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The effect of thermal history on the transparency of biaxially‐oriented polypropylene (BOPP) films was investigated. Compression molded sheets prepared with different cooling rates were biaxially oriented at several temperatures. Correlations were sought between the light transmission measured at 633 nm and the surface roughness as characterized by atomic force microscopy. It was determined that surface roughness on the 100‐μm size scale was responsible for a loss in transparency. Surface roughness on the submicron size scale did not affect the transparency. The clearest films were obtained from compression molded sheets with the most homogeneous texture and by orienting at the lowest temperature. POLYM. ENG. SCI., 47:1658–1665, 2007. © 2007 Society of Plastics Engineers

The elastic behavior of a propylene-ethylene copolymer was investigated. An initial ''conditioning'' tensile extension up to 800% strain resulted in an elastomer with low initial modulus, strong strain hardening, and complete recovery over many cycles. Structural changes that occurred in the low crystallinity propylene-ethylene copolymer during conditioning, and that subsequently imparted elastomeric properties to the conditioned material, were investigated. Thermal analysis, wide and small angle X-ray diffraction, and atomic force microscopy measurements were performed at various strains during the conditioning process. Conditioning transformed crystalline lamellae into shish-kebab fibers by melting and recrystallization. The fibers, accounting for only 5% of the bulk, were interconnected by a matrix of entangled, amor-phous chains that constituted the remaining 95%. It was proposed that the shish-kebab fibers acted as a scaffold to anchor the amorphous rubbery network. Entanglements of the amorphous chain segments acted as network junctions and provided the elastic response. The stress-strain response of materials conditioned to 400% strain or more was described by the classical rubber theory with strain hardening. The extracted value of M c , the molecular weight between network junctions, was intermediate between the entanglement molecular weights of polypropylene and polyethylene.

The effect of thermal history on the oxygen permeability of biaxially oriented polypropylene (BOPP) films was investigated. Compression-molded sheets prepared with different cooling rates were biaxially oriented at several temperatures in the range between the onset of melting and the peak melting temperature and at a strain rate similar to that encountered in a commercial film process. The stress response during stretching was found to depend on the residual crystallinity in the same way regardless of the thermal history of the compression-molded sheet. Biaxial orientation reduced the oxygen permeability measured at 23°C; however, the reduction did not correlate with the amount of orientation as measured by birefringence or with the fraction of amorphous phase as determined by density. 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 b-relaxation was demonstrated for all the films used in the study. POLYM. ENG. SCI., where DH m is heat of melting of the specimen and DH 0 m is the heat of fusion of 100% crystalline PP and is taken as 209 J g -1 [12].

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 brelaxation was demonstrated for all the polymers used in the study.

A stretching process to enhance the stiffness of an elastomeric propylene-ethylene copolymer through orientation was examined. The tensile extension was performed at various temperatures within the unusually broad melting range of the copolymer. Stretching transformed the unmelted lamellar crystals into shishkebab fibers that acted as a scaffold for an elastomeric matrix of entangled, amorphous chains. Density measurements indicated that the process did not significantly affect the amount of crystallinity, which was about 23%. If the specimen was recrystallized by cooling after it recovered from the stretched state, the amount of orientation decreased with increasing stretching temperature. How-ever, if recrystallization occurred in the stretched state, it led to the formation of a second crystalline network that prevented contraction of the oriented crystalline structure during strain recovery. It was suggested that the second network was anchored by a 0 -PP daughter lamellae that crystallized epitaxially on the a-PP mother crystals of the extended fibrils. Although the manner in which the films were stretched and recrystallized strongly affected the modulus, good elasticity of the stretched films revealed the persistence of an elastomeric network.