European Polymer Journal 48 (2012) 260–274

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Contents lists available at SciVerse ScienceDirect

European Polymer Journal

journal homepage: www.elsevier .com/locate /europol j

Macromolecular Nanotechnology

Effect of POSS on thermomechanical properties of epoxy–POSSnanocomposites

Libor Matejka ⇑, Piotr Murias, Josef PleštilInstitute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky Sq. 2, 162 06 Prague 6, Czech Republic

a r t i c l e i n f o

Article history:Received 22 July 2011Received in revised form 7 November 2011Accepted 9 November 2011Available online 25 November 2011

Keywords:Epoxy–POSS hybridPOSSThermomechanical propertiesPolymer nanocomposite

0014-3057/$ - see front matter � 2011 Elsevier Ltddoi:10.1016/j.eurpolymj.2011.11.009

⇑ Corresponding author. Tel.: +420 296809281; faE-mail address: matejka@imc.cas.cz (L. Matejka)

a b s t r a c t

The epoxy–POSS hybrid networks with POSS bound as pendant cages or with untetheredPOSS dispersed in the matrix were prepared and their structure was controlled. Formationof the hybrid network was followed by chemorheology. In situ development of physicalcrosslinks in the pregel stage of the network build-up was observed in case of the hybridswith tethered POSS. The complex effect of POSS on mechanical properties is manifested byeither increase or decrease in rubbery modulus of different hybrids. This behavior reflects(a) reinforcement due to POSS hard aggregates, (b) diminishing of crosslinking density ofthe epoxy network by tethered monofunctional POSS and (c) physical crosslinking via POSSdomains. Theories of network formation and rubbery elasticity as well as the model ofmechanical behavior of particulate composites were applied to interpret the mechanicalproperties of the hybrids.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Nanostructured polymers and nanocomposites showexcellent properties dependent on structure and morphol-ogy. Good dispersion of nanostructures within a polymermatrix is a basic polymer nanomaterials requirement. Con-trol of the structure and morphology, therefore becomescrucial in synthesis of nanocomposites. The procedure ofnanoparticles blending in a polymer matrix meets severeproblems with aggregation and a homogeneous dispersionof the nanoparticles. In situ generation of an inorganicphase in a polymer by the sol–gel process [1] makes it pos-sible to improve dispersion of nanostructures, however,the control of the structure is limited. Another approachusing well defined nanobuilding blocks is also often ap-plied to synthesize controlled nanostructured systems[2]. Polyhedral oligomeric silsesquioxanes (POSS) are themost common such nanounits. POSS involves an inorganicSi–O core and a shell of organic substituents. Moreover,POSS could include a functional group making possible a

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x: +420 296809410..

covalent attachment to a polymer matrix. POSS–polymernanocomposites often exhibit reinforcement with respectto the unmodified polymers and improved use properties;mechanical, thermal, electrical, thermo-oxygen resistance,etc.

Incorporation of POSS in a polymer can result in both anincrease [3,4] and decrease [5–7] in Tg and modulus, aswell as to unchanged [8] thermomechanical properties.Different interpretations of the POSS effect were suggestedin the literature. However, understanding of the POSS ef-fect to explain different and controversial experimental re-sults in various hybrid systems has been still lacking.Generally, the hybrids behavior is determined by two com-peting effects. Restriction of the chain mobility due to pres-ence of a robust POSS cage results in an increase in Tg and alocal chain reinforcement, while increase in free volumedue to voluminous POSS molecule leads to acceleration ofthe chain dynamics and diminishing of Tg as well as mod-ulus. POSS thus acts either as nanofiller or as a plasticizer.

The epoxy polymers are ones of the most applied matri-ces for polymer–POSS nanocomposites [9]. However, POSSunits aggregate within an epoxy network to form POSS richdomains thus limiting the POSS amount introduced with-

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out an extensive phase separation deteriorating the prop-erties. Recently we have studied the hybrids based on POSScontaining epoxy networks [10–12]. The rubbery networkfrom diglycidylether of Bisphenol A (DGEBA) and poly(oxypropylene)diamine (Jeffamine D2000) was modifiedby incorporation of POSS attached as pendant units on anetwork chain or forming polyhedral junctions of thenetwork. The hybrids with pendant POSS showed a poordispersion of POSS in the matrix and formation of amor-phous aggregates or crystalline domains. An approach toepoxy system homogenization was suggested by Williamset al. [13] and Liu and Chang [14] using the two-step syn-thesis procedure. They have shown that a large content ofpendant POSS can be incorporated in an epoxy–aminenetwork in such a way.

The rubbery modulus of the hybrids depended on thetype of the unreactive POSS substituents. The phenyl-substituted POSS form hard crystalline domains servingas physical crosslinks, thus increasing modulus. Unlike,the aggregates from POSS with flexible octyl-subtituentsare weak and plasticize the matrix.

The goal of our research in this paper consists in generalunderstanding of the POSS effect on behavior of the epoxy–POSS hybrid networks. Is it the POSS–POSS interaction andpercolation of POSS structures through the system thatdominates the hybrid properties? Are the properties deter-mined mainly by the interphase interaction between POSSand an epoxy network? Or other effects are operative? Insearching for determination of relationships betweenstructure of the hybrids and thermomechanical propertieswe have applied the strategy with two novel aspects;in situ following of the hybrid network formation and thetheoretical approach in evaluation of the mechanical prop-erties. The process of the network build-up was studied bychemorheology in order to follow in situ the POSS effect onmolecular structure evolution. The theories of network for-mation and of rubber elasticity and model of mechanicalbehavior of particulate composites, were used to interpretthe experimental data and to correlate the structure andmechanical properties of the hybrids.

Moreover, we aimed to control the structure and mor-phology of the epoxy–POSS networks in order to preparerelatively well defined hybrids. Several approaches wereapplied to regulate the structure by controlling both thePOSS–POSS and POSS–polymer interactions. We havefollowed the effect of (a) POSS substituents to adjust mis-cibility with an epoxy network and tendency to POSSaggregation, (b) POSS–polymer interaction due to chemicalbonding, (c) polymerization procedure and (d) topology ofPOSS incorporation in the network on the hybrid structureand properties (followed in the next paper).

We have studied the epoxy–POSS hybrids based on theDGEBA–poly(oxypropylene) diamine (Jeffamine D2000)network. In order to determine the effect of the covalentPOSS–polymer bonding and physical POSS–POSS interac-tion on the hybrid morphology and mechanical propertieswe have explored and compared the hybrid networks with(a) chemically bound (tethered) POSS as pendant units or(b) physically admixed (untethered) POSS. The hybridswith tethered POSS were prepared by applying either the

epoxy– or amine–functionalized POSS monomers. Thenonfunctional POSS, containing a shell of various organicinert substituents, served as untethered nanofiller in ananocomposite. Moreover, the two-step synthesis hasbeen used to improve the hybrid network homogeneityby modifying the procedure of Liu [14]. Our approachmade it possible an almost complete incorporation of POSSinto the well homogeneous hybrid network. The structure,morphology and thermomechanical properties were char-acterized by SAXS, WAXS, TEM and DMA. The molecularstructure evolution during hybrid network formation wasfollowed by chemorheology.

2. Experimental

2.1. Materials

The epoxies diglycidyl ether of Bisphenol A(DGEBA)(SYNPO a.s. Pardubice), phenyl glycidyl ether (Al-drich) and the epoxy resin DER 732 P (polypropylene oxidediglycidyl ether, MW = 640) (Norco spol. s.r.o., Dow) wereused as received. The curing poly(oxypropylene)diamineagent, Jeffamine D2000 was obtained from Hunstman Inc.

The following functionalized and nonfunctionalizedPOSSs were used as received from Hybrid Plastics Inc.,Hattiesburg, MS, USA (Scheme 1): heptaisobutyl-aminopro-pyl POSS (POSSBuA2), heptaisooctyl-aminopropyl POSS(POSSOctA2), heptaphenyl-aminopropyl POSS (POSSPhA2),heptaethyl-glycidyloxypropyl POSS (POSSEtE1), heptaphe-nyl-glycidyloxypropyl POSS (POSSPhE1), OctaMethyl POSS(POSS,Me8), OctaIsobutyl POSS (POSS,Bu8), OctaPhenylPOSS (POSS,Ph8). Indication of POSS includes the type offunctional groups (epoxy E and amine A) as well as the typeof nonfunctional organic substituents R. POSS containingreactive functional groups are marked according to thenumber of reactive functionalities as POSS,E1 for monoep-oxy–POSS monomer. The aminogroup is bifunctional andhence the monoamine–POSS is designated as POSS,A2. Theunreactive substituents R are specified as – Me (methyl),Et (ethyl), Bu (isobutyl), Oct (isooctyl), Ph (phenyl).

2.2. Preparation of epoxy–POSS networks

The epoxy–POSS hybrids based on DGEBA–D2000 net-work were prepared by a partial substitution of D2000 orDGEBA with the amine– or epoxy–functionalized POSS(POSS,A2 or POSS,E1), respectively. The systems were char-acterized by fractions x of POSS functionalities involved inthe network;

xA¼ ½NH�POSS=ð½NH�POSSþ½NH�D2000Þ; for amino�POSS;xE¼ ½epoxy�POSS=ð½epoxy�POSSþ½epoxy�DGEBAÞ; for epoxy�POSS:

In the case of the hybrid network DGEBA–D2000–POS-S,A2, e.g. three molar compositions differing in POSS con-tent were applied. The mole ratio of ANH functionalitiesin POSS,A2 and D2000 was equal to 1:1, 1:2 and 1:5, corre-sponding to xA = 0.50, 0.33 and 0.17, respectively. The[epoxy]/[NH] ratio (r) in the hybrid was kept stoichiometric, r = 1. Hence, the total mole composition with

POSSBuA2 POSSOctA2 POSSPhA2

POSSEtE1 POSSPhE1

POSS,Me8 POSS,Bu8 POSS,Ph8

O Si

O

Si

O

SiSiO

O O

OSi

O

Si

O

SiSiOO O

NH2

O Si

O

Si

O

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O O

OSi

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SiSiOO O

NH2 NH2

OSi

O

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SiO

OO

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SiSi

OO

OSi

OSi

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OO

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OO

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OO

OSi

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O Si

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O O

OSi

O

Si

O

SiSiOO O

O Si

O

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SiSiO

O O

OSi

O

Si

O

SiSiOO O

OO

Si

O

Si

O

SiO

OO

Si

O

Si

O

SiSi

OO

OSi

Scheme 1. The functionalized and nonfunctionalized POSS units.

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respect to functional groups was as follows; [epoxy]DGEBA:[NH]D2000: [NH]POSS = 2:1:1, 3:2:1 and 6:5:1, respectively,corresponding to 43–11 wt.% POSS, according to the POSSsubstituents.

2.2.1. One-step synthesisSynthesis of the nanostructured polymers should start

under optimum conditions from the homogeneous mix-ture of monomers – epoxide (DGEBA), curing agent (dia-mine D2000) and POSS. Miscibility of POSS with theorganic mixture DGEBA–D2000, however, decreases inthe series: POSSOct > POSSBu > POSSPh. The synthesis proce-dure, hence, depends on the POSS monomer. Polymeriza-tion of the hybrid containing liquid POSSOctA2 proceededin a bulk homogeneous mixture. In the case of the solidmonomers – POSSBuA2, POSSEtE1 and POSS,Bu8 – the THFsolutions were used. Due to low solubility the phenylsubstituted POSS units, such as POSSPhA2 and POSSPhE1, re-quired the reaction to be performed in the heterogeneousblend. Also network formation in the presence of nonfunc-tional POSS (POSS,Ph8 and POSS,Me8) took place in a het-erogeneous mixture. Vigorous stirring of the mixture up

to close the point of gelation of a system prevented phaseseparation and a POSS rich phase sedimentation.

The nanocomposite samples preparation was carriedout in a Teflon mold for 24 h at 120 �C followed by postcur-ing for 2 h at 150 �C. The diluted systems were prereactedat 80 �C in order to evaporate THF before filling the mold.During curing the phase/microphase separation took placein the mold.

2.2.2. Two-step synthesisIn order to avoid or suppress the phase separation and

POSS aggregation in the epoxy matrix we have appliedthe two-step synthesis procedure by modification of theLiu’s approach [14]. The procedure consists in preparationof the epoxy–POSS adducts which are more compatiblewith the initial matrix mixture then the POSS monomer.The reaction of the adducts with diamine in the secondstep resulted in a more homogeneous hybrid morphologywith well dispersed POSS domains in the epoxy matrix.Liu et al. prepared the epoxy–POSS adducts by the prereac-tion of POSS,A2 with DGEBA at a molar ratio 1:2. Ideally, inaddition to the main adduct DGEBA–POSS,A2–DGEBA,

10 12 14

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Fig. 1. SEC record of the reaction mixture POSSOctA2/DGEBA = 1:3 atT = 100 �C. Reaction time t = 0 (1), 2 h (2), 4 h (3), 6 h (4), 8 h (5), 18 h (6).E = DGEBA, PA = POSS,A2.

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abbreviated as E–PA–E, also a distribution of oligomers E–(PA–E)n was formed. This procedure, however, could sufferfrom a possible incomplete incorporation of POSS mono-mers into the hybrid network. The reactivity of the POSSfunctionalities is low [12] and mainly the secondary ANHgroup reacts very reluctantly due to a sterical hindranceleading to a possible formation of the amine-terminatedoligomers (PA–E)n–PA. Moreover, at a high reaction tem-perature TC = 160 �C used by Liu in the first step and inthe presence of catalytic basic amine groups, the epoxyhomopolymerization is strongly promoted [15], thus likelycompeting to the epoxy–amine addition. The epoxyhomooligomers of a type PA–En –PA terminated by thelow reactive ANH group of POSS will thus be formed. Suchamine-terminated POSS containing oligomers cannot reactwith a diamine in the second step of the synthesis. Conse-quently, some POSS fraction will not be incorporated intothe network.

Modification of the procedure in our system DGEBA–D2000–POSS,A2 eliminates this insufficiency by suppress-ing formation of the amine-terminated oligomer POSS ad-ducts in the first step of the hybrid network synthesis.Moreover, even in the case of build up of some amountthese oligomers will react in the second step with the mix-ture comprising also a small content of DGEBA and not adiamine only.

(i) First step – preparation of the DGEBA–POSS,A2adduct.

In contrast to Liu we have used a larger epoxy excess inthe mixture POSS,A2–DGEBA to prepare preferentiallysmall adducts with the reactive terminal epoxy groups,E–(PA–E)n. In addition, the lower prereaction temperaturewas used (TC = 100 �C) compared to Liu in order to elimi-nate the homolymerization of the epoxide.

POSS,A2 and DGEBA in molar ratios 1:2, 1:3 and 1:6were dissolved in THF. After homogenization the solventwas evaporated and the reaction proceeded in a bulk mix-ture. The kinetics of the prereaction was followed by SEC asan evolution of the oligomer molar mass distribution. Thetypical chromatogram describing the prereaction of POS-SOctA2 with DGEBA in bulk at 100 �C is given in Fig. 1.The monomers POSS,A2 (PA) and DGEBA (E) and the small-est products PA–E, E–PA–E, PA–E–PA are distinguished.The optimum composition in the synthesis was found tobe the ratio POSS,A2/DGEBA = 1:3. At the ratio POSS,A2/DGEBA = 1:2 a complete reaction of POSS,A2 was notachieved and a higher epoxy excess (1:6) did not allow toprepare hybrids with a sufficient content of POSS. Figureproves that in the case of composition POSS,A2/DGE-BA = 1:3 the POSS,A2 monomer is completely reacted at6 h and after 18 h the adduct E–PA–E becomes the mainfraction of an oligomeric product distribution.

(ii) Second step – network formation.

The oligomer adduct prepared by the prereaction at100 �C for 18 h is well miscible with the epoxy mixture DGE-BA–D2000. The total epoxy/amine composition is kept stoi-chiometric. The system underwent curing and postcuring at

120 �C and 150 �C, respectively. Both terminal epoxy groupsin the main adduct fraction E–PA–E exhibit the same reac-tivity as those in DGEBA monomer. Therefore, the reactionof both diepoxies, DGEBA and the POSS containing E–PA–Eadduct, with diamine is random and POSS is distributedrandomly along the network chain. The monoamines PA–Eand PA–E–PA–E (see Fig. 1) and even the bisamine-termi-nated adducts (e.g. PA–E–PA) are incorporated in the net-work because of presence of DGEBA in the second reactionstep.

Improvement of POSS bonding to the network in thetwo-step synthesis was proved by sol–gel analysis. Thesamples were extracted by THF at 60 �C for 24 h to deter-mine a fraction of the sol containing unbound POSS. Thesol fraction (wS) was evaluated from the weight of thedry sample (mdry) before and after extraction (mdry-ext)(wS = 1�mdry-ext/mdry). In the case of DGEBA–D2000–POS-SOctA2 hybrid containing 30 wt.% POSS the sol fractionwas determined to be 9% for the one-step procedure and4% for the two-step synthesis. The synthesized hybrid net-works are transparent.

2.3. Methods

2.3.1. Dynamic mechanical analysis (DMA)DMA was performed with a rheometer ARES (TA Instru-

ments). The temperature dependence of the complex shearmodulus of rectangular samples (40 � 10 � 2 mm) wasmeasured at a heating rate of 2 �C/min by using oscillatoryshear deformation at a frequency of 1 Hz. Chemorheologyexperiments were performed with ARES in the parallelplate geometry (diameter 40 mm). The experiments werecarried out at 120 �C using shear deformation at a fre-quency of 1 Hz. Determination of the point of gelation bychemorheology is generally based on the application ofthe power law describing the rheological behavior of thesystem in the critical state [16], G0ðxÞ � G00ðxÞ � xn.Accordingly, the loss factor tan d is independent of theexperimental frequency at the gel point. By using this ap-proach we have previously found [17] that the stoichiom-

0 2000 4000 6000 800010-310-210-1100101102103104105106

G'.

Pa

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0.1

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100

tan

δ

t, s

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5 4 1 32

0 2000 4000 6000 800010-210-1100101102103104

1

η∗ . Pa.

s

c3245

Fig. 2. Evolution of the dynamic storage modulus G0 (a), loss factor tan d(b) and dynamic viscosity g⁄ (c) during formation of polymer networks. 1– DGEBA–D2000, 2 – DGEBA–D2000–POSSOctA2 (x = 0.17), 3 – DGEBA–D2000–POSSOctA2 (x = 0.33), 4 – DGEBA–D2000–aniline (x = 0.33), 5 –DGEBA–D2000–octylamine (x = 0.33). The corresponding concentrationsof functional NH groups CNH (mol NH/l): 1–1.50, 2–1.46, 3–1.44, 4–1.98,5–1.95.

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etric DGEBA–Jeffamine systems gel at the critical value of(tan d)c � 2. Therefore, in this paper the gel point was iden-tified as a reaction time of reaching the value (tan d)C = 2during polymerization.

2.3.2. Small-angle X-ray scattering (SAXS)SAXS experiments were performed using a pinhole

camera (Molecular Metrology SAXS System) attached to amicrofocused X-ray beam generator (Osmic MicroMax002) operating at 45 kV and 0.66 mA (30 W). The camerawas equipped with a multiwire, gas-filled area detectorwith an active area diameter of 20 cm (Gabriel design).Two experimental setups were used to cover the q rangeof 0.04–11 nm�1 where q = (4p/k)sinh (k is the wavelengthand 2h is the scattering angle). The scattering intensitieswere put on absolute scale using a glassy carbon standard.

2.3.3. Size exclusion chromatography (SEC)Molecular weight growth and development of the

molecular weight distribution during the reaction werecharacterized by SEC with mixed E column filled with PLgel (polystyrene–divinylbenzene) (Polymer Laboratories)using THF as a mobile phase and RI detection.

2.3.4. Transmission electron microscopy (TEM)TEM was made with a microscope Tecnai G2 Spirit Twin

12. The samples were cut at room temperature using anultramicrotome Leica Ultracut UCT. The 50 nm ultrathinsections were collected on a microscopic grid, covered withca 4 nm carbon layer in order to limit sample damage inthe electron beam and observed in the TEM microscopeat 120 kV using bright field imaging.

3. Results and discussion

The monoepoxy–POSS (POSS,E1) or amino–POSS (POS-S,A2) monomers were used to covalently incorporate thehybrid POSS cage as a dangling block in the epoxy network.The networks with untethered POSS were prepared byusing the nonfunctional POSS compounds with inert or-ganic substituents (POSS,R8).

3.1. Networks with POSS covalently bound as pendant blocks

3.1.1. Nanostructured network formationBoth phase and molecular structure evolution are of

importance in the build-up of a nanostructured polymernetwork. As described in Experimental, the initial blendsof the phenyl substituted POSS monomers (POSSPhA2 andPOSSPhE1) with the DGEBA–D2000 mixture are heteroge-neous while the POSS with aliphatic substituents (isooctyl,isobutyl and ethyl) show a better miscibility with DGEBA–D2000. The mixture DGEBA–D2000–POSSOctA2 is initiallywell homogeneous and POSSBuA2 as well as POSSEtE1 atleast form homogeneous THF solutions. During one-steppolymerization the homogeneous systems undergo aphase/microphase separation induced by the reactionand/or by THF evaporation. As a result, a nanostructuredpolymer network involving domains of POSS aggregatesis formed. At the same time, however, a competitive effect

occurs due to a covalent bonding between the epoxy andthe POSS phase by a gradual POSS incorporation in theepoxy network leading to a partial mixture homogeniza-tion. The cured nanocomposites with POSSOctA2 and POS-SEtE1 are transparent thus revealing fine nanoscalemorphology contrary to the milky systems with POSSPh.The hybrids containing small amount of POSSBuA2 aretransparent while those with a high POSS content areopaque.

Molecular structure evolution during formation of theneat epoxy–amine and hybrid networks was followed byusing chemorheology. Fig. 2 shows an increase in dynamicviscosity g⁄ and dynamic shear storage modulus G0 at thereaction. Network formation is characterized by gelation,which is manifested by a steep increase in the modulusin Fig. 2a. Corresponding time of gelation tg is the basicinformation of a network build-up. The rate of gelationand tg of a homogeneous system are governed mainly byan average functionality of monomers and by the reactionkinetics given by a reactivity and concentration of func-tional groups. The effect of POSS on the network build-upwas determined using the homogeneous hybrid systemDGEBA–D2000–POSSOctA2. The gelation time was evalu-ated as the reaction time when the loss factor tan d reachedthe value 2 (see Fig. 2b and Experimental).

Due to a partial substitution of the diamine D2000 withthe monoamine–POSS,A2 in the DGEBA–D2000–POSSOctA2network, the average amine functionality in the hybrid isdiminished compared to the neat system. Because of the

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lower functionality and due to a relatively low reactivity ofPOSS, [12] the gelation of the hybrid network is sloweddown with respect to the neat network. The figure displaysformation of the hybrids containing 0.17 and 0.33 mol frac-tions of POSS, corresponding to 15 and 30 wt.% of POSS,respectively. The gel times increase with increasingamount of POSS,A2 introduced (curves 1–3). The figureshows also the chemorheology curves of the POSS-freenetworks containing monoamines like aniline or octyl-amine (curves 4 and 5). These systems gel faster than DGE-BA–D2000 (curve 1) despite the lower amine averagefunctionality. This fact is a result of a higher concentrationof functional groups because of a much smaller volume ofthe monoamines with respect to the replaced D2000. (Con-centration of functional groups in the neat and POSS,A2containing networks, however, is comparable; see the cap-tion in Fig. 2). Moreover, a high reactivity of the aliphaticoctylamine plays also a role in accelerating the gelation.

The Fig. 2 displays a different evolution of dynamic vis-cosity (Fig. 2c) and modulus (Fig. 2a) in the hybrids withrespect to the POSS-free systems. The viscosity increaseduring polymerization, given by growth of the polymermolar mass and branching, exhibits the similar shape andcomparable pregel viscosity values for all the studiedsystems. On the contrary, the elastic modulus developmentdiffers in both types of networks. The POSS-free polymers(DGEBA–D2000, DGEBA–D2000–octylamine, DGEBA–D2000–aniline) show a very low elastic modulus in thepregel stage. The ‘‘pregel modulus’’ of the POSS hybrids,however, is by one or two orders of the magnitude higher.The POSS monomers associated in small domains aregradually covalently attached to a polymer chain duringpolymerization. As a result, physical crosslinks are createdbetween polymer chains of the forming epoxy–aminenetwork as shown in Scheme 2. The physical crosslinkingbefore the chemical gel point is reflected by the elastic con-tribution to the modulus. The systems with POSS-freemonoamines containing dangling phenyl or octyl substitu-ents instead of the POSS cage do not show such a modulusincrease in the early reaction stage.

Rheology is often used to characterize nanocomposites.Kopesky et al. [18] studied linear entangled copolymers ofPMMA containing tethered POSS. They found a decrease inthe storage and plateau moduli in the hybrids with POSSdue to POSS induced reduction of the entanglementsdensity. However, blends of the PMMA–POSS copolymerwith an untethered POSS show an increase in viscosityand plateau modulus as a result of association betweenthe untethered and tethered POSS in the blend. Unlike

Scheme 2. Physical crosslinking by POSS aggregates. polymerchain, s POSS unit, d POSS covalently bound to the polymer chain.

the rheology behavior of these stable linear polymers, inour case we were following a dynamic evolution of therheology at formation of the hybrid network. The entangle-ments play no role in this case because of extensivebranching and a short length of network chains. Due togradual bonding of POSS,A2 units to the polymer both teth-ered and untethered POSS are present in the system duringpolymerization alike in the above blend of the linear hy-brid. The POSS association including the untethered andtethered units leads to formation of dynamic physicalcrosslinks via a path; polymer – POSS (tethered) – POSSdomain (untethered) – POSS (tethered) – polymer (seeScheme 2).

3.1.2. Structure and morphology of the hybrid networksThe hybrid networks containing pendant POSS,A2 and

POSS,E1 units show amorphous or crystalline POSS nanod-omains. The liquid POSS with isooctyl substituents, POSSOc-

tA2, forms amorphous POSS aggregates while the crystallinePOSS monomers, isobutyl-(POSSBuA2) and mainly phenyl-substituted (POSSPhA2 and POSSPhE1), tend to form partiallycrystalline domains in the network. Their crystallinity,however, is restricted in the polymer matrix. WAXS analysisproved (see Table 1) that while POSSPhA2 remained fullycrystalline in the hybrid network, the fraction of POSSPhE1and butyl substituted POSSBuA2 in the crystalline state is re-duced in the network. The crystallinity of the POSS withethyl substituents, POSSEtE1 is even completely suppressedin the polymer.

3.1.2.1. Aggregation and ordering. TEM analysis displayed ahomogeneous dispersion of POSS domains of the size up to100 nm in the POSSOct containing hybrids. The hybrid net-works with butyl- (at high contents) and phenyl-POSS,however, show large heterogeneities reaching up to�500 nm. Due to aggregation a large part of the POSSmonomers is trapped in the domains interior and cannotbe covalently tethered to the network. The sol–gel analysisproved the sol fractions wS extracted by THF from thehybrids comprising 30 wt.% of POSS; wS = 0.09 in case ofPOSSOctA2 and wS = 0.22 for POSSPhA2 and POSSPhE1,respectively. It reveals that only 70% of POSSOctA2 (i.e.1–0.09/0.30) or 27% of POSSPhA2 and POSSPhE1 units weretethered to the network, while the rest of POSS remaineduntethered within domains. This result is in agreementwith the model of physical crosslinks in Scheme 2.

The detailed internal aggregates structure was deter-mined by SAXS. The hybrid networks involving various

Table 1Crystallinity of POSS within the epoxy–POSS hybrid networks.

Hybrid network POSS crystallinitya

DGEBA–D2000–POSSOctA2 0DGEBA–D2000–POSSBuA2 0.36DGEBA–D2000–POSSPhA2 1DGEBA–D2000–POSSEtE1 0DGEBA–D2000–POSSPhE1 0.63DGEBA–D2000–POSS,Me8 0.91DGEBA–D2000–POSS,Bu8 1DGEBA–D2000–POSS,Ph8 1

a Weight fraction of POSS in the crystalline state within a hybrid.

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amount (11–43 wt.%) of POSS with octyl, butyl and phenylsubstituents are displayed in Fig. 3. The SAXS profiles of thehybrids show three characteristic regions.

(i) Increase in intensity of the scattered X-ray at low q-values proves system heterogeneities. The intensity Ienhances with growing POSS content revealing thatthe scattering is given by a POSS rich phase in a net-work. The SAXS profile provides information on thesize and inner structure of aggregates.

(ii) The peak in the high-q region, at q1 � 5–6 nm�1,appears due to presence of POSS cages and its ampli-tude increases with growing POSS content.

(iii) The region showing the maximum at q2 � 1.4 nm�1

is assigned to a correlation distance between POSSaggregates in the network (d2 � 2p/qmax � 4.5 nm).This peak features ordering in the hybrid system.The DGEBA–D2000 network was found to beordered [19] and previously we have reported[10,20] on the regular arrangement in the organic–inorganic hybrids based on the DGEBA–D2000 net-work. The stiff polyepoxy chains in the networkare separated by flexible polyoxypropylene (POP)of the diamine D2000. The correlation distancebetween polyepoxy chains in SAXS profiles corre-sponds to the end-to-end distance for D2000 chain,d � 4.5 nm. The ordered DGEBA–D2000 networkserves as a template for a regular assembling of POSSspecies in a hybrid. The correlation distance is thusemphasized by the POSS cages dangling along thepolyepoxy chain in the DGEBA–D2000–POSS,A2 net-works. We have proved that the stiff polyepoxychain in the network is crucial for ordering. Applica-tion of the flexible diepoxide DER 732 instead of stiffDGEBA resulted in disappearance of the regulararrangement in the hybrid network DER732–D2000–POSSBuA2.

0.1 1 10

0.1

1

Inte

nsity

q, nm-1

1

3

2

a

0.1

0.1

1

10

q,

3

2

1

Fig. 3. SAXS profiles of the hybrid networks DGEBA–D2000–POSS,A2 containing v15 wt.% POSS, 2–30 wt.%, 3–43 wt.%, (b) POSSBuA2, 1–11 wt.% POSS, 2–22 wt.%, 3

While a power-law is observed in low-q region for thePOSS with octyl and butyl substituents (Fig. 3a and b),the SAXS profiles of the hybrids containing phenyl-substi-tuted POSS (Fig. 3c) exhibit a knee at q � 0.2–0.5 nm�1

being more pronounced at a high POSSPh content. Such aprofile corresponds to a two-size scale structure of aggre-gates composed of smaller heterogeneity domains. The sizeof the POSS domains was determined by using the Beau-cage model [21] to be Rg = 3–4 nm, i.e. the size of8–11 nm in diameter assuming a spherical shape. Theamount of these subdomains increases with increasingPOSS content. No knee appears in the case of a high contentof POSSBuA2 and no break on the profile is apparent in thehybrids involving the more miscible POSSOctA2. This resultreveals an inner heterogeneity of the aggregates from phe-nyl-substituted POSS in contrast to those from butyl- andoctyl-substituted POSS. The size of these aggregates wasestimated to be larger than 100 nm.

The position of the interference maximum at q1 �5–6 nm�1 corresponds to the size of the POSS unit charac-terized by internal interatomic distances within the POSSincluding a shell of substituents. This value is given by sizeof the POSS substituents (d1 � 2 p/qmax = 1.1–1.3 nm), theoctyl-substituted being the largest. The sharp peak in thecase of POSSBu and mainly POSSPh characterizes well or-dered arrangement on small distances in contrast to thebroad band of the POSSoct revealing a loss of regularitydue to flexible octyl cage substituents.

The ordering in DGEBA–D2000 based hybrids is mostpronounced in the case of the most miscible monomerPOSSOctA2 (Fig. 3a), while a less regular assembly of POSSdomains was observed using POSSBuA2 and POSSPhA2. Theincreasing strength of the POSS–POSS interaction in the ser-ies POSSOct < POSSBu < POSSPh leads to a higher number ofaggregates and to a violence of the regular arrangementwithin the network. In contrast to DGEBA–D2000–POSS,A2networks the hybrids with monoepoxy–POSS monomers,

1 10

nm-1

b

0.1 1 10

1

10

100

1000

q, nm-1

12

3

c

arious amount of POSS units with different substituents. (a) POSSOctA2, 1––34 wt.%, (c) POSSPhA2, 1–13 wt.% POSS, 2–25 wt.%, 3–38 wt.%.

Fig. 4. TEM micrographs of the hybrid DGEBA–D2000–POSSBuA2 (38 wt.%) prepared by (a) one step synthesis and (b) two step procedure.

1010.1

0.01

0.1

1

Inte

nsity

q, nm-1

3

4

1

2

Fig. 5. SAXS profiles of the hybrid DGEBA–D2000–POSS,A2 (x = 0.5) withisobutyl- and isooctyl substituted POSS prepared by one and two stepprocedures. DGEBA–D2000–POSSBuA2–1 one step, 2 two step, DGEBA–D2000–POSSOctA2–3 one step, 4 two step.

L. Matejka et al. / European Polymer Journal 48 (2012) 260–274 267

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POSS,E1, exhibit no ordering. In this case the polyepoxychain is interrupted by the presence of the monoepoxymonomer. Due to the resulting network defects the order-ing is destroyed.

3.1.2.2. Effect of the two-step synthesis. The two-step syn-thesis procedure results in a reduction of aggregates sizes,a better dispersion of POSS and a more homogeneous mor-phology of the hybrid DGEBA–D2000–POSS,A2. TEMmicrograph in Fig. 4 shows a dramatic decrease in size oftypical aggregates in the case of a high content of butylsubstituted POSS. Also the SAXS scattered intensity atlow angles in Fig. 5 is significantly diminished with respectto the one-step hybrids because of a reduced size of POSSstructures. The POSS aggregates are still composed of smallparticles (Rg = 3–4 nm) which is revealed by the knee atq = 0.2–0.5 nm�1. The ordering is slightly disturbed bythe two-step synthesis as revealed by the less pronouncedmaximum at q = 1.4 nm�1 and smaller and broader POSSpeak at q = 6 nm�1. This is in agreement with the resultsof Williams et al. [6] revealing a reduced crystallinity. Amild effect of the two-step synthesis only was observedby SAXS in the case of the more miscible POSSOctA2monomer.

3.1.3. Thermomechanical propertiesThe POSS nanofiller effect on the network chain

mobility and possible network reinforcement is to beextracted from the thermomechanical behavior of thehybrid networks followed by DMA in Fig. 6–8. The hybridsDGEBA–D2000–POSSPhE1 display in Fig. 6 a mild increasein glass transition temperature with respect to the neatDGEBA–D2000 system. The glass transition temperaturewas determined from position of the maximum of the lossfactor tan d curve. Tg is pushed up by 4 �C from �27 �C ofthe neat network to �23 �C in the POSS containing systemsdue to robust POSS phenyl substituents reducing theflexible chain mobility. In addition a small transitionbroadening is observed. In contrast, an incorporation ofPOSSPhE1 in the stiff glassy and highly crosslinked network

DGEBA–MDEA (4,40methylenebis(2,6-diethylaniline)) byWilliams et al. [6] led to a significant decrease in Tg byup to 24 �C from 171 to 147 �C. This was a result of anincrease in free volume by attached bulky POSS units. Linand Khare [22] used the molecular modelling techniqueto calculate Tg of the hybrid network DGEBA–trimethyleneglycol diaminobenzoate, comprising up to 30 wt.% ofPOSSPhE1. The simulations revealed that incorporation ofPOSS,E1 in the glassy network but of a slightly lower cross-linking density compared to Williams’ did not affect thechain mobility and Tg of the system. Consequently, the of-ten discussed controversial data about POSS effect on Tg ofpolymers likely ensues from a relativity of the POSS influ-ence with respect to a polymer type. One has to take intoaccount Tg and rigidity of a neat polymer. The bulky POSScage increases the free volume and diminishes Tg in glassysystems. In the rubbery polymers, however, a restriction ofthe flexible chain mobility dominates thus increasing Tg

despite an increase in free volume.

-50 0 50 100106

107

108

109

G',

Pa

T, °C

a

423

321

-50 0 50 100

0.01

0.1

1

tan

δ

T, °C

b

4

1

Fig. 6. Storage modulus G0 (a) and loss factor tan d (b) as functions oftemperature for the hybrid networks DGEBA–D2000–POSSPhE1 withdifferent POSS contents 1 – 13 wt.%, 2–22 wt.%, 3–30 wt.%, 4 – referencenetwork DGEBA–D2000.

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106

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G',

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T, °C

a

-50 0 50 100

10-2

10-1

100

tan

δ

T, °C

4123

4

1 23

b

Fig. 7. Storage modulus G0 (a) and loss factor tan d (b) as functions oftemperature for the hybrid networks DGEBA–D2000–POSSBuA2 withdifferent POSS contents 1–11 wt.%, 2–22 wt.%, 3–34 wt.%, 4 – referencenetwork DGEBA–D2000.

268 L. Matejka et al. / European Polymer Journal 48 (2012) 260–274

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In our rubbery DGEBA–D2000 network a more pro-nounced Tg enhancement was achieved by incorporationof POSS,A2 monomers. Fig. 7 shows a shift of Tg withincreasing content of the bound POSS units up to the value�12 �C, i.e. by 15 �C, for the network with 34 wt.% of POS-SBuA2. At the same time a significant transition broadeningreveals a wider distribution of chains relaxations corre-sponding to a distribution of more and less mobile chainsequences. These changes, however, do not reflect a POSSnanofiller effect but results from a substitution of the longflexible chain of D2000 with the short segment involvingpendant POSS cage.

The effect of POSS substituents; isooctyl, isobutyl andphenyl, is demonstrated in Fig. 8 for similar POSS contents.The Tg is independent of the substituents both in POSS,A2and POSS,E1 (not displayed) containing networks. TheFig. 8b, however, reveals a new relaxation peak on the lossfactor tan d curve in the case of the octyl-substituted POSS.The POSSOct unit is miscible with the epoxy system and theinteraction with the polymer leads to a partial immobiliza-tion of the flexible poly(oxypropylene) chain of the net-work. The slowing down of the relaxation of a part of thechains results in an appearance of the loss factor maximumat a higher temperature. A weaker or no interaction is ob-served in the case of less compatible POSSPh and POSSBu,respectively. The tan d curves show a narrow symmetricaltransition or a small shoulder only.

Introducing the pendant POSS in the epoxy–amine net-work DGEBA–D2000 by using POSS,A2 or POSS,E1 leads toa modification of the network structure. Consequently, inaddition to a nanofiller reinforcing effect one has to takeinto account also change of the matrix structure affectingthe properties. Attachment of the monoepoxy POSS(POSS,E1) in the diepoxy–diamine network results in achain termination and formation of a network withdefects. Substitution of a diepoxide with a monoepoxymonomer leads to a decrease in crosslinking density ofthe network. A high mole fraction of the monoepoxymonomer will even prevent gelation of a system. The crit-ical composition for a gel formation under random reactionconditions involves the fraction of the monoepoxy unit(xE)C = 0.67. It follows from the expression for the criticalconversion (aC) at random alternating epoxy–diaminereaction [23]; aC = 1/[(fA�1) (fEw�1)]1/2, (fA and fEw arefunctionality of an amine and weight average epoxyfunctionality) and a corresponding condition for the criti-cal epoxy functionality (fEw)C = 1.33. The hybrid with ahigher POSS,E1 content does not gel. Also incorporationof the bifunctional monoamine–POSS (POSS,A2) insteadof a diamine leads to a decrease in crosslinking density.In this case, however, a linear epoxy–amine polymer isformed and a network build-up as well as density of thenetwork are affected less. The network is built even at a

-50 0 50 100

106

107

108

109

G',

Pa

T, °C

a

-50 0 50 100

10-2

10-1

100

tan

δ

T, °C

1

1

3

3

2

2

b

Fig. 8. Storage modulus G0 (a) and loss factor tan d (b) as functions oftemperature for the hybrid networks DGEBA–D2000–POSS,A2 withdifferent POSS substituents 1 – POSSOct (43 wt.%), 2 – POSSBu (34 wt.%),3 – POSSPh (38 wt.%).

0 10 20 30 40 50

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

G' ,

MPa

wt. % POSS

1

2

3

4

5

67

Fig. 9. Rubbery storage modulus G0 of the hybrid networks at 100 �C as afunction of POSS content. 1 s DGEBA–D2000–POSSPhA2, 2 h

DGEBA–D2000–POSSBuA2, 3 � DGEBA–D2000–POSSOctA2, 4 4 DGEBA–D2000–POSSPhE1, 5 theory DGEBA–D2000–POSSOctA2, 6 theory DGEBA–D2000–POSSPhE1, 7 theory DGEBA–D2000–POSS,A1, referencenetwork DGEBA–D2000.

L. Matejka et al. / European Polymer Journal 48 (2012) 260–274 269

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replacement of 99% of a tetrafunctional diamine with abifunctional monoamine. The network crosslinking densitym is related to equilibrium shear modulus Ge in the rubberystate according to the theory of rubber elasticity [24],

Ge ¼ mRTAf w2=3g ; ð1Þ

where front factor Af = (feff�2)/feff for a phantom net-work and Af = 1 for the affine network, feff is the effectivefunctionality of the crosslink, wg is fraction of the gel.

The rubbery moduli in DGEBA–D2000–POSS,A2 net-works in Fig. 7 decrease with growing POSS amount becauseof diminishing crosslinking density. On the contrary,however, the moduli increase in the case of the networkscontaining POSSPhE1 (see Fig. 6). Unlike the glass transitiontemperature the hybrids moduli depend on POSS substitu-ents as revealed in Fig. 8. The storage moduli in the rubberystate of the studied hybrids are summarized in Fig. 9. Themoduli of the hybrid networks containing a small amount(11–14 wt.%) of POSS increase in the series of the incorpo-rated POSS monomers: G0(POSSOctA2) < G0(unmodifiednetwork) < G0(POSSBuA2) < G0(POSSPhE1) < G0(POSSPhA2).Except the POSSOctA2 all other POSS monomers reinforcethe epoxy network when applied in a small content. Thefigure highlights the decrease in moduli of the POSS,A2comprising networks (curves 1–3) and the moduli enhance-ment of those with POSS,E1 (curve 4) at increasing POSScontent.

For the theoretical prediction of moduli both the struc-ture and crosslinking density of the polymer network ma-trix, as well as the reinforcing nanofiller effect were takeninto account. Theory of rubber elasticity [24], the theory ofnetwork formation, particularly the statistical theory ofbranching processes (TBP) [25], and the model for mechan-ical properties of particulate composites [26] were applied.The effect of POSS on a polymer network structure andmoduli was evaluated first under assumption of a systemhomogeneity. The theory of network formation (seeAppendix) in combination with theory of rubbery elasticitypredicts the decrease in moduli for the hybrids with POS-S,A2 (Fig. 9, curve 5) and POSS,E1 (curve 6), respectively,with growing content of POSS due to diminishing cross-linking density m. The incomplete incorporation of POSSin the network due to aggregation was considered byapplying the experimentally determined values of gel frac-tions wg (=1�wS) in Eq. (1). Differences in the theoreticalmoduli between the hybrids with octyl-, butyl- and phe-nyl-substituted POSS,A2 are not large and hence only thetheoretical curve for POSSOctA2 is displayed. The figure dis-closes a reasonable agreement of the theoretical predictionwith the experiment only in the case of the most homoge-neous POSSOctA2 containing network. Both hybrids withphenyl substituted POSS (POSSPhE1 and POSSPhA2) showsignificantly higher moduli compared to the theory forthe homogeneous networks which reflects the nanofillereffect. In the case of phenyl substituted POSS a very strongPOSSPh–POSSPh interaction results in formation of POSScrystalline domains within the epoxy network. Due tocovalent bonding of a fraction of POSSPh monomers tothe polymer (27%, as determined by the sol–gel analysis,see above), these domains of physical aggregates act likephysical crosslinks as discussed in the chemorheology sec-tion (see Scheme 2). This physical crosslinking by hard do-mains prevails over a decline in chemical crosslinking andleads to reinforcement of the network as suggested in ourprevious paper [11]. Isobutyl- and isooctyl-substituted

-100 -50 0 50 100 150

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G',

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a

1

32

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1E-3

0.01

0.1

1

tan

δ

T, oC

12

3

b

-50 0 50 100 150

106

107

G',

Pa

321

Fig. 10. Storage modulus G0 (a) and loss factor tan d (b) as functions oftemperature for the hybrid networks DGEBA–D2000–POSSBuA2 preparedby different procedures. 1 – one-step synthesis, 2 – two-step synthesis, 3– two-step synthesis in the presence of THF.

270 L. Matejka et al. / European Polymer Journal 48 (2012) 260–274

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POSS show a weaker POSS–POSS interaction thus display-ing only a low extent of crystallization or forming smallamorphous domains. These soft aggregates do not rein-force the system. With exception of the low content ofPOSSBuA2 in the network, the aggregates are not efficientenough as physical crosslinks to compensate a decrementof chemical crosslinking density.

The physical crosslinking via POSS units was proved bythe model system prepared from POSSEtE1 and D2000. Thereaction of the monoepoxide with diamine in the molarratio 4:1 leads to formation of the adduct; D2000 chain ter-minated with four POSS units. This low-molecular weightadduct was found to show a high elastic modulus in a rub-bery state (G0at 100�C = 2.7 � 106 Pa), evidencing formationof a physical network by interaction between POSS units.The ethyl-substituted POSS cages build amorphous aggre-gates at the end of D2000 oligomer chain serving as phys-ical crosslink domains. Epoxy homopolymerization ofPOSS,E1 to produce a high-molecular weight solid polymer[15], was excluded by comparing with the reference POSS-free model. The reaction of the monoepoxide phenylglyci-dylether and diamine D2000 under the same conditionsresulted in a liquid product only. The extremely densephysical network from POSS units was prepared by thereaction of the amino– and epoxy–functionalized POSSmonomers, POSSBuA2 and POSSEtE1. The hybrid composedof this monoamine and monoepoxide displayed rubberymodulus G0 = 108 Pa.

The different trend of moduli in POSS,A2 and POSSPhE1containing networks in dependence on POSS amount inFig. 9 was interpreted as follows. The POSS functionalitiesboth epoxy and amines show a low reactivity [12] leadingto a possible incomplete conversion. This effect is mainlypronounced in the case of amino groups of POSS,A2 in aheterogeneous system. Due to steric reasons given by theovercrowded neighborhood of the POSS cage particularlythe reaction of the secondary amine hydrogen is consider-ably hindered, i.e. a very strong substitution effect is oper-ative [27]. In such a case the tetrafunctional amine D2000(A4) in fact is replaced in the hybrid network by an effec-tively monofunctional reagent (A1) causing a terminationof the growing epoxy–amine chain. This actuality leads toa more pronounced diminishing of the chemical crosslink-ing density as shown by the theoretical curve 7 calculatedunder assumption that only the primary amine hydrogenin POSS,A2 has reacted. Consequently, at a high POSS,A2content in the heterogeneous network the decline of chem-ical crosslinking is too severe and the experimental modu-lus of the hybrid network decreases despite the efficientphysical crosslinking via POSSPhA2 domains. On the con-trary, an incomplete conversion of POSS,E1 does not affectthe crosslinking density. The growing chain is terminatedin any case; both if the monofunctional POSS,E1 reacts ornot.

3.1.3.1. The ‘‘two-step hybrid networks’’. The hybrid net-works prepared by the two-step procedure display an in-creased homogeneity of a POSS dispersion (see Fig. 4)leading to an enhancement of interphase interactionbetween POSS and network chains as well as to a mild in-crease in rubbery modulus in Fig. 10. The inhomogeneously

dispersed large POSS domains in the network DGEBA–D2000–POSSBuA2 prepared by one step exhibit a relativelysmall surface area. They do not interact with a networkchain as it is obvious from a narrow symmetrical band ofthe loss factor tan d in Fig. 10b and a very small loss factoramplitude above Tg. In the case of the two-step polymeriza-tion, the better dispersed POSS cages provide a larger organ-ic–inorganic interfacial area. The loss factor tan d indicates aplateau above T = 50 �C corresponding to relaxation of afraction of the network chains immobilized due to the inter-phase interaction with POSS. The broad loss factor plateauup to �120 �C reveals a wide distribution of the restrictednetwork chains forming the interphase. The fraction of theimmobilized chains, however, is small as the relative areaunder the loss factor curve attributed to the interphase islow; only �5%. According to a rough estimate [26] the vol-ume fractions of two phases in the system are approxi-mately proportional to the area under the loss factor curveat regions corresponding to relaxation of these phases.Consequently, despite a better POSS dispersion in the‘‘two-step’’ hybrids, the interfacial interaction is weakresulting in the small volume fraction of the interphase.Hence, the modulus is only slightly enhanced with respectto the ‘‘one-step’’ network (Fig. 10a). Beyond relaxation at�120 �C the loss factor decreases well below a value of

0 2000 4000 6000 800010-3

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10-1

100

101

102

103

104

105

G',

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t, s

1

23

3

45

a

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L. Matejka et al. / European Polymer Journal 48 (2012) 260–274 271

the ‘‘one-step hybrid’’. Extremely low losses remind an ide-ally elastic material. This fact is interpreted by a loosermolecular chain packing in the ‘‘two-step hybrid’’ due to alarger free volume provided by a more homogeneously dis-persed bulky POSS units. The relaxed chains are less re-stricted in movement than in the absence of POSS. This isin agreement with the results of Pissis et al. [28] and thetwo-phase model of polymer dynamics. The interfacial areais immobilized by constraints imposed to the chain motionby the bound rigid POSS unit, while the rest of the bulk poly-mer or the relaxed interphase show a higher mobility. Theeffect of homogenization on interaction and modulus wasproved also in the hybrid prepared in THF solution whilesuppressing a solvent evaporation thus delaying the micro-phase separation. Fig. 10b shows a broader loss factor pla-teau evidencing a stronger interaction.

4

5

0 2000 4000 6000 8000

10-2

10-1

100

101

102

103

η∗, P

a.s

t, s

1

3

2

3 b

Fig. 11. Evolution of the storage modulus G0 (a) and dynamic viscosity g⁄

(b) during formation of polymer hybrid networks with tethered anduntethered POSS. 1 – DGEBA–D2000, 2 – DGEBA–D2000–POSS,Bu8(11 wt.%), 3 – DGEBA–D2000–POSS,Bu8 (30 wt.%), 4 – DGEBA–D2000–POSSOctA2 (15 wt.%), 5 – DGEBA–D2000–POSSOctA2 (30 wt.%).

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3.2. Networks with untethered POSS

The nonfunctional POSS involving methyl, isobutyl andphenyl substituents (POSS,R8) were applied to prepare theepoxy hybrids with dispersed unbound POSS species. Incontrast to the POSS covalently bound in a network, theuntethered POSS blended in a matrix does not affect thechemical crosslinking density of a network. The octaIsobu-tyl POSS (POSS,Bu8) is soluble in THF and the mixture ofthe solution with the epoxy system is homogeneous andtransparent. During the reaction and THF evaporation,however, the phase separation occurs and heterogeneousnetworks are formed. On the contrary, the POSS,Me8 andPOSS,Ph8 are insoluble and even the initial mixtures areheterogeneous.

The structure evolution during formation of the DGE-BA–D2000–POSS,Bu8 hybrid characterized by chemorheol-ogy curves is displayed in Fig. 11. The comparison with thebound POSS containing hybrid DGEBA–D2000–POSSOctA2is shown as well. In the case of untethered ‘‘POSS,Bu8 sys-tem’’ first THF was evaporated before the chemorheologyexperiment. The reaction is slower and gelation occurs la-ter than in the hybrid with tethered POSSOctA2. A high dilu-tion of the reaction mixture with the nonreactive POSS‘‘diluent’’ is a probable and facile interpretation of this phe-nomenon. This influence on gelation kinetics prevails overthe fact that an unbound POSS does not decrease the sys-tem functionality contrary to POSS,A2. The heterogeneousPOSS,Bu8 containing system shows a significantly higherinitial viscosity as well as the modulus. These quantities in-crease with growing POSS content because of a hydrody-namic reinforcing effect of phase-separated nanofilleraggregates. The POSSOctA2 involving hybrid is homoge-neous and exhibits a low initial viscosity comparable tothe POSS-free mixture (DGEBA–D2000) (curve 1). The dif-ference in evolution of the polymer systems with theuntethered (curves 2 and 3) and tethered (curves 4 and5) POSS consists in the abrupt pregel growth of the initiallylow elasticity (modulus) of DGEBA–D2000–POSSOctA2polymer starting cca in 2000 s of the reaction and assignedto physical crosslinking. No such a sudden increase inmodulus in the pregel stage was observed in POSS,Bu8containing mixture. Only filler structuring (aggregation in

the initial stage), however no attachment to a networkleading to physical crosslinking occurs during the reaction.

The POSS,R8 are crystalline and the POSS domains withinthe epoxy network keep their crystallinity as determined byWAXS (see Table 1). Glass transition temperature Tg of thenetworks is not affected by the unbound POSS as shownin Fig. 12. Moreover, Tg is independent both of the POSS sub-stituents and the POSS content. The POSS domains do notinteract with the network chain with exception of the bettermiscible POSS,Bu8. In this case a small loss factor maximumat T � 70 �C is obvious in figure in addition to the main glasstransition relaxation revealing a mild interaction. However,all the dispersed POSS, forming hard crystalline domains ina matrix, efficiently reinforce the epoxy network. Fig. 13(curve 1) shows that the modulus of the ‘‘POSS,R8 hybrids’’increases with growing content of POSS, independently ofthe type of POSS substituents. 26% (or 28%) of POSS,Ph8 orPOSS,Me8 blended in the network leads to the modulus in-crease by three times.

The Kerner model modified by Nielsen [26] is often usedto predict the modulus of a heterogeneous two-phase sys-tem with hard domains dispersed in a matrix.

GC=GM ¼ ð1þ ABv f Þ=ð1� Bwv fÞ; ð2ÞA ¼ ð7� 5vMÞ=ð8� 10vMÞB ¼ ððGf=GMÞ � 1Þ=ððGf=GMÞ � AÞw ¼ 1þ v f ð1� vmaxÞÞ=v2

max

1000-100106

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108

109G

' (Pa

)

T, °C

a

1000-100

0.01

0.1

1

tan

δ

T, °C

1

2

3

12

3

b

Fig. 12. Storage modulus G0 (a) and loss factor tan d (b) as functions oftemperature for the hybrid networks DGEBA–D2000–POSS,R8 (11 wt.%POSS) with different POSS substituents 1 – POSS,Bu8, 2 – POSS,Ph8, 3 –POSS,Me8.

0 10 20 30

1

2

3

4

5

6

7

G',

MPa

vol % POSS

physical crosslinking

POSS-polymer interaction+ POSS-POSS interaction

filler structuring

POSS-POSS interaction

1

2

3

4

Fig. 13. Rubbery storage modulus G0 at 100 �C of the hybrid networkswith tethered and untethered POSS as a function of POSS content, 1DGEBA–D2000–POSS,R8, d POSS,Ph8; s POSS,Me8; 2 theory - particulatecomposite model, 3 DGEBA–D2000–POSSPhE1, 4 theory DGEBA–D2000–POSSPhE1, reference network DGEBA–D2000, Gf = 4 � 109 Pa,GM = 2.3 � 106 Pa, vmax = 0.6, vM = 0.5.

272 L. Matejka et al. / European Polymer Journal 48 (2012) 260–274

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where GC, GM, Gf are moduli of the composite, matrixand filler, respectively, vM the Poisson ratio of the matrix,vf volume fraction of the filler and vmax the maximumpacking fraction of the filler. The composite modulusdepends on volume fraction of components and hencethe volume content of POSS is applied to display the

experimental data and the theoretical curves in Fig. 13.The figure reveals higher experimental moduli comparedto the particulate composite model (curve 2) at a largePOSS content due likely to nanofiller structuring.

At a low POSS content the hybrids with bound POSSPhE1(curve 3) are slightly more efficient in the reinforcement ofthe epoxy network than the systems with dispersed unteth-ered POSS,R8. For higher POSS amounts (>15 vol.%), how-ever, the POSS,R8 containing nanocomposites (curve 1)are more beneficent. Crucial is the deteriorating effect ofdecreasing crosslinking density of the networks with teth-ered POSS units. In the hybrids with bound POSS both theeffect of diminishing crosslinking density and the nanofillerreinforcing effect were taken into account by using TBP andthe particulate composite model (curve 4). In addition,however, the POSS–polymer interaction leading to physicalcrosslinking has to be considered to fit the experimentaldata (see Fig. 13). The extent of physical crosslinkingenhances with increasing POSS content.

4. Conclusions

The structure–mechanical properties relationshipsdetermination and the structure control in the epoxy–POSSnanocomposites are the main goals of the paper. The hy-brid epoxy–POSS networks were prepared both with POSScovalently bound as pendant unit on the polymer chainand with unbound POSS dispersed in the network DGE-BA–D2000. For a system investigation we have (a) followedin situ hybrid network formation, (b) applied various ap-proaches of a structure control and (c) elucidated the POSSeffect on mechanical properties by using theoreticalmethods.

The control of the structure and morphology of thenanocomposites is governed by adjusting of POSS–polymerand POSS–POSS interactions. It was performed by (i) vary-ing organic substituents (Oct, Bu, Me, and Ph) of the POSScage, by (ii) binding of POSS units to the network and by(iii) modification of the polymerization procedure. Only oc-tyl substituted POSS is miscible with the epoxy system tobuild the transparent nanocomposite despite a limitedreaction induced microphase separation and formation ofnanosized (�100 nm) amorphous aggregates. In the caseof other substituents the low POSS–polymer compatibilityand a strong POSS–POSS interaction lead to larger POSSmainly crystalline domains (up to 500 nm) dispersed in amatrix. Covalent bonding of POSS to the network by usingepoxy- and aminofunctionalized POSS (POSS,E1 and POS-S,A2) improved homogeneity of POSS dispersion in the hy-brid compared to the systems with the untethered POSSunits (POSS,R8). The modification of the Liu’s two-step syn-thesis made it possible the almost complete POSS incorpo-ration, significant homogenization of the system andenhancement of the interaction with the matrix.

Formation of the hybrid networks was followed bychemorheology experiments. We have proved physicalcrosslinking via POSS domains occurring during the net-work build-up before the gelation point. The existence ofthe physical network in addition to chemical crosslinkingwas evidenced also by low-molecular weight models.

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The epoxy–POSS hybrids show reinforcement, charac-terized by increase in the rubbery modulus, with respectto the neat DGEBA–D2000 network. The efficiency of thereinforcement increases in the series of POSS units: POS-S,A2 < POSS,E1 < POSS,R8. The theoretical approach usingthe theory of network formation, theory of rubber elastic-ity and the model of mechanical properties of the particu-late composites was applied to evaluate the mechanicalbehavior of the nanocomposites. The following factorswere taken into account to elucidate the effect of POSSon mechanical properties of the hybrid networks; (a) fillerreinforcing effect, (b) deterioration of a network structureand decrease in chemical crosslinking density by incorpo-ration of monofunctional reagents, (c) physical crosslink-ing via POSS domains being dependent on POSSsubstituents (only hard crystalline phenyl-substitutedPOSS domains are effective as physical crosslinks).

Glass transition temperature Tg of the epoxy network isnot affected by dispersed POSS units revealing an absenceof POSS–polymer interaction. Unlike, a Tg increase occursin the case of covalently bound POSS due to partial chainimmobilization or because of replacement of the flexiblechain of D2000 by a short unit POSS,A2 with pendant POSScage.

The effect of POSS on thermomechanical properties ofpolymer networks is complex and relative with respectto the particular polymer. One can expect a decrease inTg and modulus in stiff glassy polymers due to large POSSfree volume, while reinforcement and Tg enhancementbecause of imposed constraints and a flexible chain immo-bilization by the same POSS unit in the case of rubbery sys-tems. In addition to these competitive effects another twofactors play a role; modification of the network matrixstructure by incorporation of a monofunctional POSS andthe physical crosslinking via hard POSS domains involvingboth POSS–polymer and POSS–POSS interaction.

Acknowledgments

The authors acknowledge the financial support of theGrant Agency of the Czech Republic (IAA 400500701) andthe Academy of Sciences of the Czech Republic in the frameof the Program supporting an international cooperation(M200500903). The authors thank dr. šlouf for the TEMmicrograph.

Appendix A

The theoretical moduli of homogeneous networks werecalculated according to Eq. (2) by using the theory ofbranching processes (TBP) providing the values of cross-linking density m. The network formation was describedby the epoxy–amine reaction of bifunctional diepoxideDGEBA designated as E2 and monoepoxy POSS,E1 (E1)with tetrafunctional diamine D2000 (A4) and bifunctionalmonoamine POSS,A2 (A2). The ideal random alternatingreaction was assumed.

The TBP [25] describes the system by distribution ofstructural units defined by the reaction state of thefunctional groups, i.e. number of reacted and unreacted

functionalities. The distribution developing during thereaction is obtained by using a kinetic scheme. The struc-tural units are combined at any moment of the reactionto form tree-like structures. The polymer characteristicsevolving during network formation, such as molecularmass, fraction of the gel, concentration of elastically activenetwork chains (EANC), etc. are calculated by using proba-bility generating functions (pgf) describing the number ofissuing bonds from a unit. The pgfs for a unit in a rootare as follows:

F0EðzAÞ¼ð1�aEþaEzAÞi! i¼2for the diepoxideðE2Þ; i¼1for the monoepoxideðE1ÞF0AðzEÞ¼ð1�aAþaAzEÞk! k¼4for the diamineðA4Þ; k¼2forA2andk¼1forA1F0ðzÞ¼nAF0AðzEÞþnEF0EðzAÞ

Z is a dummy variable and subscripts of z indicate thedirection of a bond, thus zA and zE indicate a bond fromunit E (epoxide) to A (amine) and from A to E, respectively.aE and aA are conversion of the epoxy and NH aminegroups, respectively (aE = aA = a for the alternating reac-tion of a stoichiometric mixture). nA and nE are mol frac-tions of amine and epoxy units.

Crosslinking density m was calculated as concentrationof EANC.

m ¼ NerfE=dðrfEMA þ fAMEÞ

Ne is number of EANCs, d - density, r – ratio of NH/epoxyfunctionalities, fE, fA – average functionalities of E and Aunits, MA and ME – molar mass of A and E unit, respectively

Determination of Ne by using TBP:

1. The system DGEBA–POSS,E1–D2000

F0ðzÞ ¼ nAð1� aþ azEÞ4 þ nE1ð1� aþ azAÞþ nE2ð1� aþ azAÞ2

nE1 and nE2 are mol fractions of E1 and E2 units in the totalcomposition (nA þ nE1 þ nE2 ¼ 1)

Ne ¼ nA½12a2ð1�vEÞ2½1�ð1�aþavEÞ2��=4

vE¼ðax1þ2ax2ð1�aþavAÞÞ=ðax1þ2ax2Þ;vA ¼ð1�aþavEÞ3

x1¼nE1=ðnE1þnE2Þ; x2¼1�x1

x1 and x2 are mol fractions of E1 and E2 units within theepoxy monomers (x1 þ x2 ¼ 1)

(2) The system DGEBA–D2000–POSS,A2F0ðzÞ ¼ nE2ð1� aþ azAÞ2 þ nA2ð1� aþ azEÞ2

þ nA4ð1� aþ azEÞ4

nA2 and nA4 are mol fractions of A2 and A4 units in the totalcomposition (nE2 þ nA2 þ nA4 ¼ 1)

Ne ¼ 3nAx4a2ð1� vEÞ2½1� ð1� aþ avEÞ2�vE ¼ 1� aþ avA;

vA ¼ ð1� aþ avEÞ½x2 þ 2x4ð1� aþ avEÞ2�=ð2� x2Þx2 ¼ nA2=ðnA2 þ nA4Þ; x4 ¼ 1� x2

x2 and x4 are mol fractions of A2 and A4 units within theamine monomers (x2 þ x4 ¼ 1)

In case of the restricted reaction of POSS,A2 units onlythe primary amines were allowed to react.

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