RESUMEN

Oriented polyethylene films, in which stacked lamellae grow perpendicular to the flow direction, can show extraordinary wavelength dispersion of birefringence; i.e., birefringence increases with wavelength. Owing to the periodical change of the refractive indices between the crystalline and amorphous layers along the flow direction, with oriented long lamellae having a high refractive index, the form birefringence has negative values. Furthermore, the refractive indices of the crystalline region show a stronger wavelength dependence than those of the amorphous region, which is responsible for the marked wavelength dispersion in the form birefringence. The combination of negative form birefringence with strong wavelength dispersion and positive orientation birefringence can give extraordinary wavelength dispersion.

RESUMEN

High-performance films with almost zero-birefringence and zero-wavelength dispersion were succeeded to prepare from pullulan esters derivatives (PLEs) without any additives. Optical transmittance analysis, birefringence measurement of PLE cast film and hot stretched films, and infrared dichroism analysis were conducted to characterize optical properties of PLE films comparing with cellulose triacetate which is commercially used as low-birefringence in optical devices. The aims of this study, characterization of optical properties of pullulan esters, can develop a deep understanding of the fundamental knowing and applicability of polysaccharides. Accordingly, authors believe this paper will open the gate for researches in the application of polysaccharides.

RESUMEN

The effect of chain packing on tensile properties was studied employing amorphous poly(lactic acid) PLA. It was found that the samples cooled in the temperature range from 60 to 80 °C, that is, slightly higher than the glass transition temperature Tg, showed ductile behavior with a low brittle-ductile transition temperature. Furthermore, the samples obtained by prolonged cooling at 56 °C also showed ductile behavior, whereas a shorter cooling time at the same temperature provided a brittle product. Even for the samples quenched at 40 °C, they showed ductile behavior after the exposure to postprocessing annealing operation at 60 °C; that is, the strain at break is larger than 3. This is an anomalous phenomenon for a glassy polymer. The dynamic mechanical analysis and thermal characterization revealed that the ductile samples show slightly higher Tg than the brittle ones, presumably due to high packing density of polymer chains. Moreover, it was found from infrared spectroscopy that the ductile samples show strong absorbance at 1267 cm(-1), ascribed to high energy gauche-gauche gg conformers. Following the classic Robertson's descriptions of plastic flow, it is concluded that the increase in the gauche-gauche gg conformers, which shows the conformation change under a low stress level, reduces the critical onset stress for shear yielding. The results demonstrated that the mechanical toughness of PLA can be controlled by the cooling conditions during processing and the postprocessing annealing operation.

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RESUMEN

Phase separation in plasticized cellulose triacetate (CTA) films is investigated to produce a microporous film that can be used in optical devices. Hot-stretched CTA films containing diisodecyl adipate (DIDA) show negative orientation birefringence similar to the hot-stretched pure CTA. After extracting DIDA from the stretched films by immersion into an organic solvent, however, the films exhibit positive birefringence. Moreover, the magnitude of the birefringence increases with the wavelength, known as extraordinary dispersion, which is an essential property in the preparation of an ideal quarter-wave plate. Numerous ellipsoidal pores with micro-scale were detected in the film after the immersion, indicating that DIDA were segregated and formed ellipsoidal domains in the CTA matrix during annealing and stretching. These results indicate that extraordinary wavelength dispersion is given by the combinations of orientation birefringence from CTA and form birefringence from micropores. Furthermore, it was found that annealing time and stretching condition affect the phase separation as well as the shape and size of pores.

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RESUMEN

Stretching vibrations of aliphatic C-D bonds at the chain end and midchain site of partially deuterated polystyrene (PS) were determined by Fourier transform infrared (FT-IR) spectroscopy. It was first found that the stretching vibration at the chain end is more rapid compared to that at the midchain site in the glassy bulk state. The difference in the frequencies of the stretching vibrations at the chain end and midchain site changed little even when the PS was dissolved in toluene. Moreover, the DFT vibrational frequency calculations showed that the C-D bonds at the end site intrinsically have higher vibrational frequency because of lower intramolecular interactions at the chain end. From these results, it was concluded that the main origin for the local rapid stretching vibration at the chain end is not intermolecular effects but reduced intramolecular interactions induced by the discontinuity of the repeating unit at the chain end.

RESUMEN

A glass transition temperature, T(g), and a ß relaxation temperature, T(ß), of spin-labeled polystyrene (PS) having a number average molecular weight (M(n)) of ca. 25 kDa were determined by the microwave power saturation (MPS) method of electron spin resonance (ESR). Spin labeling was selectively carried out at chain ends or midchain segments. This method allowed us to determine the local T(g) and the local T(ß) around the spin-labeled sites, selectively. The T(g) determined by the ESR, T(g,ESR), was in good agreement with that determined by differential scanning calorimetry, T(g,DSC); the T(g,ESR) decreased with decreasing M(n) with blending oligomers as well as the T(g,DSC). The T(g,ESR) for the end-labeled PS (PS-E) was equal to that for the midchain-labeled PS (PS-M) irrespective of the M(n). However, we previously reported that the PS-E showed distinctly higher segmental mobility than the PS-M in the temperature range 423-463 K (above T(g)). Therefore, we conclude that the chain ends intrinsically have higher segmental mobility than midchain segments due to the discontinuity of repeat units; however, the mobilities of chain ends and midchain segments are averaged out in the vicinity of T(g) due to the cooperativities with neighboring numerous chain segments. Concerning the ß relaxation, the T(ß) determined by the MPS was in good agreement with those determined by dielectric and dynamic mechanical spectroscopies and dilatometry. The T(ß) of the PS-E was the same with that of the PS-M within experimental uncertainties; the T(ß) was insensitive to the M(n) in contrast to the T(g). Therefore, we conclude that the effect of chain end is little on the ß relaxation of PS due to its local character. In addition, the effect of annealing at 353 K was found to be the same for the T(ß)s of the PS-E and PS-M.