Diglycidyl ether of bisphenol-A epoxy resin–polyether sulfone/polyether sulfone ether ketone blends: phase morphology, fracture toughness and thermo-mechanical properties

The properties of diglycidyl ether of bisphenol-A epoxy resin toughened with poly(ether sulfone ether ketone) (PESEK) and poly(ether sulfone) (PES) polymers were investigated. PESEK was synthesised by the nucleophilic substitution reaction of 4,4’-difluorobenzophenone with dihydroxydiphenylsulfone using sulfolane as solvent and potassium carbonate as catalyst at 230 °C. The T _g–composition behaviour of the homogeneous epoxy resin/PESEK blend was modelled using Fox, Gordon–Taylor and Kelley–Bueche equations. A single relaxation near the glass transition of epoxy resin was observed in all the blend systems. From dynamic mechanical analysis, the crosslink density of the blends was found to decrease with increase in the thermoplastic concentration. The storage modulus of the epoxy/PESEK blends was lower than that of neat resin, whilst it is higher for epoxy/PES blends up to glass transition temperature, thereafter it decreases. Scanning electron microscopic studies of the blends revealed a homogeneous morphology. The homogeneity of the blends was attributed to the similarity in chemical structure of the modifier and the cured epoxy network and due to the H-bonding interactions between the blend components. The fracture toughness of epoxy resin increased on blending with PESEK and PES. The increase in fracture toughness was due to the increase in ductility of the matrix. The thermal stability of the blends was comparable to that of neat epoxy resin.     

Micro phase separated epoxy/poly(ε-caprolactone)-block-poly(dimethyl siloxane)-block-poly(ε-caprolactone)/4,4′-diaminodiphenylsulfone systems: Morphology,viscoelasticity, thermo-mechanical properties and surface hydrophobicity

Epoxy resin/4,4′-diaminodiphenylsulfone (DDS) system was modified by the incorporation of poly(ε-caprolactone)-block-poly(dimethyl siloxane)-block-poly(ε-caprolactone) (PCL–PDMS–PCL) triblock copolymer (TBCP). Morphology, viscoelasticity, thermo-mechanical and surface properties of these blends were investigated. All the blends were opaque after curing. PCL blocks of the TBCP were miscible with epoxy resin while the PDMS fraction was immiscible. However in the cured state, both PCL and PDMS blocks were phase separated from epoxy/DDS matrix. The blends exhibited matrix-droplet morphology in which TBCP phase dispersed as spherical domains in epoxy matrix. Addition of TBCP had profound impact on the cure reaction kinetics. Storage modulus and glass transition temperature (T_g) decreased while impact strength significantly increased. Incorporation of 15 phr of TBCP resulted in 80% improvement in impact strength. Further, thermal stability was unaffected while surface hydrophobicity of the blends increased.     

Correlations between the morphology and the thermo-mechanical properties in poly(vinyl acetate)/epoxy thermosets

The mechanical properties of poly(vinyl acetate) (PVAc)/epoxy thermosets as a function of the PVAc content were investigated through dynamic mechanical thermal analysis from −100 to 220 °C and through tensile tests at room temperature. The morphology of the thermosets was examined by scanning electron microscopy. Cured PVAc/epoxy blends are phase separated, arising two phases that correspond to a PVAc-rich phase and to the epoxy rich-phase. The morphology evolves from nodular to inverted as the PVAc content increases. Intermediate compositions present combined morphologies, in which nodular and inverted regions are detected. The tensile properties at room temperature reveal that combined morphologies present the most ductile behaviour. The glass transition temperatures (T _g) of PVAc and of epoxy phases in the blends are different from those of the neat polymers. The profile of the loss modulus (E″)–temperature curves are correlated with the change in morphology that appears increasing the PVAc content. The storage modulus (E′)–temperature curves are highly dependent on the morphology of the samples. The E′-composition dependence is predicted using several models for two-phase composites. The low-temperature β-relaxation of the epoxy is slightly modified by the presence of PVAc. The activation energies of the α and β-relaxations are not dependent on the blend morphology.     

Mechanical properties and morphology of epoxy/poly(vinyl acetate)/poly(4-vinyl phenol) brominated system

Diglycidyl ether of bisfenol-A (DGEBA)/poly(vinyl acetate) (PVAc)/poly(4-vinyl phenol) brominated (PVPhBr) ternary blends cured with 4,4’-diaminodiphenylmethane (DDM) were investigated by differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA) and scanning electron microscopy (SEM). Homogeneous (DGEBA+DDM)/PVPhBr networks with a unique T _g are generated. Ternary blends (DGEBA+DDM)/PVAc/PVPhBr are initially miscible and phase separate upon curing arising two T _gs that correspond to a PVAc-rich phase and to epoxy network phase. Increasing the PVPhBr content the T _gof the PVAc phase move to higher temperatures as a consequence of the PVAc-PVPhBr interactions. Different morphologies are generated as a function of the blend composition.     

Morphology,toughness mechanism,and thermal properties of hyperbranched epoxy modified diglycidyl ether of bisphenol A (DGEBA) interpenetrating polymer networks

New hyperbranched poly(trimellitic anhydride‐triethylene glycol) ester epoxy (HTTE) is synthesized and used to toughen diglycidyl ether of bisphenol A (DGEBA) 4,4′‐diaminodiphenylmethane (DDM) resin system. The effects of content and generation number of HTTE on the performance of the cured systems are studied in detail. The impact strength is improved 2–7 times for HTTE/DGEBA blends compared with that of the unmodified system. Scanning electron microscopy (SEM) of fracture surface shows cavitations at center and fibrous yielding phenomenon at edge which indicated that the particle cavitations, shear yield deformation, and in situ toughness mechanism are the main toughening mechanisms. The dynamic mechanical thermal analyzer (DMA) analyses suggest that phase separation occurred as interpenetrating polymer networks (IPNs) for the HTTE/DGEBA amine systems. The IPN maintains transparency and shows higher modulus than the neat epoxy. The glass transition temperature (T_g) decreases to some extent compared with the neat epoxy. The T_g increases with increase in the generation number from first to third of HTTE and the concentrations of hard segment. The HTTE leads to a small decrease in thermal stability with the increasing content from TGA analysis. The thermal stability increases with increase in the generation number from first to third. Moreover, HTTE promotes char formation in the HTTE/DGEBA blends. The increase in thermal properties from first to third generation number is attributed to the increase in the molar mass and intramolecular hydrogen bridges, the increasing interaction of the HTTE/DGEBA IPNs, and the increasing crosslinking density due to the availability of a greater number of end hydroxyl and end epoxide functions. Copyright © 2008 John Wiley & Sons, Ltd.     

Synthesis and properties of phosphorus-containing bio-based epoxy resin from itaconic acid

A phosphorus-containing bio-based epoxy resin (EADI) was synthesized from itaconic acid (IA) and 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO). As a matrix, its cured epoxy network with methyl hexahydrophthalic anhydride (MHHPA) as the curing agent showed comparable glass-transition temperature and mechanical properties to diglycidyl ether in a bisphenol A (DGEBA) system as well as good flame retardancy with UL94 V-0 grade during a vertical burning test. As a reactive flame retardant, its flame-resistant effect on DGEBA/MHHPA system as well as its influence on the curing behavior and the thermal and mechanical properties of the modified epoxy resin were investigated. Results showed that after the introduction of EADI, not only were the flame retardancy determined by vertical burning test, LOI measurement, and thermogravimetric analysis significantly improved, but also the curing reactivity, glass transition temperature (T _g), initial degradation temperature for 5% weight loss (T _d(5%)), and flexural modulus of the cured system improved as well. EADI has great potential to be used as a green flame retardant in epoxy resin systems.     

Exploitation of introducing of catalytic centers into layer galleries of layered silicates and related epoxy nanocomposites. I. Epoxy nanocomposites derived from montmorillonite modified with catalytic surfactant‐bearing carboxyl groups

Montmorillonite (MMT) was modified with the acidified cocamidopropyl betaine (CAB) and the resulting organo‐montmorillonite (O‐MMT) was dispersed in an epoxy/methyl tetrahydrophthalic anhydride system to form epoxy nanocomposites. The intercalation and exfoliation behavior of the epoxy nanocomposites were examined by X‐ray diffraction and transmission electron microscopy. The curing behavior and thermal property were investigated by in situ Fourier transform infrared spectroscopy and DSC, respectively. The results showed that MMT could be highly intercalated by acidified CAB, and O‐MMT could be easily dispersed in epoxy resin to form intercalated/exfoliated epoxy nanocomposites. When the O‐MMT loading was lower than 8 phr (relative to 100 phr resin), exfoliated nanocomposites were achieved. The glass‐transition temperatures (T_g's) of the exfoliated nanocomposite were 20 °C higher than that of the neat resin. At higher O‐MMT loading, partial exfoliation was achieved, and those samples possessed moderately higher T_g's as compared with the neat resin. O‐MMT showed an obviously catalytic nature toward the curing of epoxy resin. The curing rate of the epoxy compound increased with O‐MMT loading. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1192–1198, 2004     

Synthesis of hydroxyl‐terminated poly(ether ether ketone) with pendent tert‐butyl groups and its use as a toughener for epoxy resins

Hydroxyl‐terminated poly(ether ether ketone) with pendent tert‐butyl groups (PEEKTOH) was synthesized by the nucleophilic substitution reaction of 4,4′‐difluorobenzophenone with tert‐butyl hydroquinone with potassium carbonate as a catalyst and N‐methyl‐2‐pyrrolidone as a solvent. Diglycidyl ether of bisphenol A epoxy resin was toughened with PEEKTOHs having different molecular weights. The melt‐mixed binary blends were homogeneous and showed a single composition‐dependent glass‐transition temperature (T_g). Kelley–Bueche and Gordon–Taylor equations gave good correlation with the experimental T_g. Scanning electron microscopy studies of the cured blends revealed a two‐phase morphology. A sea‐island morphology in which the thermoplastic was dispersed in a continuous matrix of epoxy resin was observed. Phase separation occurred by a nucleation and growth mechanism. The dynamic mechanical spectrum of the blends gave two peaks corresponding to epoxy‐rich and thermoplastic‐rich phases. The T_g of the epoxy‐rich phase was lower than that of the unmodified epoxy resin, indicating the presence of dissolved PEEKTOH in the epoxy matrix. There was an increase in the tensile strength with the addition of PEEKTOH. The fracture toughness increased by 135% with the addition of high‐molecular‐weight PEEKTOH. The improvement in the fracture toughness was dependent on the molecular weight and concentration of the oligomers present in the blend. Fracture mechanisms such as crack path deflection, ductile tearing of the thermoplastic, and local plastic deformation of the matrix occurred in the blends. The thermal stability of the blends was not affected by blending with PEEKTOH. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 541–556, 2006     

The effect of stoichiometry on curing and properties of epoxy–clay nanocomposites

Epoxy–clay nanocomposites have been prepared with an organically modified montmorillonite. The epoxy network was based on diglycidyl ether of bisphenol A (DGEBA) cured with diaminodiphenylmethane (DDM). The stoichiometry DGEBA–DDM was varied, the molar ratio of amine hydrogen/epoxy groups, r, ranged from 0.85 to 1.15. The influence of stoichiometry on curing and properties of the nanocomposites was studied using differential scanning calorimetry, dynamic mechanical thermal analysis and X-ray diffraction. All nanocomposites had intercalated clay structures. The clays accelerated the curing reaction whose rate was also increased when increasing r. The heat of reaction, −ΔH (J/g epoxy), increased as r increased, reaching a constant value for r ≥ 1. In the presence of clays −ΔH was lower than in the neat DGEBA–DDM. The glass transition temperature (T _g) of the neat epoxy thermosets reached a maximum at r = 1; however, the nanocomposites showed the T _g maximum at 0.9 < r < 1. The presence of clay lowered the T _g for r > 0.94 and raised T _g for r ≤ 0.85. The elastic modulus of neat epoxy thermosets reached a maximum in the rubber state and a minimum in the glassy state at r = 1. The nanocomposites showed similar behavior, but the maximum and the minimum values of the elastic modulus were reached at stoichiometry r < 1. The comparison of the properties of neat epoxy with those of the nanocomposites varying the stoichiometry indicates that the clay itself induces stoichiometric changes in the system.     

Dynamic mechanical and thermal properties of physically compatibilized natural rubber/poly(methyl methacrylate) blends by the addition of natural rubber‐graft‐ poly(methyl methacrylate)

The dynamic mechanical and thermal properties of natural rubber/poly (methyl methacrylate) blends (NR/PMMA) with and without the addition of graft copolymer (NR‐g‐PMMA) have been investigated. Dynamic mechanical spectroscopy is used to examine the effect of compatibilizer loading on storage modulus (E′), loss modulus (E″) and loss tangent (tan δ) at different temperatures and at different frequencies. The morphology of the blends indicates that the size of the dispersed phase decreased by the addition of a few percent of the graft copolymer followed by a leveling off at higher concentrations. This is an indication of interfacial saturation. Attempts have been made to correlate morphology with dynamic mechanical properties. Various models have been used to fit the experimental viscoelastic results. Differential scanning calorimetry has been used to analyze the glass‐transition temperatures of the blends. The thermal stability of the blends has been analyzed by thermogravimetry. Compatibilized blends are found to be more thermally stable than uncompatibilized blends. Finally the miscibility and mechanical properties of the blends annealed above T_g are evaluated. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 525–536, 2000     

Alder‐ene polymers derived from allyl aralkyl phenolic resin and bismaleimides: carbon fiber composites properties

The influence of structural variations in bismaleimides (BMIs) on Alder‐ene polymerization of O‐allyl aralkyl phenolic resin [O‐allyl Xylok (OAX)] was examined. Toward this, three BMI functional monomers, viz. 2,2′‐bis 4‐[(4′‐maleimido phenoxy) phenyl] propane (BMIP), 4,4′‐Bismaleimido diphenyl methane (BMPM), and Bis 4‐maleimidodiphenyl ether (BMPE), were blended with OAX in different molar ratios. The cure characterization revealed that the allyl‐dominated blends cure by three distinct reaction steps whereas the maleimide‐dominated blends exhibit a two‐step reaction invariable with the maleimide structure. Introduction of more maleimide functionalities increased the T_g and thermal stability of the co‐cured network. Differences in the storage modulus values and T_g of the BMI/OAX systems were correlated to the chemical structure of the BMI and crosslink density. Flexural, interlaminar shear strength (ILSS), and impact strength of the composites decreased systematically with the increase in maleimide content in the blend. Among the BMIs studied, T_g, thermal stability, and ILSS retention at elevated temperature were superior for BMPM/OAX blend owing to their high crosslink density and rigid backbone of the system. Allyl‐rich compositions exhibited improved mechanical properties owing to the better resin–reinforcement interaction as revealed from morphological analysis by scanning electron microscopy. Copyright © 2016 John Wiley & Sons, Ltd.     

Effects of novel reactive toughening agent on thermal stability of epoxy resin

A reactive amino-ended toughener was blended with different commercial epoxy resins namely, diglycidyl ether of bisphenol A, diglycidyl ether of bisphenol F, diglycidyl p-aminophenol and 1,5-naphthalenediamine as curing agent. The toughener was an aromatic amino-ended copolyethersulphone (coPES):poly(ether-sulphone)–poly(etherether-sulphone). The effect of the toughener on the thermal decomposition and char oxidation behaviour of the epoxy resins was studied by the simultaneous differential thermal analysis and thermogravimetric techniques. The glass transition temperature (T _g) as well as characteristic parameters of decomposition, initial decomposition temperature (T _i) and temperature at maximum degradation rate (T _m), in both inert and oxidative environments, were determined in order to verify the influence of toughener on the thermal degradation of the different epoxy systems. It was observed that the presence of coPES maintains the high level thermal stability of the resin and that the glass transition temperature increase with the toughener percentage.     

Synthesis and characterization of novel epoxy resins‐filled microcapsules with organic/inorganic hybrid shell for the self‐healing of high performance resins

Novel microcapsules (MCs) with organic/inorganic hybrid shell were successfully fabricated using epoxy resin as core material and nano boron nitride (BN) and mesoporous silica (SBA‐15) as inorganic shell materials in aqueous solution containing a water‐compatible epoxy resin curing agent. The morphologies, thermal properties and Young's moduli of MCs were investigated. The results indicated that epoxy resins were encapsulated by BN/SBA‐15/epoxy polymer hybrid layer, the resulting MCs were spherical in shape and the introduction of inorganic particles made MCs had rough surface morphology. The mean modulus value of MCs was from 2.8 to 3.1 GPa. The initial decomposition temperature (T_di) of MCs at 5 wt% weight loss was from 309 to 312°C. MCs showed excellent thermal stability below 260°C. The structures and properties of MCs could be tailored by controlling the weight ratio of inorganic particle. When the weight ratio of BN to SBA‐15 was 0.15:0.10, MCs had the highest T_di and modulus. The resulting MCs were applied to high performance 4,4′‐bismaleimidodiphenylmethane/O,O′‐diallylbisphenol A (BMI/DBA) system to design high performance BMI/DBA/MC systems. Appropriate content of MCs could improve the fracture toughness and maintain the glass transition temperature (T_g) of BMI/DBA system. The core materials released from fractured MCs could bond the fracture surfaces of the BMI/DBA matrix through the polymerization of epoxy resins. When the healing temperature schedule of 100°C/2h+150°C/1h was applied, 15 wt% MCs recovered 98% of the virgin fracture toughness of BMI/DBA. Copyright © 2016 John Wiley & Sons, Ltd.     

Physical aging of poly(ether sulfone)-modified epoxy resin

The physical aging process of 4,4′-diaminodiphenylsulfone (DDS) cured diglycidyl ether bisphenol-A (DGEBA) blended with poly(ether sulfone) (PES) was studied by differential scanning calorimetry (DSC) at four aging temperatures between T_g-50°C and T_g-10°C. At aging temperatures between T_g-50 and T_g-30°C, the experimental results of epoxy resin blended with 20 wt% of PES showed two enthalpy relaxation processes. One relaxation process was due to the physical aging of PES, the other relaxation process was due to the physical aging of epoxy resin. The distribution of enthalpy relaxation process due to physical aging of epoxy resin in the blend was broader and the characteristic relaxation time shorter than those of pure epoxy resin at the above aging temperatures (between T_g-50 and T_g-30°C). At an aging temperature between T_g-30 and T_g-10°C, only one enthalpy relaxation process was found for the epoxy resin blended with PES, the relaxation process was similar to that of pure epoxy resin. The enthalpy relaxation process due to the physical aging of PES in the epoxy matrix was similar to that of pure PES at aging temperatures between T_g-50 and T_g-10°C. © 1997 John Wiley & Sons, Inc.     

Cold crystallization and multiple melting behavior of poly(l-lactide) in homogeneous and in multiphasic epoxy blends

Cold crystallization and melting of poly(l-lactide) (PLLA) blended with an uncured or with an amino-cured epoxy resin (diglycidyl ether of bisphenol-A [DGEBA]) were investigated. It was found that the uncured PLLA/DGEBA blends were miscible, as they exhibited a single composition-dependent glass transition temperature (T _g). Melting point depression measurements indicated the existence of some type of interaction between the blend components, which was confirmed by Fourier transform infrared spectroscopy. Depending on the crystallization conditions and on the blend composition, a mixture of α and α′ crystals have been detected in PLLA and in uncured DGEBA/PLLA blends when crystallized from the glassy state. At high DGEBA contents, preferably imperfect α crystals are formed. On the contrary, at low DGEBA contents, the α′ form predominates and an exotherm associated to the α′–α transformation appears on the differential scanning calorimetry (DSC) scan before the main melting peak. Upon curing, the system transforms from a homogeneous mixture with a single refractive index into an opaque multiphasic one, as revealed by the existence of two T _gs in the DSC scans. These cross-linked immiscible blends displayed a single crystallization exotherm which scarcely changed with composition, and PLLA cold crystallized mainly into the α′ form from an almost pure PLLA phase; subsequently, the α′ crystals transform into the α form just before melting during the DSC scan.     

Curing kinetics of bio-based epoxy resin-toughened DGEBA epoxy resin blend

In the present study, TEIA bioresin was blended with the diglycidyl ether bisphenol A (DGEBA) epoxy resin in different ratios (i.e. 10, 20, 30, 40 mass%), cured with methylhexahydrophthalic anhydride curing agent in the presence of 2-methylimidazole catalyst. The optimized composition of DGEBA and TEIA bioresin blends system was employed as an adhesive strength. The adhesive strength of the TEIA-modified DGEBA epoxy resin blend system was increased from 4.14 to 6.31 MPa on an aluminium substrate compared to the DGEBA epoxy resin. The curing kinetics of non-isothermal, DGEBA epoxy resin and its bio-based blend systems were investigated employing differential scanning calorimetry. An increase in the peak temperature and reduction in a heat of curing as well as activation energy in DGEBA epoxy resin were observed with the addition of TEIA bioresin content. The activation energy (E_a) of the DGEBA resin and their bio-based blend system were obtained from Kissinger and Flynn–Wall–Ozawa methods.

     

Studies on phase morphology and thermo-physical properties of nitrile rubber blends

The study deals with the morphological and thermal analysis of binary rubber blends of acrylonitrile-co-butadiene rubber (NBR) with another polymer. Either ethylene propylene diene terpolymer (EPDM), ethylene vinyl acetate (EVA), chlorosulphonated polyethylene (CSM), or polyvinyl chloride (PVC) has been selected for the second phase. Depending on the relative polarity and interaction parameter of the components, the binary blends showed development of a bi-phasic morphology through scanning electron microscopy (SEM). Use of different types of thermal analysis techniques revealed that these blends are generally incompatible excepting one of NBR and PVC. Derivative differential scanning calorimetry (DDSC), in place of conventional DSC, has been used to characterize the compatibility behavior of the blends. NBR–PVC shows appearance of only one glass transition temperature (T _g) averaging the individual T _g’s of the blend components. The partially missible blend of NBR and CSM shows a broadening of T _g interval between the phase components, while the immiscible blends of either NBR–EPDM or NBR–EVA do not show any change in T _g values corresponding to the individual rubbers of their blend. The experimental T _g values were also compared with those calculated theoretically by Fox equation and observed to match closely with each other. Studies have also been made to evaluate the thermal stability of these blends by thermo-gravimetric analysis (TG) and evaluation of activation energy of respective decomposition processes by Flynn and Wall method. Thermo-mechanical analysis (TMA) was found to be effective for comparison of creep recovery and dimensional stability of the blends both at sub-ambient as well as at elevated temperatures.     

Thermal properties and toughness performance of hyperbranched‐polyimide‐modified epoxy resins

An amine‐terminated hyperbranched polyimide (HBPI) was prepared by the condensation polymerization of a commercially available triamine monomer with a dianhydride monomer. The effects of the HBPI content on the thermal and mechanical interfacial properties of diglycidyl ether of bisphenol A (DGEBA) epoxy resins were investigated with several techniques. The thermogravimetric analysis results showed that the thermal stability of the DGEBA/HBPI blends did not obviously change as the HBPI content increased. The glass‐transition temperature (T_g) of the DGEBA/HBPI blends increased with the addition of HBPI. Improvements in the critical stress intensity factor (K_IC) and impact strength of the blends were observed with the addition of HBPI. The K_IC value and impact strength were 2.5 and 2 times the values of the neat epoxy resins with only 4 wt % HBPI. The fractured surfaces were studied with scanning electron microscopy to investigate the morphology of the blends, and they showed that shear deformation occurred to prevent the propagation of cracks in the DGEBA/HBPI blends. These results indicated that a toughness improvement was achieved without a decrease in the thermal stability or T_g. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3348–3356, 2006     

The effect of masked isocyanates on the moisture absorption of MY 720/DDS epoxy resin

Several masked isocyanates were prepared with variations in both the type of isocyanate and masking group. They were characterized by elemental analysis and NMR spectroscopy, and their unblocking temperatures were determined. In general, higher unblocking temperatures were obtained using acyclic and cyclic aliphatic isocyanates and fluorinated phenols. Those with unblocking temperatures in the range of 120–180°C were incorporated into MY 720/DDS epoxy resin prior to cure. Highly fluorinated variations were incompatible with the resin. IR and DSC analyses showed that residual functional groups in the epoxy resin reacted with the masked isocyanates. Reductions in moisture absorption as high as 65% were obtained depending on the masked isocyanate. DMA studies showed that the T_g of the epoxy resin is lowered by incorporation of the masked isocyanate but the elastic modulus (E′) is relatively unchanged at temperatures below T_g.