An in situ method for preparing a redox initiated bimorphological aqueous dispersion of polymer particles comprising the steps of: a) mixing together an aqueous dispersion of phosphorus acid-functionalized acrylic-based first seed polymer particles having a volume average particle size in the range of from 40 nm to 85 nm with an aqueous dispersion of acrylic-based second seed polymer particles comprising not more than 1 weight percent of phosphorus acid monomers and having a volume average particle size in the range of from 20 nm to 80 nm; b) contacting an aqueous monomer emulsion with the aqueous dispersions of the first and second seed polymer particles under redox initiated emulsion polymerization conditions to form a dispersion of 1) polymer particles with a protuberating phosphorus acid functionalized core; and 2) polymer particles without a protuberating core; and wherein the weight-to-weight ratio, measured using AFFFF method according to the description, of particles with the protuberating phosphorus acid functionalized core to particles without a protuberating core is in the range of 25:75 to 75:25; wherein the phosphorus acid-functionalized acrylic-based first seed polymer particles comprise structural units of: - methyl methacrylate or styrene or a combination thereof; - one or more acrylate monomers selected from the group consisting of ethyl acrylate, butyl acrylate, 2-propylheptyl acrylate, and 2-ethylhexyl acrylate; and - phosphoethyl methacrylate; and wherein the monomers comprise: - methyl methacrylate or styrene or a combination thereof; - one or more acrylate monomers selected from the group consisting of ethyl acrylate, butyl acrylate, 2-propylheptyl acrylate, and 2-ethylhexyl acrylate; and - not more than 1 weight percent of a phosphorus acid monomer. a) mixing together an aqueous dispersion of phosphorus acid-functionalized acrylic-based first seed polymer particles having a volume average particle size in the range of from 40 nm to 85 nm with an aqueous dispersion of acrylic-based second seed polymer particles comprising not more than 1 weight percent of phosphorus acid monomers and having a volume average particle size in the range of from 20 nm to 80 nm; b) contacting an aqueous monomer emulsion with the aqueous dispersions of the first and second seed polymer particles under redox initiated emulsion polymerization conditions to form a dispersion of 1) polymer particles with a protuberating phosphorus acid functionalized core; and 2) polymer particles without a protuberating core; and wherein the weight-to-weight ratio, measured using AFFFF method according to the description, of particles with the protuberating phosphorus acid functionalized core to particles without a protuberating core is in the range of 25:75 to 75:25; - methyl methacrylate or styrene or a combination thereof; - one or more acrylate monomers selected from the group consisting of ethyl acrylate, butyl acrylate, 2-propylheptyl acrylate, and 2-ethylhexyl acrylate; and - phosphoethyl methacrylate; and - methyl methacrylate or styrene or a combination thereof; - one or more acrylate monomers selected from the group consisting of ethyl acrylate, butyl acrylate, 2-propylheptyl acrylate, and 2-ethylhexyl acrylate; and - not more than 1 weight percent of a phosphorus acid monomer.