Development and characterization of homogeneous membranes prepared from sulfonated poly(phenylene oxide)

This article investigates the comprehensive properties of sulfonated poly(phenylene oxide) (SPPO) membranes with different sulfonation degrees and presents the completion of previous work necessary for the application of SPPO membranes to proton‐exchange membrane fuel cells. The sulfonation level has been accurately determined by conductometric titration and ¹H‐NMR, and the glass‐transition temperature has been obtained with both differential scanning calorimetry and dynamic mechanical thermal analysis. Sulfonic groups attached to the aromatic ring in the poly(phenylene oxide) backbone split at 220–340°C, but the main‐chain splitting temperature of SPPO is similar to that of the pure polymer. In addition, the effects of sulfonic groups and water on the tensile strength of these membranes have been studied. An increase in the sulfonate groups in the polymer results in an increase in the water uptake. Atomic force microscopy phase images of the acid‐form membranes clearly show the hydrophilic domains, and the ionic regions of the membranes with a low sulfonation degree are isolated and become connected to produce a cocontinuous morphology as the degree of sulfonation increases. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1244–1250, 2005     

Improving the performance of novel polysulfone-based membrane via sulfonation method: Application to water desalination

High-performance polymers for water desalination were designed. A novel polysulfone was prepared via reaction between a new synthesized pyridine-based diol and bis(4-fluorophenyl) sulfone. Also a series of disulfonated copolymers with sulfonation content of 20–50 wt% were prepared to compare the hydrophilicity with the pristine polymer. The generated membranes were characterized by microscopic, mechanical, and thermal methods, and the influence of sulfonation degree on hydrophilicity, water flux, and salt rejection was followed. Water flux of sulfonated membranes was increased compare to pristine membrane as sulfonation increased, while the salt rejection decreased. Optimum application performance was obtained for membrane with 30 wt% sulfonation content. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48568.     

Hydrophilic and mesoporous SiO2–TiO2–SO3H system for fuel cell membrane applications

Hydrophilic and mesoporous sulfonated SiO₂–TiO₂–SO₃H systems as new additives for fuel cell electrolyte membranes are directly synthesized by the binary sol–gel reaction of TEOS–TiCl₄ and consecutive sulfonation with a hydrophilic generator, dihydroxy-m-benzenedisulfonic acid disodium salt. The sulfonation approach makes use of the simple chelating chemistry between the catecholic groups (dihydroxy benzene) and surface Ti ions of the inorganic ordered mesoporous SBA-15 structure. The system is successfully employed in fuel cell membrane applications with a composite Nafion membrane mixed with a mesoporous hydrophilic resin additive, and reveals an obvious enhancement of the proton conductivity at low humidity and elevated temperatures. This improvement was attributed to the excellent water retention capability of the hydrophilic mesoporous resin.     

Sulfonation of polysulfones: Suitability of the sulfonated materials for asymmetric membrane preparation

The effect of the starting polymer on the reaction of sulfonation of polysulfones was investigated. When concentrated sulfuric acid is used as the sulfonation reagent in an organic solvent‐free reaction, a polymer degradation equally occurs, leading to a decrease in the yield of product recovery. Poly(ether sulfone) Cardo appears to be the most resistant to chain scission in the medium and a control of the sulfonation degree can be performed via the reaction condition control. The reaction can be monitored by UV–Visible spectrophotometry. Phase inversion by immersion of a N‐methyl‐2‐pyrrolidone–polymer dope in water led to asymmetric membranes with an average pore size in the range of that of ultrafiltration and that of nanofiltration membranes. The latter membranes can only be obtained at high polymer concentrations and at moderate sulfonation degrees. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2461– 2473, 2002     

Polymer Nanocomposite Membranes of Sulfonated Poly(Styrene‐Isobutylene‐Styrene) With Sulfonated Graphene Oxide for Fuel Cell and Protective Clothing Applications

Graphene oxide (GO) and its sulfonated analog (sGO) have been incorporated into sulfonated poly(styrene‐isobutylene‐styrene) (SO₃H SIBS) in order to enhance its water retention and proton conductivity, while aiming to block permeant passage through the material. The polymer nanocomposite membranes (PNMs) were tested for two applications: direct methanol fuel cell and chemical and biological protective clothing. The transport properties of the membranes were determined as a function of SIBS sulfonation level (i.e., 37, 61, and 88 mol%), filler type (i.e., GO and sGO) and filler loading (i.e., 1, 3, 5, and 10 wt%). Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) confirmed the functionalization and incorporation of the fillers into SO₃H SIBS. No significant changes were observed in the thermal stability or FTIR spectra of the PNMs after addition of the fillers. Dissimilar behaviors were observed for the ion exchange capacity, water absorption capabilities and transport properties of the membranes after incorporation of the fillers. Atomic force microscopy (AFM) phase images and Fenton's test results indicate that the oxidative stability of the PNMs is associated to the interconnectivity between the hydrophilic domains of the fillers and SO₃H SIBS. The PNMs presented low permeability and high proton conductivity and thus, functioned adequately for both applications. POLYM. ENG. SCI., 59:E455–E467, 2019. © 2018 Society of Plastics Engineers     

Transport properties of blended sulfonated poly(styrene-isobutylene-styrene) and isopropyl phosphate membranes

This work discusses the effect of isopropyl phosphate (IP) on the transport properties of sulfonated poly(styrene-isobutylene-styrene) (SO₃H SIBS) as membranes for direct methanol fuel cell (DMFC) and chemical and biological protective clothing (CBPC) applications. The properties were determined as a function of SIBS sulfonation level (i.e., 24, 34, 49, and 84 mol %) and IP loading (i.e., 1, 3, 5, 11, and 15 wt %). A comprehensive material characterization study (e.g., FTIR, TGA, AFM, and SAXS) was performed to confirm the presence of the phosphate groups in the polymer matrix, assess the thermal stability of the proton-exchange membranes (PEMs), and understand how the unique interactions between the phosphate and sulfonic groups influenced the nanostructure of SO₃H SIBS. The transport properties, water absorption capabilities (i.e., swelling ratio, water uptake, etc.), oxidative stability, and ion-exchange capacity (IEC) were performed to evaluate the impact of IP on the properties of the resulting solvent-casted membranes. Results suggest that the morphology, thermal stability, and vapor permeability are governed by the sulfonation level, whereas the IEC, oxidative stability, water absorption capabilities, and the rest of the transport properties are dominated by the ionic content (i.e., sulfonic and phosphate groups) and their synergistic effects. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47009.     

Synthesis and characterization of chemically stable sulfonated copoly(triazole imide)s with high proton conductivity

The synthesis and characterization of a series of new sulfonated copoly(triazole imide)s (PTPQSH‐XX) are reported in this work. The PTPQSH‐XX with different degree of sulfonation (DS) were prepared by click polymerization of equimolar amounts of a diimide‐based dialkyne monomer, namely bis‐N,N′‐(prop‐2‐ynyl)pyromellitic diimide (TP) and a mixture of two different diazide monomers (one sulfonated, 4,4‐bis[3′‐trifluoromethyl‐4′{4‐azidobenzoxy} benzyl] biphenyl, and another nonsulfonated, 4,4′‐diazido‐2,2′‐stilbene disulfonic acid disodium salt [SAZ]), in different molar ratios. The copolymers showed high inherent viscosity (1.12–1.28 dL/g) in n‐methyl pyrrolidone (NMP) indicating the formation of high molar masses. Freestanding membranes were prepared from these copolymers by solution casting method. DS of the copolymers was determined from ¹H NMR signal intensities, and the values were in good agreement with the quantity of SAZ monomer used in polymer feed, indicating the successful incorporation of the sulfonated monomer. The copolymers exhibited high thermal and mechanical stabilities. The PTPQSH‐80 membrane showed proton conductivity as high as 178 mS/cm at 90°C with good oxidative and hydrolytic stability. Cross‐sectional transmission electron microscope micrographs of the membranes indicated phase segregated morphology along with interconnected hydrophilic domains with dimension in the range 15–150 nm. POLYM. ENG. SCI., 59:2279–2289, 2019. © 2019 Society of Plastics Engineers     

Synthesis and characterization of homogeneously sulfonated poly(ether ether ketone) membranes: Effect of casting solvent

Poly(ether ether ketone) (PEEK) was homogeneously sulfonated to have various degrees of sulfonation from 48 to 83%. The sulfonated PEEK (sPEEK) membranes were prepared by a solvent casting method using a few solvents such as N,N‐dimethyl formamide, N,N‐dimethyl acetamide, and 1‐methyl‐2‐pyrrolidinone. The effect of casting solvent on the membrane morphology and properties was investigated. The sulfonation degree and ion exchange capacity were determined by a back titration method, and the morphology of membrane by SEM. It has been demonstrated that the surface morphology and properties of sPEEK membranes, such as water uptake, methanol permeability, ion conductivity, and mechanical strength, were considerably affected by the type of solvent, where the DMAC‐sPEEK system showed the best performance in the polymer electrolyte membrane application for DMFC. This solvent effect on the membrane morphology and properties was caused by interaction strength (hydrogen bonding) between polymer and solvent. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Sulfonated poly(bis‐A)‐sulfones as proton exchange membranes for direct methanol fuel cell application

Sulfonated poly(bis‐A)‐sulfone (SPSF) samples were prepared by a mild postsulfonation method using trimethylsilyl chlorosulfonate as sulfonation agent, and their thermal and mechanical properties were evaluated. The serials of SPSF membranes are thermally stable up to 450°C in air. When compared with the poly(bis‐A)‐sulfone membrane, the hydrophilicity and water uptake of the SPSF membranes are enhanced. A microphase‐separated structure comprised of hydrophilic and hydrophobic polymer backbone was observed from atomic force microscopy phase images. The hydrophilic ionic clusters become continuous to form channels when ion exchange capacity (IEC) reached 1.47 mequiv/g. Moreover, the membranes showed very good proton conductivities (20°C, 0.01–0.11 S/cm) and low‐methanol permeability (0.09–3.06 × 10^?6 cm²/s), and the methanol diffusion coefficients were lower than that of Nafion112 (1.35 × 10^?6 cm²/s) with IEC values from 0.70 to 1.47 mequiv/g. However, the Fenton's reagent test revealed that the membranes exhibited very poor oxidation stability, which is the main defect limiting the application of SPSF for proton exchange membranes. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers     

Super-hydrophilic electrospun PVDF/PVA-blended nanofiber membrane for microfiltration with ultrahigh water flux

This work provides a novel approach to improve not only water flux but also fouling resistance of Polyvinylidene fluoride (PVDF) membranes. PVDF/Poly(vinyl alcohol) (PVA)-blended nanofiber membranes were prepared via electrospinning method. The structure and performance of blended nanofiber membranes were characterized by scanning electron microscopy (SEM), atomic force microscope (AFM), attenuated total reflection-Fourier-transform infrared (ATR-FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), contact angle measurement, tensile mechanical measurement, and filtration experiments. These results indicate that PVA was uniformly blend in the PVDF matrix. This blended nanofiber membranes with the ridge-and-valley structure and bicontinuous phase exhibited the hydrophilic performance and super-wettability, which is reflected in a drop of water fully spread within 1.44 s. Filtration experiments showed that the blended nanofiber membranes have ultrahigh flux and low irreversible fouling ratio. In general, this work enhances the possibility of hydrophilic modification of hydrophobic PVDF membranes. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48416.     

Novel membranes based on poly(5‐(methacrylamido)tetrazole) and sulfonated polysulfone for proton exchange membrane fuel cells

Proton‐exchange membrane fuel cells (PEMFC)s are increasingly regarded as promising environmentally benign power sources. Heterocyclic molecules are commonly used in the proton conducting membranes as dopant or polymer side group due to their high proton transfer ability. In this study, 5‐(methacrylamido)tetrazole monomer, prepared by the reaction of methacryloyl chloride with 5‐aminotetrazole, was polymerized via conventional free radical mechanism to achieve poly(5‐(methacrylamido)tetrazole) homopolymer. Novel composite membranes, SPSU‐PMTetX, were successfully produced by incorporating sulfonated polysulfone (SPSU) into poly(5‐(methacrylamido)tetrazole) (PMTet). The sulfonation of polysulfone was performed with trimethylsilyl chlorosulfonate and high degree of sulfonation (140%) was obtained. The homopolymers and composite membranes have been characterized by NMR, FTIR, thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). ¹H‐NMR and FTIR confirmed the sulfonation of PSU and the ionic interaction between sulfonic acid and poly(5‐(methacrylamido)tetrazole) units. TGA showed that the polymer electrolyte membranes are thermally stable up to ～190°C. Scanning electron microscopy analysis indicated the homogeneity of the membranes. This result was also supported by the appearance of a single T_g in the DSC curves of the blends. Water uptake and proton conductivity measurements were, as well, carried out. Methanol permeability measurements showed that the composite membranes have similar methanol permeability values with Nafion 112. The maximum proton conductivity of anhydrous SPSU‐PMTet0.5 at 150°C was determined as 2.2 × 10^?6 S cm^?1 while in humidified conditions at 20°C a value of 6 × 10^?3 S cm^?1 was found for SPSU‐PMTet2. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40107.     

Polyethersulfone Membranes: The Effect of Sulfonation on the Properties

Polyethersulfone (PES) was sulfonated using chlorosulfonic acid in order to improve proton conductivity. Incorporation of ?SO₃H groups into polymer main chain through sulfonation was confirmed using FTIR and ¹H NMR. Ion exchange capacity of sulfonated membranes was determined via titration. Morphological studies (AFM, SEM) revealed the presence of hydrophilic proton transfer channels, which became continuous at higher degrees of sulfonation. Thermal stability was observed from thermogravimetric analysis (TGA). Storage modulus and tan δ also exhibited an increase with degree of sulfonation as determined from DMA. Conductivity measurements and fuel cell performance showed that sulfonated samples possessed higher conductivity than virgin PES.     

Preparation and characterization of a proton‐exchange membrane by the radiation grafting of styrene onto polytetrafluoroethylene films

Radiation‐induced simultaneous grafting of styrene onto polytetrafluoroethylene (PTFE) films and the subsequent sulfonation in the chlorosulfonic acid/dichloroethane were investigated. The effects of the main radiation grafting conditions, such as the type of solvents, irradiation dose, dose rate, the styrene concentrations, etc., on the degree of grafting (DOG) were studied. To elucidate the influence of both the grafting and sulfonation conditions on the properties of the PTFE‐g‐polystyrene‐sulfonic acid (PSSA) membranes, the sulfonation conditions, including the sulfonation temperature and the concentration of the ClSO₃H with respect to the DOG, were systematically evaluated. The grafted and sulfonated membranes were characterized by FTIR–ATR spectra, ion‐exchange capacity (IEC), water uptake, thickness measurement, etc. The as‐prepared PTFE‐g‐PSSA membranes in this work showed a good combination of a high IEC (0.85–2.75 meq g^?1), acceptable water uptake (8.86–56.9 wt %), low thickness, and volume expansion and/or contraction. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 1415–1428, 2006     

Preparation of proton exchange membranes by radiation-induced grafting of alpha methyl styrene–butyl acrylate mixture onto polyetheretherketone (PEEK) films

Radiation-induced graft copolymerization of alpha methyl styrene (AMS)–butyl acrylate (BA) mixture onto poly(etheretherketone) (PEEK) was carried out to produce copolymer films which were subsequently sulfonated to develop proton exchange membranes. The characterization of membranes was carried out with infrared spectroscopy (FTIR), differential scanning calorimetry, thermogravimetric analysis, X-ray diffraction analysis (XRD), contact angle and electron probe microanalysis (EPMA). The presence of sulfonic acid groups within the polymer matrix was confirmed by FTIR. The crystallinity of membranes decreased significantly upon sulfonation process. The melting temperature of the membranes also decreased as compared to the virgin and the grafted films. At the same time, glass transition temperature (T _g) of membranes increased as the grafting increased. Virgin film showed stable thermogram up to ~500 °C while the grafted film had two-step degradation pattern. Sulfonation introduced one additional decomposition range in the membrane. Contact angle images showed the hydrophilic nature of the membrane surface. The EPMA showed the presence of the sulphur across the membrane matrix in a homogenous manner. The membranes showed low resistivity of 62 Ω cm for the graft level of 27 %.     

Rheology of concentrated sulfonated poly(ether ether ketone) solutions

Sulfonated polyether ether ketone (SPEEK), an ionic polymer, has been shown to be a potential candidate for fuel cell electrolyte as a proton exchange membrane. Rheological behavior of SPEEK solutions is of great interest to understand the molecular associations as well as due to implications in membrane processing. In this work, SPEEK of various degrees of sulfonation (58–80) was prepared and rheology of concentrated solutions of SPEEK was studied. The rheological properties were evaluated using steady and oscillatory shear. It was found that steady shear viscosity and storage modulus at any given concentration, is the highest for the lowest degree of sulfonation SPEEK solutions in N‐methyl‐2‐pyrrolidone. The low frequency plateau in storage modulus was observed at some combinations of degrees of sulfonation and concentrations, indicating gel‐like behavior in these SPEEK solutions. No significant change in rheological behavior was observed with different polar solvents. Increase of several orders of magnitude in viscosity, storage and loss moduli were observed with increasing concentrations. The role of hydrophobic aggregation and inter‐chain associations in determining rheology of SPEEK solutions is argued based on comparisons with other material systems. The rheological behavior of SPEEK solutions with 70 as the degree of sulfonation, suggests crossover from hydrophobic‐hydrophilic balance. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40044.     

Preparation and properties of sulfonated ETFE‐g‐polyvinyltoluene membranes for application in fuel cells

A new polymer electrolyte membrane prepared by radiation grafting of vinyltoluene into poly(ethylene‐co‐tetrafluoroethylene) (ETFE) film and subsequent sulfonation was developed for application in fuel cells. The effect of grafting condition on the degree of grafting was investigated in detail. Results indicated that the degree of grafting can be controlled over a wide range. The grafted films were sulfonated in a chlorosulfonic acid solution to obtain the polymer electrolyte membranes, which were characterized with respect to their use in fuel cells. It is concluded that the substituted methyl group on the vinyltoluene can improve the chemical stability of the resulting membranes, and the crosslinked ETFE‐g‐poly(vinyltoluene‐co‐divinylbenzene) membranes can be proposed for the future development of alternative low‐cost and high‐performance membranes for fuel cells. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2661–2667, 2006     

Fabrication and characterization of sulfonated polybenzimidazole/sulfonated imidized graphene oxide hybrid membranes for high temperature proton exchange membrane fuel cells

The sulfonated polybenzimidazole (sPBI)/sulfonated imidized graphene oxide (SIGO) was evaluated to be a potential candidate for high temperature proton exchange membranes fuel cells (HT-PEMFCs). Multifunctionalized covalently bonded SIGO is incorporated in sPBI matrix to resolve the drawbacks such as low proton conductivity, poor water uptake, and ion-exchange capacity (IEC) of sPBI polymer, synthesized by direct polycondensation in phosphoric acid for the application of proton exchange membranes. Strong hydrogen bonding among multifunctional groups established a neighborhood of interconnected hydrophobic graphene sheets and organic polymer chains. It provides hydrophobic–hydrophilic phase separation and facile proton hopping architecture. The optimized sPBI/SIGO (15 wt %) revealed 2.45 meq g⁻¹ IEC; 5.81 mS cm⁻¹ proton conductivity [120 °C and 10% relative humidity (RH)] and 2.45% bound water content. The maximum power density of the sPBI/SIGO-15 membrane was 0.40 W cm⁻² at 160 °C (5% RH) and ambient pressure with stoichiometric feed of H₂/air. This recommends that sPBI/SIGO composite membranes are compatible candidate for HT-PEMFCs. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47892.     

Polyphosphazene membranes. III. Solid‐state characterization and properties of sulfonated poly[bis(3‐methylphenoxy)phosphazene]

Poly[bis(3‐methylphenoxy)phosphazene] was sulfonated in a solution with SO₃ and solution‐cast into 100–200‐μm‐thick membranes from N,N‐dimethylacetamide. The degree of polymer sulfonation was easily controlled and water‐insoluble membranes were fabricated with an ion‐exchange capacity (IEC) as high as 2.1 mmol/g. For water‐insoluble polymers, there was no evidence of polyphosphazene degradation during sulfonation. The glass transition temperature varied from −28°C for the base polymer to −10°C for a sulfonated polymer with an IEC of 2.1 mmol/g. The equilibrium water swelling of membranes at 25°C increased from near zero for a 0.04‐mmol/g IEC membrane to 900 % when the IEC was 2.1 mmol/g. When the IEC was < 1.0 mmol/g, SO₃ attacked the methylphenoxy side chains at the para position, whereas sulfonation occurred at all available aromatic carbons for higher ion‐exchange capacities. Differential scanning calorimetry, wide‐angle X‐ray diffraction, and polarized microscopy showed that the base polymer, poly[bis(3‐methylphenoxy)phosphazene], was semicrystalline. For sulfonated polymers with a measurable IEC, the 3‐dimensional crystal structure vanished but a 2‐dimensional ordered phase was retained. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 387–399, 1999     

Thermal properties of proton-conducting radiation-grafted membranes

Proton-exchange membranes are required to exhibit chemical, mechanical, and thermal stability for fuel cell applications. The present investigation has been carried out to explore the thermal behavior of poly(ethylene-alt-tetrafluoroethylene) (ETFE)-based proton-conducting membranes, both uncrosslinked and crosslinked, prepared by radiation grafting and subsequent sulfonation. The influence of preparation steps (irradiation, grafting, sulfonation, crosslinking) on the thermal degradation, crystallinity, and melting behavior of membranes with varying degree of grafting was examined. ETFE base film and grafted films were studied as the reference materials. Furthermore, poly(tetrafluoroethylene-co-hexafluoropropylene)-based grafted films and membranes were investigated as well for comparison. Membrane preparation steps, degree of grafting, crosslinking, type of base polymer have considerable influence on the thermal properties of membranes. The crystallinity of the films decreases slightly by grafting, while a significant decrease was observed after sulfonation. For instance, crystallinity decreased from 37% (pristine ETFE) to 36% (uncrosslinked grafted film) and 23% (uncrosslinked ETFE-based membrane). On the other hand, the melting temperature of the base polymer was almost unaffected by irradiation and grafting. The crosslinked ETFE-based membranes exhibit a slightly higher melting temperature (262.5°C) than their corresponding grafted films (261.3°C) and the base film (260.6°C). © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

STRUCTURE OF POLYETHYLENE-GRAFT-POLYSTYRENE SULFONIC ACID MEMBRANES PREPARED BY RADIATION-INDUCED GRAFTING

Structural behavior of polyethylene- graft-polystyrene sulfonic acid (PE- g-PSSA) membranes prepared by simultaneous radiation-induced grafting of styrene onto low density polyethylene (PE) films followed by sulfonation was investigated. FTIR, X-ray diffraction (XRD) and differential scanning calorimetry (DSC) were used to monitor the changes in the morphology and the crystallinity taking place in the membranes during the applied two step preparation procedure (grafting and sulfonation) and the variation of the degree of grafting. Thermal properties such as melting temperature (T_m) and the heat of melting (ΔH_m) were also studied. The membranes showed a chemical structure of strongly hydrophilic nature due to the presence of (SO₃ ^?) groups with their associated water molecules. Both grafting and subsequent sulfonation were found to induce significant structural changes in the PE matrix. The overall crystallinity of the membranes was found to decrease with the increase in the degree of grafting and that is attributed to the cumulative effect of dilution and partial disruption of the inherent crystallites of the PE films.