Three types of commercially derived methylsilicone materials, Sylgard-184, QVQH (an MQ-based silicone containing no dimethylsiloxane, D units), and DVDH (a D-based silicone with no additives), were judiciously chosen to study the conditions under which long-lasting hydrophilicity after oxygen plasma treatment can be obtained. A 30 s plasma treatment time under controlled conditions was found to be optimal in terms of achieving the lowest initial advancing and receding contact angles of θA/θR = 10°/5° with undetectable surface damage. Vacuum treatment, a necessary step prior to plasma ignition that has been overlooked in previous studies, as well as room temperature curing were explored as means to remove low molecular weight species. For thin films (a few micrometers), 40 min vacuum treatment was sufficient to achieve low dynamic contact angles of θA/θR = 51-56°/38-43° on all three types of silicones measured more than 30 days after the plasma treatments. These values indicate superior hydrophilicity relative to what has been reported. The small and slow rise in contact angle over time is likely caused by the intrinsic nature of the silicone materials, i.e., surface reorientation of hydrophilic functional groups to the bulk and condensation of surface silanol groups, and is thus unavoidable. For thick films (~1 mm), room temperature curing in addition to vacuum treatment was required to reduce hydrophobic recovery and to achieve long-lasting hydrophilicity. The final contact angles for thick samples were slightly higher than the corresponding thin film samples due to the greater 'reservoir' depth and migration length for mobile species. In particular, Sylgard exhibited inferior performance among the thick samples, and we attribute this to the additives in its commercial formulation. Furthermore, unlike polydimethylsiloxane-based silicones, QVQH does not contain equilibration products of the Dn-type; its thin films perform as well as those of Sylgard and DVDH. Silicones without D units are promising materials with intrinsically low hydrophobic recovery characteristics and long-lasting hydrophilicity after oxygen plasma treatment.
Nguyen, Lien, Mimi Hang, Wanxin Wang, Ye Tian, Liming Wang, Thomas J. McCarthy, Wei Chen
ACS Appl. Mater. Interfaces