Melt strengthening of poly (lactic acid) through reactive extrusion with epoxy-functionalized chains

Abstract

International audiencePoly (lactic acid) is an industrially mature, bio-sourced and biodegradable polymer. However, current applications of this eco-friendly material are limited as a result of its brittleness and its poorly melt properties. One of the keys to extend its processing window is to melt strengthen the native material. This paper considers the chain extension as a valuable solution for reaching such an objective. An additive based on epoxy-functionalized PLA was employed during reactive extrusion. The reaction times as a function of chain extender ratios were determined by monitoring the melt pressure during recirculating micro-extrusions. Once residence times were optimized, reactive extrusion experiments were performed on a twin screw extruder. Size exclusion chromatography provided information about the molecular weight distributions (MWD) of the modified PLAs and revealed the creation of a high molecular weight shoulder. The rheological experiments highlighted the enhancement of the melt properties brought about by the chain extension. Shear rheology revealed some enlarged and bimodal relaxation time spectra for the extended materials which are in accordance with the MWD analysis. Such a modification directly amplified the shear sensitivity of modified PLAs. Regarding the rheological temperature sensitivity, it was found to be decreased when the chain extender content is raised as shown from the Arrhenius viscosity fit. The reduction of the polar interactions from neat to highly chain-extended PLAs is here proposed to explain this surprising result. Chain extension was also found to impact on the elongational melt properties where strain hardening occurred for modified PLAs. Investigation of the chain extension architecture was made from the rheological data and revealed a long-chain branched topology for the modified PLAs

PLA-OMMT nanocomposites manufactured by reactive extrusion

“…While both PLA and PLA-C samples displayed a narrow monomodal MWD, the PLA-REX spectrum was slightly broadened, the molecular weight increased, and a shoulder appeared in the high molecular weight tail. These changes were expected and are consistent with various published works [68,69]. Since the chain extender is a multifunctional-epoxide agent, these differences were attributed to an increase in the population of high molecular weight linear chains and/or the presence of a mixture of non-uniform branched structures.…”

“…According to the literature, PLA-REX samples exhibited the typical behavior of LCB polymers [62,69]. Within the experimental windows, neither |*()| reached a plateau nor G'() reached the theoretical double logarithmic slope of 2.…”

“…Since the chain extender is a multifunctional-epoxide reactive agent, this could be attributed to the chemical bonding of several PLA chains to one chain extender molecule. The thermo-oxidative degradations are therefore minimized, thus leading to an increase in molecular weight and modification of the molecular topology [69].…”

The effect of two commercial melt strength enhancer additives on the thermal, rheological and morphological properties of polylactide

“…Najafi et al [21] prepared PLA-clay nanocomposites using polycarbodiimide, tris(nonyl phenyl) phosphate and Joncryl ADR-4368 as chain extenders. Liu et al [16], Corre et al [22], Mihai et al [23] and Pilla et al [24], employed a multi-functional epoxy-based chain extender (Cesa-Extend from Clariant) to give rise to branched structures of PLA. Recently, Han et al [25] used polysilsesquioxane microspheres as novel chain extenders to improve the thermal stability of PLA.…”

Sheets of branched poly(lactic acid) obtained by one step reactive extrusion calendering process: Melt rheology analysis

“…On one side, reactive extrusion is an attractive way to minimize the degradation effects during processing and to enlarge its processing window. On the other side, Corre et al [11] as well as Pilla et al [12] reported that chain architecture modification by chain branching enhances PLA melt properties. Chain branching degree depends on the functionality of the reactive agent and generally leads to an increase in molecular weight.…”