Samples of polylactide (PLA) were synthesized by initiating lactide ring opening with glycidol using Sn(Oct) 2 catalyst. Under low-temperature solution polymerization conditions, epoxide ring opening is precluded, leading to predominately linear epoxide capped PLA. With high-temperature bulk polymerization, simultaneous lactide and epoxide ring opening leads to hyperbranched polyesters characterized by linear PLA segments separating glycerol branch points. Molecular weights and extent of branching were determined by gel permeation chromatography (GPC), and the presence of branching was confirmed by solution viscosity measurements and g′ calculations. Solution polymerizations yielded linear samples with controlled number-average molecular weights in the range 4800-19 200 g/mol. Number-average molecular weights of branched samples from bulk polymerization ranged from 19 800 to 100 800 g/mol. The branched samples exhibited relatively low polydispersity indices (PDI) in the range 1.48-2.00. Glass transition temperatures of branched samples were determined by differential scanning calorimetry and were found to be lower compared with literature values for linear PLA of similar molecular weight.
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.
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