Ran Chen

Deformation temperature and lamellar thickness dependency of Form I to Form III phase transition in syndiotactic polypropylene during tensile stretching

Phase transition from form I to form III in syndiotactic polypropylene crystallized at different conditions during tensile deformation at different temperatures was investigated by using in situ synchrotron wide angle X-ray diffraction technique. In all cases, the occurrence of this phase transition was observed. The onset strain of this transition was found to be crystalline thickness decided by crystallization temperature and drawing temperature dependent. The effect of drawing temperature on this phase transition is understood by the changes in mechanical properties with temperature. Moreover, crystalline thickness dependency of the phase transition reveals that this form I to from III phase transition occurs first in those lamellae with their normal along the stretching direction which have not experienced stress induced melting and recrystallization.

Stretching Temperature Dependency of Fibrillation Process in Isotactic Polypropylene

Isotactic polypropylene samples annealed at three temperatures were used to explore their fibrillation behaviors during tensile deformation at elevated temperatures via in situ synchrotron small and wide-angle X-ray scattering techniques. Fibrillation denotes the transition of the isotropic spherulitic morphology into a highly oriented one during tensile stretching of a semicrystalline polymer. It was found that the fibrillation was accomplished by a stress-induced melting and recrystallization process. Three regions, identified as fibrillation with the formation of only mesophase, fibrillation with the formation of both mesophase and α crystallites, and fibrillation with the formation of complete α modification, were identified in a map of deformation and annealing temperature. Such results are tightly linked to the different molecular mobilities of the samples prepared at different annealing temperatures and deformed at different temperatures. Lower annealing temperature and higher deformation temperature facilitate formation of α modification after stress-induced mechanical melting of the original crystallites. However, thicker lamellar crystallite deformed at lower temperatures presenting limited chain mobility ends up with a large amount of oriented mesophase structure.

Cavitation in Poly(4-methyl-1-pentene) during Tensile Deformation

The poly(4-methyl-1-pentene) sample was used to investigate the cavitation-induced stress-whitening phenomenon during stretching at different temperatures via the ultrasmall-angle X-ray scattering technique. Two modes of cavitation were found that mode I cavitation activated around yield point followed by mode II cavitation generated in highly oriented state. The critical strain for initiating the mode II cavitation increases with the increase of the stretching temperature, whereas the critical stress grew steadily in the lower temperature regime (30-60 °C) and reached a plateau at 70 °C. The appearance of mode II cavitation at large strains was independent of the mode I cavitation. The mode I cavitation was attributed to the competitive process between the formation of cavities and shearing yield of lamellae, whereas the mode II cavitation was proven to be related to the failure of the whole highly oriented entangled amorphous network because of the breaking of interfibrillar load-bearing tie molecules. Size distribution of cavities has been successfully calculated using a model fitting procedure. The results showed that the quantity of cavities increased heavily while the size was kept nearly constant during the propagation of the mode II cavitation.

Crystallization Temperature Dependence of Cavitation and Plastic Flow in the Tensile Deformation of Poly(ε-caprolactone)

In situ small-, ultrasmall-, and wide-angle X-ray scattering measurements were performed to investigate the structural evolution of crystalline lamellae and cavities as a function of deformation ratio during tensile deformation of isothermally crystallized poly(ε-caprolactone). The cavities were modeled as cylinder-shaped objects which are oriented along the stretching direction and randomly distributed in the samples, and their dimensions were evaluated by direct model fitting of scattering patterns. At small deformations, the orientation of these cavities at the onset of cavity formation was related to the isothermal crystallization temperature. Upon further stretching, the cavities were found to cluster in the interfibrillar regions at moderate strains where the long spacing of the newly developed lamellae along the stretching direction remained essentially constant. At large orientations, the cooperative deformational behavior mediated via slippage of fibrils was evidenced, the extent of which depended on the cavity number, which could be traced back to the significantly different coupling forces imposed by chains connecting adjacent fibrils. Furthermore, wide-angle X-ray scattering results revealed that a fraction of the polymer chains with their orientation perpendicular to the stretching direction were still preserved even at large macroscopic strains.

A deblurring procedure for two-dimensional small angle X-ray scattering patterns

A general pre-processing procedure of the measured SAXS patterns for reducing the effect of beam stop and beam stop holder is described. A proper method for automatically choosing the regularization parameter is implemented. The method works out on the two-dimensional SAXS patterns. After deblurring, the corresponding two-dimensional patterns will be converted into one-dimensional integrated intensity distribution curves. We tested the program using both calculated artificial data and real experimental data such as polystyrene and poly(methyl methacrylate) latices. The deblurred results are satisfactory showing the effectiveness of the method. The deblurring process of a typical two-dimensional SAXS pattern using the Matlab based program can be completed in few seconds on normal personal computers.