No abstract

Molecular orientation and strain-induced crystallization of vulcanized natural rubber during uniaxial deformation were studied via in situ synchrotron wide-angle X-ray diffraction (WAXD). The high intensity of synchrotron X-rays and new image analysis methods made it possible to estimate mass fractions of the strain-induced crystals and the amorphous chains in both oriented and unoriented states. Contrary to the conventional conception, it was found that, in highly stretched natural rubber, most chains remained unoriented in the amorphous phase; only a few percent of the amorphous chains were oriented and the rest of the chains were in the crystalline phase. This indicates that stress induces a network of microfibrillar crystals that is responsible for the elastic properties. The new information has prompted us to reconsider the relationships of molecular orientation, induced crystallization and mechanical behavior in natural rubber.

Electrospun poly(glycolide-co-lactide) (PLA10GA90, LA/GA ratio 10/90) biodegradable nanofiber membranes possessed very high surface area to volume ratios and were completely noncrystalline with a relatively lowered glass transition temperature. These characteristics led to very different structure, morphology, and property changes during in vitro degradation, which were examined systematically. A shrinkage study showed that the electrospun crystallizable but amorphous PLA10GA90 membranes exhibited a very small shrinkage percentage when compared with the electrospun membranes of noncrystallizable poly(lactide-co-glycolide) (PLA75GA25, LA/GA 75/25) and poly(d,l-lactide). Although the weight loss of electrospun PLA10GA90 membranes exhibited a similar degradation behavior as cast thin films, detailed studies showed that the structure and morphology changes in electrospun membranes followed different pathways during the hydrolytic degradation. After 1 day of degradation in buffer solution at 37 degrees C, electrospun PLA10GA90 membranes exhibited a sudden increase in crystallinity and glass transition temperature, due to the fast thermally induced crystallization process. The continuous increase in crystallinity and apparent crystal size, as well as the decrease in long period and lamellae thickness, indicated that the thermally induced crystallization was followed by a chain cleavage induced crystallization process. The mass loss rate was accelerated after 6 days of degradation. The increase in glass transition temperature during this period further confirmed that the degradation of PLA10GA90 nanofibers was initiated from the amorphous region within the lamellar superstructures. A mechanism of structure and morphology changes during in vitro degradation of electrospun PLA10GA90 nanofibers is proposed.