BROADBAND DIELECTRIC STUDY OF MISCIBLE POLYMER BLENDS WITH INTERMOLECULAR HYDROGEN BONDING

Open Access
Author:
Zhang, Shihai
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 30, 2003
Committee Members:
  • James Patrick Runt, Committee Chair
  • Paul C Painter, Committee Member
  • Ralph H Colby, Committee Member
  • Qiming Zhang, Committee Member
Keywords:
  • broadband dielectric spectroscopy
  • miscible polymer blends
  • hydrogen bonding
  • segmental relaxation
  • secondary relaxation
  • dynamic heterogeneity
  • concentration fluctuations
  • glass transition temperature
  • high pressure
Abstract:
The dynamics of miscible polymer blends with large differences in the components’ Tgs and with strong intermolecular hydrogen bonding were studied using, principally, broadband dielectric relaxation spectroscopy. The role of intermolecular hydrogen bonding on segmental dynamics was identified by comparing the present results with those from blends with similar chemical structures but without strong intermolecular interactions. It was found that hydrogen bonding is able to damp concentration fluctuations, which is demonstrated by the composition-insensitive segmental relaxation time distribution observed in blends with appropriate compositions. Intermolecular hydrogen bonding is also capable of coupling components’ segmental relaxations in blends with very large Tg difference, that would otherwise exhibit two segmental relaxation processes in the absence of strong intermolecular interactions. However, the coupling behavior is composition dependent and maximal coupling, resulting in a single segmental process, can only be achieved in blends dominated by inter-component interactions. Dynamic heterogeneity still exists in blends controlled by intramolecular interactions or with insufficient intermolecular hydrogen bonding. By comparing the dynamics of blends with various Tg differences, hydrogen bonding strengths and fractions, it was found that smaller dynamic asymmetry and stronger intermolecular interactions promote finer dynamic homogeneity. Whereas the above efforts are focused on the enthalpic contribution, increasing combinatorial entropy can also lead to dynamic homogeneity, as observed in polymer solutions with low molecular weight model compounds. The local relaxation of the hydrogen-bonded side groups, on the other hand, can either remain unchanged, be retarded, or suppressed, depending on the interaction strength and the mechanism of the relaxation itself. Finally, the influence of hydrostatic pressure on segmental dynamics was also studied on a selected blend, and it was found that elevated pressure lead to reduced dynamic heterogeneity due to the additional mobility gained by the hydrogen-bonded segments compared with the unassociated ones.