Metastable crystalline lamella of cetylpyridinium chloride in the Krafft transition

The metastable crystalline lamella was found in the Krafft transition of aqueous cetylpyridinium chloride (CPC) solutions. Temperature-dependent profiles of small-angle X-ray scattering (SAXS) for the CPC solution incubated for 10 min at 5 degrees C exhibited the metastable lamella structure with a lattice spacing of dL = 3.19 nm at temperatures below 12 degrees C and the stable lamella structure with a lattice spacing of dL = 2.85 nm at temperatures between 12 and 19 degrees C. The former lamella structure, however, was not observed in the temperature scanning SAXS profiles of the CPC solution incubated for 24 h at 5 degrees C. The latter lamella structure observed in the SAXS profile mentioned above started melting at 18 degrees C. The electric conductance change of the CPC solution with a time elapsed after dropping the temperature showed the existence of the temperature-dependent induction period in the Krafft transition, indicating high activation energies for the transition. In the differential scanning calorimetry measurements over temperatures ranging from 5 to 30 degrees C, a single endothermic enthalpy peak at 19 degrees C observed for the CPC solution incubated at 5 degrees C for a longer period than 6 h was split into double peaks at 14 and 19 degrees C when the same solution was incubated at 5 degrees C for a shorter period than 6 h. The observed calorimetric behavior is explained by the existence of the metastable crystalline state that grows faster and melts at a lower melting temperature than the stable crystalline state.     

Crystallization and melting of cold-crystallized poly(phenylene sulfide)

In this study, we report the melting behavior of poly(phenylene sulfide), PPS, which has been cold-crystallized from the rubbery amorphous state. We find that the crystallization kinetics are faster for cold-crystallized PPS than for melt-crystallized material, due to formation during quenching of a short-range ordered, but noncrystalline, structure. We observe that the endothermic response of cold-crystallized PPS at a large undercooling consists of a low temperature endotherm, followed by an exothermic region, and by the main higher melting endotherm. The lower melting peak temperature of cold-crystallized PPS increases as the crystallization temperature increases, but the main upper melting peak temperature remains almost the same. The size of the exothermic region is strongly related to the degree of undercooling, and must be taken into account in order properly to determine the degree of crystallinity of the material prior to the scan. When the crystallization time is varied, we see a systematic decrease in the size of the main endotherm, and an increase in size of the lower melting endotherm. This suggests that a portion of the main endothermic response is due to reorganization during the scan. Annealing will not only increase the degree of crystallinity but also improve the crystal perfection; therefore the ability of an annealed sample to reorganize decreases as the annealing time increases. However, an additional third melting peak is seen when a cold-crystallized sample is annealed at high temperature for a sufficiently long residence time. The existence of the third melting peak suggests that more than one kind of distribution of crystal perfection may occur when PPS has been cold-crystallized and subsequently annealed.     

Structural change of water by gelation of curdlan suspension

Phase transition of water restrained by curdlan suspension annealed at a temperature from 20 to 110°C was investigated by differential scanning calorimetry (DSC). The melting temperature of water restrained by annealed curdlan discontinuously decreased at around 60°C, while the amount of bound water calculated from enthalpy of melting increased at 60°C, regardless of water content. Using a highly sensitive DSC, curdlan suspension with various concentrations was studied. It was found that an endothermic transition was observed at ca. 58°C in a wide range of concentrations. The transition observed at 60°C is thermo-reversible and both temperature and transition enthalpy are constant even after gel formation. Well equilibrated suspension at a temperature lower than 60°C formed no gel.     

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.     

A new approach for the estimation of the melting enthalpy of metastable crystalline compounds using differential scanning calorimetry

This article focuses on the development of an innovative method, based on thermodynamic considerations and with the use of Differential Scanning Calorimetry (DSC), for the estimation of the melting enthalpy of crystalline compounds which are metastable near their melting temperature. The curves obtained, at various heating rates, are analysed in two steps. In the first step, the area of a zone generated by the melting endothermic peak is calculated following a specific method. In the second step, the melting enthalpy is extracted from this area through an enthalpy balance. This method is applied to both identified crystallographic forms, named form I and form II, respectively, of Etiracetam (UCB Pharma). The results show that the melting enthalpy of the stable form II compare well with the ones obtained using conventional methods. The curves of the metastable form I present thermal instabilities (partial solid–solid polymorphic transition and beta-recrystallization) near the form I melting peak leading to difficulties for a direct determination of the melting enthalpy by conventional methods. The proposed method is therefore very useful for the estimation of the form I melting enthalpy.     

Heating effect on the DSC melting curve of flaxseed oil

Flaxseed oil is rich in the alpha-linolenic acid. The effect of heating on the thermal properties of flaxseed oil extracted from flax seeds has been investigated. The flaxseed oils were heated at a certain temperature (75, 105, and 135 °C, respectively) for 48 h. The melting curve (from ?75 to 100 °C) of flaxseed oil was determined by differential scanning calorimetry (DSC) at intervals of 4 h. Three DSC parameters of exothermic event and endothermic event, namely, peak temperature (T _peak), enthalpy, and temperature range were determined. The initial flaxseed oil exhibited an exothermic peak, two endothermic peaks, and two endothermic shoulders between ?68 and ?5 °C in the melting profile. Heating temperature had a significant influence on the oxidative deterioration of flaxseed oil. The melting curve and parameters of flaxseed oil were almost not changed when flaxseed oil was heated at 75 °C. However, the endothermic peaks of melting curve decreased dramatically with the increasing of heating time when heating temperature was above 105 °C. There is almost no change of melting heat flow of flaxseed oil when heating time exceeded 32 h at 135 °C. The preliminary results suggest that the DSC melting profile can be used as a fast and direct way to assess the deterioration degree of flaxseed oil.     

The enantiotropic phase transition of antimony(III) oxide

Pure cubic senarmontite and pure orthorhombic valentite were characterised by XRD and infrared absorption spectroscopy, these techniques were also used to characterise heat treated samples. Senarmontite was shown to be the stable low temperature polymorph and valentite the stable high temperature form. The application of classical thermodynamics indicated the transition temperature to be 923K. Valentite was shown to exist below 923K but heat treatment below this temperature caused the metastable valentite to revert to senarmontite. DTA of antimony(III) oxide gave a single sharp endothermic peak at 913K, independent of the crystal modification, thus indicating that melting and phase transition were inseparable thermal events.     

DMSO对鸡蛋白溶菌酶溶液变性的影响

利用差示扫描量热(DSC)和温度调制差示扫描量热(MDSC)研究了鸡蛋白溶菌酶在纯水及二甲基亚砜(DMSO)/水混合溶剂中的热变性过程, 探讨了酶的浓度、扫描速率和共溶剂的含量对热变性行为的影响规律. 在纯水溶液中, 溶菌酶的变性焓(△Hm)随酶浓度的增大而增大. 而在DMSO/水混合溶剂中, 变性温度(Tm)随DMSO体积分数的增大向低温方向移动, 变性峰变低变宽; 当DMSO体积分数达到70%后, 热变性曲线变成了一条光滑的直线. 另外, 在纯水溶液中溶菌酶的MDSC图除了出现DSC中可观察到的主吸热峰(I)外, 在峰(I)的前面还出现一个小而对称的吸热峰(II), 并且当体系中有DMSO存在时也未能观察到此峰. 当溶菌酶浓度增大时, Tm(II)移向低温, △Hm(II)减小, Tm(I)与Tm(II)之间的距离变长. 吸热峰(II)的出现被认为是由于水溶液中溶菌酶二聚体的可逆离解造成的.     

Structural Studies on PEK/TPI Blends

Blends of poly(ether ketone) (PEK) with poly(terephthaloyl-imide) (a thermoplasticpolyimide, TPI) were studied by temperature-modulated DSC (TMDSC) and X-ray diffraction. Samples were prepared by compression moulding of the premixed materials at 400°C and quenched to prevent crystallisation.The amorphous blends showed a single glass transition but with a jump in the temperature value at 60 mass% of PEK, indicating limited miscibility of the system at both sides of the composition series in the quenched, glassy state. Two cold crystallisation peaks over the concentration range 30 to 70 mass% of PEK were observed, but only one for all other compositions. A single melting peak was observed in all systems.Blends crystallised from the glassy state showed eutectic behaviour with the presence of the crystals of both pure components. This is the first reported case of two semicrystalline polymers exhibiting eutectic co-crystallisation. The formation of eutectic crystals is proof of full miscibility of the two polymers in their liquid state, i.e. at a temperature of 400°C and above. Blends cooled from the melt at a cooling rate of 2 K min^–1 showed a single glass transition and an extended melting range.Crystallisation during a second melting run generally starts at a different temperature then during the first run indicating chemical changes occurred in the molten state. This change was also verified by an exothermic peak above the melting temperature using TMDSC.This revised version was published online in November 2005 with corrections to the Cover Date.     

Relationship between melting behavior and morphological changes of semicrystalline polymers

The present study on the case of poly(hexamethylene succinate) is to provide a basis for a better understanding of the subtle relationship between melting behavior and morphological changes of semicrystalline polymers. The melting behavior and morphological changes of poly(hexamethylene succinate) during both isothermal secondary crystallization and annealing processes were investigated by DSC and SAXS. DSC results showed that, with increasing crystallization time or annealing time, the melting endotherm continuously shifted to higher temperature, which suggested that some minor structural or morphological changes must occur. However, almost no changes at all on the crystal thickness were observed from SAXS measurements. The observed evidence confirmed that the increase in the melting temperature is not attributed to crystal thickening but crystal perfection. More exactly, the rearrangement and smoothing of tie molecules at the folding surface result in the reduction of the fold surface free energy, which dominantly contributes to the increase in the melting peak temperature. The origin of the new endothermic peak observed after annealing at elevated temperature was also discussed. TMDSC results indicated that the annealing peak resulted from the enthalpy relaxation and devitrification transition of rigid amorphous fraction formed by the driving force of thermodynamic nonequilibrium, rather than usually regarded as the melting of thin lamellae or imperfect crystals formed by annealing secondary crystallization.     

Role of solvents in stability of collagen

The influence of hydroxymethyl chain length of the solvents on collagen was established with conformational stability and thermal stability. Thermal stability of monomeric collagen and RTT fibres (rat tail tendon) treated with methanol, ethylene glycol (EG) and glycerol were reported using the melting temperature for helix-coil transition and the peak temperature for collagen-gelatin transition. Both melting temperature and peak temperature increases as the hydroxymethyl chain length increases. Conformational stability of collagen solution treated with lower and higher concentrations of methanol, ethylene glycol and glycerol indicates that aggregation of collagen molecule is more at higher concentrations of these solvents. The concentration dependence is greater for the increased number of OH groups. Since protein aggregation is associated with neuro degenerative diseases, aggregation of collagen molecule in the presence of solvents is of great importance for biomedical application.     

Influence of the Molecular Structure of Constituents and Liquid Phase Non-Ideality on the Viscosity of Deep Eutectic Solvents

Hydrophobic deep eutectic solvents (DES) have recently been used as green alternatives to conventional solvents in several applications. In addition to their tunable melting temperature, the viscosity of DES can be optimized by selecting the constituents and molar ratio. This study examined the viscosity of 14 eutectic systems formed by natural substances over a wide range of temperatures and compositions. The eutectic systems in this study were classified as ideal or non-ideal based on their solid–liquid equilibria (SLE) data found in the literature. The eutectic systems containing constituents with cyclohexyl rings were considerably more viscous than those containing linear or phenyl constituents. Moreover, the viscosity of non-ideal eutectic systems was higher than that of ideal eutectic systems because of the strong intermolecular interactions in the liquid solution. At temperatures considerably lower than the melting temperature of the pure constituents, non-ideal and ideal eutectic systems with cyclohexyl constituents exhibited considerably high viscosity, justifying the kinetic limitations in crystallization observed in these systems. Overall, understanding the correlation between the molecular structure of constituents, SLE, and the viscosity of the eutectic systems will help in designing new, low-viscosity DES.     

Thermally changing lattice distance of lamella in the hydrated solid of octadecyltrimethylammonium chloride

A temperature scanning small-angle X-ray scattering measurement was carried out for the hydrated solids of octadecyltrimethylammonium chloride (OTAC). A gradual change of the lattice spacing of lamella-like structure from 40 nm at 5 degrees C to 20 nm at 18 degrees C was observed in the melting process of the hydrated solid that was incubated at 4 degrees C for a period of 24 h in the aqueous solution, while little change of the lattice spacing of about 20 nm was observed in the same process of the hydrated solid that was incubated at 4 degrees C for a period about 10 min. This indicates structural changes of the hydrated solid during the incubation at 4 degrees C and in the melting process. Corresponding to the nanostructure changes, broad endothermic peaks were observed at temperatures from 13 to 22 degrees C for the former hydrated solid and at temperatures from 15 to 21 degrees C for the latter hydrated solid in difference scanning calorimetry measurements. The structure change at temperatures below 13 degrees C is considered to be athermal from the fact that no endothermic peak is observed there. Large dielectric dispersions at frequencies at about 10 kHz were observed for the suspensions of hydrated solids but not for the solutions of dissolved solids. It was found that the electric conductance of the hydrated solid suspensions was much lower than that of the solutions of dissolved solids. The observed electric properties indicate that an amount of the free chloride ion is very small and that the chloride ions binding to the ammonium groups are movable in the hydrated solids by responding to an applied electric field. The electric conductance of suspension of the hydrated solid being incubated at 4 degrees C for 10 min was 4 times as large as that of a suspension of the hydrated solid being incubated at the same temperature for 24 h. This indicates that the structural change of the OTAC hydrated solid at 4 degrees C is related to the chloride ion binding to the hydrated solid. The experimental results described above suggest that the lamella in the hydrated solid of OTAC is undulated and that the wavelength of undulation increases with the incubation at a temperature much lower than the melting temperature.     

Melting behavior of drawn polypropylene

The melting behavior of drawn, compression-molded isotactic polypropylene has been ex-amined in terms of the infuence of drawing conditions on the observed properties. Two endothermic peaks were observed on differential scanning calorimetry (DSC) for samples when high draw ratios and high heating rates were used during DSC tests. The peak at lower temperature is influenced by draw ratio, temperature, and rate, and exhibits a strong superheating effect. The species associated with this peak can partially recrystallize into another species associated with the peak at higher temprature during DSC measurements. The position of the peak at higher temperature depends only on draw ratio. It is proposed that the doubel-melting peaks at lower and higher temperature result from extremely thin quasi-amorphous or crystalline layers between microfibrils and the lamellar crystals within microfibrils, repectively. © 1993 John Wiley & Sons, Inc.     

Crystal formation of polyisobutylene induced at metallic interface

Polyisobutylene (PIB) is used as a nonpolar adhesive, but its mechanism of adhesion is still under discussion. Because the adhesion process of PIB requires no chemical reactions, the changes of physical properties induced at the interface play important roles in the mechanisms of the adhesion. PIB is known as an amorphous polymer, but can be crystallized by external stimulus such as stretching. Differential scanning calorimetry was carried out for thin films of PIB contacting with silver to investigate the interfacial properties and their contribution to the mechanism of adhesion. An endothermic peak appeared at the temperature which has been reported to be the melting point of PIB crystals, and this peak became clearer by thinning the PIB film. This thermal behavior can be explained by the crystal formation of PIB at the metallic interface. The formation of the crystals of PIB may contribute the good adhesive behavior of PIB. The rubbery amorphous phase of PIB with the glass transition temperature lower than room temperature can be spread on the surface, and the induced rigid crystalline phase prevent PIB from flowing. Copyright © 2011 John Wiley & Sons, Ltd.     

结晶／结晶共混体系PPS／PEEK中PPS组分的结晶熔融行为（Ⅱ）──熔融条件对退火样品的影响

结晶／结晶共混体系ＰＰＳ／ＰＥＥＫ中ＰＰＳ组分的结晶熔融行为（Ⅱ）──熔融条件对退火样品的影响麦堪成，许家瑞，梅震，曾汉民（广州中山大学材料科学研究所，广州，５１０２７５）关键词聚苯硫醚，聚醚醚酮，结晶熔融行为，熔融双峰聚苯硫醚（ＰＰＳ）的结晶熔融行...     

Preparation method of porous polymer materials by radiation technique and its application

The radiation polymerization of hydrophilic hydroxyethyl methacrylate monomer solution at temperatures below 0 °C leads to the formation of a porous structure in the polymers. The melting peak of the eutectic water–monomer composition at the eutectic point (above ?24 °C) could be distingushed and a glass transition temperature was observed at ?96 °C. The porous structure was developed after melting small ice pieces in the polymers after polymerization. The porous structure formed above 0 °C contained discontinuous pores and that formed below 0 °C had continuous pores leading to reticular structure. In a mixture of water –dioxane –monomer, the pore diameter decreased with increasing monomer concentration. Replacing dioxane with decane led to a maximum pore diameter at 70% monomer concentration. The pore diameter in 70% monomer concentration using water and dioxane was 1～4 µm, maximum activity in immobilized enzyme tablets was observed at this diameter. The porous structure was also varied by controlling the polymerization temperature. The durability of the immobilized enzyme tablets was demonstrated by the retention of high enzyme activities after repeated batch enzyme reactions. Copyright © 2001 John Wiley & Sons, Ltd.     

The effect of heating rate on the melting of polytetrafluoroethylene

When virgin polytetrafluoroethylene is heated at intermediate rates, two melting peaks are observed. As the heating rate is increased, the higher-temperature peak grows at the expense of the lower-temperature one without any significant change in the total heat of fusion. It is suggested that the higher-temperature peak represents a transition to a strained melt which subsequently changes to the more stable equilibrium melt. After recrystallization from the melt, there is only a single melting peak which occurs at a lower temperature than peaks for the virgin polymer. All of these transitions are subject to superheating.     

Process influences on the structure,piezoelectric, and gas‐barrier properties of PVDF‐TrFE copolymer

The influence of annealing between the Curie transition and the melting point of solvent cast polyvinylidene fluoride trifluoroethylene copolymer films on the crystalline structure, mechanical and electrical properties, and oxygen permeability is investigated. Annealing leads to remarkable changes in the structure and properties of the copolymer, within the first four hours of treatment, and with kinetics depending on the temperature. The crystallinity increases by 19% (relative), resulting in a 10 K increase in the Curie transition, a 4 K increase of the melting temperature and a 2 K decrease in the glass transition temperature. A crystalline phase transition from the paraelectric α‐phase to the ferroelectric β‐phase is also evidenced using in‐situ X‐ray diffraction. The elastic modulus is found to increase by more than three‐fold at room temperature and the loss peak at the glass transition is considerably reduced. The piezoelectric coefficient is found to increase by 40% and the dielectric properties are significantly changed. The most remarkable influence is the ten‐fold reduction of the oxygen permeability, with a drastic reduction of the activation energy for oxygen transport. The improvement in oxygen barrier properties of the annealed copolymer is attributed to the restricted mobility of oxygen molecules in the semicrystalline polymer with nanometer sized crystallites. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 496–506     

Thermostimulated processes in starch-bis(hydroxymethyl)propionic acid mixtures subjected to high-pressure plastic deformation

DSC and thermogravimetry are employed to study starch, bis(hydroxymethyl)propionic acid, and their mixtures of different compositions subjected to plastic deformation under pressures of 1 and 3 GPa. An endothermic peak with an enthalpy of 300 J/g observed in the thermogram of starch is most likely related to the breakage of hydrogen bonds. Treatment under pressure greatly reduces the enthalpy of this peak. For initial mixtures, the enthalpies of the endothermic peak of starch and the endothermic peak of melting of the acid are lower than those for individual components because of chemical interaction occurring between the mixture components during heating. The high-pressure treatment makes the decrease in the enthalpy of endothermic processes much more pronounced for both components.