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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.

On-line studies of structural and morphological changes during the heating and drawing process of isotactic polypropylene (iPP) fiber were carried out using synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. A unique image analysis method was used to deconvolute the two-dimensional (2D) WAXD patterns into quantitative fractions of crystal, mesomorphic, and amorphous phases. Results showed that the R-form crystals were quite defective in the initial iPP fibers and were converted into the mesomorphic modification by drawing at room temperature. Corresponding 2D SAXS patterns showed that there was no obvious long period (i.e., no lamellar structure) in the mesophase of the iPP fiber. We postulate that the constituents of the mesophase in iPP fibers include oriented bundles of helical chains with random helical hands and perhaps oriented chains with no helical structures; both have only partial packing ordering. The formation of the mesophase is through the destruction of the lamellar crystalline phase probably by pulling chains out from crystals. The R-form crystals were not converted into the mesophase by drawing at high temperatures. At higher temperatures, the R-form crystals became perfect and the crystallinity increased when the fiber was drawn. However, the draw ratio showed an inverse effect. The increase in draw ratio had a minimal effect on the crystallinity, but the transformation from the amorphous phase to the mesophase became dominant.

Deformation-induced phase transitions and superstructure formation in poly(ethylene terephthalate) (PET) were studied by means of in-situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) as well as Raman spectroscopy. The deformation conditions involved uniaxial stretching of quenched PET films at a temperature just below its glass transition temperature (T g), where a notable “plastic deformation” stage was observed. WAXD results indicated that the initial sample contained a “slush” structure (amorphous + nematic), whereby deformation induced oriented amorphous, nematic, smectic (C and quasi-A), and stable triclinic crystalline phases. SAXS results indicated that the fibrillar superstructure was formed upon the formation of oriented slush. In-situ Raman spectroscopic data revealed the orientation information on ethylene glycol and benzene ring as well as the gauche − trans transition in deformation of PET chains, which are in good agreement with X-ray results. A mechanism for deformation-induced phase transitions and for hierarchical structure formation has been proposed to correlate the structural information with the mechanical properties.

An in situ study of strain-induced crystallization in an amorphous poly(ethylene terephthalate) (PET) film was carried out by using wide-angle X-ray diffraction with synchrotron X-rays. Results indicated that the mesophase was developed during stretching, immediately upon necking below T g. A sharp meridional peak was observed during the mesophase formation. The d-spacing (10.32 Å) of this peak was smaller than the monomer length, indicating that the chains in the mesophase could be tilted with respect to the stretching direction. Prior to crystallization, the intensity of this peak was found to increase upon stretching. The triclinic PET crystalline structure began to form as the temperature was increased above T g. Then, the corresponding intensity of the d ) 10.32 Å peak was found to decrease. This observation suggests that the triclinic PET crystals form an inclined layered structure, which shifts this peak out of the meridian. A two-dimensional analytical method was used to deconvolute the diffraction pattern into isotropic and anisotropic contributions. The isotropic and anisotropic fractions remained almost constant after stretching was stopped and during crystallization, suggesting that the strain-induced crystallization occurs mainly in the mesophase, supporting the hypothesis that the intermediate mesophase acts as a precursor for crystallization in oriented PET.

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