Structure and Dynamics of Microphase Separated Polymers Containing Strong Associations

Open Access
Author:
Castagna, Alicia Marie
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 22, 2012
Committee Members:
  • James Patrick Runt, Dissertation Advisor
  • Janna Kay Maranas, Committee Member
  • Michael Anthony Hickner, Committee Member
  • Ralph H Colby, Committee Member
Keywords:
  • polymer
  • ionomer
  • Small-Angle X-ray Scattering
  • Dielectric Relaxation Spectroscopy
  • polyurea
  • polyurethane
  • molecular relaxation
Abstract:
The morphology and dynamics of microphase separated polymers containing strong associations (i.e., ionic associations and hydrogen bonding) are investigated in this dissertation. The microphase separated domains in these polymers act as physical crosslinks and are expected to strongly influence molecular dynamics. Small-angle X-ray scattering (SAXS) is utilized to quantify microphase separation characteristics and broadband dielectric relaxation spectroscopy reveals the sensitivity of polymer dynamics to the presence of microphase segregation in the polymers studied. A model ionomer, sulfonated polystyrene (SPS) is chosen to probe the effect of ionic aggregation. The microphase separation and dynamics in polyurethanes and polyureas, containing strongly hydrogen bonded hard domains, are also examined. The role of ion associations on aggregate morphology and polymer dynamics of SPS is investigated via the systematic variation of sulfonation level, neutralization, and ion type. Evidence of acid group aggregation was found at 3.5, 6.7 and 9.5 mol% sulfonation. Upon neutralization, spherical aggregates ~2 nm in diameter are revealed from SAXS and scanning transmission electron microscopy. Aggregate size is found to be independent of degree of sulfonation and neutralization level, however, aggregate composition becomes increasingly ionic with increasing neutralization. The polymer segmental relaxation process is highly sensitive to changes in ion content, neutralization and ion type. The relaxation time of this process slows with increasing ion content as the number density of ionic aggregates increases, similar to the effect of chemical crosslinking. The breadth of this process is sensitive to the interaction strength of the neutralizing ion type. For SPS neutralized with Zn2+, two distinct segmental relaxations are observed, a matrix segmental relaxation and a slow segmental process, the strength of which correlates with an increase in volume fraction of the region of restricted mobility. In addition to polymer dynamics, a process proposed to arise from Maxwell-Wagner-Sillars (MWS) interfacial polarization is observed in select ionomers and the relaxation time of this process in SPS neutralized with Zn is found to exhibit good agreement with a simple composite model. In addition to the sulfonate polystyrene ionomers, polyurethanes and polyureas based on poly(tetramethylene oxide) soft segments and methylene diphenyl diisocyanate hard segments are investigated. In polyurethanes, increasing hard segment content is found to slow down, increase the breadth, and decrease the strength of the soft phase segmental dynamics, analogous to the effect of crystalline lamellae. The strength of the MWS process observed decreases with temperature and correlates directly with an order-to-disorder transition, characterized in a previous publication. The role of soft segment molecular weight on morphology and dynamics is investigated in polyureas. The hard domains microphase separate into ribbon like domains. Phase separation is rather incomplete and its extent decreases with decreasing soft segment molecular weight. Two distinct soft phase segmental relaxations are observed in these polyureas and are proposed to arise from the soft segment rich phase and segmental motion in the slower restricted mixed phase, similar to the slow segmental process observed in SPS ionomers neutralized with Zn.