Evolution of surface free energy during thin-film polymerization of main-chain liquid crystalline polymers

Abstract

Through the use of a novel thin-film polymerization technique and the Lifshitz-van der Waals acid-base theory, we have determined, for the first time, the time evolution of contact angle and surface free energy during the polymerization (or molecular weight increase) of liquid crystalline poly(p-oxybenzoate/2,6-oxynaphthoate), poly(p-oxybenzoate), and poly(2,6-oxynaphthoate). Surface free-energy components of these main-chain liquid crystalline copolyesters and homopolyesters were calculated from contact angle measurements using a Ramé-Hart goniometer and three liquids (water, glycerol, and diiodomethane). Experimental data suggest that the Lewis base parameter, (γy-) of poly(p-oxybenzoate/2,6-oxynaphthoate) decreases rapidly with polymerization progression, but the Lewis acid parameter (γ+) and the Lifshitz-van der Waals parameter (γLW) do not vary significantly. The rapid decrease in the Lewis base parameter coincides with Fourier transform infrared spectra that indicate carboxyl and acetoxy peaks decreasing significantly in the early stages of a polycondensation reaction. In addition, since the activity of a naphthalene is less than that of a phenylene, an increase in naphthalene mole ratio in poly(p-oxybenzoate/2,6-oxynaphthoate) results in a decrease in the Lewis base parameter (γ-) of copolymers. Furthermore, the poly(p-oxybenzoate) homopolymer exhibits higher surface free energy including the γ+, γ-, and γLW parameters than does the poly(2,6-oxynaphthoate) homopolymer. This study reconfirms our previous experimental report that the wholly aromatic main-chain liquid crystalline polymer poly(p-oxybenzoate/2,6-oxynaphthoate) with a 73/27 mole ratio has a monopolar Lewis base character because its γ+ values were negligible. This conclusion can now be extended to this polymer with different mole ratios and to poly(p-oxybenzoate) and poly(2,6-oxynaphthoate) homopolymers. © 1999 American Chemical Society.