Electron beam‐induced piezoelectric phase in poly(vinylidene fluoride) nanohybrid: effect at the molecular level

A nanohybrid has been synthesized by incorporating organically modified layered silicate in a poly(vinylidene fluoride) (PVDF) matrix. Molecular‐level phenomena have been explored after exposing PVDF and its nanohybrid to an electron beam of varying doses. The electron beam interacts with polymer chains and thereby generates different free radicals, the number of which is quite high in nanohybrid as compared to pure PVDF. The stability of free radicals has been confirmed through density functional theory energy minimization, predicting stable β‐phase free radicals in the nanohybrid. Quantitative analyses of chain scission, crosslinking and double bond formation are reported and compared after irradiation for both PVDF and its nanohybrid using UV‐visible and Fourier transform infrared spectroscopies, sol–gel analyses and gel permeation chromatography, revealing both chain scission and crosslinking phenomena in irradiated PVDF and its nanohybrid, but at higher dose (>90 Mrad) crosslinking dominates in the nanohybrid due to more free radicals and proximity of radical chains on top of templated system in the nanohybrid as compared to pure PVDF. The enhanced crosslinking alters the nanostructure causing disappearance of the peak at 2θ ≈ 3°. Moreover, the electron beam induces significant piezoelectric β‐phase in the nanohybrid against only α‐phase in pure PVDF at a similar dose and raises the possibility for the use of electron‐irradiated nanohybrid as an electromechanical device. β‐Phase formation is also supported through solid‐state NMR, scanning electron microscopy and differential scanning calorimetry studies. The thermal properties in terms of heat of fusion and degradation temperature have been verified indicating steady decrease of melting point and heat of fusion for pure PVDF while considerably less effect is observed for the nanohybrid. The combined effect of chain scission and crosslinking makes both PVDF and its nanohybrid brittle, but with greater stiffness with respect to unirradiated specimens. © 2014 Society of Chemical Industry     

Pre‐thermal treatment in binary solvent systems promoting β crystalline phase of electrospun poly(vinylidene fluoride) nanofibers

Poly(vinylidene fluoride) (PVDF) nanofibers were fabricated via electrospinning with an investigation of various ratios of binary solvents at different temperatures. The amount of acetone influenced the morphology. Scanning electron microscopy showed a PVDF membrane composed of smooth and unblemished fibers without beads and dark spots with small diameters of 201 ± 54 nm at a dimethylformamide‐to‐acetone ratio of 4:6. The temperature of pre‐thermal treatment from room temperature to 120 °C was investigated to promote the β crystalline phase in electrospun PVDF nanofibers. The result was characterized using Fourier transform infrared (FTIR) spectroscopy and X‐ray diffraction (XRD). PVDF solution prepared at 80 °C was used to increase the β crystalline phase of the electrospun PVDF nanofibers due to the transformation of α to β phase occurring during the spinning process and also bead‐free PVDF nanofibers were obtained. Differential scanning calorimetry revealed crystallization behavior corresponding with that determined using FTIR spectroscopy and XRD. Therefore, the solvent proportion and pretreatment temperature were observed to affect ultrafine nanofiber and crystalline structure of PVDF, respectively. © 2020 Society of Chemical Industry     

Binary and ternary nanocomposites based on PVDF,PMMA, and PVDF/PMMA blends: polymorphism,thermal, and rheological properties

Binary and ternary nanocomposites based on poly(vinylidene fluoride) (PVDF), poly(methyl methacrylate) (PMMA), and PVDF/PMMA blends were successfully prepared through a melt-mixing process, using a commercial organoclay (15A) as the nanofiller. The 15A was more finely dispersed (intercalated/partially exfoliated) within the PVDF matrix compared with the PMMA matrix. A higher PVDF content in the ternary composite essentially led to a superior degree of 15A dispersion. The 15A addition induced the polar β-form PVDF crystals, whereas the presence of PMMA in ternary composites reduced the efficiency in promoting β-form formation by 15A. Adding 15A also enhanced the nucleation of PVDF, but the enhancement was inferior while PMMA was present. Conversely, the crystal growth of PVDF was retarded with the existence of 15A, and the PVDF/15A binary composite exhibited the greatest retardation. The equilibrium melting temperature (T_m°) of PVDF in the neat state and in the blends increased after 15A addition. The PVDF/15A binary composite possessed an evidently higher β-form T_m° than the α-form T_m° of neat PVDF (~20.1 °C rise). Similar effects on the individual components, 15A declined the thermal stability of PVDF but increased that of PMMA in the ternary composites. Rheological property measurements revealed that the ternary composites performed in-between that of individual PVDF/15A and PMMA/15A binary composites. A percolation of 15A (pseudo)network structure was developed in the composites, and a more elastic behavior was observed with increasing PVDF content in the composites.     

几种多态含能材料构型转变的DFT研究

为了从分子层面上对含能材料不同分子构型间的转变情况有一个直观认识,借助Gaussian 09软件,运用密度泛函理论(DFT),采用TS算法搜寻β-RDX→α-RDX、γ-HMX→β-HMX、ε-CL-20→β-CL-20及β-FOX-7→α-FOX-7在分子构型转变过程中的过渡态结构,确定了它们的构型转变过程;并通过计算吉布斯自由能随构型转变路径的变化,比较多态含能材料分子构型转变的难易程度。结果表明,由亚稳晶型到稳定晶型的转变首先会越过过渡态,克服自由能能垒转变为亚稳态结构,实现β-RDX→α-RDX、γ-HMX→β-HMX、ε-CL-20→β-CL-20及β-FOX-7→α-FOX-7分子构型转变分别需要克服的自由能能垒分别为5.25、22.21、9.69和10.24kJ/mol。因此,常温常压下β-RDX→α-RDX、γ-HMX→β-HMX、ε-CL-20→β-CL-20及β-FOX-7→α-FOX-7构型转变的难度大小排序为:HMX>>FOX-7>CL-20>RDX。     

Mechanism of the solvent-mediated transformation of taltirelin polymorphs promoted by methanol

Taltirelin, a central nervous system activating agent, has two crystal forms (α- and β-forms). Previously it has been reported that methanol (MeOH) as a co-solvent promoted the solvent-mediated phase transformation from α-form to β-form. In the present study, the mechanism of the promotion was investigated. Of the three processes in the solvent-mediated transformation, namely dissolution of the metastable form, nucleation of the stable form, crystal growth of the stable form, MeOH promoted the latter two processes. In particular, the nucleation was strongly promoted. The nOe (nuclear Overhauser effect) data suggested that MeOH promotes the conformational change from the α-form to the β-form. The solute–MeOH interaction resulted in the formation of pseudo-polymorphs of the β-form (2H₂O·MeOH or H₂O·2MeOH solvate), although these pseudo-polymorphs were exchangeable to the β-form (4H₂O solvate) under a humid atmosphere. Based on these findings, a possible mechanism of the transformation of taltirelin polymorphs in the presence of MeOH was discussed.     

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.     

Microinjection of poly(vinylidene fluoride) in the presence of various additives

In this study, microinjections of poly(vinylidene fluoride) (PVDF) in the presence of polar additives were investigated. The experimental results showed that compared with the static state, the strong stress during microinjection was more conducive to generate β‐PVDF with electroactive property because it could promote the transition from trans‐gauche–trans‐gauche conformation to all‐trans zigzag (TT) conformation. However, because metastable TT conformation easily relaxed back to trans‐gauche–trans‐gauche conformation with low energy, α‐PVDF coexisted with β‐PVDF in the microinjected pure PVDF. With the presence of polar additives, TT conformation formed during the microinjection was stabilized via special interaction between their polar groups and >CF₂ groups of PVDF, which was beneficial to the formation of β‐PVDF. As a result, PVDF microparts with rich β‐phases were prepared under the synergistic effects of external force field and special interaction. J. VINYL ADDIT. TECHNOL., 24:103–108, 2018. © 2016 Society of Plastics Engineers     

Influence of Spray Drying Parameters on the Formation of β-Phase Poly(vinylidene fluoride)

A simple one-step spraying method to produce poly(vinylidene fluoride) (PVDF) in the desired conformation is presented. The content of the piezoelectric β-phase is measured at different spray drying conditions and during electrospray. The influence of a strong electrical field and charges on the droplet are investigated separately from the electrospray setup with a pneumatic atomizer. For this purpose, the electric field is integrated into a pneumatic atomization process by a plate capacitor and the charge of the droplets by corona discharge. To investigate the drying properties, the drying temperature and the flow rate of dry air are examined. The presented process offers the possibility to deposit PVDF films or to produce PVDF powders, in their piezoelectric β- and γ-phases or in the nonpolar α-phase.     

Nucleation behavior of glutathione polymorphs in water

Nucleation behavior of glutathione (GSH) polymorphs in water was investigated by experimental method combined with classical nucleation theory. The solubility of α and β forms GSH in water at different temperatures, and the nucleation induction period at various supersaturations and temperatures were determined experimentally. The results show that, in a certain range of supersaturation, the nucleation of β form predominates at relatively higher temperature, while α form will be obtained at lower temperature. The nucleation kinetics parameters of α and β form were then calculated. To understand the crucial role of temperature on crystal forms, “hypothetic” nucleation parameters of β form at 283.15 K were deduced based on extrapolation method. The results show that the interfacial tension, critical free energy, critical nucleus radius and nucleus number of α form are smaller than that of β form in the same condition at 283.15 K, which implies that α form nucleates easier than β form at low temperature. This work may be useful for the control and optimization of GSH crystallization process in industry.     

Effect of syndiotactic polystyrene on the crystallization behavior of isotactic polypropylene/syndiotactic polystyrene blends with and without β‐nucleating agent

Blends of isotactic polypropylene (PP) and syndiotactic polystyrene (sPS) with and without β‐nucleating agent were prepared using a twin‐screw extruder at 290 °C. Blends of PP/sPS with β‐nucleating agent mainly show β crystalline form, irrespective of high (20 °C min^?1) or low (2 °C min^?1) previous cooling rates. This suggests that the cooling rates have little effect on the polymorphic composition of PP in PP/sPS blends. The effect of sPS on the crystallization of PP is compared with that of polyamide 6 (PA6). The increase in crystallization temperature of PP is smaller in the presence of sPS than in the presence of PA6; the fold surface free energy of PP/sPS is larger than that of PP/PA6 blends. These results reveal that compared with PA6, sPS has much weaker α‐nucleation effect on the crystallization of PP. The weak α‐nucleation effect of sPS is attributed to the high lattice mismatch between PP and sPS crystals.     

Higher order structures and mechanical properties of bacterial homo poly(3-hydroxybutyrate) fibers prepared by cold-drawing and annealing processes

Pure bacterial homo poly(3-hydroxybutyrate) (PHB) fibers were prepared by melt spinning, followed by cold-drawing in an amorphous state at a temperature just above its glass transition temperature. Cold drawn fibers obtained were further drawn at higher temperatures, followed by annealing at various temperatures under tension. Relations among the processing conditions, higher order structures and mechanical properties were investigated using wide- and small-angle X-ray diffractions (WAXD and SAXD, respectively), birefringence, differential scanning calorimetry (DSC), and tensile measurements. PHB has two different crystalline forms, 2₁ helix conformation (α-form) and planar zigzag conformation (β-form). A single broad reflection of β-form was detected even in a PHB fiber drawn once at a temperature just above its T_g immediately after quenching and it tended to be stronger after 2nd drawing at higher temperatures. Annealing under low temperature and high tension facilitates the occurrence of β-form. It is suggested that the β-form crystal is formed not only from the tie chains between α-form lamella, but also from completely free amorphous chains. Changes in the amount of two types of crystals were analyzed using the WAXD integrated intensity. Birefringence of these fibers shows negative and positive values, depending on process conditions. Changes in higher order structure on the mechanical properties are also discussed.     

Polarization study of poly(vinylidene fluoride) films under cyclic electric fields

Poly(vinylidene) fluoride (PVDF) is a prototypical piezoelectric polymer, which exhibits a strong piezoelectricity after a poling treatment. In this work, we investigate the polarization behaviors of PVDF film under cyclic electric fields by measuring polarized currents. A compensation method is used to distinguish the polarized current from the capacitive and resistive currents. The PVDF samples with a thickness of 20 or 30 μm and an electrode area of 3 × 3 or 4 × 4 mm² are employed, respectively. From the observations, we find that the electrode area does not affect the current density, but a thicker film has a higher residual polarization due to its larger fraction of effective switchable dipoles. The amplitude of the applied field primarily determines the residual polarization. Impressively, an α- to β-phase transformation occurs above 225 MV · m⁻¹ and the polarized current reaches the maximum and becomes converges at 350 MV · m⁻¹. Moreover, a high degree and reproducible residual polarization is obtained through gradually increasing applied field from 50 to 350 MV · m⁻¹, and the relative standard deviation is 3.74% for 20 available samples. Furthermore, a dipole switching model is used to describe the polarization behaviors of PVDF with a form of f(E) = Ae^{−B|E + C|} and the fitting parameters give a further illustration of polarization behaviors in PVDF. POLYM. ENG. SCI., 60:645–656, 2020. © 2020 Society of Plastics Engineers     

Relaxation behavior and activation energy of relaxation for polyimide and polyimide–graphene nanocomposite

The relaxation behavior of polyimide and its nanocomposite containing 10 wt % of graphene was studied by using the dynamic mechanical spectrometer. Dynamic mechanical analysis of polyimide and its composite was performed as a function of temperature and frequency in the temperature range of 25–480 °C and frequency range between 0.05 and 100 Hz. The effect of increasing frequency of testing from 0.05 to 100 Hz is a significant shift from the glass transition temperature, T_g, to higher temperature from 360 °C at 0.05 Hz to 420 °C at 100 Hz. The tan δ peak height for both α and β transitions decreased with increasing test frequency from 0.24 at 0.05 Hz to 0.08 at 100 Hz, due to increasing restriction to chain motion. At any given testing frequency, the T_g for the composite was shown to be higher than that for the matrix by about 5–10 °C. The Arrhenius equation was used to calculate the activation energy for both α and β transitions. The activation for α and β transitions for the composite and polyimide matrix were determined to be 688 and 537 kJ/mol and 313 and 309 kJ/mol, respectively, indicating that a significant increase in the energy barrier to chain relaxation occurred as a result of reinforcement of polyimide with low weight fraction of graphene. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43684.     

Structure, phase transition and electric properties of poly(vinylidene fluoride-trifluoroethylene) copolymer studied with density functional theory

The geometry, energy, internal rotation, vibrational spectra, dipole moments and molecular polarizabilities of poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) of α- and β-chain models were studied with density functional theory at B3PW91/6-31G(d) level and compared with those of the poly(vinylidene fluoride) (PVDF) homopolymer. The chain length and the trifluoroethylene (TrFE) concentration were examined to discuss the copolymer chain stabilities, chain conformations and electric properties. The asymmetrical internal-rotation potential energy curve shows that the angles for the g and g′ conformations in the α-chain (tg and tg′) models are 53° and −70°, respectively, and the β-chain (ttt) conformation is a slightly distorted all-trans plane with dihedral angle at 177°. The energy differences, E_β − E_α(g) and E_β − E_α(g′), between the β- and the α-conformation are 2.1 and 7.8 kJ/mol, respectively. These values are smaller than that in PVDF (8.4 kJ/mol), suggesting that the β-conformation in the copolymer will be more stable than in PVDF. The energy barriers for β → α(g) and β → α(g′) transitions are 16.2 and 5.8 kJ/mol, respectively. The former is almost twice of the energy barrier in PVDF by 8.2 kJ/mol and the latter is slightly smaller (by 2.4 kJ/mol) than that in PVDF. The respective energy barriers for α(g) → β and α(g′) → β transitions are 18.3 and 13.6 kJ/mol compared with the value 16.3 kJ/mol in PVDF. The asymmetrical energy barriers may be one of the reasons for the copolymers with 0.5-0.6 (mole fraction) VDF exhibiting complicated phase transition behavior. The conformation of α-chain P(VDF-TrFE) exhibits from a helical (containing higher TrFE) to a nearly beeline (containing lower TrFE). This behavior is different from that in the PVDF and the nearly beeline conformation might be responsible for the increasing crystallizability. The helical might also be associated with the complicated phase transition behavior and the larger lattice strain in the P(VDF-TrFE)s with higher TrFE concentration. The energy difference per monomer unit between the β- and α-chain decreases with increasing TrFE content. The ideal β-chain is curved with a radius of about 30 Å, which is similar to that in PVDF. The chain curvature and the TrFE content will affect the dipole moment contribution per monomer. The chain length and TrFE content will not significantly affect the mean polarizability. The calculations indicated that there are some additional characteristic vibrational modes that may be used in identification of the α- or β-phase P(VDF-TrFE)s with different TrFE contents.     

Pressure‐crystallized piezoelectric structures in binary fullerene C70/poly(vinylidene fluoride) based composites

The effective fabrication of polar crystalline structures of poly (vinylidene fluoride) (PVDF), such as beta and gamma, is crucial to the development of piezoelectric polymer devices. In this study, we report the effect of pressure on binary fullerene C70/PVDF‐based composite with an overall good C70 dispersion, which was prepared by an easy physical and mechanical route. The C70/PVDF composites were crystallized in a piston‐cylinder high‐pressure apparatus, and the polymeric crystalline structures totally with extended‐chain piezoelectric beta‐ or gamma‐form lamellae were successfully achieved in the composite samples by varying temperature, pressure, crystallization time, and composite composition. The c‐axis thickness of the extended‐chain beta‐form lamellae of PVDF in the composites increased and decreased with the increase of the applied temperature and pressure, respectively, and it increased with the increase of crystallization time. Although C70 was found to be negative for the rapid formation of beta‐form PVDF crystals, it played an important role in the growth of a beta‐form PVDF nanowire with extended‐chain crystalline substructures. The template‐free formation of such piezoelectric nanowires was attributed to a C70‐induced self‐assembly of the polymer, driven by physical interactions at high pressure. The pressure‐crystallized C70/PVDF composites, self‐reinforced with unique one‐dimensional piezoelectric structures, may diversify niche applications in advanced functional polymeric devices. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1823–1833, 2013     

Improving dielectric properties of poly(vinylidene fluoride) composites: effects of surface functionalization of exfoliated graphene

To investigate the effects of surface functionalization of exfoliated graphene (EG) on the crystalline form of β-phase and dielectric properties of poly(vinylidene fluoride) (PVDF), we prepared PVDF-based composites reinforced by different functionalized EG. The X-ray photoelectron spectroscopy results indicated that a wide variety of chemical functional groups such as C–OH, C–O–C, C=O, COOH and C–F could be introduced on the surface of modified EG. As confirmed by results of Fourier transform infrared spectrum and X-ray diffraction, the β-phase PVDF can be produced in the composites with the incorporation of functionalized EG. In the frequency ranging from 10² to 10⁷ Hz, the dielectric permittivity of PVDF composites shows an obvious increase owing to a variation of the carbonyl group (C=O) content. Among all the composites, the EG grafted with polymethyl methacrylate/PVDF composite has the highest dielectric permittivity and dielectric loss.     

Process analysis of phase transformation of α to β-form crystal of syndiotactic polystyrene investigated in supercritical CO2

Mechanism of solid phase transformation of α to β form crystal of syndiotactic polystyrene (sPS) was investigated in supercritical CO₂. The phase transformation occurred in the original pure α and mixed (α+β) form sPS in supercritical CO₂ was traced as a function of temperature and pressure by means of wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC). At appropriate temperature and pressure, sPS underwent solid phase transitions from α to β form. Higher temperature or higher pressure favored this transformation. Compared to the original pure α form sample, the original β form crystal in the mixed (α+β) form sample acted as the nucleus of β form crystal, so that reduced the transition temperature and pressure.     

Gold‐Nanoparticle‐ and Gold‐Nanoshell‐Induced Polymorphism in Poly(vinylidene fluoride)

PVDF nanocomposites are prepared through solution mixing of Au‐NPs or Au‐NSs with PVDF. The novel optical properties of Au‐NPs and ‐NSs are retained as confirmed from UV‐Vis spectra. Analysis of resulting nanocomposites by FT‐IR, XRD, and DSC shows an obvious polymorphism change from α‐ to β‐form compared to PVDF prepared under the same conditions. The β‐polymorph seems to be more prominent with higher concentration of Au‐NPs (0.5%) and even more so with Au‐NSs. Thermogravimetric analysis shows that both nanocomposites have better resistance toward thermal degradation. Combination of novel optical properties of Au‐NPs or Au‐NSs with induced ferroelectric‐active β‐polymorph in PVDF can lead to new design of optical, piezoelectric devices.

     

Effects of stretching on crystalline phase structure and morphology of hard elastic PVDF fibers

The phase characteristics and morphology of stretched hard elastic poly(vinylidene fluoride) (PVDF) fibers were investigated by X‐ray diffraction (XRD) and wide‐angle and small‐angel X‐ray scattering (WAXS and SAXS). It was indicated that α and β phases coexisted in stretched PVDF fibers, stretching assisted in α to β phase transformation. The β/α ratios of stretched PVDF fibers were affected by stretching temperature, rate, and ratio. The β phase content of stretched PVDF fibers had an abrupt increase when stretched near 70°C, and then it decreased with increasing stretching temperature. Besides, the β/α ratio of PVDF fibers increased with stretching rate and ratio. The total crystallinity of PVDF fibers did not change much even on different stretching conditions. WAXS results indicated that the unstretched and stretched PVDF fibers all exhibited three strong equatorial streaks, with d‐spacing (0.964, 0.488, and 0.439 nm) and (0.946, 0.494, and 0.480 nm), which suggested that PVDF fibers still remained the crystalline reflections of c‐axis orientation even after being stretched. The long periods of stretched PVDF fibers, calculated from SAXS curves, increased from 19.04 to 39.75nm. On the basis of these results, the β transformation mechanism of stretched PVDF fibers was also discussed. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 2254–2259, 2007     

Study of crystal structure and properties of poly(vinylidene fluoride)/graphene composite fibers

Poly(vinylidene fluoride) (PVDF) nascent composite fibers were prepared through the melt-spinning method with different graphene (GE) contents, following which post-stretched composite fibers were obtained using a post-stretching process under thermal treatment condition. The effects of GE content and post-stretching ratio on the crystal structure and properties of the fibers were investigated in order to obtain PVDF composite fibers with high β crystal content and high hydrophobicity and oleophilicity. The results indicated that the addition of GE led to some extent to an improvement in the content of β crystals and the orientation of crystals. Furthermore, the addition of GE obviously improved the mechanical properties, hydrophobicity and oleophilicity of nascent composite fibers. The post-stretching ratio had a significant influence on the transition of the crystal phase from α to β and the orientation of crystals. In addition, the post-stretching played a positive role in enhancement of mechanical properties and hydrophobicity. When 5 wt% GE was added and the post-stretching ratio was 6, PVDF composite fibers with high β crystal content, super-oleophilicity and high hydrophobicity could be obtained. The β crystal content of PVDF composite fibers could reach 62.8%, oil contact angle was 0° and water contact angle was 116.0°. © 2021 Society of Industrial Chemistry.