Thesis
Crystallinity and gas barrier properties in compostable polymer films for food packaging: a combined simulation and experimental study
- Creator
- Rights statement
- Awarding institution
- University of Strathclyde
- Date of award
- 2025
- Thesis identifier
- T17509
- Person Identifier (Local)
- 202189217
- Qualification Level
- Qualification Name
- Department, School or Faculty
- Abstract
- Polyhydroxyalkanoates (PHAs) are a promising class of sustainable plastics for food packaging, as environmentally friendly, compostable alternatives to conventional petroleum-based polymers. However, their barrier and mechanical properties are often limited by their crystallinity and microstructure. To address this, fillers and plasticisers are commonly added to modify these characteristics. This work investigates how such additives influence the structure, crystallinity, microstructure and ultimately the gas barrier and mechanical properties of PHAs. Molecular dynamics simulations were used to provide insight into gas diffusion in crystal, amorphous and filled systems. First, a force field was modified to enable study of any combination of polyhydroxybutyrate (PHB), polyhydroxyvalerate (PHV) and their copolymers. This force field successfully reproduced experimental trends in density, surface energy and glass transition temperatures. The simulated lattice parameters for the PHB crystal were a = 5.86 ± 0.15 ˚A, b = 12.86 ± 0.29 ˚A and c = 6.02 ± 0.12 ˚A. For PHV, these were a = 9.67 ± 0.19 ˚A, b = 10.11 ± 0.23 ˚A and c = 5.63 ± 0.11 ˚A. Both sets of crystal parameters were in good agreement with experimental values. In terms of polymer dynamics, the amorphous PHB system simulated at 300 K was deemed to be in the glass phase, with reduced chain mobility compared to the melt phase at 500 K. It was revealed, through computing diffusion coefficients (D), that PHB exhibits superior oxygen barrier performance compared to PHV, driven by its denser packing and cohesive interactions. In bulk amorphous PHB at 500 K, DH2O = 2.37 ± 0.23 and DO2 = 3.76 ± 0.25 (× 10−5cm2 s−1). Water diffusion was slower than oxygen diffusion due to interactions between the polar water molecules and the ester groups on the PHA backbones. Simulations also demonstrated that crystalline domains inhibit the mobility of permeants, with negligible long-range diffusion for infinite and finite-chain crystals. The slower polymer chain dynamics in the presence of graphite caused a reduction in D for both oxygen and water. When the filler surface was added the diffusion coefficients were reduced, with DH2O = 1.63 ± 0.36 and DO2 = 1.99 ± 0.22 (× 10−5 cm2 s −1). The slower diffusion in filled systems was attributed to both reduced polymer mobility due to densification at the surface and the accumulation of permeants at the interface. Experiments investigated thermal, mechanical and barrier properties of solvent cast PHB films with added plasticiser triacetin and the filler boron nitride (BN). Incorporating triacetin reduced the melting temperature and increased distance to burst compared to pure PHB. However, plasticised films showed a poorer water barrier performance, indicated by a higher water permeability and lower water contact angles. BN acted as a nucleating agent, significantly enhancing crystallinity in all filled samples. The film containing only 0.06 wt% BN had an average crystalline content of 71.3 ± 1.3 %, versus 41.0 ± 2.5 % in pure PHB. While this low concentration of BN also improved the water barrier and mechanical performance (both strength and elasticity), higher BN content led to significantly increased water permeability, likely due to interfacial defects, highlighting the complex role of filler-polymer interactions. Overall, this thesis provides useful insight and understanding into how molecular interactions and structural and interfacial features influence the functional performance of PHB-based materials. Taken together, the findings from simulations and experiment demonstrate that the barrier and mechanical performance of PHB-based films cannot be attributed to any single factor, but instead arise from the interplay between crystallinity, microstructure and additive interactions. By combining molecular simulation with experimental characterisation, this work provides an integrated framework for understanding and tailoring these interdependent effects, offering practical insight for the design of compostable polymer films that balance sustainability with the functional demands of food packaging.
- Advisor / supervisor
- Mulheran, Paul
- Johnston, Karen
- Resource Type
- DOI
Relations
Items
| Thumbnail | Title | Date Uploaded | Visibility | Actions |
|---|---|---|---|---|
|
|
PDF of thesis T17509 | 2025-11-07 | Public | Download |