MOLECULAR-STRUCTURES AND PERCOLATIVE PHASE-TRANSITION IN VINYLIDENE FLUORIDE-TRIFLUOROETHYLENE COPOLYMER CRYSTALS

Thermal and structural properties of vinylidene fluoride (VDF)-trifluo roethylene (TrFE) copolymers with VDF molar contents between 57 and 77 mol% are examined by differential scanning calorimetry, X-ray analysi s, IR and F-19 NMR. It is found from the experimental results that VDF sequences with three or more monomers play an important role in the p hase transition and/or phase appearance. Computer simulation for the c opolymerization suggests that the copolymer chain of 65-70% VDF contai ns ca. 50% VDF which is included in the long VDF sequences with three or more monomers. This fraction (50%) is the site-percolation threshol d for the two-dimensional triangular lattice; the transformation from the continuous structural change to the first-order phase transition w ith the increasing VDF fraction is understood as the percolative phase transition in the two-dimensional crystal system.