Tuning the Rheological, Thermal, and Solid-State Properties of Branched PLA by Free-Radical-Mediated Reactive Extrusion

Abstract

The purpose of this work is to establish structure–property relations of poly­(lactide) (PLA) formulations that were branched by peroxide-mediated reactive extrusion, in the presence of triallyl trimesate (TAM) coagent. Based on detailed evaluations of the molar mass distributions and rheological properties we deduce that modified PLA consists of a population of linear, and long chain branched (LCB) PLA chains, depending upon the amount of the peroxide and coagent. Even though peroxide alone does not induce significant changes in the architecture, TAM is very effective in producing LCB structures, which result in substantial increases in viscosity, elasticity, as well as strain hardening characteristics. At high DCP and TAM loadings, cross-linked structures are obtained. The reactively modified formulations have higher molar mass, which results in improved Izod impact strength, whereas the rest of the properties, including their capacity to degrade hydrolytically, are maintained. Furthermore, these materials develop high amounts of crystallinity, when cooled under well-controlled conditions, revealing a nucleating effect, which promotes crystallization.

Advances in peroxide‐initiated graft modification of thermoplastic biopolyesters by reactive extrusion

“…It has been documented that radical‐mediated coagent modifications in polyolefin systems yield bimodal molecular weight and branching distributions. [ 37,61–63 ] Similarly, it has been suggested that two types of chain populations are present in coagent‐modified PLA [ 42,46,64,65 ] : linear chains, which remain unreacted, and LCB structures that are generated through combination of the tri‐functional coagent with the PLA macroradicals (Scheme 2). The latter typically appear in the form of bimodal molar mass distributions, resulting in deviations from the M‐H relationship (Figure 2).…”

“…Estimation of the branching content by using the Zimm‐Stockmayer equation, revealed that branching resides in the high molecular weight fractions. [ 64,66 ] Detailed characterization showed that the efficacy of the coagents was in the order TAM~TAC>TMPTA>PETA>TMPTMA. [ 57,66 ]…”

“…Solid lines in (A) represent the respective Cross model and power‐law fit for samples at 180°C. Poly(lactide) 2500HP (PLA2) is a high molecular weight PLA used for comparison [ 64 ]…”

“…[ 42 ] It must be noted that increasing the amount of TAM above 1 wt% resulted in thermorheological complexity and a phase angle below 50° in the van Gurp plot, which is characteristic of crosslinked, gel‐containing samples. [ 64,66 ]…”

Different Influences of Two Peroxide Initiators on Structure and Properties of Poly(Lactic Acid)

“…In the preparation of PLA blends or PLA composites, free‐radical reactions initiated by various initiators are often employed to enhance the interfaces between immiscible polymers or between polymer matrix and fillers. Considering the high activities of the initiators, they are always applied in reactive extrusion or preparation of PLA blends [23,24]. Through the reactions, PLA is directly bonded with other immiscible polymers [24] or modified with small molecules, which work as a compatibilizer, such as maleic anhydride (MA) [25].…”