Thermodynamics of Melting of Polymers at High Pressures

Equations that allow determining the melting point, change in volume, and other thermodynamic parameters of melting of polymers at high pressure are proposed based on the statistical-thermodynamic theory of melting of polymers. The good agreement of the predictions of the theory and the experimental data for polyethylene is demonstrated.__________Translated from Khimicheskie Volokna, No. 1, pp. 29–32, January–February, 2005.     

Some aspects of the thermodynamics of melting and glass transition of polymers

Equations are reported for calculating the melting point and glass transition temperature of polymers at high pressure. The average value of the ratio of the glass transition temperature to the melting point is approximately 0.65. With an increase in the pressure, the ratio of the glass transition temperature to the melting point decreases weakly.     

Melting and crystallization behavior of metallic alloy in the composites with polyacrylate

Composites of poly(n‐buthyl acrylate) (PnBA) and eutectic metallic alloy composed of Bi, In, and Sn were prepared by mechanically mixing them above the melting point of the metallic alloy, and glass transition temperature of PnBA. The heating curves of differential scanning calorimetry (DSC) of the composite of PnBA and the metallic alloy showed an endothermic peak below the melting point of the metallic alloy without polymers, which indicated the formation of the interfacial phases of the metallic alloy with a lower melting point. The exothermic peaks of the cooling curves were broadened and shifted to the temperature lower than the melting point of the metallic alloy without polymers, which suggests that the crystallization of the metallic alloy was suppressed by the interaction. The mechanism of lowering the melting points and suppression of the crystallization was discussed based on the results of DSC, transmitting electron microscopy, and X‐ray diffractometry. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Nature of the thermal stability of polyheteroarylenes

Conclusions The dependence between structure and thermal stability of polyheteroarylenes has been examined.It has been shown that the thermal stability of polymers depends on two basic factors — an energy factor, determined by the breaking energy of skeletal bonds in the polymer chain, and an entropy factor, which characterizes the probability of effecting the elementary acts of chemical reactions and is expressed via the mobility of the macromolecules.Symmetry of the monomeric units in a chain, regularity in the macromolecules, the absence of bulky substituents, and a high degree of crystallinity aid in reducing the molecular mobility of heterocycloaromatic polymers and in increasing their thermal stability.Analysis of the factors which aid in thermal stability has made it possible to arrive at the conclusion that the polymer glass transition point or melting point can serve as a criterion of the thermal stability of polyheteroarylenes.Published in discussion order.Translated from Khimicheskie Volokna, No. 4, pp. 22–26, July–August, 1987.     

Temperature dependence of the thermal expansivity in polymer

The effects of temperature on the specific volumes and thermal expansivities for a range of amorphous polymers, above and below the glass transition temperature, are treated on the basis of the physical properties of polymers. The results are found to be in good agreement with observed data. The analysis of the results shows that the temperature derivative of the zeropressure thermal expansivity of the liquid polymer increases with increasing temperature. The change in the thermal expansivity, Δα = α_OL ? α_OG, decreases with increasing temperature.     

均相玻璃态高分子中溶剂扩散系数的数学模型

以自由体积理论为基础 ,提出改良的玻璃态高分子中溶剂扩散系数的数学模型 .模型推导过程中考虑了溶剂可塑化效应对高分子凝聚态的影响 ,并以明确的物理概念计算玻璃态聚合物的自由体积 .对橡胶态适用的自由体积参数在此模型中保持有效 ,所引入的表达溶剂可塑化效应的唯一参数 β可以通过计算玻璃化温度来确定 .所以 ,本模型中无可调节参数存在 ,具有完全可预测性 .以芳香族溶剂苯、甲苯、乙苯在玻璃态聚苯乙烯和聚甲基丙烯酸甲酯中的扩散系数为例对模型进行验证 ,理论计算结果和实验值取得良好一致     

Towards an understanding of the heat distortion temperature of thermoplastics

A systematic understanding of the heat distortion temperature (HDT) of amorphous and semi-crystalline polymers is possible through a direct correlation with the modulustemperature behavior. For amorphous polymers, the precipitous drop in modulus at the glass transition temperature makes the HDT a well-defined, reproducible and predictable property. Furthermore, the addition of reinforcing fillers has a negligible effect on the HDT of the amorphous polymer. For semi-crystalline polymers, however, the exact opposite may hold true. The modulus exhibits a “plateau” region between the glass transition and the melting transition. Hence the HDT often is difficult to predict, is sensitive to thermal history and may be greatly increased through the addition of fillers. More importantly, the HDT may not be an accurate measure of the upper use temperature for semi-crystalline polymers in load bearing situations since considerable stiffness may still be retained even upon exceeding the HDT.     

Compatibility studies of polystyrene–polybutadiene blends by thermal analysis

In order to examine the behavior of incompatible blends of polystyrene and polybutadiene, the glass transition temperature, the melting point, and the specific heat increment at the glass transition temperature for atactic polystyrene (a-PS), isotactic polystyrene (i-PS), polybutadiene (PBD), and blends of a-PS/PBD and i-PS/PBD were determined by use of a differential scanning calorimeter. Blends were prepared by solution casting, freeze-drying, and milling. Weight fractions of polystyrene in the blends ranged from 0.95 to 0.05. The glass transition temperature of polystyrene changed with weight fraction in the blends, and with blending preparation methods; the glass transition temperature of polybutadiene remained essentially unchanged. The specific heat increment at the glass transition temperature of PBD decreases linearly with increasing proportions of PS in the PS/PBD blend for the broad and narrow molecular weight distribution polybutadience polymers, whereas the specific heat increment for PS did not decrease with increasing proportions of PBD in the PS/PBD blend. These results suggest that the polybutadiene dissolves more in the polystyrene phase than does the polystyrene in the polybutadiene phase.     

The breaking strength of perfect polymer fibers

The breaking strength, strain at break, and work to rupture of perfect fibers prepared with polymers of finite molecular weight are calculated by treating the perfect fiber as a stressed crystal undergoing a crystal-melt phase transition. In this view, a tensile load destabilizes the crystal and depresses its melting point. When the load is sufficient to lower the melting temperature to the ambient condition the fiber melts—i.e., fails. The theoretical equations (extremely simple) are applied to several common polymer fibers. The maximum tensile strength of polyethylene, for example, is calculated to be 7 to 9 GPa, in good agreement with current experimental results.     

Influence of branching on the thermal behavior of poly(butylene isophthalate)

The thermal behavior of linear and randomly branched poly(butylene isophthalate) samples was investigated by thermogravimetric analysis and differential scanning calorimetry. As to the thermal stability, it was found to be good and similar for all the samples. The thermal analysis carried out using DSC technique showed that the melting temperature of the polymers decreased with increasing branching unit content, although the glass‐transition temperature was practically not affected by ramifications. The multiple endotherms typical of linear PBI were also observed in branched samples and were found to be influenced both by temperature and degree of branching. By applying the Hoffman‐Weeks' method, the equilibrium melting temperatures of the polymers were obtained. The presence of a crystal‐amorphous interphase was evidenced only for the branched samples and the interphase amount was found to increase as the branching unit content was increased. Isothermal melt crystallization kinetics was analyzed according to Avrami's treatment. The introduction of branching points was found to decrease the overall crystallization rate of poly(butylene isophthalate). Values of Avrami's exponent n close to 3 were obtained for all the samples, in agreement with a crystallization process originating from predetermined nuclei and characterized by three dimensional spherulitic growth. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2001–2010, 2002; DOI 10.1002/app.10517     

The effect of hydrostatic pressure on the visco-elastic properties of polymers

This paper describes an experimental study of the effect of hydrostatic pressure on the visco-elastic properties of some common thermoplastic polymers, and natural rubber. A torsion pendulum was constructed to operate inside a thick walled cylindrical pressure vessel. The pressure could be varied from atmospheric to 1500 atmospheres and the temperature from −20°C to 120°C. The pressure medium was nitrogen gas but in some cases the polymer specimen was surrounded with a hydrocarbon oil to prevent the nitrogen from coming into contact with the polymer. The results obtained show that the glass transition temperatures of rigid polymers and rubber, are raised with pressure by amounts which vary between 15 and 30°C per thousand atmospheres depending upon the material. The values of this shift, for individual polymers, are in agreement with quasi-theoretical predictions based on the ‘Free-Volume’ theory of the glass transition temperature; they also agree with experimental studies of a related nature, by other workers. The experiments have also revealed a new phenomenon with pife: this polymer is rapidly plasticized by nitrogen at pressures of a few hundred atmospheres.     

Crystallization of syndiotactic polystyrene

The crystallization of syndiotactic polystyrene (sPS) was studied over the temperature range from the glass transition (T_g) to the melting point (T_m). Using transmission electron and optical microscopies, the different lamellar and spherulitic microstructures which were formed over this temperature range were correlated with the measured crystallization kinetics. The rate of crystallization was measured isothermally using thin samples in a differential scanning calorimeter. The low temperature transformations were achieved by quenching first to the amorphous state, then reheating. The experimental measurements at both high and low temperatures of transformation could be closely fitted to the predicted rate constant. It was found that to estimate the crystallization parameters most accurately, the data must be fitted simultaneously at high and low temperatures and a relatively high value for the equilibrium melting temperature (561 K) must be used. The non-isothermal crystallization kinetics have been predicted from the isothermal experiments using Nakamura's model in a manner similar to earlier work on other polymers.     

Characterization of the thermoresponsive shape‐memory effect in poly(ether ether ketone) (PEEK)

We conducted a systematic experimental investigation to characterize the shape‐memory effect in a commercial poly(ether ether ketone) (PEEK), which is a very important high‐temperature polymers at present. The focus was on the influence of the programming conditions and heating temperature for recovery on the shape‐recovery ratio (R_r). We concluded that PEEK is not only an important engineering polymer as it is traditionally known but is also an excellent high‐temperature shape‐memory polymer. For a residual programming strain of 30%, the maximum R_r was about 90%. It was revealed that it was practically feasible to program PEEK at room temperature and to lower the recovery temperature from its melting temperature range to around its glass‐transition temperature. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39844.     

Evidence of a liquid-liquid-transition in n-Tetracosane the behavior of the specific volume

Summary Dilatometric measurements of n-Tetracosane from 330 K up to 420 K are reported. These experiments show evidence for a small kink in the specific volume — temperature curve. The correct statistical interpretation of the measured data yield a discret transition temperature above the melting point at about 378 K in close agreement with the results from other experimental methods.Dedicated to Prof. H.-J. Cantow on the occasion of his 60th birthday     

Synthesis of ethylene dichloride-based polysulfide polymers: investigation of polymerization yield and effect of sulfur content on solubility and flexibility

In this work, a series of polysulfide polymers were synthesized using organic monomer (ethylene dichloride) and sodium-based aqueous monomers via interfacial polymerization. The structural characteristics of aqueous monomers and synthesized polysulfide polymers were identified by Fourier transform infrared (FT-IR), Raman, and ultraviolet–visible-near infrared (UV–VIS-NIR) spectroscopies. The Optimum temperature of polymerization was obtained at 75°C. Benzyltriethylammonium chloride (BTEACl) and tetrabutylammonium bromide (TBAB) were used as phase transfer catalysts (PTC) where BTEACl showed better performance regarding the polymerization yield. Moreover, adding ethanol to polymerization media increased the polymerization yield significantly. The results showed that along with increasing sulfur in the structure of polymers, solubility and flexibility were increased whereas it decreased the hardness, melting point (T_m ) and glass transition temperature (T_g ) of obtained polymers.     

Dynamic mechanical and thermal properties of fire-retardant polypropylene

A study of the effect of a series of fire retardants and antimony oxide upon the dynamic mechanical and thermal properties of polypropylene suggests three categories. (1) “Inert Fillers”—These raise the elastis modulus and the heat distortion temperature of polypropylene without shifting its glass transition temperature. The melting point of polypropylene is only depressed by 1–3°C, the heat of fusion and the percentage of crystallinity of polypropylene in these composites is ～10 percent lower at additive concentrations of ～30 percent. Very poor interaction exists between the additive and the thermo-plastic which apparently exist in two separate phases. (2) “Chain Stiffener”—These raise the elastic modulus (～25 percent) and the glass transition (～11°C) for polypropylene; the melting point of polypropylene in the composite is lowered by ～6°C indicative of good interaction between the additive and polypropylene. (3) “Plasticizer”—These lower the room temperature elastic modulus (～20 percent) and the glass transition temperature (～11°C) of polypropylene; the melting point of polypropylene in the composite is depressed by ～10°C indicating good interaction. The efficacy of the “plasticizer” and “chain stiffener” are attributed partially to melting of polypropylene at the processing temperature.     

Glass Formation in the ZrF4–LaF3–BaF2–NaF System

The glass formation in the ternary ZrF₄–LaF₃–BaF₂and quaternary ZrF₄–LaF₃–BaF₂–NaF systems at a zirconium fluoride content of 50–60 mol % is investigated. The glass formation region in the ternary system has the shape of a petal and lies along the LaF₃–BaZr₂F₁₀join. The glass formation regions in the quaternary system are either localized or continuous depending on the zirconium fluoride content. The glass transition temperatures T _gfall in the range 180–290°C, and the temperatures of the onset of crystallization lie in the range 250–340°C. Glasses crystallize in one, two, or three stages. The melting temperature varies in the range from 390 to 650°C. The microhardness of glasses is measured. The compositions of the most stable glasses are determined.     

Thermal and electrical properties of polyimides containing tricyclic fused rings

Polyimides containing a series of tricyclic fused rings were synthesized by polymerization of the tricyclic diamines with aromatic tetracarboxylic acid dianhydrides. The thermal stability of a series of the polymers increased in the order of the thianthrene (SDP) containing polymers < the phenoxatiin (OSP) containing polymers < the dibenzo-p-dioxin (ODP) containing polymers. The polymers derived from 2,8-oriented tricyclic diamines showed somewhat lower glass transition temperature than those from 2,7-oriented diamines. The specific resistivity of the polymers decreased in the order of the SDP containing polymers > ODP containing polymers > OSP containing polymers. The kink temperatures in the temperature dependence curves of specific resistivity were in good agreement with the glass transition temperatures. The photoconductive properties of the polymers were measured using a surface type cell method.     

Flexibility and phase transitions of polymers

It is shown, that both the mobility of polymers as well their transition temperatures (glass transition and crystallization) depend on the “flexibility” of simple bonds, i.e., on their ability to promote by energetically stimulated rotations conformational changes to release stresses. The polymer class specific interdependence between melting temperature, T_m and glass temperature, T_g, suggests that the “flexibility” of polymers depends additionally on the probability of interactions between sequences of polymer chains. Interaction between polymer chain sequences controls at the same time the ordering necessary for crystallization. Characteristic of polymers is thus the dependence of both transition temperatures on the “mass/'flexible bond” of the monomeric (repeating) unit, μ/ρ. This experimentally observed polymer class specific behavior is reflecting the similar probabilities of interaction within a given class of polymers. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 1590–1599, 2003     

Synthesis and characterization of copolymers of poly(ethylene terephthalate) and cyclohexane dimethanol in a semibatch reactor (including the process model)

Although poly(ethylene terephthalate) (PET) has excellent basic properties, this polymer tends to crystallize rapidly and has a rather high melting temperature, a low glass‐transition temperature, and low impact on notched articles for some potential applications. Copolymerization is a reasonable method for improving the properties of PET. 1,4‐cyclohexane dimethanol (CHDM) is one of the most important comonomers for PET. In this research, PET and PET copolymers containing 5–30% CHDM were prepared from comonomer mixtures by two‐step melt polycondensation. The copolymers were synthesized in a home‐made laboratory setup. The first synthesis step was conducted under pressure, and the second was performed in vacuo at a high temperature (230–290°C). The microstructure of the synthesized copolymers was studied with Fourier transform infrared and nuclear magnetic resonance. The comonomer content in the polymer chain was determined from the nuclear magnetic resonance spectrum. The presence of the comonomer in the copolymer chain was random. Differential scanning calorimetry was used to study the thermal properties of the copolymers to detect changes in the polymer properties. CHDM reduced the heat of fusion and melting and glass‐transition temperatures of the PET copolymers. Process modeling was performed with mass balances of different functional groups and species. Equations of mass balances were integrated numerically. Numerical simulation and experimental results were in very good agreement. By modeling, the effects of the reaction temperature and feed molar ratio on the conversion and formation of diethylene glycol were studied. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009