Chemical heterogeneity in poly [styrene-co-(butyl methacrylate)] copolymer latexes prepared using different monomer addition modes. A study by isopycnic centrifugation in density gradient

 Three different styrene-butyl methacrylate copolymer latexes were prepared by a uniform procedure but introducing styrene (S), butyl methacrylate (BMA), and minor amounts of acrylic acid (AA), in three different orders: i) simultaneous monomers addition, which yielded {P(SBMA)}; ii) addition of S (and half of the AA) followed by BMA (and the remaining AA), yielding {PS/PBMA} and iii) the inverse order, {PBMA/ PS}. Product characterization was done by centrifugation in density gradients coupled to scattered light scanning photometry of the centrifugation tubes. IR and NMR spectra were obtained from bulk polymer as well as from isopycnic centrifugation fractions. In agreement with findings of other authors, the particles produced by simultaneous monomer addition {P(SBMA)} are made out of the statistical copolymer, whereas sequential monomer addition leads to the formation of latex with homopolymer domains. IR and NMR spectra of {PS/PBMA} and {PBMA/PS} are identical but isopycnic density band profiles of all three samples are distinct. Acrylic acid residues are not detected in the dialyzed latex, using both IR and NMR. Spectra of latex isopycnic fractions do also show significant differences arising from their monomer chemical compositions, but isopycnic centrifugation and spectral data do not reveal any correlation between particle density and monomer composition. Isopycnic centrifugation can thus solve two problems on latex characterization: first, it is a high-resolution preparative technique, unmatched by any other separation method. Second, it yields latex particle fingerprints, which are dependent on particle chemical characteristics, rather than on particle diameters. Received: 19 March 1996 Accepted: 29 August 1996