Real-Time Infrared–Mechano-Optical Behavior and Structural Evolution of Polypropylene and Hydroxyl-Functionalized Polypropylene during Uniaxial Deformation

Abstract

This paper focuses on the real-time mechano-optical behavior of hydroxyl-functionalized polypropylene (PPOH) copolymer containing 0.4 mol % comonomer of 10-hydroxy-1-undecene and its comparison with unmodified polypropylene (PP). The mechano-optical behavior of the two polymers was studied in the partially molten state during four different processing steps heating, stretching, annealing, and coolingwith an integrated system that combined uniaxial stretching with real-time spectral birefringence and real-time ultrarapid-scan polarized FTIR measurements. While the material response was dominated by the crystalline network in both polymers, the presence of intermolecular hydrogen bonding between the hydroxyl groups was found to affect the structural evolution of the PPOH copolymer more significantly compared to the PP homopolymer. The PPOH copolymer exhibited not only lower crystallinity and smaller crystals but also a lower extent of lamella thickening at similar deformation conditions due to steric hindrance by the copolymer C9−side chains and OH groups. Uniaxial deformation of PP up to lower strains (ε H ∼ 0.1) and subsequent cooling had the same effect on the crystal structure as annealing the polymer for longer times (≥30 min) at the same temperature with no deformation and subsequent cooling, which was a decrease in the population of smaller crystals and an increase in the average crystal size while maintaining similar crystallinity as the undeformed PP. Surprisingly, for PP both the amorphous and crystalline chain axes were oriented in the transverse direction (TD) at low strains before orienting in the machine direction (MD) at higher strains. However, no such observation was made for PPOH. This behavior was ascribed to the difference in breakup and unraveling of the spherulitic morphology between PP and PPOH upon deformation. In addition, four different regimes in the stress−optical curve were established for both the polymers. Stress relaxation immediately following uniaxial deformation of PP in the semimolten state resulted in a loss of MD orientation of crystalline segments but an increase in the MD orientation of amorphous segments. Unlike the result for PP, in PPOH the secondary H-bonded network supported the primary crystalline network during stress relaxation, and no change in the orientation of either crystalline or amorphous segments was observed. A schematic of the structural evolution was proposed based on the experimental results.

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