EFFECT OF SHEAR ON THE GELATION OF UV-CURABLE POLYMERS

Ultraviolet radiation cured polymeric systems, useful in photonic applications, are obtained through free radical polymerization of mixtures containing monomers and oligomers. We used an in situ curing technique within a rheometer to study the effect of large amplitude oscillatory shear on sample gelation. Below the critical gel point (pre-gel state), we found no effect of shearing and the samples were linearly viscoelastic for large strains. Materials in the post-gel state, on the other hand, showed strain hardening with increasing strain. Measurements of moduli versus frequency following the shearing showed significant increases in the moduli indicative of shear induced gelation. This behavior can be explained by the presence of trapped free radicals existing both as small molecules (e.g., oligomers) and as part of the polymer network. Shear (i) causes the immobile radicals, which are part of the network, to encounter other reactive sites, and, (ii) enhances the diffusion of the small molecule radicals, both of which lead to increased cross-linking. While rheological tests are consistent with this mechanism, direct confirmation of the presence of free radicals was obtained from electron spin resonance (ESR) measurements done on pre- and post-gel samples.