สำนักราชบัณฑิตยสภา

231 Suda Kiatkamjornwong et al วารสารราชบัณฑิตยสถาน ปีที่ ๓๗ ฉบับที่ ๑ ม.ค.-มี.ค. ๒๕๕๕ Likewise, the ratios of polar-to-dispersion contribution at all exposure times are approximately 1:1 and the total surface energy of the sheets is between 45-51 mN m -1 . No matter many longer exposure times of the oxygen gas plasma are given, the dispersion contribution component of surface energy could not be increased at all as expected due to the following reasons. Table 1 Surface energies of PLA sheets at varied power of the oxygen gas plasma treater Treatment Contact angle (º) Surface energy (mN m -1 ) Power Exposure Deionized Methylene γ s γ s γ s (W) time Water iodide (s) 0 0 68.1±1.6 47.8±2.6 14 26 40 30 30 51.6±2.6 41.7±1.6 25 26 51 50 30 53.8±2.2 42.8±1.5 23 25 48 70 30 56.2±2.7 42.7±1.2 21 26 4 p d When oxygen gas is ionized, electrons collide with polymer surfaces to break molecular bonds, creating free radicals which react in the presence of reactive oxygen species (ROS) to form high polarity functional groups such as carbonyl, carboxyl, hydroxyl, or amine. The ionized oxygen gas contains ions and electrons; the oxygen gas can be split to give monoatomic oxygen, O - or O + to attack the H atom on the polymer backbone. The surface energy of PLA is increased by hydrogen abstraction at the methyl groups (Figure 1) to become ions, radicals, or photon, followed by passivation by O atom, may be form- ing peroxy groups. The degree of functionalization depends upon on the gas plasma and energy density. The increase of the polar contribution portion takes place at the expense of the C-H and the C-C bonds to become the oxygen containing portions. In the present case, with the high amount of energy density (through the long time exposure), the increases in the polar contribution component is negligible as shown in Table 2. Two possible reasons can be postulated: first, a limited number of methyl groups are present in the polymer chain and, second, aging effect of the oxygen plasma treated surfaces at the longer exposure times.