Crystallization Of PVDF Research Articles

Effect of PMMA Molecular Weight on Its Localization during Crystallization of PVDF in Their Blends.

In miscible crystalline/amorphous polymer blends, the exclusion behaviors of the latter with various molecular weights during the crystallization of the former were investigated by the combination of SAXS and DSC by taking a PVDF/PMMA blend as an example. The ratio between internal crystallinity from SAXS and overall crystallinity of the entire blend from DSC was employed to characterize the exclusion of PMMA. Our results indicate that the molecular weight of the amorphous component produces a remarkable influence on the diffusion coefficient (D) and the crystal growth rate (G) of the crystalline component. There are both inter-lamellar and inter-fibrillar structures when PVDF blended with lower-molecular-weight PMMA. With increasing molecular weight of PMMA, the decrease in crystal growth rate (G) dominates the enhanced exclusion behaviors of PMMA, resulting in bigger pores after extraction. Our results are significant not only for the basic understanding of crystallization in polymer blends, but also for the fabrication and structure control of porous structures based on crystallization templates.

Hierarchically porous membranes with multiple channels: Fabrications in PVDF/PMMA/PLLA blend and enhanced separation performance

In this work, separation performance of hierarchically porous membranes (HPMs) with multiple channels have been investigated with the combination of finite element simulation and experiment. The simulation results indicate that in addition to m-channels (i.e., channels in microns) in reference (with only pores in microns), nanopores in HPMs not only contribute to n-channels (channels in nanometers), but also connect them with m-channels, producing mixed channels. In experiment, PVDF HPMs have been prepared in poly(vinylidene fluoride)/poly(methyl methacrylate)/poly(l-lactic acid) (PVDF/PMMA/PLLA) blend based on phase separation and crystallization templates. Phase separation produces bi-continuous structures of PLLA and PVDF/PMMA with size of microns. In the latter, PMMA has been expelled out during the crystallization of PVDF, resulting in the bi-continuous structures of them in nanometers. Upon etching with chloroform, both PLLA and PMMA have been removed, yielding pores in microns and nanometers respectively. Relative to reference, HPMs exhibit enhanced separation performance, which can be attributed to the shorter diffusion length, extra channels, enlarged contact angle difference and enhanced driving force for separation. Both simulation and experiment results indicate that multiple channels play more important role in the improvement of flux in the case of smaller m-channel diameter. Our results provide an efficient solution for trade-off effect in separation based on HPMs with multiple channels.

Self-Poled Poly(vinylidene fluoride)/MXene Piezoelectric Energy Harvester with Boosted Power Generation Ability and the Roles of Crystalline Orientation and Polarized Interfaces.

Micropiezoelectric devices have become one of the most competitive candidates for use in self-powered flexible and portable electronic products because of their instant response and mechanic-electric conversion ability. However, achievement of high output performance of micropiezoelectric devices is still a significant and challenging task. In this study, a poly(vinylidene fluoride) (PVDF)/MXene piezoelectric microdevice was fabricated through a microinjection molding process. The synergistic effect of both an intense shear rate (>104 s-1) as well as numerous polar C-F functional groups in MXene flakes promoted the formation of β-form crystals of PVDF in which the crystallinity of β-form could reach as high as 59.9%. Moreover, the shear-induced shish-kebab crystal structure with a high orientation degree (fh = ∼0.9) and the stacked MXene acted as the driving force for the dipoles to regularly arrange and produce a self-polarizing effect. Without further polarization, the fabricated piezoelectric microdevices exhibited an open-circuit voltage of 15.2 V and a short-circuit current of 497.3 nA, under optimal conditions (400 mm s-1 and 1 wt % MXene). Impressively, such piezoelectric microdevices can be used for energy storage and for sensing body motion to monitor exercise, and this may have a positive impact on next-generation smart sports equipment.

Highly Tunable Piezoresistive Behavior of Carbon Nanotube-Containing Conductive Polymer Blend Composites Prepared from Two Polymers Exhibiting Crystallization-Induced Phase Separation

Conductive polymer composites (CPCs) are suitable as piezoresistive-sensing materials. When using CPCs for strain sensing, it is still a big challenge to simultaneously improve the piezoresistive sensitivity and linearity along with the electrical conductivity and mechanical properties. Here, highly tunable piezoresistive behavior is reported for multiwalled carbon nanotube (CNT)-filled CPCs based on blends of two semicrystalline polymers poly(vinylidene fluoride) (PVDF) and poly(butylene succinate) (PBS), which are miscible in the melt. When cooling the homogeneous mixture of the blend components, successive crystallization of PVDF and PBS occurs, creating complex crystalline structures in a mixed amorphous phase. The morphology of the blend matrix, the crystallinity of the blend components, and the dispersion and location of the CNTs in the blend depend on the CNT content and the blend composition. Compared with PVDF/CNT composites, the substitution of 10 to 50 wt % PVDF by PBS in the composites shifts the electrical percolation concentration Φc from 0.79 wt % to filler contents as low as 0.50 wt % while improving the stretchability. The piezoresistive behavior is highly tunable by changing the PVDF/PBS ratio. The ternary composites with matrix compositions of PVDF (90 wt %)/PBS (10 wt %) and PVDF (50 wt %)/PBS (50 wt %) show either higher piezoresistive sensitivity or linearity, respectively, caused by the differences in the microstructure of the CPCs. For example, the crystallinity of PBS in the ternary composites increased from 19.8% to 52.0% as the PBS content increased from 10 wt % to 50 wt %, which is connected with altered CNT distribution and conductive network structure and substantial improvement of the linearity of the electrical response to strains up to >20%. Our findings highly contribute to the understanding of the piezoresistive properties of CPCs based on two semicrystalline polymers and are important for future studies to tune the piezoresistive behavior to achieve simultaneously improved sensitivity and linearity.

Crystallization kinetics, polymorphism fine tuning, and rigid amorphous fraction of poly(vinylidene fluoride) blends

Fast scanning calorimetry (FSC) is used to study crystallization of poly(vinylidene fluoride) (PVDF), and its blends with a random fluorinated copolymer (FCP) of poly(methyl methacrylate) and 1H,1H,2H,2H-perfluorodecyl methacrylate (PMMA-r-PFDMA). By varying the residence time at isothermal melt crystallization temperatures TMC = 80–120°C, the amount of α- and β-phase can be controlled, yielding partial or complete suppression of β-phase for all blends studied. Nonisothermal crystallization kinetics are also studied by FSC and analyzed using the Mo model. Crystallization rates increase when FCP is present. PVDF crystallizes into β-phase when cooled from the melt at rates faster than 3000 K/s giving PVDF crystal fractions of about 0.05–0.16. Only α-phase occurs at cooling rates slower than 2000 K/s yielding larger PVDF crystal fractions of about 0.30–0.43. Cooling rates between those limits result in mixed α- and β-phase. The rigid amorphous fraction (RAF) of PVDF varies with α and β crystal fraction in PVDF/FCP blends. RAF of α-phase PVDF ranges from 0.21 to 0.28 whereas RAF of β-phase PVDF spans a wider range, reaching values from 0.42 to 0.46.

Surface modified iron oxide (Fe3O4) nanosheets reinforced PVDF nanocomposites: influence on morphology, thermal and magnetic properties

ABSTRACT In this work, iron oxide (Fe3O4) nanosheets are synthesised using the solvothermal method. Transmission electron microscope (TEM) observation confirms the nanosheets-like morphology of Fe3O4. While X-ray diffraction (XRD) studies of Fe3O4 nanosheets reveal the Fe3O4 nanosheets synthesised are crystalline in nature. Vibration sample magnetometry studies of Fe3O4 nanosheets have shown superparamagnetic behaviour with saturation magnetisation (M s) value of 29.6 emu/g. The synthesised Fe3O4 nanosheets are dispersed in poly(vinylidene fluoride) PVDF matrix using the solution casting method with slow evaporation of solvent at 60 °C which facilitates the formation of β-form of PVDF crystals. The PVP modified Fe3O4 nanosheets incorporation in the PVDF matrix influences the thermal stability and crystalline behaviour. Scanning electron microscope observation shows the improved dispersion of Fe3O4 nanosheets in the PVDF matrix by modifying Fe3O4 nanosheets with PVP. The M s value is increased by ∼122% for PVP modified Fe3O4 nanosheets reinforced PVDF nanocomposites as compared to Fe3O4 nanosheets reinforced PVDF nanocomposites.

Polycarbonate/Poly(vinylidene fluoride)-Blend-Based Nanocomposites-Effect of Adding Different Carbon Nanofillers/Organoclay.

Carbon black (CB), carbon nanotubes (CNTs), and graphene nanoplatelets (GnPs) individually or doubly served as reinforcing fillers in polycarbonate (PC)/poly(vinylidene fluoride) (PVDF)-blend (designated CF)-based nanocomposites. Additionally, organo-montmorillonite (15A) was incorporated simultaneously with the individual carbon fillers to form hybrid filler nanocomposites. Microscopic images confirmed the selective localization of carbon fillers, mainly in the continuous PC phase, while 15A located in the PVDF domains. Differential scanning calorimetry results showed that blending PVDF with PC or forming single/double carbon filler composites resulted in lower PVDF crystallization temperature during cooling. However, PVDF crystallization was promoted by the inclusion of 15A, and the growth of β-form crystals was induced. The rigidity of the CF blend increased after the formation of nanocomposites. Among the three individually added carbon fillers, GnPs improved the CF moduli the most; the simultaneous loading of CNT/GnP resulted in the highest moduli by up to 33%/46% increases in tensile/flexural moduli, respectively, compared with those of the CF blend. Rheological viscosity results showed that adding CNTs increased the complex viscosity of the blend to a greater extent than did adding CB or GnPs, and the viscosity further increased after adding 15A. The electrical resistivity of the blend decreased with the inclusion of carbon fillers, particularly with CNT loading.

Crystallization and melting behavior of polyvinylidene fluoride in confined space of nanotubes

Homopolymer poly (vinylidene fluoride) (PVDF) thin films were fabricated via high pressure process and then PVDF film was melt into porous anodic aluminum oxide (AAO) templates to form hollow nanotubes and isothermally crystal at a certain temperature. Scanning electron microscopy (SEM) and Transmission Electron Microscope (TEM) is used to observed the microstructure of nanotubes. To explore the influence of nanopores on the crystallization and melting of PVDF, differential scanning calorimetry (DSC) is used. It turns out that crystal phase is restrained in the nanotubes and mainly exists and at high temperature within relatively large nanopores and the bulk. As for phase, the increase of isothermal crystallization temperature results in the increase of melting temperature.

Phase Transitions in Poly(vinylidene fluoride)/Polymethylene-Based Diblock Copolymers and Blends.

The crystallization and morphology of two linear diblock copolymers based on polymethylene (PM) and poly(vinylidene fluoride) (PVDF) with compositions PM23-b-PVDF77 and PM38-b-PVDF62 (where the subscripts indicate the relative compositions in wt%) were compared with blends of neat components with identical compositions. The samples were studied by SAXS (Small Angle X-ray Scattering), WAXS (Wide Angle X-ray Scattering), PLOM (Polarized Light Optical Microscopy), TEM (Transmission Electron Microscopy), DSC (Differential Scanning Calorimetry), BDS (broadband dielectric spectroscopy), and FTIR (Fourier Transform Infrared Spectroscopy). The results showed that the blends are immiscible, while the diblock copolymers are miscible in the melt state (or very weakly segregated). The PVDF component crystallization was studied in detail. It was found that the polymorphic structure of PVDF was a strong function of its environment. The number of polymorphs and their amount depended on whether it was on its own as a homopolymer, as a block component in the diblock copolymers or as an immiscible phase in the blends. The cooling rate in non-isothermal crystallization or the crystallization temperature in isothermal tests also induced different polymorphic compositions in the PVDF crystals. As a result, we were able to produce samples with exclusive ferroelectric phases at specific preparation conditions, while others with mixtures of paraelectric and ferroelectric phases.

Microsphere with narrow nanopores: Fabrication in PVDF/PMMA/PLLA blend and enhanced adsorption/separation performances

In this work, a novel strategy for fabricating microsphere with narrow nanopores in PVDF/PMMA/PLLA blend has been reported. In the ternary blend, phase separation results in isolated PVDF/PMMA mixed-phase and PLLA matrix. In the former, the crystallization of PVDF produces bi-continuous nanostructures of two phases. Upon etching with chloroform, both PLLA matrix and continuous PMMA in islands can be removed, yielding PVDF microspheres with narrow nanopores. According to the templates of phase separation and crystallization, the size of microsphere and narrow nanopore can be tailored precisely and separately. Relative to the reference (compact spheres), the porous PVDF microspheres exhibit enhanced performances. On one hand, the special wettability caused by hierarchical roughness and extra nano-channels endow the membrane comprising porous PVDF microspheres with excellent performance during the separation of water-in-chloroform emulsion. On the other hand, the stronger capillary effect (resulted from pore shape and pore size) and the existence of interpenetrated nanopores account for the higher capacity in the absorption of oil. Furthermore, it is facile to load nano-Fe3O4 in the porous microspheres to achieve magnetic controllability. Our results provide a novel strategy for not only the fabrication of microsphere with narrow nanopores, but also the improvement of separation and absorption performances.

Structural relaxation and dielectric response of PVDF/PMMA blend in the presence of graphene oxide

Structural relaxation of polymer chains exhibits high correlation to the macroscopic performance of polymers, and it has been well investigated and analyzed in the polymer systems. However, it is still challenging to accurately describe the structural relaxation of some complex systems, such as the nanomaterials incorporated blends. In this work, structural relaxations in the poly (vinylidene fluoride)/poly (methyl methacrylate) (PVDF/PMMA) blends in the presence of graphene oxide (GO) were systematically investigated via revealing the cooperativity concerning about segmental relaxation, chain relaxation relating to the crystallization behaviors using the broadband dielectric spectroscopy (DRS). In the presence of GO, the cooperative volume was enlarged and dynamic heterogeneity was weakened. And the chain relaxation αcmoved to higher frequency; while the αm relaxation associated with molecular motion in miscible amorphous region of PVDF/PMMA blends can be well resolved. Thus, the structure relaxations confirmed that GO could hinder the crystallization of PVDF but enhance the miscibility in the PVDF/PMMA amorphous phase. On the basis of structure relaxations, the dielectric properties as function of temperature and frequency were accordingly studied, and at room temperature PVDF/PMMA blends in the presence of GO exhibited higher constant and lower loss at low frequency regions.

Crystallization Monitoring of Semicrystalline Poly(vinylidene fluoride)/1-Ethyl-3-methylimidazolium Hexafluorophosphate [Emim][PF6] Ionic Liquid Blends

The electroactive characteristics of poly(vinylidene fluoride) (PVDF) are widely and increasingly being used in technological applications, where controlling the crystallization of the PVDF is of utmost importance. The nucleation and growth of crystals in the β or γ electroactive phases, or in the nonelectroactive α phase, depends on a number of factors that, despite the studies carried out, are still to be properly understood, in particular, when blended with specific active fillers. In this context, the crystallization of PVDF blended with the ionic liquid (IL) 1-ethyl-3-methylimidazolium hexafluorophosphate ([Emim][PF6]) has been analyzed. Both components are capable of crystallizing from the melt. The growth of the crystalline phases of PVDF during isothermal crystallization at different temperatures has been monitored using Fourier transform infrared (FTIR) spectroscopy. The isothermal crystallization kinetics of PVDF and the melting temperatures of both PVDF and IL were characterized by differential scanning calorimetry, and the microstructures of the blends were analyzed by optical and electron microscopy. It is observed for [Emim][PF6]/PVDF blends that the isothermal crystallization from the melt between 120 and 162 °C produces PVDF crystallites in the β and γ electroactive phases, while the formation of α-phase crystals is nearly suppressed. The morphology of the blends is altered by the addition of IL, which results in the separation of solid phases at room temperature. In addition, [Emim][PF6] remains liquid when mixed with the amorphous PVDF chains due to the cryoscopic descent.

Enhanced dielectric constant of PVDF-based nanocomposites with one-dimensional core-shell polypyrrole/sepiolite nanofibers

In this paper, the natural sepiolite nanofibers (Sep) were selected to be as the dispersion supporter of conductive polymer polypyrrole (PPy) owing to low cost, natural nanowires and well dispersibility in the PVDF matrix. The one-dimensional core-shell polypyrrole/sepiolite (PPy@Sep) nanofibers were synthesized by in-situ polymerization and subsequently introduced into the PVDF matrix by the solution blending method. The experimental results demonstrated that PPy was uniformly coated on the Sep surface. The PPy@Sep nanofibers could disperse in PVDF matrix well and produce the effect of heterogeneous nucleation on PVDF crystallization. With the increasing content of PPy@Sep from 1 wt% to 5 wt%, the dielectric constant of PVDF-based nanocomposites improved from 12 to 40 at 100 Hz because of the hierarchical interfacial polarization originating from the PPy/Sep interface and PPy/PVDF interface and microcapacitor effect. Meanwhile, a low dielectric loss was observed in the nanocomposites.

Tailoring of a Piezo‐Photo‐Thermal Solar Evaporator for Simultaneous Steam and Power Generation

AbstractWater and electricity shortages constitute a global energy crisis that cannot be ignored. The sun is an unlimited source of energy, and oceans provide abundant water and renewable energy resources. In this study, poly(vinylidene fluoride) (PVDF)/graphene solar evaporator membranes are fabricated for simultaneous freshwater production and power generation. Graphene addition transformed the PVDF crystal from the α‐phase to the piezoelectric self‐assembly β‐phase. The resulting membrane is used to convert the mechanical energy of waves to electrical energy. The membrane has an output voltage of 2.6 V (±1.3 V) and an energy density of 2.11 Wm−2 for 1 Hz simulated waves, which are higher than values reported in the literature. The stacked graphene and polymer formed a wood‐lumens‐like mesoporous structure with a photothermal effect. Under one sun illumination, the water production rate is 1.2 kg m−2 h−1, and the solar‐thermal energy conversion efficiency is 84%. Finally, a prototype is built to prove a single evaporator's feasibility that can simultaneously obtain freshwater and generate electricity. Thus, this membrane serves as an ocean wave power generation device that can provide all‐weather energy generation, convert stored electrical energy into thermal energy at night and on cloudy days, and continuously provide safe drinking water.

Preparation of PVDF membrane based on “In-situ Template-TIPS” technology and the investigation on membrane formation mechanism, microstructure regulation and permeability

In this work, poly(vinylidene fluoride) (PVDF) flat membrane was prepared via “In-situ template-TIPS” technology using poly(vinylpyrrolidone) (PVP) as template. The membrane formation mechanism and microstructure evolution of this method were expounded for the first time. The effect of PVP content on the crystallization behavior of the dope, the microstructure, the crystallization behavior and the permeability of the membrane was investigated in detail. Interestingly, the solidification of PVP from the diluent via air-cooling at room temperature not only hindered the PVDF crystallization, but also the growth of PVP led to the formation of porous surface and eliminated the thick and dense skin layer, and thus formed the inter-connected structure and higher porosity (The maximum porosity is 88.9%). The maximum pure water flux reached as high as 3420 L m−2 h−1. The membrane also indicated an excellent performance in rejection of bacteria and separation of “petroleum ether/water” emulsion (nearly 100%). The α crystal phase was formed for the membrane by air-cooling at room temperature. The non-isothermal crystallization kinetics revealed that PVP can act as a heterogeneous nucleating agent for PVDF crystallization and hindered the growth of PVDF crystal in three-dimension. The research presented a novel and simple idea for tailoring the microstructure of membrane via TIPS process.

Achieving Flat-on Primary Crystals by Nanoconfined Crystallization in High-Temperature Polycarbonate/Poly(vinylidene fluoride) Multilayer Films and Its Effect on Dielectric Insulation

To meet the stringent requirements of next-generation film capacitors for power electronics, multilayer films (MLFs) are fabricated with the advantage of achieving high temperature rating, high energy density, and reasonably low loss simultaneously. In this study, a high permittivity polar polymer, poly(vinylidene fluoride) (PVDF), is multilayered with a linear, low loss dielectric polymer such as high-temperature polycarbonate (HTPC). However, the dielectric loss of these MLFs was still high as compared with current state-of-the-art biaxially oriented polypropylene (BOPP) films. The goal of this work is to decrease the dielectric loss and enhance dielectric insulation by achieving flat-on primary PVDF crystals in MLFs via nanoconfined melt-recrystallization. Based on simultaneous small- and wide-angle X-ray scattering experiments, edge-on lamellar crystals were observed for all as-extruded MLFs, regardless of different PVDF layer thicknesses. However, after melting at 180 °C followed by recrystallization, flat-on primary crystals were successfully achieved when the PVDF layer thickness was below 39 nm. Above 78 nm for the PVDF layer, major edge-on primary crystals with minor flat-on secondary crystals were observed. From leakage current, breakdown, lifetime, and electric displacement-electric field loop studies, MLFs with the flat-on primary crystals exhibited reduced loss and enhanced dielectric insulation as compared to as-extruded MLFs and those with edge-on primary/flat-on secondary crystals. This was attributed to the effective blockage of charge carriers by the flat-on PVDF primary crystals and their reduced ferroelectric switching.

Influence of hydrophilic/hydrophobic protic ionic liquids (PILs) on the poly(vinylidene fluoride) (PVDF-ionic liquid) membrane properties

In this study, we investigate the effect of protic ionic liquids (PILs) on the properties of poly(vinylidene fluoride)/protic ionic liquids (PVDF/PIL) composite membranes made by solvent casting. The morphology, polymer phase, crystallinity, proton conductivities and mechanical properties were determined according to the nature and quantities of PIL added. Independently of the ionic liquids nature among EtPy][HSO4], [Py][HSO4], [(Octyl)3NH][HSO4] and [(Octyl)3NH][H2PO4], we observed the crystallization of PVDF into more stable electroactive phases (β and γ). Furthermore, the presence of PIL decreased the elastic modulus and modified the crystallization kinetics, as indicated by the size of the spherulitic microstructures. Proton conductivity results suggest the predominance of the Grotthuss-type conduction mechanism for all PVDF/PIL composites membranes supplied by the amphoteric anions, HSO4− and H2PO4−. Finally, the higher stable conductivities observed for hydrated membranes with [(Octyl)3NH][HSO4] evidenced that the Grotthuss mechanism is favored by amphiphilic cation associated with the stronger hydrogen-bonded network of the [HSO4]− anion.

Enhancement of EMI shielding effectiveness of flexible Co2U-type hexaferrite (Ba4Co2Fe36O60)-poly(vinylidene fluoride) heterostructure composite materials: An improved radar absorbing material to combat against electromagnetic pollution

In this report, for the first time, Co2U-type hexaferrite (Ba4Co2Fe36O60) nanomaterials have been considered along with their conjugation with a poly(vinylidene fluoride) (PVDF) matrix for the fabrication of heterostructure composite materials with superior electromagnetic interference shielding effectiveness (SE) that can produce better attenuation of microwave radiation. The high value of magnetizations of ∼7.21 and 10.63 emu/g is significant for the improvement of the shielding effectiveness due to absorption (SEA) of the composite materials. The unique combination of magnetic and dielectric responses of these Co2U-type hexaferrite nanomaterials helps to get significantly high total shielding effectiveness (SET) in their PVDF composite states within the frequency range of 8–18 GHz. A very high value of SET of approximately −83 dB at 14.2 GHz with >99.999 999% of attenuation of incident radiation has been observed for the Co2U-type hexaferrite–PVDF composite materials. The β-phase modified PVDF structure also plays the most important role to generate induced polarization inside the structure of composite materials. Also, this PVDF matrix provides the application friendly laminated structure of Co2U-type hexaferrite–PVDF composite materials. Structural, morphological, and chemical studies reveal the presence of the polycrystalline phase of Co2U-type hexaferrite and β-phase crystallization of polar PVDF in the multi-phase Co2U-type hexaferrite–PVDF composite materials. This study has helped us to understand the importance of magnetic Co2U-type hexaferrite nanomaterials for the fabrication of lightweight, large area, flexible, and thickness-controlled radar absorbing materials to combat against electromagnetic pollution.

Enhanced Reproducibility of Positive Temperature Coefficient Effect of CB/HDPE/PVDF Composites with the Addition of Ionic Liquid

Developing an effective method for improving the reproducibility of positive temperature coefficient (PTC) effect is of great significance for large-scale application of polymer based PTC composites, owing to its contribution to the security and reliability. Herein, we developed a carbon black (CB)/high density polyethylene (HDPE)/poly(vinylidene fluoride) (PVDF) composite with outstanding PTC reproducibility, by incorporating 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([OMIm][NTf2]) into the composite. After multiple repeated temperature cycles, the PTC performance of as-prepared material keeps almost unchanged and the varition of resistance at room temperature is less than 7%. Our studies revealed that [OMIm][NTf2] contributes to the improvement of PTC reproducibility in two ways: (i) it acts as an efficient plasticizer for refining the co-continuous phase morphology of HDPE/PVDE blends; (ii) it inhibits the crystallization of PVDF through the dilution effect, leading to more overlaps of the volume shrinkage process of HDPE and PVDF melt which results in the decrease of interface gap between HDPE and PVDF. This study demonstrated that ionic liquids as the multifunctional agents have great potential for improving the reproducibility in the application of the binary polymer based PTC composites.

Tuning the Piezoresistive Behavior of Poly(Vinylidene Fluoride)/Carbon Nanotube Composites Using Poly(Methyl Methacrylate).

In conductive polymer composites (CPCs), which can be used as both strain sensors and materials with self-diagnosis capabilities for structural health monitoring, the piezoresistive sensitivity can be tuned by changing the electrical filler network structure, mainly influenced by the conductive filler content. Typically, the electrical resistance increases exponentially with strain, and the piezoresistive sensitivity and linearity cannot be improved simultaneously. In this work, we report a facile method to tune the piezoresistive behavior of melt-mixed poly(vinylidene fluoride) (PVDF)/carbon nanotube (CNT, 0.75-2.0 wt %) composites using blending with poly(methyl methacrylate) (PMMA, 5-30 wt %). PVDF and PMMA are completely miscible in the melt state regardless of the proportion. For PVDF-rich blends, the crystallization of PVDF induces separation of the PVDF crystal region from the miscible PVDF/PMMA amorphous blend part during the cooling process. Addition of PMMA tuned the piezoresistive strain behavior and improved the electrical conductivity and toughness at the same time. The PVDF/PMMA/CNT composites show higher sensitivity at low strains than their PVDF/CNT counterparts with comparable initial resistivity. For example, ΔR/R0 at 5% strain is 18.6% for the PVDF(80)/PMMA(20) blend containing 0.75 wt % CNT versus 11.0% for PVDF containing 1 wt % CNT, both having a volume resistivity of around 104 Ω·cm. The PVDF/PMMA/CNT blend composites also show a less steep exponential increase in the sensing response at higher strains, indicating better linearity. These differences are due to the altered microstructure of the composites and the more homogeneous distribution of CNTs between the smaller and less numerous PVDF crystallites when PMMA is added. The concept of modifying the composite microstructure by adding another commercially available miscible polymer offers a simple and effective way to tune the piezoresistive behavior and improve mechanical properties of CPC sensor materials.