Title

Synthesis and characterization of novel phenylethynyl end-capped fluorinated polyimides

Date of Completion

January 2002

Keywords

Chemistry, Polymer

Degree

Ph.D.

Abstract

Aromatic polyimides exhibit good thermal stability, chemical and radiation resistance, and have been shown to be promising as high-temperature structural adhesives. These aromatic-based polyimide materials generally display high glass transition temperatures in the range of 200°C to 400°C or higher, which greatly depends on the stiffness of the backbone chain, and require high pressures and temperatures for adequate processing. While polyimides of reasonably high molecular weight are required for the development of adequate mechanical strength, it often preferred, for ease of production, and other reasons to employ lower molecular weight reactive, oligomeric imides containing substituents which will undergo chain extension and crosslinking reactions during thermal processing. One such system of is a series of phenylethynyl-terminated imide (PETI) oligomers, prepared from 3,3,4,4-biphenyltetracarboxylic dianhydride (BPDA) and aromatic diamines. However, due to the high melt viscosity encountered in these materials several approaches were investigated to address methods to reduce viscosity and to develop an understanding of the structure-viscosity relationship. ^ Therefore, a series of phenylethynyl end-capped polyimide oligomers based on 4,4-(2,2,2-trifluoro-1-phenylethylidene) diphthalic anhydride (3FDA) and 4,4-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) were synthesized. These oligomers, especially those with 3FDA in the backbone, had significantly lower melt viscosity than PETI-5 and other similar oligomers from the more planar BPDA monomer. The presence of the bulky trifluoromethyl and pendant phenyl groups of 3FDA suppresses interchain packing and introduces free volume to provide for a separation of the polyimide chains. The improved viscosity result comes with no significant compromise in the cured glass transition temperatures. In isothermal viscosity studies, oligomers containing 3FDA or 6FDA in the oligomer repeat unit had far better melt stability at 310°C relative to oligomers from BPDA, this is believed to be caused by a slowing of the free radical induced thermal cure of the ethynyl group. It is proposed that electron-withdrawing substituents in the 3FDA and 6FDA oligomides slow down the cure reaction, thereby stabilizing the oligomer melt. ^