Effect of post-cure on the glass transition temperature and mechanical properties of epoxy adhesives

The effects of post-curing and cure temperature on the glass transition temperature, T _g, and the mechanical properties of epoxy adhesives were studied. T _g was measured by a dynamic mechanical analysis apparatus developed in-house and the mechanical properties of the adhesives (yield strength, Young’s modulus and failure strain) were measured by a tensile machine. The relationships between T _g and mechanical performance under various post-cure conditions were investigated. The curing process was the same for all tests, consisting of an initial stage performed at different temperatures followed by cooling at room temperature. Three sets of specimens were considered, sharing the same initial cure process, but with a different post-curing procedure. In the first set, the specimens were only subjected to a curing process; in the second set, the specimens were subjected to a curing process followed by a post-cure performed at a temperature below the T _g of the fully cured network, T _g∞; and in the third set, the specimens were subjected to a curing process followed by a post-cure performed at a temperature above the T _g∞. When post-cured at a temperature above T _g∞, the mechanical and physical properties tend to have a constant value for any cure temperature.     

Correlation of the glass transition temperature of plasticized PVC using a lattice fluid model

A model has been developed to estimate the glass transition temperature of polymer+plasticizer mixtures (up to 30 wt% plasticizer). The model is based on the Sanchez-Lacombe equation of state and the Gibbs-DiMarzio criterion, which states that the entropy of a mixture is zero at the glass transition. The polymers studied included polystyrene and poly(vinyl chloride). The plasticizers studied included a wide range of chemicals from methyl acetate to di-undecyl phthalate. The model qualitatively accounted for the effect of different plasticizers on the mixture glass transition temperature.     

Influence of co-solvent content on electro-deposition behavior of acrylic lattices of different glass transition temperatures

The influence of co-solvent content on the constant-voltage electro-deposition behavior of three acrylic lattices was investigated. It was found that co-solvent effect is system-specific and the type and the extent of the influence depend on the specific features of the latex itself, specially its glass transition temperature, T_g. For a low-T_g high molecular weight latex, the addition of co-solvent makes the electro-coagulates more sticky but it does not show a quick influence on the deposition behavior. In contrast for higher T_g lattices the influence is more pronounced. For medium and high-T_g lattices, the co-solvent was found to have a significant influence on the electro-deposition behavior and the film morphology.     

Quantitative structure-property relationships for composites: prediction of glass transition temperatures for epoxy resins

The glass transition temperatures of nine stoichiometric resin systems of tetraglycidyl-4,4′-diamino-diphenylmethane (TGDDM), triglycidyl p-amino phenol and diglycidyl ether of bisphenol A with 4,4′-diaminodiphenylsulphone (DDS), diethyl-toluenediamine and dimethylthiotoluenediamine were calculated using group interaction modelling (GIM) and atomic additivity (AA) methods. The input parameters were generated from kinetics simulation, which outputs the structure information for the cured systems. The modelling parameters were also applied to four non-stoichiometric systems of TGDDM and DDS. The predicted values from GIM were in good quantitative agreement with measured results from temperature modulated differential scanning calorimetry for all systems studied. Compared to GIM, the AA method gave inferior predictions for the highly crosslinked systems, especially for those, where epoxy was in excess.     

The role of network architecture on the glass transition temperature of epoxy resins

A series of epoxy networks were synthesized in which the molecular weight between crosslinks (M_c) and crosslink functionality were controlled independent of the network chain backbone composition. The glass transition temperature (T_g) of these networks was found to increase as M_c decreased. However, the rate at which T_g increased depended on crosslink functionality. The dependency of M_c on T_g is well described by two models, one based on the concept of network free volume while the other model is based on the principle of corresponding states. Initially, neither model could quantitatively predict the effect of crosslink functionality in our networks. However, our tests indicated that both the glass transition and the rubbery moduli of our networks were dependent on M_c and crosslink functionality, while the glassy state moduli were independent of these structural variables. The effect of crosslink functionality on the rubbery modulus of a network has been addressed by the front factor in rubber elasticity theory. Incorporation of this factor into the glass transition temperature models allowed for a quantitative prediction of T_g as a function of M_c and crosslink functionality. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 387–395, 1997     

Absorption and glass transition temperature of adhesives exposed to water and toluene

If adhesives are to be used in practical applications, such as for aerospace or marine components, it is essential that their long-term behaviour when exposed to aggressive environments, natural or man-made, should be known. The objective of this paper is to determine the liquid absorption behaviour of epoxy adhesives under complete immersion in two solvents (water and toluene) at two different temperatures (room temperature and 60 °C) and the effect on their glass transition temperature (T_g). The specimens were periodically removed from their environment for weighing and T_g measurement, the latter by using a rapid method which was developed previously by the authors. The rate of weight gain of the specimen (sorption) and the equilibrium level were higher for the higher temperature immersion than for the low temperature. The equilibrium level reached for toluene was higher than that of water. The values of T_g were significantly influenced by the liquid uptake. Finally, heating to a temperature above the T_g of the adhesive, even in a very short time, was found to significantly influence the results of the liquid uptake.     

The effect of the glass transition temperature on the toughness of photopolymerizable (meth)acrylate networks under physiological conditions

The purpose of this study is to evaluate how the toughness of photopolymerizable (meth)acrylate networks is influenced by physiological conditions. By utilizing two ternary (meth)acrylate networks, MA-co-MMA-co-PEGDMA and 2HEMA-co-BMA-co-PEGDMA, relationships between glass transition temperature (T_g), water content and state, and toughness were studied by varying the weight ratio of the linear monomers (MA to MMA or 2HEMA to BMA). Differential scanning calorimetry and thermogravimetric analysis were performed to evaluate the thermal behavior and water content as a function of either MA or 2HEMA concentration while tensile strain-to-failure tests were performed at 37 °C to determine network toughness. Both networks exhibited a maximum in toughness in PBS in the composition corresponding to a T_g close to the testing temperature. This toughness maximum was achieved by adjusting the glass transition temperature and/or hydrophilicity through changes in chemistry. These relationships may be utilized to design tough photopolymerizable networks for use in mechanically rigorous biomedical applications.     

Variations in the glass transition temperature of polyester with special architectures confined in thin films

Variations of the polymer dynamic of different systematically varied polyester architectures in the confinement of thin films were studied by temperature dependent spectroscopic vis-ellipsometry. The architectures were tailored in order to evaluate (a) the impact of different polymer backbones (hyperbranched, branched or linear and aromatic, aromatic-aliphatic or aliphatic), (b) the influence of functional groups (hydroxyl, benzoyl, tert-butyldimethylsilyl) and (c) the role of interfacial interactions (attractive, repulsive) with the silicon substrate. Possible reasons for the deviation of the glass transition temperature T_g in thin polymer films (10-800 nm) from the bulk value are described and compared to the literature. It was found that the functional groups of the applied polymers have the largest effect on T_g. Beside interfacial interactions, chemical and physical reactions in the polymer film are playing a significant role.     

Curing of epoxy systems at sub‐glass transition temperature

Three epoxy‐amine thermoset systems were cured at a low ambient temperature. Evolution of the reaction kinetics and molecular structure during cure at the sub‐glass transition temperature was followed by DSC and chemorheology experiments. The effect of vitrification and the reaction exotherm on curing and final mechanical properties of the epoxy thermosets was determined. Thermomechanical properties of the low‐temperature cured systems depend on the reaction kinetics and volume of the reaction mixture. Curing of the fast‐reacting system in a large volume (12‐mm thick layer) resulted in the material with T_g exceeding the cure temperature by 70–80°C because of an exothermal temperature rise. However, the reaction in a too large volume (50‐mm layer) led to thermal degradation of the network. In contrast, thin layers (1.5 mm) were severely undercured. Well‐cured epoxy thermosets could be prepared at sub‐T_g temperatures by optimizing reaction conditions. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 99: 3669–3676, 2006     

Improvement of the glass transition temperature in novel molybdenum carbide doped polyaniline nanocomposites

Several studies have revealed that the glass transition temperature (T_g) of polymer nanocomposites (PNCs) may change upon the loading of even a small number of nanoparticles (NPs). However, the exact phenomenon behind such shifting in T_g is still not well known. This study uses experimental and MD simulation approaches to explore the physics of the T_g of polyaniline (PANI) nanocomposite with molybdenum carbide (Mo₂C) nanoparticles at different weight percentage loading of NPs. The experimental study shows that, at a lower wt-% loading of NPs, the T_g of the resulting PNCs decreases slowly, and above a particular loading wt-%* (30 wt-%), the depression in T_g becomes more serious. The SEM images show that for wt-% < wt-%*, NPs remain diffuse in the polymer matrix, and for wt-%t > wt-%* a large number of NPs surround the polymer. The MD simulation reveals a slow increase in the diffusion of polymer and NPs for wt-% < wt-%* and a sudden increase for wt-%t > wt-%*. The measurement of the radius of gyration of the polymer reveals swelling of the polymer for wt-% < wt-%* and contraction of the polymer for wt-%t > wt-%*. Moreover, the variations of the diffusion constants and the radius of gyration of the polymer are consistent with the T_g behaviour of PNCs.     

The effect of confinement in nanoporous polymers on the glass transition temperature

The glass transition temperature (T_g) of nanoporous polyetherimide (PEI) and PEI thin films was investigated. The T_g decreased from its bulk value in both of these confined systems. Monte Carlo simulations were performed to calculate the nearest neighbor pore-to-pore distances in the nanoporous PEI. A quantitative analogy between the nanoporous PEI and PEI thin films is proposed through an equivalence of nearest neighbor pore-to-pore distances and thin film thickness. The effect of confinement is believed to be due to the interface regions, which possess higher chain mobility than the bulk. When these high mobility interface regions are sufficiently close together, the excess mobility at the interface region affects the dynamics of the system by restraining percolation of the slow domains resulting in the observed decrease in T_g.     

Correlation between glass transition temperature and molecular mass in non-polymeric and polymer glass formers

It is shown that the glass transition temperature T_g of molecular (non-polymeric) glass formers correlates with molecular mass M as T_g(M) ∝ M^α, α = 0.51 ± 0.02. The subclasses of molecular glasses with homologous chemical structure but different M exhibit a similar universal correlation with significantly lower scatter. A possible explanation of T_g vs M correlation in molecular glasses is suggested. Comparing molecular glasses with polymers we found that in polymers T_g(M) dependence at small M (short chains) is similar to that in molecular glasses. At further increasing of the chain length the T_g(M) dependence in polymers begin to deviate from the universal T_g(M) correlation of molecular glasses and eventually saturates at some polymer specific T_g∞ value. We conclude that at least a substantial part of T_g(M) dependence of low-M polymers is common with molecular glasses mechanism that does not require chain-like structure. In particular, the model of T_g(M) dependence in polymers based on additional free volume on chain ends is not fully adequate at small M. Our picture provides an alternative explanation that in polymers a mechanism is in action which leads to a saturation of the normal T_g(M) dependence common with molecular glasses.     

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.     

Effects of freeze-drying on the glass temperature of cyclic polystyrenes

The calorimetric glass temperature was measured for three cyclic polystyrenes with apparent molecular weights ranging from 4.0×10³ to 195.5×10³ g/mol for both bulk material and for samples freeze-dried from dilute solution. Freeze-drying from dilute solution was found to reduce the glass temperature by 7–14 K depending on the sample. These T_g depressions are 5–12 K greater than those found previously for freeze-dried linear polystyrene. Annealing at 403.2 and 443.2 K (130 and 170 °C) resulted in recovery of the T_g back to the bulk value with the time scales depending on both temperature and the magnitude of the T_g reduction; the low apparent activation energy dependence of the recovery of T_g precludes its being due to viscous flow.     

Determination of the glass transition temperature of polymer/layered silicate nanocomposites from positron annihilation lifetime measurements

The glass transitions of acrylonitrile-butadiene rubber (NBR)/organoclay nanocomposites with various silicate contents were investigated using positron annihilation lifetime spectroscopy (PALS). The nanocomposites were prepared through melt intercalation of NBR with various concentrations of organoclay (OC30B) modified with the organic modifier, methyl tallow bis(2-hydroxyethyl) quaternary ammonium (MT2EtOH), i.e., Cloisite^® 30B. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM) measurements of the NBR/OC30B nanocomposites showed that the NBR chains were intercalated between the silicate layers, thereby increasing the gallery heights of the organosilicates. The glass transition temperature of NBR was determined using differential scanning calorimetry (DSC). However, it seemed to be very difficult to clearly resolve the very small differences in T_gs caused from various loading of nanosized silicate in NBR/OC30B nanocomposites. Hence, we performed positron annihilation lifetime spectroscopy (PALS) on NBR/OC30B nanocomposites containing various amounts of OC30B (1-10 wt%). Significant changes in the temperature dependencies of free volume parameters (i.e., lifetimes and intensities) were observed at the transition temperature, T_g,PALS, and the T_g,PALS values were found to increase with increasing organoclay content in the samples. These observations are consistent with PALS having a higher sensitivity in the detection of very small changes in free volume properties. The present findings thus highlight the usefulness of PALS for studying phase transition phenomena in polymeric materials with nanoscale structural variations.     

Influence of alternating sequential fraction on the melting and glass transition temperatures of ethylene-tetrafluoroethylene copolymer

The melting (T_m) and glass transition (T_g) temperatures of a series of ethylene (E)-tetrafluoroethylene (TFE) copolymer (ETFE) have been found to show unique dependence on the TFE content with the minimal and maximal points. These behaviors have been interpreted successfully on the basis of the degree of alternation of E and TFE monomeric units along the skeletal chain. The melting point of a perfectly alternating copolymer is estimated to be 295 °C on the basis of the dependence of T_m using a modified Flory’s equation. The corresponding T_g was estimated as 145 °C by applying a modified Gibbs-Damnation’s equation.     

纳米粒子改性环氧树脂玻璃化转变温度的研究

采用示差扫描量热分析(DSC)研究了纳米A l2O3粒子改性的环氧树脂基体玻璃化转变温度与纳米粒子含量之间的关系以及纳米粒子含量对改性体系固化剂用量的影响。结果表明,随着纳米粒子含量的提高,改性树脂的玻璃化转变温度逐渐下降,由纯树脂的224℃下降到182.5℃(纳米粒子用量30%,固化剂添加量70%)。并且纳米粒子的加入会影响树脂基体的固化反应。达到玻璃化转变温度峰值时的固化剂用量并非按照改性体系中环氧树脂含量等当量比加入,而是与纳米粒子含量有关,纳米粒子含量越高,达到玻璃化转变温度峰值时固化剂用量越少。