Molecular structures and percolative phase transition in vinylidene fluoride-trifluoroethylene copolymer crystals

Thermal and structural properties of vinylidene fluoride (VDF)-trifluoroethylene (TrFE) copolymers with VDF molar contents between 57 and 77 mol% are examined by differential scanning calorimetry, X-ray analysis, 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 phase transition and/or phase appearance. Computer simulation for the copolymerization suggests that the copolymer chain of 65-70% VDF contains ca. 50% VDF which is included in the long VDF sequences with three or more monomers. This fraction (50%) is the site-percolation threshold for the two-dimensional triangular lattice; the transformation from the continuous structural change to the first-order phase transition with the increasing VDF fraction is understood as the percolative phase transition in the two-dimensional crystal system.