Synthesis and Surface Analysis of Siloxane-Containing Amphiphilic Graft Copolymers, Poly(2-hydroxyethyl methacrylate-<i>g</i>-dimethylsiloxane) and Poly(2,3-dihydroxypropyl methacrylate-<i>g</i>-dimethylsiloxane) (2002)

A series of novel amphiphilic graft copolymers having a hydrophilic poly(2-hydroxyethyl methacrylate) [poly(HEMA)] or water-soluble poly(2,3-dihydroxypropyl methacrylate) [poly(DHPMA)] backbone and hydrophobic poly(dimethylsiloxane) (PDMS) side chains have been synthesized to develop a model system to examine surface properties under dry and wet conditions. The direct radical copolymerization of PDMS macromer with HEMA generated graft copolymers poly(HEMA-g-DMS) with broad molecular weight distributions (MWD; Mw/Mn = 2.85−3.55). The anionic copolymerization of the same PDMS macromer with silyl-protected HEMA, namely, 2-(trimethylsiloxy)ethyl methacrylate (SiEMA), produced graft copolymers, poly(SiEMA-g-DMS). Successive hydrolysis eliminated the protecting trimethylsilyl groups in the backbone to afford amphiphilic graft copolymers poly(HEMA-g-DMS) with a narrow MWD (Mw/Mn = 1.09−1.16). For the well-defined graft copolymer with a water-soluble poly(DHPMA) backbone and hydrophobic PDMS side chains, anionic copolymerization of allyl methacrylate (AMA) with a PDMS macromer was carried out first, generating graft copolymers consisting of a poly(AMA) backbone and PDMS side chains. The unreacted allyl side groups of AMA units in the backbone were subsequently reacted with N-methylmorpholine N-oxide (NMO) in the presence of a trace amount of osmium tetroxide (OsO4) to afford a new well-defined amphiphilic graft copolymer, poly(DHPMA-g-DMS). Using the angle-dependent X-ray photoelectron spectroscopy (XPS) technique, the surface compositions of solution-cast films of the graft copolymers were measured under both dry and wet (frozen) conditions. The effects of PDMS graft lengths, PDMS bulk content, and MWD on the surface composition were investigated. It was found that, for poly(HEMA-g-DMS) graft copolymers, the PDMS surface concentration increases with increasing PDMS bulk content. For a given PDMS bulk content, a longer PDMS graft gives a surface richer in siloxane. The effect of graft length is more significant than PDMS bulk content. Comparing radically to anionically prepared graft copolymers, the MWD has little effect on the dry air (free) surface composition but does affect the degree of reorganization upon exposure to water. In addition, the backbone hydrophilicity has a great effect on the PDMS segregation to the surface, and a significant surface composition difference between poly(HEMA-g-DMS) and poly(DHPMA-g-DMS) graft copolymers of similar PDMS graft length was found. Poly(DHPMA-g-DMS) with more hydrophilic backbone exhibited less PDMS concentration at the dry air (free) surface, even though its PDMS bulk content was higher than that of the poly(HEMA-g-DMS) graft copolymer. Furthermore, for poly(DHPMA-g-DMS), the effect of PDMS bulk content on its surface segregation was little even over a wide range of 9.0−26.0 wt %. However, as expected, upon exposure to water, poly(DHPMA-g-DMS) underwent a dramatic surface reorganization.