Regulation of Entanglement Networks under Different Shear Fields and Its Effect on the Properties of Poly(l-lactide)

Abstract

Entanglement will lead to a sharp increase in viscosity and a decrease in flowability of polymers and affect their crystallization behavior adversely. Thus, it seems that disentanglement becomes a feasible way to improve the flowability and crystallization ability of polymers without adding other substances. Poly(L-lactide) (PLLA), as a renewable polymer material, has attracted increasing attention in the past decades. Accelerating crystallization of PLLA is extremely important to improve its performance. The present study utilizes a self-designed polymer melt disentanglement machine (PMDM) with PLLA as the experimental material, which can effectively promote polymer disentanglement by applying a unique complex shear field (superposition of rotational and vibrational shear fields) to a polymer melt. The experimental results show that compared with the untreated material, the disentangled ones have a lower viscosity and a higher melt flow rate and can fill the mold cavity at a lower pressure and a lower melt or mold temperature during the injection molding process. Simultaneously, the crystallization kinetics of the disentangled materials is accelerated, the crystallinity is higher under the same secondary processing conditions, and the heat resistance and mechanical properties of prepared products are better. This study provides new ideas for resolving the conflict between mechanical properties and processing flowability, improving the heat resistance of PLLA, and expanding its application range.

Development of Unstable Flow and Morphological Analysis of Flow Marks in Injection Molding of Polypropylene and Polyolefin Elastomer Blend

“…All of the results were recorded during the first heat process. Sample crystallinity was calculated by the following equation X C = normalΔ H i normalΔ H i m ϕ i where Δ H i is the fusion enthalpy of PP, directly obtained from DSC results, Δ H i m is the fusion enthalpy of completely crystallized PP and is chosen as 207 J/g in this work, and ϕ i represents the mass fraction of PP in the tested sample.…”

Crystallization of Polymers with a Reduced Density of Entanglements

“…The crystallization of PLA in shear-disentangled samples was also investigated [125]. The full crystallization time at Tc = 110 °C shortened from 14 min to 10 min for the mostdisentangled sample, so the differences were much smaller than in Wang's research [122].…”

“…The temperature of the maximum crystallization effect shifted upwards by 1.5 °C between the entangled and The non-isothermal crystallization of polymers in which the disentangled state was obtained by melt shear has been the topic of several publications. For example, Hu et al [125] examined PLA samples previously disentangled by applying different shearing procedures. The molten samples were cooled at a slow rate of 3 • C/min.…”

“…Faster [107,119,120,125,126] Faster [32,44,57,67,106,107,119,122,125,126,128] Slower [124] Crystallization temperature Higher [57,67,101,106,107,119,121,122,125,126] Lower [124,127] --Melting temperature…”

“…[57] Lower [124] --Equilibrium melting temperature --Higher [133] Lower [67] Crystallinity degree Higher [57,67,106,119,121,125] Lower [124] Higher [122] Growth rate of spherulites --Faster [30,31,44,67,106,107,119,123,130,131] Slower [124] Regimes of crystallization --Shift of temperature [67,119,131,133] Lamellae thickness Increased [57,143] Increased [57] Nucleation density --Increased [30,31,67,106,130] Decreased [44,107,124,131] Cold-crystallization rate --Faster [105,106] Avrami's k coefficient Increased [122,125] Decreased [57] Early research on disentangled polymers focused on expanding the knowledge on polymers. Currently, it is possible to disentangle polymers on an industrial scale, either by polymerization or in extrusion processing.…”