Structural Change in Ferroelectric Phase Transition of Vinylidene Fluoride Copolymers as Studied by WAXS, SAXS, IR, Rama
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35 Mat. Res. Soc. Symp. Proc. Vol. 600 ©2000 Materials Research Society
Paraelectric Phase
Ferroelectric Phase
Tc
C
L 0
A , •(100) .
Low-Temperature
ColdPaeC)
Phase(LT)
High -Temperature Phase(HT)
Figure 1. Crystal structures of VDF-TrFE copolymers. (a) Low-temperature (LT) phase, (b)Cooled (CL) phase, and (c) High-temperature (HT) phase. Change of Domain Size in the Crystallite through Phase Transition As the temperature increases and the LT phase transfers into the HT phase, the X-ray profile changes remarkably, as shown in Figure 2 [15]. The reflection of the LT phase is in general broad and that of the H-T phase generated at the early stage of transition is also relatively broad and rather close to that of the LT phase. The reflection width of the HT phase becomes remarkably sharper as the transition proceeds furthermore at higher temperature, while the half width of the reflection of the LT phase remains broad and almost constant during the transition. These observations suggest an existence of the domain structure and its growth during the phase transition. The crystallite of the LT phase is assumed to have a multi-domain structure, where the domains of approximately 100 A size in the lateral direction, as evaluated from the X-ray reflection width, are gathered with their dipoles oriented into different directions to form a large crystallite as illustrated in Figure 2. As the temperature increases and approaches the Tc, the HT phase begins to appear randomly in some domains and the domain size of the HT phase reflects directly that of the LT phase, giving the reflection width close to that of the LT phase. As the temperature increases furthermore, the number of domains of the HT phase increases and these domains arc fused together into a large single domain, because the large-amplitude rotation of the chains makes each domain nonpolar and so the boundary between the domains becomes unambiguous and these domains fuse into one large domain. The large size of the thus created domain reflects on the sharpness of the X-ray reflection. VDF Content Dependence of the Transition Behavior The transition behavior is affected by many factors. For example, the transition behavior changes depending upon the VDF content of the copolymers [4,10-13]. For the copolymers with
36
4
-LT -~~-
..
v
domains
A-ray pattern
phase
crystallite
-Tc
early stage of transition LT
Figure 2. Temperature dependence
T>>Tc
"phase (_D high-temp phase
of X-ray diffraction profile taken for VDF 73 % copolymer and the
corresponding change in domain
size of the crystallite during the phase transition of VDF copolymer.
2thet,
the VDF content higher than 70 molar %, the transition occurs almost reversibly and discontinuously in the thermodynamically first ordered fashion between the regular LT phase and the HT phase. For the samples with VDF 70 - 60 %, the crystalline phase, obtained after the heat treatment above Tc, is a mixture of the LT phase and the CL phase at room temperature, and the complicated transitions are ob
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