Crystallographic changes characterizing the Curie transition in three ferroelectric copolymers of vinylidene fluoride and trifluoroethylene: 2. Oriented or poled samples

The effects of uniaxial drawing or poling on the structural changes involved in the ferroelectric-to-paraelectric phase transition in copolymers of vinylidene fluoride and trifluoroethylene were examined and compared to the behaviour of as-crystallized films. The compositions studied were $\text{65}\text{35}$, $\text{73}\text{27}$ and $\text{78}\text{22}\text{mol}\text{\%}$ vinylidene fluoride/trifluoroethylene, all of which crystallize from the melt with a molecular conformation and packing analogous to those of the common piezoelectric β-phase of poly(vinylidene fluoride). Contrary to the previously described behaviour of a $\text{52}\text{48}\text{mol}\text{\%}$ copolymer, orientation did not induce any significant changes in the structure of these copolymers or in its variation with temperature, primarily because these already crystallize directly from the melt in well-ordered, compact unit cells. On the other hand, electrical poling caused the all-trans chains of the ferroelectric phase to be packed more compactly and to survive to higher temperatures, thus shifting the Curie transition closer to the melting points of these copolymers. As a result, competition from melting interfered with the later stages of this solid-state transformation in the $\text{73}\text{27}\text{mol}\text{\%}$ composition, and aborted it at a very early point in the $\text{78}\text{22}\text{mol}\text{\%}$ samples. The Curie temperature was found to exhibit hysteresis between heating and cooling parts of the thermal cycle, to extend over a broad range of temperatures, and to involve intramolecular changes to the same disordered conformation found in melt-crystallized samples. Our results have allowed reasonable implications to be made concerning the existence and nature of a Curie transition in the piezoelectric β-phase of poly(vinylidene fluoride).     

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.     

Phase transition at a temperature immediately below the melting point of poly(vinylidene fluoride) from I: A proposition for the ferroelectric Curie point

A phase transition at a temperature immediately below the melting point of poly(vinylidene fluoride) form I has been found by means of differential scanning calorimetry (d.s.c.) and infra-red (i.r.) vibrational spectroscopy. An endothermic d.s.c. shoulder has been observed at a temperature about 10°C below the melting point, in the vicinity of which the i.r. crystalline trans bands decrease in intensity steeply and the crystalline gauche bands increase in intensity, indicating the conformational change from all-trans to T₃GT₃G type. These observations have been found to be detectable more clearly for samples subjected to the poling treatment under a d.c. high voltage. The transition shows the characteristic behaviour essentially identical to those observed for ferroelectric copolymers of vinylidene fluoride and trifluoroethylene, except for the irreversibility of the structural change, suggesting that the phase transformation revealed here may be a ferroelectric-to-paraelectric phase transition of polar form I crystal and the the Curie point may be about 172°C. It is consistent with Micheron's measurement of the temperature dependence of the dielectric constant. Other structural changes in the form I sample occurring in the temperature range from 20° to 170°C have also been discussed based on the i.r. spectral measurements.     

Understanding polymorphism formation in electrospun fibers of immiscible Poly(vinylidene fluoride) blends

Effects of electric poling, mechanical stretching, and dipolar interaction on the formation of ferroelectric (β and/or γ) phases in poly(vinylidene fluoride) (PVDF) have been studied in electrospun fibers of PVDF/polyacrylonitrile (PAN) and PVDF/polysulfone (PSF) blends with PVDF as the minor component, using wide-angle X-ray diffraction and Fourier transform infrared techniques. Experimental results of as-electrospun neat PVDF fibers (beaded vs. bead-free) showed that mechanical stretching during electrospinning, rather than electric poling, was effective to induce ferroelectric phases. For as-electrospun PVDF blend fibers with the non-polar PSF matrix, mechanical stretching during electrospinning again was capable of inducing some ferroelectric phases in addition to the major paraelectric (α) phase. However, after removing the mechanical stretching in a confined melt-recrystallization process, only the paraelectric phase was obtained. For as-electrospun PVDF blend fibers with the polar (or ferroelectric) PAN matrix, strong intermolecular interactions between polar PAN and PVDF played an important role in the ferroelectric phase formation in addition to the mechanical stretching effect during electrospinning. Even after the removal of mechanical stretching through the confined melt-recrystallization process, a significant amount of ferroelectric phases persisted. Comparing the ferroelectric phase formation between PVDF/PSF and PVDF/PAN blend fibers, we concluded that the local electric field-dipole interactions were the determining factor for the nucleation and growth of polar PVDF phases.     

Dielectric properties and ferroelectric behavior of poly(vinylidene fluoride‐trifluoroethylene) 50/50 copolymer ultrathin films

Ultrathin films of poly(vinylidene fluoride‐trifluoroethylene) copolymer [P(VDF‐TrFE), with a content (mol %) ratio of 50/50 VDF/TrFE] were fabricated on silicon wafers covered with platinum by a spin‐coating technique, ranging in thickness from 20 nm to 1 μm. The effect of thickness on dielectric properties and polarization behavior was investigated. A critical thickness was found to be about 0.1 μm. An abrupt drop of dielectric constant was observed, although there is no significant change in dielectric loss at this thickness. Square and symmetric hysteresis loops were obtained in films thicker than 0.1 μm. However, for films thinner than 0.1 μm, fewer square hysteresis loops were observed. SEM and X‐ray results demonstrate that the effect of thickness on dielectric and ferroelectric properties could be explained by the changes of crystal structure in these films. In addition, the effects of irradiation on dielectric property and polarization response for ultrathin P(VDF‐TrFE) films were also presented. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 2259–2266, 2001     

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.     

Crystallization behavior of nano-composite based on poly(vinylidene fluoride) and organically modified layered titanate

To understand the effect of the nano-filler particles on the crystallization kinetics and crystalline structure of poly(vinylidene fluoride) (PVDF) upon nano-composite formation, we have prepared PVDF/organically modified layered titanate nano-composite via melt intercalation technique. The layer titanate (HTO) is a new nano-filler having highly surface charge density compared with conventional layered silicates. The detailed crystallization behavior and its kinetics including the conformational changes of the PVDF chain segment during crystallization of neat PVDF and HTO-based nano-composite (PVDF/HTO) have been investigated by using differential scanning calorimetric, wide-angle X-ray diffraction, light scattering, and infrared spectroscopic analyses. The neat PVDF predominantly formed α-phase in the crystallization temperature range of 110-150 °C. On the other hand, PVDF/HTO exhibited mainly α-phase crystal coexisting with γ- and β-phases at low T_c range (110-135 °C). A major γ-phase crystal coexists with β- and α-phases appeared at high T_c (=140-150 °C), owing to the dispersed layer titanate particles as a nucleating agent. The overall crystallization rate and crystalline structure of pure PVDF were strongly influenced in the presence of layered titanate particles.     

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.     

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.     

Colloidal Crystalline Arrays of β‐Polymorphic Poly(vinylidene fluoride)

Stable layers of nearly monodisperse spheres of β‐polymorphic poly(vinylidene fluoride) with iridescent properties are prepared. The colloidal crystalline arrays (CCAs) were characterized by optical microscopy, differential scanning calorimetry (DSC), and FT‐IR spectroscopy. FT‐IR spectroscopic and wide‐angle X‐ray scattering (WAXS) studies revealed a β‐polymorphic PVF₂ structure, the DSC study showed that the level of crystallinity in the CCA was much higher than that in the melt‐crystallized sample, and UV‐visible spectroscopy showed extinction peaks at 323 and 510 nm in the CCAs. The β‐polymorphic PVF₂ structure, along with the optical extinction properties of these CCAs, raises the prospect of their application in optical filters and/or piezoelectric sensors.

Optical micrograph of PVF₂ CCA films cast on glass substrates.     

Effect of poly(vinylidene fluoride) binder crystallinity and graphite structure on the mechanical strength of the composite anode in a lithium ion battery

We have evaluated the mechanical strength of a series of composites consisting of carbon particles bound together by poly(vinylidene fluoride) (PVDF), which is closely related to the carbonaceous anode in a lithium ion battery. We used a balanced beam scrape adhesion tester and evaluated the influence of carbon particle structure, the chemical properties of PVDF, and the processing parameters of annealing temperature and casting solvent on the adhesion of the composite film to a copper substrate. The composite prepared with amorphous carbon shows over 10 times higher adhesion strength than those fabricated from other graphite materials. This results from chemical binding that is intermediate between semi-ionic and covalent C-F bonds, as detected by X-ray photoelectron spectroscopy. To address the effect of the crystalline phase of the binder on the adhesion strength, we investigated PVDF crystallinity in the composite films using differential scanning calorimetry. Samples with higher crystallinity show higher adhesion strength, independent of annealing temperature and casting solvent. The scratch adhesion was also measured for swollen electrodes immersed in 3:7 volume ratio of ethylene carbonate:ethyl methyl carbonate (EC:EMC) at different temperatures. After being swollen, the composite films prepared from PVDF modified with hydroxyl functional groups show higher adhesion strengths than the others due to their low uptake of the electrolyte solvent.     

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.     

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.     

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.     

Crystal transformation and thermomechanical properties of poly(vinylidene fluoride)/clay nanocomposites

The crystal transformation and thermomechanical properties of melt‐intercalated poly(vinylidene fluoride) (PVDF)/clay nanocomposites are reported in this study. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to study the thermal properties of PVDF and its nanocomposites with various clay concentrations. The incorporation of clay in PVDF results in the formation of β‐form crystals of PVDF. DSC study of melting behavior suggested the presence of only α‐phase crystals in neat PVDF and both α‐ and β‐phase crystals in the nanocomposite. This conclusion was corroborated by findings from Fourier‐transform infrared (FTIR) spectroscopy and X‐ray diffraction (XRD). Dynamic mechanical analysis (DMA) indicated significant improvements in storage modulus over a temperature range of 20–150 °C. The coefficient of thermal expansion (CTE) decreases with increasing clay loading. Copyright © 2004 Society of Chemical Industry     

Dielectric constant and energy density of poly(vinylidene fluoride) nanocomposites filled with core-shell structured BaTiO3@Al2O3 nanoparticles

Ceramics-polymer nanocomposites consisting of core-shell structured BaTiO₃@Al₂O₃ (BT@Al₂O₃) nanoparticles as the filler and poly(vinylidene fluoride) (PVDF) as the polymer matrix were fabricated by solution casting. At the same volume fraction, the BT@Al₂O₃/PVDF nanocomposites, with larger dielectric constant and higher energy density, outperformed the BT/PVDF nanocomposites. The 2.5 vol% BT@Al₂O₃/PVDF nanocomposites at 360 MV/m had a double more energy density than pure PVDF at 400 MV/m (6.19 vs. 2.30 J/cm³), and a remarkably 42% lower remnant polarization than the 2.5 vol% BT/PVDF nanocomposites (0.99 vs. 1.69 μC/cm² at 300 MV/m). Such significant enhancement was closely related to the surface modification by Al₂O₃, which improved the insulation of BT nanoparticles and reduced the contrast of dielectric constant between the filler and the PVDF matrix.     

Doubly oriented β-form films of poly(vinylidene fluoride)

PVDF sheets, rapidly quenched, were (1) two-step transversely stretched at various temperatures and (2) stretched at various temperatures, rolled at room temperature and then annealed. The orientation patterns of the β-form crystal (which contains the polar b-axis) in these films were analysed on the basis of X-ray diffraction photographs taken with flat and cylindrical cameras. In the case of (1), when both of the two-step transversely stretching temperatures were below 100°C, a doubly oriented film with the plar b-axis oriented parallel to the film surface was obtained. In the case of (2), when the stretching temperature was below 100°C, the sheets then rolled without annealing, another doubly oriented film with the polar b-axis preferentially oriented at 30° to the film surface was obtained. On the other hand, when these films were annealed above 100°C, or the stretching temperatures were above 100°C, orientation patterns in which the polar b-axis was partially rotated through 60° were obtained. The orientation mechanisms of these films are discussed using the measurements of the lattice spacings of the β-form crystal.     

Controlled grafting from poly(vinylidene fluoride) microfiltration membranes via reverse atom transfer radical polymerization and antifouling properties

A reverse atom transfer radical polymerization (RATRP) with benzoyl peroxide (BPO)/CuCl/2,2-bipyridine (Bpy) was applied onto grafting of poly(methyl methacrylate) (PMMA) and poly(poly(ethylene glycol) methyl ether methacrylate) (PPEGMA) from poly(vinylidene fluoride) (PVDF) microfiltration (MF) membrane surfaces, including the pore surfaces. The introduction of peroxide and hydroperoxide groups onto the PVDF membranes was achieved by ultraviolet (UV) irradiation in nitrogen, followed by air exposure. RATRP from UV pretreated hydrophobic PVDF membranes was then performed for attaching well-defined homopolymer. The chemical composition of the modified PVDF membrane surfaces was characterized by attenuated total reflectance (ATR) FT-IR spectroscopy and X-ray photoelectron spectroscopy (XPS). The surface and cross-section morphology of membranes were studied by scanning electron microscopy (SEM). The pore sizes of the pristine PVDF and the PMMA grafted PVDF membranes were measured using micro-image analysis and process software. With increase of graft concentration, the pore size of the modified membranes decreased and became uniform. Kinetic studies of homogeneous (in toluene solution) system revealed a linear increase in molecular weight with the reaction time and narrow molecular weight distribution, indicating that the chain growth from the membrane surface was a “controlled” or “living” grafting process. The introduction of the well-defined PMMA on the PVDF membrane gave rise to hydrophilicity. Protein adsorption and protein solution permeation experiments revealed that the UV pretreated hydrophobic PVDF membrane subjected to surface-initiated RATRP of methyl methacrylate (MMA) and poly(ethylene glycol) methyl ether methacrylate (PEGMA) exhibited good antifouling property.     

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.     

Improving the dielectric performance of poly(vinylidene fluoride)/polyaniline nanorod composites by stretch‐induced crystal transition

The introduction of conductive polyaniline (PANI) can significantly improve the dielectric constant of polymer‐based materials. However, there is a drawback of high dielectric loss. Herein, a simple and efficient stretching process was applied to improve the dielectric performance of poly(vinylidene fluoride)/PANI (PVDF/PANI) nanorod films through the stretch‐induced crystal transition from non‐polar α‐crystal to polar β‐crystal in PVDF and the oriented distribution of PANI nanorods. XRD, DSC and Fourier transform IR analyses indicate that the stretched PVDF and stretched PVDF/PANI films possess a high content of β‐crystal at the stretching temperature of 135 °C under a stretching ratio of 200%–400%. Furthermore, the stretched PVDF/PANI film with 10 wt% PANI displays a high dielectric constant of 338 at 100 Hz, which is increased by 20% compared to non‐stretched PVDF/PANI film (281). More importantly, the corresponding dielectric loss is reduced from 0.31 for the non‐stretched film to 0.17 for the stretched film. © 2018 Society of Chemical Industry