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Chain extension of poly(lactic acid) aiming the foams development by SSP and reactive processing

Abstract

Poly (lactic acid) (PLA) is a promising candidate, with many studies and commercial availability, among biodegradable polymers; which are constantly on the spotlight nowadays due to environmental issues. However, the application of PLA on foaming has some hindrances as its low melt strength and melt elasticity. Several strategies are approached to overcome the problem, such as chemical modification, blending and compounding. High melt elasticity may be achieved by the introduction of long chain branching or by the increase of molecular weight of the polymer. For this, obtaining processes can be used, among which there are solid state polymerization (SSP) and reactive processing. Solid state polymerization (SSP) is a traditional method of increasing molar mass for polyesters, although little explored for PLA, which generates products with high purity and adjustable molar masses, in long reaction times. Reactive processing is presented as a polymer modification technique, which combines processing with the reaction in a few minutes, which economically enables its production, but, unlike SSP, the products obtained are less pure and in which side reactions are recurrent. In this project, the main objective is to investigate SSP as a chain extension process to increase the PLA's extensional viscosity. In order to do this, the influence of material variables (as initial molecular weight, catalysts grafting), and processing variables (time, temperature, pre-crystallization) will be investigated to obtain materials with properties suitable to foaming. For comparison, reactive processing in a torque rheometer will also be performed, using two different extenders, glycidyl methacrylate and a carbodimide-based compound. The characterization techniques that will be employed, before and after reactions, will be size exclusion chromatography (SEC), acid group titration, as well as rheological analysis, in permanent regimes (shear viscosity, first difference in normal stresses), transient ( stress growth, stress relaxation, elastic recovery and elongational viscosity) and oscillatory (linear viscoelastic properties). The materials with the most promising properties will then be foamed, and the morphology of the foams obtained will be characterized by scanning electron microscopy (SEM). (AU)