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43042-G7
New Approaches to Self-Assembled Electroactive Polymers Exhibiting High Electromechanical Responses
We performed the detailed studies on the synthesis and structural characterization of a library of 47 piezoelectric terpolymers poly(vinylidene fluoride trifluoroethylene chlorotrifluoroethylene), P(VDF-TrFE-CTFE) with varied chemical compositions. The microstructure, chain conformation, and thermal and dielectric properties have been investigated as a function of the polymer composition and compared with the polymers prepared via direct ter-polymerization.
The 1H and 19F NMR studies revealed that CTFE is attacked exclusively at its CF2 carbon by the growing CH2CF2 radical, leading to tail-to-tail placements. It was found that the main structure of the terpolymer includes VDF-VDF head-to-tail sequence and tail-to-tail sequences of VDF-CTFE and VDF-TrFE (-CF2-CH2-CClF-CF2-, -CF2-CH2-CHF-CF2-). It is of interest to note that the terpolymers prepared via the reductive route exhibit different microstructures from the ones produced by direct ter-polymerization of three co-monomers. VDF-VDF head-to-tail sequences account for 46% of the polymer chains synthesized by direct ter-polymerization, while the terpolymer prepared by the reduction reaction contains about 28% VDF-VDF head-to-tail sequences. Beside 6.4% VDF-TrFE tail-to-tail structures, there is about 8.9% VDF-TrFE head-to-tail (-CH2-CF2-CHF-CF2-) sequence existing in the terpolymer prepared by direct ter-polymerization; this is in marked contrast to the terpolymers generated from reduction reaction, which show no evidence of the existence of VDF-TrFE head-to-tail structures from the 19F NMR measurements. VDF-CTFE exists predominantly as tail-to-tail structures in the both terpolymers. The discrepancy in microstructure leads to significantly different thermal and dielectric properties in these two terpolymers. In general, with the same chemical composition, the terpolymers prepared via direct ter-polymerization possess a lower percentage of sequence defects such as head-to-head and tail-to-tail structures in polymer chain, resulting in a higher degree of crystallinity and Curie transition temperature compared to the terpolymers synthesized via the dechlorination reaction.
As identified in the FTIR measurements, the polymer chain conformation is strongly influenced by the polymer composition. P(VDF-TrFE) copolymers yielded from complete reduction of P(VDF-CTFE)s exhibit largely an all-trans conformation of the &beta-phase. On the other hand, the peaks corresponding to all-trans conformation are almost absent and the peaks from tttg+tttg- conformation in the -phase become prominent in the P(VDF-CTFE-TrFE) terpolymers containing less than 10 mol% TrFE. This indicates that the crystalline form of the terpolymers changes from ferroelectric &beta- to &gamma-phases upon introduction of bulky CTFE components.
WAXD is used to investigate the evolution of crystalline structures of the terpolymers with chemical compositions. Due to replacement of bulkier chlorine atoms by fluorine, the spacing and volume of the crystal lattice progressively decrease with the increase of TrFE content. Consistent with FTIR results, degree of crystallinity increase with decreasing CTFE concentration, further confirming that the existence of bulky CTFE components in the terpolymers reduces crystallinity and crystalline size.
The observed relatively sharp melting and Curie phase transitions in DSC studies can be interpreted as an indication of uniform polymer crystalline morphology, owing to the random distributed CTFE and TrFE components generated by the reductive reaction. It is shown that both melting and Curie temperatures increase with increasing TrFE content under certain VDF concentrations. These results agree quite well with the FTIR spectra showing all-trans conformation of the &beta-phase for high concentration of TrFE component in the polymers. Compared to tttg+tttg- conformation of the &gamma-phase, the &beta-phase possessing longer trans sequence in the polymer chains results in an increased activation energy barrier for phase transition and in turn high melting and Curie temperatures. For example, for the terpolymers containing 78.8 mol% VDF, as the TrFE concentration increases from 5 to 21 mol%, Curie transition and melting temperatures shift from 23 and 63 0C to about 98 and 152 0C, respectively, coincide with the transformation of primary conformation from &gamma- to &beta-phases.
The dielectric properties of the terpolymer appear to be strongly correlated to the chemical compositions. Under the certain amount of VDF content, with decreasing TrFE concentrations in the terpolymers, the room-temperature dielectric constants correspondingly increase from 11 to 50 as a result of a gradual shift of Curie transition temperatures from about 100 0C to ambient temperature. The highest room-temperature dielectric constant of about 50 at 1 kHz was revealed on the terpolymer having 78.8 mol% VDF, 7.2 mol% TrFE, and 14 mol% CTFE. This composition is quite different from the one prepared by direct ter-polymerization, which shown a dielectric constant in the vicinity of 50 at a composition of 62 mol% VDF, 26 mol% TrFE and 12 mol% CTFE. For the polymers exhibiting the same dielectric constant, the one from the reductive reaction contains about 3-4 times less TrFE contents than the terpolymers prepared via ter-polymerization. This difference can be explained by the statement above, which TrFE is present primarily as structural defects in the case of the terpolymer yielded from the reduction of P(VDF-CTFE)s, while TrFE exists as both head-to-tail normal and tail-to-tail defect sequences in the terpolymer from the direct ter-polymerization process.
