The crystalline phase transformation of poly(vinylidene fluoride)/poly(vinyl fluoride) blend films

Poly(vinylidene fluoride) (PVDF), poly(vinyl fluoride) (PVF), and their blends were prepared by solution casting, followed by quenching in ice water after melting to obtain an α-crystalline phase. The films were drawn by solid state extrusion at two different drawing temperatures, 50°C and 110°C. The crystalline phases were analyzed by DSC and FTIR. In the undrawn films, the content of β-crystalline phase in the blend of PVDF/PVF 88.5/11.5 was higher than in the PVDF homopolymer, but it was lower than in the PVDF film with a draw ratio higher than 4. The α-crystalline phase in PVDF/PVF blends was mostly transformed into the β-crystalline phase beyond a draw ratio of 4, regardless of the draw temperature and PVF content. The α-crystalline phase of PVDF systematically transformed into the β-crystalline phase with increasing draw ratio. The crystallinity of PVDF/PVF blend films drawn at 110°C was higher than those drawn at 50°C. In the drawn blend films, characteristic IR bands of the α form were shifted to those of the β form and completely changed into those of β form at draw ratio of 4, regardless of the draw temperature and PVF content.     

Determination of intrinsic birefringence of smectic phase in isotactic polypropylene

When isotactic polypropylene is quenched at 0°C from the molten state, a biphasic material, amorphous-smectic, is obtained. The smectic phase has an intermediate order between the amorphous and the crystalline. In this study the intrinsic birefringence of the smectic phase was determined. Tapes with different draw ratio were prepared at room temperature, at which the smectic phase does not transform into the crystalline phase. The amount of the smectic phase was determined with measurements of density and sorption of vapors of dichloromethane. The value of the intrinsic birefringence for the smectic phase is: δ°n_sm = 40 · 10⁻³. This value is in good agreement with the values proposed by Samuels for the intrinsic birefringence of the amorphous and the crystalline phases. © 1996 John Wiley & Sons, Inc.     

Effect of the temperature on the drawing behavior of poly(ether ether ketone)

This study examines the drawing behavior of amorphous films of poly(ether ether ketone) (PEEK) and the effect of the drawing and the drawing temperature on some physical properties of the drawn samples. The thermal analysis shows that the amorphous films drawn at 200% of deformation and at temperatures of 80, 120, and 140°C crystallize by heating and the crystallization occurs at lower temperature with a lower crystallization enthalpy. The first effect can be related to the influence of orientation on the crystallization rate and the second to strain-induced crystallization. The dynamic—mechanical behavior of the drawn samples is in good agreement with the thermal analysis and confirms the presence of strain-induced crystallization at drawing temperatures well below 180°C. © 1992 John Wiley & Sons, Inc.     

Shrinkage behavior after the heat setting of biaxially stretched poly(ethylene 2,6‐naphthalate) films and bottles

Amorphous preforms of poly(ethylene 2,6‐naphthalate) (PEN) were biaxially drawn into bottles up to the desired volume under industrial conditions. These bottles were used to characterize the shrinkage behavior of the drawn bottles with or without heat treatment and to study structural variations during heat setting. During drawing, a rigid phase structure was induced, and the amount of the induced rigid phase structure was linearly related to the square root of the extra first strain invariant under equilibrium conditions. During the production of these bottles, this equilibrium was not attained because of high stretching conditions and rapid cooling after stretching. The structure after orientation contained a rigid amorphous phase and an oriented amorphous phase. The shrinkage behavior was a function of the temperature and time of heat setting. Long heat‐setting times, around 30 min, were used to characterize the possible structural variations of the oriented PEN after heat setting at equilibrium. Under the equilibrium conditions of heat setting, the start temperature of the shrinkage was directly related to the heat‐setting temperature and moved from 60°C without heat treatment up to a temperature of 255°C by a heat‐setting temperature of 255°C; this contrasted with poly(ethylene terephthalate) (PET), for which the start temperature of shrinkage was always around 80°C. For heat‐setting temperatures higher than 220°C, the structural variations changed rapidly as a function of the heat‐setting time, and the corresponding shrinkage of the heat‐set samples sank below 1% in a timescale of 30–60 s for a film thickness of 500 μm. The heat treatment of the oriented films taken out of the bottle walls with fixed ends stabilized the induced structures, and the shrinkage of these heat‐set films was zero for temperatures up to the heat‐setting temperature, between 220 and 265°C, if the heat‐setting time was sufficient. According to the results obtained, a heat‐setting time of 30 s, for a film thickness of 500 μm, was sufficient at a heat‐setting temperature of 255°C to stabilize the produced biaxially oriented PEN bottles and to take them out the mold without further shrinkage. During the drawing of PEN, two different types of rigid amorphous phases seemed to be induced, one with a mean shrinkage temperature of 151°C and another rigid amorphous phase, more temperature‐stable than the first one, that shrank in the temperature range of 200–310°C. During heat setting at high temperatures, a continuous transformation of the less stable phase into the very stable phase took place. The heat‐set method after blow molding is industrially possible with PEN, without the complicated process of subsequent cooling before the molds are opened, in contrast to PET. This constitutes a big advantage for the blow molding of PEN bottles and the production of oriented PEN films. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1462–1473, 2003     

Morphology of quenched polypropylene

The morphology of the quenched form of polypropylene was investigated. Thin films of the quenched form consist of small ball-like structures which we suggest are defective hexagonal crystals. In thicker films they aggregate to form spherulites with no observable fine structure. Quenching of thin films of molten polypropylene to a temperature below 0°C results in the formation of numerous, isolated monoclinic spherulites with a better defined morphology than the smectic spherulites obtained by quenching above 0°C. This result suggests that the films (quenched below 0°C) were quenched to the glass and subsequently crystallized to the monoclinic form upon heating to room temperature.     

Plastic deformation of polypropylene in relation to crystalline structure

The tensile drawing behavior of quenched and annealed films of isotactic polypropylene is investigated as a function of draw temperature and strain rate. A strain‐induced structural change from the smectic to the monoclinic form is observed for the quenched films. A kinetic interpretation is proposed for the phenomenon. Data of thermal activation volume at the yield point indicate two regimes of plastic flow for the quenched sample, between 25 and 60°C, but only one regime for the annealed sample. Homogeneous and heterogeneous crystal slip processes are proposed to account for these regimes in relation to the nucleation and propagation of screw dislocations. The basic mechanism of molecular motion in the polypropylene crystal is suggested to be a wormlike motion of conformational defects along the 3/1 helix chains that allows a 120° rotation and a c/3 translation. The occurrence of the smectic form as a transitory state in the deformation pathway is discussed in terms of plasticity defect generation. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1873–1885, 1999     

Effect of structure on the deformation and failure of oriented polymer films

This study compares the effect of anisotropic structure on the room temperature (23°C) deformation and failure behavior of two distinctly different polymers. One a single phase nematic polymer, PMDA-ODA polyimide [PI] with a glass transition, T_g, near 400°C; the other a previously studied anisotropic two phase molecular polymer composite, isotactic polypropylene [IPP] with a glass transition near 14°C. Although isotactic polypropylene has a two phase crystal-noncrystalline structure its deformation behavior is controlled by the noncrystalline phase. A series of oriented PMDA-ODA polyimide [PI] films are fabricated and their orientation characterized. The present study investigates whether the deformation and failure behavior of these anisotropic single phase nematic PMDA-ODA polyimide films (tested at a very low strain rate at room temperature) is similar to that previously observed for isotactic polypropylene at its low strain rate failure envelope limit.     

Effects of initial structure on the deformation behavior of PP hollow fiber in continuous drawing

The effects of the microcrystalline structure of undrawn fibers on the rheological behavior in the process of the polypropylene (PP) hollow fiber formation system were studied by a simple model describing the continuous drawing process. The predicted and observed drawing behaviors were explained in view of a simple approach based on the concept of strain-rate sensitivity and strain-hardening parameters. Various parametric studies showing the interactive nature of the strain-rate sensitivity and strain-hardening parameters on the drawing behavior were also numerically performed. Strain-rate sensitivity affects mainly the intensity of the neck, and strain hardening has more effects on the position of the inflection point in continuous drawing. Details of the necking mechanism in the drawing process were studied by observing the deformation behavior of the specimens with different initial microcrystalline structures. The distinctive initial structure of undrawn PP hollow fibers can be formed by controlling the quenching condition in melt spinning. It was shown that the water-quenched PP fiber exhibited an unstable smectic form, whereas the stable monoclinic phase was observed for the fiber prepared without quenching by forced convection. The experimental results of the drawing behavior indicated that the strain-rate sensitivity of the fiber with a smectic form was larger than that with the monoclinic form. It was also shown that the hollowness was affected by the quenching condition in melt spinning and the deformation behavior in the drawing process. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2170–2182, 2001     

Long-Range Interactions in Thin Smectic Films on Substrates: X-Ray Reflectivity Studies

Long-range forces between interfaces of thin and ultrathin smectic films are responsible for their thickness and stability. We present here X-ray reflectivity studies of the forces in spin-coated thin films of three different commercial liquid crystal (LC) mixtures as well as in main-chain LC polymers based on polysiloxane. All these LC materials possess a smectic C* phase at room temperature. We demonstrate spontaneous molecular self-assembly after spin coating into a nearly perfect smectic layer structure on various substrates. However, annealing at room temperature is essential to achieve an equilibrium state. Measurements on ultrathin annealed films of the LC mixtures show dramatic variation of the smectic layer spacing, L, as a function of the number of smectic layers, n (or film thickness, d). The functional dependence of L(n) for all three different liquid crystal mixtures suggests a long-range interaction between the interfaces that decays algebraically as 1/n^κ where κ = 2 ± 0.3. This decay is consistent with a van der Waals type of interaction, although its magnitude cannot be explained by the existing mechanisms. X-ray studies of thick and thin annealed polysiloxane films allow determination of the phases, the phase transition temperatures, and the temperature dependence of the tilt angle in the smectic C* phase. Thin (300 Å to 600 Å) polysiloxane films far above the bulk smectic–isotropic phase transition temperature show the formation of smectic film at the film–substrate interface due to surface freezing phenomena. Preliminary investigations of the temperature dependence of the smectic film thickness indicate that the interaction between the interfaces decays algebraically, with an exponent κ = 1.5 ± 0.5.     

Effect of thermal history on mechanical properties of polyetheretherketone below the glass transition temperature

Tensile properties of polyethertherketone (PEEK) have been studied at 125, 25, and ?100°C for thin films prepared with different thermal histories. Initial morphology was controlled by rate of cooling from the melt. Amorphous films resulted from quenching the melt, while semicrystalline films were obtained by cooling the melt at different rates, or by crystallization of the rubbery amorphous state. The films were characterized using density, X-ray scattering, differential scanning calorimetry, and infrared spectroscopy. Scanning electron microscopy was used to examine fracture surfaces. Degree of crystallinity and rate of cooling from the melt affected the tensile properties at all test temperatures. For films with nearly the same degree of crystallinity, those which were more slowly cooled from the melt fractured at the lowest strain. The amorphous films were most tough, drawing to 233% at ?100°C and to over 500% at 125°C. Films crystallized from the rubbery amorphous state had stress–strain behavior intermediate between that of the amorphous and melt-crystallized films at all test temperatures. Density measurements on the drawn material indicate that void formation occurs simultaneously with the formation of fibrillar crystals. Necking resulted in density increases for amorphous films, and density decreases for the semicrystalline films.     

Two-stage drawing of poly(ethylene 2,6-naphthalate)

Initially amorphous and semicrystalline films of poly(ethylene 2,6-naphthalate) with different molecular weights were drawn by two-stage drawing, that is, coextrusion at low temperatures (25–160°C) followed by tensile drawing at high temperatures (200–245°C). Both films could be drawn up to a draw ratio of 8–10 by this method under controlled conditions. The tensile modulus and strength of drawn samples were greatly affected by the draw temperature for the first stage, predrawn morphology, and molecular weight. The remarkable effects of these variables on the tensile properties are closely related to the difference in the resultant amorphous chain orientation of the samples, reflecting the disentanglements and chain slippage during drawing, and the dissipation of chain orientation after processing.     

In situ studies on the temperature‐related deformation behavior of isotactic polypropylene spherulites with uniaxial stretching: The effect of crystallization conditions

The deformation behavior of isotactic polypropylene (iPP) spherulites with uniaxial stretching was investigated at different drawing temperatures via in situ polarized optical microscope (POM) observation. The iPP spherulites were prepared by two procedures: cooled to the room temperature from melt and annealed at 135, 140, and 145°C for 3 h. It was found that the crystallization conditions dominate the crystalline morphology and even the tensile properties of iPP. For iPP which crystallized during cooling progress, the spherulites were imperfect and the boundaries of the spherulites were diffuse, displaying good toughness at various drawing temperatures. For iPP annealed at high temperatures displayed the brittle fracture‐modes and the crack happened between spherulites, which due to the large and perfective spherulites have thick lamellas and weak connection at interspherulitic boundary. The shape and size of the iPP spherulites formed at 140 and 145°C are affected with uniaxial stretching till to the fracture of the samples at different drawing temperatures. The spherulites obtained at 135°C are deformed along the drawing direction at 100°C but not affected at low drawing temperatures, indicating the toughness increased with the increase of the drawing temperatures. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Effect of flow history on poly(vinylidine fluoride) crystalline phase transformation

This study was devoted to the effect of extensional flow during film extrusion on the formation of the β‐crystalline phase and on the piezoelectric properties of the extruded poly(vinylidine fluoride) (PVDF) films after cold drawing. The PVDF films were extruded at different draw ratios with two different dies, a conventional slit die and a two‐channel die, of which the latter was capable of applying high extensional flow to the PVDF melt. The PVDF films prepared with the two‐channel die were drawn at different temperatures, strain rates, and strains. The optimum stretching conditions for the achievement of the maximum β‐phase content were determined as follows: temperature = 90°C, strain = 500%, and strain rate = 0.083 s^?1. The samples prepared from the dies were then drawn under optimum stretching conditions, and their β‐phase content and piezoelectric strain coefficient (d₃₃) values were compared at equal draw ratios. Measured by the Fourier transform infrared technique, a maximum of 82% β‐phase content was obtained for the samples prepared with the two‐channel die, which was 7% higher than that of the samples prepared by the slit die. The d₃₃ value of the two‐channel die was 35 pC/N, which was also 5 pC/N higher than that of the samples prepared with the slit die. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Morphology and texture development of uniaxially stretched poly(ethylene naphthalene‐2,6‐dicarboxylate)

The texture development of PEN films with different semicrystalline morphologies have been studied by X‐ray diffraction. These different structures have been obtained by uniaxially stretching PEN amorphous films at 100 and 160°C (below and above T_g) at different drawing ratios. Samples have also been characterized by DSC to determine the crystallinity ratios, the crystallization, and melting temperatures. To define the orientation of crystallites in the oriented samples, pole figures have been constructed, as a function of temperature and drawing ratio (DR) in the range 1.5–4. In the range from DR = 2 to 4 the orientation is clearly uniplanar‐axial. At T_draw = 100°C the crystallinity shown by DSC analysis is higher than the sample stretched at 160°C. The orientation is also higher when samples are stretched at 100°C. The naphthalene rings mainly stay in the plane of the film with a lower fraction perpendicular to the plane of the film. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 395–401, 2007     

Mechanical and thermal properties of polycarbonate,part 1: Influence of free quenching

The effect of different thermal treatments on the mechanical and thermal properties of polycarbonate was investigated. The first quenching procedure which involves the quench of the samples from the melt state to different temperatures allowed improving impact strength and elongation at break for a quenching temperature of 0°C. A second quenching procedure, corresponding to specimens heated again at 160°C (T_g + 15°C) and quenched a second time, showed a better enhancement of the impact strength and elongation at break to the detriment of other properties such as elastic modulus, density, yield stress, and heat distortion temperature, for a quenching temperature of 40°C. This effect was associated to the existence of a relaxation mode around 35°C. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Effect of montmorillonite on orientation of drawn polypropylene films

Films of polypropylene/organically modified montmorillonite (PP/OMMT) nanocomposites were drawn at two different temperatures with various draw ratios. The effect of OMMT on the orientations of the crystalline and amorphous phases was studied using polarized infrared spectroscopy. It is found that OMMT layers always retard the orientation of the crystalline phase. The higher the OMMT loading, the stronger the retardance effect. In contrast, the effect of OMMT layers on the orientation of the amorphous phase depends on draw temperature and OMMT loading. A favorable effect on the orientation of the amorphous phase is observed at low OMMT loading and high draw temperature, but the retardance prevails at high OMMT loading and low draw temperature. The favorable effect on orientation at high draw temperature is attributed to the stabilization effect of OMMT layers on the conformation of amorphous PP chains. Such an effect was further verified by comparing the crystallization behavior and the morphologies of drawn PP and PP/OMMT films crystallized from 180°C. Memory effect is observed for crystallization of drawn PP/OMMT film, but it is not obvious for the drawn film of neat PP. Spherulites are formed for orientated neat PP films cooled from 180°C, but cylindrites are still formed after the drawn PP/OMMT films undergo melting at 180°C and recrystallization. The stabilization effect disappears at higher temperature (230°C). © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Ultradrawing behavior of one- and two-stage drawn gel films of ultrahigh molecular weight polyethylene and low molecular weight polyethylene blends

The ultradrawing behavior of gel films of plain ultrahigh molecular weight polyethylene (UHMWPE) and UHMWPE/low molecular weight polyethylene (LMWPE) blends was investigated using one- and two-stage drawing processes. The drawability of these gel films were found to depend significantly on the temperatures used in the one- and two-stage drawing processes. The critical draw ratio (λ_c) of each gel film prepared near its critical concentration was found to approach a maximum value, when the gel film was drawn at an “optimum” temperature ranging from 95 to 105°C. At each drawing temperature, the one-stage drawn gel films exhibited an abrupt change in their birefringence and thermal properties as their draw ratios reached about 40. In contrast, the critical draw ratios of the two-stage drawn gel films can be further improved to be higher than those of the corresponding single-stage drawn gel films, in which the two-stage drawn gel films were drawn at another “optimum” temperature in the second drawing stage after they had been drawn at 95°C to a draw ratio of 40 in the first drawing stage. These interesting phenomena were investigated in terms of the reduced viscosities of the solutions, thermal analysis, birefringence, and tensile properties of the drawn and undrawn gel films. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 149–159, 1998     

Effect of the unidirectional drawing on the thermal and mechanical properties of PLA films with different L‐isomer content

This work simulates the thermoforming process of two different L ‐isomer content PLA grades (95.6 and 98%), studying the influence on the induced morphology and the variation in the thermal and tensile properties. The thermoforming process was simulated with uniaxial tensile tests performed at different temperatures and strain rates, reporting the tensile behavior before, during, and after the test. The resulting structural changes were analyzed by wide‐angle X‐ray scattering and Fourier transformed infrared spectroscopy. Thermal characterization was carried out with differential scanning calorimetry and dynamomechanical thermal analysis. The results showed that, regardless the L ‐isomer content, drawing at 70°C produced a stable mesomorphic phase that showed greater tensile properties than the original films. This mesomorphic phase, indeed, could be reordered into a crystalline structure under mild annealing conditions (5 min/75°C), if compared with those needed for similar amorphous PLA specimens (60 min/120°C), thus providing a processing opportunity for obtaining thermally stable PLA products at temperatures above 100°C. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Phase stability and melting behavior of the α and γ phases of nylon 6

The phase stability and melting behavior of nylon 6 were studied by high‐temperature wide‐angle X‐ray diffraction and differential scanning calorimetry (DSC). The results show that most of the α phase obtained by a solution‐precipitation process [nylon 6 powder (Sol‐Ny6)] was thermodynamically stable and mainly melted at 221°C; the double melting peaks were related to the melt of α crystals with different degrees of perfection. The γ phase formed by liquid nitrogen quenching (sample LN‐Ny6) melted within the range 193–225°C. The amorphous phase converted into the γ phase below 180°C but into the high‐temperature α phase at 180–200°C. Both were stable over 220°C. α‐ and γ*‐crystalline structures were formed by annealing but were not so stable upon heating. Typical double melting peaks were shown on the DSC curve; melt recrystallization happened within the range 100–200°C. The peak at 210°C was mainly due to the melting of the less perfect crystalline structure of the γ phase and a fraction of the α phase; the one at 219°C was due to the high‐temperature α‐ and γ‐phase crystals. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Properties of syndiotactic-rich poly(vinyl alcohol) thin films in water. VI. Elastic behavior of undrawn/annealed thin films by repeated elongation/contraction in water

The elastic behavior of undrawn/annealed swollen thin films of syndiotactic-rich poly(vinyl alcohol), derived from vinyl trifluoroacetate, was studied by repeated elongation/contraction in water. For the films annealed at temperatures below 175°C, the characteristic of deformation was divided into low-drawn and high-drawn regions. The elastic deformation was dominant over the plastic deformation in the low-drawn region and vice versa in the high-drawn region. The effect of heat treatment on the elastic behavior of the swollen films in water almost independent of annealing temperatures below 125°C. The elastic deformation in water at 70°C was especially remarkable in the low-drawn region for the films annealed at temperatures below 125°C and at 80°C for the films annealed at 150°C. For the films annealed at 200°C, considerable plastic deformation occurred in addition to elastic deformation from the initial drawing; the films were broken in the low-drawn region.