Preparation of Microcellular Epoxy Foams through a Limited‐Foaming Process: A Contradiction with the Time–Temperature–Transformation Cure Diagram

3D cross‐linking networks are generated through chemical reactions between thermosetting epoxy resin and hardener during curing. The curing degree of epoxy material can be increased by increasing curing temperature and/or time. The epoxy material must then be fully cured through a postcuring process to optimize its material characteristics. Here, a limited‐foaming method is introduced for the preparation of microcellular epoxy foams (Lim‐foams) with improved cell morphology, high thermal expansion coefficient, and good compressive properties. Lim‐foams exhibit a lower glass transition temperature (T_g) and curing degree than epoxy foams fabricated through free‐foaming process (Fre‐foams). Surprisingly, however, the T_g of Lim‐foams is unaffected by postcuring temperature and time. This phenomenon, which is related to high gas pressure in the bubbles, contradicts that indicated by the time–temperature–transformation cure diagram. High bubble pressure promotes the movement of molecular chains under heating at low temperature and simultaneously suppresses the etherification cross‐linking reaction during post‐curing.     

Effect of norbornene content on deformation properties and hot embossing of cyclic olefin copolymers

The dynamic mechanical behaviour of a series of cyclic olefin copolymers (COCs) with varying norbornene content has been examined in the vicinity of the glass transition temperature, T _g. Using dynamic mechanical thermal analysis (DMTA), the temperature of the glass transition in COC increased linearly with increase in % norbornene. Above T _g, the magnitude of the elastic storage modulus, E′, decreased exponentially with rise in temperature for all of the copolymers. The loss modulus, E″, has also sharply decreased at temperatures above the transition with a levelling-off in E″ at ≥20 °C above T _g for all grades. The results of DMTA have been used in the identification of the optimum conditions for hot embossing experiments. Hot embossing of COC at ≥20 °C above the transition temperature in a region of viscous liquid flow has resulted in a full replication of channel depth without cracking or distortion.     

The influence of residual stresses on the mechanical and thermal properties of injection moulded ABS copolymer

The mechanical and thermal properties of acrylonitrile-butadiene-styrene copolymer (ABS) were determined for as-moulded and annealed specimens in order to assess the influence of residual stresses on these properties. Mouldings were heat treated at temperatures below and above the glass transition temperature, T _g. Annealing below the glass transition removes most of the residual stresses arising during the moulding process, while annealing above the glass transition temperature removes both the residual stresses and molecular orientation. Residual stresses have a strong bearing on the end-use properties. The thermal behaviour of the mouldings are mostly affected by the stimulation of molecular motion caused by the relaxation of the residual stresses near the glass transition temperature.     

Thermoanalytical characterization of epoxy matrix-glass microballoon syntactic foams

Syntactic foams are finding new applications where their thermal stability and high temperature response are important. Therefore, the high temperature response of these advanced composites needs to be characterized and correlated with various material parameters. The present study is aimed at evaluating the effect of microballoon (hollow particle) volume fraction (Φ) and wall thickness (w) on thermoanalytical characteristics of epoxy matrix syntactic foams containing glass microballoons. These composites are characterized to determine the glass transition temperature (T _g), the weight loss, and the char yield. It is observed that T _g decreases and the char yield increases due to the presence of microballoons in the resin. The T _g is increased with an increase in Φ but is not significantly affected by w. The thermal stability is increased by increasing w and is relatively less sensitive to Φ. Understanding the relations between thermal properties of syntactic foams, the microballoon wall thickness, and microballoon volume fraction will help in developing syntactic foams optimized for mechanical as well as thermal characteristics. Due to the increased interest in functionally graded syntactic foams containing a gradient in microballoon volume fraction or wall thickness, the results of the present study are helpful in better tailoring these materials for given applications.     

Influence of post-curing and temperature effects on bulk density,glass transition and stress-strain behaviour of imidazole-cured epoxy network

The effects of post-curing and temperature on the glass transition, bulk density and stress-strain behaviour in the glassy and rubbery state of 2-ethyl-4-methyl imidazole-cured epoxy network have been evaluated by differential scanning calorimetry (DSC), water displacement and tensile testing. The glass transition temperature,T _g, was found to increase with increasing post-cure temperature and the size of the base line shift in the glass transition region on the DSC thermogram can serve as an indicator of the extent of cure. At room temperature, the decrease in bulk density with increasing extent of cure may be attributed to the additional cross-linking, adding molecular constraints to the thermal constraints. Thus, a higher free volume atT _g can be expected to remain in the glassy state as the sample is slowly cooled through the glass transition temperature. In the investigation on the temperature dependence of the tensile mechanical properties, a fracture envelope was obtained. The tensile strength, Young's modulus and ultimate elongation in the glassy and rubbery state are discussed in detail.     

Deformation, stress relaxation, and crystallization of lithium silicate glass fibers below the glass transition temperature

The deformation and crystallization of Li₂O·2SiO₂ and Li₂O·1.6SiO₂ glass fibers subjected to a bending stress were measured as a function of time over the temperature range 50 to 150°C below the glass transition temperature (T _g). The glass fibers can be permanently deformed at temperatures about 100°C below T _g, and they crystallize significantly at temperatures close to, but below T _g, about 150°C lower than the onset temperature for crystallization for these glasses in the no-stress condition. The crystallization was found to occur only on the surface of the glass fibers with no detectable difference in the extent of crystallization in tensile and compressive stress regions. The relaxation mechanism for fiber deformation can be best described by a stretched exponential (Kohlrausch-Williams- Watt (KWW) approximation), rather than a single exponential model.The activation energy for stress relaxation, E _s, for the glass fibers ranges between 175 and 195 kJ/mol, which is considerably smaller than the activation energy for viscous flow, E (400 kJ/mol) near T _g for these glasses at normal, stress-free condition. It is suspected that a viscosity relaxation mechanism could be responsible for permanent deformation and crystallization of the glass fibers below T _g.     

Strain rate- and temperature-dependent tensile properties of an epoxy-based, thermosetting, shape memory polymer (Veriflex-E)

In this study, the inelastic deformation behavior of an epoxy-based, thermally triggered shape memory polymer resin, known as Veriflex-E, was investigated. The experimental program was designed to explore the influence of strain rate on monotonic loading at various temperatures which is needed to establish the design space of SMPs in load bearing applications. Thermally actuated shape memory polymers can be thought of as having two phases separated by the glass transition temperature (T _g ). At temperatures below the T _g , Veriflex-E exhibits a high elastic modulus and positive, non-linear strain rate sensitivity in monotonic loading. The Poisson’s ratio at room temperature is independent of the strain rate, but dependent upon the strain magnitude. As the temperature is increased, the strain rate sensitivity in monotonic loading decreases. Well above the T _g , the elastic modulus drops by several orders of magnitude. In this high temperature region, the material achieves strain levels well above 100% and Poisson’s ratio is constant at 0.5 regardless of strain rate or strain magnitude.     

Effects of thermal rates on the thermomechanical behaviors of amorphous shape memory polymers

Shape memory polymers (SMPs) are polymers that can recover a large pre-deformed shape in response to environmental stimuli, such as temperature, light, etc. For a thermally triggered (or activated) amorphous SMP, the pre-deformation and recovery of the shape require the temperature of the material to traverse the glass transition temperature T _g under constrained or free conditions. In this paper, effects of thermal rates on the thermomechanical behaviors of amorphous SMPs are investigated. Under uniaxial compression, during a temperature cycle (cooling followed by heating), the stress decreases to zero as the temperature decreases to below the glass transition temperature, and increases to a value larger than the initial stress (termed stress overshoot) as the temperature is raised above the glass transition temperature. These observations are examined by a thermoviscoelasticity model that couples the nonequilibrium structural relaxation and temperature dependent viscoelastic behavior of the material. In addition, using this model, stress-temperature behaviors during temperature cycles with various thermal rate conditions and tensile loading conditions are studied.     

Dielectric properties of solution-grown-undoped and acrylic-acid-doped ethyl cellulose

Dielectric capacities and losses were measured, in the temperature (50–170°C) and frequency (01–100 kHz range), for undoped and acrylic acid (AA) doped ethyl cellulose (EC) films (thickness about 20 μm) with progressive increase in the concentration of dopant in the polymer matrix. The variation of capacity with temperature is attributed to thermal expansion in the lower temperature region to the orientation of dipolar molecules in the neighbourhood of glass transition temperature (T _g) and random thermal motion of molecules aboveT _g. The dielectric losses exhibit a broad peak. Doping with AA is found to affect the magnitude and position of the peak. AA is found to have a two-fold action. Firstly, it enhances the chain mobility and secondly, it increases the dielectric loss by forming charge transfer complexes.     

Switching between Crystallization from the Glassy and the Undercooled Liquid Phase in Phase Change Material Ge2Sb2Te5

Controlling crystallization kinetics is key to overcome the temperature–time dilemma in phase change materials employed for data storage. While the amorphous phase must be preserved for more than 10 years at slightly above room temperature to ensure data integrity, it has to crystallize on a timescale of several nanoseconds following a moderate temperature increase to near 2/3 T_m to compete with other memory devices such as dynamic random access memory (DRAM). Here, a calorimetric demonstration that this striking variation in kinetics involves crystallization occurring either from the glassy or from the undercooled liquid state is provided. Measurements of crystallization kinetics of Ge₂Sb₂Te₅ with heating rates spanning over six orders of magnitude reveal a fourfold decrease in Kissinger activation energy for crystallization upon the glass transition. This enables rapid crystallization above the glass transition temperature T_g. Moreover, highly unusual for glass‐forming systems, crystallization at conventional heating rates is observed more than 50 °C below T_g, where the atomic mobility should be vanishingly small.     

The effect of thermal ageing on carbon fibre-reinforced polyetheretherketone (PEEK)

The static and dynamic mechanical properties of carbon fibre-reinforced PEEK (APC-2) laminates subjected to long-term thermal ageing and cycling treatments have been studied using three-point bend flexure tests. Results are discussed with respect to morphological changes and degradation analysis. S/N curves were modelled using fatigue modulus degradation data. Ageing laminates at high temperatures, for long time periods, between the glass transition temperature, T _g, and the melting temperature, T _m, caused a significant reduction in mechanical properties. However, for short ageing periods, a crystal-perfection process occurs which enhanced the low stress level fatigue resistance of both laminate geometries.     

The anomalous lowering of the glass transition of an epoxy resin by plasticization with water

The validity of the assertion that 6 to 7 wt% absorbed water lowers the glass transition,T _g, (~250° C) of a highly cross-linked, high temperature epoxy resin (NARMCO 5208) by 100 to 150° C was investigated by a number of thermoanalytical techniques. This plasticization is of significance to the use of organic matrix composition skins on supersonic aircraft which experience a sudden thermal spike, and supposedly a sudden loss in modulus. The study was complicated by the loss of water above 100° C during the tests and the decomposition of the polymer at about the temperature of its apparent glass transition. No glass transition could be observed by differential scanning calorimetry and the thermal mechanical analyser gave inconclusive results. The dynamic mechanical analyser gave a clear indication of a glass-like transition at about 250° C which was reversible, reappearing upon cooling and reheating. This transition could not be related to the decomposition of the sample. Absorption of 7 wt% water broadened and lowered the transition by about 50° C. However, after studying the rate of change of the modulus of the resin from the plasticized state to the dry state it was concluded that the absorbed water lowersT _g by only 50° C and not the 100 to 150° C claimed by others. Moreover, the mechanism of this apparent glass transition differs from that of a normalT _g, and may involve the breaking of hydrogen bonds.     

Internal stress of epoxide resin modified with spiro ortho-ester type resin

The internal stress and shrinkage of bisphenol type epoxide resin compounded with spiro ortho-ester resin were investigated by measuring the change of density and the strain of the steel ring embedded in the cured resins. Internal stress was absent in the rubbery region and was mainly induced by shrinkage in the glassy region. The shrinkage in the glassy region seems to be directly converted to internal stress because the motion of network segments is restricted in this region. The internal stress decreased with increasing fraction of spiro ortho-ester resin in the cured system. The reduction of internal stress was independent of the expansion of the cured resin caused by the reaction of spiro rings during the curing process. This reduction was due to the decrease in the glass transition temperatureT _g of the cured resins, that is the decrease in the shrinkage occurring in the cooling process fromT _g to room temperature.     

Effect of annealing on the thermal expansion and residual stresses of bidirectional thermoplastic composite laminates

Thermal expansion measurements have been conducted on various [(±30)_N]_S laminates fabricated by combining different types of matrix and fibers. The main objective was to link the coefficient of thermal expansion (CTE) of these laminates to the thermophysical characteristics of the matrix. The results show how these laminates can be used to analyze the influence of annealing and glass transition temperatures on the CTE and the release of residual stresses. It is shown that to ensure a reproductive expansion during thermal cycling, the composite must be annealed at a temperature high enough over T_g of the matrix (below the melt temperature) in order to completely release the residual stresses induced during the molding phase and uniformize the crystalline structure of the matrix. Once this is done, the response of the material under thermal cycling gives reproducible coefficient of thermal expansion (CTE) with almost no distortion after each cycle.     

Discontinuities in Amplitudes of Particles Micromotions Characterizing Phase Transitions in Liquid State

The behavior of amorphous solids below Vogel's (T _V ) or the glass transition (T _g ) temperature, as well as the solid-liquid transition, have been analyzed taking into consideration the anharmonicity of motion of microparticles forming the amorphous bodies. The T _g transition is explained within the logical association of this transition with the higher-temperature transitions, which can eventually involve the process of particle release into the gas phase through the process of a sudden vibrational amplitude growth. It follows from the mathematical solution of the anharmonicity problems that the pulses and the double amplitudes will always be present in aliquid matrix. The T _g temperature is considered as the boundary point for the liquid state at which the dynamical microcracks of a solid state matrix start to proceed. The processes at T _g are accompanied by appearance of new, highly agitated spots and the first microcracks (vacancies) filled up with the ‘semi-evaporated’ particles. In the mechanical sense, these vacancies form a new particle species characterized by quite different properties (different thermal expansion coefficient) as compared with the particles of the original matrix. It is assumed that a number of new mechanical units are growing up to the critical temperature when the original liquid frame, bonding the particles to lower amplitudes, is completely destroyed. The approach proposed does not contradict the traditional views reflected in the famous Adam-Gibbs-Di Marzio or WLF approaches, but allows a different approach to these theories. This revised version was published online in August 2006 with corrections to the Cover Date.     

Optimum mixing ratio of epoxy for glass fiber reinforced composites with high thermal stability

The optimum condition of glass fiber/epoxy composites was investigated according to mixing ratio of two epoxy matrices. Novolac type epoxy and isocyanate modified epoxy were used as composites matrix. Based on chemical composition of mixing matrix, optimum mixing ratio of epoxy resins was obtained through FT-IR instrument. In order to investigate thermal stability and interface of epoxy resin, glass transition temperature was observed by DSC instrument, and static contact angle was measured by reflecting microscope. Change of IR peak and T_g was conformed according to different epoxy mixing ratios. After fabrication of glass fiber/epoxy composites, tensile, compression, and flexural properties were tested by UTM by room and high temperature. The composites exhibited best mechanical properties when epoxy mixing ratio was 1:1.     

Kinetics of two-stage crystallization in Metglas 2826A

Crystallization kinetics of MSI and MSII stages in Metglas 2826A has been investigated by quantitative transmission electron microscopy. The volume fraction of crystallization against time curve has been found to show a plateau in the lower temperature region of annealing. It has been found that the glass transition temperature, T _g, has no effect on the sequence reversal of the crystallization reaction. It has been shown that the sequence reversal of transformation stages occurs due to the large difference in the activation energy of crystallization of MSI and MSII. In addition, the change in the morphology of the crystals obtained by annealing the amorphous alloy below and above the glass transition temperature, T _g, is reported.     

Crystallization study of Te-Bi-Se glasses

Crystallization studies are carried out under non- isothermal conditions with samples heated at several uniform rates. The dependence of the glass transition temperature (T_g), the crystalline temperature (T_c) and the peak temperature of crystallization (T_p) on the composition and heating rate (β) has been studied. For a memory/switching material, the thermal stability and ease of glass formation are of crucial importance. The glass transition temperature, T_g, increases slightly with the variation of Bi content. From the heating rate dependence of T_g, the activation energy for glass transition (E_t) has been evaluated. The results are discussed on the basis of Kissinger’s approach and are interpreted using the chemically ordered network model (CONM).     

Volume relaxation in amorphous and semicrystalline PET

Two types of polyethyleneterephthalate (PET) were investigated, one nearly amorphous and the other highly crystallized. DSC analysis and mercury-in-glass dilatometry were used to determine the effect of crystalline phase content on the thermal behavior of amorphous phase. Increasing portion of crystals caused an increase in glass transition temperature (T _g) and broadening of the transition zone. Thermal expansion coefficient and specific heat decreased. The amount of rigid amorphous fraction, RAF, was calculated to be around 21–26%. Volume relaxation measurements initiated by temperature down-jump from the equilibrium above T _g to several temperatures in the vicinity of T _g showed considerably reduced relaxation rate for semicrystalline PET.     

Formation and properties of SnO–MgO–P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses

A new glass system SnO–MgO–P₂O₅ with low viscosity has been developed by a melt-quenching method. Formation, thermal properties, and chemical durability of these glasses have been investigated. For a constant P₂O₅ concentration, the glass formation ability is enhanced with the increasing Sn/(Sn + Mg) ratio. The glasses exhibit low glass transition temperature (T _g = 270–400 °C), low dilatometric softening temperature (T _DS = 290–420 °C), and high thermal expansion coefficient (CTE = 110–160 × 10⁻⁷ K⁻¹). With the increasing Sn/(Sn + Mg) ratio, T _g and T _DS decrease, and CTE increases. When Sn/(Sn + Mg) ratio is varied, the relationship between chemical durability and thermal properties of the present glasses is not consistent with what expected in general cases. It is noted that the glasses with 32–32.5 mol% P₂O₅ exhibit excellent chemical durability and tunable T _g, T _DS, and CTE (by varying Sn/(Sn + Mg) ratio).