Phase separation of trimethylolpropanetriacrylate/poly( styrene-co-methylmethacrylate) induced by UV-irradiation ( photo-PIPS) has been studied. Three poly( St-co-MMA) with St:MMA monomer ratio of 25: 75, 50: 50 and 75: 25 were synthesized with average molecular weight of Mw approximate to 5 x 10(4) and average Tg being approximate to 345 K, due to stereoirregularity in their structure. Difference in TMPTA/poly( St-co-MMA) blends compatibility caused different extents of photo-PIPS and final conversion status depended on the copolymer type and amount. Optical microscopy used to study time-evolution of photo-PIPS. SEM observation showed that, within studied range of copolymer content, droplet domains of copolymer-rich phase appeared in TMPTA-rich matrix through viscoelastic photo-PIPS and in copolymer-rich domains network-like structures of TMPTA were formed. Also some columns were created due to copolymer extrusion to the surface of the sample. Size of the copolymer-rich domains, their formation procedure and size of the columns depended on the copolymer type and amount. SEM Carbon-mapping confirmed copolymer extrusion to the surface. Based on AFM phase-mode images medium-, low-and high-modulus regions were detected and assigned to the TMPTA-rich matrix phase, copolymer-rich phase and TMPTA-rich phase of copolymer-rich domains, respectively. These corresponds to peaks of tan delta curves used to determine mentioned phases composition. (C) 2018 Published by Elsevier Ltd.