Study On Crystallization And Melting Behavior Of P(VDF-TrFE) Copolymer In Ultrathin Films

Copolymers of vinylidene fluoride and trifluoroethylene,P(VDF-TrFE),have attracted intense scientific and technological interest because of their strong piezoelectric,ferroelectric,and pyroelectric properties.For the purpose of comprehending the relationship between the crystalline structures and properties of P(VDF-TrFE),we studied the crystalline morphology evolution and melting behavior of P(VDF-TrFE)crystals in ultrathin films which were prepared by the spin coating method utilized in optical microscopy(OM)and atomic force microscopy(AFM).The main contents are briefly summarized as follows:(1)Crystallization morphology of P(VDF-TrFE)ultrathin films depended on many factors such as crystallization temperature(T_c),film thickness(d),and molecular weight(M_w)of the copolymer.With increasing T_c from 126? to 138?,the crystal morphology in the P(VDF-TrFE)film(with the thickness of ca.13 nm)evolved from seaweed-like crystals to lath-like lamellar crystals with periodic bifurcation of edges.Two sectors with different thickness could be identified in lath-like lamellar crystal:ca.37 nm for the thicker sector and 24 nm for the thinner sector.Different growth front instability cycles were found in the edges of two sectors.Furthermore,when the film thickness increased from 8 nm to 35 nm,the lath-like crystals curved gradually and showed a periodic splitting of the growth face,resulting in many irregular branches typical of the dense branching morphology or compact seaweed.The envelope of this branching structure is nearly circular.One can also observe that the complete transition temperature of the crystalline morphology from seaweed-like to lath-like was about 130? when the M_w was 370?530 kg/mol,while that was about 136? when the M_w was 520?860 kg/mol.It is considered that the P(VDF-TrFE)lath-like lamellar crystals with different instability periods in two sectors probably were caused by the difference of growth rate of the sectors.(2)Observation of the melting behavior of P(VDF-TrFE)lamellar crystals in the ultrathin film was carried out by using an AFM.The effects of annealing temperature(T_a),annealing time(t_a),heating protocols,and initial crystallization temperature(T_c)on the melting and morphological evolution of P(VDF-TrFE)lath-like lamellar crystals were further explored.Melting and thickening in P(VDF-TrFE)lamellar crystals were found when T_a gradually rose from 136? to 147?.Different thermal stabilities were observed in two sectors within a lath-like crystal.For example,the melting point of the thinner sector was about 144?,while the thicker one was about146?.It is suggested that the thermal stability of P(VDF-TrFE)lamellar crystal is strongly dependent on the history of crystallization,as well as the thickening and perfection of crystals during melting process.The thermal stability of P(VDF-TrFE)lamellar crystals can be improved by heating in multi-steps and increasing initial crystallization temperature.