Dynamic viscoelasticity and FT-IR measurements of end-crosslinking alpha,omega-hydroxyl polybutadiene solutions near the gel point in the gelation process

Dynamic viscoelasticity of alpha,omega-dihydroxyl polybutadiene/p-xylene system is studied through gelation process by end-crosslinking reaction with crosslinker which has three isocyanate groups. The gel point is determined as the reaction time t = t(c), at which the storage and the loss shear moduli, G'(omega) and G ''(omega), become both proportional to omega(n) over the whole omega range measured, where omega is the angular frequency and n is a critical exponent at the gel point. The relative extent of chemical reaction p(t) is estimated from a decrease in the absorption peak of the isocyanate residue by FT-IR measurement to estimate precisely epsilon = \p(t) - p(c)\/p(c), the relative distance from the gel point (p = p(c)). Effects of polymer concentration C on the increase of zero shear viscosity eta(0) before the gel point and on the increase of equilibrium modulus G(cq) after the gel point are examined in terms of critical exponents of k and z for power laws, eta(0) similar to epsilon(-k) and G(cq) similar to epsilon(=), respectively. The exponents n and z are found to take universal values of n = 0.67 +/- 0.02, and z = 2.5 +/- 0.1 independent of C in the lower concentration of C < 0.6 where intermolecular interaction, i.e., hydrogen bonding between hydroxyl groups at chain ends are negligible. The k monotonously increases with C, and the scaling relation among the three exponents appears only applicable in the limited C range. These anomalous critical behaviors are discussed in terms of hydrogen bond coupling in the prepolymer solutions and chemical reaction scheme of this system.