UV intensity,temperature and dark-curing effects in cationic photo-polymerization of a cycloaliphatic epoxy resin

A difunctional cycloaliphatic epoxy monomer was cationically photo-polymerized in the presence of a diaryliodonium salt photoinitiator and an isopropyl thioxanthone photosensitizer at different temperatures and UV intensities. The photo-polymerization kinetics and structure formation were analysed using photo-DSC, IR spectroscopy and photo-rheology. An autocatalytic relation was used to model the conversion state with Arrhenius and power-law relationships for temperature and light intensity dependence. Conversion was found to depend on sample thickness, following the Beer–Lambert law. Photo-rheology measurements showed that the system vitrified before gelation at ambient temperature, and after gelation at high temperature under intense UV illumination. Time temperature transformation and time intensity transformation diagrams were built. Moreover, isothermal dark-curing enabled significant conversion increases up to the occurrence of vitrification, while thermal post-curing above T_g led to conversion as high as 71%. Thermo-mechanical measurements enabled to quantify T_g and the effects of the increase in conversion provided by thermal post-curing.     

Synthesis and properties of polybenzoxazole-clay nanocomposites

A novel polybenzoxazole (PBO)/clay nanocomposite has been prepared from a PBO precursor, polyhydroxyamide (PHA) and an organoclay. The PBO precursor was made by the low temperature polycondensation reaction between isophthaloyl chloride (IC) and 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane with an inherent viscosity of 0.5 dl/g. The organoclay was formed by a cation exchange reaction between a Na⁺-montorillonite (Na⁺-Mont) clay and an ammonium salt of dodecylamine. The PHA/clay was subsequently thermal cured to PBO/clay. Both X-ray diffraction and transmission electron microscope analyzes showed that the organoclay was dispersed in the PBO matrix in a nanometer scale. The in-plane coefficient of thermal expansion (CTE) of PBO/clay film decreased with increasing amounts of organoclay. The CTE of PBO/clay film containing 7 wt% clay was decreased by 21% compared to the pure PBO film. Both of the glass transition temperature (T_g) and the thermal decomposition temperature of PBO/clay increased with increasing amounts of organoclay. The thermal decomposition temperature and the T_g of PBO/clay containing 7 wt% clay increased to 12 and 16 °C, respectively.     

Glass‐transition temperature in the curing process of bismaleimide modified with diallylbisphenol A

The T_g‐conversion relationship, during the thermal curing of different stoichiometric formulations of 1,1′‐(methylene‐di‐4,1‐phenylene) bismaleimide (BMI), modified with o,o′‐diallyl bisphenol A (DABA), was investigated. The DiBenedetto equation was used to model this relationship for the formulation of DABA‐1 (BMI : DABA, 1 : 1). Based on this model, the T_g‐conversion relationship of formulation DABA‐0.5 (BMI : DABA, 1 : 0.5) was modeled. The high consistency between the model curve and experimental data showed that the change of T_g, attributed to copolymerization between BMI and DABA in DABA‐0.5, in the low‐conversion regime, was the same as that in DABA‐1. This also verifies that, for the formulation DABA‐0.5, copolymerization and homopolymerization do not overlap with each other. The reactions progressed sequentially and homopolymerization occurred after completion of copolymerization. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 3244–3247, 2004     

Effect of cross-linker length on the thermal and volumetric properties of cross-linked epoxy networks: A molecular simulation study

Volumetric and thermal properties of cross-linked epoxy systems consisting of diglycidyl ether of bisphenol A (DGEBA) and poly(oxypropylene) (POP) diamines of four different lengths ranging from 3 to 68 units were investigated by molecular dynamics (MD) simulations. The cross-linked structures were built by using the simulated annealing polymerization approach. The density, coefficients of volume thermal expansion and glass transition temperature (T_g) of each of the four cross-linked epoxy systems were obtained from their volume–temperature behavior. The density obtained in the simulations agreed well with the experimental value, whereas the coefficients of volume thermal expansion were at least 30% lower than their corresponding experimental results. The predicted T_g values were higher than the experimental values due to the considerably faster cooling rates that are used in the simulations. It was observed that an increase in the chain length of the cross-linker POP-diamines led to a larger difference between the predicted and experimental values of T_g. Three different approaches were used to estimate the expected shift in the experimental T_g to higher values had these measurements been made at cooling rates comparable to those used in MD simulations. It is shown that, in general, the T_g values obtained in MD simulations are consistent with such shifted T_g values that account for the difference in the cooling rates, although no one particular shift approach worked well for all four epoxy systems studied.     

Molecular dynamics predictions of thermal and mechanical properties of thermoset polymer EPON862/DETDA

We use molecular dynamics (MD) to perform an extensive characterization of the thermo-mechanical response of a thermoset polymer composed of epoxy EPON862 and curing agent DETDA. Our simulations, with no adjustable parameters, show that atomistic simulation can capture non-trivial behavior of amorphous thermosets including the role of polymerization degree, thermal history, strain rate and temperature on the glass transition temperature (T_g) and mechanical response (including ultimate properties) and lead to predictions in quantitative agreement with experiments. We find a significant increase in T_g, Young’s modulus and yield stress with degree of polymerization while yield strain is significantly less sensitive to it. For structures cured beyond the gel point (percolation of a 3D network) conversion degree and temperature affect yield stress in a similar way with yield stress linearly dependent on T−T_g; however, we find non-linear and non-universal relationship below the gel point. Our results show that a relative small variation in polymerization degree (∼5%) can explain the spread in experimental measurements of T_g and elastic constants available in the literature.     

Crystallization and vitrification effect in a poly(styrene)-g-poly(ethylene oxide) graft copolymer

Crystallization and solid state structure of a poly(styrene)-graft-poly(ethylene oxide) (PS-g-PEO) graft copolymer with crystallizable side chains were studied using simultaneous small angle X-ray scattering/wide angle X-ray scattering/differential scanning calorimetry (SAXS/WAXS/DSC). It is found that the glass transition temperature (T_g) of PS main chain is remarkably higher than that of PS homopolymer. The start cooling temperature (T_o) has a great influence on crystallization of the PEO side-chain. When the graft copolymer is cooled from the temperature above T_g, phase separation is suppressed due to the low mobility of the PS main chain and the homogeneous melt is vitrified. The unfavorable conformation of the rigid main chain results in a single crystallization peak and lower crystallinity. When PS-g-PEO is only heated to a temperature lower than the T_g and then cooled, phase separation is retained. Both the PEO side chains with high and low crystallizability can crystallize in the phase-separated state, leading to double crystallization peaks and higher crystallinity. The effect of solvent on crystallization of the graft copolymer was also examined. It is observed that addition of toluene reduces the T_g of the PS main chain and leads to the disappearance of the vitrification effect.     

The apparent activation energy and dynamic fragility of ancient ambers

According to the literature, different types of materials have different glass transition temperature (T_g) dependences of apparent activation energy (E_g) and dynamic fragility (m). In previous work we found that for different ambers, there were different glass transition temperatures. These same samples provide an opportunity to study the T_g dependence of E_g and m for amber, which has not been reported previously in the literature. In this work, nine pieces of amber from different locations and having different ages were investigated by differential scanning calorimetry. Six cooling rates were used to provide the different thermal histories, and the corresponding limiting fictive temperatures were determined using Moynihan's area matching method. From the cooling rate dependence of the limiting fictive temperature both the apparent activation energy and dynamic fragility were calculated. We find that as glass transition temperature increases, both the apparent activation energy and dynamic fragility increase. The T_g dependence of m for amber shows a similar trend with temperature as do the metallic glass formers and compares favorably with the m–T_g dependence of other aromatic polymers.     

Temperature dependence of the zero-shear melt viscosity of oligomeric epoxy resins

The melt viscosities of a homologous series of five epoxy resins based on bisphenol A and epichlorohydrin have been measured over a wide range of temperatures. It is shown that the temperature dependence of the zero-shear viscosity can be well described by the three-parameter Vogel equation. The Vogel parameters B and T_o were found to increase with increasing glass transition temperature, T_g, of the resin, while the ratios $\text{B}\text{T}{}_{\mathrm{<mtext>g</mtext>}}$ and $\text{T}{}_{\mathrm{<mtext>o</mtext>}}\text{T}{}_{\mathrm{<mtext>g</mtext>}}$ remained approximately constant. This constancy of the reduced Vogel parameters implies that T_g can be used as a corresponding states parameter to superimpose the viscosity-temperature data of all five resins onto a single master curve. The existence of such a master curve is of practical use, as it enables melt viscosities of epoxy resins to be predicted over a fairly extensive temperature range from only two quantities, viz. the value of T_g and one single viscosity value at a given temperature.     

Anomalous behavior of thermosetting systems after cure vs. chemical conversion: A normalized conversion–temperature–property diagram

Isothermal properties of thermosetting materials after cure, such as density and modulus, pass through maximum and minimum values with increasing chemical conversion. In this report observed decreases in modulus and density at isothermal temperatures below the glass-transition temperature, T_g, are termed “anomalous.” Four diepoxide (diglycidyl ether of bisphenol A) and tetrafunctional diamine (trimethylene glycol di-p-aminobenzoate) high T_g thermosetting systems with different ratios of amine to epoxy were investigated for the purpose of analyzing the evolution of the isothermal properties with increasing conversion. The density, T_g, and modulus of the materials with increasing conversion were measured by a combination of dilatometric, differential scanning calorimetry, and torsional braid analysis techniques. The results are presented in the form of conversion–temperature–property (T_gTP) diagrams with modulus and density as the properties. T_g is used as a direct measure of conversion based on the one-to-one relationship between T_g and conversion. The property-conversion behavior of the systems with different ratios of amine to epoxy show similar behavior if T_g is used as the measure of conversion and the data are normalized with respect to T_g at a conversion corresponding to the lower limit of the conversion range at which a maximum in the isothermal modulus occurs. The conversion corresponding to molecular gelation, _gelT_g, correlates with the lower limit of the conversion range at which the maximum in isothermal modulus occurs; _gelT_g also marks a change in the behavior of the sub-T_g mechanical relaxations vs. conversion. The conversion corresponding to the maximum in isothermal modulus vs. conversion correlates with the conversion corresponding to the maximum in isothermal density vs. conversion. © 1995 John Wiley & Sons, Inc.     

Novel eco-friendly random copolyesters of poly(butylene succinate) containing ether-linkages

Poly(butylene succinate/diglycolate) random copolymers (P(BSxBDGy)) of various compositions were synthesized and characterized from the molecular, thermal, structural and mechanical point of view. All the polymers showed a good thermal stability and at room temperature they appeared as semicrystalline materials. The main effect of copolymerization was a lowering in the crystallinity and a decrease of T_m respect to homopolymers. The dependence of T_m on composition for copolymers with high butylene succinate unit content was well described by Baur’s equation. WAXD measurements indicated that two different crystalline phases are present, depending on composition: copolymers with high BS unit content are characterized by PBS crystal phase, whereas those rich in BDG co-units crystallized in PBDG lattice. Amorphous samples showed a monotonic increment of T_g as the content of BDG units is increased and this can be explained on the basis of interchain interactions, due to the high electronegativity of ether–oxygen atoms. A Fox-type equation was found to fit the T_g data of completely amorphous samples, permitting the extrapolation of pure PBS T_g value for the completely amorphous polymer.     

Determination of the glass-rubber transition of poly(vinyl chloride) and poly(ethylene terephthalate) by small-angle X-ray scattering

Semicrystalline polymers can be regarded as systems of two phases, an amorphous and a crystalline phase, with different electron densities and different thermal expansion coefficients. The small-angle X-ray scattering of these systems is proportional to the square of the difference between the electron densities of the phases and so increases with temperature. As Fischer and Kloos have shown¹, this increase is discontinuous at the glass-rubber transition temperature, T_g, so providing a method for determining T_g and the difference α_ar-α_ag between the thermal expansion coefficients of the amorphous phase above and below T_g. This method was applied to poly(vinyl chloride) (PVC) and poly (ethylene terephthalate) (PETP). For the latter, small-angle X-ray scattering in addition constitutes a method for following and interpreting physical aging.     

Aging of poly(lactide)/poly(ethylene glycol) blends. Part 1. Poly(lactide) with low stereoregularity

Poly(lactide) (PLA) is rapidly gaining interest as a biodegradable thermoplastic for general usage in degradable disposables. To improve mechanical properties, a PLA with low stereoregularity was blended with polyethylene glycol (PEG). Blends with up to 30 wt% PEG were miscible at ambient temperature. Blending with PEG significantly decreased the T_g, decreased the modulus and increased the fracture strain of PLA. However, the PLA/PEG 70/30 blend became increasingly rigid over time at ambient conditions. The mechanism of aging primarily under ambient conditions of temperature and humidity was studied. Changes in mechanical properties, thermal transitions and solid state morphology were examined over time. Aging was caused by slow crystallization of PEG. Crystallization of PEG depleted the amorphous phase of PEG and gradually increased the T_g. As T_g approached the aging temperature, reduced molecular diffusivity slowed the crystallization rate dramatically. Aging essentially ceased when T_g of the amorphous phase reached the aging temperature. The increase in matrix T_g and the reinforcing effect of the crystals produced a change in mechanical properties from elastomer-like to thermoplastic-like.     

Glass transition temperature (Tg) versus fractional conversion for a linear thermosetting polyamic acid ester–polyimide system

The relationship of the glass transition temperature, T_g, to fractional conversion, x, for the conversion of the cyclohexylmethyl ester of the benzophenone tetracarboxylic dianhydride–oxydianiline polyamic acid to polyimide has been investigated using dynamic mechanical analysis (torsion pendulum, TBA). The glass transition temperature and conversion measurements were obtained on a single solvent-free polyamic acid ester specimen during cooling after heating to successively higher cure temperatures. Conversion was determined from the intensity of a sub-T_g mechanical relaxation peak that has been assigned to a relaxation of the cyclohexyl group. The resulting T_g versus x relationship is nonlinear. The T_g versus x relationship is adequately modeled using an expression derived by Couchman. © 1994 John Wiley & Sons, Inc.     

Conversion—temperature—property relationships in thermosetting systems: Property hysteresis due to microcracking of an epoxy/amine thermoset—glass fiber composite

A single specimen of an epoxy/amine thermoset—glass fiber composite was examined, using a freely oscillating torsion pendulum operating at ∼ 1 Hz, for different conversions (as measured by T_g) from T_g0 = 0°C to T_g∞ = 184°C during cooling and heating temperature scans. T_g was increased for successive pairs of scans by heating to higher and higher temperatures. The data were used in two ways: (i) vs. temperature for a fixed conversion to obtain transitions, modulus, and mechanical loss data, and (ii) by crossplotting to obtain isothermal values of the mechanical parameters vs. conversion (T_g). Hysteresis between cooling and subsequent heating data was observed in temperature scans of essentially ungelled material (T_g < 70°C) and was attributed to spontaneous microcracking. Hysteresis was analyzed in terms of the following three parameters: T_crack, the temperature corresponding to the onset of microcracking on cooling; T_heal, the temperature at which the specimen heals on subsequent heating; and the difference between isothermal cooling and heating data vs. conversion. Results were incorporated into a more general conversion—temperature—property diagram which serves as a framework for relating transitions (relaxations) to macroscopic behavior. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 39–53, 1997     

Synthesis and thermal characterization of poly(butylene terephthalate-co-thiodiethylene terephthalate) copolyesters

Poly(butylene terephthalate-co-thiodiethylene terephthalate) copolymers of various compositions were synthesized and characterized in terms of chemical structure and molecular weight. The thermal behavior was examined by thermogravimetric analysis and differential scanning calorimetry. All the polymers under investigation show a good thermal stability. At room temperature they appear as semicrystalline materials: the main effect of copolymerization was a lowering in the amount of crystallinity and a decrease of melting temperature with respect to homopolymers. A pure crystalline phase has been evidenced at high content of butylene terephthalate or thiodiethylene terephthalate units and Baur's equation was found to describe well the T_m-composition data. Amorphous samples (containing 50-100 mol% of thiodiethylene terephthalate units) showed a monotonic decrease of T_g as the content of sulfur-containing units is increased, due to the presence of flexible C-S-C bonds in the polymeric chain. Finally, the Fox equation described well the T_g-composition data.     

Atomistic molecular simulations of structure and dynamics of crosslinked epoxy resin

Many excellent thermal and mechanical performances of cured epoxy resin products can be related to their specific network structure. In this work, a typical crosslinked epoxy resin was investigated using detailed molecular dynamics (MD) simulations, in a wide temperature range from 250 K to 600 K. A general constant-NPT MD procedure widely used for linear polymers failed to identify the glass transition temperature (T_g) of this crosslinked polymer. This can be attributed to the bigger difference in the time scales and cooling rates between the experiments and simulations, and specially to the highly crosslinked infinite network feature. However, by adopting experimental densities appropriate for the corresponding temperatures, some important structural and dynamic features both below and above T_g were revealed using constant-NVT MD simulations. The polymer system exhibited more local structural features in case of below T_g than above T_g, as suggested by some typical radial distribution functions and torsion angle distributions. Non-bond energy, not any other energy components in the used COMPASS forcefield, played the most important role in glass transition. An abrupt change occurring in the vicinity of T_g was also observed in the plots of the mean squared displacements (MSDs) of the crosslinks against the temperature, indicating the great importance of crosslinks to glass transition. Rotational dynamics of some bonds in epoxy segments were also investigated, which exhibited great diversity along the chains between crosslinks. The reorientation functions of these bond vectors at higher temperatures can be well fitted by Kohlrausch-Williams-Watts (KWW) function.     

Photo-induced deformation behavior depending on the glass transition temperature on the surface of urethane copolymers containing a push-pull type azobenzene moiety

Urethane copolymers containing a push-pull type azobenzene moiety with the same dye content were synthesized to investigate the relationship between photo-induced deformation and molecular mobility. The copolymers exhibit different glass transition temperatures (T_g), from 46 to 143 °C, due to their different main chain structures. An indented nanostructure induced by the optical near field around the polystyrene microspheres and a surface relief grating (SRG) induced by exposure to a two-beam interference pattern were examined using films of copolymers. We found the dependency of the deformation efficiency on T_g was inverted depending on the irradiation power. The deformation depth increased with T_g under high power irradiation in both the indented nanostructure and the SRG forming experiments. In contrast, the deformation depth of the SRG decreased with increasing T_g under low power irradiation. The discovery of this inverted tendency suggests that, in addition to the molecular mobility, we should consider other factors in the deformation mechanism, such as the recovery of deformation, the degree of plasticization, and the thermal effects.     

Fast physical drying,high water and salt resistant coatings from non-drying vegetable oil

This paper reports fast physical drying, high water and salt resistances of coating materials from non-drying palm oleic acid. Short oil-length alkyd was synthesized and copolymerized with methyl methacrylate. Three copolymers of the alkyd and methyl methacrylate with different alkyd/MMA ratios were prepared via free radical polymerization. The copolymers were characterized by FTIR and H NMR spectroscopy, and glass transition temperatures (T_g) were measured by DSC. The decreasing amount of alkyd was noticed to increasing conversion and T_g. The overall thermal stability has increased with higher amount of alkyd in the copolymer. Moreover, incorporation of alkyd has improved the adhesion and film hardness of the coatings.     

Effect of diffusion control in the glass transition region on critical conversion at the gel point during curing of epoxy resins

An investigation into curing of Bisphenol A diglycidyl ether with an amine curing agent was carried out both above and below the glass transition temperature, T_g, of the reacting system. At T_g curing was still partly controlled by chemical kinetics. The critical gel point conversion was the same, irrespective of whether curing occurred above or below T_g. Such independence indicated that all reaction steps were retarded to the same degree when viscosity increased.     

Time-temperature–transformation (TTT) diagrams of high Tg epoxy systems: Competition between cure and thermal degradation

The cure behavior and thermal degradation of high T_g epoxy systems have been investigated by comparing their isothermal time-temperature-transformation (TTT) diagrams. The formulations were prepared from di- and trifunctional epoxy resins, and their mixtures, with stoichiometric amounts of a tetrafunctional aromatic diamine. The maximum glass transition temperatures (T_g∞) were 229°C and > 324°C for the fully cured di- and trifunctional epoxy materials, respectively. Increasing functionality of the reactants decreases the times to gelation and to vitrification, and increases the difference between T_g after prolonged isothermal cure and the temperature of cure. At high temperatures, there is competition between cure and thermal degradation. The latter was characterized by two main processes which involved devitrification (decrease of modulus and T_g) and revitrification (char formation). The experimentally inaccessible T_g∞ (352°C) for the trifunctional epoxy material was obtained by extrapolation from the values of T_g∞ of the less highly crosslinked systems using a relationship between the glass transition temperature, crosslink density, and chemical structure.