N0-dimethyl-1,6-hexanedi-amine (HDSA), dibutylamine (DBA), phenyl glycidyl ether(PGE), butyl glycidyl ether (BGE) and 1,2-epoxyoctane (C8)were purchased from Aldrich.

The POSS octaepoxide (POSS,E8) and tetraepoxide(POSS,E4) were synthesized as described by us earlier [38]from the POSS octasilane Q8M8H8 (octakis(dimethylsily-loxy)-T8-silsesquioxane) and from 5,6-epoxyhex-1-ene or 5,6-epoxyhex-1-ene and 1-hexene, respectively, under catalysisby the platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxanecomplex solution in xylene, containing w2 wt% of Pt. Thesechemicals were purchased from Aldrich.

2.2. Kinetics samples preparation

The kinetics samples for the reactions of POSS,E8 withJeffamine D400 and D2000 were prepared in an analogousway like the networks described further below. The conversionof epoxy groups was determined via 13C NMR (solid stateNMR experiment setup).

All the remaining reactions for kinetics investigations (1HNMR) were carried out in sealed ampoules at 120 �C. The start-ing concentrations of epoxy groups were 0.63 mol/L, those ofamino groups 1.26 mol/L, or, in the case of more diluted exper-iments, 0.17 and 0.34 mol/L. The epoxide and the amine com-ponents were mixed and diluted by the necessary amount(needed to achieve the above concentrations) of 1-methyl-naphtalene or e in the case of the more diluted experiments eby toluene with a small amount of 1-methylnaphthalene (asinternal standard). The solvents also served as an internal stan-dard. After mixing the components, the reaction mixture wasdivided into reaction vessels, which were sealed and put into

a 120 �C oil bath. After the desired reaction time was reached,the samples were frozen instantly at �35 �C. Just before theNMR experiment they were dissolved in CDCl3. (The reactionsinvestigated proceed very slowly at room temperature andstands still in the timescale of our experiments at the freezingtemperature �35 �C.) The conversion of epoxy groups wasdetermined via 1H NMR (NMR in CDCl3 solution).

2.3. Synthesis of networks

The polymer networks were prepared as described by theauthors previously [38]. In the case of POSS,E8-based net-works, the components were simply mixed in the desired stoi-chiometric ratio and cured at 120 �C. In the case of POSS,E1or POSScp-DGEBA-based networks, a compatibilisation of thereaction mixture e either by reaction blending or by using asolvent (toluene) in the initial curing stage e was necessary.These latter networks are referred to use the following designa-tion: e.g. DGEBAePOSS,E1(x¼ 0.30)eD2000 means astoichiometric epoxy-amine network, in which 30% (0.30)of epoxy groups originate in POSS,E1 and the remaining(70%) in DGEBA.

2.4. NMR spectroscopy (for kinetics)

1H NMR spectra used for kinetic investigations were mea-sured with a Bruker (Karlsruhe, Germany) Avance DPX 300spectrometer at 300 MHz.

The relative concentration of the epoxy groups wasdetermined by following the relative intensity (¼ ratio (signal-integral/integral-of-standard-signal)) of the Oxirane-Ring-Signal denoted as ‘‘H1’’, see Scheme 2. This signal (locatedbetween 2.98 and 3.32 ppm in the cases investigated) was al-ways well separated from all the other signals of starting com-pounds and of products. As internal standard the non-volatile1-methyl-naphtalene was used, namely the integral of its wellseparated aromatic signal at 8.97 ppm.

Quantitative 13C NMR SP-MAS spectra were measuredwith a Bruker (Karlsruhe, Germany) Avance DPX 300 spec-trometer at 125.8 MHz (intense pulse: 90�, length 3 ms; intensedipolar decoupling (DD): 84 kHz; long relaxation time: 30 s).This method was used to evaluate the epoxy concentration inthe gel forming reaction systems of octaepoxy-POSS(POSS,E8) with Jeffamines D2000 and D400. The oxirane-ring signals at 51.5 and 46.3 ppm were followed, while theCH3eSi-signal (methyl groups on ‘‘outer’’ Si atoms of theoctaepoxy-POSS used) at 0 ppm served as internal standard(see Scheme 2).

2.5. Determination of the fraction of gel

The solegel analysis of the network samples prepared wascarried out as follows: the POSS-free samples and thosecontaining POSS,E8 were swollen and extracted with acetone,the other networks (with POSS,E1 or POSScp-DGEBA) with tol-uene/tetrahydrofurane (1:1). The samples were extracted for3 days while the extraction solvent was changed every day


for a pure charge. After the extraction, the samples were dried(vacuum, 100 �C) till weight constancy and the fraction of gel(wg) was determined as:

wg¼massðdry; after extractionÞ=massðdry; before extractionÞ:

2.6. Determination of POSS content as SiO2 ash

The POSS content was determined as SiO2 ash mass for theprepared networks in the following way: the samples were putinto a platinum pot together with the double of their weight ofsulfuric acid and this mixture was slowly pyrolysed in air. Theremaining ash was heated to ca. 1000 �C for 15 min. The py-rolysis with the sulfuric acid was repeated once more with theash. The dry SiO2 ash was then weighed, yielding the POSScontent as mass of SiO2. From the amount of SiO2 the amount

OH1

H2

H3

C2C1

(b)

(a)

(c)

Scheme 2. (a) The hydrogen atom H1 and the carbon atom C1 of the oxirane

rings, whose (b) 1H and (c) 13C NMR signals were used to determine the

relative epoxy groups’ concentration.

of Si was calculated, which in turn led to the calculation of theamount of POSS itself (using the weight fraction of silicon inthe known respective POSS compound).

2.7. Dynamic mechanical thermal analysis (DMTA)

The DMTA was performed on an ARES rheometer fromRheometric Scientific (now TA Instruments), USA. The tem-perature dependence of the complex shear modulus of rectan-gular samples was measured by using an oscillatory sheardeformation at a frequency of 1 Hz.

2.8. Transmission electron microscopy (TEM)

TEM experiments were performed on a JEM 200CX micro-scope from JEOL, Japan. The microphotographs were taken atan acceleration voltage of 100 kV, recorded on a photographicfilm, and digitized with a PC-controlled digital cameraDXM1200 (Nikon, Japan). Ultrathin sample sections for TEM,approximately 50 nm thick, were cut with the cryo-equippedultramicrotome Leica Ultracut UCT at the temperatures �80and �50 �C of the sample and of the knife, respectively.

2.9. Chemorheology (for time of gelation)

Chemorheology experiments were performed with anARES rheometer from Rheometric Scientific (now TA Instru-ments), USA, in order to follow the network formation and todetermine the point of gelation of reaction mixtures. The ini-tially liquid samples were measured in the parallel plate geom-etry (diameter 40 mm, thickness ca. 1 mm). The experimentswere carried out at 120 �C using oscillatory multifrequencyshear deformations.

2.10. Determination of the critical molar ratios

The critical molar ratio of the networks studied was deter-mined as the molar ratio of amino protons to epoxy groups r(¼(N)H/epoxy), just at which the reaction mixture alreadydid not gel in 1 week at 120 �C (a typical gel time of thestoichiometric networks at 120 �C was 1e5 h).

3. Results and discussion

3.1. Reactivity of the POSS-based epoxides

The formation process and the final properties of the POSS-containing epoxy networks strongly depend on the reactionkinetics of the POSS monomers used. Usually, the miscibilityof these monomers with the other components of the epoxy-amine systems, i.e., DGEBA and Jeffamine, was problematic(except POSS,E8 and POSS,E4). Solvent addition or e insome cases e only an increased mixing temperature (e.g.120 �C) led to homogeneous reaction mixtures. At higher con-versions, the solubility of free POSS in the forming polymernetwork typically decreased. Therefore, the network formation


inorganic core

organic shell: substituentson POSS

(a)

(b)

(c) (d)

group covalently linking POSS to the network

Scheme 3. (a) Symbolic representation of the formation of additional physical crosslinks in a network by aggregation of pendant POSS units (the aggregates are

encircled); (b) coreeshell structure of a pendant POSS unit; (c) crystallite-like POSS nano-aggregate; (d) liquid-like POSS nano-aggregate.

was determined by the competition of the chemical incorpora-tion of POSS into the growing structure and of the nano-scaled(in the cases studied) phase separation favoring the formationof small POSS domains. These domains (typically several peraggregate [38,39]), which in the final products contain net-work-bonded as well as some free POSS (see further below),were previously shown to play a key role in the effect ofPOSS,E1 on the mechanical properties of the modified net-works [38,39]. If these nano-domains were hard and crystal-lite-like, then they acted as strong physical crosslinks, thusincreasing the rubber modulus (Scheme 3).

The reactivity of the POSS-based epoxides shown inScheme 4 was compared, including monoepoxy (POSS,E1:Scheme 4bed), pendant POSS diepoxy (Scheme 4e and f)and polyepoxy (POSS,E8, Scheme 4a) compounds. The rela-tive reactivity of these epoxides was at first characterized bytheir reaction with dibutylamine (DBA, Scheme 5a), a simplesecondary (and hence monofunctional) amine at 120 �C, at theamino-H/epoxy ratio of 2, at the concentration of epoxygroups of 0.63 mol/L (see Fig. 1). The less soluble epoxidesPOSSphE1, POSScp-DGEBA and POSScp-DGEBA,olig were investi-gated (reaction with DBA) at the epoxy concentration of0.17 mol/L (Fig. 2) and compared to butyl glycidyl ether(BGE) and DGEBA. Fig. 3 compares the reactions of selectedepoxides with two primary amines, Jeffamine D230 andhexylamine.

3.1.1. Comparison of POSS epoxides with their respectivemodel compounds

The specific ‘‘POSS-effect’’ on the reaction kinetics wasdetermined by comparing the POSS epoxides with modelcompounds, which were analogous to the epoxide-carryingsubstituents on the POSS molecules (Scheme 4gek). BGE(Scheme 4g) served as a model for POSS,E1 monomers, while1,2-epoxyoctane (‘‘C8’’, Scheme 4h) is a good model for

POSS,E8. Fig. 1 shows the decrease of epoxy groups’ concen-tration during the reaction of selected epoxides with DBA. Itwas found that generally, the reactivity of the POSS-epoxies is lower than that of their corresponding model com-pounds. This can be explained by steric effects, like direct hin-drance of the epoxy carrying substituents by the neighbouringinert or reactive substituents of the POSS core, and by thelower mobility of the POSS-bound epoxide units, which inturn leads to a reduced efficiency of the reactive approach, be-cause only certain approaches of amino groups to an epoxygroup lead to a reaction. An electronic influence of the POSScore on the epoxy groups should be nearly negligible, becausethe chains, which connect the epoxy groups with Si fromPOSS, are too long. A catalytic or inhibiting influence of thesiloxane structure of POSS is also very improbable, as thesestructures stay inert during the reaction and are neither basicnor acidic.

The relations between the reaction rates v are as fol-lows: v(POSS,E8)< v(C8)� v(POSSoctE1) w v(POSSiBuE1)� v(DGEBA) w v(BGE) w v(PGE). These results also ex-pectedly confirm the decisive role of positive induction effectof the ether oxygen in the glycidyl ethers (the ether oxygen isin b-position relative to an oxirane C-atom): v(BGE) [v(C8). Therefore, the epoxy groups in all of the POSS,E1investigated (all contain propyloxy-glycidyl substituents as ep-oxy group carriers) are much more reactive than epoxy groups(1,2-epoxyhexane segments as epoxy carriers) in POSS,E8 dueto an absence of the ether oxygen in POSS,E80 functionalsubstituents.

3.1.2. Effect of epoxy-POSS’ inert organic substituents onits reactivity

The effect of the inert organic substituents, which areattached to the POSS core, on the reactivity of POSS mono-epoxides was studied by comparing the reactions with DBA


(in gelationexperiments only)

(a)

(e) (f)

(g) (h) (j) (k) (l)

(b) (c) (d)

Scheme 4. The epoxy-POSS compounds investigated: (a) POSS,E8; (b) POSSoctE1; (c) POSSiBuE1; (d) POSSphE1; (e) POSScp-DGEBA; (f) POSScp-DGEBA,olig;

(g) butyl glycidyl ether (BGE); (h) 1,2-epoxyoctane (C8); (j) diglycidylether of bisphenol A (DGEBA); (k) phenyl glycidyl ether (PGE); (l) POSS,E4.

of otherwise analogous POSS,E1 compounds, which differedonly in the ‘‘inert’’ substituents. POSSiBuE1 (Scheme 4c)with isobutyl, POSSoctE1 (Scheme 4b) with larger isooctyl,and POSSphE1 (Scheme 4d) with phenyl ‘‘inert’’ substituentswere investigated. While POSSoctE1 and POSSiBuE1 showapproximately the same reactivity towards DBA (see Fig. 1),POSSphE1 reacts significantly faster (see Fig. 2, comparison

with POSSiBuE1), practically as fast as its model compoundBGE (whose curve is omitted from the graph), but still slowerthan DGEBA. It seems that the branched isobutyl and isooctylsubstituents cause a similar sterical hindrance in spite of theirdifferent sizes, while the flat and rigid phenyl rings shield theepoxy rings of the propyloxy-glycidyl substituents to a lesserextent.

NH NH

HN

OH

H n-1

NH

HN

H

H

(in reactivity investigations only)

OCH3

NO

H

H n-1

N

H

H

N

H

N

H

(a)

(d) (e) (f)

(b) (c)

Scheme 5. Amines used in this work: (a) dibutylamine (DBA); (b) Jeffamine D230 (n¼ 3.7), D400 (n¼ 7.2) and D2000 (n¼ 33.6); (c) 1,6-hexanediamine (HDA);

(d) Jeffamine M600 (n¼ 9.8); (e) n-hexylamine (HA); (f) N,N0-dimethyl-1,6-hexanediamine (HDSA).


3.1.3. Reactivity of POSScp-DGEBA and of POSScp-DGEBA,olig

Fig. 2 also shows the comparison of the reactivity ofPOSScp-DGEBA, POSScp-DGEBA,olig and DGEBA. The reactivityof both former compounds is similar to that of the POSS,E1epoxides but lower than that of DGEBA (probably forsterical and mobility reasons): v(DGEBA)> v(POSSphE1)>v(POSScp-DGEBA)> v(POSScp-DGEBA,olig)> v(POSSiBuE1).

3.1.4. Reactivity of epoxy-POSS in comparison withDGEBA

The lower reactivity of all POSS epoxides investigated incomparison with DGEBA (see Figs. 1 and 2) is an importantfinding, as the latter is their reaction ‘‘competitor’’ during

0 50 100 150 2000.0

0.2

0.4

0.6

0.8

1.0

(6)

(5)

(4)

(3)(2)

(1)rel.

Epox

ide

conc

. (c

/ c0)

time [min]

Fig. 1. Consumption of epoxy groups during the reaction of epoxides

with DBA. Reaction conditions: r(H(eN)/Ep)¼ 2, c0(Ep)¼ 0.63 M,

c0(Am)¼ 1.26 M, T¼ 120 �C, diluted by 1-methyl-naphtalene; reactions

from bottom: (1) DGEBA, (2) BGE, (3) POSSoctE1, (4) POSSiBuE1, (5) 1,2-

epoxy-octane (C8), and (6) POSS,E8.

0 400 800 12000.0

0.2

0.4

0.6

0.8

1.0

(5)

(4)

(3)(2)

(1)

rel.

Epox

ide

conc

. (c

/ c0)

time [min]

Fig. 2. Consumption of epoxy groups during the reaction of epoxides:

(1) DGEBA, (2) POSSphE1 preceded closely by the not depicted BGE,

(3) POSScp-DGEBA, (4) POSScp-DGEBA,olig and (5) POSSiBuE1 with DBA.

Reaction conditions: r(H(eN)/Ep)¼ 2, c0(Ep)¼ 0.17 M, c0(Am)¼ 0.34 M,

T¼ 120 �C, diluted by toluene (reacted in sealed ampoules).

the formation of the DGEBAeJeffamine networks modifiedwith epoxy-POSS. This lower POSS reactivity impliesa slow incorporation of POSS into these networks, whichoccurs mainly in the late (post-gel) stages of the networkformation. This in turn favors the formation of POSS-richdomains due to the tendency of POSS to phase separation.

3.1.5. Reactivity of POSS,E1 towards JeffaminesThe kinetics of the reaction of the epoxides POSSoctE1 and

BGE with primary amines, n-hexylamine (HA, Scheme 5e)and poly(oxypropylene)diamine (Jeffamine) D230 (Scheme5b), is illustrated in Fig. 3. Poly(oxypropylene)diamines aretypical components of the POSS-modified epoxy networksprepared in this and in previous works. The results showthat the reactions of both epoxides with the Jeffamine are sig-nificantly slower than their reactions with both HA (fastestreaction in the comparison) and DBA. This finding can beexplained by the electronic properties of Jeffamines (presenceof electronegative oxygen atoms in the chain backbones) andby the known tendency of the Jeffamines’ amino groups toform hydrogen bridges with oxygen atoms of the polypropyl-ene oxide chains [42,49]. The also known negative substitutioneffect on primary amines, which is mainly due to stericalcrowding on the nitrogen atoms (after the first amino-Hatom was substituted), did not manifest itself in the describedexperiments, because the reactions were carried out in doubleexcess of amine.

3.1.6. Kinetics of gel forming POSS,E8 reactions withJeffamines

The investigation of the reactions of POSS,E8 with Jeff-amine D2000 and D400 (see Scheme 5b) in stoichiometricratios showed (see Fig. 4) that these reactions are alwaysslower in the later stages than analogous reactions of the

0 100 2000.0

0.2

0.4

0.6

0.8

1.0

(5)

(4)

(6)

(3)(2)

(1)

rel.

Epox

ide

conc

. (c

/c0)

time [min]

Fig. 3. Consumption of epoxy groups during the reaction of epoxides with

DBA and D230. Reaction conditions: r(H(eN)/Ep)¼ 2, c0(Ep)¼ 0.63 M,

c0(Am)¼ 1.26 M, T¼ 120 �C, diluted by 1-methyl-naphtalene; reactions

from bottom: (1) BGEþ n-hexylamine, (2) BGEþDBA, (3) BGEþD230,

(4) POSSoctE1þDBA, (5) POSSoctE1þD230, and (6) 1,2-epoxy-

octaneþDBA.


model epoxide C8 with the same Jeffamines. Both systemsPOSS,E8eD400 and POSS,E8eD2000 gelate at ca. 40%conversion of the epoxy groups. In the case of the reactionPOSS,E8eJeffamine D400 (see Fig. 4) the POSS,E8 reactssomewhat faster than the model epoxide C8 in the beginningstages. A possible explanation could be a more efficient auto-catalysis by close OH groups formed by previous reaction onthe same POSS,E8 molecule. In the later stages, around epoxyconversion a¼ 0.5, the reactivity of POSS,E8 decreases rap-idly and is much lower than in the case of the model com-pound, obviously due to strong sterical hindrance by bulkyJeffamine chains attached to POSS,E8. The sterical differencein comparison to the beginning stage (approach of an un-reacted primary NH2 group to a practically unreactedPOSS,E8 molecule) is apparent. The POSS,E8þD400 reac-tion reaches a nearly complete conversion after a sufficientlylong time (97% after 4320 min¼ 3 days), however. In thecase of the POSS,E8eD2000 (see Fig. 4) reaction, the resultsare similar, but the steric effects dominate from the beginningstages (even the unreacted NH2 groups are attached to a verybulky Jeffamine molecule (34-mer, D400 is a 7-mer)). In theabove-discussed epoxy-amine systems the substitution effectsboth in epoxy and amine monomers should be taken into ac-count and moreover, these effects are mutually dependent [43].

3.2. Microphase separation of POSS,E1 and POSScp-DGEBA

The nano-scaled heterogeneous structure of DGEBAeJeffamine networks modified by the addition of POSS,E1 orPOSScp-DGEBA plays a key role in their thermomechanicalbehavior and was reported by the authors in a previous paper[38]. The tendency of the above POSS co-monomers to phase

0 400 800 12000.0

0.2

0.4

0.6

0.8

1.0

C8 + D2000

POSS,E8 + D2000

POSS,E8 + D400

C8 + D400

rel.

Epox

ide

conc

. (c

/ c0)

time [min]

Fig. 4. Consumption of epoxy groups during the reactions of Jeffamine D400

with POSS,E8 (-) and epoxyoctane (B); and of Jeffamine D2000 with

POSS,E8 (:) and epoxyoctane (C). Reaction conditions: r(H(eN)/

Ep)¼ 1, T¼ 120 �C. Epoxyoctane was diluted by 1-methyl-naphtalene, the

reactions of POSS,E8 were carried out in bulk. Concentrations: reactions

with D400: c0(Ep)¼ c0(Am)¼ 3.19 M. Reactions with D2000: c0(Ep)¼c0(Am)¼ 0.72 M. Both POSS,E8 reactions gelate at ca. 40% conversion in

37.5 min (POSS,E8eD400) and in 240 min (POSS,E8eD2000).

separation is strongly pronounced. The aggregation of POSSto crystallite-like nano-particles containing several network-bonded POSS units was found previously to have a strong re-inforcing effect [38,39] (physical crosslinking, see Scheme 6band c).

The nano-phase separation and aggregation of the POSSco-monomers (alone or in the presence of solvent) observablytake place during the reaction (for a similar example seeRef. [44]). The reactive compatibilization performed in thesynthesis procedure was found not to be complete, and nano-meter-sized POSS aggregates were observed after the cure wasfinished [38,39]. The nano-phase separation is also supportedby the lower reactivity of the POSS epoxides (see above kinet-ics experiments), which are incorporated in the network in thelate reaction stages, mainly after the gel point, where thePOSS solubility in the mixture decreases. In that stage, how-ever, homogenization by reaction blending is difficult. Thestatistics also favor the late incorporation of POSS,E1, as theDGEBA epoxy groups are in fourfold excess. Some POSS-epoxy groups hence remain unreacted and inaccessible, as theyare buried in aggregates. The final phase structure of the abovenetworks containing POSS,E1 or POSScp-DGEBA is determinedby the competition between microphase separation, gelationand the compatibilizing grafting reaction of POSS with the or-ganic matrix. It involves nano-sized amorphous or crystallinePOSS aggregates [38] containing also unbound POSS (asshown by extraction experiments further below, and as de-picted schematically in Scheme 7 and more precisely inScheme 6b or c). If the POSS domains are crystalline, or atleast solid, they form additional physical crosslinks in thenetwork and display a strongly reinforcing effect. IsolatedPOSS units, as for example in the previously synthesizedPOSS,E8eD2000 networks [39], which were found (see fur-ther below) to be incorporated as monomeric and oligomericnetwork junctions, were previously shown not to bring abouta ‘‘POSS-specific’’ reinforcement, but a mild reinforcementby high functionality [39] (see Section 1). The same wasthe case for aggregated network-bonded POSS units withweak POSSePOSS interaction, e.g. POSSoctE1 (liquid-likeaggregates) [39].

3.2.1. Effect of unbound POSS and of topological exclusionby POSS on the thermomechanical properties of thehybrid networks

The results of the solegel analysis of most of the above-discussed networks with POSS incorporated as dangling unitsare shown in Table 1 (for network nomenclature see Section2.3 in the experimental part) and compared with thePOSS-free reference network. Generally, the POSS-modifiednetworks display high sol fractions (with high POSS content,often crystallizing from the sol solution), typically between ca.30 and 50% (reference: 4%; network with PGE instead ofPOSS,E1: 10%). Table 1 lists also the content (wt%) of thePOSS co-monomer as well as the determined percentage ofcovalently bonded POSS, which remained in the samples afterextraction. In the case of the networks containing POSScp-

DGEBA and POSScp-DGEBA,olig, the increasing POSS content


NN

N

NN

N

N

N

NN

N

NN

NN

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

(a)

N

N

N

NN

N

NN

N

N

N

NN

N

NN

NN

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

(b)

N

N

N

N

N

N

N

N

NN

N

NN

N

N

N

N

NN

N

N

N

N

N

N

N

N

N

N

N

N

N

N

(c)

NN

N

N DGEBA N DGEBA N

D2000D2000

symbolizesor

(d)

symbolizes POSS units

Scheme 6. Schematic structures of the prepared networks: (a) DGEBAeD2000 reference network; (b) the similar POSScp-DGEBAeD2000 network; (c) the

DGEBAePOSS,E1(x)eD2000 network with interrupted DGEBA chains; (d) explanation of structure symbols.

leads to an increasing sol fraction, which was found to consistmainly of POSS (crystallizing from the extraction solution),meaning more unbound POSS in the aggregates. Comparingthe networks with POSScp-DGEBA and such with POSS,E1, itcan be seen that in the case of a similar POSS content inwt%, the higher functional POSScp-DGEBA is better incorpo-rated (higher gel fraction and higher POSS incorporation de-gree). The comparison of the networks with POSS,E1 showsthat the incorporation of the less reactive POSSoctE1 leads toa slightly higher sol fraction than in the case of the morereactive POSSphE1, but the difference is small; in the caseof POSSoctE1, the sol fraction contains more Jeffamine andDGEBA. Both networks with POSS,E1 contained approxi-mately the same wt% of POSS, which in both cases waschemically attached to a medium degree (see Table 1). The

molecular weight of POSSoctE1 is slightly higher than thatof POSSphE1. POSSoctE1 is also better compatible with thenetwork matrix: it dissolves in the network components atca. 100 �C without solvent, while POSSphE1 always needsthe addition of a solvent for compatibilization. Hence nano-phase separation of POSSphE1 from the reaction mixture startsmuch more readily than in the case of POSSoctE1, leading tomore unbound POSSphE1 and to a more concentrated remain-ing reaction mixture. The POSSoctE1 stays longer dissolved inthe reaction mixture, thus exerting a diluting effect (more cy-clizations) for a longer time than POSSphE1. For the same rea-son the less reactive but better compatible POSSoctE1 becomesincorporated into the network to a higher degree. If POSSinertia or its affinity to the polymer chains were the decisivefactors, then POSSoctE1 should be the stronger reinforcing


agent. The opposite was found experimentally [38,39]:POSSoctE1 acted as a softening agent while POSSphE1 asa strongly reinforcing one (as reference for this comparison,the network DGEBAePGE(x¼ 0.2)eD2000 was used, withthe monofunctional small PGE in place of POSS,E1). The lat-ter finding strongly supported the ‘‘aggregation mechanism’’of reinforcement: only POSSphE1 forms crystallite-likenano-aggregates, while POSSoctE1 forms similar, but liquid-like ones (see Scheme 3c vs. d, and Scheme 6c). Both POSSadditives only negligibly influenced the Tg (see Table 1), butstrongly shifted the rubber modulus [39] of the modifiednetworks. This means only a minimal effect of topologicalconstraint by POSS, reducing only minimally the elastic chainmobility in the above samples (containing ca. 30% POSS).

A very interesting result is the comparison of the main glasstransition temperatures (Tg, see Tg change in Table 1 andFig. 5), corresponding to the relaxation of poly(propylene ox-ide) chains, in the series of POSScp-DGEBA containing networks.These networks also display a second phase transition [39], inthe range 100e180 �C (partly visible in Fig. 5), which corre-sponds to the disordering of the lamellae-like POSScp-DGEBA

aggregates [39], which act as physical crosslinks. Their chem-ical structure is somewhat complex, with POSS units along theDGEBAeNeDGEBA chains, see Scheme 6b; this chemical

Scheme 7. Symbolic representation of the formation of additional physical

crosslinks in a network by aggregation of pendant POSS units attached cova-

lently to the network with unbound POSS units acting as ‘‘cement’’ in the

aggregates.

structure leads to the two-phase lamellar structure, whichwas found previously by the authors in these materials [38].Fig. 5 shows the corresponding DMTA spectra (dynamic me-chanical thermal analysis) of these samples. Up to a certainPOSS content, the Tg value (main transition) practically doesnot increase with POSS content, only the modulus in the rubberplateau increases due to physical crosslinks. In the stoichiomet-ric POSScp-DGEBA(x¼ 1)eD2000 network, the main Tg value isstill practically the same as in the reference network, but a rel-atively small off-stoichiometry, a 15% excess of POSScp-DGEBA

in the network POSScp-DGEBA(x¼ 1.15)eD2000 (POSS-mono-mer content increases from 68 to 71 wt%), leads to a markedchange in the DMTA spectrum, increasing the Tg (main transi-tion) by 21 �C and increasing the rubber modulus substantiallyas well (Fig. 5); the increased sol fraction consisting practicallyof POSS rules out a homopolymerization of the epoxy groupsbeing the key effect. Obviously, the crystallite-like POSScp-

DGEBA aggregates (which act as physical crosslinks, seeScheme 6b) form easier (modulus increase), if they containhigher amounts of unbound POSS which can ‘‘cement’’ to-gether network-bonded POSS units unable to associate

-100 -50 0 50 100 150105

106

107

108

109

1010

5

4

3

2

1

G' [

Pa]

T [°C]

Fig. 5. (1) DMTA profiles of the reference DGEBAeJeffamine D2000

network, and of network with DGEBA partly or fully replaced by

POSScp-DGEBA or POSScp-DGEBA,olig: (2) DGEBAePOSScp-DGEBA(x¼ 0.33)eD2000, (3) POSScp-DGEBA(x¼ 1)eD2000, (4) POSScp-DGEBA(x¼ 1.15)e

D2000, and (5) DGEBAePOSScp-DGEBA,olig(x¼ 1)eD2000.

Table 1

POSS content, gel fraction and shift in glass temperature for selected networks

Sample wt% of POSS component wg DTg % of POSS bonded to the network

DGEBAeD2000 (reference Network) 0 0.96 0 (Tg¼�29 �C) e

DGEBAePOSScp-DGEBA(x¼ 0.33)eD2000 36.4 0.81 þ3 66

POSScp-DGEBA(x¼ 1)eD2000 68.1 0.70 þ5 61

POSScp-DGEBA(x¼ 1.15)eD2000a 71.3 0.62 þ21 52

DGEBAePOSScp-DGEBA,olig(x¼ 1)eD2000 87.8 0.52 þ49 49

DGEBAePOSSphE1(x¼ 0.2)eD2000 25.4 0.80 �1 33

DGEBAePOSSoctE1(x¼ 0.2)eD2000 29.7 0.75 �2 53

DGEBAePGE(x¼ 0.2)eD2000 0 0.90 �2 e

POSS,E8eD2000 31.4 0.97 �15 ca. 100

a Off-stoichiometric network with 15% excess of epoxy groups.


otherwise (see Scheme 7). Even more importantly, at a POSScp-

DGEBA content of ca. 70 wt%, the numerous hard POSS domainsoccupy the most volume, and begin to immobilize a rapidlyincreasing fraction of the Jeffamine chains, thus strongly in-creasing main Tg. The effect of topological constraint causinga reduced elastic chain mobility (see also Ref. [25]) hence startsto play a very strong role in the rubbery samples at high POSSloadings. In the case of the network POSScp-DGEBA,olig(x¼ 1)eD2000, the originally hard and glassy network becomes rubberyand elastic after losing more than the half of its’ POSS in the ex-traction experiment. The remaining network-bonded POSS,whose content is lower and whose aggregation is poorer, rein-forces the network much less efficiently.

3.3. Microphase separation in POSS,E8-based networks

The components of the reaction mixture POSS,E8eD2000are well compatible (well miscible clear initial reaction mix-ture). SAXS experiments performed by the authors earlier[38] were at first interpreted as the occurrence of nano-phaseseparation in this system. The POSS,E8 nano-aggregates wereoriginally assigned to a SAXS maximum near q¼ 4.5 nm�1.The formation of an ordered network structure was

manifesting itself by a growing interference maximum atq¼ 1.5e1.0 nm�1. The latter maximum has been assigned[38] to scattering on POSS domains (crosslinks of the net-work) separated by a regular distance given by D2000 chains.The ordering was found to continue in the post-gel state [38].However, the structure of the POSS,E8eD2000 network wasnot elucidated and the morphology of the eventual POSS do-mains remained also uncharacterized. An interesting questionremained concerning the formation of the POSS,E8 domains,because this compound was very well compatible with theother network component, Jeffamine D2000. The SAXS datareported in Ref. [38] could be explained by the complex chem-ical structure of the growing network (see Scheme 8): its bothcomponents have a high crosslinking functionality (POSS:8, D2000: 4) and additionally, D2000 has a specific shape(see Scheme 8): two linear unit precursors (NH2 groups) areconnected by a long elastic chain (poly(propyleneoxide)).POSS,E8, on the other hand, is a compact symmetrical octaep-oxide. The nano-phase separation observed by SAXS couldmean the formation of a cluster structure in the mixture asshown in Scheme 8b, containing several POSS,E8 unitsbonded together by their epoxy groups and N-atoms fromthe D2000. These ‘‘POSS,E8 oligomer domains’’ are

NH

H

NO

H

H+

OR OR

n-1

N

NN

N NN

NN

N

N

N N

N N

N

NN

NN

N

N

NN

NN

N

(a)

N

N

N

N

N

N

N

NN

N

N N

N

HH

(b)

N

N

N

N

(c)

Scheme 8. possible structures of the POSS,E8eD2000 network: (a) ideally crosslinked network of a homogeneous structure (network from POSS,E8 connected by

N-atoms, the N-atoms are connected by D2000 chains which fill the space between the POSS,E8); (b) network with oligomeric POSS junctions connected together

by elastic D2000 chains (this structure contains numerous cyclic bonds between two neighbouring epoxy substituents of a POSS,E8 and D20000 nitrogen atoms);

(c) structure containing isolated POSS,E8 junctions connected together by exclusively ‘‘cyclic-bonded’’ D2000 chains (similar to structure (b) but monomeric

junctions).


(a)

N

H

H

N

+

(b)

N

NN

N

+ +

(c)

N

H

NH

N

H

NHH

H

+

(d)

N

H

N+

Scheme 9. Reactions of the POSS,E8 with amines: (a) short-range cyclization with a primary amino group; (b) intermolecular network build-up reaction with

HDSA; (c) long-range cyclization with a diamine; (d) reaction of the second amino-H atom of a formerly primary NH2 group now attached to POSS,E8 with

another POSS,E8 molecule.

surrounded by the poly(propyleneoxide) elastic chains ofD2000 which later connect ellastically all POSS,E8 oligo-meric domains together. In this way POSS,E8 would formoligomer-sized junctions in an elastic network. A fully homo-geneous and ideally crosslinked structure as shown Scheme 8awould be an infinite network from POSS,E8 connected by N-atoms of D2000, with the elastic chains of D2000 filling thehollow space in this network and connecting pairs of N-atomstogether. The D2000 chains would lose their elastic function in

such a network, which would be hard and brittle. Its elasticitywould be given by the skeleton from octafunctional hardSi8O12 building blocks connected by 17-atom-long chains.Moreover, the accommodation of the sterically demandingJeffamine chains in the hollow rooms of the network wouldbe extremely disadvantageous from the sterical point of view.The structure shown in Scheme 8c would mean the totalabsence of POSS,E8ePOSS,E8 ‘‘short-connections’’ by N-atoms. In that structure, all POSS,E8 units would be isolated


and connected by elastic D2000 chains ‘‘cyclically attached’’(see Scheme 9a) to the POSS,E8 units. The previously citedSAXS data support the formation of the structure with oligo-meric junctions (as in Scheme 8b): the maximum at q¼4.5 nm�1 (which has been now determined to correspond tomonomeric POSS,E8) shifts to about 3.5 nm�1 (assignableto POSS,E8 oligomers) and somewhat broadens, with theinterference at q¼ 4.5 nm�1 still remaining. The maximumnear q¼ 1.5 nm�1 corresponds well to scattering on POSSdomains separated by randomly conformed D2000 chains. Therubbery appearance of the POSS,E8eD2000 network and itsdetermined rubbery shear modulus G0, as found in Ref. [39],also support the structures from Scheme 8b and c and ruleout the improbable structure from Scheme 8a. The obtainednetwork hence contains oligomeric and probably also mono-meric POSS,E8 junctions. In order to further elucidate themorphology of the POSS,E8eD2000 network, this materialwas investigated by transmission electron microscopy (TEM,see Fig. 6). The POSS,E8eD2000 network was found to bepractically homogeneous till the nanometer scale, in contrastto the above-discussed POSS,E1 and POSScp-DGEBA networks,the morphology of which was investigated in our previouspapers [38,39]. These dangling-chain POSS networks displaya less homogeneous morphology, especially those withPOSScp-DGEBA: the latter show a complex lining-up of POSSnano-aggregates, due to the POSS units’ chemical attachmentto the DGEBAeNeDGEBA chains, as shown in Scheme 6b(this structure leads to the lamellar two-phase morphologyreported in Ref. [38]). In the POSS,E8eD2000 TEM image,the larger dark spots sized up to several nanometers couldbe assigned to the oligomeric POSS,E8 junctions as proposedin Scheme 8b. The assumed tendency of POSS,E8 to cycliza-tion reactions with the primary amine D2000, as well as stericeffects, was the key factor favoring the formation of a network

Fig. 6. TEM (transmission electron microscopy) image of the POSS,E8eD2000 network.

with a structure according to Scheme 8b and c. These effects,as well as the influence of the topology of amino groups (e.g.primary vs. disecondary groups), were further investigated inthis work (see below).

3.4. Network gelation

The gelation of hybid epoxy networks containing POSS andthe gel fraction in the final products (stoichiometric and non-stoichiometric networks) was studied in order to elucidate thereaction mechanisms. The formation and the chemicalstructure of the networks DGEBA-POSS,E1(x)eD2000 (fornetwork nomenclature see Section 2.3 in the experimentalpart) and POSScp-DGEBA(x)eD2000 are practically analogouslike in the case of the well-investigated DGEBAeD2000reference network (see Scheme 6a), differing mainly in thepresence of some additional monofunctional low-reactiveepoxide in the case of POSS,E1, which also tends to formnano-aggregates.

On the other hand, the networks based on POSS,E8 anddiamines have a different, more complex chemical structure(as discussed above, Scheme 8b and c) than the referencenetwork. The starting mixture is fully, and the final productpractically homogeneous in the case of the POSS,E8ediaminenetworks. The nano-scaled phase differentiation (POSS,E8‘‘oligomerization’’, given by the forming complex chemicalstructure) during the network build-up is much less pro-nounced than in the case of the POSS,E1 and POSScp-DGEBA-based networks so that the reaction mixture can be consideredpractically homogeneous. For this reasons, the investigationsof the gelation of the POSS-modified networks were focusedmostly on the POSS,E8-based system. The topology of theamino functions (e.g. primary vs. disecondary) reacting withPOSS,E8 was shown to be a very important factor. The cross-linking density of the POSS,E8 networks was controlled by us-ing Jeffamines of various molecular weights, D230, D400 andD2000. Various hexylamines (monoamine, diamine, disecon-dary diamine; see Scheme 5 depicting all the amines) werealso applied as crosslinkers instead of Jeffamine in order tofurther clarify the network formation mechanism. The ratio r(¼(N)H/epoxy) of the total amino-H amount and the total ep-oxy groups amount was varied on selected POSS,E8 systemsin order to obtain additional information on the reactionmechanism.

3.4.1. Chemorheology: determination of the time of gelationThe formation of both reference (POSS-free) and of POSS-

containing (POSS,E1, POSS,E4 (see Scheme 4l) and POSS,E8)epoxy networks was followed by chemorheology experiments.Fig. 7a shows the evolution in time of the storage shear modu-lus G0 during the crosslinking of the POSS,E8eD2000 mixture.The steep increase in the modulus corresponds to the gelationof the system. The gel point is a crucial parameter of the net-work formation, which is often quantitatively characterizedby the time of gelation, tg. A more precise determination ofthe gel point in time (tg) was performed by multifrequency dy-namic experiments. The latter experiments apply the Winter


and Chambon method [45] of the frequency independence ofthe loss angle d at the gel point, where G0 w G00 w un

(tan d¼G00/G0; G0 and G00 are the dynamic storage and lossmoduli, respectively, and u is the shear frequency). The cross-over of the tan d¼ f(time) curves for different frequencies(see chemorheology in Fig. 7b) hence corresponds to the gelpoint. The gelation times tg of POSS-containing and referenceepoxy networks investigated are given in Table 2 (for networknomenclature see Section 2.3 in the experimental part).

The POSS-containing networks with POSS,E1 or withPOSS,E8, were formed more slowly (increased time of gela-tion) than the respective reference epoxy networks due to alower reactivity of POSS epoxides with respect to DGEBA.The comparison of the networks DGEBAeD2000, DGE-BAePOSSoctE1(x¼ 0.2)eD2000 and DGEBA(x¼ 0.8)eD2000 (non-stoichiometric network with D2000 excess)shows (Table 2) that replacing 20% of epoxy groups ofDGEBA by POSSoctE1 epoxy groups leads to a longer delayin gelation than the removal of these 20% of DGEBA epoxy

10000 11000 12000 13000 140001E-3

0.01

0.1

1

10

100

G' [

Pa]

time [s]

10000 11000 12000 13000 14000time [s]

(a)

0.1

1

10

100

1000

tan(delta

)

(b)

Fig. 7. (a) The development of the storage shear modulus G0 in time; (b) tan d

as function of time at 8 rad/s (,), 16 rad/s (:) and 32 rad/s (�). Reaction:

POSS,E8þD2000, T¼ 120 �C, r¼ 1.

groups (this simulates chain termination by monoepoxide).This result demonstrates the diluting effect of POSSoctE1(much larger volume per epoxy group than in DGEBA). Thegelation was accelerated by using shorter Jeffamines, becausethe concentration of functional groups increases with decreas-ing molecular weight of the amine crosslinker (e.g. the seriesD2000<D400<D230 in Table 2). The replacement of Jeff-amines with the more reactive and even shorter aminohexanesled to a further acceleration of the gelation.

3.4.2. Reactant functionality effect on the time of gelationof POSS,E8 networks

3.4.2.1. Theory. The components’ functionality and thereaction mechanism play a determining role in the networkformation. The gelation occurs at the critical conversion offunctional groups aC. For the epoxy-amine reaction in theideal case of the random reaction it holds [46]

ðaCÞEðaCÞA¼ ½ðfE� 1ÞðfA� 1Þ��1 ð1Þ

where (aC)E and (aC)A are the critical conversions of epoxyand NH groups at the gel point, and fE and fA correspond tothe functionality of the epoxy (E) and amine (A) monomers,respectively. An increasing functionality of a system resultsin a decrease in the critical conversion (gelation at a lowerconversion), and hence in a faster gelation at the same kineticrate of the reaction. For instance in the case of the stoichiomet-ric diepoxyediamine system (e.g. DGEBAeD2000) for r(¼(N)H/epoxy)¼ 1 and the ideal random reaction, we haveaE¼ aA¼ a, fE¼ 2, fA¼ 4, and from Eq. (1) it follows: aC¼3�1/2¼ 0.56. For ideal reactions in the case of the stoichiomet-ric POSS,E8eprimary diamine system it holds: fE¼ 8, fA¼ 4and aC¼ (7� 3)�1/2¼ 0.22, and for POSS,E8eprimarymonoamine (or disecondary diamine) ( fA¼ 2), aC¼ 0.38.

3.4.2.2. Amine functionality effect. Our experimental results(Table 2) prove that the decrease in functionality of an amineusually delays gelation in its reaction with POSS,E8. The

Table 2

Gelation times tg of POSS-epoxy and reference networks, r¼ 1, T¼ 120 �C

Sample tg [min]

DGEBAeD230 5.0

DGEBAeD400 7.1

DGEBAeD2000 68.0

POSS,E8eD230 20

POSS,E8eD400 37.5

POSS,E4eD400 343

DGEBAePOSS,E8(x¼ 0.6)eD400 20.4

POSS,E8eD2000 203

DGEBA(x¼ 0.8)eD2000 (r¼ 1.25)a 97

DGEBAePOSSoctE1(x¼ 0.2)eD2000 143

POSS,E8eHDA 7.6

POSS,E8eHA 14.7

POSS,E8eHDSA 5.7

POSS,E8eM600 217.0

a Off-stoichiometric network as model for DGEBAePOSS,E1(x¼ 0.2)eD2000, without POSS,E1.


replacement of a tetrafunctional ( fA¼ 4) diamine (HDA(Scheme 5c) or D400) by an otherwise similar primary bifunc-tional ( fA¼ 2) monoamine (HA or M600, Scheme 5d) in POS-S,E8eamine networks leads to a significant increase in thegelation time from 8 to 15 min and from 38 to 217 min, re-spectively (see Table 2). This effect is more pronounced inthe case of the longer and bulkier amines D400 (MW¼ ca.430 g/mol) and M600 (MW¼ ca. 600 g/mol).

3.4.2.3. Epoxide functionality effect on time of gelation ofPOSS,E8 systems. An increase in functionality of the epoxidemonomer by replacement of the diepoxide DGEBA with theoctaepoxide POSS,E8 did not accelerate gelation with Jeff-amines because the kinetics effect of a much lower POSS,E8reactivity prevailed. If POSS,E8 and the similar POSS,E4are compared, the increase in epoxy functionality expectedlyleads to a much shorter time of gelation (see Table 2). The crit-ical conversion for the systems POSS,E8eD400 and POS-S,E8eD2000, determined by using our kinetics data (Section3.1.6) and the measured gelation times (Table 2), was foundto be aC¼ 0.40 (in both cases) instead of the theoretical valueof 0.22. This delay of gelation with respect to the ideal casecan be considered as a result of a non-random reaction andof cyclizations (non-ideal reaction mechanism).

3.4.2.4. Topology of functional amino groups’ effect on gela-tion kinetics of POSS,E8ediamine. The effect of the func-tional groups’ topology, which strongly influences thereaction mechanism, on the network formation from POSS,E8is obvious from the comparison of its gelation with differ-ent hexane amines (see Table 2). The slower gelation ofPOSS,E8eHA than of POSS,E8eHDA networks is easily ex-plained by the lower functionality of the bifunctional primarymonoamine HA. However, the system cured with the bifunc-tional disecondary hexanediamine, HDSA ( f¼ 2, see Scheme5f) gelates much faster than that with HA of the same func-tionality, despite HDSA having only secondary amino groups,which are generally less reactive than primary ones. Thisexperimental finding is a result of a different structure ofboth amines: the primary amino groups of HA are bifunctionaland possess one very reactive amino-H. The second amino-Hbecomes secondary and much less reactive after the reaction ofthe first one, due to the attachment of a bulky POSS,E8 groupto the amino-N atom. The conversion of these secondaryamino-H (as shown in Scheme 9d) is necessary for the forma-tion of any branched structures from POSS,E8 and HA. More-over, the second amino-H of HA is very likely sterically proneto an intramolecular reaction with neighbouring epoxy groupsof POSS,E8 monomer as shown in the Scheme 9a. Cycliza-tion, i.e., the intramolecular reactions (Scheme 9a and c), donot contribute to a molecular structure growth, and thereforean increasing extent of cyclization results in a slowing downof the structure development, and a delay of gelation. On thecontrary, a larger distance between amino-H functional(CH3eNH) groups in HDSA favors an intermolecular reactionwith POSS,E8 (Scheme 9b). Furthermore, the CH3eNHgroups of HDSA are sterically less deactivated than the

secondary NH groups formed from HA and POSS,E8 (seeScheme 9b vs. d).

A seemingly surprising finding was the faster gelation ofPOSS,E8 with HDSA than with HA. In analogy to the compar-ison of HA and HDSA, the key factors explaining this resultare the lower reactivity of the second H-atoms on both aminogroups of HDA for sterical reasons: a secondary amino groupwith one alkyl and one very bulky POSS,E8 substituent has toreact with one more very bulky POSS,E8 (Scheme 9d). Thestrong tendency of the primary groups of HDA to cyclizationreactions with the epoxy group-carrying substituents ofPOSS,E8 (Scheme 9a) is probably strongly favored by theabove mentioned steric effect in addition to the easy geomet-rical possibility of the cyclization. In the similar case of thePOSS,E8eD2000 network, the primary diamine D2000 addi-tionally has a high sterical demand. The observed structure ofthe latter network (discussed further above Scheme 8b), withPOSS,E8 oligomers connected by D2000 chains, is the resultof sterical hindrance for non-cyclic bonding and of easy cyclicbonding of primary amino groups on POSS,E8, in analogy tothe reaction of POSS,E8 with HDA. A network made fromPOSS,E8 and a (commercially not available) D2000-baseddisecondary amine would presumably display a simple chem-ical structure and a simple gelation behavior in analogy to thePOSS,E8eHDSA network.

3.4.3. Gel fraction and critical ratios in the POSS,E8-basednetworks

Information on the reaction mechanism and on the networkformation can be obtained also by the determination of thecritical molar ratios of functional groups above which thegel is not formed [47]. In the case of a random alternating re-action of epoxide with amine, an ideal network without defectsis built at a stoichiometric composition, r (¼(N)H/epoxy)¼ 1.The stoichiometric network shows the highest crosslinkingdensity and a fraction of gel wg¼ 1, i.e., all reactants are in-corporated in the network. At an excess of any reactant, a de-fective network is formed and a fraction of sol wS (¼ 1� wg)appears. An increasing off-stoichiometry results in a growth ofthe sol fraction and above the critical ratio rC the gel is notformed at all. Eq. (2) holds for an ideal random alternatingreaction [47]:

rC ¼ ðfA� 1ÞðfE� 1Þ ð2Þ

Fig. 8a illustrates the dependence of the gel fraction wg onthe composition of the networks prepared from POSS,E8 andvarious polyoxypropylene-based amines. The comparison ofthe networks from POSS,E8 and the Jeffamines D230 andD2000 shows the effect of an increasing amine chain length(dilution by a larger reactant), which leads to a narrower‘‘gel window’’. Dilution of the reaction system by a solventalso strongly reduces wg (comparison of POSS,E8eD2000and POSS,E8eD2000e20 vol% solvent). Replacing Jeff-amine D (primary diamine) with Jeffamine M (primarymonoamine) leads to a marked decrease of wg and rC due tothe lower amine functionality. Furthermore, the negative


substitution effect on the monoamine hinders the network for-mation much more than on the diamine. The gel fraction at agiven ratio r decreases in the series of networks POSS,E8eD230> POSS,E8eD2000> POSS,E8eD2000e20% solvent>POSS,E8eM600. The solegel results for POSS,E8eaminenetworks at epoxide excess are often distorted (high gelfractions) due to the epoxy groups’ easy homopolymerization.The system POSS,E8eD2000 did not undergo this sidereaction and could be characterized in the full range of the rratios.

3.4.3.1. Comparison of theoretical and experimental gel frac-tions for POSS,E8eJeffamine networks. Fig. 8b compares ex-perimental results of the dependence wg¼ f(r) with theoreticalvalues calculated for the POSS,E8eD230 network. The exper-imental gel fractions are lower than the theoretical values cal-culated for an ideal random reaction (see Appendix A). Onlysmall deviations of experimental data from the theory are ob-served in the cases of the stoichiometric network and in those

0.1 1 10

0.0

0.2

0.4

0.6

0.8

1.0

Wg

0.0

0.2

0.4

0.6

0.8

1.0

Wg

r

0.1 1 10r

(a)

(b)

Fig. 8. Gel fraction wg as function of the stoichiometric ratio r: (a) experimental

wg¼ f(r) dependence determined for: POSS,E8eD2000 (C), POSS,E8eD230

(7), POSS,E8eD2000e20 wt% solvent (B), POSS,E8eM600 (@); (b) theo-

retical (ideal) wg¼ f(r) for POSS,E8eD230 vs. experimental data (7).

with a small excess of the epoxide, r< 1. However, the net-works with higher amine excess (r> 1) exhibit significantlylower gel fractions. The critical ratio rC for POSS,E8eD230was determined to be 10 (see Fig. 8b) instead of the theoreticalvalue of 21 (Eq. (2)) for a randomly reacted system from oc-tafunctional epoxide and tetrafunctional amine. These devia-tions of POSS,E8eJeffamine networks from the randomsystem are obviously caused by an unequal reactivity of func-tional groups (especially amino-H), by intramolecular reac-tions as shown in Scheme 9a and c (see also Refs. [42,48])and by steric exclusion.

3.4.3.2. Unequal reactivity effects in POSS,E8eJeffamine sys-tems. The unequal reactivity involving both the amine and theepoxy components is composed of several effects: it includesthe negative substitution effect on the amine group of Jeff-amine [48,49] causing a lower reactivity of the hydrogen ina secondary amine group, eNHe, with respect to a hydrogenin a primary amine group, eNH2. The reactivity of POSS,E8epoxy groups can be influenced by both positive and negativesubstitution effects. The positive substitution effect resultsfrom the activation of an epoxy group by a hydroxyl groupformed by the conversion of a nearby epoxy group (observedfor POSS,E8eD400 in the early stage, see Fig. 4). On theother side the sterical hindrance from former reaction partnersattached to reacted epoxy groups leads to deceleration in laterreaction stages of POSS,E8eD400 in Fig. 4.

3.4.3.3. Cyclization in POSS,E8ediamine networks. The au-thors suppose that cyclization is the dominant factor causingdeviations of POSS,E8ediamine networks from a structureevolution of the random system. These systems can easilyundergo the short-range cyclization as shown in Scheme 9a,because the octafunctional POSS,E8 contains a close packingof epoxy groups. A cyclization of this type was also describedin the literature on the reaction of tetraglycidyl diamino-diphenylmethane (TGDDM) with amines [42]. TGDDM hassimilarly closely packed epoxy groups like POSS,E8. Theabove mentioned positive substitution effect in polyepoxides(activation of an epoxy group by a reacted one in the closeneighborhood) presumably also plays an important role inthe short-range cyclization shown in Scheme 9a.

Long-range cyclization should also be taken into account,involving the reaction of both NH2 groups of a Jeffaminewith the epoxy groups of the same POSS molecule (seeScheme 9c). Such cyclizations were found in the case of thereaction of DGEBA with long Jeffamines, in contrast to thefully cyclization-free reaction of DGEBA with short amines[31,48].

Dilution of the reaction mixture generally promotes cycli-zation reactions. The results displayed in Fig. 8a are in goodagreement with this rule. The long D2000 chain in thePOSS,E8eD2000 system brings about the dilution of the reac-tion mixture with respect to the short D230 in the POSS,E8eJeffamine networks. Hence, the more diluted systemPOSS,E8eD2000 undergoes a more extensive cyclizationleading to a lower gel fraction and to a lower critical ratio.


The effect of dilution is obvious also in the POSS,E8eD2000network prepared in the presence of 20% of toluene. This net-work shows lower wg than an analogous one prepared in bulk.The dilution of the reaction mixture also results in the dimin-ishing of the steric exclusion effect. The decreasing gel frac-tion in the diluted networks implies that the latter effect isnot decisive for deviations from the random system in thestudied POSS,E8 systems.

4. Conclusions

The reactivity of mono- and polyfunctional POSS-epoxymonomers was studied and its effect on the formation of ep-oxy-amine networks containing POSS as pendant units or asjunctions was evaluated. The kinetic experiments revealed thatthe POSS epoxide monomers investigated are less reactive to-wards amines than DGEBA and than corresponding model com-pounds, due to the sterical crowding around the epoxy groupscaused by inert POSS substituents and also due to reduced epoxygroups’ mobility. The reactivity of the pendant POSS epoxidesdepends on the inert POSS substituents, i.e., on their sterical de-mand. The reactivity order for the epoxide species investigatedis: v(POSSoctE1) w v(POSSiBuE1)< v(POSScp-DGEBA,olig)<v(POSScp-DGEBA)< v(POSSphE1)< v(BGE)< v(DGEBA). Thereactivity of POSS,E8 is much lower than that of the otherPOSS epoxides investigated and of DGEBA but similar to thatof its model compound, 1,2-epoxyoctane (C8). This is due tothe less reactive (alkyl-oxirane type instead of glycidyl ethertype) epoxy groups in POSS,E8 and C8.

The formation of organiceinorganic networks from thePOSS epoxides and the final nanostructure of these networksare controlled by the competition between the aggregationtendency of POSS monomers and their reactive incorporation(and hence compatibilization). The reaction kinetics (exceptthe POSS,E8 system) favors the aggregation by incorporatingPOSS in the later stages. The nano-aggregation in turn canhave e if POSS forms crystallites e a crucial reinforcing influ-ence on the thermomechanical properties of the final networks.The aggregates form easily, if they contain unbound POSS. Athigh POSS loadings (above ca. 70%), the crystallite-like aggre-gates occupy most of the samples’ volume, which leads toa marked immobilization of elastic chains by topological con-straint, thus yielding the strongest reinforced networks (by theaddition of the physical crosslinking and immobilizationeffects).

The network POSS,E8eD2000 with octafunctional POSScrosslinks shows a slow gelation due to the low reactivity ofPOSS,E8, sterically induced substitution effects and a strongtendency to short-range cyclization. The topology of the aminofunctions reacting with POSS,E8 was found to be a veryimportant factor influencing the cyclizations and determiningthe network growth mechanism. At higher conversions thereaction slows down due to the sterical crowding around thereactive centers (on both amino and epoxy groups). Duringthe reaction of POSS,E8 with D2000 a phase differentiation ap-pears in the mixture after some initial conversion, obviously due tothe complex network structure being formed. Finally a network

with well-dispersed and regularly arranged oligomeric andmonomeric POSS,E8 junctions is built, whose homogeneitytill the multi-nanometer scale was checked by TEM.

Acknowledgments

The authors thank Ms. Dana Kankova for recording theNMR spectra, Ms. Iveta Vlasakova for preparing the polymernetworks in molds and for the extraction experiments, andMs. Miloslava Plichtova for determining the silicon contentin selected samples.

The authors thank the Grant Agency of the Czech Republic,Grant Nr. 203/05/2252, the Grant Agency of the Academy ofSciences of the Czech Republic, Nr. IAA 400500701 and theEuropean Commission, Grant EU-HPRN-CT-2002-0036 forfinancial support of this work.

Appendix A

The fraction of gel wg in ideal epoxy-amine networks wascalculated using the theory of branching processes (TBP) [50].The epoxy-amine reaction of POSS,E8 and a diamine was de-scribed as the reaction of the octa- and tetrafunctional mono-mers, epoxides and amines, E8þA4. The ideal randomalternating reaction was taken into account.

The statistical theory describes the system by distribution ofthe structural units defined by the reaction state of the functionalgroups, i.e., number of reacted and unreacted functionalities.These structural units of the system are combined anew at anymoment of the reaction to form tree-like structures. The distri-bution of structural units at a different reaction state developingduring the reaction is obtained by using TBP. The number ofbonds issuing from a unit is given by the probability generatingfunction (p.g.f.). The p.g.f. for a unit in the root is as follows:

F0AðzEÞ ¼ ð1� aAþ aAzEÞ4 for the diamine

F0EðzAÞ ¼ ð1� aEþ aEzAÞ8 for the octaepoxide

z is a dummy variable and subscripts of z indicate the directionof a bond, thus zE and zA indicate a bond from unit A (amine)to E (epoxide) and from E to A, respectively.

aE and aA are conversions of the epoxy and NH aminegroups, respectively.

The sol of the system contains the material with no bondscontinuing to infinity. Fraction of the sol wS (1� wg) is char-acterized by the extinction probability v, i.e., the probabilitythat the bond has no continuation to infinity.

v¼ F1ðvÞ ¼ ð1� aþ avÞ F1ðvÞ ¼ dF0ðzÞ=dF0ð1Þ

wS ¼ mAF0AðvEÞ þmEF0EðvAÞ¼ mAð1� aA þ aAvEÞ4þmEð1� aE þ aEvAÞ8

mA and mE are mass fractions of A (amine) and E (epoxide)units, respectively.


The extinction probabilities for the amine and epoxideunits, vA and vE, were determined numerically for the corre-sponding conversions from equations

vE ¼ ð1� aEþ aEvAÞ7

vA ¼ ð1� aAþ aAvEÞ3:

References

[1] Scott DW. J Am Chem Soc 1946;68:7302e4.

[2] Agaskar PA. Inorg Chem 1991;30:2707e8.

[3] Crivello JV, Malik R. J Polym Sci Part A Polym Chem 1997;35:407e25.

[4] Brown JF, Vogt LH. J Am Chem Soc 1965;87:4313e7.

[5] Feher FJ, Budzichowski TA, Rahimian K, Ziller JW. J Am Chem Soc

1992;114:3859e63.

[6] Lichtenhan JD, Otonari YA, Carr MJ. Macromolecules 1995;28:8435e7.

[7] Gravel MC, Laine RM. Polym Prepr (Am Chem Soc Div Polym Chem)

1997;38:155e6.

[8] http://www.hybridplastics.com/press.html.

[9] Lichtenhan JD, Vu NQ, Carter JA, Gilman JW, Feher FJ. Macro-

molecules 1993;26:2141e2.

[10] Haddad TS, Lichtenhan JD. Macromolecules 1996;29:7302e4.

[11] Tsuchida A, Bolln C, Sernetz FG, Frey H, Muelhaupt R. Macromolecules

1997;30:2818e24.

[12] Liu YL, Lee HC. J Polym Sci Part A Polym Chem 2006;44:4632e43.

[13] Baldi F, Bignotti F, Ricco L, Monticelli O, Ricco T. J Appl Polym Sci

2006;100:3409e14.

[14] Pyun J, Matyjaszewski K, Wu J, Kim GM, Chun SB, Mather PT. Polymer

2003;44:2739e50.

[15] Sellinger A, Laine RM. Macromolecules 1996;29:2327e30.

[16] Lin EK, Snyder CR, Mopsik FI, Wallace WE, Wu WL, Zhang CX, et al.

Organiceinorganic hybrid materials, vol. 519. MRS; 1998. p. 15e21.

[17] Pellice SA, Fasce DP, Williams RJJ. J Polym Sci Part B Polym Phys

2003;41:1451e4.

[18] Li GZ, Wang LC, Toghiani H, Daulton TL, Koyama K, Pittman CU.

Macromolecules 2001;34:8686e93.

[19] Lichtenhan JD, Haddad TS, Schwab JJ, Carr MJ, Chaffee KP, Mather PT.

Polym Prepr (Am Chem Soc Div Polym Chem) 1998;39:489e90.

[20] Romo-Uribe A, Mather PT, Haddad TS, Lichtenhan JD. J Polym Sci

Part B Polym Phys 1998;36:1857e72.

[21] Zheng L, Waddon AJ, Farris RJ, Coughlin EB. Macromolecules 2002;

35:2375e9.

[22] Bharadwaj RK, Berry RJ, Farmer BL. Polymer 2000;41:7209e21.

[23] Fu BX, Gelfer MY, Hsiao BS, Phillips S, Viers B, Blanski R, et al.

Polymer 2003;44:1499e506.

[24] Haddad TS, Lee A, Phillips SH. Polym Prepr (Am Chem Soc Div Polym

Chem) 2001;42:88e9.

[25] Lee A, Lichtenhan JD. Macromolecules 1998;31:4970e4.

[26] Tamaki R, Tanaka Y, Asuncion MZ, Choi J, Laine RM. J Am Chem Soc

2002;123:11420e30.

[27] Huang JC, He CB, Liu XM, Xu JW, Tay CSS, Chow SY. Polymer

2005;46:7018e27.

[28] Huang JC, Xiao Y, Mya KY, Liu XM, He CB, Dai J, et al. J Mater Chem

2004;14:2858e63.

[29] Kim GM, Qin H, Fang X, Sun FC, Mather PT. J Polym Sci Part B Polym

Phys 2003;41:3299e313.

[30] Liu YL, Chang GP, Hsu KY, Chang FC. J Polym Sci Part A Polym Chem

2006;44(12):3825e35.

[31] Liu YH, Zheng SX, Nie KM. Polymer 2005;46:12016e25.

[32] Ni Y, Zheng SX, Nie KM. Polymer 2004;45:5557e68.

[33] Ni Y, Zheng SX. Macromol Chem Phys 2005;206:2075e83.

[34] Liu YL, Chang GP. J Polym Sci Part A Polym Chem 2006;44(6):

1869e76.

[35] Liu HZ, Zheng SX, Nie KM. Macromolecules 2005;38:5088e97.

[36] Bizet S, Galy J, Gerard JF. Macromolecules 2006;39(7):2574e83.

[37] Turri S, Levi M. Macromolecules 2005;38:5569e74.

[38] Matejka L, Strachota A, Plestil J, Whelan P, Steinhart M, Slouf M.

Macromolecules 2004;37:9449e56.

[39] Strachota A, Kroutilova I, Kovarova J, Matejka L. Macromolecules

2004;37:9457e64.

[40] Urbanova M, Brus J, Strachota A, in preparation.

[41] Xu HY, Kuo SW, Lee JS, Chang FC. Macromolecules 2002;35:

8788e93.

[42] Matejka L, Tkaczyk M, Pokorny S, Dusek K. Polym Bull 1986;15:

389e96.

[43] Matejka L, Dusek K, Dobas I. Polym Bull 1985;14:309e15.

[44] Abad MJ, Barral L, Fasce DP, Williams RJJ. Macromolecules 2003;

36:3128e35.

[45] Chambon F, Winter HH. J Rheol 1987;31:683e92.

[46] Stockmayer WH. J Polym Sci 1952;9:69e76.

[47] Matejka L, Dusek K. Polym Bull 1980;3:489e95.

[48] Matejka L. Macromolecules 2000;33:3611e9.

[49] Dusek K, Ilavsky M, Stokrova S, Matejka L, Lunak S. In: Sedla�cek B,

Kahovec J, editors. Crosslinked epoxies. West Berlin: Walter de Gruyter

Publ.; 1987. p. 279e89.

[50] Dusek K. Adv Polym Sci 1986;78:1.

http://www.hybridplastics.com/press.html

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