formed by the treatment (16). The present study focuses on the effect of temperature and crosslinking on the molecular mobility of the plasma-
The dynamic behavior of the oxygen-containing polar functional treated polymer surfaces. groups on the plasma-treated polymer surfaces has been studied As for temperature dependence, Taru et al. and Occhello et al. have in relation to the polymer-chain mobility with regard to the effect reported on plasma-treated polystyrene and/or polypropylene, but they have of the aging temperature and the crosslinking density of the polynot excluded the effects of low molecular-weight products. mer. The films of polystyrene (PS) were treated by O 2 plasma, In this study, the dynamic behavior of molecular chains is investigated and aged in dry nitrogen gas or distilled water at 25 and 75ะC for with O 2 -plasma treated polystyrene without low-molecular-weight com- 480 h. Second, the styrene-divinylbenzene (St-DVB) copolymers pounds. with different crosslinking densities were synthesized and aged in dry nitrogen for 480 h after the O 2 -plasma treatment. The low EXPERIMENTAL molecular-weight oxidation products formed by the plasma treatment were removed by washing the films with methanol, which Materials made it possible to eliminate the effects of the products. The wettability of PS film acquired by the O 2 -plasma treatment became Polystyrene films (PS; thickness 100 mm; M g n ร
85,900; M g w ร
247,000; poorer during the aging in nitrogen. Especially the aging at 75ะC T g ร
106ะC) were kindly supplied by Idemitsu Petrochemical Co. Styrene (St) -divinylbenzene (DVB) copolymers were synthesized using styrene in nitrogen led the contact angle of water close to that of the (Wako Pure Chemical Ind.), divinylbenzene, and 2,2-azobis-isobutyroniuntreated PS, which suggested that the surface molecules of the trile (AIBN: Tokyo Chemical Ind.). The mixtures of St as the monomer, treated PS were beyond the glass transition point even at the DVB (0, 5, 10, 15, 20 mol%) as the crosslinking agent, and AIBN (0.1 temperature. On the other hand, the restriction of the hydrophobic mol%) as the initiator were filled into the frames (made of poly(tetrafluorecovery by crosslinking was, contrary to the expectations, rather roethylene)) and heated at 45ะC for 17 h, then at 100ะC for 24 h, resulting mild: The St-DVB copolymer with as much as 20 mol% DVB in copolymer plates (1 mm thick). still increased the contact angles of water by 30ะ from the value
The polymers were cut into 40 1 20 mm stripes and washed with methaimmediately after the plasma treatment. These results suggested nol in a ultrasonic washer and in a Soxhlet extractor for 30 s and 24 that the molecular mobility on the surface of polystyrene was h, respectively. The purified samples were desiccated in vacuo at room increased by the O 2 -plasma treatment compared with that of the temperature for more than 24 h. untreated state, which enhanced the migration of the polar functional groups introduced into the PS.