Effect Of Tensile Strain On The Morphology And Dielectric Property Of Pvdf Based Copolymers
Open Access
- Author:
- Li, Mengfang
- Area of Honors:
- Materials Science and Engineering
- Degree:
- Bachelor of Science
- Document Type:
- Thesis
- Thesis Supervisors:
- Qing Wang, Thesis Supervisor
Evangelos Manias, Thesis Honors Advisor - Keywords:
- PVDF-BTFE
crystal structures
morphology
stretching
dielectric. - Abstract:
- Electro-active polymers have great impact on many current technologies such as sensors and actuators, energy storage and flat panel display. An important families of electro-active polymer: polyvinylidene fluoride (PVDF) based polymer, are studied in this thesis to investigate the effect of mechanical stretching on its microstructure and dielectric properties. In comparison with other linear dielectric polymers, PVDF-based polymers have a much higher polarization responses under an external applied electric field. The maximum polarization level of PVDF-based polymer can be high as 0.1 C/m2. And an estimated energy density of 30J/cm3 is expected in this class of polymers [1][2]. This is greater than energy density achieved by PVDF homopolymers and copolymers because of the polarization hysteresis in these polymers. PVDF and its copolymers with ferroelectric nature are often chosen because of they can admit an efficient electrical to mechanical actuation in response to external electrical stimulation. The optimization of these properties is related to the degree of crystallinity, structure, and orientation of the crystalline part of polymer. These parameters are heavily depending on the processing conditions. When cooling from melt, PVDF based copolymers commonly crystallize in the non-polar crystalline α-phase. However, the electroactive properties depend on polar β-phase and its structural characteristics. The β-phase can be induced by several methods, the one investigated in this study being the mechanical stretching of the α-phase at suitable temperature [1][2]. The α to β phase transformation of PVDF-BTFE copolymer through the stretching process at different temperatures was investigated. The optimum stretching conditions were studied and characterized by infrared spectroscopy, WAXD and differential scanning calorimetry. The maximum β phase content was achieved at 110 C with a stretch ratio of 5. An orientation of the polymer chains with specific packing preferences in crystalline structure is observed in the experiment. FTIR spectroscopy was used to examine the structural variation as function of strain. DSC data allowed the measurement of melting temperatures and enthalpies of material before and after stretching. Energy and intensity variation was also investigated at different deformations. Reorientation of the chain from perpendicular to parallel to stress direction was observed. The lamellae thickness does not change significantly during stretching.