Reaction-Induced Strain in Rigid-Rod Polymeric Fibers

Abstract

The thermomechanical behavior of several rigid-rod polymeric fibers has been investigated. Up to 300 °C, all fibers exhibited the expected axial shrinkage on heating (CTE ≈ -6 × 10 -6 °C-1 ). However, a pronounced increase in thermal contraction was observed at temperatures where polymer degradation or cleaving of the pendant group occurred. A degradation mechanism has been proposed that accounts for the evolved gases in PBZT and MePBZT. The accelerated shrinkage in these rigid-rod polymers is a result of decrease in the c-axis lattice parameter as measured via WAXD. The enhanced axial shrinkage and accompanying decrease in lattice parameter are attributed to chemical changes, and consequent cross-linking taking place within these systems which, in turn, serves to perturb the crystalline structure. The implications of this phenomenon of accelerated axial contraction, with regards to morphology, are discussed.

The effect of hydrogen bonding on the physical and mechanical properties of rigid-rod polymers

“…Other factors, such as chain flexibility (i.e., conformational flexibility) and consequent morphological differences, may also influence the compressive strength of these materials, as discussed elsewhere. 7 A linear correlation has been observed between the compressive strength and torsional modulus for a number of highly oriented fibers. 13,15 The fiber torsional modulus can give a macroscopic measure for the shear modulus between chains, but it is greatly influenced by voids or any transverse texture present in the fiber.…”

“…On the basis of the crystal structures of PPTA, 3 MePBI, 7 and PIPD, 5 the intermolecular hydrogen-bond strengths calculated with ab initio techniques are given in Table III. The repeat units of PIPD and PPTA have four groups involved in hydrogen bonding, whereas MePBI has two.…”

“…Crystalline structures have been reported for PPTA, 3 MePBI, 7 and PIPD. 5 The head and tail atoms were fixed in space to prevent molecular conformations impossible to realize in the bulk polymer.…”

“…6 The structure and properties of methyl pendant poly-(p-phenylene benzobisimidazole) (MePBI), which forms a sheetlike arrangement of hydrogen bonds, have been reported elsewhere. 7 The compressive strengths and torsional moduli of MePBI and PIPD are greater than those of other rigidrod polymeric fibers that do not have the capacity to form intermolecular hydrogen bonds. Attempts have been made to introduce hydrogen bonding in PBZT through the incorporation of hydroxy groups on the phenyl ring.…”

Rigid‐Rod Polymers

“…The accelerated shrinkage in these rigid-rod polymers is a result of decrease in the c-axis lattice parameter. The enhanced axial shrinkage and an accompanying decrease in the lattice parameter are attributed to chemical changes, and consequent crosslinking taking place within these systems which, in turn, serves to perturb the crystalline structure (309). Thermal-induced chemical changes and cross-linking are considered the reasons for the accelerated shrinkage (286).…”