Sánchez Valencia, Andrea
(2018)
Polystyrene blends: a rheological and solid-state study of the role of molecular weight distribution.
PhD thesis, University of Nottingham.
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Abstract
Commercial polymers are typically classified according to their melt flow indices, measures of their viscosities. These properties are known to depend on a material’s molar mass distribution, on its averages and its degree of polydispersity. In determining a polymer’s performance, both the molar mass distribution and the process employed to produce the part are highly relevant, since the balance in the mass fractions from its distribution will determine the flow characteristics in the mould, and influence the material’s performance. The compromise polymer manufacturers have to make is to maintain the mechanical properties known to improve with increased molar mass at the same time as a sufficiently low viscosity, known to reduce with decreasing molar mass, to enable part production. This is often achieved by judicious blending of homopolymers. This thesis examines how varying molar mass and distribution in blends leads to changes in the thermal, rheological, and mechanical properties in polystyrene, and discusses and develops physical models to capturing the observed experimental responses.
Chromatographic and calorimetric studies were carried out on monodisperse, bimodal blends of monodisperse, polydisperse, and blends of polydisperse polystyrenes. They revealed that changes in molar mass distributions and glass transition temperatures, Tg, could be directly attributed to the blending procedure of choice. In polydisperse blends, higher contents of low molar mass fractions, and corresponding lower Tgs were observed in the blends produced using a melt mixing method compared with solution-blended equivalents. Thermal degradation, accelerated by the large number of chain ends, was suggested as the cause for the increase in low molar mass fractions in the melt-mixed blends. The filtration and precipitation stages characteristic of solution blending instead promoted oligomer loss and evaporation, resulting in reductions in the low molar mass tails of the distributions.
Craze initiation stress was measured in 3-point bending isochronal creep tests on the same polymers and blends, and was found to in-crease rapidly with additions of a higher molar mass component, reaching a plateau at 20 wt%. A simple model based on a weighted addition of the crazing stress contributions of individual weight fractions was developed from an established piecewise linear crazing law in order to enable predictions of the crazing stress in the blends, using a power law exponent of 2.59 (90% CI [1.75 17.34]). In highly poly-disperse systems, where short unentangled chains dilute the polymer, it was necessary to include dynamic tube dilution theory. Dilution leads to a change in the entanglement length and hence in the molar mass at which transitions in the crazing mechanisms (disentanglement and chain scission) occur. With the improved model, crazing stress could be predicted even for highly polydisperse blends with wide and bimodal distributions.
Linear and non-linear rheological measurements were carried out in shear and extensions on the same materials. Existing rheological models for linear viscoelasticity including Likhtman-McLeish (L-M), Rubinstein-Colby (R-C) and polydisperse double reptation (pDR) theory were applied to the linear experimental data, exposing some of the fundamental difficulties of modelling the structure of systems where multiple chain-lengths interact. R-C was found applicable to bi-modal blends of monodisperse, whereas pDR was better able to model broad polydisperse blends. New non-linear shear and extensional rheology was recorded experimentally on all polymers and blends, and should enable future non-linear theories to be compared to experiment.
Item Type:
Thesis (University of Nottingham only)
(PhD)
Supervisors:
De Focatiis, Davide Graham, Richard S.
Keywords:
Molar mass, molar mass distribution, crazing, polymer blends, polydispersity, rheology
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