Formation of nanostructured epoxy networks containing polyhedral oligomeric silsesquioxane (POSS) blocks

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<ul><li><p>Miroslav Slouf,Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovskeho nam. 2, CZ-162 06 Prague, Czech Republic</p><p>Received 25 September 2006; received in revised form 5 March 2007; accepted 21 March 2007</p><p>Available online 27 March 2007</p><p>Abstract</p><p>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).</p><p>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 octaepoxides 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.</p><p>Keywords: POSS; Epoxy network formation; Kinetics</p><p>1. Introduction</p><p>Epoxy-functional POSS (polyhedral oligomeric silses-quioxanes), whose reactivity and incorporation into epoxy net-works are discussed in this work, are nano-sized inorganic</p><p>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-Formation of nanostructured epoxoligomeric silsesquio</p><p>Adam Strachota*, Paul Whelan, Jir</p><p>Polymer 48 (2007) 3building blocks (size ca. 1.5 nm). Polymer networks with in-corporated POSS can be regarded as an organic matrix with</p><p>* Corresponding author. Tel.: 420 296 809 264; fax: 420 296 809 410.E-mail address: (A. Strachota).</p><p>0032-3861/$ - see front matter 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.polymer.2007.03.052y networks containing polyhedralxane (POSS) blocks</p><p>Krz, Jir Brus, Martina Urbanova,Libor Matejka</p><p>, 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</p></li><li><p>compounds of this type are nowadays commercially availableand industrial scale production of POSS has been announcedby Hybrid Plastics [8].</p><p>Incorporation of POSS into polymers affects the polymerchain mobility through POSS aggregation (authors 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.</p><p>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-</p><p>O Si</p><p>O</p><p>Si</p><p>O</p><p>SiSiO</p><p>O O</p><p>OSi</p><p>O</p><p>Si</p><p>O</p><p>SiSiOO O</p><p>RR</p><p>R</p><p>RR</p><p>RR</p><p>R</p><p>Scheme 1. T8-type (octameric) POSS of the composition R8Si8O12, RH ororganic group.</p><p>3042 A. Strachota et al. / Polymical constraint to chain movement by POSS was shown tobecome a very important factor at high POSS loadings inthe 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 anyPOSS-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.</p><p>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.</p><p>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 byPOSSs 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.</p><p>In order to elucidate the mechanism of the gelationreactions, the formation of DGEBAePOSS,E1eJeffamineD2000 (pendant POSS) and of several POSS,E8eamine</p><p>er 48 (2007) 3041e3058networks (junction-POSS) was followed by chemorheologymeasurements, yielding exact time of gelation. The</p></li><li><p>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.</p><p>2. Experimental part</p><p>2.1. Materials</p><p>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 (MW 2000, 430 and 230 g/mol, respectively) weredonated by Huntsman Inc., as well as the poly(oxypropylene)monoamine M600 (MW 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.</p><p>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.</p><p>2.2. Kinetics samples preparation</p><p>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).</p><p>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-</p><p>A. Strachota et al. / Polymdard. After mixing the components, the reaction mixture wasdivided into reaction vessels, which were sealed and put intoa 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).</p><p>2.3. Synthesis of networks</p><p>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(x 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.</p><p>2.4. NMR spectroscopy (for kinetics)</p><p>1H NMR spectra used for kinetic investigations were mea-sured with a Bruker (Karlsruhe, Germany) Avance DPX 300spectrometer at 300 MHz.</p><p>The relative concentration of the epoxy groups wasdetermined by following the relative intensity ( 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.</p><p>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 o...</p></li></ul>