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 molecular mass and chemical composition. The characterization of these joint distributions by a single technique [e.g., size-exclusion chromatography (SEC)] is hindered by the effects of both variables on the separation mechanism. Separation emphasizing chemical composition heterogeneity can be efficiently performed by gradient elution high-performance liquid chromatography (HPLC) combining precipitation and adsorption retention. Comparison of Fourier transform IR and evaporative light-scattering detection indicated decreasing polydimethylsiloxane (PDMS) macromonomer 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 solvent for PDMS (tetrahydrofuran) with low-angle laser-light scattering, differential viscometry, and differential refractive index detection allowed determination of the molecular mass of both the whole copolymer and that of the PMMA backbone for each HPLC fraction. Comparison with an independent SEC determination of the PDMS macromonomer molecular mass allowed estimation of the number of pendant PDMS chains per graft copolymer molecule across the HPLC chromatogram. Results indicated a relatively constant incorporation of the number of PDMS side chains with increasing PMMA backbone molecular mass, leading to a relative decrease in weight fraction PDMS incorporation with increasing molecular mass of the whole graft copolymer molecule.