Role of Stably Entangled Chain Network Density in Shish-Kebab Formation in Polyethylene under an Intense Flow Field

Influence of Strong Shear Field on Structure and Performance of HDPE/PA6 In Situ Microfibril Composites

As one of the most widely applied general-purpose plastics, high-density polyethylene (HDPE) exhibits good comprehensive performance. However, mechanical strength limits its wider application. In this work, we introduced the engineering plastic PA6 as a dispersed phase to modify the HDPE matrix and applied multiple shears generated by vibration to the polymer melt during the packing stage of injection molding. SEM, 2D-WXRD and 2D-SAXS were used to characterize the morphology and structure of the samples. The results show that under the effect of a strong shear field, the dispersed phase in the composites can form in situ microfibers and numerous high-strength shish-kebab and hybrid shish-kebab structures are formed. Additionally, the distribution of fibers and high-strength oriented structures in the composites expands to the core region with the increase in vibration times. As a result, the tensile strength, tensile modulus and surface hardness of VIM-6 can reach a high level of 66.5 MPa, 981.4 MPa and 72, respectively. Therefore, a high-performance HDPE product is successfully prepared in this study, which is of great importance for expanding the application range of HDPE products.

Verification of thermodynamic theories of strain-induced polymer crystallization

We studied the crystallization of nearly "homogeneous" polyethylene glycol (PEG) networks prepared by end-crosslinking of tetra-armed PEG. The influence of stretching ratio and strand length on the melting and crystallization temperature was investigated. The relation of melting temperature and elongation ratio verifies the thermodynamic theories of strain-induced polymer crystallization.

Flow-Induced Precursor Formation of Poly(l-lactic acid) under Pressure

For the first time, the influences of two inevitable processing fields (pressure and flow fields) on the crystallization of a semirigid molecular chain polymer, that is, poly(l-lactic acid) (PLLA), were explored using a homemade pressuring and shearing device. The results reveal that the shear rate facilitated the generation of precursor because it induced oriented segment formation. It was found that the most sensitive shear temperature for the generation of PLLA precursor under 100 MPa was 180 °C. When the shear temperature was higher (e.g., 190 °C), the relaxation of shear-induced oriented segments was too quick to induce the generation of PLLA precursor. Oppositely, at a lower shear temperature (170 °C), the oriented segments were hard to relax within the whole shear rate range (3.1-31.4 s^-1). Annealing treatment was infaust to the PLLA precursor formation because it promoted the relaxation of oriented segments. Different from the shear and annealing, pressure played a more complicated role in the formation of PLLA precursor. Pressure decreased the free volume between PLLA molecular chains and meantime increased the supercooling of PLLA melt. In addition, PLLA chains tended to form locally oriented segment bundles to adapt to the pressurized state, which facilitated the formation of PLLA precursor and the following crystallization process. These two factors lowered the movability of PLLA chains and suppressed the relaxation of chain, so shear-induced orientation facilitated PLLA precursor formation under pressure. In that case, pressure and shear flow showed a synergetic promoting effect on the generation of PLLA precursor and the following crystallization process. These meaningful results could be helpful for comprehending the relationship between crystallization conditions and the crystallization behavior of PLLA and thus would provide guidance to fabricating the final products through controlling the crystallization process of PLLA.