CHAPTER 1: Films of two isotactic propylene homopolymers prepared with different catalysts and a propylene/ethylene copolymer were biaxially oriented under conditions of temperature and strain rate that were similar to those encountered in a commercial film process. The draw temperature was varied in the range between the onset of melting and the peak melting temperature. It was found that the stress response during stretching depended on the residual crystallinity in the same way for all three polymers. Biaxial orientation reduced the oxygen permeability of the oriented films, however the reduction did not correlate with the amount of orientation as measured by birefringence, with the fraction of amorphous phase as determined by density, or with free volume hole size as determined by PALS. Rather, the decrease in permeability was attributed to reduced mobility of amorphous tie molecules. A single one-to-one correlation between the oxygen permeability and the intensity of the dynamic mechanical β-relaxation was demonstrated for all the polymers used in the study.
CHAPTER 2: This study examined the effect of chain microstructure on adhesion of ethylene-octene copolymers to polypropylene. The copolymers were candidates for compatibilization of polypropylene (PP) and high density
polyethylene (HDPE) blends, and included a blocky copolymer, a statistical copolymer that had the same composition as the soft segment of the blocky copolymer, and a statistical copolymer that had the same comonomer content and crystallinity as the blocky copolymer. The compatibilized melt blend was modeled by a microlayered, 1-dimensional structure consisting of alternating layers of PP and HDPE, each separated by a thin tie-layer. The microlayered structure made it possible to directly measure the adhesion using the T-peel test. Infrared analysis of matching peel fracture surfaces established that fracture occurred adhesively at the interface between PP and the tie-layer. Direct observation of the damage zone at the crack tip and microscopic examination of the fracture surfaces revealed that the tie-layer was highly deformed before final separation occurred at the interface. The substantially higher delamination toughness of the blocky copolymer compared to the statistical copolymers could be accounted for by considering both interspherulitic mechanically interlocking influxes and intraspherulitic entrapment of interdiffused tie-layer chains. The blocky copolymer also retained delamination toughness to a higher temperature due to the greater stability of lamellar crystals compared to fringed micellar crystals.
CHAPTER 3: A process to enhance the stiffness of propylene-ethylene copolymers through orientation was shown to increase the modulus of the film by up to one order of magnitude. The uniquely broad endotherm of this material allowed for semi-solid state drawing over a rather large range of temperatures. Increasing drawing temperature during the process caused melting, with increases in the amount of molten material with increasing temperature. The density data suggest that crystallinity is conserved whether the molten material recrystallizes in the stretched state upon or in the recovered state. If the sample was recrystallized before after recovery, orientation decreased with temperature. However, if recrystallization occurred in the stretched state, it led to the formation of a crystalline network that prevented contraction of the oriented crystalline structure during strain recovery. This suggested that the secondary structure acted as a scaffold that froze orientation in the elastic film. The recrystallized scaffold is shown to be in the form of epitaxial crystallization of α'-PP daughter lamellae upon α-PP mother fibrillar crystals.