Crosslinking of mixtures of DGEBA with 1,6-dioxaspiro[4,4]nonan-2,7-dione initiated by tertiary amines. Part IV. Effect of hydroxyl groups on initiation and curing kinetics

The anionic homopolymerization of DGEBA epoxy resin and its anionic copolymerization with a bislactone was studied using two alternative tertiary amines, 1-methylimidazole (1MI) and dimethylaminopyridine (DMAP) as initiators. 1MI caused slower cure than DMAP in neat DGEBA and DGEBA-bislactone formulations. Studies of the influence of the hydroxyl concentration in the DGEBA oligomer on its homopolymerization explain descrepancies in the literature regarding the ability of these initiators to produce full cure of the epoxy groups. In contrast, in the copolymerization of DGEBA-bislactone formulations, full cure could be readily achieved with either 1MI or DMAP as initiators, irrespective of the hydroxyl content. FTIR and DSC experiments show that this behaviour is associated with the formation of the carboxylate anion which plays an important part on the curing kinetics and the completion of cure.     

Poly(phenylene ether)/epoxy thermoset blends based on anionic polymerization of epoxy monomer

Low molar mass poly (phenylene ether) (LMW‐PPE) with phenol‐reactive chain ends was used as modifier of epoxy thermoset. The epoxy monomer was diglycidylether of bisphenol A (DGEBA), and several imidazoles were used as initiators of anionic polymerization. The curing and phase separation processes were investigated by different techniques: Differential Scanning Calorimetry, Size Exclusion Chromatography, and Light Transmission measurements. The final morphology of blends was observed by Environmental Scanning Electron Microscopy and Transmission Electron Microscopy. The epoxy network is obtained by imidazole initiated DGEBA homopolymerization. Initial LMW‐PPE/DGEBA mixtures show an UCST behavior with cloud point temperatures between 40 and 90°C. PPE phenol end‐groups can react with epoxy, leading to a better interaction between phases. The curing mechanism and phase separation process are not influenced by the chemical structure of initiators, except when reactive amine groups are present. The phase inversion is observed at 30 wt % of PPE. The mixtures with amine‐substituted imidazole present important differences in the initial miscibility and curing process interpreted in terms of fast room temperature amine‐epoxy reaction during blending. Final domain size is affected by this prereaction. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2678–2687, 2004     

Macro- and microgelation in the homopolymerization of diepoxides initiated by tertiary amines

Summary The homopolymerization of an epoxy resin based on diglycidylether of bisphenol A (DGEBA), initiated by benzyldimethylamine (BDMA), was analyzed in the 80°C–140°C temperature range. An heterogeneous network characterized by regions of different glass transition temperature, was obtained. Microgels appeared early in the polymerization while an increase in the reactivity of the second epoxy group of a DGEBA molecule after reaction of the first one, was inferred from size exclusion chromatograms (SEC), obtained at different overall conversions. Both experimental findings were qualitatively explained through an intramolecular chain transfer step that regenerates the initiator in the proximity of pendant epoxy groups. The increase in the polymerization temperature produced an increase in the macroscopic gel conversion and a decrease in the glass transition temperature of regions of high crosslink density. This was ascribed to the increase in the ratio of intramolecular chain transfer over propagation rates, leading to shorter primary chains.     

Crosslinking of mixtures of diglycidylether of bisphenol‐A with 1,6‐dioxaspiro[4.4] nonan‐2,7‐dione initiated by tertiary amines: III. Effect of hydroxyl groups on network formation

BACKGROUND: Blends of epoxy resin oligomers, diglycidylether of bisphenol‐A (DGEBA), and a bislactone, 1,6‐dioxaspiro[4.4]nonan‐2,7‐dione (s(γBL)), were anionically copolymerized using two tertiary amines as anionic initiators. Their curing rheology and gelation behaviour were studied to provide a more comprehensive knowledge of the curing of these previously studied systems. RESULTS: The activation energy for gelation was found to be similar to that previously measured using differential scanning calorimetry and appeared to increase in the presence of the bislactone. The reaction rate during copolymerization of DGEBA with s(γBL) was slower than DGEBA homopolymerization alone because the alkoxide attack on the epoxide is faster than the reaction of the carboxylate ion and the epoxy group. The effect of the initiator type on the gel conversion was small and was presumably due to differences in the kinetic chain length caused by amine regeneration from the quaternary amine. For the same initiator and at a constant ratio of DGEBA/s(γBL), an increase in the hydroxyl concentration of the DGEBA oligomer raised the gel conversion. For a DGEBA oligomer with low hydroxyl levels, an increase in the concentration of s(γBL) increased the gel conversion; however, for a DGEBA oligomer with high hydroxyl levels, increasing s(γBL) concentration decreased the gel conversion. CONCLUSION: These results are interpreted in terms of the effect of initiation rate and chain transfer rate on the kinetic chain length. The glass transition temperature of the gel was found to be controlled by the fraction of the aliphatic s(γBL) and the amount of plasticizing sol in the matrix. Copyright © 2009 Society of Chemical Industry     

Effect of backbone moiety in epoxies on thermal conductivity of epoxy/alumina composite

We chose two commercial epoxies, bisphenol A diglycidyl ether (DGEBA) and 3,3′,5,5′‐tetramethyl‐4,4′‐biphenol diglycidyl ether (TMBP), and synthesized one liquid crystalline epoxy (LCE), 4′4′‐bis(4‐hydroxybenzylidene)‐diaminophenylene diglycidyl ether (LCE‐DP) to investigate the effect of backbone moiety in epoxies on the thermal conductivity of epoxy/alumina composite. The DGEBA structure shows an amorphous state and the TMBP structure displays a crystal phase, whereas the LCE‐DP structure exhibits a liquid crystalline phase. The curing behaviors of them were examined employing 4,4′‐diaminodiphenylsulfone (DDS) as a curing agent. The heat of curing of epoxy resin was measured with dynamic differential scanning calorimetry (DSC). Alumina (Al₂O₃) of commercial source was applied as an inorganic filler. Thermal conductivity was measured by laser flash method and compared with value predicted by two theoretical models, Lewis‐Nielsen and Agari‐Uno. The results indicated that the thermal conductivity of the LCE‐DP structure was larger than that of the commercial epoxy resins such as TMBP and DGEBA and the experimental data fitted quite well in the values estimated by Agari‐Uno model. POLYM. COMPOS., 2013. © 2013 Society of Plastics Engineers     

Physical and Mechanical Interfacial Properties of Epoxy Copolymers Initiated by Latent Thermal Catalysts

Summary: The epoxy copolymers containing sulfone groups, diglycidyl ether of bisphenol‐A – Bisphenol‐S (DGEBA‐S) were synthesized by a hot‐melt method. The thermal properties of the epoxy systems initiated by two cationic latent catalysts, i.e., N‐benzylpyrazinium hexafluoroantimonate (BPH) and N‐benzylquinoxalinium hexafluoroantimonate (BQH), were investigated by using a dynamic DSC, DMA, and TGA. The mechanical properties were measured by single‐edge‐notched (SEN) beam fracture toughness tests. As a result, the thermal stability and mechanical interfacial properties of the DGEBA‐S/catalyst system were found to be higher than those of the DGEBA/catalyst. This was probably due to the fact that the introduction of sulfone groups with a polar nature to the main chain of the epoxy resins led to an improvement of thermal stability and toughness of the cured epoxy copolymers.

Conversion of the epoxy/catalyst systems as a function of curing temperature.     

Epoxy Networks Modified by a New Class of Oligomeric Silsesquioxanes Bearing Multiple Intramolecular Rings Formed through Si?O?C Bonds

Summary: A new class of silsesquioxane (SSO), containing species with two to nine Si atoms bearing multiple intramolecular rings formed through Si? O? C bonds, was synthesized as a glassy powder. It was characterized by UV‐MALDI‐TOF MS, ²⁹Si NMR and FT IR. Solutions containing different amounts of SSO in the diglycidyl ether of bisphenol A (DGEBA), were homopolymerized in the presence of (4‐dimethylamino)pyridine (DMAP) as initiator, leading to SSO‐modified epoxy networks. SSO species were covalently bonded to the epoxy network without any evidence of phase separation. The SSO addition provoked an increase in the elastic modulus in the glassy state explained by an increase in the cohesive energy density. The SSO addition gave also place to an increase in the intensity of tan δ and a decrease in both the glass transition temperature and the elastic modulus in the rubbery state. This was explained by a decrease in crosslink density associated with the flexibility of SSO structures. DMAP was much more effective than other usual initiators (like benzyldimethylamine, BDMA), in increasing the crosslink density of the resulting epoxy network. This led to high values of the glass transition temperature and the elastic modulus in the rubbery state.

Schematic representation of the chemical structure of the most significant species containing three Si atoms, present in the silsesquioxane.     

Crosslinking reaction of epoxy resin (diglycidyl ether of bisphenol A) by anionically polymerized polycaprolactam: I. Mechanism and optimization

Different ratios of epoxy resin, diglycidyl ether of bisphenol A (DGEBA) and ?‐caprolactam (starting from 10:90 DGEBA and vice versa), were used to synthesize reactive DGEBA and polycaprolactam blends by the anionic polymerization of ?‐caprolactam at 140°C. Anionic polymerization was conducted with a strong base such as sodium hydride as a catalyst along with a cocatalyst such as N‐acetyl caprolactam. The reaction mechanism, possible cure reactions, and reaction conditions of the reactive blends were studied with Fourier transform infrared spectroscopy and differential scanning calorimetry. The experiments were carried to study the optimization ratio and the effect of the composition on properties such as hardness and tensile strength of the reactive blends. The DGEBA was crosslinked by polycaprolactam through the reaction of the oxirane group with the amide nitrogen, and the reaction was very fast. A ratio of 80:20 (DGEBA:?‐caprolactam) was optimum, and the resulting blend showed the highest tensile strength and hardness. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3237–3247, 2003     

Nonisothermal cure kinetics of DGEBA with novel aromatic diamine

The effect of the molar ratio of diglycidyl ether of a bisphenol‐A based epoxy (DGEBA) and synthesized 4‐phenyl‐2,6‐bis(4‐aminophenyl)pyridine (PAP) as curing agent during nonisothermal cure reaction by the Kissinger, Ozawa, and isoconversional equations was studied. The cure mechanism was studied by FTIR analysis. Kinetic analysis of the curing reaction of DGEBA at two different concentrations (42 and 32 phr) of the curing agent was studied by using DSC analysis. With an increasing PAP content, the pre‐exponential factor increased by increasing collision probability between epoxide and primary or secondary amine groups in noncataltyic or catalytic modes. The activation energy also increased because of the increasing content of crosslink density. The activation energies obtained from three equations were in good agreement. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3076–3083, 2007.     

A novel flame retardant of spirocyclic pentaerythritol bisphosphorate for epoxy resins

A novel flame retardant for epoxy resins, bisdiglycol spirocyclic pentaerythritol bisphosphorate (BDSPBP) was synthesized from the reaction of diglycol with spirocyclic pentaerythritol bisphosphorate diphosphoryl chloride, which was obtained from the reaction of phosphoryl chloride with pentaerythritol. Flammability of the cured epoxy resin systems consisted of diglycidyl ether of bisphenol A (DGEBA), low‐molecular‐weight polyamide and BDSPBP are investigated by vertical burning test (UL‐94) and limiting oxygen index test (LOI). The results indicate that BDSPBP has good flame retardance on epoxy. The thermogravimetric analysis (TGA) shows that the epoxy resin containing BDSPBP has a high yield of residual char at high temperatures, indicating that BDSPBP is an effective charring agent. From the SEM observations of the residues of the flame retardant systems burned, the compact charred layers can be seen, which form protective shields to protect effectively internal structure, and inhibit the transmission of heat and heat diffusion during contacting fire. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4978–4982, 2006     

Synthesis and Spectroscopic Studies of DGEBA–Grafted Polyaniline

N-grafted polyaniline with diglycidyl ether bisphenol A (DGEBA) was synthesized via anionic copolymerization technique. Metalation reaction was performed to cause grafting to occur between DGEBA with emeraldine salt polyaniline (PANI-ES) and leucoemeraldine base polyaniline (PANI-LEB), respectively. Fourier transform infrared (FTIR), ¹³C-nuclear magnetic resonance (NMR), and ¹H-NMR spectroscopies and gel permeation chromatography (GPC) were performed to characterize the PANI-EB-g-DGEBA and PANI-ES-g-DGEBA copolymers. The gel content of the resultant copolymers was determined by Soxhlet extraction. Results of spectroscopic studies showed that grafting took place together with the side reaction, cross-linking, and DGEBA homopolymerization. Oxidation states of PANI were found to influence the gel content and molecular weight distribution of copolymers produced.     

Reaction and miscibility of two diepoxides with poly(ethylene terephthalate)

Poly(ethylene terephthalate) (PET) was melt‐blended at 270°C with two epoxy monomers, diglycidyl ether of bisphenol A (DGEBA) and 3,4‐epoxycyclohexyl‐methyl‐3,4‐epoxycyclohexyl carboxylate (ECY). Intermediate proportions of the epoxy in the range of 20–0.5 wt % were used. If the epoxy monomers were added in a high proportion (10–20%), a large fraction did not react with PET. Calorimetric experiments showed that the unreacted fractions of both epoxies were miscible with the amorphous phase of the polyester. Only one glass‐transition temperature was detected. It was depressed as the epoxy content was increased. The transition was broad when the PET component was crystalline, and it was narrow when the PET component was made amorphous by quenching of the blend. These features were confirmed by dynamic thermal mechanical analysis. As is often the case for crystalline blends, the crystallization and melting temperatures decreased when the proportion of the epoxy was increased. Concerning the reactivity of the epoxy with PET, the behavior differed according to the nature of the epoxy. The DGEBA monomer showed a low reactivity. It was not effective for the chain extension of PET, and no increase in the intrinsic viscosity was observed under the experimental conditions. However, some functionalization of the chain ends may be possible at a high concentration of the epoxy. ECY was more reactive, and the molecular weight of the processed PET increased, although the value of the commercial untreated polyester was not attained. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1995–2003, 2003     

Azo initiator selection to control the curing order in dimethacrylate/epoxy interpenetrating polymer networks

Differential Scanning calorimetry (DSC) and Fourier‐transform infrared (FT‐IR) spectroscopic studies have been undertaken of the cure of interpenetrating polymer networks (IPNs) formed with imidazole‐cured diglycidyl ether bisphenol‐A (DGEBA) and with either diethoxylated bisphenol‐A dimethacrylate (DEBPADM) or bisphenol‐A diglycidyl dimethacrylate (bisGMA), polymerized by a range of azo initiators (AIBN64, VAZ088, VR110 and AZO168). Due to the differing decomposition rates of the azo initiators, the neat dimethacrylate resin either cured faster than (with AIBN64 and VAZO88), or similar to (VR110), or slower than (AZO168), the neat epoxy resin. In the neat DGEBA/1‐methyl imidazole (1‐MeI), DEBPADM/AIBN64, DEBPADM/VAZO88 and DEBPADM/VR110 resins, close to full cure was achieved. For the neat, high‐temperature DEBPADM/AZO168 resin, full cure was not attained, possibly due to the compromise between using a high enough temperature for azo decomposition while avoiding depolymerization or decomposition of the methacrylate polymer. IPN cure studies showed that, by appropriate initiator selection, it was possible to interchange the order of cure of the components within the IPN so that either the dimethacrylate or epoxy cured first. In the isothermal cure of the 50:50 DEBPADM/AIBN64:DGEBA/1‐Mel IPN system, the cure rate of both species was less than in the parent resins, due to a dilution effect. For this system, the dimethacrylate cured first and to high conversion, due to plasticization by the unreacted epoxy, but the subsequent cure of the more slowly polymerizing epoxy component was restricted by the high crosslink density developed in the IPN. After post‐curing, however, high conversion of both reactive groups was observed and the fully cured IPN exhibited a single high‐temperature T_g, close to the T_g values of the parent resins. In the higher‐temperature, isothermal cure of the 50:50 DEBPADM/VR110:DGEBA/1‐Mel IPN system, the reactive groups cured at a similar rate and so the final conversions of both groups were restricted, while in the 50:50 DEBPADM/AZO168:DGEBA/1‐Mel system it was the epoxy which cured first. Both of these higher‐temperature azo‐initiated IPN systems exhibited single T_gs, indicating a single‐phase structure; however, the T_gs are significantly lower than expected, due to plasticization by residual methacrylate monomer and/or degradation products resulting from the high cure temperature. Copyright © 2004 Society of Chemical Industry     

Polymerization of epoxy resins initiated by metal complexes

BACKGROUND: Processing parameters and material properties of epoxy resins can be vastly influenced by choice of curing agent. In this work, metal complexes were investigated as initiators for anionic and cationic epoxide polymerization. Systems for thermally induced and electron beam‐induced curing are described. RESULTS: Zinc or cobalt imidazole complexes of the type [M(imidazole)₂(anion)₂] are efficient initiators for anionic polymerization of glycidyl‐based epoxy resins. The complexes can be employed to prepare tailored resin systems ranging from fast curing systems at slightly elevated temperatures to systems with very high thermal latencies curable at temperatures far above 150 °C. Silver complexes [Ag(L)_n]SbF₆ (L = crown ether or alkene) are highly efficient initiators for cationic curing and low initiator contents of around 1% are sufficient to reach high degrees of crosslinking. The complexes are excellent initiators for both thermally induced and electron beam‐induced polymerizations. CONCLUSION: Metal complexes are powerful initiators for the homopolymerization of epoxy resins and can be designed not only for anionic and cationic polymerization but also for thermal and radiation curing. Based on this study and additional work, a library can be compiled which allows retrieval of optimized metal–ligand–anion combinations and adjustment of the initiators to the respective processing and material demands. Copyright © 2009 Society of Chemical Industry     

Synthesis of photoaddressable polymeric networks having azobenzene moieties and alkyl‐chain‐containing compounds

We discuss the synthesis of new optically active polymeric networks containing azobenzene moieties and different alkyl‐chain‐containing compounds. An epoxy resin based on diglycidyl ether of bisphenol A (DGEBA) was reacted with metaxylylenediamine (MXDA). An azo prepolymer (TAZ) was synthesized by reaction between Disperse Orange‐3 and DGEBA. Reaction between palmitic acid (PA) and DGEBA was performed using triphenylphosphine as catalyst of the epoxy–acid reaction employing variable molar ratios of epoxy to carboxyl groups (r = 1, 2, 4). These precursors were called PA1, PA2 and PA4. Crosslinked epoxy‐based azopolymers containing variable PA‐based precursor content and constant chromophore concentration equal to 20 wt% TAZ were synthesized. Their reversible optical storage properties were studied and compared. It was found that the optical response is a direct consequence of the morphologies generated, and that crystallization of PA‐based precursor can take place. When the PA‐based precursor is not covalently bonded to the matrix, e.g. PA1, the remaining birefringence is high. PA4‐modified materials present a completely different response, showing a behaviour that could be of great importance in the development of optical switchers. In this case, the organic tails remain dissolved in the matrix and unable to crystallize, giving a typical ‘on–off’ response. © 2012 Society of Chemical Industry     

Curing of DGEBA epoxy resin by low generation amino‐group‐terminated dendrimers

Low generation amino‐group‐terminated poly(ester‐amine) dendrimers PEA1.0 (NH₂)₃ and PEA1.5 (NH₂)₈, and poly(amido‐amine) dendrimer PAMAM1.0 (NH₂)₄ were used as diglycidyl ether of bisphenol A (DGEBA) epoxy resin hardeners. Thermal behavior and curing kinetics of dendrimer/DGEBA systems were investigated by means of differential scanning calorimetry (DSC). Compared with ethylene diamine (EDA)/DGEBA system, the dendrimer/DGEBA systems gradually liberated heat in two stages during the curing process, and the total heat liberated was less. Apparent activation energy and curing reaction rate constants for dendrimer and EDA/DGEBA systems were estimated. Thermal stabilities and mechanical properties of cured thermosetting systems were examined as well. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 3902–3906, 2006     

A new epoxy resin/hardener system with variable processing and end properties

A combination of a standard bisphenol-A liquid epoxy resin and a zinc–pyrrolidone–carboxylate complex with a long chain poly(oxypropylene diamine) (Jeffamine D 2000) has been found to give variable processing and end properties over a very wide range, e.g., pot life < 1–70 days, Martens Heat-Distortion Temperature < 25–124°C and elongation at break 35–1%. The variation in properties is achieved simply by changing the epoxy/complex ratio. The very reactive high elongation formulation is thought to be the result of a very fast zinc catalyzed amine/epoxy polyaddition reaction and the low reactivity rigid formulation to be due to zinc-complex initiated anionic homopolymerization of the epoxy resin. The borderline concentrations are approximately 200 and 10 phr of the complex, respectively.     

Structures and properties of polycarbonate‐modified epoxies from two different blending processes

It has been proved in our previous study that during the melt‐blending of an epoxy oligomer based on the diglycidyl ether of bisphenol‐A (DGEBA) with polycarbonate (PC) at 200°C, the secondary hydroxyl groups in the DGEBA react with the carbonate groups in PC through transesterification, resulting in degraded PC chains with phenolic end groups and also in PC/DGEBA copolymers. Yet, in the same study, it was found that the prereactions between DGEBA and PC can be minimized or eliminated if a solution‐blending process was used. Therefore, it was expected that, after being cured with a curing agent, different epoxy‐network structures should result as a consequence of the two different premixing processes of DGEBA and PC. In addition, we also expect that in the melt‐blending process, the fracture toughness of epoxies should be increased due to the incorporation of ductile PC chains into the epoxy network. In this study, therefore, we attempted to examine and compare the structures and properties of PC‐modified epoxies through these two different blending processes. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2510–2521, 1999     

Novel thermosets based on DGEBA and hyperbranched polymers modified with vinyl and epoxy end groups

The cationic polymerization of DGEBA with two hyperbranched polymers (HBPs) with epoxy or vinyl end groups, using ytterbium triflate as initiator, has been studied. These HBPs have been obtained from commercial Boltorn H30 of which terminal hydroxyl groups have been replaced with long aliphatic ester chains having vinyl or epoxy end groups. Differences between the HBPs added as modifiers are observed with respect to the curing kinetics, network development, properties and morphology of the cured materials. While terminal epoxy groups ease the solubility of the HBP in DGEBA and allows its covalent incorporation into the network structure, the HBP with vinyl terminal groups is only miscible at high temperature and phase-separates during curing. As a consequence, morphology and thermal–mechanical properties are strongly dependent on the HBP employed.     

Effect of curing order on the curing kinetics and morphology of bisGMA/DGEBA interpenetrating polymer networks

The curing kinetics and morphology of an interpenetrating polymer network (IPN) formed from an epoxy resin (DGEBA) cured by an imidazole (1‐MeI) and a dimethacrylate resin (bisGMA), cured by low‐ and high‐temperature peroxide initiators (TBPEH and DHPB, respectively) have been studied by temperature‐ramping DSC, isothermal near‐infrared (NIR), DMTA and small‐angle neutron scattering (SANS). bisGMA and DGEBA are polar and chemically similar thermosetting resins which should enhance the miscibility of their IPNs. The phase structure was controlled by varying the curing procedure: the order of gelation of the components is dependent on the choice of low‐ and high‐temperature initiators for bisGMA and this affects the morphology formation. In the cure of the bisGMA/TBPEH:DGEBA/1‐MeI system, the dimethacrylate cures first. For isothermal cure studies at 80 °C, the final conversion of the epoxy is reduced by high crosslinking of the methacrylate groups in the IPN causing vitrification before full cure. The dimethacrylate conversion is enhanced due to plasticisation with unreacted DGEBA, and its cure rate is increased due to accelerated decomposition of TBPEH initiator by 1‐MeI. SANS revealed that phase separation occurs in these IPNs with domains on the scale of 6–7 nm. In the cure of the bisGMA/DHBP:DGEBA/1‐MeI system, the epoxy cures at a similar rate to that of the methacrylate groups. For isothermal cure studies at 80 °C, similar final conversions of the epoxy have been observed except for the 75:25 IPN. The cure rate of the methacrylate groups in the IPN is increased also due to accelerated decomposition of DHBP initiator by 1‐MeI, and the extent of accelerated decomposition for DHBP is stronger than that in the TBPEH‐based systems. SANS studies revealed that this system is more homogeneous due to the rapid formation of the dimethacrylate gel in the presence of the preformed epoxy network which interlocks the networks at low degrees of methacrylate conversion. Copyright © 2006 Society of Chemical Industry