Reversible and irreversible heat capacity of poly[carbonyl(ethylene-co-propylene)] by temperature-modulated calorimetry (original) (raw)
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Journal of Polymer Science Part B: Polymer Physics, 2006
Quantitative temperature-modulated differential scanning calorimetry (TMDSC) and superfast thin-film chip calorimetry (SFCC) are applied to poly(butylene terephthalate)s (PBT) of different thermal histories. The data are compared with those of earlier measured heat capacities of semicrystalline PBT by adiabatic calorimetry and standard DSC. The solid and liquid heat capacities, which were linked to the vibrational and conformational molecular motion, serve as references for the quantitative analyses. Using TMDSC, the thermodynamic and kinetic responses are separated between glass and melting temperature. The changes in crystallinity are evaluated, along with the mobile-amorphous and rigid-amorphous fractions with glass transitions centered at 314 and 375 K. The SFCC showed a surprising bimodal change in crystallization rates with temperature, which stretches down to 300 K. The earlier reported thermal activity at about 248 K was followed by SFCC and TMDSC and could be shown to be an irreversible endotherm and is not caused by a glass transition and rigid-amorphous fraction, as assumed earlier.
Journal of Applied Polymer Science, 1986
Thermal analysis has been carried out on polyester (PET) fibers after subjecting them to different physical modifications, such as drawing and heat setting. The relationship between structure and the various thermal transitions observed in the thermograms of poly(ethy1ene terephthalate) has been examined. It has been shown that the endothermic transition near the glass transition region and the exothermic transition a t about 140°C, observed for amorphous PET fibers, may be associated with mesomorphic phase changes. The premelting endotherm is sensitive to the orientation, crystallite size distribution, and thermal prehistory. This transition actually represents melting of smaller crystals and recrystallization into larger crystals. Heat of fusion does not always necessarily represent the actual crystallinity, or order of the fiber prior to differential scanning calorimetry and may be influenced by several factors. The fusion curves give more information regarding crystallite size distribution than crystallinity.
Non-isothermal melt-crystallization kinetics of poly(trimethylene terephthalate)
2004
Non-isothermal melt-crystallization kinetics and subsequent melting behavior of poly(trimethylene terephthalate) (PTT) have been investigated by differential scanning calorimetry (DSC). The Avrami, Tobin and Ozawa equations were applied to describe the kinetics of the crystallization process. Both of the Avrami and Tobin crystallization rate parameters (i.e. K A and K T , respectively) were found to increase with increasing cooling rate. The Ozawa crystallization rate K O was found to decrease with increasing temperature. The ability of PTT to crystallize from the melt under a unit cooling rate was determined by the Ziabicki's kinetic crystallizability index G Z , which was found to be ca. 0.98. The effective energy barrier describing the non-isothermal melt-crystallization process DE of PTT was estimated by the differential iso-conversional method of Friedman and was found to increase with an increase in the relative crystallinity. In its subsequent melting, PTT exhibited triple endothermic melting behavior when it was cooled at cooling rates lower than ca. 20 v C min À1 , while it exhibited double endothermic melting behavior when it was cooled at cooling rates greater than ca. 20 v C min À1 .
Heat capacity of poly(trimethylene terephthalate)
Journal of Polymer Science Part B: Polymer Physics, 1998
Thermal analysis of poly(trimethy1ene terephthalate) (PTT) has been carried out using standard differential scanning calorimetry and temperature-modulated differential scanning calorimetry. Heat capacities of the solid and liquid states of semicrystalline PTT are reported from 190 K to 570 K. The semicrystalline PTT has a glass transition temperature of about 331 K. Between 460 K and 480 K, PTT shows an exothermic ordering. The melting endotherm occurs between 480 K and 505 K with an onset temperature of 489.15 K (216°C). The heat of fbsion of typical semicrystalline samples is 13.8 kJ/mol. For 100% crystalline PTT the heat of fusion is estimated to be 28-30 kJ/mol. The heat capacity of solid PTT is linked to an approximate group vibrational spectrum, and the Tarasov equation is used to estimate the skeletal vibrational heat capacity (0, = 542 K and 0, = 42 K). A comparison of calculation and experimental heat capacities show agreement of better than *2% between 190-300 K. The experimental heat capacity of liquid PTT can be expressed as a linear fhction of temperature: C, "(exp) = 21 1.6 + 0.434 T J/(K mol) and compares well with estimations from the ATHAS data bank using group contributions of other polymers with the same constituent groups (*0.5%). The change of heat capacity at Tg of amorphous PTT has been estimated from the heat capacities of liquid and solid to be 86.4 J/(K mol). Knowing C, of the solid, liquid, and the transition parameters, the thermodynamic functions: enthalpy, entropy and Gibbs function were obtained. With these data, one can compute the crystallinity changes with temperature and the mobile and rigid amorphous fractions.
Thermal memory of polyethylenes analyzed by temperature modulated differential scanning calorimetry
Journal of Applied Polymer Science, 2003
Temperature modulated differential scanning calorimetry (TMDSC) was employed to study the melting and crystallization behavior of various polyethylenes (PEs). Samples of high density PE (HDPE), low density PE (LDPE), linear low density PE (LLDPE), and very low density PE (VLDPE) with different crystal structures and morphologies were prepared by various thermal treatments (isothermal crystallization and slow, fast, and dynamic cooling). The reversing and nonreversing contributions, measured on the experimental time scale, were varied, depending on the crystal stability. A relatively large reversing melt contribution occurs for unstable crystals formed by fast cooling compared to those from slow cooling treatments. All samples of highly branched LDPE, LLDPE, and VLDPE showed a broad exotherm before the main melting peak in the nonreversing curve, suggesting crystallization and annealing of crystals to more stable forms. Other samples of HDPE, ex-cept when cooled quickly, did not show any significant crystallization and annealing before melting. The crystallinity indicated that dynamically cooled polymers were much more crystalline, which can be attributed to crystal perfection at the lamellar surface. A reversible melting component was also detected during the quasiisothermal TMDSC measurements. Melting is often accompanied by large irreversible effects, such as crystallization and annealing, where the crystals are not at equilibrium. Such phenomena during a TMDSC scan provide information on the polymer thermal history.
Multiple melting behavior in isothermally crystallized poly(trimethylene terephthalate)
European Polymer Journal, 2004
Melting behavior of poly(trimethylene terephthalate) (PTT) after isothermal crystallization from the melt state was studied using differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) techniques. The subsequent melting thermograms for PTT isothermally crystallized within the temperature range of 182-215°C exhibited triple (for crystallization temperatures lower than %192°C), double (for crystallization temperatures greater than %192°C but lower than %210°C), or single (for crystallization temperatures greater than %210°C) endothermic melting phenomenon. These peaks were denoted peaks I, II, and III for low-, middle-, and high-temperature melting endotherms, respectively. For the triple melting phenomenon, it was postulated that the occurrence of peak I was a result of the melting of the primary crystallites, peak II was a result of the melting of recrystallized crystallites, and peak III was a result of the melting of the recrystallized crystallites of different stabilities. In addition, determination of the equilibrium melting temperature T 0 m for this PTT resin according to the linear and non-linear Hoffmann-Weeks extrapolation provided values of 243.6 and 277.6°C, respectively.
Melting and reorganization of poly(ethylene terephthalate) on fast heating (1000 K/s
Polymer, 2004
For poly(ethylene terephthalate) (PET) and other polymers the origin of the multiple melting peaks observed in differential scanning calorimetry (DSC) curves is still controversially discussed. This is due to the difficulty to investigate the melting of the originally formed crystals exclusively. Recrystallization is a fast process and most experimental techniques applied so far do not allow fast heating in order to prevent recrystallization totally. Developments in thin-film (chip) calorimetry allow scanning rates as high as several thousand Kelvin per second. We utilized a chip calorimeter based on a commercially available vacuum gauge, which is operated under non-adiabatic conditions. The calorimeter was used to study the melting of isothermally crystallized PET. Our results on melting at rates as high as 2700 K/s give clear evidence for the validity of a melting -recrystallization -remelting process for PET at low scanning rates (DSC). At isothermal conditions PET forms crystals, which all melt within a few dozens of K slightly above the isothermal crystallization temperature. There is no evidence for the formation of different populations of crystals with significantly different stability (melting temperatures) under isothermal conditions. Superheating of the crystals is of the order of 10 K at 2700 K/s. q
Journal of Polymer Research
Poly(ethylene terephthalate) (PET) materials with different molecular weights were isothermally crystallized from melt by systematically varying the temperature and duration of the treatment performed in the differential scanning calorimeter (DSC). Multiple endotherm peaks were observed on the subsequent heating thermograms that were separated from each other on the basis of their melting temperature versus crystallization temperature and melting temperature versus crystallinity function. By this new approach five sub-peak sets were identified and then comprehensively characterised. Wide-Angle X-Ray Diffraction (WAXD) analyses revealed that the identified sub-peak sets do not differ in crystalline forms. By analysing the crystallinity and the melting temperature of the sub-peak sets as a function of crystallization time, crystallization temperature and intrinsic viscosity, it was concluded that below the crystallization temperature of 460 K the sub-peak sets that were formed during ...
Polymer, 2004
Amorphous poly(ethylene terephthalate) fibers in which the skin was removed were studied to expressly study the effect of amorphous molecular orientation on crystallization behavior. Thermal analysis was carried out on fibers with a wide range of molecular orientation using differential scanning calorimetry (DSC) under constrained and unconstrained conditions. The thermal behavior was correlated with structural characteristics such as amorphous orientation determined using wide-angle X-ray diffraction. We show for the first time a quantitative inverse linear relationship between the degree of amorphous orientation and the cold crystallization temperatures and heat of crystallization. Crystallization begins at a critical amorphous orientation of 0.18, and extrapolation shows that even at modest amorphous orientation of 0.27, the cold crystallization can start spontaneously at T g and with no change in free energy.
Journal of Polymer Science Part B: Polymer Physics, 2000
After isothermal crystallization, poly(ethylene terephthalate) (PET) showed double endothermic behavior in the differential scanning calorimetry (DSC) heating scan. During the heating scans of semicrystalline PET, a metastable melt which comes from melting thinner lamellar crystal populations formed between the low and the upper endothermic temperatures. The metastable melt can recrystallize immediately just above the low melting temperature and form thicker lamellae than the original ones. The thickness and perfection depends on the crystallization time and crystallization temperature. The crystallization kinetics of this metastable melt can be determined by means of DSC. The kinetics analysis showed that the isothermal crystallization of the metastable PET melt proceeds with an Avrami exponent of n ϭ 1.0 ϳ 1.2, probably reflecting one-dimensional or irregular line growth of the crystal occurring between the existing main lamellae with heterogeneous nucleation. This is in agreement with the hypothesis that the melting peaks are associated with two distinct crystal populations with different thicknesses.