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The strain hardening behavior of polymers has important roles in processing such as foaming, film formation, and fiber spinning. The most common method to enhance strain hardening is to introduce a long-chain branching structure on the backbone of a linear polymer, but this method is costly and challenging to tailor the behavior. We hypothesized that in situ shrinking fibers can increase the strain hardening of linear polymers, and the degree can be efficiently controlled. In this study, we show that heat-activated shrinking fibers compounded in linear polypropylene enhance strain hardening and foamability. Moreover, changing processing conditions, such as temperature, can amplify the degree of enhancement. Rheological measurements and physical foaming tests are shown to support our hypothesis. [ABSTRACT FROM AUTHOR]

Strain hardening has important roles in understanding material structures and polymer processing methods, such as foaming, film forming, and fiber extruding. A common method to improve strain hardening behavior is to chemically branch polymer structures, which is costly, thus preventing users from controlling the degree of behavior. A smart microfiber blending technology, however, would allow cost-efficient tuning of the degree of strain hardening. In this study, we investigated the effects of compounding polymers with microfibers for both shear and extensional rheological behaviors and characteristics and thus for the final foam morphologies formed by batch physical foaming with carbon dioxide. Extensional rheometry showed that compounding of in situ shrinking microfibers significantly enhanced strain hardening compared to compounding of nonshrinking microfibers. Shear rheometry with linear viscoelastic data showed a greater increase in both the loss and storage modulus in composites with shrinking microfibers than in those with nonshrinking microfibers at low frequencies. The batch physical foaming results demonstrated a greater increase in the cell population density and expansion ratio with in situ shrinking microfibers than with nonshrinking microfibers. The enhancement due to the shrinkage of compounded microfibers decreasing with temperature implies that the strain hardening can be tailored by changing processing conditions. [ABSTRACT FROM AUTHOR]