COMPOSITIONAL DISTRIBUTION CHARACTERIZATION OF POLY(METHYL METHACRYLATE)-GRAFT-POLYDIMETHYLSILOXANE COPOLYMERS

Graft copolymers prepared by radical polymerization of a low-molecular -mass monomer with a macromonomer display heterogeneity in both molecu lar mass and chemical composition. The characterization of these joint distributions by a single technique [e.g., size-exclusion chromatogra phy (SEC)] is hindered by the effects of both variables on the separat ion mechanism. Separation emphasizing chemical composition heterogenei ty can be efficiently performed by gradient elution high-performance l iquid chromatography (HPLC) combining precipitation and adsorption ret ention. Comparison of Fourier transform IR and evaporative light-scatt ering detection indicated decreasing polydimethylsiloxane (PDMS) macro monomer incorporation corresponding to increasing retention time for a poly(methyl methacrylate) (PMMA)-graft-PDMS copolymer. More detailed information was obtained by multidetector SEC of composition fractions from gradient elution HPLC. SEC separation in an isorefractive solven t for PDMS (tetrahydrofuran) with low-angle laser-light scattering, di fferential viscometry, and differential refractive index detection all owed determination of the molecular mass of both the whole copolymer a nd that of the PMMA backbone for each HPLC fraction. Comparison with a n independent SEC determination of the PDMS macromonomer molecular mas s allowed estimation of the number of pendant PDMS chains per graft co polymer molecule across the HPLC chromatogram. Results indicated a rel atively constant incorporation of the number of PDMS side chains with increasing PMMA backbone molecular mass, leading to a relative decreas e in weight fraction PDMS incorporation with increasing molecular mass of the whole graft copolymer molecule.