F. Romero Colomer

Structural relaxation of glass‐forming polymers based on an equation for configurational entropy, 4. Structural relaxation in styrene‐acrylonitrile copolymer

The structural relaxation process in styrene-acrylonitrile copolymer has been characterized by means of differential scanning calorimetry (DSC) experiments. The results in the form of heat capacity, cp(T), curves are analyzed using a model for the evolution of the configurational entropy during the process recently proposed by the authors.11,12 The model simulation allows one to determine the enthalpy (or entropy) structural relaxation times and the β parameter of the Kohlrausch-Williams-Watts equation characterizing the width of the distribution of relaxation times. This material parameters are compared with their analogues determined from the dielectric and dynamic-mechanical relaxation processes.

Structural relaxation of glass-forming polymers based on an equation for configurational entropy: 3. On the states attained at infinite time in the structural relaxation process. Results on poly(ether imide)

The aim of this paper is to discuss the limit state attained at infinite time in the structural relaxation process. This state usually is identified with the equilibrium state extrapolated from the experimental data obtained at temperatures above the glass transition. The analysis is conducted with the help of a phenomenological model with fitting parameters, based on an equation for the evolution of the configurational entropy during the process. The model avoids the use of the fictive temperature, which makes it easier to introduce a different hypothesis on the limit states of the process.

Blends of styrene–butadiene–styrene triblock copolymer and isotactic polypropylene: morphology and thermomechanical properties

A set of blends of styrene–butadiene–styrene triblock copolymer (SBS) and isotactic polypropylene (i-PP) in a composition range 0–100 % polypropylene by weight was prepared in a twin screw extruder. The morphology of the blends has been studied by transmission electron microscopy. The blends present phase separation. Dynamic mechanical measurements show an improvement of the mechanical properties of SBS when i-PP is the dispersed phase. This reinforcing effect can be observed even at high temperatures when i-PP is in the rubbery state. The mechanical properties of the blends have been interpreted using Takayanagis block model. The melting and crystallization behaviour of the i-PP in the blends has been studied by differential scanning calorimetry. The fractionated crystallization phenomenon has been observed in the blends where i-PP forms the dispersed phase. The results are consistent with the morphology shown by the blends, in particular, with its phase inversion, which occurs at a composition near to 50% i-PP. © 2000 Society of Chemical Industry

Structural relaxation of poly(γ-benzyl-l-glutamate)

Abstract A structural relaxation process similar to that in amorphous polymers has been found in poly(γ-benzyl- l -glutamate). Its kinetics has been determined by differential scanning calorimetry and modelled using a phenomenological model. The parameters characterizing the shape and temperature dependence of the distribution of recovery times have been compared with those describing the dielectric relaxation process, occurring in the same range of temperatures. The process is ascribed to conformational rearrangements of the side chains and is responsible for the glass transition and the WLF behaviour of the dielectric relaxation described in the literature.

Side-chain crystallization and mobility of poly(γ-stearyl-L-glutamate)

Abstract Two phase transitions have been found in poly(γ-stearyl- l -glutamate) by using differential scanning calorimetry (d.s.c.) and X-ray diffraction techniques. These transitions define the existence of three phases, A, B and C, in order of increasing temperature. The A-B transition involves both the melting of the side-chain crystallites and the rearrangement of the helices. The kinetics of side-chain crystallization and those of the formation of phase B are determined by d.s.c. The side-chain mobility in phase A has been studied by measuring the dielectric relaxation spectrum of the sample as a function of the degree of crystallization.

Morphology and thermomechanical properties of blends of styrene–butadiene–styrene triblock copolymer with polystyrene and syndiotactic polybutadiene homopolymers

Abstract The reinforcing effect of a dispersed phase of polystyrene or syndiotactic 1,2-polybutadiene on the mechanical properties of a styrene–butadiene–styrene triblock copolymer (SBS) has been studied. The blends of any of the homopolymers with the styrene–butadiene–styrene triblock copolymer present phase separation. The morphology of the blends has been characterized by transmission electron microscopy. The blends with polystyrene homopolymer show a dispersion of small homopolymer domains in the SBS matrix with poor adhesion between the polystyrene homopolymer domains and the polybutadiene blocks of SBS. This conclusion can be reached through the low, or nearly null, reinforcing effect in the low deformation regime studied by dynamic-mechanical experiments. The blends of SBS with syndiotactic polybutadiene on the contrary, show a characteristic increase of the modulus in the temperature region in which the SBS is in the rubber state and the syndiotactic polybutadiene is in the glassy state. Some cavitation phenomena in this region, due to the separation of both phases under deformation, cannot be ruled out. Dynamic-mechanical relaxation spectrum of the blends has been analyzed with the help of a simple block model.

Study of Polymer Glass Transition and Segmental Motions in Partially Crystallized Poly(ethyl acrylate)/p‐xylene Mixtures

A poly(ethyl acrylate network), PEA, was swollen with several contents of p‐xylene, cpx, from xerogel until saturation (0≤cpx≤0.86). Differential Scanning Calorimetry (DSC), Thermally Stimulated Depolarization Currents (TSDC) and Dielectric Relaxation Spectroscopy (DRS) were performed to probe the thermal transitions and the polymer molecular dynamics in these mixtures. The results show, that the mixtures are homogeneous in the whole temperature range studied for p‐xylene contents cpx 0.30 during cooling. The partially crystallized mixtures exhibit a critical p‐xylene content estimated at 0.12–0.15 that remains un‐crystallized regardless of the p‐xylene composition. The respective polymer glass transition temperature remains co...