Real-time investigation of crystallization in poly(vinylidene fluoride)-based nano-composites probed by infrared spectroscopy

Via time-resolved Fourier transform infrared spectroscopy (FTIR), we examined the real-time investigation of the conformational changes of poly(vinylidene fluoride) (PVDF) chain segment during crystallization of neat PVDF and the corresponding nano-composites having intercalated structure. It was shown that in the following crystallization processes the crystal growth was virtually the same in both nano-composites and neat PVDF. We have examined an annealing at an infinitely long time at 200 °C (∼20 min) to erase the thermal history in the nano-composites. The dispersed titanate nano-filler particles exhibited strong contribution to enhance the heterogeneous nucleation for the formation of both γ- and β-phase crystals.     

Nanocomposites of poly(vinylidene fluoride) with organically modified silicate

We report a study of the impact of cold crystallization on the structure of nanocomposites comprising poly(vinylidene fluoride) (PVDF) and Lucentite STN™ organically modified silicate (OMS). Nanocomposites were prepared from solution over a very wide composition range, from 0.01 to 20% OMS by weight. Thermal preparation involved cold crystallization at 145 °C of quenched, compression-molded plaques. Static and real-time wide and small angle X-ray scattering (WAXS, SAXS), Fourier transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC) were used to investigate the crystalline phase of PVDF. For OMS content greater than 0.50 wt%, WAXS studies show that that the silicate gallery spacing increases modestly in the nanocomposites compared to neat OMS film, indicating a level of polymer intercalation.Using Gaussian peak fitting of WAXS profiles, we determine that the composition range can be divided into three parts. First, for OMS greater than 0.5 wt%, alpha phase fraction, ?_alpha, is insignificant (?_alpha∼0-0.01). Second, at the intermediate range, for OMS between 0.5 wt% down to 0.025 wt%, beta phase dominates and the beta fraction, ?_beta, is related to alpha by ?_beta>?_alpha. Third, below 0.025 wt% OMS, alpha dominates and ?_alpha>?_beta. The ability of small amounts of OMS (≥0.025 wt%) to cause beta crystal domination is remarkable. Overall, crystallinity index (from the ratio of WAXS crystal peak area to total area) ranges from about 0.36 to 0.51 after cold crystallization. Real-time WAXS studies during heating of initially cold crystallized nanocomposites show that there is no inter-conversion between the alpha and beta phase PVDF crystals, where these crystals coexist at room temperature. While all samples showed a strong SAXS Bragg peak, indicating existence of two-phase lamellar stacks, the sample containing predominantly beta phase had poorly correlated lamellar stacks, compared to samples containing predominantly alpha phase.     

Effect of crystallization rate on the formation of the polymorphs of solution cast poly(vinylidene fluoride)

A systematic study was carried out to investigate the effect of solvent type and temperature on the formation of the α and β phases from solution cast PVDF. Three solvents with different boiling points were used: N,N, dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP) and hexamethylphosphoramide (HMPA). Infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) revealed that the type of phase formed depends on the crystallization rate of PVDF, which in turn is determined by the evaporation rate of the solvent. Low rates result predominantly in the trans-planar β phase, high rates predominantly in the trans-gauche α phase and intermediate rates in a mixture of these two phases, regardless of solvent and temperature used. Since evaporation rate of the solvent is intimately related to temperature, PVDF films can be obtained predominantly in either one of these phases, or a mixture of these, by an adequate choice of the evaporation temperature range for a given solvent. The possible solubility curves of the two polymorphs α and β of PVDF were sketched. The formation of different types of spherulites, associated with the two different PVDF polymorphs, could be verified by surface micrographs of the cast films.     

Morphology, polymorphism behavior and molecular orientation of electrospun poly(vinylidene fluoride) fibers

The morphology, polymorphism behavior and molecular orientation of electrospun poly(vinylidene fluoride) (PVDF) fibers have been investigated. We found that electrospinning of PVDF from its N,N-dimethylformamide/acetone solutions led to the formation of β-phase. In contrast, only α- and γ-phase was detected in the spin-coated samples from the same solutions. In the aligned electrospun PVDF fibers obtained using a rotating disk collector, the β-phase crystallites had a preferred orientation along the fiber axis. The degree of orientation did not, however, vary significantly with the speed of the rotation disk collector, and the β-phase was also not significantly enhanced with the increase in the rotation speed or the decrease in the size of spinnerets. These facts indicated that the orientation was likely to be caused by Columbic force rather than the mechanical and shear forces exerted by the rotating disk collector and spinnerets. The Columbic force may induce local conformational change to straighter TTTT conformation, and hence promote the β-phase. The addition of 3 wt.% of tetrabutylammonium chloride (TBAC) into the polymer solutions effectively improved the morphology of the electrospun fibers, and led to almost pure β-phase in the fibers. With spin coating, PVDF-TBAC did not, however, show any strong β-phase diffraction peak. The synergistic β-enhancement effect of TBAC and electrospinning is possibly due to the fact that while TBAC could induce more trans conformers, electrospinning promotes parallel packing, and hence inter-chain registration.     

Proton exchange membranes based on poly(vinylidene fluoride) and sulfonated poly(ether ether ketone)

Blend membranes were obtained by solution casting from poly(vinylidene fluoride) (PVDF) and sulfonated poly(ether ether ketone) (SPEEK) in N,N-dimethylacetamide (DMAc). DSC and XRD were used to characterize the structure of the blend membranes. The effect of PVDF content on the membrane properties was investigated. The methanol permeability, water uptake and the swelling ratio of blend membranes decreased with the increase of PVDF content. Though the proton conductivity decreased upon the addition of PVDF, they were still comparable to that of Nafion^® 117 membrane. Higher selectivities were also found for most blend membranes in comparison with Nafion^® 117 membrane. The effect of methanol concentration on solution uptake, swelling ratio and methanol permeability of the blend membranes was also studied.     

Effect of salt additive on the formation of microporous poly(vinylidene fluoride) membranes by phase inversion from LiClO4/Water/DMF/PVDF system

The effect of a salt additive, lithium perchlorate, on the morphology and crystal structure of PVDF membranes prepared by wet phase inversion process was studied. The gelation phase boundaries of the quaternary system, LiClO4/water/DMF/PVDF, were determined at 25 °C. It was found that the gelation lines shifted up progressively with increasing salt contents in this system. For a salt-free casting dope, the formed membrane exhibited a typical asymmetric structure characterized by the skin, parallel columnar macrovoids, and cellular pores. WAXD analysis indicated that PVDF crystallized into ‘α’ (type II) structure in this membrane. By contrast, when PVDF was precipitated from high salt-content dopes (e.g. ≥5 wt%), the macrovoids bent and extended towards the bottom region while the original cellular pores evolved into very large voids. The PVDF crystallites became ‘β’ form (type I) in these membranes. Thermal analysis (DSC) of all membranes showed dual melting peaks at low heating rates (≤5 °C/min), suggesting that the crystallites formed in the immersion-precipitation process were imperfect and they underwent re-crystallization during the heating process. Using low voltage SEM at high magnifications (e.g. 100 KX at 0.55 KV) on uncoated samples, the fine structures (10-20 nm) of the PVDF crystallites were observed. And at very high magnifications (225 KX at 0.59 KV), it was observed that the skin region of the membrane prepared from high salt-content dopes actually contained many nano-pores (e.g. 20 nm). This contributes to the high permeation rate and low solute rejection as revealed from the water-flux measurements.     

Formation of nanostructured composites with environmentally-dependent electrical properties based on poly(vinylidene fluoride)-polyaniline core-shell latex system

Submicron poly(vinylidene fluoride) (PVDF)/polyaniline (PANI) core-shell latex particles are synthesized and examined as an active component in a simple conductometric chemical sensor. The structure and physical properties of these particles and nanostructured composite PVDF-PANI polymer films built of them are characterized with transmission electron, atomic force, and helium ion microscopy techniques, differential scanning calorimetry, and conductivity measurements. The nanostructured composite films with conductivity of about 4 × 10⁻⁴ S/cm suitable for sensor applications are prepared by casting from the core-shell particles dispersions on glass substrates patterned with silver electrodes followed by annealing at 180 °C, i.e. above T_m of the PVDF component. Sensor properties of these films are tested by measuring current-voltage (I-V) characteristics in response to varying concentration of NH₃ or HCl vapors. The developed thin film sensor heterostructures with electrically conductive percolation network of PANI as an active component and employing the conductometric detection scheme show high sensitivity to both analytes. However, the polymer material is especially efficient for application to NH₃ sensing with the detection limit as low as 100 ppb, and good reproducible recovery behavior upon repeated exposure to NH₃ at ambient conditions.     

Crystallization and thermal conductivity of poly (vinylidene fluoride)/boron nitride nanosheets composites

ABSTRACT

In this work, boron nitride (BN) and exfoliated boron nitride nanosheets (BNNs) were employed as thermal conductive fillers to improve the thermal conductivity of poly(vinylidene fluoride) (PVDF) composites. Results suggested that the thermal conductivity of PVDF increases significantly with an increase in loading content of functional fillers. When the mass ratio of fillers was more than 30 wt%, the heat conduction network was formed. BNNs were capable of forming denser heat conduction network as per the SEM observations. In this scenario, PVDF/BNNs composites demonstrated excellent thermal conductivity. For example, the thermal conductivity of PVDF/BNNs (60/40) was 0.82 W/mK, which was 2.4 times and 17% higher than that of neat PVDF and PVDF/BN (60/40) counterpart, respectively. The non-isothermal crystallization of corresponding composite was studied by Mo method. Combining with XRD results, both BN and BNNs acted as the nucleation agents but had no effect on crystal forms.     

Conferring pH-sensitivity on poly (vinylidene fluoride) membrane by poly (acrylic acid-co-butyl acrylate) microgels

In this paper, cross-linked poly (acrylic acid-co-butyl acrylate) microgels were utilized to impart pH-sensitivity to poly (vinylidene fluoride) membranes by phase separation of a casting solution of poly (vinylidene fluoride)/poly (acrylic acid-co-butyl acrylate)/DMF in aqueous solution. The effect of microgels content on morphologies, surface composition, and chemistry of the as-prepared membranes was studied by varieties of spectroscopic and microscopic characterization techniques. By using the filtration of water and protein aqueous solution, the performance of the membrane was evaluated. Results indicated that the as-prepared membrane was pH-sensitive to water flux, bovine serum albumin rejection and antifouling property. Besides, the as-prepared membrane showed an obvious improvement of water flux and proper bovine serum albumin rejection ratio, compared to the pristine PVDF membrane. Meanwhile, dynamic bovine serum albumin fouling resistance and flux recovery property were also greatly enhanced due to the improvement of surface hydrophilicity. Hopefully, the hydrophilic microgels additive would be favorable to fabricate other polymer membranes for water treatment.     

Temperature- and pH-sensitive membrane formed from blends of poly(vinylidene fluoride)-graft-poly(N-isopropylacrylamide) and poly(acrylic acid) microgels

Temperature- and pH-responsive membranes prepared from blends of poly(vinylidene fluoride)-graft-poly(N-isopropylacrylamide)(PVDF-g-PNIPAM) copolymer and poly(acrylic acid) (PAA) microgels in N,N-dimethylformamide (DMF) solution by phase inversion method. PAA microgels help PNIPAM chains largely enrich onto membrane surface. Furthermore, adding PAA microgels increases the porous size, porosity and hydrophilic property of the blend membrane. The membranes show temperature-sensitivity between 30 and 35 °C, and pH-sensitivity between pH 3 and 5 on permeating aqueous solutions. Meanwhile, the blend membranes keep good antifouling property even if one of the hydrophilic components becoming hydrophobic in response to temperature or pH stimuli, which is superior to single-sensitive PVDF membrane.     

Impact of nanosilicates on poly(vinylidene fluoride) crystal polymorphism: Part 1. Melt-crystallization at high supercooling

Polymorphism of poly(vinylidene fluoride), PVDF, in the presence of Lucentite STN organically modified silicate (OMS) is investigated for PVDF nanocomposites melt-crystallized at high supercooling temperatures where neat PVDF crystallizes exclusively in the alpha crystalline phase. Nanocomposites were prepared from solution with 0-1.0 wt% OMS composition. Here we observed that clay addition promotes gamma phase formation in nanocomposites melt-crystallized at high supercooling (i.e., at low crystallization temperature), whereas previously we showed that even small amount of nanosilicates resulted in beta phase formation in cold-crystallized PVDF nanocomposites [1].Wide-angle X-ray scattering (WAXS), Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) studies showed that α- and γ-phases co-existed in nanocomposites containing up to 0.1 wt% OMS, and the amount of α-crystals substantially diminished for higher OMS content. Formation of γ-crystal phase was confirmed with morphologic observation of spherulites of low-birefringence using polarizing optical and atomic force microscopies, and their crystalline structures were verified by FTIR and Raman microscopic spectroscopy. We also address in this work the ambiguities in assessing PVDF crystallographic phases, and correct the phase identification errors which have persisted up to this point in the literature based on melting point confusion. The crystal phase identification for PVDF nanocomposites is discussed and clarified, based on X-ray scattering, vibrational spectra, and thermal analysis. For reference, we provide a vibrational band list, indicating the close, or overlapping bands, of the three phases of PVDF: α, β and γ.     

Synthesis of transparent poly(vinylidene fluoride) (PVdF)/zirconium oxide hybrids without crystallization of PVdF chains

Transparent and homogeneous organic-inorganic hybrids with poly(vinylidene fluoride) (PVdF) could be prepared by addition of zirconium oxide nanocrystals (ZrO₂-NCs) in a polar aprotic solvent and the subsequent solvent evaporation. The polar aprotic solvents such as DMF, DMAc and DMSO would form hydrogen bonds with Zr-OH groups of the ZrO₂-NC and play a role as compatibilizers between the PVdF and ZrO₂-NCs. The interpenetration between PVdF and ZrO₂-NCs resulted in the nanometer dispersion of PVdF chains in a ZrO₂-NC matrix. High dosage of the ZrO₂-NCs as physical inhibitors between PVdF polymer chains sufficiently prevented the PVdF chain mobility in the internal of hybrids. The transparency of the PVdF/ZrO₂-NC hybrids was dramatically improved by controlling the content of ZrO₂-NCs. Novel multifunctional hybrids with high transparency, high refractive index and good mechanical property were obtained by hybridization of PVdF and ZrO₂-NCs.     

Thermal behaviour and annealing of poly(vinylidene fluoride)

In this study, we report the melting behavior of poly(vinylidene fluoride) (PVF₂) annealed in a differential scanning calorimeter. PVF₂ annealed under isothermal conditions often shows double or triple melting endotherms depending on the annealing temperature (T_a) and the heating rate. The lower melting peak temperature increases as T_a increases. When the annealing time is varied, there is a systematic increase in the size of the lower endotherms. This suggests that a portion of the main endothermic response is due to reorganization during the scan. Annealing PVF₂ not only increases the degree of crystallinity, but also improves the crystal perfection. The ability of an annealing sample to reorganize decreases as the annealing time increases. However, an additional third melting peak appears when PVF₂ is annealed at 140°C for a sufficiently long time. The existence of this peak suggests that more than one kind of distribution of crystal perfection may occur when PVF₂ is quenched from the melt into liquid nitrogen and subsequently annealed.     

A large enhancement in dielectric properties of poly(vinylidene fluoride) based all-organic nanocomposite

A nanocomposite was fabricated using poly(vinylidene fluoride) (PVDF) as matrix and poly(p-chloromethyl styrene) (PCMS) grafted with high dielectric constant copper phthalocyanine oligomer (CuPc) (PCMS-g-CuPc) as filler. Transmission electron microscopic morphologies reveal that the PCMS-g-CuPc particle size of ca. 80 nm in average are dispersed in PVDF matrix, while in PCMS-g-CuPc particles the PCMS acts as “matrix” which contains dispersed CuPc balls with a average size of ca. 25 nm [1/20 of that of CuPc in simple blend of PVDF and CuPc (PVDF/CuPc)]. The nanocomposite with only 15 wt% CuPc can realize a dielectric constant of 325 at 100 Hz, about 7 times larger than that of PVDF/CuPc, and nearly 40-fold enhancement with respect to that of the pure PVDF. The significant enhancement of dielectric response can be attributed to the remarkably strengthened exchange coupling effect as well as the Maxwell-Wagner-Sillars polarization mechanism.     

Cooperative effect of shear and nanoclay on the formation of polar phase in poly(vinylidene fluoride) and the resultant properties

The polar crystalline phase is the most important crystal mode for poly(vinylidene fluoride) (PVDF); its high content is urgently desired in the large-scale processing fabrication likes injection-molding. In this study, we proposed a convenient pathway to achieve large amount of polar phase in injection-molding part through cooperation of exerting oscillatory shear field and adding nanoclay. The effects of these two factors on the polymorphic composition were well demonstrated by infrared spectroscopy and X-ray diffraction. The increment of polar phase content was limited when shear field was solely imposed or only less amount of nanoclay, 1 wt%, was added. Whereas, by simultaneously exerting shear field and adding 1 wt% nanoclay, an extremely high polar phase fraction was achieved. So a positive cooperative effect of shear and nanoclay on the formation of polar phase can be proved absolutely. The simultaneously exerting shear and adding nanoclay leaded to not only high content of polar phase but also highly oriented structure. With this unique structure, an order-of-magnitude increase in the ductility (elongation) as well as good piezoelectric property has been achieved for the molded parts of PVDF/nanoclay nanocomposites.     

Interfacial polarization and layer thickness effect on electrical insulation in multilayered polysulfone/poly(vinylidene fluoride) films

In this study, we report layer thickness effect on the electrical insulation property of polysulfone (PSF)/poly(vinylidene fluoride) (PVDF) multilayer films having a fixed composition of PSF/PVDF = 30/70 (vol./vol.). Breakdown strength, dielectric lifetime, and electrical conductivity were studied for 32- and 256-layer films having various total film thicknesses. Among these films, those having thinner PVDF and PSF layers exhibited lower breakdown strength, shorter lifetime, and higher electrical conductivity than those having thicker layers. These experimental results were explained by Maxwell–Wagner–Sillars interfacial polarization due to contrasts in dielectric constant and electronic conductivity for PVDF and PSF, respectively. When both PVDF and PSF layers were thick (ca. > 100–200 nm), more space charges were available in PVDF and no electronic conduction was allowed for PSF. These accumulated interfacial charges could serve as effective traps for injected electrons from metal electrodes under high electric fields. As a result, reduced electrical conductivity and enhanced breakdown strength/dielectric lifetime properties were obtained. When both layers were thin (ca. < 100 nm), fewer space charges were available in PVDF and significant electronic conduction through PSF resulted in low interfacial polarization. Consequently, higher electrical conductivity, lower breakdown strength, and shorter lifetime were observed. These results provide us insights into potential physics to enhance electrical insulation property of polymer films using a multilayered structure having large dielectric constant contrast.     

Stretching induced phase separation in poly(vinylidene fluoride)/poly(butylene succinate) blends studied by in-situ X-ray scattering

The structure evolution of poly(vinylidene fluoride)/poly(butylene succinate) (PVDF/PBS) blends during stretching above the melting point of PBS is investigated by synchrotron-based simultaneous wide angle and small angle X-ray scattering (WAXS/SAXS). Before stretching, PVDF crystallizes into the α-form, whereas the chains of molten PBS locate at the inter-lamellar amorphous phase of PVDF. Crystal transition from α to β of PVDF is observed in all samples during stretching. The morphological transformation from a lamellar structure into a fibrillar structure occurs at low and intermediate strains. With further deformation, a “stretching induced phase separation” phenomenon is observed. The final microstructure of PVDF/PBS blends contains PVDF microfibrils with PBS chains preferentially distributed in the inter-fibrillar region. The PBS molecular weight influences the onset and end strain for the transition. A new “two-step model” is proposed to describe the deformation process.     

CO2 pressure effects on melting, crystallization, and morphology of poly(vinylidene fluoride)

Supercritical CO₂ fluids (SCF CO₂) assisting melting of poly(vinylidene fluoride) (PVDF) and the SCF CO₂ pressure affecting surface and bulk morphology, melting and crystallization of PVDF were investigated by means of SEM, AFM, FTIR, WAXD, DSC and SAXS. Three SCF CO₂ conditions at 84, 283, and 476 atm all at 140 °C for 30 min were studied. Morphological changes, induced by melting of PVDF under SCF CO₂ and recrystallization during depressurization of CO₂, were found. The level of the CO₂-assisted melting of PVDF was found to increase with increasing pressure. SEM and AFM images showed that the 84 atm of CO₂ assisted melting on the surface of PVDF film while both 283 and 476 atm of CO₂ gave rise to melting of the whole film. FTIR spectra and WAXD patterns found that the hot-pressed PVDF film exhibited predominant α-crystalline form, which is one of the reported four crystalline forms including α, β, γ, and δ forms, and did not transform to other crystalline form(s) upon the SCF CO₂ treatments although they lowered the bulk crystallinities of PVDF. SEM images showed that the SCF CO₂ treatments at 283 and 476 atm resulted in foam formations in PVDF, with smaller foam cells resulting from the lower pressure treatment. SAXS data found that the thickness of crystalline layer in the lamellar stacks increased while that of amorphous layers insignificantly changed after SCF CO₂ treatments at 283 and 476 atm, as compared with untreated PVDF. SAXS and DSC data suggested the presence of a bimodal distribution of crystal size of PVDF after SCF CO₂ treatments.     

聚偏二氟乙烯注塑成型技术

从注塑成型制品设计、模具设计、成型工艺参数的选择与控制等方面对聚偏二氟乙烯(PVDF)进行了讨论;重点介绍了成型工艺流程和工艺参数的设定,并对成型工艺条件进行了分析;系统介绍了聚偏二氟乙烯注塑成型工艺中常见问题及解决方法。     

Crystallization behavior and hydrophilicity of poly(vinylidene fluoride)/poly(methyl methacrylate)/poly(vinyl pyrrolidone) ternary blends

Ternary blends composed of matrix polymer poly(vinylidene fluoride) (PVDF) with different proportions of poly(methyl methacrylate) (PMMA)/poly(vinyl pyrrolidone) (PVP) blends were prepared by melt mixing. The miscibility, crystallization behavior, mechanical properties and hydrophilicity of the ternary blends have been investigated. The high compatibility of PVDF/PMMA/PVP ternary blends is induced by strong interactions between the carbonyl groups of the PMMA/PVP blend and the CF₂ or CH₂ group of PVDF. According to the Fourier transform infrared and wide‐angle X‐ray difffraction analyses, the introduction of PMMA does not change the crystalline state (i.e. α phase) of PVDF. By contrast, the addition of PVP in the blends favors the transformation of the crystalline state of PVDF from non‐polar α to polar β phase. Moreover, the crystallinity of the PVDF/PMMA/PVP ternary blends also decreases compared with neat PVDF. Through mechanical analysis, the elongation at break of the blends significantly increases to more than six times that of neat PVDF. This confirms that the addition of the PMMA/PVP blend enhances the toughness of PVDF. Besides, the hydrophilicity of PVDF is remarkably improved by blending with PMMA/PVP; in particular when the content of PVP reaches 30 wt%, the water contact angle displays its lowest value which decreased from 91.4° to 51.0°. Copyright © 2011 Society of Chemical Industry