TRANSITION IN THE MELT OF FEP COPOLYMER

The nature of the transition in molten FEP copolymer was examined in relation to the enthalpy change, mechanical damping and melt viscosity. For a pre-heat-treated FEP copolymer sample a small endothermic peak appeared at 309—312 ℃in DSC trace with enthalpy change 0.03—0.05 cal/g. A peak was also detected in damping versus temperature curve at the same temperature range. The theological property of FEP copolymer melt was similar to that of liquid crystal, but no birefrigence was viewed in the melt. Therefore the transition was explained as the melting of small crystallites which persist in typical copolymer beyond its melting temperature. These crystallites can act as nuclei for crystallization upon cooling.     

Hydration and drying of various polysaccharides studied using DSC

The hydration of cellulose, chitosan, schizophyllan, hyaluronan, and carboxymethyl cellulose was studied using differential scanning calorimetry (DSC). In the first part, the classical freezing/thawing approach was used to determine the amount of non-freezing water. The inconsistency in enthalpies obtained during crystallization and melting of freezable water was discussed with respect to the DSC experimental conditions. Our interpretation questions the recent conclusions about competitive processes occurring during melting which are hypothesized to influence the determined melting enthalpy. In the second part, the hydration and drying were studied using the evaporation enthalpy of water. The dry mass normalized dependency of vaporization enthalpy on water content confirmed an abrupt break at low water content in hyaluronan sample which was attributed to the sudden appearance of a parallel process taking part during the drying. The rest of polysaccharide samples showed only a linear decrease in evaporation enthalpy. The renormalization of enthalpies by the water content revealed the increase in evaporation enthalpy with decreasing water content in most samples which was ascribed to the strong interaction between polysaccharide and water. The exceptions were carboxymethyl cellulose which showed a decrease in evaporation enthalpy. This indicates the existence of a simultaneous process occurring during drying, but unlike in hyaluronan, the processes do not appear abruptly but accompany the evaporation in the wide concentration range. Comparison of determined hydration numbers showed that part of non-freezing water in hyaluronan is not bound to sorption sites but occurs presumably in small temporary pores. In contrast, water-soluble schizophyllan forms temporary pores as well but presumably with higher dimension and the non-freezing water is formed mostly by water molecules interacting with sorption sites.     

Importance of bound water in hydration-dehydration behavior of hydroxylated poly(N-isopropylacrylamide)

In this study, a differential scanning calorimetric analysis was performed to investigate the role of water existing around the polymer chains on their thermoresponsive behaviors using the novel functional temperature-sensitive copolymer, poly(N-isopropylacrylamide-co-2-hydroxyisopropylacrylamide) (poly(NIPAAm-co-HIPAAm)). The HIPAAm comonomers were incorporated into the polymeric chains as hydrophilic parameters, and then the hydration states of poly(NIPAAm-co-HIPAAm) with various HIPAAm compositions were examined. Bound water, which is affected by the polymeric chains to some extent, was produced by adding the copolymers to the water, and the temperature due to the melting of the bound water decreased as the HIPAAm content increased. On the basis of this result, we considered that the interaction between the bound water and the polymeric chains is reinforced by the increasing HIPAAm composition. In addition, the cloud points of the copolymers shifted to a higher temperature, and the endothermic enthalpy for the phase transition decreased with increasing the HIPAAm content, suggesting that the number of water molecules disassociated from the polymeric chains decreased. Based on these results, we postulate that the changes in the interaction between the thermosensitive polymer and bound water exert a strong influence on its phase transition and/or separation, such as the cloud point or dehydration behavior.     

Hydration of humic and fulvic acids studied by DSC

Qualitative and quantitative aspects of hydration of four humic acids (HA) and three fulvic acids (FA) originating from different sources were investigated. DSC experiments at subambient temperatures were carried out in order to monitor differences in ice behavior originating from freezable water surrounding humic molecules. It was found that kinetic effects play a significant role in hydration processes of both HA and FA. In fact, the hydration took part over 21?days which was detected as a progressive decrease in ice melting enthalpy. Simultaneously, the peak shapes and positions changed indicating structural changes in the physical structure of the humic substances. In case of FA, the dependency of melting enthalpy on water concentration showed a linear trend resembling a complete hydration previously observed for water-soluble hydrophilic polymers. In contrast, the melting enthalpy of some HA increased in a step-like way with increasing water content, suggesting preservation of original hydrophobic scaffold during the hydration. The differences between the rather young FA and the rather old HA lead to the conclusion that water can play a significant role in processes of humification. We assume that separation of hydrophobic and hydrophilic domains and thus increase in nanoscale heterogeneity represents an important physical contribution to the overall humification process. It was also demonstrated that the higher content of oxygen in humic molecules is not the only indicator of higher water holding capacity. Instead the porosity of humic matrix seems to contribute as additional parameter into these processes.     

Specific reversible melting of polyethylene

The specific reversibility of the crystallization and melting of linear and branched polyethylene has been determined as function of temperature by temperature‐modulated differential scanning calorimetry. The specific reversibility of crystallization and melting is defined as the ratio of the reversible enthalpy to the total enthalpy of the transition, both measured at the same temperature. This definition emphasizes a close connection between the reversible and irreversible parts of the transition. As one would expect, the crystal‐to‐melt transition of a given portion of a sample can only be reversible at a temperature close to its own temperature of irreversible melting. Reversible melting is absent at temperatures far from irreversible melting, and this is usually seen by experimentation as its zero‐entropy production melting temperature. The reversible change in the fold length, in contrast, is observed far from the melting temperature of the crystal involved. The specific reversibility of the crystallization and melting of polyethylene crystals may exceed 50% outside the temperature range of the main crystallization and melting. The specific reversibility seems rather independent of the branch concentration, and this points to similar mechanisms of the reversible transition in linear polyethylene of high crystallinity and in branched polyethylene of low crystallinity. The reversible transition is due to a local equilibrium at the crystal surface and is, therefore, largely independent of the overall morphology of the sample. In this study, a model is developed that is based on partial molecular melting, which avoids the need of molecular nucleation and permits, therefore, reversible melting as seen for small molecules in the presence of crystal nuclei. It provides an explanation of the rather large number of the crystals that may participate in reversible melting and allows a connection to the fully reversible crystallization of paraffins and the fully irreversible crystallization of extended‐chain crystals of high crystallinity. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2157–2173, 2003     

静电场诱导十二烷基硫酸钠结晶行为的研究

用透射电镜、X射线衍射及DSC等方法 十二烷基硫酸钠（SDS）极稀水溶液（溶液介于临界聚集浓度和临界胶束浓度之间）在静电场作用下的结晶行为,静电场作用诱导SDS形成规则的四方单晶与从甲醇中重结晶所得样品的晶体结构相同,DSC结果表明,从有序到无序结构变化的一级相转变热焓相同,但由于电场诱导结晶的晶体较小而表现为转变温度的降低,没有施加静电场处理的样品只具有较低的有序程度。     

Magnetically alignable phase of phospholipid "bicelle" mixtures is a chiral nematic made up of wormlike micelles

We have studied the phase behavior of binary mixtures of long- and short-chain lipids, namely, dimyristoyl phosphatidylcholine (DMPC) and dihexanoyl phosphatidylcholine (DHPC), using optical microscopy and small-angle neutron scattering. Samples with a total lipid content of 25 wt %, corresponding to ratios Q ([DMPC]/[DHPC]) of 5, 3.2, and 2, are found to exhibit an isotropic (I) --> chiral nematic (N) --> lamellar phase sequence on increasing temperature. The I-N transition coincides with the chain melting transition of DMPC at Q = 5 and 3.2, but the N phase forms at a higher temperature for Q = 2. All three samples form multilamellar vesicles in the lamellar phase. Our results show that disklike "bicellar" aggregates occur only in the lower temperature isotropic phase and not in the higher temperature magnetically alignable N phase, where they were previously believed to exist. The N phase is found to consist of long, flexible wormlike micelles, their entanglement resulting in the very high viscosity of this phase.     

Physicochemical properties and solubility of alkyl-(2-hydroxyethyl)-dimethylammonium bromide

Quaternary ammonium salts, which are precursors of ionic liquids, have been prepared from N,N-dimethylethanolamine as a substrate. The paper includes specific basic characterization of synthesized compounds via the following procedures: nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectra, water content, mass spectroscopy (MS) spectra, temperatures of decompositions, basic thermodynamic properties of pure ionic liquids (the melting point, enthalpy of fusion, enthalpy of solid-solid phase transition, glass transition), and the difference in the solute heat capacity between the liquid and solid at the melting temperature determined by differential scanning calorimetry (DSC). The (solid + liquid) phase equilibria of binary mixtures containing (quaternary ammonium salt + water, or + 1-octanol) has been measured by a dynamic method over wide range of temperatures, from 230 K to 560 K. These data were correlated by means of the UNIQUAC ASM and modified nonrandom two-liquid NRTL1 equations utilizing parameters derived from the (solid + liquid) equilibrium. The partition coefficient of ionic liquid in the 1-octanol/water binary system has been calculated from the solubility results. Experimental partition coefficients (log P) were negative at three temperatures.     

聚L-乳酸/4,4'-二羟基二苯硫醚共混物的分子间相互作用及结晶和熔融行为

利用红外吸收光谱(FTIR)研究了聚乳酸(PLLA)/4,4'-二羟基二苯硫醚(TDP)熔融共混物的分子间相互作用,结果表明,PLLA的羰基与TDP的羟基之间形成了分子间氢键.通过差示扫描量热(DSC)研究了共混物的玻璃化转变行为及非等温结晶和熔融行为.结果表明,样品的玻璃化转变温度(Tg)随TDP含量的增加呈线性下降.共混物的熔融结晶温度(Tc)、结晶焓(ΔHc)、熔融温度(Tm)及熔融焓(ΔHm)均随TDP含量的增加呈下降趋势,而冷结晶温度的变化趋势则相反.当TDP达到40%(质量分数)时,共混物的DSC曲线既未出现结晶峰,也未出现熔融峰,表明该样品已完全成为非晶态物质.广角X射线衍射(WAXD)分析结果表明,TDP的加入未改变PLLA的晶型,但导致其晶面间距变大,晶体结构变得松散.因此共混物熔点的下降归因于分子间氢键的形成降低了PLLA分子链的运动能力及晶体的紧密程度而非晶型的改变.     

Enthalpy, heat of fusion and specific electrical resistivity of pure silver, pure copper and the binary Ag-28Cu alloy

Within the present work, recent investigation carried out with a fast pulse-heating technique on silver, copper and the Ag-28Cu binary alloy in the solid and the molten states are presented.Properties like enthalpy or electrical resistivity of a pulse-heated sample can be obtained for a wide temperature range (solid state up into the liquid state) from the directly measured base quantities, namely: current through the sample, voltage drop across the specimen and pyrometric determined temperature. As a further result, enthalpy of fusion is computable from the enthalpy values at the melting transition or the solidus/liquidus transition. These thermophysical properties (mainly of the melting transition and the subsequent liquid phase) are commonly used as input data for numerical casting simulations.The measurements presented within this work deal with group¹ 11 elements silver, copper and the binary eutectic 72-28 (wt.%) alloy of the two elements, respectively. One of the main goals of this work is to investigate to which extent the thermophysical properties of the two pure materials influence or determine the properties of its corresponding alloy. It is to proof if data for pure materials can be used to predict the thermophysical properties of simple alloys. For this specific copper-silver alloy, there is a certain mutual solid solubility with a quite large miscibility gap. Ag-28Cu is not a single phase alloy but an eutectic alloy with two phases.     

Structural investigation of water‐induced phase transitions of poly(ethylene imine). III. The thermal behavior of hydrates and the construction of a phase diagram

The thermal behavior of poly(ethylene imine) (PEI) hydrates in a water vapor atmosphere was investigated through temperature‐dependent measurements of infrared spectra and X‐ray diffraction. Almost perfectly dried anhydrate melted at about 60 °C during the heating process. Anhydrate containing a small amount of water showed a phase transition to a mixture of hemihydrate and sesquihydrate around 40 °C, at which point the ethylene imine (EI)/water ratio was 1/0.5 in the hemihydrate and 1/1.5 in the sesquihydrate. The hemihydrate transferred to the sesquihydrate around 60 °C, and the latter melted above 80 °C. When the starting PEI sample contained a greater amount of water and consisted of hemihydrate and sesquihydrate, the hemihydrate transferred to the sesquihydrate via heating, and the latter melted around 75 °C. For a sample of dihydrate (EI/water ratio = 1/2) containing an appreciably large amount of water, it transferred to the sesquihydrate around 65 °C, and the latter melted above 90 °C. A sample of dihydrate with a much higher water content existed up to 110 °C and then melted; during this period, no transition to the sesquihydrate was observed. In this way, the starting crystalline phases were found to change for anhydrate and various types of hydrates. Their transition behaviors varied according to the water content. From these data, a phase diagram was successfully derived as a function of the temperature and water content. This phase diagram allowed us to predict the transition behavior during the hydration process at various constant temperatures. For example, at 60 °C, a molten sample should crystallize into a mixture of hemihydrate and sesquihydrate at first, and the hemihydrate should transfer to the sesquihydrate with increasing water content. The latter should change to the dihydrate in the final stage. This prediction was checked with time‐resolved measurements of X‐ray diffraction and infrared spectra during the hydration process at the corresponding temperature; this led to the establishment of the phase diagram. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2937–2948, 2003     

A view and a review of the melting of alkali metal halide crystals

Summary A representational model, proposed to account for the physical changes that accompany the melting of alkali halides, was described in Part 1 [1]. The liquid is portrayed as undergoing continual dynamic structural reorganization of its constituent ions between individual small domains, zones of various regular, crystal-type arrays. These alternative arrangements are stabilized by the enthalpy of melting, which, in liquids, relaxes the restriction for solids that only the single, most stable, crystal structure can be present. The dynamic character of the melt accounts for its fluid character and the loss of long-range order [1, 2]. This model is extended here to consider the phase diagrams of binary, common ion, alkali halide mixtures comprehensively reviewed in [3]. Factors determining whether each of these yields a eutectic, or a solid solution, on cooling are discussed and several trends in the 70-phase diagrams are identified. Eutectic formation, involving maintenance of the liquid state below the melting points of the pure components, is ascribed to the participation, in an extended dynamic equilibrium, of additional domains having the regular structures characteristic of double salts. The known crystalline double binary halides [3], Li/Cs or Rb/F, Cl, Br or I, melt at temperatures well below those of the simpler pure component salts. It is concluded that the set/liq model for melting, proposed in [1, 2], accounts for some important properties of the phase diagrams presented in [3].     

Heat Capacity and Enthalpy of Fusion of Crystalline Pyrimethanil Decylate (C22 H33 N3 O2)

IntroductionPyrimidinamines are heterocyclic compounds withnovel biological activities[1].They have been applied tomedicine and pesticide syntheses for their importantphysiological-action[2,3].Compared with the free py-rimethanil,pyrimethanil salts have a reduced vapourpressure,which increases the persistence of the com-pounds on crops and they have increased activities inmany aspects.Pyrimethanil decylate(molecular formu-la:C22H33N3O2)is an important new compound.Itcan be synthesized by the…     

Thermophysical and electrophysical properties of conductive organic composites based on salts of TCNQ and methyl-TCNQ

Conductive complex salts of methyl-TCNQ, melting without decomposition, have been synthesized for the first time. Composites of TCNQ and methyl-TCNQ salts with low melting points (below 353 K) and high stability have been obtained. It has been shown that the half-decomposition time of the salt melt varies exponentially with the temperature. The transition to the liquid phase, which is possibly a liquid crystal phase, is not accompanied by any great jump in electrical resistance, and the activation energy of electrical conduction remains the same.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 29, No. 4, pp. 361–368, July–August, 1993.     

Formation and properties of reverse micellar cubic liquid crystals and derived emulsions

The structure of the reverse micellar cubic (I2) liquid crystal and the adjacent micellar phase in amphiphilic block copolymer/water/oil systems has been studied by small-angle X-ray scattering (SAXS), rheometry, and differential scanning calorimetry (DSC). Upon addition of water to the copolymer/oil mixture, spherical micelles are formed and grow in size until a disorder-order transition takes place, which is related to a sudden increase in the viscosity and shear modulus. The transition is driven by the packing of the spherical micelles into a Fd3m cubic lattice. The single-phase I2 liquid crystals show gel-like behavior and elastic moduli higher than 104 Pa, as determined by oscillatory measurements. Further addition of water induces phase separation, and it is found that reverse water-in-oil emulsions with high internal phase ratio and stabilized by I2 liquid crystals can be prepared in the two-phase region. Contrary to liquid-liquid emulsions, both the elastic modulus and the viscosity decrease with the fraction of dispersed water, due to a decrease in the crystalline fraction in the sample, although the reverse emulsions remain gel-like even at high volume fractions of the dispersed phase. A temperature induced order-disorder transition can be detected by calorimetry and rheometry. Upon heating the I2 liquid crystals, two thermal events associated with small enthalpy values were detected: one endothermic, related to the "melting" of the liquid crystal, and the other exothermic, attributed to phase separation. The melting of the liquid crystal is associated with a sudden drop in viscosity and shear moduli. Results are relevant for understanding the formation of cubic-phase-based reverse emulsions and for their application as templates for the synthesis of structured materials.     

Hydration-dependent enthalpy changes in the helix–coil transition in tendon

The shrinkage phenomenon in bovine achilles heel tendon was studied calorimetrically as a function of water content of the tendon. The transition temperature was found to increase from 67°C to 98°C as the water content decreased from 53% by weight to 26% by weight. The enthalpy change was found to increase with increasing percentages of water. The data is interpreted by considering the possibility of water binding to the polar sites on the solute molecule which become free after the transition. A calculation is carried out in which the melting shifts are taken to be related to the free energy of binding. The increase in enthalpy with an increase in water content is shown to have a semiquantitative interpretation in terms of the structural aspects of water.     

The amount of immobilized polymer in PMMA SiO2 nanocomposites determined from calorimetric data

For semicrystalline polymers there is an ongoing debate at what temperature the immobilized or rigid amorphous fraction (RAF) devitrifies (relaxes). The question if the polymer crystals are melting first and simultaneously the RAF devitrifies or the RAF devitrifies first and later on the crystals melt cannot be answered easily on the example of semicrystalline polymers. This is because the crystals, which are the reason for the immobilization of the polymer, often disappear (melt) in the same temperature range as the RAF. For polymer nanocomposites the situation is simpler. Silica nanoparticles do not melt or undergo other phase transitions altering the polymer-nanoparticle interaction in the temperature range where the polymer is thermally stable (does not degrade). The existence of an immobilized fraction in PMMA SiO₂ nanocomposites was shown on the basis of heat capacity measurements at the glass transition of the polymer. The results were verified by enthalpy relaxation experiments below the glass transition. The immobilized layer is about 2 nm thick at low filler content if agglomeration is not dominant. The thickness of the layer is similar to that found in semicrystalline polymers and independent from the shape of the nanoparticles. Nanocomposites therefore offer a unique opportunity to study the devitrification of the immobilized fraction (RAF) without interference of melting of crystals as in semicrystalline polymers. It was found that the interaction between the SiO₂ nanoparticles and the PMMA is so strong that no devitrification occurs before degradation of the polymer. No gradual increase of heat capacity or a broadening of the glass transition was found. The cooperatively rearranging regions (CRR) are either immobilized or mobile. No intermediate states are found. The results obtained for the polymer nanocomposites support the view that the reason for the restricted mobility must disappear before the RAF can devitrify. For semicrystalline polymers this means that rigid crystals must melt before the RAF can relax.     

Ultrahigh-molecular-weight polyethylene: Raman spectroscopic study of melt anisotropy

The Raman spectrum of ultrahigh-molecular-weight polyethylene (UHMWPE) has been obtained in the temperature interval 135–208°C, a region where optical anisotropy was observed to exist. On the basis of our spectroscopic evidence, we believe that ordered regions persist in the melt above the calorimetrically determined melting point, and that part of the polyethylene chain is in an enviroment which is similar to that of the orthorhombic crystal. These ordered domains disappear with increasing temperature, but no calorimetric phase transition is associated with this change. We postulate that the very long relaxation times associated with the highly viscous melt keep the polyethylene chains in ordered environments which persist until decreased viscosity at increased temperature allows long-range segmental motion. Our evidence supports the view that the melt anisotropy of UHMWPE arises from oriented slowly melting superheated crystals and not from a smectic liquid-crystalline phase.     

磷脂DHPC和短杆菌肽D相互作用的DSC研究

Changes of the behavior of the thermotropic phase transitions of the phospholipid DHPC by the action of miniprotein gramicidin D (G.D) have been studied by differential scanning calorimetry. The experimental results demonstrated that G.D could not remove the pretransition of DHPC, but the pretransition peak and the main transition peak of DHPC were incorporated into a broad peak. The pretransition temperature and the main transition temperature of DHPC decreased to different ex-tent. The sums of the pretransition and main transition enthalpies of DHPC showed a lineally decreasing relationship with increasing the mole fraction x_g of gramicidin D.