Melting and crystallization behavior of poly(tetrafluoroethylene) by temperature modulated calorimetry

Temperature-modulated differential scanning calorimetry (TMDSC) in the quasi-isothermal mode is applied to investigate melting and crystallization of poly(tetrafluoroethylene) (PTFE) obtained from aqueous dispersion, both melt-crystallized and native (as-polymerized). The differences, shown in the past between the melting behavior of melt-crystallized and native PTFE, have been confirmed and further evidenced through this technique. A large reversing heat capacity is present in the melting and crystallization regions. As proposed by many authors, the presence of the reversing heat capacity can be related to surface melting and crystallization. It can occur on the growth and/or fold surfaces and it has been shown that it is larger for those macromolecules having higher chain mobility that allows rearrangements on the crystal surface. In the present case, the large observed reversing heat capacity can be related to the very high sliding ability of PTFE chains in the pseudohexagonal phase, which is much larger than that of most semicrystalline polymers. Due to the crystal-crystal transition at 30 °C, which can be described as a fusion in the longitudinal direction, melting of PTFE can be considered intermediate between the irreversible melting of macromolecules and the completely reversible isotropization of liquid crystalline polymers.     

Melting and crystallization behavior of poly(trimethylene 2,6-naphthalate)

The melting and crystallization behavior of poly(trimethylene 2,6-naphthalate) (PTN) are investigated by using the conventional DSC, the temperature-modulated DSC (TMDSC), wide angle X-ray diffraction (WAXD) and polarized light microscopy. It is observed that PTN has two polymorphs (α- and β-form) depending upon the crystallization temperature. The α-form crystals develop at the crystallization temperature below 140 °C while β-form crystals develop above 160 °C. Both α- and β-form crystals coexist in the samples crystallized isothermally at the temperature between 140 and 160 °C. When complex multiple melting peaks of PTN are analyzed using the conventional DSC, TMDSC and WAXD, it is found that those arise from the combined mechanism of the existence of different crystal structures, the dual lamellar population, and melting-recrystallization-remelting. The equilibrium melting temperatures of PTN α- and β-form crystals determined by the Hoffman-Weeks method are 197 and 223 °C, respectively. When the spherulitic growth kinetics is analyzed using the Lauritzen-Hoffmann theory of secondary crystallization, the transition temperature of melt crystallization between regime II and III for the β-form crystals is observed at 178 °C. Another transition is observed at 154 °C, where the crystal transformation from α- to β-form occurs.     

Isothermal crystallization of poly(ethylene-terephthalate) of low molecular weight by differential scanning calorimetry: 1. Crystallization kinetics

The isothermal crystallization of poly(ethylene-terephthalate) (PETP) fractions, from the melt, was investigated using differential scanning calorimetry (d.s.c.). The molecular weight range of the fractions was from 5300–11750. Crystallization temperatures were from 498–513 K. The dependence of molecular weight and undercooling on several crystallization parameters has been observed. Either maxima or minima appear at a molecular weight of about 9000, depending on the crystallization temperature. The activation energy values point to the possibility of different mechanisms of crystallization according to the chain length. A folded chain process for the higher $\text{M}\text{?}{}_{\mathrm{<mtext>n</mtext>}}$ chains and an extended chain mechanism for the lower $\text{M}\text{?}{}_{\mathrm{<mtext>n</mtext>}}$ chains. The values of the Avrami equation exponent n vary from 2–4 depending on the crystallization temperature; non-integer values are indicative of heterogeneous nucleation. The rate constant K depends on T_c and $\text{M}\text{?}{}_{\mathrm{<mtext>n</mtext>}}$, showing maxima related to the T_c used. The plot of log K either vs. (ΔT)^?1 and (ΔT)^?2 or $\text{T}{}_{\mathrm{<mtext>m</mtext>}}\text{T(ΔT)}$ and $\text{T}{}^2{}_{\mathrm{<mtext>m</mtext>}}\text{T(ΔT)}{}^2$ is linear in every case.     

Melting behavior and crystallization kinetics of sulfonated poly(butylene isophthalate)

The melting behavior and the crystallization kinetics of sulfonated poly(butylene isophthalate) random copolymers were investigated by means of differential scanning calorimetry. The multiple endotherms, commonly observed in polyesters, were found to be influenced both by composition and crystallization temperature. By applying the Hoffman‐Weeks method, the equilibrium melting temperatures of the copolymers under investigation were obtained. The presence of a crystal‐amorphous interphase was evidenced and its amount was found to increase as the sulfonated unit content was increased. Isothermal melt crystallization kinetics of the sample containing the lowest amount of sulfonated units was analyzed according to the Avrami treatment. The introduction of such units was found to decrease the overall crystallization rate of poly(butylene isophthalate). Values of Avrami's exponent n close to 3 were obtained, independently of crystallization temperature, in agreement with a crystallization process originating from predetermined nuclei and characterized by three‐dimensional spherulitic growth.     

Melting and crystallization behavior of poly(ethylene terephthalate) and poly(m‐xylylene adipamide) blends

The crystallization and melting behaviors as well as the crystalline morphologies of Poly(ethylene terephthalate)/Poly(m‐xylylene adipamide) (PET/MXD6) blends have been examined and characterized with the aid of differential scanning calorimetry (DSC) and wide angle x‐ray diffraction (WAXD). The isothermal and nonisothermal crystallization behaviors of the blends were studied as functions of the contents of MXD6, catalyst concentrations, and the effects of the interchange reactions between PET and MXD6. Wide angle x‐ray scattering has been used to examine the crystalline morphologies of the PET/MXD6 blends, to characterize their crystalline and amorphous phases, and to determine crystallite sizes in the blends. Results indicate that the catalyst has both catalyzing and nucleation effects on the PET/MXD6 blends, with the extents of each effect dependent upon the content of catalyst. In addition the crystalline morphology was found to be dominated by the MXD6 content as well as the crystallization temperature. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Effect of molecular weight on crystallization isotherms of high molecular weight poly(ethylene oxide) fractions

Dilatometric crystallization isotherms have been determined for a set of poly(ethylene oxide) fractions ranging in molecular weight from 2 × 10⁴ to 1.6 × 10⁶. For a given fraction the isotherms obtained for different crystallization temperatures can be superimposed over most of the crystallization. For a given crystallization temperature the degree of crystallinity obtained in the primary stage of the crystallization varies greatly with molecular weight, and superimposition of the isotherms is not possible. Secondary crystallization processes are pronounced when the molecular weight ($\text{M}\text{?}{}_{\mathrm{v}}$) exceeds 10⁵.     

Effect of molecular weight on crystallization and melting of poly(trimethylene terephthalate). 1: Isothermal and dynamic crystallization

The crystallization behavior of poly(trimethylene terephthalate) as a function of molecular weight was investigated under isothermal and dynamic cooling conditions using a differential scanning calorimeter (DSC) and polarized light optical microscopy (POM). THe overall rate of bulk crystallization increased with molecular weight. An Avrami analysis of the isothermal crystallization kinetics indicated that the crystallization rate constant increased with increasing molecular weight. The Avrami exponent, n, approached 2 and was nearly independent of both molecular weight and temperature. The modified Avrami analysis developed by Jeziorny and Ozawa was applied to the dynamic crystallization data. At the same cooling rate, higher molecular weight resulted in a narrower crystallization peak, higher onset crystallization temperature, and larger rate constant (Z_t)^1/n. Higher molecular weight resulted in larger cooling function of dynamic crystallization K(T) and lower Ozawa exponent m. For dynamic crystallization, the average value of the Avrami exponent varied from 3.4 to 3.8 and the average value of the Ozawa exponent changed from 2.3 to 2.6 as the number‐average molecular weight changed from 13,000 to 67,000. Morphology studies indicated that both the isothermal crystallization and the dynamic crystallization of PTT from the melt were thermal nucleation processes, and for a fixed temperature between 190°C and 210°C, the nucleation density increased with increasing the molecular weight.     

The rate of crystallization of poly(ethylene terephthalate) by differential scanning calorimetry

The value of t_max in differential scanning calorimetry is correlated with the crystallization kinetics of poly(ethylene terephthalate) (PET). The Avrami exponent, n, obtained varies as a result of a change in slope of the curve at the point t_n, a secondary crystallization transition. The plot of t_n vs. t_max shows a linear relationship. The rate of crystallization depends upon both molecular weight and crystallization temperature. Under a nucleation controlling step, the plot of log t_max vs. \documentclass{article}\pagestyle{empty}\begin{document}$ t_{\max } vs.\frac{1}{{T^2 \Delta T}} $\end{document}gives a linear relationship. Theoretical concepts of the treatment are discussed.     

Melting behavior of ultra-high modulus and molecular weight polyethylene (UHMWPE) fibers

Superheating and double-melting endotherms are the characteristic melting behaviors of gel-spun ultra-high modulus and molecular weight polyethylene (UHMWPE) fibers in differential thermal analysis (DTA). A mostly orthorhombic structure with very little amorphous content is indicated by wide angle x-ray scattering (WAXS) data. The melting temperatures are elevated and believed to result from superheating and incomplete chain relaxation in the highly oriented and crystalline structure. The melting behavior of the fibers is shown to be strongly affected by changes in polymorphic transformations and in intercrystalline disordered domains. Compression in the direction perpendicular to fiber axis causes significant increases in the 110 and 200 dimensions of the orthorhombic structure. Such lateral compression increases monoclinic forms and perhaps amorphous content, and decreases the degree of transformation to hexagonal phase. Superheating, which is related to the intercrystalline stress, can be reduced by the presence of interacting solvents such as trichlorbenzene. © 1994 John Wiley & Sons, Inc.     

Melting of low molecular weight poly(ethylene oxide) with acetoxy- and trimethylsiloxy-end-groups

Melting points and lamellar spacings are reported for samples of low molecular weight poly(ethylene oxide) with acetoxy- and trimethylsiloxy- end-groups. The results, together with others reported earlier for hydroxy-, phenoxy- and chloro-ended polymers, show that the melting point can be much affected by the end-group. The main causes of the effects are ascribed to differences in end-end and end-chain interactions and to end-group dimensions.     

Influence of molecular weight on the crystallization of poly(vinylidene fluoride)

Five samples of Poly (vinylidene fluoride) with a molecular weight from 45,000 to 260,000 were analyzed by means of FT-IR spectroscopy in order to study the influence of molecular weight on the crystallization process. The spectra of films obtained from methylethylketone solutions revealed the prevalence of form II () in the case of lower and of form III () in the higher molecular weight samples. The amount of form III was found to increase as the Mv decreased and as annealing temperatures increased. The deconvolution technique of vibrational spectra allowed detection of small amounts of other forms accompanying the prevailing one.     

Number-average molecular weight of radiation-degraded poly(tetrafluoroethylene). An end group analysis based on solid-state NMR and IR spectroscopy

Changes in the chemical structure of poly(tetrafluoroethylene) (PTFE) induced by electron beam irradiation at room temperature in air have been studied by ¹⁹F solid-state NMR and IR spectroscopy. Chain scission associated with the formation of trifluoromethyl (-CF₃), carboxylic acid fluoride (-COF) and carboxylic acid (-COOH) end groups was confirmed to be the predominant process under these conditions. The number-average molecular weight of radiation-degraded PTFE as a function of irradiation dose was calculated based on quantitative end group analysis. It strongly decreases with increasing dose to approximately 4000 g/mol at a irradiation dose of 4 MGy. The molar ratio of CF₃ end groups to oxygen-containing (-COF, -COOH) end groups was found to depend on the irradiation dose and the irradiation conditions. For low dose level, the molar ratio determined experimentally coincides well with the mechanism proposed for radiation-induced degradation of PTFE.     

Study of melting and crystallization behavior of polyacrylonitrile using ultrafast differential scanning calorimetry

The thermal behavior of polyacrylonitrile (PAN) has been investigated using X-ray diffraction, differential scanning calorimetry (DSC), and ultrafast DSC. In conventional DSC, it is difficult to prevent the concurrent occurrence of the exothermic reactions of PAN with melting. However, in the ultrafast DSC curve, the exothermic peak due to these reactions disappears over the temperature range 0–400 °C at heating rates above 250 °C s⁻¹. Alternatively, the glass transition and the melting of PAN are observed over the temperature range 109–129 °C and 335–362 °C, respectively. Moreover, upon cooling from the molten state at a rate of −7500 °C s⁻¹, PAN crystallization is observed at 204 °C. These findings were observed repeatedly during heating and cooling measurements. From the extrapolation analysis, the zero-entropy-production melting temperature of PAN is found to lie in the temperature range 320–350 °C. Finally, the equilibrium melting temperature of PAN is estimated to be ca. 465 °C.     

Simultaneous measurement of light transmission and heat flow in a modified differential scanning calorimeter

A commercial DSC cell has been modified to allow simultaneous measurement of heat flow and light transmission. A special fiber optic probe capable of transmitting and receiving light is located immediately above the sample. The sample is held in a reflective aluminum pan during the measurement. Maximum optical signal intensity is recorded when the sample becomes completely transparent. The light from the probe will pass through the sample, reflect off the bottom of the pan, and then pass up through the sample again and back into the probe. The method was developed to augment the normal DSC heat-flow measurement. We find that the additional optical information is useful in elucidating the nature of thermal transitions in polymeric materials. Any transition, physical or chemical, that is accompanied by an appreciable change in optical transmission can be studied by this technique. The probe is useful even in investigations where only the optical signal is desired, because one can make use of the temperature programming and data aquisition capabilities of the DSC instrument. The presence of the probe tip near the sample does not seriously degrade the heat-flow signal. Good calorimetric accuracy is maintained, albeit some baseline curvature is introduced. Several applications of the technique are described. These include melting and crystallization of polymers, solubility behavior of polymer–polymer and polymer–additive blends, and sintering of polymer particles during oxidative degradation.     

乳液聚合法合成高相对分子质量聚醋酸乙烯酯的研究

本文采用过硫酸铵/亚硫酸氢钠氧化还原体系低温引发醋酸乙烯酯乳液聚合,分别讨论了聚合温度、单体用量、十二烷基硫酸钠(SDS)用量、过硫酸铵用量、亚硫酸氢钠用量等因素对聚醋酸乙烯酯粘均相对分子质量的影响。通过调节单体浓度、SDS浓度、过硫酸铵浓度、亚硫酸氢钠浓度和聚合温度,合成出高相对分子质量的聚醋酸乙烯酯,用GPC法对其进行表征,粘均相对分子质量为1.07×106,相对分子质量分布为2.75。     

Melting behavior of poly(trimethylene terephthalate)

In this study, the melting behavior of isothermally crystallized polytri‐ methylene terephthalate (PTT) was investigated. Multiple melting behaviors in DSC heating trace were found because two populations of lamellar stacks were formed during primary crystallization and the recrystallization at heating process, respectively. This fact could be also confirmed from the result of optical microscopy observation. The Hoffman–Weeks equation was applied to obtain equilibrium melting temperature (T). The T value of PTT is about 525 K, which is 10 K higher than that reported. Combining the enthalpy of fusion from the DSC result and the degree of crystallinity from WAXD result, the value of the equilibrium‐melting enthalpy ΔH was deduced to be approximately 28.8 kJ mol^?1. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2426–2433, 2002     

Effect of molecular weight on the crystallization kinetics of poly(hexamethylene oxide)

Dilatometric crystallization isotherms have been analysed for poly(hexamethylene oxide) fractions ranging in molecular weight from 2200 to 33 500. Previously, the influence of the temperature and the time of melting in the reproducibility of the isotherms were studied. Deviations from the Avrami or Göler-Sachs free growth formulations are systematic with molecular weight and become more pronounced as the molecular weight increases. The Avrami exponent is an integral number, 4, and is independent of temperature and molecular weight. The crystallization rate goes through a maximum as a function of molecular weight and the location of this maximum depends on the undercooling. The crystallization temperature coefficient was studied using the three dimensional nucleation theory and it was found that the crystallization is described by a unique function of the free energy for nucleation when the change of the interfacial free energy with molecular weight is considered.     

Heterogeneous structure of poly(glycolic acid) fiber studied with differential scanning calorimeter, X-ray diffraction, solid-state NMR and molecular dynamic simulation

The heterogeneous structures of poly(glycolic acid) (PGA) fibers which have been used as bio-degradable suture were studied by differential scanning calorimeter (DSC),X-ray diffraction and ¹³C solid state NMR. The ¹³C cross polarization NMR spectra without magic angle spinning of the stretched fibers observed by changing the angle between the fiber axis and the magnetic field clearly showed the heterogeneous structures which consist of three components; well-oriented, poorly-oriented and isotropic amorphous components. The local structure, distribution of the fiber axis and fraction of each component were determined quantitatively. Change in the heterogeneous structure by changing the stretching method in the sample preparation and by changing the stretching ratio was also monitored. The X-ray diffraction data of the fibers are in good agreement with the ¹³C CP NMR data. Change in the heterogeneous structures correlate with change in the thermal properties observed by DSC method. The molecular dynamic simulation showed the generation of trans conformation of PGA chain and also change in the fraction of other conformations by stretching, which supports the experimental results obtained above and gives additional structural information.     

Evaluation of number-average molecular weight of poly(tetrafluoroethylene) irradiated with γ rays

The number-average molecular weight of poly(tetrafluoroethylene) (PTFE) irradiated from a ⁶⁰Co source in air at room temperature has been estimated from the experimental results of tensile creep measurements and electron microscopy. The viscoelastic method which makes use of the maximum relaxation time was used for samples with low radiation doses. The mean chain length of highly irradiated samples, which can form fully extended chain crystals upon crystallization from the melt, was obtained from the bandwidth distribution on micrographs of the fracture surfaces. The dependence of the average molecular weight on the radiation dose is derived on the basis of a statistical treatment in which effects of cages in the process of chain scission and of molecules evolved out of the specimen, together with random decomposition of polymer backbone chains, are considered. Good agreement between the experimental and calculated results is attained over a wide range of radiation doses.