Synthesis and Electric Property of VDF/TrFE/HFP Terpolymers
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With the increase of HFP content, both polarization and dielectric constant decreased at the Curie temperature, and the switching time was very much independent on the HFP content. Although there are experimental results on switching time, little information is available on the structure and the piezoelectric and pyroelectric properties of the VDF/TrFE/HFP terpolymers. It is very interesting to understand their structure-property relationship, especially the sample containing only a very small content of HFP termonomer (enough to reduce the size of polar crystalline domains, but not the overall crystallinity). The interested terpolymers have the VDF/TrFE mole ratio between 50/50 and 70/30, that provide high polarization at low Curie temperature (- 60 *C). In this study, we will investigate their thermal and electrical properties, such as dielectric constant, polarization hysteresis loop, and electrostrictive response. RESULTS AND DISCUSSION Table I summarizes several VDF/TrFE/HFP terpolymers that were prepared by borane/0 or trichloroacetyl peroxide initiators at low temperature. Two commercial copolymers with VDF/TrFE mol% of 50/50, and 62/38 were also investigated for comparison.
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Table 1 A Summary of VDF/TrFE/HFP Co- and Ter-polymers. Polymer Comp.(%)
Tm (OC)
AHm (J/g)
Tc (°C)
AHc (J/g)
Mw (x 10 4)
158.2
28.1
63.8
6.3
19.67
-
152.0
32.7
103.1
22.7
34.30
1.83
128.3
18.0
40.6
4.1
3.40
0.84
137.9
25.1
44.8
7.4
3.35
0.71
139.6
28.8
61.2
15.3
3.43
2.1
138.4
19.6
40.9
4.7
3.36
26.5
0.14
143.0
27.5
55.6
13.3
11.3
57.7
41.96
0.3
142.0
25.7
45.5
5.7
8.89
114
52.59
46.63
0.8
139.3
23.2
49.5
9.3
8.61
115
63.4
35.8
0.8
139.7
22.7
42.4
4.6
18.20
119
60.49
37.2
2.3
131.5
20.9
42.4
4.8
5.19
121
55.17
42.35
2.46
129.1
15.6
35.6
2.6
17.20
Run No.
VDF
TrFE
50/50
50
50
62/38
62
38
103
54.3
43.8
104
69.2
30.0
106
65.9
33.4
108
59.7
38.2
112
73.3
113
HFP
Thermal Properties DSC measurement was used to investigate the thermal behaviors of the terpolymers. Figure 1 compares three terpolymers having VDF/(TrFE+HFP) mole ratio in the vicinity of 55/45 (run no. 114,103, 121) with the copolymer having VDF/TrFE 50/50 mole ratio. It is very clear that as the HFP molar content increased the Curie and melting temperatures of the terpolymer
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decreased. The Curie temperature decreased from 63 °C (commercial VDF/TrFE 50/50 copolymer) to 49.5, 40.6, 35.6°C (our VDF/ITrFE/HFP terpolymers with 0.18, 1.8, 2.46 mol%HFP, respectively). The bulky HFP units clearly effect the crystalline domains. The smaller polar domains may result in the lower energy for the conformation transition, hence the Curie transition temperature decreases. In addition, the bulky groups may also reduce the intermolecular interaction between the polymer chains. 50.0 48.0 46.0 Peak 64.84 *C AN 10.25 J/g
44.0 42.0 40.0 38.0 36.0
Peak 49.41 "C A 8.471 J/g ___-_-_'_-_"--___"___.....
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Peak 139.27g C AN 24.48 J/g
Peak 40.58 'C AN
A1 3.816 J/g
Peak 160
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