Reversible Melting of Semi-crystalline Polymers Frequency dependence of the reversible melting enthalpy

Modulated DSC (MDSC) has been used to study the heat flow during melting and crystallisation of some semi-crystalline polymers i.e. different grades of polyethylene (LDPE, LLDPE and HDPE), and polypropylene (PP). The heat capacities measured by MDSC are compared with the hypothetical complex heat capacities of Schawe and it is shown that numerically they are equivalent; nevertheless, the concept of the complex heat capacity is problematic on a thermodynamic basis. A reversing heat flow (proportional to the experimental heat capacity of the material) was present at all conditions used for the study. In the melting zone of the polymers it depends on the modulation frequency and on the amplitude. Higher amplitude and frequency of modulation reduce the ratio of the reversing heat flow to the total heat flow, the latter is nearly independent on these parameters. The reversible component of the melting enthalpy of polymers depends on the modulation frequency, the modulation amplitude and the type of the polymer. It increases by increasing the branching in polyethylene. The existence of the reversible heat flow during the crystallisation and melting is contrary to the current hypotheses and theories of polymer crystallisation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Annealing of Polypropylene/Polyethylene Blends Near to the Melting Points in TMDSC

Annealing experiments have been carried out just below the melting temperature of both polyethylene (LLDPE) and polypropylene (PP) and their blends. The total melting enthalpy measured after the annealing cycle was greater by 10-15% with respect to the value having been measured before it. During the annealing period the heat capacity decreases to a lower value within the first 2-3 min. Heat capacities of PP (either in pure form or in the blends) measured during the heating cycle following the annealing cycle have the same value as during the cooling section. The heat capacities of the LLDPE in the heating cycle following the annealing were those of the preceding heating cycle. The total heat flows in the cooling section following the annealing cycle were greater than those in another cooling cycle at the same temperatures indicating that the crystallisation takes place during the cooling rather than during the annealing periods. The presence of LLDPE decreases the crystallisation temperature of PP. The presence of SEBS in the blend results in a greater crystallisation temperature than that of pure PP. The crystallisation temperature of LLDPE increases with increasing levels of PP. This revised version was published online in July 2006 with corrections to the Cover Date.     

A Method for Analysis of the Measured Curves of TMDSC Investigation in the Melting Region of Polymers

The measured signal of the temperature-modulated differential scanning calorimetry (TMDSC) is discussed in the case of polymer melting. The common data evaluation procedure of TMDSC-signals is the Fourier analysis. The resulting information is the amplitude and the phase shift of the first harmonic of the periodic heat flow component. It is shown that this procedure is not sufficient for quantitative discussions if deviations from the symmetric curve shape occur in the measured heat flow curves. For polymer melting it is demonstrated that asymmetric curves will be measured if the experimental temperature amplitude is too large. In this paper a data evaluation method is presented, which is based on the Fourier transform of the measured curves. The peaks of the first and second harmonics in the resulting spectra are used for the analysis of the asymmetry of the measured curves. In the case of polymer melting this analysis yields the maximum temperature amplitude which follows a correct linear data evaluation. This maximum temperature amplitude depends on the material. This revised version was published online in July 2006 with corrections to the Cover Date.     

On the determination of the melting point of semicrystalline polymer by DSC

This is a study for criteria to judge the melting point of semi-crystalline polymers from the DSC endotherm for polymer melting. Beyond standard indium DSC melting results an evaluation has been made on a series of polyethylenes for which crystal sizes were measured and predicted from Raman LAM analysis. The results confirm the conclusion of Prof. Wunderlich that the DSC content of melting is the proper basis of reporting melting points.Dedicated to Professor Bernhard Wunderlich on the occasion of his 65th birthday     

Stress‐Induced Melting of Crystals in Natural Rubber: a New Way to Tailor the Transition Temperature of Shape Memory Polymers

Lightly cross‐linked natural rubber (NR, cis‐1,4‐polyisoprene) was found to be an exceptional cold programmable shape memory polymer (SMP) with strain storage of up to 1000%. These networks are stabilized by strain‐induced crystals. Here, we explore the influence of mechanical stress applied perpendicular to the elongation direction of the network on the stability of these crystals. We found that the material recovers its original shape at a critical transverse stress. It could be shown that this is due to a disruption of the strain‐stabilizing crystals, which represents a completely new trigger for SMPs. The variation of transverse stress allows tuning of the trigger temperature T_trig(σ) in a range of 45 to 0 °C, which is the first example of manipulating the transition of a crystal‐stabilized SMP after programming.     

Challenge and Solution of Characterizing Glass Transition Temperature for Conjugated Polymers by Differential Scanning Calorimetry

Thermomechanical properties of polymers highly depend on their glass transition temperature (T _g). Differential scanning calorimetry (DSC) is commonly used to measure T _g of polymers. However, many conjugated polymers (CPs), especially donor–acceptor CPs (D–A CPs), do not show a clear glass transition when measured by conventional DSC using simple heat and cool scan. In this work, we discuss the origin of the difficulty for measuring T _g in such type of polymers. The changes in specific heat capacity (Δc _p) at T _g were accurately probed for a series of CPs by DSC. The results showed a significant decrease in Δc _p from flexible polymer (0.28 J g^?1 K^?1 for polystyrene) to rigid CPs (10^?3 J g^?1 K^?1 for a naphthalene diimide‐based D–A CP). When a conjugation breaker unit (flexible unit) is added to the D–A CPs, we observed restoration of the Δc _p at T _g by a factor of 10, confirming that backbone rigidity reduces the Δc _p. Additionally, an increase in the crystalline fraction of the CPs further reduces Δc _p. We conclude that the difficulties of determining T _g for CPs using DSC are mainly due to rigid backbone and semicrystalline nature. We also demonstrate that physical aging can be used on DSC to help locate and confirm the glass transition for D‐A CPs with weak transition signals. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1635–1644     

Study on the Melting Process of Nitrocellulose by Thermal Analysis Method

The melting process of NC is studied by using modulated differential scanning calorimetry (MDSC) technique, the microscope carrier method for measuring the melting point and the simultaneous device of the solid reaction cell in situ/RSFT-IR. The results show that the endothermic process in the MDSC curve is reversible. It is caused by the phase change from solid to liquid of the mixture of initial NC, decomposition partly into condensed phase products. The values of the melting point, melting enthalpy (H_m), melting entropy (S_m), the enthalpy of decomposition (H_dec) and the heat-temperature quotient (S_dec) obtained by the MDSC curve of NC at a heating rate of 10 K min^–1 are 476.84 K, 205.6 J g^–1, 0.4312 J g^–1 K^–1, –2475.0 J g^–1 and –5.242 Jg^–1K^–1, respectively. The MDSC results of NC with different nitrogen contents show that with increasing the nitrogen content in NC, the absolute values of H_m, S_m, H_dec and S_dec increase.This revised version was published online in November 2005 with corrections to the Cover Date.     

Constant-Volume Heat Capacity of H2O + Na2CO3 Solutions Near the Critical Point of Pure Water

The results of new isochoric heat capacity measurements of H₂O + Na₂CO₃ solutions as a function of temperature along several isochores in the near-critical region of pure water are reported. The measurements cover temperatures from 331 to 661 K at Na₂CO₃ mole fraction 0.008869. The experiments were performed at seven densities between 245 and 875 kg-m^–3. The measurements were made in a high-temperature, high-pressure, adiabatic, and nearly constant-volume calorimeter. Uncertainties of the isochoric heat capacity measurements are estimated to be 1–2% in the liquid phase, 2–3% in the vapor phase, and 4–5% near the critical point. Measurements were made in the two-phase (vapor–liquid, liquid–solid, vapor–solid) and three-phase (vapor–liquid–solid) regions. Two peaks in isochoric heat capacity have been found near the critical point of pure water. One of the peaks at T _S1 occurs on the three-phase (L–V–S) curve and another peak at T _S2 corresponds to a two-phase (L–S or V–S) curve. The experimental values of phase transition temperatures T _S() on each isochore was determined. Uncertainty in the phase transition temperature measured was no greater than ±0.03 to 0.05 K.     

Effect of Molecular Mass on the Melting Temperature,Enthalpy and Entropy of Hydroxy-Terminated PEO

This paper studies the effect of molecular mass on the melting temperature, enthalpy and entropy of hydroxy-terminated poly(ethylene oxide) (PEO). It aims to correlate the thermal behaviour of PEO polymers and their variation of molecular mass (MW). Samples ranging from 1500 to 200,000 isothermally treated at 373 K during 10 min, were investigated using DSC and Hot Stage Microscopy (HSM). On the basis of DSC and HSM results, melting temperatures were determined, and melting enthalpies and entropies were calculated. Considering the melting temperatures, it was found that the maximum or critical value of MW was found around 4000, and then these remain almost constant. This behaviour was interpreted assuming that lower MW fractions (MW<4000) crystallize in the form of extended chains and higher MW fractions (MW>4000), as folded chains. The melting enthalpies showed a scattering effect at least up to MW 35,000. It was difficult to obtain any relationship between melting enthalpies in J g^–1 and MW. These variations seem to be of statistical nature. Corrected enthalpy data on a molar basis (kJ mol^–1) exhibited a linear relationship with MW. Considering the solid—liquid equilibrium, the melting entropies (in kJ mol^–1) were calculated. These values were more negative as compared with molar enthalpy increases. It was explained because the changes in melting temperatures are much smaller than those observed in the enthalpy values. Linear relationship between enthalpies andentropies as a function of MW was deduced.This revised version was published online in November 2005 with corrections to the Cover Date.     

Detection of multiple nanophases by DSC

Polymer molecules have contour lengths which may exceed the dimension of microphases. Especially in semicrystalline samples a single molecule may traverse several phase areas, giving rise to structures in the nanometer region. While microphases have properties that are dominated by surface effects, nanometer-size domains are dominated by interaction between opposing surfaces. Calorimetry can identify such size effects by shifts in the phase-transition temperatures and shapes, as well as changes in heat capacity. Specially restrictive phase structures exist in drawn fibers and in mesophase structures of polymers with alternating rigid and flexible segments. On several samples shifts in glass and melting temperatures will be documented. The proof of rigid amorphous sections at crystal interfaces will be given by comparison with structure analyses by X-ray diffraction and detection of motion by solid state NMR. Finally, it will be pointed out that nanophases need special attention if they are to be studied by thermal analysis since traditional ‘phase’ properties may not exist.     

Reversible Switching of the Shear Modulus of Photoresponsive Liquid‐Crystalline Polymers

Manipulation makes light work : The morphology and rheological properties of a liquid‐crystalline system can be dynamically manipulated with UV light by attaching photoresponsive liquid‐crystalline moieties to a siloxane‐based polymer. Stimulation with UV light induces a conformational change in the molecule, which disrupts the liquid‐crystalline mesophase (see picture), and results in a dramatic change in its rheological properties.

     

Synthesis and Characterization of Functional Thienyl‐Phosphine Microporous Polymers for Carbon Dioxide Capture

A novel kind of functional organic microporous polymer is designed by introducing polar organic groups (P=O and P=S) and electron‐rich heterocyclic into the framework to obtain high carbon dioxide capture capacity. The estimated Brunauer–Emmett–Teller (BET) surface areas of these polymers are about 600 m² g⁻¹ and the highest CO₂ uptake is 2.26 mmol g⁻¹ (1.0 bar/273 K). Interestingly, the polymer containing P=O groups shows greater CO₂ capture capacity than that containing P=S groups at the same temperature. In addition, these polymers show high isosteric heats of CO₂ adsorption (28.6 kJ mol⁻¹), which can be competitive with some nitrogen‐rich networks. Therefore, these microporous polymers are promising candidates for carbon dioxide capture.

     

Reversible and irreversible heat capacity of poly(trimethylene terephthalate) analyzed by temperature‐modulated differential scanning calorimetry

The heat capacity of poly(trimethylene terephthalate) (PTT) has been analyzed using temperature‐modulated differential scanning calorimetry (TMDSC) and compared with results obtained earlier from adiabatic calorimetry and standard differential scanning calorimetry (DSC). Using quasi‐isothermal TMDSC, the apparent reversing and nonreversing heat capacities were determined from 220 to 540 K, including glass and melting transitions. Truly reversible and time‐dependent irreversible heat effects were separated. The extrapolated vibrational heat capacity of the solid and the total heat capacity of the liquid served as baselines for the analysis. As one approaches the melting region from lower temperature, semicrystalline PTT shows a reversing heat capacity, which is larger than that of the liquid, an observation that is common also for other polymers. This higher heat capacity is interpreted as a reversible surface or bulk melting and crystallization, which does not need to undergo molecular nucleation. Additional time‐dependent, reversing contributions, dominating at temperatures even closer to the melting peak, are linked to reorganization and recrystallization (annealing), while the major melting is fully irreversible (nonreversing contribution). © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 622–631, 2000     

Crystal Structures and Melting Behaviors of 2D and 3D Anionic Coordination Polymers Containing Organometallic Ionic Liquid Components

Although coordination polymers generally do not melt, several that do melt have been synthesized recently and have drawn much attention. In this study, two- and three-dimensional coordination polymers that melt were synthesized, [Ru(Cp)(C₆H₅R)][M{C(CN)₃}₂] (R=H, Me, Et; M=K, Rb; Cp=C₅H₅), which are complex salts comprising M[C(CN)₃] and organometallic ionic liquids [Ru(Cp)(C₆H₅R)][C(CN)₃]. They have anionic [M{C(CN)₃}₂]_n⁻ coordination polymer frameworks, whose dimensionalities depend on the size of the organometallic cation inside. Their melting points decreased with increasing cation substituent length and size of the alkali metal ion (T_m=102–239 °C), and these low-melting-point coordination polymers exhibited incongruent melting, forming mixtures of solid M[C(CN)₃] and ionic liquid upon melting. Using the same method, coordination polymers were synthesized with various bridging ligands, [Co(Cp)₂][MX₂] (X=B(CN)₄, C(CN)₃, N(CN)₂; M=K, Na), as well as a paramagnetic coordination polymer, [Fe(Cp)₂][K{C(CN)₃}₂].     

Biomimetic Polymers Responsive to a Biological Signaling Molecule: Nitric Oxide Triggered Reversible Self‐assembly of Single Macromolecular Chains into Nanoparticles

Novel nitric oxide (NO) responsive monomers (NAPMA and APUEMA) containing o‐phenylenediamine functional groups have been polymerized to form NO‐responsive macromolecular chains as truly biomimetic polymers. Upon exposure to NO—a ubiquitous cellular signaling molecule—the NAPMA‐ and APUEMA‐labeled thermoresponsive copolymers exhibited substantial changes in solubility, clearly characterized by tuneable LCST behavior, thereby inducing self‐assembly into nanoparticulate structures. Moreover, the NO‐triggered self‐assembly process in combination with environmentally sensitive fluorescence dyes could be employed to detect and image endogenous NO.     

水溶性聚合物和两亲聚合物18．2－丙烯酰胺基烷磺酸类两亲单体

２－丙烯酰胺基烷磺酸的溶解行为和胶体化学性质明显随烷基长短而异。由于手性碳原子的存在，与其相邻亚甲基的质子峰在~１ＨＮＭＲ谱上清楚地发生分裂。研究了２－丙烯酰胺基十六烷磺酸（ＡＭＣ１６Ｓ）在水／二氧六环混合溶剂中的聚合动力学和胶体化学性质。发现聚合速度和所得聚合物分子量均在水／二氧六环体积比为１～１／２时出现最小值，临界胶束浓度随混合溶剂中二氧六环含量的增加而增加，当水／二氧六环体积比为１／２～１／４时，观察不到胶束化现象。     

Anomalous Melting of Poly(Phenylene Oxide)

Slow evaporation of toluene from poly(2,6-dimethyl-1,4-phenylene oxide), PPO, at 23°C produced crystals which melt between 185 and 265°C and have a crystallinityof 0.58. About 8 wt% of the crystals appear to melt temperatures below 220°C. The lattertemperature marks the end of the glass transition interval for this polymer.This revised version was published online in November 2005 with corrections to the Cover Date.     

水溶性聚合物和两亲聚合物18.2—丙烯酰胺基烷磺酸类两亲单体

２－丙烯酰胺基烷磺酸的溶解行为和胶体化学性质明显随烷基长短而异，由于手性碳原子的存在，与其相邻亚甲基的质子峰在ＨＮＭＲ谱上清楚地发生分裂，研究了２－丙烯酰胺基十六烷磺酸（ＡＭＣ１６Ｓ）在水／二氧六环混合溶剂中的聚合动力学和胶体化化学性质，发现聚合速度和所得聚合物分子量均均在水／二氧六环体积比为１～１／２时出现最小值，临界胶束浓度随混合溶剂中二氧六环含量的增加而增加，当水／二氧六环体积比为１／２～１     

Thermal Conductivity of CuSbSe2 and CuBiSe2 in the Solid and Liquid State

Abstract

Thermal Conductivity of CuSbSe₂ and CuBiSe₂ have been studied in the solid and liquid state in a wide range of temperatures. Measurements of thermal conductivity were carried out using the concentric cylinder method which is based on the flow of heat through a cylindrical wall.