No abstract

Polymer blends with co-continuous structures in the micrometer scale were obtained by photo-cross-linking of poly(2-chlorostyrene)/poly(vinyl methyl ether) (P2CS/PVME) mixtures undergoing the spinodal decomposition process. The reaction was carried out with UV light at 365 nm to induce the photodimerization of anthracenes labeled on the P2CS chains (P2CS-A). To obtain the blends with cocontinuous structures of varying length scales (A), P2CS-A/PVME blends having the same cross-link density were prepared by irradiation and subsequently annealed over different time intervals. The resulting morphology was observed by using phase-contrast optical microscopy and was analyzed by digital image analysis. It was found that the time evolution of these co-continuous structures follows the power law A « ta, where a is close to 1/4. This value is smaller than 1/3, as predicted by the Lifshitz-Slyozov-Wagner law, indicating that the spinodal decomposition was slowed down by the presence of cross-links. The physical properties of these photo-cross-linked blends were examined by dynamic mechanical measurements. For the same cross-link density, the storage modulus E' of P2CS/PVME blends increases with A, whereas the loss peaks of these co-continuous structures shift to the high temperature side. These experimental results suggest that the combination of photo-cross-linking reactions and the phase separation kinetics of polymer blends has the potential of providing a new way to design multiphase polymer materials with controllable co-continuous structures in the micrometer range.

No abstract

It is demonstrated that runaway of electrons in polar semiconductors in a strong magnetic field H begins when the value of the total electric field E:: exceeds the critical value E& for which an electron velocity in crossed fields is given by rc = C Et0,:H = +(,I2l)ro (ca = (2tiw,/m*)1'2 is an electron velocity at an optical phonon energy). This result was found on the basis of experimental investigations of avalanche breakdown in n-lnbb.

Photocrosslinking reaction kinetics of poly(2-chlorostyrene) performed inside the spinodal region of poly(2-chlorostyrene)/poly(vinyl methyl ether) (P2CS/ PVME) blends was investigated by means of ultraviolet (UV)-visible absorption spectroscopy. The reaction was performed via photodimerization of anthracene moieties chemically labeled on the P2CS chains. The crosslinking kinetics of (P2CS/PVME) blends submitted to a temperature jump from the one-phase into the spinodal regions was observed by monitoring the irradiation time dependence of the absorbances of anthracene as well as of the blend in two regions of wavelengths. One is inside and the other is outside the absorption range of anthracene. The contribution of the sample cloudiness to the absorbance of anthracene was subtracted from the absorption data by using an empirical power law experimentally established between the incident wavelengths and the absorption of the blends. It was found that the reaction kinetics approximately follows the mean-field kinetics inside the spinodal region, resembling the behavior of the crosslinking reaction performed in the miscible region at relatively low crosslinking densities. On the other hand, the method described here fails to estimate the crosslinking densities when the phase separation proceeds rapidly, overcoming the reaction.