A comparative assessment of poly(vinylidene fluoride)/conducting polymer electrospun nanofiber membranes for biomedical applications

Piezoelectric polymers, especially poly(vinylidene fluoride) (PVDF) are increasingly receiving interest as smart biomaterials for tissue engineering, energy harvesting, microfluidic, actuator, and biosensor applications. Despite possessing the greatest piezoelectric coefficients among all piezoelectric polymers, it is often desirable to increase the electrical outputs from PVDF for several of these applications. Blending with intrinsically conducting polymers (CP) in the form of nanofiber membranes is one of the facile methods to achieve the same. However, these polymers and their composites have so far been primarily investigated only for their physical property enhancements and in applications like energy storage while their biomedical applications and comparative assessment of their biocompatibility properties have not been yet explored. In this report, electrospinning of PVDF blends with polypyrrole (PPy), polyaniline (PANI), and a modified PANI with l -glutamic acid (PANI-LGA/P-LGA) is performed to obtain different electrically active material membranes. The PVDF:CP composite nanofibers are compared with respect to their nanostructures, β-phase content, and electrical conductivity. Further, biocompatibility of all the membranes was compared. It was found that incorporation of PPy, PANI, and P-LGA increased the electrical conductivity of PVDF while the β-phase content was also substantially enhanced. The highest biocompatibility with a pre-osteoblast cell line (MC3T3) was exhibited in the order p-LGA/PVDF > PANI/PVDF > PPy/PVDF, all being significantly higher than PVDF (p < .001). Although P-LGA/PVDF showed higher electrical conductivity, biocompatibility with MC3T3, it was found to be highly cytotoxic to a HeLa (cancer) cell line. It is concluded that such structure property relations would help in selection of materials for specific biomaterial applications.     

Highly conductive proton exchange membranes based on sulfonated poly(phthalazinone ether sulfone) and cerium sulfophenyl phosphate

Polymer composite membranes based on sulfonated poly(phthalazinone ether sulfone) (SPPES) and cerium sulfophenyl phosphate (CeSPP) are prepared. Three CeSPP concentrations are used: 10, 20, and 30 wt.%. The membranes are characterised by infrared spectroscopy (IR), X-ray diffraction spectroscopy, thermal gravimetric analysis, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The IR results indicate the formation of intense hydrogen bonds between CeSPP and SPPES molecules. The SEM micrographs show that CeSPP well dispersed in composite membrane. The properties of the membranes are evaluated by their water uptake, ionic exchange capacity, proton conductivity and methanol permeability. The proton conductivity of the SPPES (DS 91%)/CeSPP (30 wt.%) composite membrane (I) reaches 0.384 S/cm at 130 °C and 100% relative humidity, which is three times more than Nafion®117. CeSPP improves the conductivity of composite membranes at a low humidity. At 105 °C and 70% RH, the proton conductivity of membrane (I) is 9.1 × 10⁻² S/cm, while Nafion®117 8.8 × 10⁻³ S/cm. The methanol permeability of membrane (I) is 10⁻⁸ cm²/s. That is much lower than Nafion®117.     

Influence of oxygen plasma treatment parameters on poly(vinylidene fluoride) electrospun fiber mats wettability

Electrospun poly(vinylidene fluoride) (PVDF) fiber mats find applications in an increasing number of areas, such as battery separators, filtration and detection membranes, due to their excellent properties. However, there are limitations due to the hydrophobic nature and low surface energy of PVDF. In this work, oxygen plasma treatment has been applied in order to modify the surface wettability of PVDF fiber mats and superhydrophilic PVDF electrospun membranes have been obtained. Further, plasma treatment does not significantly influences fiber average size (∼400 ± 200 nm), morphology, electroactive β-phase content (∼80–85%) or the degree of crystallinity (X_c of 42 ± 2%), allowing to maintain the excellent physical–chemical characteristics of PVDF. Plasma treatment mainly induces surface chemistry modifications, such as the introduction of oxygen and release of fluorine atoms that significantly changes polymer membrane wettability by a reduction of the contact angle of the polymer fibers and an overall decrease of the surface tension of the membranes.     

Preparation and Characterization of Thermal Conductive Composite Membranes of Aligned Esterified Carbon Nanotubes/Poly(vinylidene fluoride)

Esterified carbon nanotubes (MWCNT-COOC₁₆H₃₃) were prepared. Composite membranes were fabricated. An alternating current (AC) electric field was applied to the membrane structure orientation. The structures and properties of composite membranes were investigated. The fillers induced the β phase of PVDF; the electric field further enhanced the β phase and made more orderly structures. The fillers arranged along with the electric field and formed a thermal channel. The thermal conductivities of composite membranes were improved by MWCNT-COOC₁₆H₃₃. When the MWCNT-COOC₁₆H₃₃ content reached 5%, the thermal conductivity of composite membrane was 56.05% higher than pure PVDF and even 57.18% higher after alignment.     

Desirable electrical and mechanical properties of continuous hybrid nano-scale carbon fibers containing highly aligned multi-walled carbon nanotubes

The continuous highly aligned hybrid carbon nanofibers (CNFs) with different content of acid-oxidized multi-walled carbon nanotubes (MWCNTs) were fabricated through electrospinning of polyacrylonitrile (PAN) followed by a series of heat treatments under tensile force. The effects of MWCNTs on the micro-morphology, the degree of orientation and ordered crystalline structure of the resulting nanofibers were analyzed quantitatively by diversified structural characterization techniques. The orientation of PAN molecule chains and the graphitization degree in carbonized nanofibers were distinctly improved through the addition of MWCNTs. The electrical conductivity of the hybrid CNFs with 3 wt% MWCNTs reached 26 S/cm along the fiber direction due to the ordered alignment of MWCNTs and nanofibers. The reinforcing effect of hybrid CNFs in epoxy composites was also revealed. An enhancement of 46.3% in Young’s modulus of epoxy composites was manifested by adding 5 wt% hybrid CNFs mentioned above. At the same time, the storage modulus of hybrid CNF/epoxy composites was significantly higher than that of pristine epoxy and CNF/epoxy composites not containing MWCNTs, and the performance gap became greater under the high temperature regions. It is believed that such a continuous hybrid CNF can be used as effective multifunctional reinforcement in polymer matrix composites.     

Conductivity and anticorrosion performance of polyaniline/zinc composites: Investigation of zinc particle size and distribution effect

Polyaniline/zinc composites and nanocomposites were prepared using solution mixing method. Zinc (Zn) particles with an average particle size of 60 μm and zinc nanoparticles with an average particle size of 35 nm were used as fillers in polyaniline (PANI) matrix. Films and coatings of PANI/Zn composites and nanocomposites were prepared by the solution casting method. Electrical conductivity and anticorrosion properties of PANI/Zn composite and nanocomposite films and coatings with different zinc loadings were evaluated. According to the results, electrical conductivity and anticorrosion performances of both PANI/Zn composites and nanocomposites were increased by increasing the zinc loading. Also results showed that the PANI/Zn nanocomposite films and coatings have better electrical conductivity and corrosion protection effect on iron coupons compared to that of PANI/Zn composite.     

Effect of organo clay content on proton conductivity and methanol transport through crosslinked PVA hybrid membrane for direct methanol fuel cell

In the present study, crosslinked poly(vinyl alcohol) (PVA) membranes were prepared using poly(styrene sulfonic acid-co-maleic acid) (PSSA_MA) (PVA:PSSA_MA = 1:7). The PSSA_MA was used both as a crosslinking agent and as a donor of the hydrophilic group (–SO₃H and/or –COOH). The hybrid membranes were prepared by modified clay such as Clay Na⁺, Clay 30B, and Clay 15A. The thermal, water uptake, proton and methanol transport properties of the hybrid membrane were found to be sensitive to the clay type and content. The hybrid membrane with Clay 30B shows higher proton conductivity than other hybrid membranes due to hydroxyethyl group. The membrane with Clay 15A showed the lowest methanol permeability due to lower specific gravity than other clay. Compared to the membrane without modified, the PVA/PSSA_MA/Clay 15A containing 4 wt% of Clay 15A showed both high proton conductivity (0.023 S/cm) and low methanol permeability (2.19 × 10^?7 cm²/s).     

Suppression of AC conductivity by crystalline transformation in poly(vinylidene fluoride)/carbon nanofiber composites

Poly(vinylidene fluoride) (PVDF) is an important ferroelectric semi-crystalline polymer with multiple-phase behavior. In this study, remarkable effects of the various crystalline structures of PVDF nanocomposites on alternating current (AC) conductivity were discovered using carbon nanofibers (CNF). It was found that the transformation from α-phase to β-phase in PVDF, induced by the addition of CNFs, had a surprisingly suppressive effect on the AC conductivity of the nanocomposites. These unexpected results indicate that the decline in conductivity occurs after re-crystallization treatment (annealing) of the nanocomposites, and the reduction levels increase with increasing amounts of CNFs. Interestingly, the AC conductivity of annealed 5 wt% CNF/PVDF composites becomes even lower than that of re-crystallized nanocomposites with 3 wt% CNFs. These findings are believed to be very significant for fabrication and long-term service of PVDF composites in industry, which often involves exposure to repeated thermal cycling.     

Influence of molecular weight of polydimethylsiloxane precursors and crosslinking content on degree of ethanol swelling of crosslinked networks

Using PDMS (polydimethylsiloxane) as a basic polymeric matrix to the preparation of ethanol-permselective pervaporation membranes is a vibrant field of research. In this paper, a detailed study of the effects of the molecular weight of PDMS precursors and the content of the TEOS (tetraethyl orthosilicate) crosslinker on the degree of swelling in ethanol and ethanol contact angle is reported. Five PDMS precursors with molecular weights of 26.6 K, 35.5 K, 50.2 K, 71.7 K, and 110.4 K, and five crosslinking contents (1 wt%, 2 wt%, 5 wt%, 10 wt%, and 15 wt%) were chosen to prepare twenty-five PDMS networks. Considering only the maximum tensile strength of the networks, the optimum molecular weight of the precursor was found to be 35.5 K and the optimum crosslinker content was 5 wt%. The average Young’s modulus of the PDMS network prepared under these conditions reached 0.63 MPa after using toluene to extract the network. Some uncrosslinked precursors always occur in the networks, and have some influence on the molecular weight of the precursors and the crosslinker content that is used. It was found that the content of the uncrosslinked precursors has direct effect on the contact angle of ethanol sessile drops at the surface of the extracted PDMS networks, and higher extraction corresponded to a smaller ethanol contact angle. A combined parameter (S), defined as the quotient of the extraction amount (A_E) and the tensile elastic modulus (E_Y), gives a good linear relationship with the increase in weight of networks swelled in ethanol. This means that the degree of equilibrium swelling of the networks is simultaneously strongly influenced by the tensile modulus and the content of the uncrosslinked precursors.     

Enhanced dielectric permittivity and thermal conductivity of hexagonal boron nitride/poly(arylene ether nitrile) composites through magnetic alignment and mussel inspired co-modification

In this work, we present novel hexagonal boron nitride (h-BN)/poly(arylene ether nitrile) nanocomposites with high dielectric permittivity and thermal conductivity. For this purpose, the interfacial adhesion and orientation of nanofillers are the two key factors that need to be considered. Firstly, iron oxide was attached onto the surface of h-BN to obtain magnetically responsive property, which would realize the orientation of h-BN by applying an external magnetic field during the preparation process of PEN composites. Secondly, the magnetic h-BN was further modified by mussel-inspired method with dopamine and secondary functional monomer (KH550). It was found that the alignment of h-BN and improvement of interfacial adhesion resulted in the interesting properties of PEN composites. With addition of 30 wt% modified h-BN, the dielectric permittivity of PEN composites was increased from 3.2 of neat PEN to 16.4 (increased by 413%), and the low dielectric loss was remained. Meanwhile, the thermal conductivity was enhanced to 0.662 W/m K (increased by 140%) at the same loading content. In addition, the resulting h-BN/PEN nanocomposites maintained high mechanical strength and thermal stability even the nanofillers loading content reached 30 wt%. Therefore, the dielectric and thermally conductive h-BN/PEN composites with high mechanical strength and thermal stability have big advantages in the area of energy storage devices.     

Polymer-assistant ceramic nanocomposite materials for advanced fuel cell technologies

In this study,nanocomposites of LaCePr-oxide (LCP) and Ni_0.8Co_0.15Al_0.05LiO_2-δ (NCAL) with different contents of polyvinylidene fluoride (PVDF) were prepared and applied to solid oxide fuel cells. The composite materials were characterized by X-ray diffraction analysis (XRD), scanning electron microscope (SEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and electrochemical impedance spectrum (EIS). The effect of PVDF concentration on the conductivity and performance of the fuel cells was investigated. It was found that PVDF plays a template role of pore forming in the nanocomposites, and the changed microstructure by as-formed pores greatly influences the electrochemical property of the nanocomposites. The cell with 3 wt% PVDF heat-treated at 210 °C achieved the highest power density of 982 mW cm⁻² at 520 °C, which enhanced performance by more than 57% than when no heat-treatment was implemented. It is 66% higher than the cell with no PVDF and no heat-treatment. Pores formed by PVDF after heat-treatment enlarged the triple phase boundary (TPB), which results in improved fuel cell performance.     

Preparation of high styrenic sulfonated polySEPS/clay composite film for proton exchange membranes (PEMs)

High styrenic sulfonated polystyrene-block-poly(ethyl-ran-propylene)-block-polystyrene (S-polySEPS) containing 65% styrene groups was prepared by sulfonation at the phenyl group. Also, S-polySEPS/clay composite film was produced by mixing organic clay with S-polySEPS in organic blending solvent (THF/DCE/IPA). The proton conductivity of the pure S-polySEPS film and S-polySEPS/clay composite films was ranged from 10^?2 to 10^?1 S cm^?1. In particular, the S-polySEPS/clay 1 wt% composite film was shown higher proton conductivity, higher ion exchange capacity (IEC) and lower water uptake than Nafion^® 117 membrane. However, the proton conductivity of the S-polyseps/clay composite films slightly was decreased with increasing the contents of organic clay. Thermogravimetric analysis (TGA) was carried out to investigate the thermal stability of S-polySEPS/clay composite films. The ¹H NMR and FT-IR analysis is used to verify the sulfonation reaction on the phenyl groups of S-polySEPS. The micro-phase separated images and dispersed organic clay state of the prepared films were confirmed by transmission electron microscopy (TEM) and X-ray diffraction (XRD).     

Non-fluorinated hybrid composite membranes based on polyethylene glycol functionalized polyhedral oligomeric silsesquioxane [PPOSS] and sulfonated poly(ether ether ketone) [SPEEK] for fuel cell applications

Novel hybrid composite membranes were prepared by blending poly(ethylene glycol) functionalized polyhedral oligomeric silsesquioxane [PPOSS] as nanofiller in varying concentration ranging from 1 to 5% (w/w) into sulfonated poly(ether ether ketone) [SPEEK] with degree of sulfonation ～55% for proton exchange membrane fuel cells [PEMFCs]. The effect of incorporation of PPOSS into SPEEK matrix was investigated in terms of thermomechanical and morphological properties, water uptake and proton conductivity of SPEEK. All the composite membranes were thermally and mechanically stable up to 250 °C. Transmission electron microscopy (TEM) revealed that the smallest particle size (～100 nm) of PPOSS was found for SPEEK membranes containing 2% (w/w) PPOSS where as agglomeration (～300 nm) was observed at higher loadings of PPOSS. The proton conductivity was found to be dependent on the morphology and was independent of the amount of water present in the membranes. At 100 °C and 100% RH, the highest proton conductivity (47 mS/cm compared 34 mS/cm for neat SPEEK i.e. an increase of ～51%) was recorded at 2% (w/w) PPOSS contents followed by a decrease on further addition of PPOSS.The water uptake of composite membranes increased with concentration of PPOSS while maintaining their hydrolytic stability at 100 °C for more than 24 h.     

Theophylline molecular imprint composite membranes prepared from poly(vinylidene fluoride) (PVDF) substrate

Theophylline molecular imprint composite membranes were prepared on the PVDF membrane substrate through the free radical polymerization method using theophylline as a template, methacrylic acid (MAA) as a functional monomer, and ethylene glycol dimethacrylate (EDMA) as a cross-linker. The binding constant (K) for the formation of monomer–template adduct was determined by means of infrared spectroscopy titration and nonlinear least-squares method. Theophylline (K=140 M^?1) can form more specific binding sites with MAA than caffeine (K=83 M^?1), therefore was chosen as the template. An effective ultrasonic cleaning method was used to remove the bound theophylline templates from the polymerized PVDF membrane. The reaction conditions were investigated to optimize the maximum binding capacity of theophylline templates to the PVDF membrane. Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and contact angle measurement were used to study the surface chemistry, morphological structure and hydrophilicity of theophylline molecular imprint composite membranes. The specific binding capacities of theophylline imprint membranes were investigated by both single molecule and multi-molecule solution filtration experiments, respectively.     

Highly thermal conductive composites with polyamide-6 covalently-grafted graphene by an in situ polymerization and thermal reduction process

The thermal conductive polyamide-6/graphene (PG) composite is synthesized by in situ ring-opening polymerization reaction using ε-caprolactam as the monomer, 6-aminocaproic acid as the initiator and reduced graphene oxide (RGO) as the thermal conductive filler. The generated polyamide-6 (PA6) chains are covalently grafted onto graphene oxide (GO) sheets through the “grafting to” strategy with the simultaneous thermal reduction reaction from GO to RGO. The homogeneous dispersion of RGO sheets in PG composite favors the formation of the consecutive thermal conductive paths or networks at a relatively low GO sheets loading, which improves the thermal conductivity (λ) from 0.196 W m⁻¹ K⁻¹ of neat PA6 to 0.416 W m⁻¹ K⁻¹ of PG composite with only 10 wt% GO sheets loading.     

Sulfonated poly(arylene ether sulfone)/Laponite-SO3H composite membrane for direct methanol fuel cell

Polymer nanocomposite membranes based on sulfonated poly(arylene ether sulfonate) (SPAES) containing a flake filler (Laponite) with varying degrees of sulfonation, were prepared and characterized for application in direct methanol fuel cells (DMFCs). Unlike most other clays, Laponite crystals are very small in size with a very low aspect ratio (diameter to thickness ratio) of 25–30. They improve the mechanical, thermal properties and decreased the fuel permeability. However, polymer composite membranes containing non-proton conducting inorganic particles tend to show low proton conductivity, as compared with pristine polymer membranes. To resolve this problem, prior to the preparation of the composite membranes, Laponite-Na+(NLa) was sulfonated with various amounts of organo silanes (3-Mercaptopropyl trimethoxysilane (SH-silane)) via an ion exchange method. Functionalized Laponite with the organic silane compound showed higher ion exchange capacity and ion conductivity, respectively. In order to minimize the loss of proton conductivity while reducing the methanol permeability, various amounts (0.5–2.0 wt%) of the organically sulfonated Laponite (SLa) were introduced into the SPAES matrices. The performances of hybrid membranes for DMFCs in terms of mechanical properties, behavior of water in membranes, proton conductivity and methanol permeability were investigated.     

High electrical anisotropy in hydrochloric acid doped polyaniline/phyllosilicate nanocomposites: Effect of phyllosilicate matrix,synthesis pathway and pressure

Pressed tablets from polyaniline/phyllosilicate nanocomposites have been prepared under various conditions in order to optimize anisotropic conductivity of composite by ordering of flat phyllosilicate particles intercalated with polyaniline (PANI). Powder samples of PANI/phyllosilicate nanocomposites have been prepared using two phyllosilicates, montmorillonite and vermiculite, with a different layer charge. Two precursors were used, anilinium hydrochloride and anilinium sulfate. Prepared PANI/phyllosilicate composites were subsequently doped by hydrochloric acid via rinsing after polymerization process and for the DC conductivity measurements pressed into tablets. Applied pressure was 28 MPa and 128 MPa. Highly anisotropic conductivity has been achieved in pressed tablets. The in-plane conductivity for PANI/montmorillonite was 0.084 S/cm, i.e., 1000 × higher than in the direction perpendicular to the tablet plane. Increase of pressure up to 128 MPa led to dramatic decrease of conductivity.     

Exceptional dielectric properties of chlorine-doped graphene oxide/poly (vinylidene fluoride) nanocomposites

Herein, we report a facile method to significantly enhance the dielectric performance of reduced graphene oxide-based polymer composites. Addition of thionyl chloride into graphene oxide (GO) dispersion induces synergistic modifications of the structure, chemistry, charge carrier density and electrical conductivity of GO, as well as the interfacial interaction and phase of the surrounding matrix in the poly (vinylidene fluoride) (PVDF) composite. The composites reinforced with a very low reduced chlorinated GO (Cl-rGO) content of 0.2 vol% deliver an exceptional dielectric constant of 364 with a moderate dielectric loss of 0.077 at 1 kHz. These values are well contrasted with the corresponding properties of the neat PVDF polymer with a constant of 28 and a loss of 0.0029. Synergistic effects arising from chlorination are identified, including the much enhanced electrical conductivity of Cl-GO sheets by more than 3 orders of magnitude through introducing charge-transfer complexes, the improved interfacial interactions between the fillers and the PVDF matrix through hydrogen bonds, and the transformation of PVDF to β-phase with an inherently high dielectric constant due to dipolar interaction. The comparison with the literature data confirms superior dielectric performance of the present Cl-rGO/PVDF composites.     

The effect of graphite oxide on the thermoelectric properties of polyaniline

Graphite oxide (GO)/ordered polyaniline (PANI) composites have been prepared through an in situ polymerization. TEM, XRD, FTIR and XPS analyses show that the PANI grew along the surface of exfoliated GO as a template to form a more ordered structure with high crystallinity during polymerization. Compared with pure PANI, both higher electrical conductivity and higher Seebeck coefficient of GO/PANI composites result from the increased carrier mobility, which is confirmed by Hall measurement. Strong interactions exist between graphene oxide and PANI, including electrostatic forces, hydrogen bonding and π–π stacking. There is no significant difference in thermal conductivity between GO/PANI composites and PANI. The maximum electrical conductivity and Seebeck coefficient of the composites reach 751 S m^?1 and 28.31 μV K^?1, respectively. The maximum thermoelectric figure of merit is up to 4.86 × 10^?4, 2 orders of magnitude higher than that of pure PANI.     

Research on the preparation technology of polyaniline nanofiber based on high gravity chemical oxidative polymerization

Polyaniline (PANI) nanofibers with higher yield and homogeneous morphology were successfully prepared in larger scale by multi-step oxidation process with high gravity chemical oxidative polymerization (HGCOP) method in a rotating packed bed (RPB) under a higher initial aniline concentration of 0.5 M. The influence of oxidation times and ammonium peroxydisulfate (APS) dosages on the morphology, yield and conductive property of PANI were investigated, the products were characterized by SEM and UV–vis. Moreover, the anti-corrosion property and water dispersity of the as-prepared PANI nanofibers were also studied. The results showed that two-step oxidation process was an efficient way for mass production of PANI nanofibers by HGCOP, in which the optimum molar ratio of APS/aniline in the first and second oxidation stage was 0.5 and 0.25, respectively. PANI nanofibers with yield of 76.1%, diameters of 50–80 nm and average aspect ratio of 9.7 were obtained under the optimized condition. The PANI nanofibers were highly dispersible in water and exhibited an outstanding anti-corrosion effect, which could be applied to the environment-friendly processing and applications.