Origin of Excess Light Scattering in Poly(methyl methacrylate) Glasses

The origin of the excess light scattering invariably observed in purified poly(methyl methacrylate) (PMMA) glasses was investigated. The isotropic heterogeneous structure with a dimension of about 1000 Å, which increases the scattering loss by at least 1 order of magnitude (to several hundred dB/km), is generated during polymerization below T_g at high conversion and is not due to high molecular weight, specific tacticities, formation of cross-links, or aging. It is clearly shown that the remaining monomer in concentrations of several weight percent does not aggregate spontaneously at high conversion to cause excess scattering unless polymerization takes place. The excess scattering is, we believe, mainly due to voids caused by the volume shrinkage accompanying polymerization of the remaining monomer trapped inside the polymer glass. The excess scattering model proposed in this paper shows that if a slight amount of monomer in the PMMA glass is polymerized in situ at high conversion, generating slightly localized voids due to the aggregation of the unreacted initiator, a rise of only 1 wt % in the conversion increases the scattering loss by several hundred dB/km, which can reasonably explain the experimental results obtained in this paper.