Combining the Incompatible: Melt State Grafting between\nHigh-Density Polyethylene and Isotactic Polypropylene without a Coupling\nAgent

In this study, we\nreport the coupling of high-density polyethylene\n(HDPE) and isotactic polypropylene (iPP) into grafted block copolymers\n(HDPE-<i>g</i>-iPP) from HDPE and iPP precursors subjected\nto reactive extrusion with an organic peroxide, however, without a\ncross-linker. Coupling of macroradicals in the melt state is confined\nto a small interfacial volume at the HDPE/iPP domain interfaces of\nthis immiscible two-phase blend system. The tendency of HDPE macroradicals\nto branch and cross-link into a solid-like network, together with\nthe tendency of iPP macroradicals to simply cleave, is a common problem\nto overcome. Moreover, to prove HDPE-<i>g</i>-iPP molecules\nin reactive HDPE/iPP blends is difficult due to the low grafting yields\nand the additional challenge to analytically distinguish HDPE-<i>g</i>-iPP molecules from the HDPE/iPP matrix. In this study,\nwe work with a low-viscosity blend system, which, combined with the\nlow unsaturation content of the HDPE, made cross-linking negligible.\nWe identify HDPE-<i>g</i>-iPP molecules via interaction\nchromatography by iPP components in the HDPE elution range and by\nanalytical temperature-rising elution fractionation, where we find\nHDPE in the iPP elution range. Physical characterization by thermal,\ndynamic mechanical, and morphological analyses confirmed that the\nreactive blend is compatibilized by HDPE-<i>g</i>-iPP molecules,\nseen by shifts in crystallization temperature and glass transition,\nand by diffuse domain interfaces. With improved grafting yields, reactive\nblends could potentially be used directly as compatibilizers.