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Title: Chain-length dependent rheology and relaxation dynamics in glass-forming polymers
Author: Reynolds, Matthew
Awarding Body: University of Leeds
Current Institution: University of Leeds
Date of Award: 2018
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Glassy materials differ greatly from crystalline solids; their lack of long range order makes it difficult to model their behaviour. While a lot of work has been done regarding the properties of glass-forming polymers, their exact nature is not well understood. This thesis primarily focuses on the chain-length dependence of glassy dynamics, in particular poly(methyl methacrylate) (PMMA), which is of interest due to its commercial and industrial applications. Using dielectric spectroscopy, rheology, and calorimetry, the relaxation behaviour of chain-modes and the segmental, alpha, relaxation were determined as a function of chain-length. Time-temperature superposition adequately describes the rheology data, even though decoupling between chain-modes and segmental relaxations were observed. Changes occur in the behaviour of the glass transition temperature, T_g, at the molecular weight, M, of the "dynamic bead", M_R. Relaxation times of the alpha relaxation and chain-modes of PMMA and other polymer systems collapse when renormalized by T_g, suggesting universal behaviour. This occurs when the number of correlated monomers, N_a, in the alpha relaxation corresponds to M_R. N_a was determined using modulated calorimetry and dielectric spectroscopy. A clear change in N_a was observed at M_R for less flexible PMMA and polystyrene, whereas this was less pronounced for the more flexible poly(dimethyl siloxane). This may relate to a change from intermolecular to mainly intramolecular behaviour. Furthermore, for PMMA the activation enthalpies of the alpha and beta relaxations below T_g are approximately equal at M_R, suggesting these relaxations act on similar lengthscales. The activation enthalpy of the beta relaxation also becomes M invariant for M > M_R, suggesting M_R characterises the beta relaxation. Finally, the ionic conductivity was determined for PMMA and two poly(propylene glycol) (PPG) chain-length systems. The alpha relaxation and conductivity were coupled for PPG, whereas for PMMA decoupling occurred for M > M_R. This demonstrates that polymer behaviour leads to this decoupling in PMMA. We also show that non-polymeric systems do not exhibit this decoupling behaviour.
Supervisor: Mattsson, Johan ; Hine, Peter Sponsor: EPSRC
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available