Hyperbranched Aramids by the A2 + B3 versus AB2 Approach: Influence of the Reaction Conditions on Structural Development

Abstract

Hyperbranched aromatic polyamides (pPDT) from A₂ (p-phenylene diamine)+ B₃ (trimesic acid) reactants have been prepared using direct polycondensation in solution. The influence of various polymerization parameters on the polymer structure development and the sol content, as well as on gel formation, has been thoroughly investigated. Prior to the attainment of the gel point, whose occurrence is linked quite strongly to the reaction conditions, the resultant hyperbranched polymers are fully soluble even at room temperature, not only in concentrated H₂SO₄, but also in aprotic polar solvents (DMF, NMP, DMAc, DMSO). At 80°C, under typical experimental conditions, gel formation starts just under 120 min. At a higher temperature (115°C), the attainment of crosslinking reactions is accelerated (gel point occurs at about 20 min). Both the presence of added salt (LiCl) to the reaction medium at 80°C and the use of highly purified reagents promotes earlier gel occurrence. By increasing the molar ratio of primary amino to carboxyl groups from two-thirds to one, the network formation is delayed. A molar ratio of 1/2 leads to the formation of oligomers only.

Both the polymer structure development and the degree of branching of pPDT have been evaluated by NMR spectroscopy as functions of reaction conditions. As compared to pPDT, polymers prepared from the homologous AB₂ monomer show similar intrinsic viscosity and glass transition temperature, but higher molecular weights and distinct differences in the relative ratio of the various structural units. The lyotropic and thermotropic behaviour of both polymers has been found.

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