Computational and Experimental Approaches to Characterise the Barrier Performance of Polymer Systems, Towards the Development of Sustainable Packaging Materials

Abstract

The alarming quantity of waste generated through single-use plastic has been
brought to public attention in recent years. Much of this is produced for packaging, an industry which is responsible for 38 % of global plastic consumption. In this application, the barrier performance of materials, usually polymeric films, is essential to avoid the transmission of gases to, and subsequent spoiling of the packaged item. This project explores the characterisation of barrier performance in polymers computationally and experimentally, towards the development of sustainable packaging materials.

Gas permeability and diffusion through plastic membranes could be derived experimentally from transient permeation experiments, employing a differential
pressure methodology. An affordable method of deriving these parameters was
developed, from which a wide range of commercial plastics and bioplastics could
be analysed accurately. The consistency in testing procedure across the large
characterisation study allowed for permeability and diffusion values to be reliably compared to one another and used as industrial benchmarks against which
to assess the performance of novel bioplastics. The extrapolation of oxygen diffusion coefficients, in polymers currently employed in packaging applications, could also be used directly to inform computational modelling.

Computational methods to predict gas solubility and diffusion in polymers were
investigated. Whilst grand canonical Monte Carlo simulations could be used
effectively to analyse the free volume distributions within a system, solubility
coefficients derived from these simulations were overestimated. The use of mean
squared displacement to derive oxygen diffusion from the trajectories of molecular dynamics simulations was studied, having been largely unsuccessful in the past when applied to penetrant movement in bulk polymer systems. Minimum requirements were established to obtain accurate predictions of gas diffusion, which ensured an adequate simulation length in which to attain Einstein diffusion, and sufficient statistics gathered to obtain meaningful results.

This computational method was employed in a comparative study assessing barrier properties in polyethylene terephthalate (PET) and biopolymer, polyethylene furanoate (PEF). Models of both polymers were generated and verified against published experimental data and complementary quantum calculations. The tenfold decrease in oxygen diffusion through PEF relative to PET was correctly identified from simulation, where coefficients were predicted in close agreement with experimental data, as 2.88×10−9 and 3.24×10−8
cm2.s−1 respectively. Converse to common opinion, a decrease in ring flipping activity was demonstrated to have a minimal effect on penetrant diffusion in these polyesters. These simulations indicated that a difference in fractional free volume was likely to be responsible for the observed variation in permeability between the two plastics.

The barrier properties of poly(l-lactic acid) (PLA) were studied computationally,
in order to assess the effect of clay nanoparticles on oxygen diffusion. Composite
systems comprising of a single layer of pyrophyllite in contact with amorphous
PLA were generated, and characterised relative to an equivalent neat system.
Oxygen diffusion was found to be three times lower in clay composite systems,
in line with experimental observations, with calculated diffusion coefficients exhibiting close agreement with those determined experimentally. This reduction
was attributed to structural changes in amorphous PLA chains at the clay interface, which exhibited significantly higher density and lower mobility. Models
where finite crystalline or clay domains were incorporated within amorphous systems were found to be unstable, most likely due the small size of the inserted
structures.

Date of Award	14 Sept 2022
Original language	English
Awarding Institution	University of Bath
Supervisor	Steve Parker (Supervisor), Antoine Buchard (Supervisor) & Bernardo Castro Dominguez (Supervisor)