Morphological Structure And Properties Of Polyvinylidene Fluoride/Multi-walled Carbon Nanotubes Composites

Carbon nanotubes(CNTs) have excellent mechanical and electrical properties dueto the high strength, toughness and electrical conductivity. The addition of CNTs topolymers in certain ways may combine easy processing characteristics of polymerswith excellent mechanical and electrical properties of CNTs effectively. It is benificialto expand the applications of polymers and prepare new types of composites withhigh performance and multi-function. Currently, there have been many reseaches onpolymer/CNTs composites, however, polyvinylidene fluoride/multi-walled carbonnanotube(PVDF/MWCNT) composite was seldom recorded in literatures. Manyissues have not been resolved, for instance, there is still lack of scientific explanationon the formation mechanism of PVDF/MWCNT composite with a spherical structureprepared by ultrasonic dispersion method. In addition, some studies on mechanicaland electrical properties of PVDF/MWCNT composite are not in-depth enough.PVDF has been widely used in many fields as an important kind of engineeringplastics. Based on systematical researches on the structure and performance ofPVDF/MWCNT composite under different experimental conditions, the applicationsof PVDF/MWCNT composite on sensors, energy materials, biomaterials, andengineering materials would be spread greatly.In this work, the morphological structure, mechanical and electrical properties ofPVDF/MWCNT composites were investigated by X-ray diffraction(XRD),differential scanning calorimeter(DSC), polarizing optical microscopy(POM), tensiletest and resistivity measurement, respectively. It was found that there was interactionbetween MWCNTs and PVDF by ultrasonic dispersion method. Spherical particleswere formed in PVDF/MWCNT composite. MWCNTs dispersed in PVDF acted asheterogenous nucleus for the crystallization of PVDF. The addition of MWCNTs toPVDF accelerated the nucleation of PVDF. In the meantime, the interaction betweenMWCNTs and PVDF also confined the movements of polymer chains and hinderedthe growth of PVDF spherulites, which resulted in the decrease in the degree of crystallinity. Nonisothermal crystallization data were analyzed using Ozawa, Jeziorny,and Mo methods. The Jeziorny and Mo methods successfully described thenonisothermal crystallization behaviors of PVDF/MWCNT nanocomposite. Theactivation energy for nonisothermal crystallization of PVDF and PVDF/MWCNTcomposite determined using Kissinger method were estimated to be218.135kJ/moland306.741kJ/mol, respectively. The MWCNTs loading was favorable to producethe piezoelectric β phase in PVDF matrix. The α phase coexisted with β phase, whichresulted in polymorphic structure in PVDF/MWCNT nanocomposite.The mechanical tensile test results showed that the strength of PVDF/MWCNTcomposite was improved and the elongation at break decreased with the increase ofMWCNTs concentration. The SEM observations showed that the fracture surface ofpure PVDF appeared ductile fracture, while PVDF/MWCNT composite appearedbrittle fracture. The experimental and theoretical calculation values for tensilemodulus of PVDF/MWCNT composite were1320MPa and1214.4MPa, respectively.The close agreement, within about10%, between the experimental and theoreticallypredicted modulus of composite indicated that the external tensile loads weresuccessfully transmitted to the MWCNTs across the MWCNTs-PVDF interface.When PVDF and PVDF/MWCNT composite were drawn at the same temperature anddrawning ratio in a temperature rang from20℃to160℃, we found that MWCNTshad been having strengthening effect in PVDF matrix, however, the effect ofstrengthening decreased with temperature increasing. At room temperature, no stressrelaxation was observed in PVDF and PVDF/MWCNT composite. While temperaturereached at70℃, both of them occurred stress relaxation, in addition, the interactionbetween MWCNTs and PVDF matrix made PVDF/MWCNT composite more difficultto occur stress relaxation than PVDF.Electrical measurments showed that PVDF/MWCNT composite had betterconductivity compared to PVDF/carbon black(CB) composite. The electricalconductivity of PVDF/MWCNT composite with8wt.%MWCNTs was nearly equalto that of PVDF/CB composite with20wt.%CB. In addition, the increase ofresistivity of PVDF/MWCNT composite with strain was lower compared to PVDF/CB composite. For PVDF/MWCNT composites with a segregated structureprepared by high-speed mechanical mixing method, a low percolation threshold(0.078wt.%) and high electrical conductivity were obtained. Compared to ultrasonicdispersion method, this effective preparation method is simpler, low-cost, and lesstime consuming without environmental pollution.