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

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 (γ^LW) 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 γ^LW 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.