熔体中转变对四氟乙烯-六氟丙烯共聚物熔体流变性能的影响

用差示扫描量热仪与扭辨分析检出了四氟乙烯-六氟丙烯共聚物熔体中的转变。为了阐明转变的性质,用毛细管流变仪和微型平板粘度计研究了其熔体粘度的温度依赖性。发现,当温度上升到转变温度附近,熔体粘度先升高,然后下降,其粘度行为与热致型小分子液晶相似。但用偏光显微镜观察FEP共聚物熔体,未发现双折射现象。根据实验结果对转变的机制提出了解释。     

全氟乙丙烯共聚物的超分子结构与结晶性质

本文用激光小角光散射技术研究了全氟乙丙烯共聚物的超分子结构。发现它的结晶形态主要依赖于熔体温度。当共聚物从315—320℃以下降温结晶时得到无规排列晶片所固有的圆对称散射图象。从315—320℃以上温度冷却时,形成球晶结构。这些结果与DSC测得的结晶性质之间存在对应关系。结晶性质在上述温度区间上下也有突变。这些结果可用熔体中存在有序微区来解释,它在结晶时起晶核的作用,在温度高于315—320℃时消失。     

嵌段共聚物PVCH-b-PE-b-PVCH的多重结晶行为

通过采用差示扫描量热仪(DSC)主要研究了结晶-非晶嵌段共聚物聚乙烯基环己烷-b-聚乙烯-聚乙烯基环己烷(PVCH-b-PE-b-PVCH)溶液结晶样品的熔融与非等温再结晶过程.探讨了溶液结晶样品中微相分离结构的形成对嵌段共聚物受限结晶的影响,并发现样品在熔融后的非等温结晶过程中出现了多重结晶峰.通过对嵌段共聚物有序、...     

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     

DSC study of phase transition of anhydrous phospholipid DHPC and influence of water content

The phase transition behavior of 1,2-di-n-heptadecanoyl phosphatidylcholme (DHPC)with and without water has been studied by use of differential scanning calorimetry It was found by experiment that the glass transition occurred at first during the first heating of a sample of DHPC without water and then the sample underwent melting as an ordinary crystal.Therefore the sample of DHPC without water was a glassy crystal However,the DHPC sample crystallizing from melt was an ordinary crystal From the relationship between the total melting enthalpy Qf of freezable water and the water content h,it was concluded that the water contained in the DHPC samples might exist in three states recognizable thermodynamically.The water in the first state was an unfreezable water It was the water bound directly with the head groups of the phospholipid,i.e.the primary hydration water Every head group might bind seven such molecules of water.The water in the second state was the secondary hydration water,us melt ing point was     

X‐ray studies on the double melting behavior of poly(butylene terephthalate)

The double melting behavior of poly(butylene terephthalate) (PBT) was studied with differential scanning calorimetry (DSC) and wide‐angle X‐ray analysis. DSC melting curves of melt‐crystallized PBT samples, which we prepared by cooling from the melt (250 °C) at various cooling rates, showed two endothermic peaks and an exothermic peak located between these melting peaks. The cooling rate effect on these peaks was investigated. The melt‐crystallized PBT sample cooled at 24 K min^?1 was heated at a rate of 1 K min^?1, and its diffraction patterns were obtained successively at a rate of one pattern per minute with an X‐ray measurement system equipped with a position‐sensitive proportional counter. The diffraction pattern did not change in the melting process, except for the change in its peak height. This suggests that the double melting behavior does not originate from a change in the crystal structure. The temperature dependence of the diffraction intensity was obtained from the diffraction patterns. With increasing temperature, the intensity decreased gradually in the low‐temperature region and then increased distinctly before a steep decrease due to the final melting. In other words, the temperature‐dependence curve of the diffraction intensity showed a peak that is interpreted as proof of the recrystallization in the melting process. The peak temperature was 216 °C. The temperature‐dependence curve of the enthalpy change obtained by the integration of the DSC curve almost coincided with that of the diffraction intensity. The double melting behavior in the heating process of PBT is concluded to originate from the increase of crystallinity, that is, recrystallization. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2005–2015, 2001     

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.     

差示扫描量热法研究全氟乙丙烯共聚物的化学组成分布

本文从共聚物结晶的两种模型出发,总结了共聚物的熔点、熔化热与组成的关系,提出了从熔化曲线得到组成分布的方法。然后以全氟乙丙烯共聚物作为研究对象,用实验方法对上述关系进行了验证,再用来测定乳液及悬浮聚合试样的组成分布,得到了初步结果。发现乳液聚合试样的组成分布要比悬浮聚合为窄;对于平均组成相近的试样,组成分布宽的,其应力开裂性能较差。     

聚全氟乙丙烯的转变

<正> 聚全氟乙丙烯(F_s-46)是四氟乙烯(TFE)与六氟丙烯(HFP)的共聚物。McCrum用扭摆法测得α和β转变与HFP含量有关。Eby等用超声波法发现γ松弛也与HFP含量有关。 本文用动态力学法发现HFP含量对α、β、γ转变都有影响。用介电方法得到了α、γ、δ三个转变峰。δ峰属于端基运动,Eby等认为是—CF_2H基运动;我们从红外光谱图中检测到—COOH基的存在,认为与—COOH基运动也有关。     

Phase behavior of crystalline blends of poly(tetrafluoroethylene) and of random fluorinated copolymers of tetrafluoroethylene

In the present article, we investigate by differential scanning calorimetry (DSC) the thermal behavior (melting, crystallization, and crystal–crystal transitions) far from equilibrium of blends constituted of two crystalline polymers. In particular, the following blends are examined: PTFE–PFMVE, PTFE–FEP, and FEP–PFMVE where PTFE is poly(tetrafluoroethylene), PFMVE is poly(tetrafluoroethylene‐co‐perfluoromethylvinylether), and FEP is poly(tetrafluoroethylene‐co‐hexafluoropropylene). The two last ones are random tetrafluoroethylene copolymers with small amounts of comonomer. Our results indicate that, under the experimental investigated conditions, the blends containing PTFE do not give cocrystallization on cooling from the melt, although under very rapid crystallization conditions, quenching, the presence of the copolymer would seem to slightly influence PTFE crystallization (lower peak temperatures are observed for the crystalline transitions and the melting with respect to those of the neat homopolymer). The behavior of the FEP–PFMVE blend is completely different; in fact, our results indicate the occurrence of cocrystallization, then miscibility in the crystalline phase, for almost all compositions and all investigated experimental conditions. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 679–689, 1999     

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     

Mechanism of polymorph selection during crystallization of random butene-1/ethylene copolymer

Starting from an initial sample of butene-1/ethylene copolymer with stable form I’, we examined the nucleation of different crystalline polymorphs (here metastable form II and stable form I’) at different isothermal crystallization temperatures after being melted at different melt temperature (T _melt). When T _melt was just above the melting temperature (T _m) of the crystallites, self-seeding took place. There, residue crystallites served as nuclei leading to the crystallization of the same crystalline phase. When T _melt was a few degrees above the T _m, self-seeding was disabled due to complete melting of the initial crystals. Upon crystallization, the selection of the different polymorphs in this random copolymer was found to depend on an interplay between the domain size of segregated long crystallizable sequences and the size and energy barrier of the critical nucleus of the respective crystalline forms. Our results provide a clear understanding of the polymorphs selection during crystallization of a random copolymer as well as homo-polymers under confinement.     

Detection of obscured glass transitions by QiDSC

Determination of compatibility in the amorphous phase for a two component blend is usually accomplished by analyzing for whether one notes one or two glass transitions. This can be complicated when one of the components is semicrystalline and its melting peak obscures the second glass transition. Quasi-isothermal differential scanning calorimetry (QiDSC) can be used to detect an obscured glass transition by allowing the semicrystalline component to melt and relax revealing the underlying glass transition of the other component. QiDSC is accomplished by performing a modulated temperature DSC experiment at a particular temperature and step ramping through the transitions of interest. For this study two systems are investigated. The first system is a model system based on a blend of polystyrene (PS) and a copolymer of vinylidene fluoride and hexafluoropropylene, P(VF₂/HFP). The glass transition for the PS occurs at the same temperature as the melting point for the fluoro-copolymer. The second system is a fluoro-copolymer/acrylic dried latex. In both cases the hidden glass transition can be noted in the reversing heat capacity of the QiDSC analysis.     

Temperature dependent wide angle X-ray diffraction studies on the crystalline transition in water saturated and dry polyamide 6/66 copolymer

The crystalline transition in water saturated and dry polyamide 6/66 copolymer (ratio 4:1 by mol) was investigated by means of temperature dependent wide angle X-ray diffraction (WAXD). The polyamide 6/66 copolymer (PA6/66) exhibits a poorly developed α-phase at room temperature after being cooled down from the melt. Uptake of water improves the crystalline structure by mobilizing the amorphous phase, thus facilitating chain relaxation in the crystalline phase. Upon heating, the effect of the glass transition on the change of the crystal lattice constants is seen. Further heating leads to a gradual crystalline transition from the α-phase to a pseudohexagonal phase. Different from the behavior in polyamide 6, this pseudohexagonal phase does not further transform to a high temperature α′-phase before melting. The delay of the crystalline transition in the water saturated PA6/66--as compared to the dry material can be understood as a result of the better ordered crystalline structure in it.     

Specific reversible melting of polymers

Temperature‐modulated differential scanning calorimetry can detect a certain amount of reversible latent heat in flexible macromolecules. In short, one can identify a reversible melting in such polymers earlier thought to exhibit only fully irreversible crystallization and melting. Details of the reversible melting of isotactic polypropylene and ethylene‐1‐octene copolymers of low and medium densities have newly been measured and linked to the crystallization, annealing, or melting temperature. It is possible to assign the experimental reversibility of melting to specific crystal fractions that ultimately melt irreversibly at higher temperatures; that is, it is suggested that reversible melting mainly occurs only between the temperatures of their formation and their zero‐entropy‐production melting temperature, at which they change to a melt of the same degree of metastability. This is supported by the almost complete absence of reversibility below the temperature of crystal formation and the observation of a distinct relationship between the amount of irreversibly by annealing reorganized material and reversibility in the case of isotactic polypropylene. A given crystal fraction, characterized by its formation temperature and zero‐entropy‐production melting temperature, has a specific reversibility of the melt‐to‐crystal transition, which is represented by the ratio of the reversible latent heat to the total enthalpy change when the crystal fraction of interest ultimately melts. This specific reversibility is, for ethylene‐1‐octene copolymers, at least 25% at temperatures in the primary crystallization range, and this indicates that the reversible contribution to the total of the melting processes is much larger than expected from simple calculations by the excess apparent reversible heat capacity being referred to the heat of fusion of the polymer, as is commonly done. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2039–2051, 2003     

Conformational transition of the copolymer of maleic acid and styrene in aqueous solution. II

The temperature dependence of the conformational transition from a compact to expanded-coil form of the alternating copolymer of maleic acid with styrene in aqueous 0.03M NaCl solution was studied at 15 to 40°C by pH titration, optical titration, and calorimetric measurements. The pH titration curve scarcely changed with temperature. The degree of dissociation of primary carboxyl groups in the copolymer at the midpoint of the conformational transition, determined by the optical titration, was almost independent of temperature. The standard enthalpy change of the transition at 25°C estimated from the calorimetric data was 0.36₃ kcal/mole, which may mean that the difference between the standard free energy changes of the transition at 20°C and 40°C does not exceed experimental error. The enthalpy change agrees well with the heat of transfer of benzene from the hydrophobic to the aqueous medium, and thus the compact form of the copolymer is considered to be stabilized by the hydrophobic interaction between the phenyl residues in the interior of the molecule.     

Multiple melting behavior of poly(butylene succinate)

The multiple melting behavior of poly(butylenes succinate) (PBS) isothermally crystallized from the melt was investigated using differential scanning calorimetry (DSC), temperature modulated DSC (MDSC) and polarized optical microscopy. PBS exhibits at most four melting endotherms (denoted as T_m1, T_m2, T_m3, and T_m4 from high to low temperatures) and a crystallization exotherm (denoted as T_re) in the DSC heating trace. Multiple melting endotherms were observed even at high heating rates. The origins of each endothermal and exothermal peak were discussed in detail. It is suggested that: (i) the crystallization exothermic peak, T_re, relates to the recrystallization of the melt of the crystallites with lower thermal stability; (ii) the T_m1 is ascribed to the melting of crystallites formed through recrystallization; (iii) two crystal populations with different thermal stability are responsible for the T_m2 and T_m3; (iv) the T_m4, which is the annealing peak, represents the transition of the rigid amorphous fraction (RAF) from solid-like RAF into liquid-like amorphous fraction.     

Melting behavior of a poly(ether‐block‐amide) copolymer melt‐crystallized under quiescent,isothermal conditions

Segmented poly(ether‐block‐amide) copolymers are typically known as polyamide‐based thermoplastic elastomers consisting of hard, crystallizable polyamide block and flexible, amorphous polyether block. The melting characteristics of a poly(ether‐block‐amide) copolymer melt‐crystallized under various quiescent, isothermal conditions were calorimetrically investigated using differential scanning calorimetry (DSC). For such crystallized copolymer samples, their crystalline structures under ambient condition and the structural evolutions upon heating from ambient to complete melting were characterized using ambient and variable‐temperature wide‐angle X‐ray diffractometry (WAXD), respectively. It was observed that dependent of specific crystallization conditions, the copolymer samples exhibited one, two, or three melting endotherms. The ambient WAXD results indicated that all melt‐crystallized copolymer samples only exhibited γ‐form crystals associated with the hexagonal habits of the polyamide homopolymer, whereas variable‐temperature WAXD data suggested that upon heating from ambient, a melt‐crystallized copolymer might exhibit so‐called Brill transition before complete melting. Based on various DSC and variable‐temperature WAXD experimental results obtained in this study, the applicability of different melting mechanisms that might be responsible for multiple melting characteristics of various crystallized PEBA copolymer samples were discussed. It was postulated that the low (T _m1) endotherm was primarily because of the disruption of less thermally stable, short‐range ordered structure of amorphous polyamide segments of the copolymer, which was only formed after the completion of primary crystallization via so‐called annealing effects. The intermediate (T_m2) and high (T_m3) endotherms were attributed to the melting of primary crystals within polyamide crystalline microdomains of the copolymer. The appearance of these two melting endotherms might be somehow complicated by thermally induced Brill transition. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2035–2046, 2008     

Crystallization and melting of isotactic polypropylene crystallized from quiescent melt and stress‐induced localized melt

Temperature dependency of crystalline lamellar thickness during crystallization and subsequent melting in isotactic polypropylene crystallized from both quiescent molten state and stress‐induced localized melt was investigated using small angle X‐ray scattering technique. Both cases yield well‐defined crystallization lines where inverse lamellar thickness is linearly dependent on crystallization temperature with the stretching‐induced crystallization line shifted slightly to smaller thickness direction than the isothermal crystallization one indicating both crystallization processes being mediated a mesomorphic phase. However, crystallites obtained via different routes (quiescent melt or stress‐induced localized melt) show different melting behaviors. The one from isothermal crystallization melted directly without significant changing in lamellar thickness yielding well‐defined melting line whereas stress‐induced crystallites followed a recrystallization line. Such results can be associated with the different extent of stabilization of crystallites obtained through different crystallization routes. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 957–963     

Thermal behavior of co[poly(ethylene terephthalate)-p-oxybenzoate]

In a previous investigation of a co[poly(ethylene terephthalate)-p-oxybenzoate] containing 30 mole % oxybenzoate units (referred to as T2/30), Krigbaum and Salaris identified the endotherm at t_III = 244°C as a nematic → isotropic transition. Subsequent investigation revealed that the t_III endotherm disappeared if the polymer is heated to the isotropic melt or dissolved and reprecipitated. The loss of the t_III transition might be due to molecular weight reduction, sequence randomization, or erroneous identification of the transition. Viscosity and high-field NMR data eliminate the first two explanations. Annealing studies at temperatures higher than those of the earlier work demonstrate that the t_III transition, in fact, represents the melting of crystallites formed during a high-temperature annealing operation. Moreover, we cannot determine the thermodynamic melting temperature of the copolymer from solid-state annealing studies. Hence, the principal objective of the study of Krigbaum and Salaris, to compare the enthalpies and entropies of the nematic → isotropic and crystal → isotropic transitions, must be fulfilled in future work.