Ionic naphthalene thermotropic copolyesters: Divalent salts and tensile mechanical properties

Novel ionic naphthalene thermotropic polymers (NTPs), based on wholly aromatic copolyesters, were synthesized, and their tensile mechanical properties were investigated. These ionic NTPs, containing 1 mol % ionic groups with divalent metal counterions (Ba, Ca, Mg, and Zn), exhibited thermotropic liquid crystallinity. Among them, the Ca salt NTP maintained a high molecular weight and exhibited excellent thermal and mechanical properties. Systematic comparison was made among nonionic NTP, ionic NTP (Na salt), and ionic NTP (Ca salt), since they possessed similar molecular weights and since their film specimens were made under similar processing conditions. Ionic NTP with monovalent Na ions showed a moderate increase in tensile modulus and strength over nonionic NTP. However, ionic NTP with divalent Ca ions showed a significant increase: i.e., 76% increase in modulus and 147% increase in strength. An increase in tensile properties arises from enhanced lateral support, via ionic bonds (cross-links), between highly aligned NTP chains. Ionic cross-links can effectively enhance tensile properties of the LCPs, unlike covalent cross-links, which are less effective. This arises from the nondirectional nature of ionic bonds, which makes the bonds thermally labile; thus chains aligned under elongational flow at high processing temperature are reinforced effectively upon cooling. Tensile fracture surface morphologies of ionic NTPs were also investigated as a function of counterion.