DSC and DMTA study of annealed cold-drawn PET: a three phase model interpretation

Cold-drawn poly(ethylene terephthalate) (PET) samples annealed at different undercoolings are studied by means of differential scanning calorimetry and dynamic mechanical thermal analysis. When heating from room temperature, the onset of the glass transition region in cold-drawn, un-annealed samples is found to be significantly lower than in the case of un-oriented PET. On the contrary, the presence of crystalline lamellae in oriented PET cause a shift (and spread out) of the glass transition region towards higher temperatures. The crystal thickening process caused by heating above the annealing temperature, is suggested to take place after a rigid amorphous phase linked to the basal surface of the lamellae has softened. It is found that the low-temperature (between 100 and 140 °C) annealed samples have a glass dispersion region ranging significantly above the annealing temperature itself. This circumstance leads to envisage vitrification as a possible mechanism able to limit lamellar thickening during the annealing process at these low temperatures.     

Structural characterization of Celgard® microporous membrane precursors: Melt-extruded polyethylene films

“Row nucleated lamellar” structures are formed when highly crystalline polymers are melt-extruded and recrystallized under high stress. Polyethylene (PE) and polypropylene (PP) films with row lamellar structures have been utilized to produce microporous membranes. Birefringence measurements of melt-extruded PE films show that improved film orientation can be achieved by annealing, extruding at higher speed, and using higher molecular weight polymers. Images from scanning tunneling, atomic force, and field emission scanning electron microscopy (STM, AFM, and FESEM) clearly show the lamellar structures in the melt-extruded PE and PP films. Microscopy results also show that surface lamellar textures are more pronounced with thicker lamellae and are better aligned along the extrusion direction after annealing. X-ray diffraction results show that the increase in film orientation can be attributed to increased lamellar perfection and orientation during annealing and also to better crystallite alignment along the machine direction with higher extrusion speed or with higher molecular weight. High-resolution capabilities of STM, AFM, and FESEM prove to be very effective tools in elucidating lamellar structures in polymeric membrane precursors and can be used as an aid in establishing structure–process–property relationships in making microporous membranes. © 1994 John Wiley & Sons, Inc.     

Effect of high‐temperature annealing on the microstructure and mechanical properties of polypropylene with shish kebab or spherulite structure

Various annealing temperatures below, near, or above the melting temperature were used to anneal polypropylene with oriented shish kebab and isolated spherulite structures in this work. The results showed that a high annealing temperature decreases the time needed to achieve the ideal material property. When the annealing temperature is near or above the melting temperature, the impact strength would be 1.6 times improved by partial melting and recrystallization. The crystal structure of the oriented shish kebab or isolated spherulite structures was improved when annealed at 150 °C, whereas annealing at 165 or 170 °C recombined the crystal lamellae of the structure. Moreover, the high crystallinity and thick lamellae improved the impact and yield strength values of the spherulite structure. However, excessively high crystallinity and thick lamellae in the oriented shish kebab structure did not result in good mechanical performance. Therefore, the prediction of mechanical properties for the shish kebab structure based on crystallinity and lamellar thickness is not feasible. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46465.     

Secondary crystallization and premelting endo‐ and exotherms in oriented polymers

Crystallization in polyamide 6 (nylon 6) fibers during annealing was studied in detail by following the changes that occurred in the neighborhood of crystalline relaxation temperatures, by using wide‐ and small‐angle X‐ray scattering and differential scanning calorimetry (DSC). Two distinct crystallization regimes were observed depending on whether annealing was carried out below or above onset of crystalline relaxation at ～190°C. In fibers annealed below 190°C, minor melting peaks were followed by exothermic transitions. These were attributed to ～1.5% (by weight) of microcrystals formed during annealing that melt and recrystallize during the DSC scan. These microcrystals are nucleated from unoriented amorphous chains between the lamellar stack within a fibril, and are shown to account for the observed increase in the crystalline orientation and decrease in permeability. Fibers annealed above 190°C did not show the exotherm and had significantly fewer microcrystals. The crystallization in this regime was attributed to the growth of existing lamellae, as evidenced by the increase in crystallite size, crystalline density, crystalline orientation, lamellar spacing, and lamellar intensity. The changes at annealing temperatures >190°C are accompanied by increased dyeability, indicative of more open amorphous regions. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 447–454, 2006     

Lamellar crystalline structure of hard elastic HDPE films and its influence on microporous membrane formation

The development of hard elastic high-density polyethylene (HDPE) precursor films and its influence on the microporous membrane formation have been investigated. As a first step, the HDPE precursor films with ‘row-nucleated lamellar crystalline’ structure were prepared by applying elongation stress to the HDPE melt during T-die cast film extrusion and subsequently annealing the extruded films. This unusual crystalline structure was analyzed in terms of lamellar crystalline orientation, long-period lamellar spacing, crystallite size, and degree of crystallinity. The processing (melt extension and annealing temperature)-structure (lamellar crystalline structure)-property (hard elasticity) relationship of HDPE precursor films was also investigated. The uniaxial stretching of hard elastic HDPE precursor films induces the bending of crystalline lamellae, which leads to the formation of micropores between them. The observation of morphology and air permeability for the HDPE microporous membranes have revealed that the well-developed porous structures characterized by superior air permeability were established preferably from the precursor films prepared by the high stress levels and the high annealing temperatures. Finally, the relationship between the hard elasticity of HDPE precursor films and the air permeability of corresponding microporous membranes was discussed.     

Determination of the crystalline phases of poly(vinylidene fluoride) under different preparation conditions using differential scanning calorimetry and infrared spectroscopy

A method with good precision has been developed to quantitatively measure the degree of α‐, β‐, and γ crystallinity in poly(vinylidene fluoride) (PVDF) by means of infrared spectroscopy. The phase composition of solution‐deposited PVDF films was found to be strongly influenced by the presence of hydrophilic residues on the silicon substrate, the relative humidity present at film deposition, the spatial position on the substrate, and the thermal treatment of the deposited film. Films produced on pristine surfaces gave predominantly α‐phase PVDF, but when a layer of polar solvent (acetone or methanol) remained on the surface, the films produced were predominantly γ phase. Higher humidity promoted a higher fraction of γ crystallinity in the solution‐deposited PVDF films. Solution‐cast films had highly variable composition across the substrate, whereas spin‐cast films were uniform. High‐temperature annealing of PVDF films normally converts the polymer to the γ phase, but annealing the film while still attached to the silicon substrate inhibited this phase transformation. Low‐temperature annealing of freestanding films led to a previously unreported thermal event in the DSC, a premelting process that is a kinetic event, assigned to a crystalline relaxation. Higher‐temperature annealing gave a double endotherm, assigned to melting of different‐sized crystalline domains. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1093–1100, 2003     

Isothermal annealing of poly(lactide‐co‐glycolide) (PLGA) and its effect on radiation degradation

The purpose of this study was to examine how the presence of crystals can retard electron‐beam (e‐beam) radiation degradation, and their effects on the thermal and morphological properties of poly(lactide‐co‐glycolide) (PLGA) upon e‐beam irradiation. Isothermal annealing at 115 °C was carried out on PLGA films and the effect of different annealing times on the degree of crystallinity (DOC) of PLGA was recorded. The DOC increased with annealing time to a maximum value, and remained unchanged with further annealing. The annealed films were then e‐beam irradiated at doses of 5, 10, 20 and 30 Mrad. The degradation of the films was studied by measuring the changes in their molecular weight, DOC, thermal properties and FTIR spectra. It was observed that, regardless of the DOC of the films, the molecular weight of PLGA generally decreased with increasing radiation dose, indicating that chain scission is dominant. However, the extent of degradation is less for the films with a higher DOC. The thermal properties of PLGA also decreased with increasing radiation dose. Radiation increases the DOC for films with initial crystallinity below 5 % but decreases the DOC for films with initial crystallinity above 5 %. Crystals in PLGA films decreased the extent of radiation degradation. Copyright © 2005 Society of Chemical Industry     

Effects of melt‐extension and annealing on row‐nucleated lamellar crystalline structure of HDPE films

The row‐nucleated lamellar crystalline structure of high‐density polyethylene (HDPE) films was prepared by applying elongation stress to HDPE melt during T‐die cast film extrusion and subsequently annealing the extruded films. This unusual crystalline structure was analyzed in terms of lamellar crystalline orientation, long‐period lamellar spacing, crystallite size, and degree of crystallinity. The contribution of melt‐extension represented by draw‐down‐ratio (DDR) to the overall orientation was found to be most noticeable than other processing variables. Meanwhile, the long‐period lamellar spacing, the crystallite size, and the degree of crystallinity were influenced predominantly by the annealing temperature. Finally, the processing (melt extension and annealing temperature) – structure (lamellar crystalline structure) – property (hard elasticity) relationship of HDPE films was investigated. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3326–3333, 2007     

Plasticizer effect of dibutyl phthalate on the morphology and mechanical properties of hard elastic poly(vinylidene fluoride) fibers

To improve the structure and hard elasticity of poly(vinylidene fluoride) (PVDF) fibers, a small amount of the plasticizer dibutyl phthalate (DBP) was added to PVDF. The PVDF/DBP blend fibers were prepared by melt spinning and subsequent annealing. The crystalline structure and thermal properties of the blend fibers were analyzed in terms of the long‐period lamellar spacing, crystal structure, and degree of crystallinity with X‐ray diffraction, differential scanning calorimetry, and small‐angle X‐ray scattering. The results indicated that stacked crystalline lamellae, which were aligned normal to the fiber axis, existed in the blend fibers, and they were in the form of an α‐crystal phase. The total crystallinity of the blend fibers was higher than that of the pure PVDF fibers, and it reached its highest value when the DBP concentration was 2 wt %; then, it decreased with an increase in the DBP content. The morphology and mechanical properties of the fibers were also investigated with scanning electron microscopy and electronic tensile experimentation. The results of scanning electron microscopy apparently exhibited a small porous structure on the surface of the blend fibers, and the more DBP there was in the PVDF fibers, the more porous structure was obtained. Mechanical experiments indicated that the fibers with a 5 wt % concentration of DBP had better elastic recovery and breaking strain than the pure PVDF fibers. These results all indicated that DBP‐modified PVDF fibers have potential applications in preparing microporous membranes by a melt spinning and stretching process. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Nanoscale localization of poly(vinylidene fluoride) in the lamellae of thin films of symmetric polystyrene-poly(methyl methacrylate) diblock copolymers

We employed thin film blends of diblock copolymers with functional homopolymers as a simple strategy to incorporate organic functional materials into nanodomains of diblock copolymers without serious synthesis. A blend pair of polystyrene-poly(methyl methacrylate) (PS-PMMA) diblock copolymers and poly(vinylidene fluoride) (PVDF) was selected as a model demonstration because PVDF is a well-known ferroelectric polymer and completely miscible with amorphous PMMA. Thin films of symmetric PS-PMMA copolymers provided the nanometer-sized PMMA lamellae, macroscopically parallel to the substrate, in which PVDF chains were dissolved. Thus, amorphous PVDF chains were effectively confined in the PMMA lamellae of thin film blends. The location of PVDF chains in the PMMA lamellae was investigated by the dependence of the lamellar period on the volume fraction of PVDF, from which we found that PVDF chains were localized in the middle of the PMMA lamellae. After the crystallization of PVDF, however, some of PVDF migrated to the surface of the film and formed small crystallites.     

A Study on the Relationship Between the Crystal Structure and Hard Elasticity of PVDF Fibers

Summary: Poly(vinylidene fluoride) (PVDF) fibers were prepared by melt-spinning process. The crystal structure of annealed PVDF fibers was characterized by wide- and small-angle X-ray diffraction (WAXD and SAXD) and scanning electron microscopy (SEM). Crystalline reflections of c-axis orientation of annealed PVDF fibers were illustrated by WAXD pattern. The stacked lamellar structure aligned in the direction normal to the fiber axis was found in SAXD pattern and the d-spacing of the lamellae was 13.4 nm. Such lamellar structure was supported by SEM micrographs as well. The elastic recovery of annealed PVDF fibers was above 80% from 50% extension, which was much higher than that of unannealed fibers on the first cycle. The initial elastic modulus of annealed fibers reached to a value of 3.5 GPa. The morphological and mechanical properties, all indicated that the annealed PVDF fibers had the characteristic of hard elasticity. A typical stress-strain curve at a very low strain rate indicated the deformation of crystal lamellae in the fibers and a suggested structural deformation mechanism detailed the characteristics of hard elasticity.

Cyclic loading to 50% extension of annealed PVDF fibers.     

Tuning the aggregation of polyimide thin films by modification of their molecular interactions

Thin films of an organo‐soluble polyimide based on 1,4‐(3,4‐dicarboxyphenoxy)benzene dianhydride (HQDPA) and 2,2′‐dimethyl‐4,4′‐methylene dianiline (DMMDA) have been studied. A prism coupler was used to measure the refractive indices. The average refractive indices of thin films prepared by annealing at different temperatures and times were chosen to characterize the condensation states of thin films. Thin films annealed at 200 °C show irreversible changes in physical properties, eg solubility. FTIR spectroscopy showed that the chain structures of the above thin films remained unchanged. It is proposed that specific molecular interactions induce the irreversible changes revealed by fluorescence spectroscopy. © 2000 Society of Chemical Industry     

Effect of crystallinity on enthalpy recovery peaks and cold‐crystallization peaks in PET via TMDSC and DMA studies

Poly(ethylene terephthalate) (PET) sheets of different crystallinity were obtained by annealing the amorphous PET (aPET) sheets at 110°C for various times. The peaks of enthalpy recovery and double cold‐crystallization in the annealed aPET samples with different crystallinity were investigated by a temperature‐modulated differential scanning calorimeter (TMDSC) and a dynamic mechanical analyzer (DMA). The enthalpy recovery peak around the glass transition temperature was pronounced in TMDSC nonreversing heat flow curves and was found to shift to higher temperatures with higher degrees of crystallinity. The magnitudes of the enthalpy recovery peaks were found to increase with annealing times for samples annealed ≤30 min but to decrease with annealing times for samples annealed ≥40 min. The nonreversing curves also found that the samples annealed short times (≤40 min) having low crystallinity exhibited double cold‐crystallization peaks (or a major peak with a shoulder) in the region of 108–130°C. For samples annealed long times (≥50 min), the cold‐crystallization peaks were reduced to one small peak or disappeared because of high crystallinity in these samples. The double cold‐crystallization exotherms in samples of low crystallinity could be attributed to the superposition of the melting of crystals, formed by the annealing pretreatments, and the cold‐crystallizations occurring during TMDSC heating. The ongoing crystallization after the cold crystallization was clearly seen in the TMDSC nonreversing heat flow curves. DMA data agreed with TMDSC data on the origin of the double cold‐crystallization peaks. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Morphological and dielectric properties of barium chloride‐filled poly(vinylidene fluoride) films

Morphological characteristics of poly(vinylidene fluoride) (PVDF) films, filled with mass fractions (w ≤ 20%) of Barium Chloride (BaCl₂), were investigated by X‐ray diffraction (XRD), Fourier transform infrared (FTIR) absorption spectra and differential scanning calorimetry (DSC) measurements. The dielectric properties of films were measured from 250 Hz to 1 MHz range between 100 and 400 K as a function of frequency and temperature. Spectroscopic data revealed that the filled and unfilled films include α‐, β‐, and γ‐crystalline phases. By a 20% filling, 73% increase was obtained in the total degree of crystallinity. Since the BaCl₂ formed fluorine bridges over the chain segments on the crystal lamellar surface, the γ‐crystalline phase decreased, whereas the total degree of crystallinity increased. Dielectric measurements showed that maximum of the dielectric loss factor belonging to β‐relaxation transition decreased linearly with filling level. The filling process did not have any effect on the real dielectric constant till α‐relaxation transition region. However, in the α‐relaxation transition region, it was determined that the real dielectric constant increased linearly with filling level. POLYM. COMPOS., 31:1782–1789, 2010. © 2010 Society of Plastics Engineers.     

The impact of new crystalline lamellae formation during annealing on the properties of polypropylene based films and membranes

Cast films, based on polypropylene (PP), were prepared via melt extrusion and, then, annealed below the melting temperature. The effect of annealing conditions on the properties of the films and the microporous membranes formed by stretching was investigated. It is shown that annealing is an effective method to improve the physical properties of semi-crystalline polymers by promoting chain rearrangement and creating secondary lamellae in the amorphous region. DSC results for annealed samples revealed the appearance of a shoulder endothermic peak and a new peak in the correlation function from the SAXS patterns was observed. The annealed films exhibited double yield points in the tensile deformation curves. A direct linear relationship was found between the strength of the second yield point and the fraction of the lamellae. From mercury porosimetry and SEM images of the membranes larger pore sizes and porosity were observed as the annealing time and temperature increased.     

Dual lamellar crystal structure in poly(vinylidene fluoride)/acrylic rubber blends and its biaxial orientation behavior

The miscibility for melt-mixed poly(vinylidene fluoride) (PVDF)/acrylic rubber (ACM) blends and the crystal morphology of PVDF in the blends were investigated over the whole composition ranges by dynamic mechanical analysis (DMA), wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), and transmission electron microscopy (TEM). DMA measurements revealed that PVDF is miscible with ACM in ACM-rich system, and partially miscible in PVDF-rich system. Two kinds of PVDF lamellar structures with different long periods were detected by SAXS and TEM for the partially miscible blends. In the miscible system, only one kind of crystal lamellae with enlarged long period is found. The two kinds of lamellar structures in the blend show different orientation behavior during the uniaxial stretching to result in a biaxial orientation. The lamellae with short long period are oriented vertical to the stretching direction, while those with large long period were found to be oriented parallel to the stretching direction.     

Nanocrystalline ZnO Thin Film Synthesis Using Glycerol in Aqueous Polymeric Precursor Processing

Synthesis of high-quality ZnO thin films via simple and cost effective processing technique is a major challenge. In this work, the preparation of nanocrystalline ZnO thin films by a novel polymeric precursor processing using glycerol as chelating agent is presented. The process has advantages of being cost-effective and environment friendly. ZnO thin films were prepared by a single spin-coating deposition of aqueous polymeric precursor prepared with zinc nitrate [Zn(NO₃)₂] and glycerol as chelating agent. The thermal decomposition of polymeric precursors was studied by thermogravimetric analysis and Fourier transform infrared (FTIR) spectroscopy. Annealing of these films were performed over the range of 300°–600°C, and the effect of annealing on the degree of crystallization, surface morphology, crystallite size, and optical properties was investigated. X-ray diffraction analysis shows that the thin films are polycrystalline with wurtzite structure. The thin films are 80% dense, have crack free microstructure, and transparency of >85% in the visible region. These films exhibit absorption edge at 375 nm. On measuring at room temperature, the optical band gap energy of ZnO thin films, annealed at 450° and 600°C, was determined to be 3.295 and 3.267 eV. Room temperature photoluminescence spectra of these films show strong UV emission and a broad yellow-green emission in the range 525–600 nm. The intensity of UV emission peak increases with increase in annealing temperature that is attributed to an improvement in crystallinity.     

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

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

Crystallization and melting behavior of amorphous poly(iminosebacoyl iminodecamethylene)

Multiple melting behavior was observed in the differential scanning calorimetry (DSC) scans for the isothermally crystallized poly(iminosebacoyl iminodecamethylene) (PA1010) samples. Coexistence of crystal populations with different lamellar thickness in PA1010 was discussed by means of DSC, wide‐angle X‐ray diffraction (WAXD), and small‐angle X‐ray scattering techniques. During crystallization of the polymer, a major lamellar crystal population developed first, which possessed a higher melting temperature. However, a small fraction of the polymer formed minor crystal population with thinner lamellae, which was metastable and, upon post‐annealing, could grow into more stable and thicker lamellae through melting and recrystallization process. Lamellae insertion or stacks would develop during the post‐annealing at a lower temperature for the isothermally crystallized samples; thus, multiple crystal populations with different thickness could be produced. It is the multiple distribution of lamella thickness that gives rise to multiple melting behavior of crystalline polymers. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 993–1002, 2000     

Transport properties and their correlation with the morphology of thermally conditioned polypropylene

Time-lag and static sorption experiments were employed to measure permeability, diffusivity and solubility constants of He, A, and CF₄ in polypropylene films cooled at various rates from the melt and subsequently annealed at varying temperatures near the melting point. While solubility constants in films annealed above 90°C showed the normal variation with the amorphous content of the polymer, solubility constants for all unannealed, quenched films were remarkably constant and independent of the rate of cooling. In fact, all quenched films appeared to have the same amorphous content (ca. 41%). The remaining material is believed to be a mixture of monoclinic and hexagonal crystallinity, the volume ratio of the two being a function of the rate of quenching, and changing on annealing, in favor of the more stable, monoclinic form; the transition occurring rather sharply at 90°C. X-ray diffraction provided supporting evidence for the presence of the hexagonal crystals. The diffusion behavior in crystalline polypropylene is normal and Fickian but instead of the usual decline with increasing crystallinity, diffusivities showed definite enhancement in the case of the annealed films, i.e., the expected monotonic decline of D with increasing crystallinity is not observed. This behavior is attributed to a reduction in diffusional impedance through formation of defects in existing crystallites, as the lamellae thicken, in a manner similar to that observed on annealing of polyethylene single crystals. The apparent activation energies of diffusion were essentially constant and independent of thermal history. This suggests that in a highly crystalline polymer diffusion is not so much impeded by the restricted mobility of chain segments but rather by the extremely small dimensions of the available diffusive pathways. In support of the argument that the transport properties of polypropylene are controlled at a level of microstructure well below the characteristic dimensions of spherulities, it was observed that bulk-crystallized polypropylene has a spherulitic structure whose size and texture do not change significantly on annealing.