ABSTRACT
In a wide variety of industrial technologies, surface modification of polymers is important and has attracted considerable interest in modifying polymer materials related to several engineering fields [1, 2]. Particularly, active gaseous species have been widely used to modify surfaces of various polymers. These modification methods are available as corona, flame and plasma treatments, which are well known as convenient and practical techniques. Among these, ozone (O3) is known as a convenient species for surface modification of polymers. This is because of the decom-
position of the gaseous O3 to safe O2 by thermal reaction: O3 → O2 +O. However, only little is known about the modification process for polymer-O3 systems. The O3 treatment of hydrocarbons produced intermediate radicals and decomposed into molecular oxygen and hydroxyl radicals [3-5]. Similar reaction process might proceed in organic polymer systems such as polyethylene, when the polymer surface is exposed to gaseous O3. From these research results, it is more challenging to study surface modification of polymers by O3, since O3 is expected to react with polymer surface. There have been reports on the reaction of O3 with different polymers: polypropylene [6, 7], polysiloxanes [8], and poly(methyl methacrylate) [9]. Thus, the O3 treatment of polymers enhanced the oxidative reaction, which caused the formation of C=O groups. For example, Rabek et al. [6] and Walzak and coworkers [7, 10] studied O3 treatment of polypropylene. Their data indicated that O3 treatment of polymers depended on the chemical structure of the polymer used. In their works, the mechanism of the O3 reaction was based on atomic oxygen produced from O3. Also, they focused mainly on UV-O3 effect to produce such reactive species from O3, since UV light effectively decomposes O3 to reactive species. It has been known for alkane-O3 reaction in the gas phase that the O3 treatment of hydrocarbons produces intermediate radicals, which decompose into molecular oxygen and hydroxyl radicals [10-12]. Similar reaction process might proceed in other organic polymer systems, when the polymer surface is exposed to O3. However, there have been only limited experimental studies on the reaction of O3 with polymers. For this reason, we studied surface modification of polymers by O3 to compare with polyethylene (PE) and polystyrene (PS) treated by thermal O3 method [13]. The modification of PS surface was analyzed with in situ FT-IR spectroscopy, as thermal-ozone treatment was taking place [14]. Namely, when O3 was heated, the species generated were effective for PE modification. Relative to PE, PS surface was highly modified by gaseous O3 with weak dependence on the modification temperature. This result indicated that O3 attacked PS with a different reaction mechanism than PE. However, a systematic investigation is needed to clarify O3-polymer reaction. In the present study, we focused on O3 modification of PS and its derivatives at different temperatures. The reaction process of O3 and PS derivatives was investigated by directly recording the FT-IR spectra of polystyrene (PS) and its derivatives, poly(4-methyl styrene) (P4MS) and poly(α-methylsyrene) (PαMS) (Scheme 1) at different temperatures under O3 exposure. Furthermore, the reaction process of the polymer surface was followed by AFM and contact angle measurements.
