We present the synthesis and characterization of silica particles modified by a poly(ionic liquid), namely, poly[2-(methacryloyloxy)ethyl]trimethylammonium tetrafluoroborate (P[MATMA][BF 4 ]), which has a strong carbon dioxide absorption, and its application as a nucleation agent in polymer foaming. Silica particles with different particle sizes were fabricated by the sol–gel method and then grafted with (P[MATMA][BF 4 ]) brushes by surface-initiated atom-transfer radical polymerization. The modified silica particles (SiO 2 P[MATMA][BF 4 ]) were introduced into a polystyrene matrix by solution blending. The (P[MATMA][BF 4 ]) brushes grafted onto the silica particles were able to slightly increase the carbon dioxide solubility, and consequently, SiO 2 P[MATMA][BF 4 ] showed a higher heterogeneous nucleation efficiency in the foaming process compared to amino-functionalized silica particles (SiO 2 NH 2 ) used as control.