Relaxation Behavior of Polyimides Based on 2,2'-Disubstituted Benzidines

Abstract

The mechanical relaxation behavior of polyimides based on a variety of 2,2'-disubstituted benzidines and rigid dianhydrides was investigated. Two transitions were observed in these polyimides. The glass relaxation process is relatively weak and occurs at high temperatures due to the linear and rigid nature of these polyimides. The subglass relaxation is very prominent in these polyimides and is due to main-chain rotational motion localized within the diamine (benzidine) segment. Changes in the dianhydride moiety have little effect on the temperature of the subglass transition and result in only minor changes in the magnitude of this relaxation. The presence of 2,2'-CF3 substituents on the benzidine moiety increases the magnitude and shifts the subglass relaxation approximately 150 °C to higher temperatures versus Cl or CHs in these positions. Incorporating a flexible ether linkage between the phenyl rings of the benzidine and the CF3 side group (e.g., OCF3) substantially reduces the temperature and to some extent the magnitude of the subglass relaxation. Replacement of the 2,2'-disubstituted benzidine unit (two phenyl rings) with one (benzene) or three (terphenyl) unsubstituted phenyl rings results in a substantial decline in both the temperature and magnitude of the subglass relaxation. Molecular modeling was used to clarify the nature of the subglass relaxation. Rotational energy barriers for the 2,2'-disubstituted benzidines, calculated from both semiempirical and density functional quantum mechanical calculations, are comparable in magnitude to the experimentally determined activation energies for the subglass relaxation.

High thermal stable polyimide resins derived from phenylethynyl-endcapped fluorenyl oligoimides with low melt viscosities

“…Additionally, the presence of bulky pendent fluorene groups restricted the segment mobility, enhanced the rotational barrier of polymer chains and increased the time to complete glass process, which also led to a higher T g . Because of the rigidity of the polymer chains, the subglass relaxation was hardly observed in the DMA tan curve [29] . Figure 12 depicts the dependence of dielectric constants of the thermoset polyimide resins on frequencies in the range of 812 GHz at ambient temperature.…”

The effect of zeolite L content on dielectric behavior and thermal stability of polyimide thin films

“…Typically, three relaxation processes are observed with increasing temperature designated as c, b, and a, respectively, with a corresponding to the glass-rubber relaxation. There are some reports, which refer to the characterization and analysis of the electrical and dynamical properties of polyimide hybrid films [36][37][38][39][40][41][42][43][44]. The properties of these materials are dependent on the interphase between filler and the polymer matrix.…”

Synthesis and properties of aromatic polyimides derived from 2,2,?3,3?-biphenyltetracarboxylic dianhydride

“…It has been demonstracted that the 2-and 2Ј-substituted 4,4Ј-diaminobiphenyl produces polyimides with optical transparence, high birefringence and enhanced solubility. [7][8][9][10] The introduction of pendant groups at the 2-and 2Ј-positions of the biphenyl dianhydride also improves the solubility and the thermal stability of the resulting polyimides. [11][12][13] The steric repulsion of the substituents in the 2-and 2Ј-positions of the biphenyl moieties apparently forces the two phenyl rings into adoption of a non-coplanar conformation, which, in turn, inhibits chain packing and crystallization, and thus increases solubility.…”