An Investigation of the Importance of Screening Effects in Polymer Solutions and Blends

Open Access
Author:
Berg, Lennart Paulmann
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 22, 2005
Committee Members:
  • Paul C Painter, Committee Chair
  • Ronald C Hedden, Committee Member
  • James Patrick Runt, Committee Member
  • Ramanathan Nagarajan, Committee Member
Keywords:
  • intramolecular screening effects
  • thermodynamic modelling
  • concentration dependence of the interaction parame
Abstract:
In this work a closer look will be taken at the importance of screening effects in both polymer solutions as well as polymer blends. It is essentially a natural extension of earlier work in this group with functionalized polymer blends, where it was proposed that the number of intramolecular contacts far exceeded what one would expect from the classical mean field theory. Models with varying degrees of complexity were investigated, where intramolecular contacts, in each case, were treated explicitly. First a very simple modification of the classical Flory-Huggins theory is investigated, and it is shown that it is possible to reproduce the concentration dependence of the interaction parameter in polymer solutions with surprising accuracy. It is also shown that the calculated spinodals were in good agreement with experimentally reported data, especially at higher molecular weights. Free volume effects are then accounted for through a modification of the Flory-Orwoll-Vrij Equation of State model. The model was tested for a number of systems at θ-conditions as well as non θ-conditions, for its ability to reproduce the concentration dependence of the interaction parameter, using solubility parameters as well as internal pressures, with varying degrees of accuracy. Another simple modification of the Flory-Huggins theory for polymer blends is presented. It is shown that the model is able to reproduce the very peculiar phenomenon observed in the behavior of the interaction parameters determined from Small Angle Neutron Scattering (SANS) in isotopic polymer blends, by varying the screening factor for one of the components by less than 3 %. Finally it is shown that a further modification to accommodate copolymer blends, leads to a model which is equally well equipped to describe the strong concentration dependence of the interaction parameter observed experimentally. A few assumptions are made in these calculations, and the most important of these is that the copolymers are treated as homopolymers. Since only isotopic copolymer systems were investigated it is presumably a safe assumption. It was found that a small change of less than 4 % in the screening factor of one of the components was enough to obtain extremely good correspondence with the experimentally observed data.