Frequency Dependence of the Dynamic Heat Capacity at the Melting Temperature of Polyethylene Crystals

The dynamic heat capacity of polyethylene was measured in the heating process over two decades of the modulating frequency using the light heating modulated temperature DSC. The dynamic heat capacity exhibited clear frequency dependence from 95°C to the end of the melting of the crystals. Frequency dependence of this work was compared with that of the quasi-isothermal measurement. The relaxation time estimated in this work was much shorter than that of the quasi-isothermal measurement. It was found that notable heat exchange between the sample and reference sides occurred between 120 and 135°C. Frequency dependence of the heat exchange was studied.This revised version was published online in November 2005 with corrections to the Cover Date.     

Quasi-isothermal measurement by TMDSC in isotactic polypropylene

We measure the frequency dependences of complex heat flows for isothermally crystallized isotactic polypropylene (iPP) by the quasi-isothermal TMDSC. Regarding the quasi-isothermal melting processes as a kind of the single relaxation process, we analyze them by the Debye model. The resultant heat capacity of iPP is larger (about 11%) than usual thermodynamic heat capacity. We also found that the excess of the heat capacity, C _p (excess), has non-monotonous temperature dependence. A simple model introducing some kinetic modes into amorphous producing after and during temperature modulation can reproduce the temperature dependence of C _p (excess) very well.     

On the kinetics of melting and crystallization of poly(l-lactic acid) by TMDSC

The crystallization and melting process of poly(l-lactic acid), PLLA, is investigated by temperature modulated differential scanning calorimetry, TMDSC. The sample is cooled from the melt to different temperatures and the crystallization process is followed by subjecting the material to a modulated quasi-isothermal stage. From the average component of the heat flow and the application of the Lauritzen–Hoffman theory two crystallization regimes are identified with a transition temperature around 118 °C. Besides, the oscillating heat flow allows calculating the crystal growth rate via the model proposed by Toda et al., what gives, in addition, an independent determination of the transition temperature from modulated experiments. Further, the kinetics of melting is studied by modulated heating scans at different frequencies. A strong frequency dependence is found both in the real and imaginary part of the complex heat capacity in the transition region. The kinetic response of the material to the temperature modulation is analyzed with the model proposed by Toda et al. Finally, step-wise quasi-isothermal TMDSC was used to investigate the reversible surface crystallization and melting both on cooling and heating and a small excess heat capacity is observed.     

Crystallization and melting of a branched polyethylene with precisely controlled chemical structure

The heat capacity of a linear polyethylene with dimethyl branches, at every 21st backbone atom was analyzed by differential scanning calorimetry (DSC) and quasi-isothermal temperature-modulated DSC. This novel copolyethylene (PE2M) is relatively difficult to crystallize from the melt. On subsequent heating, a first, sharp melting peak is followed by a sharp cold-crystallization and crystal perfection and a smaller endotherm, before reaching the main melting at 315–320 K, close to the melting temperatures of eicosane and tetracontane. The low-temperature melting is sensitive to the cooling rate and disappears below 1.0 K min⁻¹. The cold crystallization can be avoided by heating with rates faster than 80 K min⁻¹. The PE2M exhibits some reversing and reversible melting, which is typical for chain-folded polymers. The glass transition of semicrystalline PE2M is broadened and reaches its upper limit at about 260 K (midpoint at about 0.355 K). Above this temperature, the crystals seem to have a heat capacity similar to that of the liquid. A hypothesis is that the melting transition can be explained by changes in crystal perfection without major alteration of the crystal structure and the lamellar morphology. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3461–3474, 2006     

步进扫描差示扫描量热法研究不同链结构的聚乙烯类聚烯烃热力学特性

采用步进扫描差示扫描量热法研究了几种具有不同链结构的聚乙烯类聚烯烃的热力学性质.结果表明,不同链结构的聚烯烃在热力学平衡熔融态下,其绝对比热容的温度依赖性在实验误差范围内几乎相同;而无论链结构的变化如何,在极低温度下稳定结晶态的热容也非常接近.但在熔融温度范围内,比热容对不同聚烯烃的链结构非常敏感,这是由于在远低于聚烯烃主熔化峰温度范围内,聚乙烯晶体结构中的不同支链的影响.在时间-热流的步进扫描曲线中,具有长支链结构的聚烯烃表现出独特的熔融-再结晶行为,这可能是由于交联点之间较长的受约束链段的运动在熔融过程中得到释放并重排所致.     

The Relationship Between the TMDSC Curve of Frozen Sucrose Solutions and Collapse During Freeze-Drying

This study compares measurements of the collapse temperature of sucrose solutions by freeze drying microscopy with features of TMDSC curves both in the scanning and quasi-isothermal modes. The objective was to determine which feature of the TMDSC curve is predictive of collapse and to provide additional evidence for recent interpretations of the physical significance of the low temperature transitions for sucrose solutions. Interpretations based on the heat capacity signal and the kinetic heat flow using TMDSC are consistent with previous reports based on total heat flow measurement, where the lower temperature transition is the glass transition and the higher temperature transition is associated with the onset of ice melting. Quasi-isothermal experiments further support these conclusions, since additional crystallization of ice is observed only in the region of the lower temperature transition. Collapse of sucrose solutions during freeze-drying begins at the approximate midpoint between the end of the glass transition region and the onset of ice melting. This revised version was published online in July 2006 with corrections to the Cover Date.     

Water effect on the physicochemical properties of oligomeric polysaccharide inulin

For oligomeric polysaccharide inulin from chicory roots, the heat capacity in the range of 80–330 K is measured, and values of standard enthalpy of combustion and formation were determined. Water concentration in inulin in the solution saturated at water melting temperature was determined by means of a calorimetric method from the melting enthalpy of water excess over its solubility in the oligosaccharide. Using the technique of differential thermal analysis, a temperature of relaxation transitions in inulin and an effect of water on these transitions are determined.     

Temperature-modulated calorimetry of hexacontane and oligomer fractions of poly(oxyethylene) and poly(oxytetramethylene)

The paraffin hexacontane, C₆₀H₁₂₂, and oligomeric fractions of poly(oxyethylene), POE, and poly(oxytetramethylene), POTM, of varying low molar masses were studied with temperature-modulated calorimetry. The analyses were by standard differential scanning calorimetry (DSC) and quasi-isothermal, temperature-modulated DSC, TMDSC. Small sample masses were examined with temperature amplitudes from 0.05 to 2.5 K, using periods of 60 s. The supercooling decreases with molar mass for all three types of samples. The melting varied between fully irreversible, reversing, and largely reversible. There are no major differences in supercooling between extended- and folded-chain crystals. Due to conformational contributions, all crystals increase their heat capacities within the melting range from the level set by the vibrational spectrum to that of the liquid.     

Differential thermal analysis of polyethylene under high pressure

The design of a differential thermal analysis apparatus for use at elevated pressure is described. Experiments on melting and crystallization of folded-chain crystals of polyethylene and poly(ethylene–butene-1) copolymer, and melting of extended-chain polyethylene crystals have been conducted at pressures up to 4200 bars. The precision in transition temperature measurement was ±1°C. The Clausius-Clapeyron equation predicts the melting point increase with pressure at atmospheric pressure to be 32.0°C/kb. The melting point depression due to copolymerization remained constant over the complete pressure range analyzed on the poly(ethylene–butene-1) used in this study. Crystallization of polyethylene is retarded at elevated pressures, and a 50% larger degree of supercooling is necessary at 5000 bars to give a crystallization rate equal to that observed at atmospheric pressure. The difference in melting point between folded-chain and extended-chain polyethylene increases from 8.4°C at 1 bar to 25.6°C at 3000 bars.     

Nanoscale calorimetry of isolated polyethylene single crystals

We used thin‐film differential scanning calorimetry to investigate the melting of isolated polyethylene single crystals with lamellar thicknesses of 12 ± 1 nm. We observed the melting of as few as 25 crystals. Over a wide number of crystals (25–2000 crystals), the heat of fusion was 40% larger than the bulk value. The melting temperature of the isolated single crystals was 123 ± 2 °C, 9 °C lower than that of the bulk material. We also measured the heat of fusion of quenched crystals (±15%) over a wide range of heating rates (20,000–100,000 K/s). Annealing the quenched crystals resulted in shifts in the endotherm peak by as much as 15 °C. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 1237–1245, 2001     

Reaction-induced phase separation in polyethersulfone-modified epoxy-amine systems studied by temperature modulated differential scanning calorimetry

In epoxy-amine systems with a thermoplastic additive, the initially homogeneous reaction mixture can change into a multi-phase morphology as a result of the increase in molecular weight or network formation of the curing matrix. Temperature modulated DSC (TMDSC) allows the real-time monitoring of this reaction-induced phase separation. A linear polymerizing epoxy-amine (DGEBA–aniline) and a network-forming epoxy-amine (DGEBA–methylene dianiline), both with an amorphous engineering thermoplastic additive (polyethersulfone, PES), are used to illustrate the effects of phase separation on the signals of the TMDSC experiment. The non-reversing heat flow gives information about the reaction kinetics. The heat capacity signal also contains information about the reaction mechanism in combination with effects induced by the changing morphology and rheology such as phase separation and vitrification. In quasi-isothermal (partial cure) TMDSC experiments, the compositional changes resulting from the proceeding phase separation are shown by distinct stepwise heat capacity decreases. The heat flow phase signal is a sensitive indication of relaxation phenomena accompanying the effects of phase separation and vitrification. Non-isothermal (post-cure) TMDSC experiments provide additional real-time information on further reaction and phase separation, and on the effect of temperature on phase separation, giving support to an LCST phase diagram. They also allow measurement of the thermal properties of the in situ formed multi-phase materials.     

Reversible local melting in polymer crystals

The melting of poly(ethylene terephthalate) is analyzed by quasi-isothermal, temperature-modulated differential scanning calorimetry. The measurement is done by sinusoidally changing the temperature in the melting range (± 1.0 K). In the melting range a small portion of the sample melts reversibly. This observation is taken as a direct observation of the reversibility of melting of specific macromolecules as long as they are melting only partially and need no molecular nucleation for recrystallization.     

TMDSC analysis of single-site copolymer blends after thermal fractionation

Temperature-modulated differential scanning calorimetry (TMDSC) has been used to study the melting of a series of blends containing linear low-density polyethylene (LLDPE) and very low-density polyethylenes (VLDPE) with long chain branches. After the blends were subjected to different thermal histories including thermal fractionation by stepwise isothermal cooling, they were examined by TMDSC. TMDSC curves have been interpreted in terms of a combination of the reversing and non-reversing specific heats that result from reversible and irreversible events at the time and temperature, which they are detected, respectively. It was found that crystals formed at different crystallisation conditions had different internal order; hence they showed different amounts of reversing and non-reversing contributions. There is no exothermic activity seen in the non-reversing signal for the thermally fractionated polymers and their blends suggesting formation of crystals approaching equilibrium. In contrast, polymers and blends cooled at 10°C min^-1 cooling rate showed large exothermic contributions corresponding to irreversible effects. In addition, a true reversible melting contribution is also detected for both fast-cooled and thermally-fractionated samples during the quasi-isothermal measurements. This revised version was published online in July 2006 with corrections to the Cover Date.     

Longitudinal growth of polymer crystals from flowing solutions. VI. Melting behavior of continuous fibrillar polyethylene crystals

The melting behavior of continuous fibrillar crystals of high-molecular-weight polyethylene has been investigated. The macrofibers were grown from dilute solutions in xylene subjected to Couette flow in the temperature range between 103 and 118.5°C. The thermograms, as determined by differential scanning calorimetry, exhibit three melting endotherms with peak temperatures at 141, 150.5, and 159.5°C after extrapolation to zero scan speed. All peaks were found to be strongly superheatable. Reduction of fiber length, in particular by etching with fuming nitric acid, led to the disappearance of the melting peaks at 150.5 and 159.5°C. The remaining peak at 136°C appeared not to be superheatable. The heat of fusion of the fragmented fibers was 69.8 cal/g. Wide-angle x-ray diffractograms taken on a macrofiber while gradually heated at a rate of 0.35°C/min at constant length showed that the triclinic phase present in the fiber disappeared at 130°C and that the orthorhombic cell transformed into the hexagonal modification at 150°C. This hexagonal phase was still observable at 180°C. The retractive force developed on heating at constant length displays first a slight decrease followed by a maximum at 150°C. Beyond the latter temperature the stress decays abruptly corresponding to the temperature at which fracture of the fiber could be observed visually. From all these observations it is inferred that the first melting endotherm in the differential scanning calorimeter (DSC) thermograms arises from the melting of unconstrained fibrillar crystal regions which are able to shrink during fusion. Moreover, the melting of lamellar overgrowths on the elementary fibrils on shish-kebab type may contribute to this endotherm. The second melting endotherm at about 150°C is associated with the transformation of the orthorhombic into the hexagonal lattice in constrained parts of the sample. This latter “rotator” phase allows slippage of the polymer chains past each other, giving rise to stress relaxation. The third endotherm arises from melting of this hexagonal phase and the heat take-up connected with the formation of higher energy gauche states upon randomization of the chains in the melt. Almost smooth, fully constrained fibrillar crystals grown at high temperature absorb more than 15.5 cal/g during this process, indicating that the polymer chains in such fibers must be highly extended.     

Effect of soft segment length on the thermal behaviors of fluorinated polyurethanes

Differential scanning calorimetry (DSC) and temperature modulated DSC (MDSC) have been applied to investigate the thermal behaviors of fluorinated polyurethanes (FPU), which were obtained using 2,2,3,3-tetrafluoro-1, 4-butanediol as the chain extender and based on various soft segments—polytetramethyl oxides (PTMO) with molecular weights of 650, 1000, 1400 and 2000. An exothermic peak and/or multiple melting endotherms were observed during the heating to melting temperature of soft and hard segments. Attributed to the simultaneous recrystallization and melting processes during heating, these features have been confirmed via MDSC, where an endotherm and an exotherm were noted in reversing and non-reversing components of the heat flow. Separating the non-reversing components from the reversing curves, the dependencies of polyurethane morphology on the length of the soft segment could be clarified using MDSC analysis. Soft segment lengthening significantly influences the morphology of soft segment domains in FPUs. The phase separation and crystallinity of the soft segment increased with its length. However, soft segment length exerted a minor influence on the dissociation temperature of the short-range ordered hard segment domain and on the melting temperature of hard segment crystals. Examination of the heats of melting based on the quasi-isothermal MDSC experiments indicated that the crystallinity of hard segment domains declined with increasing soft segment length.     

Low temperature heat capacity of Nd2Zr2O7 pyrochlore

Heat capacity of neodymium zirconate (Nd₂Zr₂O₇) with pyrochlore structure was measured by adiabatic calorimetry and the hybrid adiabatic relaxation method in the temperature range (0.45 to 400) K. Its excess component was obtained by comparison with the heat capacity of the lanthanum zirconate. A thermal anomaly was observed below T=7.2 K. From the heat capacity measurements, the thermodynamic functions of Nd₂Zr₂O₇ were determined.     

Reversible Melting During Crystallization of Polymers Studied by Temperature Modulated Techniques (TMDSC, TMDMA)

Quasi-isothermal temperature modulated DSC and DMA measurements (TMDSC and TMDMA, respectively) were performed to determine heat capacity and shear modulus as a function of time during crystallization. Non-reversible and reversible phenomena in the crystallization region of polymers can be observed. The combination of TMDSC and TMDMA yields new information about local processes at the surface of polymer crystals, like reversible melting. Reversible melting can be observed in complex heat capacity and in the amplitude of shear modulus in response to temperature perturbation. The fraction of material involved in reversible melting, which is established during main crystallization, keeps constant during secondary crystallization for PCL PET and PEEK. This shows that also after long crystallization times the surfaces of the individual polymer crystallites are in equilibrium with the surrounding melt. Simply speaking, polymer crystals are ‘living crystals’. This revised version was published online in August 2006 with corrections to the Cover Date.     

Heat capacity and thermodynamic properties of N-(2-cyanoethyl) aniline (C9H10N2)

The low temperature heat capacities of N-(2-cyanoethyl)aniline were measured with an automated adiabatic calorimeter over the temperature range from 83 to 353 K. The temperature corresponding to the maximum value of the apparent heat capacity in the fusion interval, molar enthalpy and entropy of fusion of this compound were determined to be 323.33 ± 0.13 K, 19.4 ± 0.1 kJ mol⁻¹ and 60.1 ± 0.1 J K⁻¹ mol⁻¹, respectively. Using the fractional melting technique, the purity of the sample was determined to be 99.0 mol% and the melting temperature for the tested sample and the absolutely pure compound were determined to be 323.50 and 323.99 K, respectively. A solid-to-solid phase transition occurred at 310.63 ± 0.15 K. The molar enthalpy and molar entropy of the transition were determined to be 980 ± 5 J mol⁻¹ and 3.16 ± 0.02 J K⁻¹ mol⁻¹, respectively. The thermodynamic functions of the compound [H_T − H_298.15] and [S_T − S_298.15] were calculated based on the heat capacity measurements in the temperature range of 83–353 K with an interval of 5 K.