Modification of Silicone Elastomers Using Silicone Comonomers Containing Hydrophilic Surface Active Endgroups
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Modification of Silicone Elastomers Using Silicone Comonomers Containing Hydrophilic Surface Active Endgroups Jonathan Goff1, Barry Arkles1 and Santy Sulaiman1 1 Gelest Inc, 11 E Steel Road, Morrisville, PA 16067, U.S.A. ABSTRACT A facile technique was developed for a long-term increase in silicone elastomer surface hydrophilicity, eliminating the need for post-cure surface treatment (e.g. oxygen plasma or surface grafting). Well-defined silicones (1-4 kDa) with a central vinyl functionality and discrete PEG2, PEG3 and tetrahydrofurfuryl (THF) pendant endgroups were synthesized, characterized and used as comonomers in addition-cure, platinum catalyzed 2-part silicone elastomer formulations. The modified silicone elastomers were optically clear and maintained the mechanical performance characteristic of this class of material with up to 20 wt.% comonomer in the 2-part formulation. Contact angle measurements of deionized water on the silicone elastomer surface showed improved wettability with comonomer content. The elastomer surface shifted from hydrophobic (contact angle ~120oC) to hydrophilic (contact angle < 900C) at ~5 wt.% comonomer loadings for extended time frames (> 5 months). Coefficient of friction measurements of the modified silicone elastomers revealed an increase in surface lubricity with comonomer loadings. INTRODUCTION Silicone elastomers have a myriad of uses in medical applications and microfluidics. The untreated surface of a silicone elastomer, however, is hydrophobic, with water contact angles reported in the 105-120o range.1,2 Long-term surface modification of silicones to improve wettability and reduce protein adsorption is desirable. Silicone elastomers are low Tg, low modulus materials with no secondary forces and negligible water adsorption and are often considered to be a paradigm for soft hydrophobic materials. The elastomers are formed by crosslinking highly flexible polymeric silicone fluids. The majority of elastomeric silicone products utilized in medical devices and microfluidics are based on the crosslinking of reactive silicones that possess vinyl and hydride substitution and typically incorporate inorganic fillers or resinous products for improved mechanical properties.1-3 Commercially available addition-cure, platinum catalyzed silicone RTVs undergo a crosslinking hydrosilylation reaction yielding silicone elastomers. Typically in an addition cure 2-part RTV formulation, a Part A ‘Base’ of a vinyl functional silicone, filler and catalyst is mixed with a Part B hydride functional silicone ‘Activator’ in specific proportions and subsequently heated for cure. The stoichiometry of reactive functional groups in the formulation is set for an excess of hydride to ensure the quantitative consumption of vinyl groups and compensate for side reactions involving hydride functionality during the elastomer cure.2 CH 3 CH 3 CH 3 CH 3 Si CH 3
CH CH 2
+ H Si
CH 3
Si CH 3
CH 2 CH 2 Si CH 3
Treatment of the silicone elastomer surface with O2 plasma improves the surface wettability due to forma
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