SYNTHESIS AND CHARACTERIZATION OF MIXED-SUBSTITUENT POLY(ORGANOPHOSPHAZENES)

Open Access
Author:
Maher, Andrew Elessar
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
November 20, 2003
Committee Members:
  • Harry R Allcock, Committee Chair
  • Alan James Benesi, Committee Member
  • Karl Todd Mueller, Committee Member
  • James Patrick Runt, Committee Member
Keywords:
  • polymers
  • elastomers
  • ion-conductive
  • polyphosphazenes
Abstract:
This thesis focuses on the synthesis and characterization of mixed-substituent poly(organophosphazenes). The work in chapters 2 through 4 examines mixed-substituent polyphosphazenes with fluoroalkoxy side groups. Chapters 2 and 3 involve a synthetic route to mixed-substituent polyphosphazenes via side group replacement of fluoroalkoxy substituents. The thermal and mechanical properties of mixed-substituent poly(fluoroalkoxyphosphazenes) are examined through varying the ratios of two fluoroalkoxy substituents. These structure-property relationships and the potential use of these materials as fluoroelastomers are the subjects of chapter 4. The specifics of chapters 2-4 are summarized below. The work in chapter 5 concerns the synthesis and evaluation of mixed-substituent polyphosphazenes as single-ion conductors. The synthesis of a sulfonimide functionalized side group for proton conducting fuel cell applications is the subject of the appendix and is also utilized in the work in chapter 5. The specific details of the ionic conducting polymer studies are also summarized below. In chapter 2, several poly(alkoxyphosphazenes) and poly(fluoroalkoxyphosphazenes) were synthesized and allowed to react with various organic nucleophiles. Reactions were carried out at room temperature (25 oC) and at 67 oC in tetrahydrofuran (THF) and were monitored by 31P NMR spectroscopy. The possible use of single-substituent polymers as hydrolytically stable macromolecular intermediates for mixed-substituent polyphosphazene synthesis is discussed and factors that influence side group replacement in poly(organophosphazenes) are examined. In addition, evidence for a random, irreversible SN2-like mechanism of side group replacement is presented. The work in chapter 3 is complementary to chapter 2 and examines the equilibrium exchange reactions of fluoroalkoxy substituents and fluoroalkoxy nucleophiles. In chapter 4, a series of poly(fluoroalkoxyphosphazenes) containing a mixture of 2,2,3,3,4,4,5,5-octafluoropentoxy and 2,2,2-trifluoroethoxy substituents was synthesized. The series included polymers with 25-94% incorporation of the trifluoroethoxy substituent, as well as single-substituent polymers with 100% octafluoropentoxy or trifluoroethoxy side groups. The polymers were analyzed by multi-nuclear NMR spectroscopy, gel permeation chromatography (GPC), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and were subjected to limiting oxygen index and microtensile testing. The variation in thermal and mechanical properties as a function of the side group ratios is discussed and polymers that may be suitable for fabrication into low temperature elastomer materials are identified. The use and evaluation of mixed-substituent polyphosphazenes as single ion conductors is described in chapter 5. Polyphosphazenes with both 2-(2-methoxyethoxy)ethoxy and lithiated sulfonimide functionalized side groups were synthesized and characterized using multi-nuclear NMR, GPC, flame atomic absorption (FAA) spectroscopy, elemental analysis (EA), and DSC. The sulfonimide functional group was lithiated via dialysis with aqueous lithium chloride solutions. Lithiation was confirmed by FAA spectroscopy and EA. The polymers were examined for their behavior as single ion conductors using impedence analysis. The dependence of the conductivity on the macromolecular motion of the polymers and on the amount of sulfonimide functionalized side group is discussed. Chapter 6 briefly describes potential future research directions based on the work presented in chapters 2 through 5. The appendix describes the synthesis of the sulfonimide functionalized side group. This work also examines this side group for use in polyphosphazene proton conducting fuel cell membranes.