How molecular weight distribution and drawing temperature affect polypropylene physical properties and morphology

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Abstract
Three polypropylene polymer samples possessing three different molecular weight distributions (MWD) ranging from narrow to broad, but having the same melt flow, were studied. The three resins were used to produce uniaxially oriented stretched tapes, which were evaluated for physical properties and morphological characteristics. In addition, five different drawing temperatures were used to investigate the effect of temperature on the physical properties and molecular morphology of the tape. As the MWD became broader, the tape modulus and tenacity increased slightly and the percent elongation and shrinkage decreased slightly. These effects were attributed to decreases in molecular mobility and increases in the number of tie molecules present in broad MWD resins. Morphological studies determined that the crystalline orientation was not affected by MWD. However, the amorphous orientation, at higher draw temperatures, demonstrated a small increase as the MWD became broader. MWD did not have a large effect on the percent crystallinity but the long period spacing (LPS) did exhibit a small decrease as the MWD broadened.

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