J O U R N A L O F C H E M I S T R Y Materials Curing and morphology of epoxy resin–silica hybrids Leno Mascia* and Tao Tang Institute of Polymer Technology and Materials Engineering, Loughborough University, Loughborough, UK LE11 3TU Received 3rd July 1998, Accepted 25th August, 1998 Hybrids of epoxy resin and silica cured respectively with methyl nadic anhydride (MNA) and 4,4¾-diaminodiphenyl sulfone (DDS) were prepared.Control of the morphology was achieved through functionalisation of a diglycidyl ether resin respectively with monofunctional and difunctional secondary amine trialkoxysilanes prior to being mixed with a solution of tetraethoxysilane (TEOS) and hardener. Scanning and transmission electron microscopy examinations (SEM and TEM) were carried out to study the morphology of the samples.The results have shown that the preparation conditions and nature of solvent play a vital role in the compatibilisation of the final hybrid. The addition of a hydrolysed TEOS solution into the epoxy resin to produce the corresponding hybrid was found to interfere with the cross-linking reactions with the hardener, inevitably resulting in a reduction in the Tg of the epoxy resin component.This was attributed to side reactions of MNA with the ethanol released from hydrolysis and condensation of the TEOS, producing monofunctional and difunctional esters which act as plasticisers and to decreased functionality of the epoxy resin from the reaction with the HCl used for the hydrolysis of TEOS. Recently, Nishijima et al.10,11 have reported the preparation Introduction of a hybrid material based on an epoxy resin/silica system, Prevention of phase separation and morphology control are using tetraglycidyl-meta-xylenediamine (TGMXDA) as the crucial factors in the preparation of organic–inorganic hybrid resin and 1,2-cyclohexanedicarboxylic anhydride (HHPA) as materials (also known as ceramers or nanocomposites) pro- curing agent. In this case, the hybrid was prepared by producing duced by the in situ polymerisation of a hydrolysed metal first a silica filler by the sol–gel method, containing N-balkoxide within the bulk of an organic component.aminoethyl-c-aminopropylmethyldimethoxysilane as a coupling It has been shown that highly transparent hybrid materials, agent which was subsequently incorporated in the epoxy resin with phase domains <100 nm, can be produced by a variety mixture. of compatibilisation techniques.e.g. (1) Through functionalis- Several workers have used the intercalation process to ation of the polymer or polymerisable oligomer with a trialkox- produce an epoxy resin–clay hybrid12–15 utilising the well ysilane.1–4 (2) By adding a functional trialkoxysilane, as a defined dimensions of the structure layers of the chosen clay coupling agent, to the precursor mixture to chemically link or (i.e.montmorillonite). Other workers have shown that the provide strong physical interactions between the organic and important factor in controlling the dimensions of the inorganic inorganic domains.5 (3) By using another polymer as compati- phase is the nature of the intercalating agent.16 biliser, which is miscible with the primary polymer component In this work the possibility of producing hybrids of a cured and is capable of producing strong interactions with the epoxy resin with silica is being investigated with the scope of siloxane component.6 determining the influence of curing agents and reaction con- Homogeneous hybrids have been prepared also without the ditions.Although the occurrence of some adverse reactions use of coupling agents from solutions of polymers containing between the components of the alkoxysilane precursor for the pendant carbonyl groups along the chains, such as polyvinyl inorganic phase and either the hardener or the epoxy resin acetate (PVAc) and poly(methyl methacrylate) (PMMA).7,8 can be anticipated, the extent to which they will aVect both David and Scherer9 have shown that hybrids of poly(ethy- the compatibilisation mechanism and the final properties loxazoline) and silica form a molecular semi-interpenetrating cannot be predicted. This work seeks to identify the major network, as a result of extensive hydrogen bonding being diYculties that are likely to be encountered in attempting to formed between the silanol groups of the siloxane network produce epoxy–silica hybrids.and the carbonyl groups in the polymer. Very little has been reported on the preparation of epoxy resin/silica hybrid. Landry et al.4 have prepared a hybrid Experimental material from a very high molecular weight epoxy resin (Mw= Materials 47000), functionalised with c-aminopropyltriethoxysilane and (a) Resin components.The resins used were all bisphenol-A silica. A dioxane solution of the reactants catalysed by 0.15 M types, namely Epikote 828, Epikote 1001 and Epikote 1009 HCl was cast in open air and subsequently allowed to gel and (manufactured by Shell Chemicals). These have a number the solvent to evaporate over a period of two days.This was average molecular weight of ca. 370, 880 and 5000, correspond- followed by further drying at 60 °C for 2 h and a curing step ing to an average degree of polymerisation of 0.1, 2 and 16, at 150 °C for 15 h. No specific mention was made, however, expressed in terms of central CH2CHOHCH2 units per mol- of any likely crosslinking reaction taking place within the organic polymer phase.ecule. Their general structure is as shown below: J. Mater. Chem., 1998, 8(11), 2417–2421 2417Two hardeners were used, respectively methyl nadic It is important to note that only partial functionalisation of the epoxy resin is being considered in these studies so that the anhydride (MNA), corresponding to methyl 4-endo-methylenetetrahydrophthalic anhydride, in conjunction with N-benzyldi- composition of the organic phase will remain essentially similar to that of a cured epoxy resin.This confers to the resulting methylamine (BDMA) as catalyst, and 4,4¾-diaminodiphenyl sulfone (DDS). hybrids features that are substantially diVerent from earlier systems in which the difunctional organic oligomer was part of the siloxane network, forming a separate phase from the (b) Alkoxysilanes.Tetraethoxysilane (TEOS) (purity> pure silica network. 95%), obtained from Fluka, was used as the precursor The expected advantages of using the epoxy functionality for the formation of the silica phase. The compatibilising to produce a network with conventional hardeners for epoxy agents were trialkoxysilanes types, N-phenylaminopropylresins are twofold: (a) it makes it possible to produce hybrid trimethoxysilane (Y-9669) (purity>98.5) and bis(cmaterials in which the organic component predominates so trimethoxysilylpropyl )amine (A-1170) (purity>95%), both that the inorganic phase acts primarily as a reinforcing obtained from OSI Specialities.component. (b) The formation of a purely organic network would make it possible to produce larger organic domains, (c) Auxiliaries.A 32 wt.% solution of HCl in water was thereby reducing the level of brittleness normally experienced used as catalyst for the hydrolysis of the TEOS component. with conventional oligomer based organic inorganic Dimethylformamide (DMF) and tetrahydrofuran (THF) were hybrids.1,2 used as solvents.Preparation of epoxy resin ceramers Functionalisation of the epoxy resin The trimethoxysilane functionalised epoxy resin was dissolved Preliminary attempts to functionalise the epoxy resin with in an anhydrous solvent, either DMF or THF, at ca. 25 wt.% either a primary amine silane (i.e. c-aminopropyltriconcentration and measured amounts of water and TEOS at ethoxysilane) or with an isocyanate silane (i.e. cmolar ratio of 3–451 were added.This was followed by the isocyanatopropyltriethoxysilane), from reactions in bulk at addition of the HCl solution to bring the pH in the range of 90 °C produced rapid gelation and, therefore, were not 2–3. The reactants were stirred until the mixture became clear studied further. (ca. 12 h) and finally the resin and the MNA hardener were Aliphatic primary amines are known to react very rapidly added.The ceramer solutions were cast in PTFE moulds and with epoxy groups and, despite their bifunctionality, they can the solvent allowed to evaporate slowly at room temperature promote the formation of highly branched species by catalysing for ca. 24 h to induce gelation and then cured at 80 °C for 48 h.the internal condensation reactions via the newly formed Further curing was carried out in several steps, i.e. 24 h at hydroxyl groups. 120 °C, 5 h at 150 °C and 3 h at 180°C. The gelation resulting from the reaction with the isocyanate The above reactions involve the formation of two primary silane is due to the very rapid reaction of –CNO with the networks, an epoxide–ester network and a high density silox- pendant hydroxyl groups in the –CH2CH(OH)CH2O segment ane network (silica), each forming two separate phases.The in the epoxy oligomer, and the subsequent reaction of the two main phases are linked by species consisting of a network urethane groups so formed with the epoxy groups, and containing the two components. with itself, to produce the corresponding allophonate A schematic example of this type of network is shown below: groups.Subsequent preliminary experiments have identified secondary amines to be suitable for the functionalisation of the chosen epoxy resin. The chemical structure of the silanes used is described in the materials section above. In a round bottom flask was added the epoxy resin and the secondary amine coupling agent at diVerent molar ratios.The mixture was stirred for 2 h at 90 °C and the progress of the reaction was followed by FTIR analysis and by observing qualitatively any increase in viscosity. 1H NMR measurements were used to determine the reaction yield using a 300 MHz equipment. The reaction scheme for the functionalisation of the epoxy Characterisation of the hybrids resin is as shown below: (a) The compatibility of hybrids was first assessed from a (a) Reaction with N-phenylaminopropyltrimethoxysilane visual inspection of the state of cast films in order to obtain a qualitative assessment of the dimensions of the phases.Cloudiness and opacity was taken to indicate the presence of heterogeneous structures of the order of ca. 0.5 mm or larger. (b) The morphology of the cured resins was characterised by examining fractured surfaces using a Cambridge stereoscan electron microscope (360 Model). Some samples were etched with a 10 wt.% aqueous HF solution to enhance the contrast between the two phases. Examinations were also made by (b) Reaction with bis(trimethoxysilylpropyl )amine 2418 J. Mater. Chem., 1998, 8(11), 2417–2421A molar ratio of epoxy resin to silane A1170 of 1051 was found to be suYcient to achieve compatibility as compared to a molar ratio of 451 for the monofunctional aminosilane Y9669.It is not known, however, to what extent, the higher degree of conversion in the functionalisation reaction is responsible for the above behaviour relative to the bifunctionality resulting from the use of silane A1170.The enormous eVect that an increase in the molecular weight of the epoxy resin has on the compatibility of the hybrid is very clearly demonstrated from the data in Table 1, which shows that the amount of Y-9669 is reduced by a factor of seven when the molecular weight of the epoxy resin is increased from 370 to 5000. This is expected to result from the increased number of pendant hydroxyl groups along the chains of the epoxide oligomer which provide stronger H-bonding interactions with the silanol groups of the silica phase, thereby reducing the rate of phase Fig. 1 Conversion of NH groups in the secondary amine silane (Y- separation and subsequent growth during the drying stage of 9669) in the reaction with Epikote 828 at a molar ratio of 251.the preparation of the ceramer films. For ceramers based on Epikote 1001 (M=880) using THF as solvent an appreciable eVect was observed for the stirring transmission electron microscopy (TEM-100CX apparatus time after adding the hardener, i.e., the longer the stirring time manufactured by JEOL Ltd) on thin slices microtomed from the higher is the compatibility of the hybrids.This eVect is cast films cast in epoxy resin. shown in the SEM micrographs in Fig. 2 and TEM micro- (c) The curing reactions of the functionalised epoxy resin graphs in Fig. 3. with hardener were followed by FTIR analysis (Mattson 3000 No eVect was observed on the compatibility of the cast films FTIR spectrometer) and the glass transition temperature (Tg) when THF was used to replace DMF as the solvent for the of the epoxide network in the ceramers was measured using a ceramer solution. However, the stirring time needed to compa- Dupont DSC 9000 apparatus.tibilise the ceramer increased considerably. Replacing DMF (d) Various model experiments were carried out to elucidate with methyl ethyl ketone (MEK) produced opaque systems the mechanism of the reactions involved in the curing process. even after prolonged stirring, e.g. 6 h at room temperature. (These are outlined in the next section.) This solvent related kinetic eVect for the compatibilisation of the ceramer is related to the solubility of the products of Results and discussion the reaction in the precursor ceramer solution. In other words, a more advanced stage of reaction between the functionalised Functionalisation of epoxy resins epoxy resin and the hydrolysed TEOS is required when the The reaction of the low molecular weight resin, Epikote 828, H-bonding power of the solvent is reduced.with the secondary aromatic amine silane (Y-9669) was monitored by FTIR analysis, measuring the intensity ratio of the Curing reactions of the epoxy resin in the corresponding silica NH peak at 3396 cm-1 for the silane component to the OH ceramers peak at ca. 3500 cm-1 formed in the epoxide network. EVects on glass transition temperature. It was observed that Plots of the NH/OH absorbance peak ratio against reaction ceramers based on the epoxy resin functionalised with the time at 80 and 90 °C are shown in Fig. 1. From these it is aromatic amine silane (Y-9669) containing 25 wt.% of silica, evident that the reaction is not complete, and that it reaches and cured with MNA under standard conditions, i.e. 24 hours about 50% conversion after 1–2 h. Since the reaction yield, at 120 °C, 5 hours at 150 °C and 3 hours at 180 °C, displayed even at 90 °C, did not increase appreciably after 2 h, this a glass transition (Tg) of ca. 75°C, as compared with the value condition was chosen for the final preparation of the telechelic of 106 °C for the pure epoxy resin system. functionalised epoxy resin. For the same system, i.e. Epikote Table 2 lists Tg values for the two modified epoxy resins, 828/Y-9669 at molar ratio of 251, 1H NMR measurements one with silane A-1170 and the other with dibutylamine (DBA) revealed a 44% conversion of the Y-9669.When the epoxy at various molar ratios. These show that systems containing resin was reacted with bis(c-trimethoxysilylpropyl )amine (ADBA displayed an increase in Tg from 96 to 125 °C and 1170) at molar ratio of 351, FTIR measurements showed that remained constant even after increasing the level of modifi- the conversion of A-1170 was much higher than with Y-9669 cation from 1051 to 1052 molar ratio of epoxy to amine and NMR data confirmed that the conversion was 85% after groups.The equivalent ceramer system also displayed a small only 1 h at 90 °C. In both cases the unreacted aminosilane was increase in Tg at 1051 molar ratio functionalisation, but the not removed from the functionalised oligomer as this would Tg dropped to 62 °C when the molar ratio of the epoxy to participate in the subsequent reactions with the hydrolysed amine groups was increased to 1052.TEOS in the sol mixture and in the post curing reactions of FTIR analysis showed, however, that when a TEOS solution the epoxy resin. in THF was mixed and heated for 2 h at 80 °C with MNA, there was no change in the anhydride absorption band at EVect of preparation conditions on the compatibility† of epoxy- 1780 cm-1. When, however, this was carried out with a TEOS silica hybrids The results have shown that for the low molecular weight Table 1 EVect of molecular weight of bisphenol-A epoxy resins on the resin (i.e., Epikote 828), the amount of secondary difunctional amount of Y-9669 (50% conversion) for compatibilization of epoxy resin/silica hybrids at room temperature aliphatic aminosilane A-1170 required in functionalised epoxy resin in order to obtain a compatible hybrid was much lower Molecular weight of than for the monofunctional aromatic aminosilane (Y-9669).epoxy resin 370 880 5000 †The term compatible is used to indicate that the morphological Molar ratio of Y-9669 structure consists of domains considerably less than the wavelength to epoxy resin 0.50 0.17 0.07 of visible light.J. Mater. Chem., 1998, 8(11), 2417–2421 2419Fig. 3 TEM micrographs of morphology of ceramers with diVerent stirring time after adding hardner. (a) 0 h (opaque), (b) 5 h (transparent). Table 2 EVect of functionalization of epoxy resin on the Tg (°C) of the resulting networka Epoxy resin: modifiers (molar ratio) 1050 1051 105 2 DBA 96 125 125 A-1170 96 100 62 aSystem: MNA hardener and BDMA catalyst.Curing conditions: 120 °C, 5 h; 150 °C, 3 h; 180 °C, 1 h. Side eVects of HCl used as catalyst for the precursor siloxane solution. It is worth noting also that DSC measurements showed that the presence of water did not aVect the Tg of Fig. 2 SEM micrographs showing the changes in morphology brought about by increasing stirring time.(a) 0 h (opaque), (b) 3 h epoxy resin cured by MNA. When C2H5OH was deliberately (translucent), (c) 5 h (transparent). added, on the other hand, to the epoxy resin mixture, the Tg of the cured epoxy resin decreased from 119 to 55 °C. When HCl was also added to the epoxy resin mixture containing C2H5OH, the Tg of the cured epoxy resin, however, decreased solution hydrolysed in the presence of HCl and then mixed from 119 °C to only 90 °C.This indicates that the reaction under the same conditions, a part of the anhydride groups between the anhydride and hydroxyl groups proceeds to a was converted to ethyl carboxylate groups (absorption band lower degree of conversion when HCl is present.It is known, at 1737 cm-1) as a result of the reaction with the ethanol in fact, that esterification reactions occur more readily under formed from the hydrolysis of TEOS (Fig. 4). basic conditions.17 It is noted in Fig. 4 that spectra (a) and (b) are similar, FTIR analysis of the products extracted from films after whereas spectrum (c) displayed a large reduction in the immersion in THF for 24 h at 60 °C has indicated the presence intensity of the peak at 1780 cm-1 and the formation of a of ester groups due to formation of monoesters and possibly large peak at 1737 cm-1.This means that, under the curing some biesters from the reaction of MNA with ethanol. The conditions used in this work, only ethanol formed from the evidence is provided by the appearance of a strong absorption hydrolysis of TEOS can react with MNA and not the SiOH peak at 3500 cm-1, which is attributed to hydroxyl groups in the carboxylic acid produced from the anhydride.groups. 2420 J. Mater. Chem., 1998, 8(11), 2417–2421experiment carried out under acidic condition revealed a large increase in the intensity of the hydroxyl groups in the mixture.From this it is deduced that the large reduction in the Tg of the epoxy phase in the ceramer has to be attributed primarily to the reduction in concentration of epoxy groups as a result of the reaction with HCl used for the hydrolysis of TEOS, irrespective of the type of hardener used. Conclusions The following conclusions can be drawn: (1) it is possible to produce organic–inorganic hybrid materials based on epoxy resins through the sol–gel method, provided that the epoxy resin is functionalised with an organo trialkoxysilane. (2) Secondary aliphatic aminosilanes are more eVective than aromatic/aliphatic aminosilanes for the compatibilisation of epoxy resin–silica hybrids.(3) The level of compatibilisation achievable for epoxy resin–silica hybrids can be enhanced by increasing the magni- Fig. 4 FTIR spectra for mixtures of MNA and TEOS solution in tude of the following parameters: (a) molecular weight of the THF. (a) mixture of MNA and TEOS solution dried at room resin, (b) degree of functionalisation of the epoxy resin, temperature, (b) mixture of MNA and TEOS in the presence of HCl, dried at room temperature and heated at 80 °C for 2 h, (c) mixture of (c) polarity of the solvent and (d) processing temperature.MNA and TEOS, heated at 80 °C for 2 h, then cast film at room (4) The glass transition temperature of the epoxy network temperature and dried at room temperature under vacuum. in a ceramer is lower than the value achievable in the absence of the inorganic phase. This is attributed primarily to the Table 3 EVect of nature of hardner on the Tg of the epoxy network following reasons: (a) side reactions between the hardener and in the ceramera by-products of the hydrolysis of TEOS, and (b) the reduction in eVective concentration of epoxy groups as a result of their SiO2 content (wt.%) MNA DDS reaction with the acid catalyst used for the hydrolysis of TEOS. 0 103 124 2 80 93References 10 63 89 1 H. H.Huang, B. Orler and G. L. Wilkes, Macromolecules, 1987, aCuring conditions: 120 °C, 24 h; 150 °C, 5 h; 180 °C, 3 h. 20, 1322. 2 D. E. Rodrigues, A. B. Brennan, C. Betrabet, B. Wang and G. L. Wilkes, Chem. Mater., 1992, 4, 1437. It was noted that under the same curing conditions, the Tg 3 B. K. Coltrain, C. J. T. Landry, J. M. O’Reilly, A. M. of the epoxy resin crosslinked with DDS was higher than Chamberlain, G.A. Rakes, J. S. Sedita, L. W. Kelts, M. R. when MNA was used as hardener (Table 3). The resulting Landry and V. K. Long, Chem. Mater., 1993, 5, 1445. ceramer, however, exhibited a similar reduction in Tg as for 4 M. R. Landry, B. K. Coltrain, C. J. T. Landry and J. M. O’Reilly, the MNA cured system, despite the lack of any possibilities J.Polym. Sci., Polym. Phys. Ed., 1995, 33, 637. 5 A. Kioul and L. Mascia, J. Non-Crystal. Solids, 1994, 175, 169. for the formation of network diluents as side products. To 6 C. J. T. Landry, B. K. Coltrain, D. M. Teegarden, T. E. Long and verify further that the main cause for the reduction in the Tg V. K. Long, Macromolecules, 1996, 29, 4712. of the organic phase is the reaction of the epoxy groups with 7 J.J. Fitzgerald, C. J. T. Landry and J. M. Pochan, HCl, an epoxy resin/DDS mixture was prepared with the Macromolecules, 1992, 25, 3715. addition of volumetric amounts of HCl and DMF correspond- 8 C. J. T. Landry, B. K. Coltrain and B. K. Brady, Polymer, 1992, ing to the levels used in the production of the corresponding 33, 1486. 9 I. A. David and G.W. Scherer, Chem. Mater., 1995, 7, 1957. ceramers and cured under the same conditions. 10 S. Nishijima, M. Hussain, A. Nakahira, T. Okada and K. Niihara, Thin films were prepared to ensure that there was no residual Mater. Res. Soc. Symp. Proc., 1996, 435, 243. solvent left in the material after curing. DSC analysis was 11 M. Hussain, S. Nishijima, A. Nakahira, T. Okada and K. Niihara, used to obtain confirmation of the total absence of residual Mater. Res. Soc. Symp. Proc., 1996, 435, 369. solvent in the film by showing that the Tg of the resin was the 12 M. S. Wang and T. J. Pinnavaia, Chem. Mater., 1994, 6, 468. same irrespective of the amount of DMF used in the resin 13 P. B. Messersmith and E. P. Giannelis, Chem. Mater., 1994, 6, 1719. mixture, i.e. 193 °C. 14 P. Kelly, A. Akelah, S. Qutubuddin and A. Moet, J. Mater. Sci., When both HCl and water were also added to the resin 1994, 29, 2274. mixture, Tg decreased to 160 °C, irrespective of the water 15 T. Lan and T. J. Pinnavaia, Mater. Res. Soc. Symp. Proc., 1996, content. The possibility of water reacting with the epoxy 435, 79. groups was also discounted by model experiments in which 16 A. Akelah and A. Moet, J. Appl. Polym. Sci., Appl. Polym. Symp., small amounts were mixed with the epoxy resin in a THF 1994, 55, 153. 17 T. Tang, H. Y. Li and B. T. Huang, Eur. Polym. J., 1994, 30, 479. solution and kept at 80 °C for 18 h. FTIR measurements showed that hydroxyl group intensity did not increase in Paper 8/05144E comparison to the pure epoxy resin. Conversely a similar J. Mater. Chem., 1998, 8(11), 2417–2421 2421