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Title: Simulation of molecular behaviour at polymeric interfaces
Author: Anderson, K. L.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2002
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Monte Carlo computer simulation methods were used to understand molecular behaviour of polymeric materials in the condensed state near an interface. The 'interfaces' studied were between an amorphous polymer and (a) air, (b) another amorphous polymer, and (c) a polymer crystal. Three specific cases were examined: interfacial induced ordering and dynamics, diffusion across an interface and crystallisation and phase transitions. A calibrated model of a polymer system illustrates the computational possibilities that now exist in what has mainly been an experimental and theoretical approach to polymer surface science. Molecular ordering at the polymer-air interface was examined for various molecular weights of atactic poly(styrene), and directly compared to experiment and theory. The actual radius of gyration of polymer chains was shown to be directly observable using scanning force microscopy and lattice chain simulations, leading to a new method of molecular weight characterisation. Furthermore, dynamic analysis of this system hinted at a depression of the glass transition temperature near the free surface. Dynamic simulation of polymer-polymer interfaces, where diffusion across the interface leads to a loss of order and eventual disappearance of the interface, led to very promising results. The diffusion and disordering processes were examined in the cases of polydisperse and semi-flexible chain systems, in which there is inherent ordering. In polydisperse systems, the lower molecular weight polymer solubilises larger polymers, leading to shorter weld-times. However, this does not necessarily lead to increased weld strength. Chain stiffness had a tremendous effect on weld strength, with stiff chains oriented normal to the weld plane producing the most interdiffusion and strength development. A modified version of the Sadler/Gilmer model was used to probe interfacial growth kinetics and thermodynamics for forming a polymer crystal. A study of the phase transitions from amorphous polymer to mobile polymer crystal, and then to stable crystal, was undertaken within the context of competing energies of the crystal-amorphous interface and bulk crystalline regions. The model reproduced the theoretical and the experimental results published to date. Given more experimental data, these predictions could be tested even more rigorously.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available