Rheo-optical FTIR spectroscopic investigation of elastomeric behavior in metallocene polypropylenes

Rheo-optical experiments provide a better understanding of the deformational response of the polymer microstructure and its influence on the material property changes. Rheo-optical FTIR spectroscopy, a novel technique, is a combination of polarimetry, rheometry and infrared spectroscopy. It is a versatile material characterization technique as it can simultaneously obtain the responses of individual morphological structures (i.e. crystalline and amorphous) as well as individual components in blends or copolymers during deformation. A thermoplastic elastomeric form of polypropylene (ePP), synthesized from the metallocene catalyst bis (2-(3, 5-di-t-butylphenyl) indenyl) hafnium dichloride by Waymouth, et al., was characterized along with its solvent-extracted fractions. The solvent-extracted fractions [heptane soluble (HS), heptane insoluble (HI), and ether soluble (ES)] have varying degrees of isotacticity. In polyolefins, tacticity has a prominent influence on the material mechanical and physical properties. Experimental results indicate that ePP and HS undergo a strain-induced crystallization and mesophase formation. Specifically, an increase in the normalized 841 cm ^-1 peak intensity indicates increased crystallinity and mesophase formation, and the 998 cm ^-1 peak indicates crystallite formation. In contrast, at least for the strains obtainable, HI did not demonstrate any deformation induced crystallinity or mesophase formation. However, the appearance of these structures has been observed in X-ray diffractometry of HI films deformed to greater extents. ES fraction also demonstrated no strain-induced crystallization or mesophase formation. Interestingly, at high strains (400%) the crystallinity of ES decreases. This result is unexpected because ES is nearly atactic (very little crystallization present) and one would expect the order to increase with strain rather than decrease. In ePP and HS, at higher strains there is higher degree of orientation anisotropy of the mesophase than the amorphous or crystalline structures. An increase and subsequent saturation in the degree of orientation are observed in HI for all the morphological structures, except mesophase.