ABSTRACT:The structural gradients developed along and across the flow direction of injection-molded PVDF and PVDF/PMMA parts were investigated by optical microscopy and thermal analysis techniques. The spatial variation of crystallinity across the thickness direction was found to be insensitive to the process variables: injection speed and mold temperature. This relatively flat crystallinity profile across the thickness of the parts was found to decrease with the increase of PMMA concentration. The blends become noncrystallizable beyond about 40-45% PMMA concentration. The influence of flow history on the structural evolution across the thickness was observed in the peak position of the cold crystallization region. This peak temperature showed a minimum at depths where shear effects are at their maximum. This was attributed to the increased levels of chain orientation frozen in the amorphous portions of these regions which crystallize at lower temperatures upon heating. ᭧ 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: [909][910][911][912][913][914][915][916][917][918][919][920][921][922][923][924][925][926] 1998

SYNOPSISThe bacterial copolyesters poly(3-hydroxybutyrate-co-3-hydroxyvalerate) have been successfully commercialized by ICI and are currently being distributed worldwide. Because of their bacterial origin, they are completely biodegradable. This has opened up numerous opportunities to develop new environmentally friendly products. The solid-state extrusion of a series of biodegradable copolyesters (P(3HB-3HV)) was performed in our laboratory with the aim of gaining fundamental understanding about their processability below their melting temperatures. The extrudability windows were found to span the temperature range from 135 to 150°C, depending on the composition of the samples under our experimental setup. The solid-state extrudates were found to exhibit an extra melting endotherm about 15-20°C above their normal melting temperature. This high temperature melting peak increasingly became dominant at lower extrusion temperatures. Wide angle X-ray diffraction studies did not indicate any phase change that might be responsible for this increase in the melting point. Contrary to the expectations, the solid-state extruded samples did not show significant chain orientation along the extrusion direction. This might be a result of fracture of the mass in the barrel into smaller pieces and their randomization during the course of their passage through the die. When the extrusion temperature was raised closer to the melting temperature, the quality of the extrudates was improved, and this was reflected in improvement of their mechanical properties.

SynopsisThe use of thin polymeric films in applications such as flexible circuit boards and dish membrane solar collectors has been gaining popularity. In these and many other applications the films are used under constant loading conditions which subjects them to long-term creep. In this paper, we present detailed experimental tensile creep results on unoriented films of varying crystallinities and unequal and ,equal biaxially oriented poly (ethylene terephthalate) (PET) films. The results indicate that the increase of crystallinity, stretch ratios, and annealing causes reduction in longterm creep strains. Unequal biaxially stretched films exhibited in-plane anisotropy in their tensile creep behavior. In these films the lowest creep strains are observed in the direction along which the film stretched to the highest stretch ratio.

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SYNOPSISIn this article, we present detailed processing characteristics and structure development in a thermoplastic polyimide BTDA-DMDA in the solid-state extrusion process. This fully imidized polyimide polymer is known to crosslink at fast rates when it is brought to a molten phase even for short periods of time. This characteristic makes it difficult to process it in the molten phase and attempts at melt processing result in melt fracture and highly distorted extrudates. However, this polymer can be shaped into high-quality extrudates when it is processed below its melting temperature directly from its postpolymerization powdered state. The solid-state extrusion of precompacted BTDA-DMDA powder was studied in the temperature range from 250 to 320°C. At the temperatures from 290 to 320"C, high-quality extrudates were obtained. Below 290"C, solid-state extrusion was not possible due to the limitation of the load cell capacity of the capillary rheometer used in this research. Above 320"C, the extrudates were found to be of poor quality as a result of degradation and crosslinking in the molten phase. Structural characteristics of the samples produced by solid-state extrusion was investigated by the microbeam X-ray diffraction technique. The thermal behavior of the extrudates was also characterized by differential scanning calorimetry (DSC). The DSC results show that at low extrusion temperatures the samples exhibit dual endothermic peaks and are highly crystalline in an extruded state. The higher melting peak located at about 350°C is due to the melting of the new crystalline phase that has developed partially during the solid-state extrusion process and partially during the recrystallization process that takes place at temperatures a t and slightly above the primary melting process during the DSC heating scan. This has been confirmed by DSC, depolarized light hot-stage video microscopy, and wide-angle X-ray diffraction studies. The long spacing of the higher melting crystals was found to be much larger than that of the lower melting crystals, as evidenced by the small angle X-ray scattering studies. 0 1995