Study on reconstruction mechanism at the surface of a glassy polymer

The introduction of hydrophilic functional groups at various depths in the functionalized interfacial region of poly(4-methylstyrene) (P4MS) provided a system for studying the surface reconstruction mechanism of this glassy polymer. The degree of surface reconstruction (RD) and the rate of surface reconstruction (1/t1/2) were employed to compare the surface reconstruction behavior for various samples. The results showed that 1/t1/2 decreased with increase of the depth in the functionalized interface region at a temperature below Tg of poly(4-methylstyrene). By studying the relation of water contact angles and the surface free energy of the samples with temperature, it was found that surface reconstruction of surface-oxidized P4MS samples took place in two steps when the samples were heated in air. The first step took place below 80 °C, in which polar side groups turned into the bulk, leaving a relatively nonpolar backbone projecting out of the surface to form a “hydrophobic conformation”. The second step occurred above the P4MS Tg (110 °C), in which P4MS molecular chains migrated to the surface in order to minimize the interfacial free energy between surface-oxidized P4MS film and air, since oxidized P4MS molecules containing carbonyl groups have higher surface free energy than the unmodified P4MS molecules. As the depth of the functionalized interfacial region increases, a longer time is needed for the polar side groups to reorient (the first step) and for unoxidized P4MS molecules to migrate to the surface (the second step), which resulted in the sample with deeper functionalized region having lower reconstruction rate and RD using the same treatment condition.