Formation of kink bands by compression of the extrudate of solid linear polyethylene

Extrudates of solid linear polyethylene prepared under proper pressure and temperature conditions have a high c axis orientation along the extrusion direction, with lamellar crystals and amorphous layers stacked alternately along the extrusion direction. Kink bands were formed by compressing the oriented extrudate at room temperature along the extrusion direction. Inspection of the kink bands by wide-angle and small-angle x-ray diffraction and electron microscopy revealed that the fiber axis was rotated from the original axis direction by 70–75° and that the lamellar crystals were inclined to the fiber axis in the kink band by 55–60° and stacked nearly parallel to the kink boundary. The superstructural change during the formation of the kink band could not be interpreted in terms of uniform c axis shear alone. In addition to such a mechanism, it was necessary to take into account intermicrofibril and/or intercrystallite slip.     

Development of oriented structure and properties on drawing of poly(vinylidene fluoride) by solid‐state coextrusion

Gel films of poly(vinylidene fluoride) (PVDF) consisting of α‐form crystals were drawn uniaxially by solid‐state coextrusion to extrusion draw ratios (EDR) up to 9 at an optimum extrusion temperature of 160 °C, about 10°C below the melting temperature (T_m). The development of an oriented structure and mechanical and electrical properties on coextrusion drawing were studied as a function of EDR. Wide‐angle X‐ray diffraction patterns showed that the α crystals in the original gel films were progressively transformed into oriented β‐form crystals with increasing EDR. At the highest EDR of 9 achieved, the drawn product consisted of a highly oriented fibrous morphology with only β crystals even for the draw near the T_m. The dynamic Young's modulus along the draw direction also increased with EDR up to 10.5 GPa at the maximum EDR of 9. The electrical properties of ferroelectricity and piezoelectricity were also markedly enhanced on solid‐state coextrusion. The D–E square hysteresis loop became significantly sharper with EDR, and a remanent polarization P_r of 100 mC/m² and electromechanical coupling factor along the thickness direction k_t of 0.27 were achieved at the maximum EDR of 9. The crystallinity value of 73–80% for the EDR 9 film, estimated from these electrical properties, compares well with that calculated by the ratio of the crystallite size along the chain axis to the meridional small‐angle X‐ray scattering (SAXS) long period, showing the average thickness of the lamellae within the drawn β film. These results, as well as the appearance of a strong SAXS maximum, suggest that the oriented structure and properties of the β‐PVDF are better explained in terms of a crystal/amorphous series arrangement along the draw axis. Further, the mechanical and electrical properties obtained in this work are the highest among those ever reported for a β‐PVDF, and the latter approaches those observed for the vinylidene fluoride and trifluoroethylene copolymers. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1371–1380, 2001     

Morphological and structural features of heat-treated drawn polypropylene/high-density polyethylene blends

The ion etching technique has been applied to a morphological study of mechanically blended polypropylene (PP) with high-density polyethylene (HDPE). Samples blended to PP/HDPE compositions of 65/35 and 85/15 by weight were highly drawn and then heat treated for 30 min at selected temperatures up to 163°C. When these samples are carefully ion-etched several features are observed in electron micrographs, namely (i) crosshatched, and (ii) twisted or layered textured inclusions of HDPE crystals within arrays of lamellalike PP crystals situated perpendicular to the direction of drawing. X-ray diffraction measurements of the drawn samples heat treated in the range 145–163°C for 30 min shows that oriented HDPE crystallizes with b-axis orientation along the drawing direction. Supporting evidence is obtained from electron diffraction measurements. The molecular weight of the HDPE component is a major factor in the b-axis-oriented growth of HDPE crystals in PP/HDPE blends.     

Nanoscale calorimetry of isolated polyethylene single crystals

We used thin‐film differential scanning calorimetry to investigate the melting of isolated polyethylene single crystals with lamellar thicknesses of 12 ± 1 nm. We observed the melting of as few as 25 crystals. Over a wide number of crystals (25–2000 crystals), the heat of fusion was 40% larger than the bulk value. The melting temperature of the isolated single crystals was 123 ± 2 °C, 9 °C lower than that of the bulk material. We also measured the heat of fusion of quenched crystals (±15%) over a wide range of heating rates (20,000–100,000 K/s). Annealing the quenched crystals resulted in shifts in the endotherm peak by as much as 15 °C. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 1237–1245, 2001     

X-ray study of microstructure of slightly plasticized poly-(vinylchloride) (PVC)

Small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) as well as transmission electron microscopy (TEM) techniques have been applied to investigate the microstructure of a number of commercial poly-(vinylchloride) (PVC) samples, stretched 200% and annealed for 1 h at 110 °C. From SAXS analysis, the microstructure is described as an ensemble of quasi-spherical particles one-dimensionally ordered (in the fiber axis direction) and with large distance fluctuations in the equatorial plane. The superstructure is described as fibrillar or nematic-like. TEM micrographs confirm the SAXS data.SAXS meridional patterns present 001 and 003 reflections of Ca-stearate added as stabilizer to the samples, while WAXS profiles do not show any crystalline reflection of Castearate.An interaction of Ca-stearate molecules with PVC chains is postulated, which could partially account for the phenomenon called antiplasticization of PVC.     

Effect of constrained annealing on the mechanical properties of electrospun poly(ethylene oxide) webs containing multiwalled carbon nanotubes

In this work, flexible nanofibrous membranes (mats) of poly(ethylene oxide) (PEO) with and without multiwall carbon nanotubes (MWNTs) were fabricated by electrospinning. The effects of annealing and MWNT concentration on mat morphology, MWNT dispersion within the nanofibers, and the mechanical properties of electrospun mats were studied. Annealing temperatures ranged from 60 °C to 64 °C [near the melting temperature (64 °C via differential scanning calorimetry)] for 4 minutes. Samples were annealed with and without applied tension (constrained and unconstrained annealing). Annealing at the highest temperature (64 °C), before the loss of fibrous morphology, significantly improved fiber–fiber bonding and therefore the tensile strength of the mats. Compared with unconstrained annealing, constrained annealing introduced fiber alignment (and therefore molecular orientation) along the tensile axis (direction of constraint) during annealing and resulted in a significant increase in modulus for all samples (with and without MWNTs). The use of constrained annealing may be a facile approach to enhance modulus in nanofibrous mats while maintaining high porosity. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 787–796     

Nanostructure reorganization in a thermotropic copolyester. A simultaneous WAXS and SAXS study

Thermally induced Angstrom and nanometer‐scale reorganization in thermotropic liquid crystalline polymer based on (1,4)‐hydroxybenzoic acid (B) and (2,6)‐hydroxynaphthoic acid (N) was investigated by simultaneous wide‐angle and small‐angle X‐ray scattering (SAXS, respectively). Extruded tapes 50 µm thick were annealed at 240°C under dry air conditions. The as‐received tape exhibited fiber‐like structure with crystalline order, whereas the SAXS patterns exhibited diamond‐shaped diffuse scattering elongated along the equatorial axis elucidating nanovoid morphology oriented along the extrusion axis. Guinier analyses showed that the radius of gyration R_g of nanovoids were ca. 17 nm along the extrusion axis. Heat treatment produced a sharpening of the 002 meridional reflection and the 110 equatorial reflection suggesting an improvement of molecular register and packing. The molecular alignment, as quantified by the order parameter , increased as well as the degree of crystallinity χ. On the other hand, SAXS intensity along the equatorial axis decreased evidencing reduction of R_g, i.e. lateral compression of the nanovoids and better molecular packing. Thermal treatment increased the thermal stability and the uniaxial tensile Young's modulus, E, along extrusion axis. However, the tapes exhibited microhardness anisotropy and the indentation anisotropy, ?H, gradually decreased suggesting reduction of elastic recovery in the molecular chain direction. Scanning electron microscopy evidenced an outer skin with an internal layered morphology that transformed into sheet‐like morphology with meandering fibrils. This investigation evidenced microstructure and morphology reorganization correlating with improved thermal and mechanical properties. Copyright © 2015 John Wiley & Sons, Ltd.     

Orientation-induced crystallization in isotactic polypropylene melt by shear deformation

Development of orientation-induced precursor structures (nuclei) prior to crystallization in isotactic polypropylene melt under shear flow was studied by in-situ synchrotron small-angle X-ray scattering (SAXS) and rheo-optical techniques. SAXS patterns at 165°C immediately after shear (rate = 60 s⁻¹, t_s = 5 s) showed emergence of equatorial streaks due to oriented structures (microfibrils or shish) parallel to the flow direction and of meridional maxima due to growth of the oriented layer-like structures (kebabs) perpendicular to the flow. SAXS patterns at later times (t = 60 min after shear) indicated that the induced oriented structures were stable above the nominal melting point of iPP. DSC thermograms of sheared iPP samples confirmed the presence of two populations of crystalline fractions; one at 164°C (corresponding to the normal melting point) and the other at 179°C (corresponding to melting of oriented crystalline structures). Time-resolved optical micrography of sheared iPP melt (rate = 10 s⁻¹, t_s = 60 s, T = 148°C) provided further information on orientation-induced morphology at the microscopic scale. The optical micrographs showed growth of highly elongated micron size fibril structures (threads) immediately after shear and additional spherulities nucleated on the fibrils at the later stages. Results from SAXS and rheo-optical studies suggest that a stable scaffold (network) of nuclei, consisting of shear-induced microfibrillar structures along the flow direction superimposed by layered structures perpendicular to the flow direction, form in polymer melt prior to the occurance of primary crystallization. The scaffold dictates the final morphological features in polymer.     

Morphology of polyethylene crystallized by solution stirring

Dilatometric and calorimetric studies have been made of the fusion process of linear polyethylene crystallized by stirring xylene solutions at elevated temperatures. It is shown that the melting point of the crystals increases rapidly from 139.5°C to 145°C in the crystallization temperature range of 100–103°C and levels off to 146 ± 0.5°C, provided that very slow heating rates are employed. Stirrer-crystallized samples treated with fuming nitric acid show higher crystalline contents. Comparison of their enthalpies of fusion and melting points indicate that higher molecular order along the fiber axis is associated with higher crystallization temperatures. This is in general agreement with corresponding results of other modes of crystallization. The attack of fuming nitric acid on stirrer crystals is characterized by weight-loss curves similar to those of dilutesolution crystals and bulk polyethylene. The linear molecular weight dependence on time of exposure to nitric acid suggests that the oxidation proceeds mainly from the chain ends at a constant rate for samples stirred in the lower crystallization range, but an increased rate is observed for a sample stirred from xylene at 105°C. It is suggested that the lamellar overgrowths, most evident at low crystallization temperatures, are epitaxially attached to the fiber axis, whereas the smaller crossbandings observed at higher crystallization temperatures are possibly made up of elements of chains that are only partly incorporated in the highly ordered fibrous core.     

Stretching temperature and direction dependency of uniaxial deformation mechanism in overstretched polyethylene

In order to elucidate microscopic deformation behavior at different locations in isotropic semicrystalline polymers, the structural evolution of a preoriented high‐density polyethylene sample during tensile deformation at different temperatures and along different directions with respect to the preorientation was investigated by means of combined in situ synchrotron small‐angle X‐ray scattering (SAXS) and wide‐angle X‐ray diffraction (WAXD) techniques. For samples stretched along preorientation, two situations were found: (1) at 30 °C, the sample broke after a moderate deformation, which is accomplished by the slippage of the microfibrils; (2) at 80 and 100 °C, fragmentation of original lamellae followed by recrystallization process was observed resulting in new lamellar crystals of different thickness depending on stretching temperature. For samples stretched perpendicular or 45° with respect to the preorientation, the samples always end up with a new oriented lamellar structure with the normal along the stretching direction via a stress‐induced fragmentation and recrystallization route. The thickness of the final achieved lamellae depends only on stretching temperature in this case. Compared to samples stretched along the preorientation direction, samples stretched perpendicular and 45° with respect to the preorientation direction showed at least several times of maxima achievable stress before macroscopic failure possibly due to the favorable occurrence and development of microdefects in those lamellar stacks with their normal parallel to the stretching direction. This result might have significant consequence in designing optimal procedure to produce high performance polyethylene products from solid state. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 716–726     

Drawing of single-crystal mats of linear polyethylene

Drawing of single-crystal mats of linear polyethylene has been investigated. Drawing is possible at temperatures higher than about 90°C. The drawing is accompanied by distinct necking, with a large decrease in the thickness of the mat and a very high maximum draw ratio, sometimes over 30. The maximum draw ratio is approximately proportional to the thickness of the lamellae. This behavior strongly suggests the unfolding of chains during drawing. A change of orientation of crystal axes occurs before necking without change of lamellar orientation. The a axis orients in the drawing direction; the b axis orients perpendicular to the direction of drawing; and the chain axis tilts away from the thickness direction of the mat. The structure of films drawn from mats is characterized by a distinct double orientation of crystals. This biaxial orientation in the drawn films has a high degree of correlation with the orientation of crystal axes observed before necking, and suggests that necking takes place in such a way that the chain tilts gradually about the b axis and ultimately unfolds. The postulate of formation of transitory two-dimensional crystals in necking seems useful in explaining the double orientation in the drawn film. The orientation behavior of crystal axes observed before necking is not always similar to that observed in the deformation of a single crystal. The difference is thought to be due to the effect of forces induced by drawing that act in the direction normal to the lamellae within a mat.     

Crystal structure and morphology of biaxially oriented polyamide 12 films

The structural and morphological characteristics of biaxially oriented polyamide 12 films are described on the basis of the results from differential scanning calorimetry, wide‐angle X‐ray diffraction (WAXD), polarized FT‐IR spectroscopy, and small angle X‐ray scattering (SAXS). The WAXD patterns of the oriented polyamide 12 films indicated only the monoclinic γ crystal with little dimensional changes of its unit cell depending on the stretching conditions. The crystallographic angles (α = γ = 90°, β = 121°) that were determined via the WAXD patterns confirmed the monoclinic symmetry of the γ crystal. Annealing the films stretched at 115 °C in boiling 20% formic acid solution did not result in structural changes of the crystalline unit cell. The chain‐axis repeat distance of 31.9 Å for the γ crystal was experimentally obtained with (0 26 0) planes. It was shortened as compared with that of all‐trans conformation. For films having primary orientation to MD, normals to the basal plane of folded‐chain lamellae were parallel to MD (primary stretch direction) resulting in two‐point SAXS patterns. Growth in long spacing with an increase of stretch temperature was discovered. Annealing the films induced further elongation in long spacing. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1189–1200, 2002     

Flow mechanisms in the face-centred cubic micellar gel of a diblock copolymer

The mechanisms of flow of a face-centred cubic micellar phase were investigated using small-angle X-ray scattering (SAXS) for samples under either steady or oscillatory shear in two different geometries: Couette cell and planar shear sandwich. The system studied was a gel formed by a poly(oxyethylene)–poly(oxypropylene) diblock copolymer in water. SAXS indicated that under steady shear in a Couette cell, flow occurs via sliding of hexagonal close-packed (hcp) layers with a close-packed [110] direction along the shear direction. Under oscillatory shear in the planar shear sandwich, coexistence between this orientation and one in which the hcp layers are rotated by 30° (and flow is in a [211] direction) was observed; however, when subject to oscillatory shear in the Couette cell, flow only occurred along a [110] direction. This observation of flow in a non-close-packed direction may be due to alignment induced by the walls of the shear sandwich. Received: 24 February 2001 Accepted: 21 March 2001     

Structural evolution of melt-drawn transparent high-density polyethylene during heating and annealing: Synchrotron small-angle X-ray scattering study

The morphological development of melt-drawn transparent high-density polyethylene during heating was investigated employing in-situ synchrotron small-angle X-ray scattering (SAXS) technique. The results confirm that at lower temperatures only meridional scattering peaks aligned perpendicular to the extensional flow direction can be observed, indicating a highly oriented lamellar crystallite structure; whereas at higher temperatures an equatorial streak additional to the layer-like meridional scattering pattern develops, reflecting the presence of shish-kebab-like objects in the specimen under investigation. Upon heating, the average thickness of the kebab crystals remains essentially unaffected below 110 °C, and subsequently the selective melting of the less stable kebabs proceeds yielding thicker layered lamellar crystals. When the temperature is raised to 131 °C, the shish-like formation and the thermally stable kebab crystals melt simultaneously. In addition, the microstructure of the melt-drawn specimen subjected to annealing at elevated temperatures was probed at room temperature. As opposed to the SAXS patterns registered at high temperatures, the SAXS diagram measured after annealing shows no equatorial streak, suggesting that the cylindrical structures could be re-formed. This observation can be explained by assuming that the plate-like kebab crystals with their normal parallel to the stretching direction grow and impinge during cooling to room temperature due to secondary crystallization, which can be verified by in-situ SAXS experiments during annealing and subsequent cooling.     

Effect of drawing on the α relaxation of poly(vinyl alcohol)

The α relaxation of isotropic and drawn poly(vinyl alcohol) dried gel films was studied using dynamic mechanical analysis. The temperature of the relaxation T_α increased from 160°C in the isotropic gel to 220°C in a fiber drawn 19 ×. The relaxation, which is associated with the crystalline regions of the material, also decreased continuously in magnitude as drawing proceeded, although crystallinity increased. At draw ratios over 12 ×, the relaxation became difficult to resolve, and no relaxation was observed in fibers drawn over 19 ×. The melting points of the fibers increased with draw ratio, but not enough to account for the large change in T_α. Crystal thickness in the fiber direction also increased with draw ratio. An analogy is drawn to the case of polyethylene where crystal thickness has been found to control T_α. The absence of a resolvable α relaxation is one reason why it is difficult to draw poly(vinyl alcohol) gels to extremely high ratios.     

Uniaxial drawing of polytetrafluoroethylene virgin powder by extrusion plus cold tensile draw

Polytetrafluoroethylene (PTFE) virgin powder was ultradrawn uniaxially by a two-stage draw. A film, compression molded from powder below the melting temperature (T_m), was initially solid-state coextruded to an extrudate draw ratio (EDR) of 6–20 at an established optimum extrusion temperature of 325°C, near the T_m of 335°C. These extrudates from first draw were found to exhibit the highest ductility at 45–100°C for the second-stage tensile draw, depending on the initial EDR and draw rate. The maximum achievable total draw ratio (DR_{t, max}) was 36–48. Such high ductility of PTFE, far below the T_g (125°C) and T_m, is in sharp contrast to other crystalline polymers that generally exhibit the highest ductility above their T_g and near T_m. The unusual draw characteristics of PTFE was ascribed to the existence of the reversible crystal/crystal transitions around room temperature and the low intermolecular force of this polymer, which leads to a rapid decrease in tensile strength with temperature. The structure and tensile properties of drawn products were sensitive to the initial EDR, although this had no significant influence on DR_t,max. The most efficient and highest draw was achieved by the second-stage tensile draw of an extrudate with the highest EDR 20 at 100°C, as evaluated by the morphological and tensile properties as a function of DR_t. The efficiency of draw for the cold tensile draw at 100°C was a little lower than that for solid-state coextrusion near the T_m. However, significantly higher tensile modulus and strength along the fiber axis at 24°C of 60 ± 2 GPa and 380 ± 20 MPa, respectively, were achieved by the two-stage draw, because the DR_t,max was remarkably higher for this technique than for solid-state coextrusion (DR_t,max = 48 vs. 25). The increase in the crystallite size along the fiber axis (D₀₀₁₅), determined by X-ray diffraction, is found to be a useful measure for the development of the morphological continuity along the fiber axis of drawn products.© 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2551–2562, 1998     

Crystalline structure and morphology of biaxially oriented polyamide‐11 films

The effect of the uniaxial and biaxial stretching and subsequent solution annealing of extrusion‐cast polyamide‐11 films on the crystalline structure and morphology was investigated with differential scanning calorimetry, wide‐angle X‐ray diffraction (WAXD), Fourier transform infrared spectroscopy, and small‐angle X‐ray scattering (SAXS). The extrusion‐cast polyamide‐11 films exhibited elevations in the glass‐transition and cold‐crystallization temperatures with a constant crystallinity and a constant melting point during aging under room conditions (20–26 °C and 20–31% relative humidity). WAXD and SAXS suggested that chain‐folded lamellae of coexisting α‐ and β‐crystals existed in all the stretched polyamide‐11 films. WAXD pole figures indicated that hydrogen bonds in the hydrogen‐bonded sheets of these two crystalline forms apparently formed between antiparallel chain molecules. The unit cell parameters [a = 9.52 Å, b = 5.35 Å, c = 14.90 Å (chain axis), α = 48.5°, β = 90°, and γ = 74.7° for a triclinic α form and a = 9.52 Å, b = 14.90 Å (chain axis), c = 4.00 Å, α = 90°, β = 67.5°, and γ = 90° for a monoclinic β form] for polyamide‐11 crystals were proposed according to the results of this study and the results of previous investigators. The unit cell parameters of the stretched extrusion‐cast polyamide‐11 films varied, depending on the stretching conditions (the stretch temperature and stretch ratio). As the stretch temperature and stretch ratio were increased, the crystal became more similar to the form described previously and was accompanied by an increase in the long spacing of crystalline lamellae. Annealing the stretched films in a boiling 20% formic acid solution made slightly more perfected crystals. The hydrogen‐bonding α(010) + β(002) planes, which are nearly parallel to both amide group planes and zigzag methylene sequence planes of the biaxially stretched films were found to be parallel to the film surface. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2624–2640, 2002     

Development of internal fine structure in stretched rubber vulcanizates

Small‐angle X‐ray scattering (SAXS) pattern and tensile stress during relaxation of stretched rubber vulcanizates (synthetic polyisoprene) were measured simultaneously at room temperature and at 0 °C. The samples were quickly stretched to the prefixed strain and then allowed to relax for 1 h. In every SAXS pattern, the intensity distribution was elongated along the equator, indicating the formation of structures elongated in the stretching direction. The so‐called two‐spots pattern corresponding to the long period of stacked lamellar crystals did not appear even when the critical strain to induce crystallization was exceeded. On the other hand, even below the critical strain, additional development of equatorial streaks was detected in the differential SAXS patterns. This result suggests the growth of the density fluctuation elongated in the stretching direction, which is not directly related to strain‐induced crystallization. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Fibrillar structure development of polyacrylonitrile fibers treated by ultrasonic etching in oxidative stabilization

Polyacrylonitrile fibers were oxidatively stabilized through 10 gradient‐elevated temperature zones in sequence. The ultrasonic etching method was used for fibril separation of fibers heated at different temperatures, and the fibrillar structure development was studied by scanning electron microscopy. The voids among fibrils are the weak combination points. Under ultrasonic etching, the voids are enlarged. Subsequently, the solvent enters and spreads among fibrils, which results in the separation of fibrils. Separated fibrils with diameters of 100–400 nm appear in fibers heated at less than 235°C. Fibrils in fibers heated from 195°C to 235°C tend to adhere to each other, and the observed macrofibrils are composed of several to dozens of fibrils. For fibers heated from 195°C to 245°C, only a few fibril bundles emerge on the skin near the fiber end, and the fibrils manifest themselves as numerous protuberances on the cross section. In the ranges of 255–275°C, fibrils compactly combine with each other, which suggests insolubility and infusibility, and no separated fibrils appear. The fibrils arrange in a systematic way along the fiber axis and grooves parallel to the fiber axis on the fibers' surface. These grooves are the macro behavior of fibrils arranging on the fiber surface. Copyright © 2017 John Wiley & Sons, Ltd.     

Epitaxial growth of low molecular weight polypropylene from vapor phase

Polypropylene was evaporated on the (001) cleavage faces of alkali halide single crystals, NaCl, KCl, KBr, and KI, under a vacuum of 10^?5 torr. Films evaporated on the substrate were found to be composed of low molecular weight polypropylene with a narrow molecular weight distribution. The polypropylene film evaporated on the substrate maintained at 100°C, was composed of rod-like crystals of α form, the (010) plane of which was parallel to the (001) plane of alkali halide and their long axes were oriented in the (110) direction of the substrate. When the substrate was kept above 150°C, the film included three crystalline forms, α, γ, and δ forms. Fiber-like crystals of the α form lay parallel to the (110) direction of NaCl. In the initial stage of growth, lamellar crystals were observed in the film. The lamellar (001) plane lay flat in the film and branching occurred on the (010) lateral faces. Both the γ-form and the (predominately) new δ-form crystals were observed in the same area of the film prepared at temperatures above 150°C.