Ashraf A. Mansour

Dielectric investigation of cold crystallization of poly(3-hydroxybutyrate)

Abstract The dielectric behaviour of poly(3-hydroxybutyrate) (PHB) has been investigated for the glass (α) relaxation in the temperature and frequency ranges of −60 to 100°C and 103–105 Hz, respectively. Also, the crystallization kinetics of PHB has been studied dielectrically as a function of heating rate, time and temperature. Crystallinity has a marked influence on the glass relaxation characteristics owing to the relative constraint imposed on the amorphous phase by the crystallites. Furthermore, the results obtained showed that: (1) the optimum crystallization temperature is about 25°C; (2) the time at which crystallization starts is a function of the working temperatures and apparently is reduced to just a few minutes at 40°C; and (3) the crystallization process is completed within a period of less than 1 h in the temperature range 20–40°C. The bulk crystallization phenomena could be also described by the Avrami equation with an exponent ∼ 3.8.

Detection of compatibility of some rubber blends by DSC

The compatibility of some technically important polymer blends, namely BR/NR, NR/NBR and CR/NBR, has been investigated using the DSC method. In addition, dynamic mechanical measurements have been carried out for the NR/NBR blends over the frequency range of 10−4 Hz –200 Hz and temperatures ranging from −70 to +70°C.The results obtained show that the three rubber blends are not compatible over the entire composition range as proven by the DSC and mechanical measurements. By analyzing the heat capacity increases at the glass transitions of the separate phases in the NR/BR blend, it was possible to suggest the presence of a limited compatibility at the boundaries of the two phases.By comparing this work with prior measurements, it was possible to conclude that the calorimetric method is a more efficient tool for the study of compatibility of polymer blends when compared to ultrasonic and viscosity methods.Furthermore, it was found that polymers that show compatibility when measured with an ultrasonic method could behave compatible, semicompatible or incompatible when analyzed by DSC. On the other hand, blends that show incompatibility by the ultrasonic method are always incompatible by the DSC method.

DIELECTRIC INVESTIGATION OF SBR-NBR AND CR-NBR BLENDS

SBR-NBR and CR-NBR blends were prepared with different con centrations, namely 0, 20, 40, 60, 80 and 100%, and measured dielectrically over a wide frequency range (10-2 -106Hz) and at different temperatures (-10°-25°C). The aim of the present work is to employ the dielectric method to investigate the compatibility of two different types of rubber blends. The first type consists of nonpolar and polar polymers (SBR and NBR), while the second is blended from two polar polymers (CR and NBR). It was expected that the styrene (in SBR) and the nitrile groups (in NBR could act as a donor-acceptor system and therefore allow compatibility of SBR and NBR, while the second series was expected to be compatible due to their polarity.

Effect of Crosslinking on Block and Random Copolymers of Styrene and Butadiene

The effect of crosslinking on the molecular dynamics of two triblock copolymers and a random copolymer is investigated and compared. Dynamic-mechanical measurements were done to evaluate both real and imaginary parts (J′, J″) of shear compliance in wide temperature and frequency windows, –95 to 120°C and 10–4 to 10 Hz, respectively. Moreover, DSC as well as stress-strain measurements were carried out. The materials used in this study are two triblock copolymers, mainly styrene-butadiene-styrene, (SBS) triblock copolymer, in addition to styrene-butadiene-rubber (SBR) random copolymer. The materials were crosslinked by means of dicumylperoxide (DCUP) up to 10 parts per hundred of rubber (phr) to obtain different crosslinking concentrations. Complete master curves were constructed for all samples and analyzed using Cole-Cole processes. It is shown that crosslinking of block copolymers has a much stronger effect on the dynamics of the polybutadiene (PB) phase in block copolymers than in random copolymers having the same styrene content. The results were discussed using the meander model of Pechhold, and the Mooney-Rivlin and Mullins equations.

Effect of Method of Preparation on Molecular Packing of TMPC/PS Blends

Dielectric and differential scanning calorimetry measurements of polystyrene/tetramethyl polycarbonate (PS/TMPC) blends have been carried out for two series of samples prepared by a melt mixing method, using a Brabender, and a solvent casting method using methylene chloride. The dielectric measurements of the samples were carried out over wide ranges of temperature (50-220°C) and frequency (10 -2 -10 5 Hz). The composition ratios of the samples measured were 12.5, 25, 50, 75 and 87.5wt% TMPC for solvent casting and 10, 25, 40, 50 and 60wt% TMPC for melt mixing. It has been found that melt mixing under the conditions used produces compatible blends for all the composition ratios measured without causing any pronounced degradation. The glass transition temperatures and dielectric relaxation behaviour of the blends prepared by the different methods showed differences in molecular packing and dynamics. The results obtained could be attributed to a variation in the size of structural units responsible for compatibility, which depended on the method of preparation.

Dielectric investigation of molecular dynamics of blends : III. Effect of molecular weight in TMPC/PS blends

Dielectric and calorimetric measurements have been carried out for tetramethyl polycarbonate/polystyrene (TMPC/PS) blends with different compositions. The effect of varying the molecular weight of the weakly polar component (PS) on the molecular dynamics of the polar segments of TMPC has been thoroughly studied over wide ranges of frequency (10 -2 -10 5 Hz), temperature (50-220°C) and number average molecular weight, M n , (6500-560000 g mol -1 ). All blends were found to be compatible regardless of the composition ratio and the molecular weight of PS. Some new and interesting experimental findings have been observed concerning the effect of molecular weight on the glass temperature and on the broadness of the glass transition and relaxation. Neither the kinetics nor the distribution of relaxation times of the local process observed in pure TMPC was affected by blending with PS, regardless of the composition ratio or the molecular weight of PS. It has been concluded that the mixing of the polymeric components to form a homogeneous single phase (compatible blend) does not take place on a segmental level but on a structural one. The size of this structural level has been suggested to have the same volume as the cooperative dipoles, which is assumed to be the minimum volume responsible for a uniform glass transition (10-15 nm). The molecular weight dependence of the relaxation characteristics of the glass process and temperature could be attributed to the variation in the size and packing of the structural units.

Effect of block position on the dynamics of the glass processes in carbonate/styrene triblock copolymers, CSC and SCS

Molecular dynamics of specially prepared triblock copolymers of polystyrene and polycarbonate were studied by dielectric spectroscopy over wide ranges of frequency; 10-1 - 105 Hz and temperature; 50-200°C. Two block copolymers are used in the current investigation; carbonate-styrene-carbonate, CSC and styrene-carbonate-styrene, SCS, which have blocks with the same molecular weights but with different positions. The dielectric relaxation spectra and d.s.c scans showed that these block copolymers CSC and SCS exhibit two relaxation processes corresponding to the glass transitions of the two microseparated PS- and PC-phases. It is surprising to find that the molecular dynamics of either PS- or PC-blocks are different in CSC and SCS and also differ from the dynamics of the corresponding homopolymers with same molecular weight, i.e. the molecular dynamics of the blocks depends on their position within the chain. The results are assessed and discussed in terms of the different factors that could affect the glass transition and its dynamics for different blocks; namely: the number of free end groups per block and the morphological confinement that influence the size of the cooperative regions responsible for the glass transition.

Dielectric Investigation of Molecular Dynamics of Blends: IV. Effect of Blending on the Normal Mode Process of Polyisoprene/Polystyrene Blends

Polyisoprene/polystyrene (PI/PS) blends have been prepared and investigated for compatibility using dielectric and calorimetric measurements. Various blends were prepared from polystyrene (number average molecular weight, M n = 160 000 g mol -1 ) and polyisoprene with M n values of 13 800, 40 500 and 130 000gmol -1 . Dielectric measurements have been carried out over a wide frequency range (10 -2 10 6 Hz) and in the temperature range of the glass and normal mode processes (-70 to +70°C). The glass transition, as well as the corresponding relaxation process, of polyisoprenes were shifted to higher temperatures in the different blends, indicating compatibility. The blends showed a lower critical solution temperature (LCST) at temperatures above 105°C. It was surprising to find that blending of polyisoprene with polystyrene led to a great shift to higher values in the relaxation frequency of the normal mode process for the isoprene segments. The measurements showed that the relaxation time of the normal mode process in the blends was longer than that of the glass process by a constant factor (3.2 decades), regardless of the molecular weight of the polyisoprenes used in the blends. This finding implied that the domain length responsible for the compatibility of the two polymers was consistent regardless of the molecular weight used (where M n > M c , the critical molecular weight). In view of the results obtained, and by using a molecular model, it was possible to determine the size of the structural domains responsible for the compatibility. The value obtained (16.7nm) is very similar to that suggested to be responsible for the glass transition in pure polymers.

Dynamic Mechanical Relaxation Behavior of Block Copolymers: Styrene-Butadiene-Styrene Rubber, SBS

The real and imaginary parts of the complex shear compliance (J0 and J00) of styrene-butadiene-styrene block copolymer, SBS, have been measured isothermally over the frequency range; 0.03-30 Hz, at different temperatures ranging from -70 to 130°C. SBS is taken as representative example of thermoplastic rubbers, which are not susceptible for direct construction of master curves by the usual time-temperature-superposition, tTs, technique. A method has been developed to construct master curves, which we call “step mastering technique”. The step mastering was carried out by constructing different master curves at different temperatures covering both glass transitions which were then superpositioned as whole to maintain the individual shift factors of different processes unchanged. The relaxation frequency of the low temperature glass process can be obtained at any given reference temperature by fitting and extrapolating the activation curve in order to superposition its low temperature master curve (as whole) on the high temperature master one at the correct position of frequency that corresponds the new reference temperature. Accordingly, it was possible for the first time to construct a complete master curve for J0 and J00 for SBS at 30°. The validity of the master curves obtained by the step mastering technique were confirmed by real frequency measurements in the MHz region using the Quartz method. The analysis showed that the relaxation spectrum consists of four relaxation processes. Using a molecular (meander) model, it was possible from the plateau compliance to determine the size of the structural units of the two phases, which was estimated to be 6.5 and 19.5 nm for PB- and PS-phases, respectively. A possible geometrical packing of the different domains is suggested and compared with the literature data.

Dielectric Relaxation Behavior of Polyethylene Grafted with Acrylic Acid, 2-Hydroxyethylmethacrylate and their Binary Mixtures

Dielectric investigation of low density polyethylene grafted with acrylic acid (AA) and 2-hydroxyethylmethacrylate (HEMA) and their mixtures was carried out. The dielectric properties of different samples with different graft contents lower than 30% wt. were measured in the frequency range of 10 -2-106 Hz and the temperature range of 50°-175°C.Only one relaxation pro cess is observed in the case of polyethylene grafted with acrylic acid (PE-PAA). The observed process is attributed to the cooperative reorientation of the poly acrylic acid chains modified by their connections to the polyethylene. The melt ing of the polyethylene crystalline phase does not influence the relaxation behavior of the polyacrylic acid phase.