Regulation Of Heterogeneous Melting State Of Poly(vinylidene Fluoride)and Its Influence On Crystallization Behavior

Polymer electroactive materials,due to their unique flexibility and deformability,are gradually replacing traditional ceramic materials to be the leader in various interdisciplinary fields,such as electronic technology,medical health,intelligent textile,energy storage,and environmental monitoring.Among numerous polymer electroactive materials,polar-phase（mainlyβ-andγ-phase）poly（vinylidene fluoride）（PVDF）stand out,with excellent dielectric,ferroelectric,piezoelectric properties,and biocompatibility.Therefore,exploring simple and efficient methods for fabricating PVDF materials in polar phases of high content,and clarifying the inherent mechanisms related to crystal structures,has theoretical guidance significance for the development of polymer electroactive materials.In this thesis,we have systematically studied the influence of heterogeneous melting state and initialα-phase crystal morphology on the fraction ofγ-phase crystals,based on an experimental phenomenon we discovered before that partial melting and recrystallization ofα-PVDF crystals are possible to formγ-PVDF crystals.Toward this end,oriented PVDF ultrathin film and bulk PVDF thin film systems are selected as object of study.Furthermore,isothermal experiments were used to further observe and study the transition from heterogeneous melting state toγcrystals.Finally,the dielectric,ferroelectric,and piezoelectric properties of the bulk PVDF films prepared by this strategy were characterized.The main results of this thesis are as follows:1.Highly oriented pureα-PVDF ultrathin film was prepared through melt-drawing,to study the influence of partial melting degree,i.e.heterogeneous melting state,on the recrystallization behavior.Theγ-phase fraction after melting for 1 min and recrystallization has a significant dependence on the melting temperature,reaching 100%with an optimal melting temperature window of 181～186°C.In-situ observations during the heating process revealed that the crystals have been completely molten and the chain orientation has been completely relaxed within the optimal melting temperature window.Thermal analysis demonstrated that the characteristic heterogeneous structure in the optimal melting temperature window is locally ordered nanostructure.On the contrary,incompletely molten initial crystals,residual initial nuclei,and random molecular chains in homogeneous melts,are beneficial for growth ofαcrystals.The above results indicate that the polar-phase crystals are entirely induced by locally ordered structure.Only if sufficient locally ordered structure exclusive in the heterogeneous melt,can 100%γ-phase crystals be formed.Also,orientedγ-PVDF films cannot be fabricated by this strategy.2.By eliminating the thermal history of solution-cast PVDF bulk film,pureα-PVDF samples obtained through isothermal or non-isothermal crystallization are used as the initial.It was found that a strong dependence of polar phase content on the melting temperature was observed for non-isothermal crystallization samples,with an optimal melting temperature of 175°C.Then,by alternating the（ending）crystallization temperature,we found thatγrecrystallization would only occur when the（ending）crystallization temperature reached below 120°C,andγ-phase fraction gradually increases as（ending）crystallization temperature decreasing.The in situ structural evolution during the cooling process from 120 to 35°C revealed that after the completion of primary crystallization,secondary crystals with thinner thickness were formed in the interfacial region at the surface of primary lamellae.Forα-PVDF samples formed at 120～69.7°C,theγ-phase fraction obtained after melting at175°C for 1 min and recrystallization is perfectly positively correlated to the crystallinity of secondary crystals,while the secondary crystals formed at lower temperatures are too thin to further promote recrystallizedγ-phase fraction.The thermal analysis for the melting of secondary crystals indicates that thermally stable conformational disordered crystals are transformed from secondary crystals with TGTG’conformation during the slow melting process,and then spontaneously turn into T₃GT₃G’conformation at high temperatures around175°C.As a result,γ-phase crystals are induced by the locally ordered nanostructures with T₃GT₃G’conformation during the recrystallization process.The above results indicate that secondary crystals in the initialα-phase PVDF samples is the key to forming sufficient locally ordered structures with T₃GT₃G’conformation and thus inducingγ-phase recrystallization.3.The non-isothermally crystallizedα-PVDF bulk films were used as the initial samples,and structural evolution over time was observed during the isothermal process at annealing temperatures around 175°C.It was found that pureαphase crystallization gradually changes to pureγcrystallization as isothermal temperature elevated from 173 to 175°C,and pureγcrystallization keeps until 178 ^oC.Take isothermal annealing at 175°C as an example,the induction period forγnucleation is very short,only about 5 minutes.While no trace of crystals was observed even after isothermal crystallization at 175°C for20 hours from a homogeneous melt,precursors with T₃GT₃G’conformation must have been formed during the heating process in our system.The observation on phase structures shows that the conformational disordered crystals form aggregation within the first 3 min and nucleate after 5 min,followed by crystal growth after 10 min.The Avrami equation was used to study the crystallization kinetics at different annealing temperatures,and it was found that the crystallization rate constant K of theγcrystals reaches maximum at176°C.We infer that within 173-176°C the growth ofα-andγ-phase crystals is competitive.At relatively higher temperature,random molecular chains will move faster to the surface ofγnucleus and adhere to the surface,accelerating theγ-phase crystallization.At the annealing temperatures above 177°C,due to the strong mobility of molecular chains,the desorption rate of chains on the surface of theγnucleus will gradually approach the adsorption rate,so that the crystallization rate ofγcrystals significantly slows down.Then,when the sample was cooled down to the room temperature after the isothermal annealing at 175°C,α-phase microcrystals and scrolledγ-phase crystals with approximately 600 nm in size are uniformly dispersed in the PVDF sample,demonstrating thatγ-phase locally ordered structures can only induceγcrystals with a finite size.The above results indicate that during the high-temperature annealing process,the locally ordered structure with the T₃GT₃G’conformation serves as a precursor for nucleation through aggregation.Nevertheless,the inducedγcrystals are extremely small andα-phase formation cannot be avoided upon cooling to the room temperature.4.The strategy for sufficientγ-phase crystallization via heterogeneous melting state was implemented for hot-pressed PVDF bulk films and found to be applicable to such non-solvent system.The optimum thermal treatment temperature for fabricatingγ-phase non-solvent PVDF film is 177°C.With a thermal treatment temperature between 175 and 179°C,γ-phase fraction can reach～80%.Compared to the films prepared with solvent,non-solvent PVDF bulk films have no pore defects caused by solvent evaporation and exhibits more uniform structure.The non-solventγ-PVDF films possess fine dielectric breakdown,ferroelectric,and piezoelectric properties,which are comparable to those in the literatures.Therefore,this strategy is expected to be applied in more PVDF systems to achieve more ideal application performance.