Method development for the analysis of biopolymers as polylactic acid with gel permeation chromatography

Polylactic acid (PLA) is one of the most promising sustainable plastics. It can be produced on an industrial scale using simple, bacteria-driven, bio-based synthesis whilst also being biodegradable, meaning it could help solve problems such as long-term pollution of the environment by nondegradable plastics. It is the most widely used plastic filament material in 3D printing and used in a wide range of applications from medical applications to packaging materials.
The macroscopic properties of a polymer, such as melting point, hardness and elasticity, are mainly determined by the number average molar mass (Mn), the weight average molar mass (Mw) and the polydispersity. The method of choice to determine these parameters is Gel Permeation Chromatography (GPC). In this study, three GPC methods were investigated to determine the molecular weight of different PLA samples. The selected column was a styrene-divinylbenzene copolymer (StyDiViBe) stationary phase, and the detection was based on refractive index detection (RID) and diode array detection (DAD). Calibrations curves of polystyrene (PS) and polymethyl methacrylate (PMMA) in three different solvents, tetrahydrofuran (THF), chloroform and ethyl acetate (EtOAc), were recorded. A 20 kDa PLA standard and two real samples, PLA filaments for 3D-printing and a PLA disposable drinking bottle, were evaluated using these calibrations and compared to the NMR results. It could be shown that EtOAc is suitable as an environmentally friendly alternative, compared to THF and chloroform, for the analysis of low molecular weight PLA samples.