The effect of sulfonation treatment on the structure and properties of isotactic polypropylene fibers prior to the carbonization stage

The effect of sulfonation treatment was investigated on the molecular structure and mechanical properties of isotactic polypropylene fibers extruded at a take up speed of 2500 m/min. It was found that at extensive sulfonation times, the sulfonated structure showed the characteristic features of carbonized structure as indicated by the results of the density and the X‐ray diffraction measurements. Mechanical properties of the sulfonated samples were found to be adversely affected by the sulfonation conditions. Scanning electron microscopy observations showed surface irregularities at low sulfonation times and fiber fractures at high sulfonation times. Polarized infrared spectroscopy measurements analysed by curve fitting procedure showed increasing molecular orientation of long helical chain segments represented by the IR band at 841 cm^?1 whereas amorphous structure represented by the IR band at 2723 cm^?1 showed gradual loss of orientation with the progress of sulfonation. IR bands assigned to the sulfonic acid groups formed during sulfonation treatment showed perpendicular polarization and low molecular orientation characteristics indicating the initiation and the development of crosslinking process being perpendicular to the fiber axis direction. Analysis of the equatorial X‐ray diffraction traces showed the loss of crystallinity where the paracrystalline phase disappeared faster than the crystalline α‐monoclinic phase. During the sulfonation treatment, content of amorphous phase showed gradual increase in line with decreasing crystallinity. In accordance with the loss of crystallinity, apparent crystallite sizes corresponding to the 110, 040 and 130 planes of the α‐monoclinic phase also decreased gradually with increasing sulfonation time. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Molecular structure and orientation of gel‐spun polyethylene fibers

Characterization of molecular structure and orientation of six commercially available gel‐spun polyethylene fibers have been carried out using infra‐red and Raman spectroscopy, thermal and X‐ray diffraction analysis, together with optical microscopy techniques. Thermal and X‐ray diffraction analysis revealed the existence of highly oriented orthorhombic and monoclinic crystallites together with a highly oriented intermediate phase known as pseudohexagonal mesophase structure. The results suggest the existence of a three‐phase structure consisting, at room temperature, of orthorhombic and monoclinic polymorphic crystallites, oriented noncrystalline and un‐oriented noncrystalline (amorphous) phases, respectively. The crystallinity measurements have been carried out using density, thermal and X‐ray diffraction analysis together with infra‐red and Raman spectroscopy techniques whereas the molecular orientation measurements have been carried out using birefringence and polarized IR spectroscopy, respectively. The results obtained from density, thermal analysis, and Raman spectroscopy based on a simple two‐phase modeling approach lead to the overestimated amorphous fractions and appears to ignore the presence of an intermediate phase known as oriented noncrystalline structure. X‐ray analysis has also been used for the measurement of the apparent crystallite sizes. The birefringence values have been used to determine the overall orientation parameters whereas the dichroic measurements of IR bands have been used to determine the crystalline and oriented noncrystalline orientation parameters. The results show that the orthorhombic and monoclinic phases are more highly oriented than the oriented pseudohexagonally packed noncrystalline chains. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1317–1333, 2006     

The role of dry and wet isothermal annealing treatment on the structure and the mechanical properties of isotactic polypropylene fibers

Dry and wet annealing of isotactic polypropylene fibers was carried out under constant length at 120°C in air and in glycerine environments with annealing times ranging from 1 to 30 h. A detailed analysis of the infrared spectrum of samples annealed, especially in air, showed clear evidence of the surface oxidation as indicated by the appearance of oxygen containing functional groups. Annealing was found to lead to an improved structural organization as indicated by the crystallinity, crystallite size, and orientation measurements using X‐ray diffraction and infrared spectroscopy methods. Analysis of the X‐ray diffraction measurements showed a gradual transformation of metastable smectic phase to a more stable α‐monoclinic phase with increasing annealing time. Crystallinity, crystallite size, and orientation measurements performed for the samples annealed in air and in glycerine environments showed no distinct difference. Mechanical properties of the annealed samples were influenced by the annealing environment. Annealing in an air environment resulted in a continuous loss of tensile strength up to the annealing time of 12 h due to an oxidation related chain scission mechanism. On the other hand, annealing in glycerine environment resulted in a continuous and gradual increase of tensile strength without loss of physical form up to the annealing time of 30 h. It is suggested that wet annealing in glycerine environment should be used to obtain improved tensile strength values. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Evolution of aggregation structure of polyacrylonitrile fibers in the cyclization reaction

The aggregation structure of polyacrylonitrile precursor evolves gradually with progress of cyclization. In this work, the variety of cyclization degrees were determined by Fourier transform infrared spectroscopy and the evolution of aggregation structure of PAN fibers were characterized by wide‐angle X‐ray diffraction. Experimental results showed that the cyclization occurred first in the amorphous parts when the heating temperature was below 200°C. After heated at 200°C for 30 min, molecular chains in the pseudo‐crystalline regions started to pack into crystalline regions due to the increasing stress which was produced by cyclization occurred in the amorphous phase, and the crystallinity and crystallite size increased slowly. When the temperature reached to 220°C, pseudo‐crystalline regions rearranged obviously under stress, while molecular chains in the crystalline region started to participate in cyclization, and the original crystalline structures were destructed. The two competitive processes induced that the crystallinity and crystallite size grew to the maximum values at 30 min. When the temperature up to 240°C, the cyclization occurred in the crystalline region became more intensely, while the crystallinity and crystallite size decreased out of synchronize. A scheme of evolution of aggregation structure in cyclization was modified based on the above results. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

High modulus polypropylene fibers. II. Influence of fiber preparation upon structure and morphology

The influence of drawing on the limiting draw ratio upon formation of the morphological structure of fibers spun from binary polypropylene (PP) blends was studied. Fibers were spun from a fiber‐grade CR‐polymer and from the blends of a fiber‐grade CR‐polymer with a molding‐grade polymer in the composition range of 10–50 wt % added. As‐spun fibers were immediately moderately and additionally highly drawn at the temperature of 145°C. The structure and morphology of these fibers were investigated by small‐angle X‐ray scattering, wide‐angle X‐ray scattering, differential scanning calorimetry, scanning electron microscopy, density, birefringence, and sound velocity measurements. It was shown that continuously moderately drawn fibers are suitable precursors for the production of high tenacity PP fibers of very high modulus, because of so called oriented “smectic” structure present in these fibers. With drawing at elevated temperature, the initial metastable structure of low crystallinity was disrupted and a c‐axis orientation of monoclinic crystalline modification was developed. Hot drawing increased the size of crystallites and crystallinity degree, the orientation of crystalline domains, and average orientation of the macromolecular chains and resulted in extensive fibrillation and void formation. It was found that the blend composition has some influence on the structure of discontinuously highly drawn fibers. With increasing the content of the molding‐grade polymer in the blend, the size of crystalline and amorphous domains, density and crystallinity, as well as amorphous orientation decreased. Relationship has been established between the mechanical properties, crystallinity, and orientation of PP fibers. It was confirmed that by blending the fiber‐grade CR‐polymer by a small percentage of the molding‐grade polymer, maximization of elastic modulus is achieved, mainly because of higher orientation of amorphous domains. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1067–1082, 2006     

Effect of annealing on the structure and properties of polyvinylidene fluoride hollow fiber by melt‐spinning

Polyvinylidene fluoride hollow fibers were prepared by melt‐spinning technique under three spinning temperatures. The effects of annealing treatment on the structure and properties of hollow fiber were studied by differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction (WAXD), tensile test, and scanning electron microscopy (SEM) measurements. DSC and WAXD results indicated that the annealing not only produced secondary crystallization but also perfected primary crystallization, and spinning and annealing temperature influenced the crystallinity of hollow fiber: the crystallinity decreased with the increase of spinning temperature; 140°C annealing increased the crystallinity, and hardly influenced the orientation of hollow fiber; above 150°C annealing increased the crystallinity as well, and furthermore had a comparative effect on the orientation. The tensile tests showed that the annealed samples, which did not present the obvious yield point, exhibited characteristics of hard elasticity, and all the hollow fiber had no neck phenomenon. Compared with the annealed sample, the precursor presented a clear yield point. In addition, the annealed samples had a higher break strength and initial modulus by contrast with the precursor, and the 140°C annealed sample showed the smallest break elongation. SEM demonstrated the micro‐fiber structure appeared in surface of drawn sample. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 935–941, 2007     

Influence of alkali treatment on the structure of newcell fibers

Alkali treatment can change the structures and properties of cellulosic fibers. The aim of this work was to study the mechanism of structural changes of Newcell fibers treated with different alkali concentrations and two treatment methods. Raman spectra showed that the molecular conformation of Newcell fibers remained unchanged. X‐ray diffraction indicated that the crystal structure of Newcell cellulose II, treated with different alkali concentrations and different methods, did not change. With the increase of alkali concentration the crystallinty and crystallinity orientation index of Newcell fibers in their original length decreased slightly, whereas those of fibers in relaxed condition substantially decreased, and the crystallite size of 101¯ and 002 increased in both methods. The quasi‐crystallite disassociation and recrystallization in the quasi‐crystalline phase, during the process of alkali treatment, led to changes of crystallinity and orientation index of Newcell fibers. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1731–1735, 2004     

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     

Application of CO2 laser heating zone drawing and zone annealing to nylon 6 fibers

Nlon 6 fibers were zone drawn and zone annealed by using a continuous wave carbon dioxide laser to develop their mechanical properties. A laser‐heating zone drawing was carried out under a applied tension of 35.4 MPa at a power density of 9.65 W · cm^?2, and then the zone‐drawn fiber was annealed. A laser‐heating zone annealing was carried out in two steps at a power density of 9.65 W · cm^?2; the first step was carried out under 423 MPa and the second under 517 MPa. The treating temperature of the fiber heated by the CO₂ laser was measured by using an infrared thermographic camera equipped with a magnifying lens. The treating temperature at the zone drawing is 138°C, and those at the first and the second zone annealing are 121 and 125°C, respectively. The second laser‐heated zone‐annealed fiber has a birefringence of 65.2 × 10^?3, a degree of crystallinity of 54%, and a storage modulus of 21 GPa at 25°C. Wide‐angle X‐ray diffraction patterns for the laser‐heated zone‐drawn and the zone‐annealed fibers show (200) reflection and (002/202) doublet due to only an α form on the equator. The laser‐heated zone‐drawn fiber has a melting endotherm peaking at 216°C and a trace of shoulder on the higher temperature side of its peak, and the laser‐heated zone‐annealed fibers have a single melting endotherm peaking at 216°C. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1711–1716, 2002     

Effects of annealing on the hierarchical crystalline structures and mechanical properties of injection‐molded bars of high‐density polyethylene

The effect of annealing on the microstructural evolution and mechanical properties of high‐density polyethylene parts molded via gas‐assisted injection molding was investigated using scanning electron microscopy, differential scanning calorimetry, two‐dimensional wide‐angle X‐ray diffraction and tensile testing. The results indicated that a variety of annealing temperatures could induce considerable variations in the hierarchical structures, crystallinity, lamellar thickness and yield stress of the molded bars. According to these results, the annealing temperatures could be divided into three regions. In the low‐temperature region of annealing at 80 °C, the spatial variation of the superstructure developed along the thickness direction and mechanical properties of the annealed sample were mainly unchanged and similar to those of the original specimen. At 100 and 120 °C, the intermediate temperature region of annealing, the thickness of the crystals, degree of orientation and yield stress of annealed samples were greatly improved. Finally, at 127 °C, the degree of orientation decreased and yield stress slightly improved, an indication of the high‐temperature annealing region being characterized by increasing melting/recrystallization and causing relaxation of oriented molecular chains. A model is proposed to interpret the mechanism of the annealing treatment of the samples at various temperatures. © 2013 Society of Chemical Industry     

Effects of Annealing Temperature on the Structure and Capacitive Performance of Nanoscale Ti/IrO2–ZrO2 Electrodes

Electrodes consisting of coating of the Iridium oxide–Zirconium oxide (70%IrO₂–30%ZrO₂) binary oxide were formed on Ti substrates by thermal decomposition and annealing at 340°C–450°C. The effects of the annealing temperature on the structure, surface morphology, surface composition, and capacitive performance of the coatings were investigated using X‐ray diffraction analysis (XRD), transmission electron microscopy (TEM), scanning electron microscopy, X‐ray photoelectron spectroscopy, cyclic voltammetry, and electrochemical impedance spectroscopy (EIS). The XRD and TEM analyses showed that 360°C is greater than but very close to the crystallization temperature of the 70%IrO₂–30%ZrO₂ oxide coating. The 70%IrO₂–30%ZrO₂ oxide coatings annealed at this temperature consisted of an amorphous matrix containing a few IrO₂ nanocrystalline particles (diameter of 1–2 nm). The degree of crystallinity of the coatings was approximately 13.2%. EIS analysis showed that the electrode annealed at 360°C exhibited the highest specific capacitance, which was much higher than that of the electrode annealed at 340°C (which had a purely amorphous structure) as well as those of the electrodes annealed at 380°C and 400°C (which had higher degrees of crystallinity). On the basis of the obtained results, the following conclusion can be drawn: oxide coatings prepared at temperatures slightly higher than the crystallization temperature of the oxide and containing conductive nanocrystalline particles exhibit the best capacitive performance. We suggest that this phenomenon can be explained by the fact that the electronic conductivity of the coating is greatly improved by the presence of the homogeneously distributed conductive nanocrystalline particles in the amorphous matrix. Furthermore, the protonic conductivity and loose atomic configuration of the amorphous structure of the electrode are not adversely affected by the annealing treatment.     

Strain‐induced crystallization in uniaxially drawn PETG plates

The copolyester poly(ethylene glycol‐co‐cyclohexane‐1,4‐dimethanol terephthalate) (PETG) is used industrially as an uncrystallizable polymer, whereas PET is an inherently crystallizable polymer. Nevertheless, a crystalline phase could appear in the material. To create a strain‐induced crystalline phase in an initially amorphous PETG material, plates were placed in the heating chamber of a tensile machine at 100°C and uniaxially drawn to obtain different samples with various draw ratios. During DSC analysis of highly drawn samples, perturbations of the baseline appear above the glass‐transition temperature, consisting of weak exothermic and endothermic phenomena. Comparison of DSC and X‐ray diffraction analysis of drawn PETG and PET shows that a strain‐induced crystalline phase appears in this copolyester. A spherulitic superstructure could also appear after lengthy annealing. Analysis of this semicrystalline material allowed estimation of the degree of crystallinity, about 3% after a drawing at high draw ratio and about 11% for undrawn annealed material. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 3405–3412, 2001     

Microstructural Development of Uniaxial Stretched Anneal-Oriented Syndiotactic Polystyrene and its Blend with Poly (2,6-dimethyl-1,4-phenylene oxide): DSC,WAXD, SAXS and FTIR Studies

Studies have been done on strain-induced microstructure development in syndiotactic polystyrene (s-PS) and its blends with poly (2,6-dimethyl-1,4-phenylene oxide) (PPO) in 70/30 and 50/50 compositions of stretched annealed samples. Wide-angle X-ray showed that crystal orientation is less in annealed blend samples compared to annealed pure s-PS for a higher draw ratio. It increases with annealing, and relaxation occurs after a certain annealing temperature at above 180° for both s-PS and s-PS/PPO 70/30 blends. No crystal orientation was observed in the blend of s-PS/PPO 50/50 stretched samples. Small angle X-ray scattering (SAXS) shows the inclusion of amorphous PPO chains in between s-PS crystals lamella. Fourier transform infrared (FTIR) spectroscopy shows that the s-PS molecular chain packing band at 905 cm^?1 is enhanced due to annealing in oriented samples and saturates to around 0.63. The crystal chain relaxation is lower than amorphous chains of s-PS. The molecular chains of amorphous PPO are less oriented into the blend matrix, whereas its relaxation is enhanced during heat treatment and reaches an optimum value after full relaxation. The different behaviors of orientation and relaxation of s-PS and PPO chains into the blend matrix produce superstructures.     

Recrystallization Kinetics of 3C Silicon Carbide Implanted with 400 keV Cesium Ions

Polycrystalline 3C silicon carbide (SiC) was implanted at room temperature with 400 keV cesium ions to a dose of 10¹⁶ ions/cm². The samples were annealed at 600°C–1000°C for times up to 48 h to observe changes in the implantation zone crystallinity and density. The implanted regions were characterized by transmission electron microscopy (TEM) and secondary ion mass spectroscopy (SIMS) before and after annealing. It is shown that the implantation resulted in a 217 ± 2 nm amorphous region with microstructural damage extending to ~250 nm below the surface. Recrystallization of the amorphous region was observed to begin at 725°C. Densification was determined indirectly through changes in the measured implantation zone thickness. Measurable thickness, or densification, of the implanted region was not observed until temperatures greater than ~800°C. The SiC recrystallization began at the interface between the amorphous, damaged region, and the underlying polycrystalline material. Image analysis was used to quantify the fraction of crystalline phase as a function of time and temperature. The recrystallization kinetics exhibited Arrhenius dependency with an apparent activation energy of 480 kJ/mol. SIMS demonstrated that 60%–70% of the cesium was retained within the recrystallized microstructure after thermal annealing.     

Effect of annealing temperature on the morphology and antibacterial activity of Mg-doped zinc oxide nanorods

Herein, magnesium-doped zinc oxide nanorods (MgZnO NRs) were synthesized by the co-precipitation method and annealed at different temperatures in the range of 100–500?°C. The increase in the annealing temperature was found to influence both chemical and morphological structures of the MgZnO NRs: Ultraviolet–visible diffuse reflectance spectroscopy showed an increase in band gap with increase in the annealing temperature. Fourier-transform infrared spectra showed that two characteristic peaks at 487?cm^?1 and 442?cm^?1 corresponding to a weak Zn–O stretching initially decreased and then disappeared with increase in the annealing temperature. Moreover, the MgZnO NRs annealed at 100?°C had large crystallite size, high aspect ratio, and narrow edges. Remarkably, the MgZnO NRs annealed at 100?°C exhibited the highest antibacterial activity against both S. aureus and E. coli strains, attributed to the high aspect ratio and diffusion ability of the Zn²⁺ ions and large surface charge, crystallite size, and surface area. The MgZnO NRs annealed at the relatively low temperature of 100?°C could be easily produced commercially, in large quantities, and effectively used to prevent the growth of foodborne microbes in food packaging applications.     

Annealing effect on the microstructure of poly(vinyl chloride)

The effect of annealing on the microstructure of commercial grade poly(vinyl chloride) was investigated by calorimetric, X-ray and viscoelastic measurements. The degree of crystallinity increases with increasing annealing temperature from above the glass transition temperature up to 130°C, at which point the degree of crystallinity takes on a maximum value. Also, the crystal melting temperature increases with increasing annealing temperature. Thermal analysis and X-ray study suggest that the crystallite of poly (vinyl chloride) decomposes by thermal degradation when annealed, above 170°C. The isothermal crystallization process is analyzed using Avrami's equation employing the degree of crystallinity as a function of annealing time at various annealing temperatures. The crystallization rate has a maximum value at around 140°C. It is expected that the crystalline texture grows in the shape of a lineal-like habit, judging from the magnitude of Avrami's constant and from a study of the X-ray intensity distribution. The α_f-transition was observed to occur at temperatures 5 to 10°C lower than the crystalline melting temperatures for annealed specimens of poly(vinyl chloride) using a dynamic spring analysis. The α_f-transition may be attributed to thermal molecular motions with a long time scale, resulting from the cross-link points introduced by the small crystallites.     

Thermally induced crystallization and enzymatic degradation studies of poly (L‐lactic acid) films

Poly(L ‐lactic acid) (PLLA) films with different crystallinities were prepared by solvent casting and subsequently annealed at various temperatures (T_a) (80–110°C). The effects of crystallinity on enzymatic degradation of PLLA films were examined in the presence of proteinase K at 37°C by means of weight loss, DSC, FTIR spectroscopy, and optical microscopy. DSC and the absorbance ratio of 921 and 956 cm^?1 (A₉₂₁/A₉₅₆) were used to evaluate crystallinity changes during thermally induced crystallization and enzymatic hydrolysis. The highest percentage of weight loss was observed for the film with the lowest initial crystallinity and the lowest percentage of weight loss was observed for the film with highest crystallinity. FTIR investigation of degraded films showed a band at 922 cm^?1 and no band at 908 cm^?1 suggested that all degraded samples form α crystals. The rate of degradation was found to depend on the initial crystallinity of PLLA film and shown that enzymatic degradation kinetics followed first‐order kinetics for a given enzyme concentration. DSC crystallinity and IR absorbance ratio, A₉₂₁/A₉₅₆ ratio, showed no significant changes with degradation time for annealed PLLA films whereas as‐cast PLLA film showed an increase in crystallinity with degradation; this revealed that degradation takes place predominantly in the free amorphous region of annealed PLLA films without changing long range and short range order © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

The effect of a liquid isothermal bath in the threadline on the structure and properties of poly(ethylene terephthalate) fibers

The process of melt-spinning poly(ethylene terephthalate) (PET) filament at high speeds was modified through the inclusion of a liquid isothermal bath (LIB) in the spinline. A wide range of positions, temperatures, and depths associated with the operation of the LIB were utilized in this study. The structural characteristics and mechanical properties of the as-spun fibers were characterized by birefringence, wide-angle X-ray diffraction (WAXD), infrared spectroscopy, and tensile testing. Experimental results showed that the structure and mechanical properties of the as-spun fibers were significantly influenced by the LIB operating conditions. The as-spun fibers prepared under optimum LIB conditions exhibit high birefringence and excellent mechanical properties. Results suggest the development of a critical value of threadline stress that is determined primarily by LIB depth and take-up velocity. Below this critical value, raising of LIB temperature, LIB depth, and take-up velocity resulted in increases of the apparent crystallite size, sample crystallinity, and both the crystalline and amorphous orientation. As would be expected, the mechanical properties of the fiber samples were improved in a corresponding manner. Above this critical stress value, molecular chains in the amorphous phase are stretched tautly, but the crystal growth process is restricted, resulting in a decrease in crystallite size and crystallinity, as well as a continued increase in mechanical properties. The fiber properties were also found to be very responsive to the relative location of the LIB. A unique structure, believed never before obtained in a one-step high-speed PET melt-spinning process, has been achieved that combines high amorphous orientation, low crystallinity, and high tenacity. © 1995 John Wiley & Sons, Inc.     

An optical investigation of high‐tenacity polyester (H‐T PET) fibers annealed at different temperatures

A two‐beam “Interphako” interference microscope was used to study the effect of annealing on the physical properties of high‐tenacity poly(ethylene terephthalate) H‐T PET fibers. The PET fibers were annealed with free ends for 1 h at temperatures ranging from 100 to 200°C. The shrinkage, refractive indices, and orientation angle of the PET fibers were determined for different annealing temperatures. The measured birefringence and orientation function were found to have decreased with increasing temperature, whereas the degree of crystallinity and the onset temperature (DSC) increased. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

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