UV Curing Behavior of Five Heteroleptic POSS Bearing Methacrylate and Glycidyl Groups and Evaluation of Their Potential for Hard Yet Flexible Coatings

Abstract

Octakis(3-methacryloxypropyl) silsesquioxane (MPOSS) photocured using a free-radical initiator under N 2 has a high hardness H of 0.99 ± 0.05 GPa but has poor bendability. Octakis(3-glycidyloxypropyl) silsesquioxanes (GPOSS) photocured in air using a cationic initiator is more flexible than the cured MPOSS coating but has a lower H value of 0.65 ± 0.02 GPa. In this study, a family of heteroleptic POSSs (G m M n POSS) bearing 2, 4, or 6 glycidyl (G) groups and 6, 4, or 2 methacrylate (M) groups are photocured in N 2 , in air, in air then N 2 (air/N 2 ), and in N 2 /air. The hardest coating is obtained from a given precursor G m M n POSS when it is cured in N 2 /air. The G 2 M 6 POSS coating offers the best balance between H and bendability. At a thickness of 50 μm, this coating has an H value of 0.95 ± 0.07 GPa, a pencil hardness rating of >9H, and a critical bending radius of <1 mm for inward bending on the inner surface of a poly(ethylene terephthalate) substrate film. At a thickness of 10 μm, the G 2 M 6 POSS coating has a critical outward bending radius of 1.5 mm. This study provides experimental guidelines for the proper photocuring of G m M n POSS and reveals that G 2 M 6 POSS is the best candidate for hard yet flexible coatings, which may be used to protect the touchscreens of foldable smartphones among other applications.

Facile Preparation of a Transparent and Rollable Omniphobic Coating with Exceptional Hardness and Wear Resistance

“…Results in Table suggest that our coating from LASQ with a much higher molecular weight should have hardness substantially greater than that of Bae’s work. While this conclusion is supported by the higher H value of our LASQ coating than their value of 0.78 ± 0.01 GPa, we caution against this direct comparison because H values reported by different research groups can vary substantially even for the same sample due to the use of different instruments and test methods. − …”

“…The E and w e values have been included because H / E > 10% and w e > 60% have been previously cited as criteria for hard flexible coatings. While our LASQ-FP coatings meet such criteria, we note that these criteria should be used with caution because the H value reported for a given sample by different groups can vary substantially, as was discussed in a recent paper …”

“…The finding that the critical outward bending diameters are larger than the inward bending diameters suggests that the coatings are more susceptible to extension failure during outward bending than to compression failure during inward bending . The bending properties of the LASQ coatings are comparable with those of cured POSS coatings …”

“…Table 1 shows that the H values are similar for coating samples of F1, F2, and the unfractionated LASQ, which all have M w > 1.0 × 10 4 Da, but decrease substantially from the F3 coating to the F4 coating when the LASQ M w decreases below ∼1.0 × 10 4 Da. Because H has been shown to be directly correlated with the wear resistance of coatings and LASQ fractionation is troublesome, 39 we focused our subsequent studies on coatings that had been prepared from unfractionated LASQ or m-LASQ-FP that had been obtained by reacting FP-COOH with unfractionated LASQ.…”

“…While our LASQ-FP coatings meet such criteria, we note that these criteria should be used with caution because the H value reported for a given sample by different groups can vary substantially, as was discussed in a recent paper. 39 3.6.2. Pencil Hardness.…”