Correlation between nanohole volume and mechanical properties of amine‐cured epoxy resin blended with poly(ethylene oxide)

A set of diglycidylether of bisphenol‐A (DGEBA)/4,4′‐diaminodiphenylmethane (DDM) epoxy matrix modified with poly(ethylene oxide) (PEO), pre‐cured at two different temperatures, was examined by positron annihilation lifetime spectroscopy (PALS). The aim was to investigate the correlation between local free volume and mechanical properties. A negative deviation from the linear additivity rule of the local free volume is observed at both cure schedules. Using together the local free volume and mechanical results allows to conclude that the cure temperature makes small contribution to the flexural strength and modulus of blends but is responsible for the composition‐dependent rise of the fracture toughness. It is proposed that this behavior is a consequence of the nearest‐neighbor intrachain contacts or self‐association of the epoxy‐OH groups during cure leading to a non‐uniform space distribution of the DGEBA–PEO contacts, which causes the deflection of the crack path. Copyright © 2008 John Wiley & Sons, Ltd.     

α‐ and β‐Relaxations in neat and antiplasticized polybutadiene

Dielectric spectroscopy was carried out to measure the α‐relaxation (local segmental motion) and the higher frequency, secondary relaxation (β‐mode) in 1,4‐polybutadiene, both neat and containing a nonpolar diluent, mineral oil. The α‐relaxation shifted to lower frequencies (antiplasticization) in the presence of the diluent, suggesting the glass temperature of the latter is higher than the T_g of the polymer (i.e., >187K). The T_g of neat mineral oil cannot be determined directly, due to crystallization. While the diluent increased the magnitude of the α‐relaxation times, it had no effect on the β‐relaxation. Moreover, neither the shape of the α‐relaxation function nor its temperature dependence was influenced by the diluent. From this we conclude that the main effect of the mineral oil was to increase the local friction, without changing the degree of intermolecular cooperativity of the molecular motions. We also find that near the glass temperature, there is rough agreement between the time scale of the secondary relaxation process and the value of a noncooperative relaxation time estimated from theory. This approximate correspondence between the two relaxation times also holds for 1,2 polybutadiene. However, the β‐process cannot be identified with the noncooperative α‐relaxation, and the relationship between them is not quantitative. © 2000 John Wiley & Sons, Inc.* J Polym Sci B: Polym Phys 38: 1841–1847, 2000     

Role of polyester resin on the epoxy matrix system

Interpenetrating polymer network (IPN) as a polymer blend matrix was prepared using different ratios of polyester (PE) to epoxy (E) resins. It was observed that 11.33 wt % PE in the blend provides maximum mechanical properties, particularly the modulus for the casting, and such a blend is referred to as “optimum casting” (OC). The density of the blends containing up to 11.33% PE, especially of OC, was found to have increased by the presence of PE in the E network significantly more than would have been estimated from the calculation based on the rule of additivity. The thermal expansion coefficient parameter (Δα_i) of the OC was found to be 2.68 × 10^?4K^?1. The scanning electron micrograph (SEM) of the fracture surface of OC only showed a glass-like smooth surface. The change in the mechanical properties due to the varying proportions of PE in the blend was studied critically on the basis of failure mechanisms and morphological features. The differential thermogravimetric analysis (DTGA) of OC indicated only one peak in spite of the presence of two resin constituents, whereas castings with an excess of PE in the blend showed multiple peaks. Interestingly, as far as the DTGA is concerned, a domain of seemingly OC origin was found to remain, even in the blends containing PE resin above its optimum level (> 11.33 wt %). ©1995 John Wiley & Sons, Inc.     

A quantitative model for the specific volume of polymer–diluent mixtures in the glassy state

A mathematical model to describe the specific volume of glassy mixtures of a polymer and a low molecular weight diluent or additive is presented. The model is based on understandable physical assumptions and relies on parameters that can be determined experimentally or estimated from methods available in the literature. The predictions of the model show good agreement with the experimental data for mixtures of four polymers with diluents that in the pure state are liquid, glassy, or crystalline. The observed negative departure from volume additivity, as defined by simple additivity of the specific volume of the pure glassy polymer and the pure amorphous diluent, is the result of the relaxation of the excess volume of the glassy mixture relative to the equilibrium state caused by mixing two components with different glass transition temperatures. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1037–1050, 1998     

Secondary retardation modes in diglycidyl ether of bisphenol A–diamino diphenyl methane networks

Thermally stimulated creep (TSCr) has been used to follow the viscoelastic behavior of some amine-cured epoxy networks below the glass transition. The investigation of the -180/+40°C temperature range has revealed two essential retardation modes characterizing localized motion of chain segments: the γ mode centered at ?155°C in all samples, and the well-known β mode observed around ?40°C in the stoichiometric network. The magnitude of the β mode was seen to decrease unexpectedly with the cross-link density, whereas its peak temperature and glass transition temperature both decreased. This evolution was confirmed by thermally stimulated currents (TSC) measurements and discussed on the basis of the antiplasticization concept. Water desorption under vacuum yielded additional information on the nature of the β mode and TSCr fractional loading experiments brought evidence that two types of relaxing units participate in β motions and furnished activation enthalpy data. © 1994 John Wiley & Sons, Inc.     

X-ray study of microstructure of slightly plasticized poly-(vinylchloride) (PVC)

Small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) as well as transmission electron microscopy (TEM) techniques have been applied to investigate the microstructure of a number of commercial poly-(vinylchloride) (PVC) samples, stretched 200% and annealed for 1 h at 110 °C. From SAXS analysis, the microstructure is described as an ensemble of quasi-spherical particles one-dimensionally ordered (in the fiber axis direction) and with large distance fluctuations in the equatorial plane. The superstructure is described as fibrillar or nematic-like. TEM micrographs confirm the SAXS data.SAXS meridional patterns present 001 and 003 reflections of Ca-stearate added as stabilizer to the samples, while WAXS profiles do not show any crystalline reflection of Castearate.An interaction of Ca-stearate molecules with PVC chains is postulated, which could partially account for the phenomenon called antiplasticization of PVC.     

Effect of ionic additives on deformation behavior of bisphenol a polycarbonate

The deformation behavior of bisphenol A polycarbonate containing only a small amount of oligoionomeric additives in the range of a few parts per hundred parts of resin was examined. The impact strength of polycarbonate markedly decreased as the content of additive increased, and brittle fracture of polycarbonate was observed in tensile tests when the concentration of additive was above 2.5 phr. The ductile‐to‐brittle transition that was determined using a comparison of the critical shear yield stress and the critical craze stress appeared to exist in the range of 2.5–3.5 phr of additive. The measured entanglement density was also found to decrease significantly with the addition of a few parts per hundred parts of resin of additives, and the change of the dominant deformation mechanism from ductile to brittle failure was recognized as a result of the change of the entanglement density of polycarbonate. Therefore, it was concluded that the presence of a small amount of ionomeric additives caused the loss of entanglement density that induced transition of the deformation mechanism of polycarbonate from ductile to brittle failure and led to the corresponding deterioration of impact strength. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2635–2643, 2001