A series of polyimides was synthesized to facilitate the study of several structure/property relationships, including relative permittivity, moisture absorption, free volume, and viscoelastic transitions. This series was based on hexafluoroisopropylidene bis(phthalic anhydride) and various diamines, with the intention of having a number of fluorine/hydrogen and symmetric/asymmetric analog sets. Since several of the fluorinated diamines were exceptionally unreactive, a multistep polycondensation polymerization technique was developed to obtain materials of sufficient molecular weight for property evaluation. Effects of fluorine incorporation on the aforementioned properties are reported.

The often observed decrease in relative permittivity ( ′) and refractive index (n) that accompanies fluorine substitution is the result of several effects. Fluorine replacement for hydrogen is known to decrease local electronic polarization and is thought to frequently increase fractional free volume. Both of these effects can independently decrease ′ and n, and this paper attempts to assign the observed decreases between these mechanisms for a series of hydrogen/fluorine polyimide analogs.

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SynopsisEpoxy resins of varying epoxy equivalent weight were crosslinked with 4,4*-dithidianiline (DTDA) at a 2/1 mole ratio. Resin of equivalent weight 190 (Epon 828) was cured with DTDA at mole ratios varying from 2/1 to 1.25/1. Properties of cured resins were evaluated and their reduction was investigated. Under the conditions used, reduction to the point of complete solubilization was possible providing crosslink density did not exceed a threshold value tentatively established as M, = 400 -500. Recurring of reduced resins was accomplished by oxidation and by reaction of thiol groups with polyfunctional reagents, including bismaleimides and pol yepoxides.

SynopsisThe objective of the research is to investigate epoxy resins where crosslinks formed in curing reactions can be cleaved and reformed without significant sacrifice in resin properties. In this phase of the investigation, experimental disulfide-containing crosslinking agents have been studied, including dihydrazides of dithiodicarboxylic acids (aliphatic and aromatic) and dithio-aromatic diamines. Conditions for curing reactions and for subsequent cleavage of crosslinks by reduction have been determined in model systems and confirmed for a prototype epoxy resin. A preliminary comparative evaluation of cure kinetics and of crosslink density and thermal and mechanical properties has been carried out for this resin cured with aromatic curing agents (4,4'-dithiodianiline and methylene dianiline) and with aliphatic compounds (3,3'-dithiopropionic acid dihydrazide and hexamethylene diamine). It has been shown that in a resin fully cured with disulfide-containing crosslinking agents, crosslinks could be ruptured by reduction, and reestablished by oxidation under mild conditions.