Rigid-rod polymers with enhanced lateral interactions

Abstract

Poly{(benzo-[1,2-d4,5-d′]-bisthiazole-2,6-diyl)-1,4-phenylene} (PBT) and poly{(benzo-[1,2-d:5,4-d′]-bisoxazole-2,6-diyl)-1,4-phenylene} (PBO) are rigid-rod molecules with extended chain conformation. Fibers and films processed from these polymers have very high tensile strength and tensile modulus, but their performance under compression has been disappointing. Much work has been done in various ways to correct this problem. Most workers have focused on approaches that enhance lateral interaction of these polymers to increase the compressive strength of the fibers and films. The methods included cross-linking by coupling of free radicals generated by heat-treatment at above and below PBT and PBO decomposition temperature and through o-quinodimethane intermediate from benzocyclobutene, imbedding sol–gel glass or a thermoset matrix, synthesis of two-dimensional PBO, and introduction of hydrogen bonding in PBT fiber. Despite the apparent achievement of crosslinking and intercalation, modest changes in fiber compressive strength have been reported, often at the expense of tensile strength. On the contrary, results on improving PBT film compressive strength by inclusion of sol–gel glass and PBO film delimination resistance by imbedding a thermoset matrix were very encouraging. Recent reports on a new rigid-rod fiber claimed to have outstanding compressive strength are also included in this review.

Introduction

The US Air Force (USAF) was engaged in and sponsored the rigid-rod polymer research program in the 1970s and 1980s and directed it toward the development of new structural materials having low density, high strength, high modulus, and long-term retention of these properties at elevated temperatures [1], [2], [3], [4]. The focus was on two linear polymers: poly{(benzo-[1,2-d:4,5-d′]-bisthiazole-2,6-diyl)-1,4-phenylene} (PBT) and poly{(benzo-[1,2-d:5,4-d′]-bisoxazole-2,6-diyl)-1,4-phenylene} (PBO), which are rodlike polymers with extended chain conformation. These polymers have excellent thermal and oxidative stability and solvent resistance [5], [6], [7]. Fibers prepared from these polymers have superior tensile strength and modulus, cut and abrasion resistance, and flame retardance [8].

A shortcoming of PBT and PBO (PBX is used for both PBT and PBO) fibers is their low compressive strength, which may result in compressive failure in structural composites reinforced by these fibers. Improvement of PBX fiber compressive strength has been an active research area since the beginning of the rigid-rod polymer program at the USAF [4], [9], [10]. Many workers believe that the PBX fiber compressive failure mechanism is by buckling of the chain [11]. Binding the chains together by crosslinking or some other means would laterally stabilize the chain and thereby increase its resistance to buckling. As a result, the compressive strength of the fiber would increase.

PBX polymers can be processed into films, which have also been investigated [12]. Although PBX films have exceptional strength, stiffness, and thermal stability, their compressive strength and delamination resistance is low. Binding the polymer chains together may also improve these properties of PBX films.

This article reviews different approaches to enhancing the lateral interaction of the PBX polymers to increase compressive strength of the fibers and compressive strength and delamination resistance of these films. Section 2 is a brief review of PBX chemistry to provide readers with background information. 3 PBX fiber with enhanced lateral interaction, 4 PBX film with enhanced lateral interaction discuss the development of enhanced lateral interaction in PBX fiber and film, respectively. Section 5 covers the work by McGarry and Moali on coating PBO fiber with a stiff ceramic material to improve its compressive strength, although their approach might arguably not involve enhanced lateral interaction across the fiber [13]. Methods for PBO fiber compressive strength improvement by surface modification [14] and by changing processing conditions [15] are not within the scope of this paper. Recently, workers at Akzo Nobel reported the synthesis of poly{2,6-diimidazo[4,5-b:4′–5′-e]pyridinylene-1,4-(2,5-dihydroxy)phenylene (PIPD). Heat-treated PIPD fiber is reported to have an outstanding compressive strength among organic fibers. A review on PIPD is included in Section 6.