FERROELECTRIC TERPOLYMERS, BASED ON SEMICRYSTALLINE VDF/TRFE/CHLORO-CONTAINING TERMONOMERS: SYNTHESIS, ELECTRICAL PROPERTIES, AND FUNCTIONALIZATION REACTIONS
Open Access
- Author:
- Petchsuk, Atitsa
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 23, 2003
- Committee Members:
- Tze Chiang Chung, Committee Chair/Co-Chair
Ian Roland Harrison, Committee Member
James Patrick Runt, Committee Member
Kenji Uchino, Committee Member - Keywords:
- Chloro-containing termonomers
Fluoropolymer
Functionalization
Ferroelectrics
VDF
TrFE
synthesis and electrical properties - Abstract:
- In the past decades, many researches have devoted their efforts related to PVDF and VDF/TrFE copolymers to the general goal of reducing the Curie temperature, narrowing the polarization hysteresis loop, and generating a large electromechanical response at ambient temperature. The direct correlation between reduced polar domain size and lower energy barrier, shown in ferroelectric ceramic materials, led to many attempts to alter copolymer morphology. The methods include mechanical deformation, electron irradiation, uniaxial drawing, and crystallization under high pressure or high electric field. By using these physical methods, many shortfalls have been addressed such as the limitation of the processability of the material, the instability of the crystal phase, the extremely high electric field in the strain measurement. In this thesis, a new chemical method is performs, by incorporating the small amount (~7 mol %) of the chloro-containing termonomers, including chlorotrifluoroethylene (CTFE), 1,1 and 1,2-chloro-fluoroethylene (CFE), 1-chloro-2,2-difluoro ethylene (CDFE), into VDF/TrFE copolymer chains. Along with this study, we also synthesize a functional fluoropolymer that contains the functional group on either one end or both ends, and their diblock or triblock copolymers. The long term objective is to utilize these reactive functional groups to connect fluoropolymer with other materials to further improve the electrical properties of the terpolymers. A novel polymerization method involves a combination of a low temperature borane/oxygen initiator and a bulk polymerization process. This process allows the thorough mixing of all monomers during the terpolymerization reaction, resulting in the homogeneous distribution of the termonomer units in the polymer chain. A chloro-containing termonomers, especially CDFE and CTFE, having comparative reactivity ratios with VDF and TrFE, lead to terpolymers with relatively narrow molecular weight and composition distributions. The homogeneous incorporation of the chloro-containing termonomers in the VDF/TrFE copolymer chain is crucial in modifying its polymer chain conformation and crystal structure. With ~7 mol% of the termonomer units, the copolymer alters its crystalline phase from the ferroelectric ƒÒ phase (all-trans conformation) toward another ferroelectric ƒ× phase (tttg+tttg- conformation), without significant reduction of the overall crystallinity. The new ferroelectric ƒ× phase of the terpolymer, with shorter trans-sequence and smaller crystalline domains, reduces the Curie phase transition temperature since it has a much smaller energy barrier for phase transition. The unique combination in this new ferroelectric ƒ× phase, with a small energy barrier and the large unit expansion or contraction during the crystal phase transition, results in high electromechanical response and behaves like a ferroelectric relaxor. The terpolymer shows high dielectric constant, as high as 100, with narrow hysteresis and high field-induced longitudinal strain (up to 4-5% at 150 MV/m) at ambient temperature. For the functionalization of fluoropolymer, the telechelic VDF/TrFE/chloro-containing terpolymers having functional (polar and reactive) terminal groups, and triblock copolymers containing a VDF/TrFE/chloro-containing terpolymer and functional polymers was developed. These telechelic and block polymers not only provide interactive properties but also preserve the existing desirable properties due to the undisturbed polymer backbone. Two chemical methods have been studied, including (i) the use of functional borane/oxygen initiators that carry functional groups and exhibit living radical polymerization mechanism, and (ii) the investigation of an improved iodine transfer polymerization (ITP) method to prepare telechelic fluoropolymers containing two terminal iodine groups. The former method although has some advantages in terms of ease and high purity of the polymer with the desirable reactive functional groups such as acetyl (can be converted to hydroxy group), silane and bromine groups, the polymerization yield is not very high due to the intramolecular interaction between the boron atom and the functional groups. The latter method, the iodine transfer polymerization (ITP), provides a very effective method with high yield, good catalyst efficiency, and well-defined telechelic polymer structure with two reactive terminal iodine functional groups on both chain ends. The chemistry involved an improved iodine transfer polymerization process using the combination of the AIBN imitator and a diiodo-compound chain transfer agent. Both initiation systems also show a living radical polymerization characteristic leading to the well-defined polymer with narrow molecular weight and composition distribution. The living characteristic was demonstrated by the block copolymerization of the terpolymer with the non-fluoropolymer (such as the PMMA homopolymer), or the fluoropolymer (including the PVDF homopolymer and the VDF/TrFE copolymer). The material from the latter case is very useful in terms of the improvement of the mechanical strength of the terpolymer.