Positron annihilation lifetime spectra have been measured on styrene-methyl methacrylate, styrene-acrylonitrile, and styrene-butyl methacrylate copolymers. The results show that the longest lifetime tau(3) remains constant during extended PALS measurement in all experiment copolymers, and relative intensity I-3 decreases to a certain extent with measured time in weak polar copolymers and remain almost unchanged in strong polar copolymers as well as ST-BMA copolymers. The observed decrease in I-3 have been found to be unrelated to the microstructural change of copolymers, and, instead, to be more likely a result of the buildup of an electric field inside the copolymers during the prolonged PALS measurement. The field effect can result in the decrease of I-3 due to the increase in the positrons and electrons diffusing out of the spur, but the influence of the electric field on I-3 decrease with increasing the polarity of the copolymers and the softness of side groups of macromolecular chains in the copolymers.