Surface Tensions

In adding the difunctional PFHDA as filler into PFPE matrix the mechanical strength is enhanced while simultaneously preserving the fluorinated nature of the resulting PFPE network materials, which served to keep the surface tension for these materials at a minimum. Surface tensions of the p(PFHDA-PFPETMA) materials were calculated based on the Owens-Wendt-Kaelble method (OWK). As discussed in Chapter 2, this method can distinguish the contributions of dispersive and polar components to the overall surface tension. The static contact angles of water and n-hexadecane were measured at air interfaces of crosslinked samples cured at room temperature. As summarized in Table 4-2, the surface tension of the neat PFPE-TMA sample is 14.6 mN/m, which is comparable to that of a crosslinked difunctional PFPE-DMA surface (13-16 mN/m). When incorporating 20 wt % of PFHDA into the crosslinked PFPE system, the surface tension remained low (14.3 mN/m). Further increasing the PFHDA content slightly decreased both the water and hexadecane contact angles, resulting in slightly increased contributions from the dispersive and polar components to the overall surface tension. When 80 wt % of PFHDA was incorporated, the overall surface tension was increased to 17.5 mN/m. Furthermore, the surface tension of a homogeneous p(PFHDA) sample was measured (23.6 mN/m). A large contribution of dispersive component was observed that contributed to a relatively higher overall surface tension for this material. Compared to other samples, the increased dispersive component was mainly attributed to the less hydrophobic nature of the PFHDA crosslinker. Along with the low surface tension, the greater mechanical durability of these PFPE materials would be advantageous since one of limiting factors in soft silicone-based fouling-release performance

such as silicone is its softness which makes it prone to physical damages in long term ocean trials.

Static Contact Angle (degree) Surface Tension (mN/m) Water Hexadecane d S γ p S γ γS PFPE-TMA 104.8 ± 1.7 70.5 ± 0.6 12.3 2.3 14.6 p(PFHDA0.2-PFPETMA0.8) 105.5 ± 2.1 70.8 ± 1.0 12.2 2.1 14.3 p(PFHDA0.4-PFPETMA0.6) 103.5 ± 1.3 69.3 ± 0.5 12.6 2.5 15.1 p(PFHDA0.6-PFPETMA0.4) 102.0 ± 2.6 69.0 ± 0.8 12.7 2.9 15.6 p(PFHDA0.8-PFPETMA0.2) 98.3 ± 3.0 65.8 ± 2.2 13.7 3.8 17.5 p(PFHDA) 96.1 ± 1.8 41.0 ± 2.1 21.2 2.4 23.6

Table 4-2. Surface tensions of PFPE materials determined by Owens-Wends-Kaelble method.

4.5 Conclusions

In this work, we were able to synthesize tetramethacrylate-modified perfluoropolyethers which can be photochemically crosslinked by UV irradiation in one step to yield a fluorinated network with improved mechanical strength. With hydrogen bonds and dispersive interactions between the urethane ether methacrylate and fluorinated acrylate groups, PFHDA shows partial miscibility with the PFPE-TMA macromonomer. As a small amount of PFHDA (< 40 wt %) is mixed with the PFPE-TMA macromonomer, a homogeneous solution can be formed at room temperature. However, when the PFHDA becomes the major component of the liquid binary system, the self-aggregation of the PFHDA begins to dominate the intermolecular interactions between these two components and results in an incompatible system. The cloud-point temperature data indicates that as the

content of the PFHDA in the binary system is increased, the miscibility between the two components is decreased, and a homogeneous clear solution becomes more difficult to achieve. Because of the partial immiscibility of these two components, only optically transparent materials with a PFHDA content of less than 40 wt % can be obtained via UV curing at room temperature. However, optically transparent samples with a PFHDA content larger than 40 wt % can be achieved by controlling the cure temperature above the corresponding cloud-point temperature of the system. The DMTA spectra indicate that the partial immiscibility of the two components results in microphase separation even in the cured clear samples. The domain size of these microheterogeneous domains was revealed to vary from < 10 nm to 30 nm by SAXS. AFM contrast images further confirmed the microphase separation morphology of these optically transparent materials.

The Young’s moduli of these materials (155 – 458 MPa) are much higher than those reported for PDMS (2.5 MPa) and other fluorinated elastomer materials (1 – 90 MPa). Along with the low surface tensions (14.3 – 17.5 mN/m), these materials should provide enhanced mechanical strength for fouling-release coating applications. Studies on these two- component systems will help to fundamentally understand the miscibility issues associated with fluorinated monomers and lead to an optimized design for more compatible fluorinated materials with better mechanical durability.

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