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     

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     

Analysis of reversible melting in polytetrafluoroethylene

Summary The reversibility of crystallization and melting of polytetrafluoroethylene (PTFE) has been investigated as function of crystallization conditions and temperature by temperature-modulated differential scanning calorimetry (TMDSC). The total and average specific reversibility of the melt-crystallized PTFE is considerably larger than in case of as-polymerized powder. This experimental observation must be attributed to different coupling between crystallized sequences of the molecules within the globally semi-crystalline superstructure. The crystallinity of as-polymerized PTFE is close to 100%, and the crystals melt in a narrow temperature interval close to the equilibrium melting temperature. Melt-crystallized PTFE, in turn, shows a crystallinity of about only 40% and melts at lower temperatures. The morphology of the melt-crystallized PTFE allows molecule segments to melt and crystallize reversibly as a function of temperature. The extended-chain conformation, evident in as-polymerized powder, inhibits reversible melting due to required molecular nucleation after complete melting of a molecule. The experimental findings are discussed within the framework of a similar investigation on polyethylene of different crystal morphology and support both the concepts of lateral-surface activity and molecular nucleation.     

MULTIPLE MELTING TRANSITIONS IN POLYURETHANE-UREA ELASTOMERS UNDER EXTENSION

Two melting transitions were observed in linear segmented polyurethane-urea elastomers underextension using thermal, mechanical and X-ray diffraction techniques, and the results are compared.These data indicate both strain-induced and temperature-induced crystallization in the stretchedclastomers, which may result from two different types of crystallites with different melting tempera-tures. These have been assigned as type 1 appearing around 60℃, and type 2 around 30℃. Thetype 1 crystallization can be induced by stretching at room temperature to large strain, and is mechani-cally reversible, but the type 2 crystallization is mainly induced by cooling below its crystallizationtemperature. These two crystalline structures are interchangeable under suitable conditions. Atelongations greater than 300%, the low temperature peak observed on fusion thermograms disappearsor combines with the high temperature peak. When the temperature of the sample is over the meltingpoint of the type 1 crystal, irreversible melting occurs and only the type 2 crystal develops on cooling.The results of stress-strain and stress hysteresis experiments at different temperatures indicate therelative importance of strain-induced and temperature-induced crystallization on the mechanicalproperties of these materials.     

Evidence for coupling and decoupling of parts of macromolecules by temperature‐modulated calorimetry

Equilibrium crystals of linear macromolecules have an extended‐chain macroconformation. They can melt at the equilibrium melting temperature, whereas crystallization needs considerable supercooling, even in the presence of crystal nuclei, making the overall phase transition irreversible. The same molecules with a metastable, chain‐folded macroconformation may have a large amount of specific reversibility, that is, a fraction of the same polymer molecule that melts irreversibly may also show decoupled, reversible melting. The overall metastable, nanophase structure of such semicrystalline polymers may thus support local equilibria. The tool for the quantitative analysis is quasi‐isothermal temperature‐modulated calorimetry that can separate reversible from irreversible processes. A major review of the study of crystals of more than 20 polymers has been published. On the basis of this extensive body of information, a first discussion of decoupling of parts of macromolecules is attempted and linked to previous studies of phase equilibria. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1275–1288, 2004     

Influence of deformation on irreversible and reversible crystallization of poly(ethylene‐co‐1‐octene)

The effect of uniaxial deformation and subsequent relaxation at ambient temperature on irreversible and reversible crystallization of homogeneous poly(ethylene‐co‐1‐octene) with 38 mol % 1‐octene melt‐crystallized at 10 K min was explored by calorimetry, X‐ray scattering, and Fourier transform infrared spectroscopy. At 298 K, the enthalpy‐based crystallinity of annealed specimens increased irreversibly by stress‐induced crystallization from initially 15% to a maximum of, at least, 19% when a permanent set of more than 200% was attained. The crystallinity increased by formation of crystals of pseudohexagonal structure at the expense of the amorphous polymer, and as a result of destruction of orthorhombic crystals. The stress‐induced increase of crystallinity was accompanied by an increase in the apparent specific heat capacity from 2.44 to about 2.59 J g^?1 K^?1, which corresponds to an increase of the total reversibility of crystallization from, at least, 0.10 to 0.17% K^?1. The specific reversibility calculated for 100% crystallinity increased from 0.67 to 0.89% K^?1 and points to a changed local equilibrium at the interface between the crystal and amorphous phases. The deformation resulted in typical changes of the phase structure and crystal morphology that involve orientation and destruction of crystals as well as the formation of fibrils. The effect of the decrease of the entropy of the strained melt on the reversibility of crystallization and melting is discussed. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1223–1235, 2002     

A calorimetric study of normal high density and ultra-high molecular weight polyethylene blends

The melting and the crystallization of blends of ultra-high molecular weight polyethylene (UHMWPE) and polyethylene high density with normal molecular weight (NMWPE) are investigated by means of differential scanning calorimetry (DSC). Mixing the components at a temperature below the flow temperature of UHMWPE (215 °C) results in segregated melting and crystallization. The segregated melting and crystallization temperatures of both components do not depend on composition of the blend. The extreme enthalpy dependence on blend composition is explained in terms of mutual influence exhibited by the components with respect to each other. It is due to the inner stresses in nonflowing UHMWPE characterized with a lot of entangled tie molecules. Mixing the components above the flow temperature of UHMWPE results in only one peak of melting and crystallization respectively. Complete mixing and probably co-crystallization between the components takes place on mixing NMWPE with flowing UHMWPE.     

Reversible melting of high molar mass poly(oxyethylene)

The heat capacity, C_p, of poly(oxyethylene), POE, with a molar mass of 900,000 Da, was analyzed by temperature-modulated differential scanning calorimetry, TMDSC. The high molar mass POE crystals are in a folded-chain macroconformation and show some locally reversible melting, starting already at about 250 K. At 335 K the thermodynamic heat capacity reaches the level of the melt. The end of melting of a high-crystallinity sample was analyzed quasi-isothermally with varying modulation amplitudes from 0.2 to 3.0 K to study the reversible crystallinity. A new internal calibration method was developed which allows to quantitatively assess small fractions of reversibly melting crystals in the presence of the reversible heat capacity and large amounts of irreversible melting. The specific reversibility decreases to small values in the vicinity of the end of melting, but does not seem to go to zero. The reversible melting is close to symmetric with a small fraction crystallizing slower than melting, i.e., under the chosen condition some of the melting and crystallization remains reversing. The collected data behave as one expects for a crystallization governed by molecular nucleation and not as one would expect from the formation of an intermediate mesophase on crystallization. The method developed allows a study of the active surface of melting and crystallization of flexible macromolecules.     

Crystallization and melting of isotactic polypropylene in response to temperature modulation

Isotactic polypropylene (iPP) was crystallized using temperature modulation in a differential scanning calorimeter (DSC) to thicken the crystals formed on cooling from the melt. A cool-heat modulation method was adopted for the preparation of the samples under a series of conditions. The effect of modulation parameters, such as temperature amplitude and period was monitored with the heating rate that followed. Thickening of the lamellae as a result of the crystallization treatment enabled by the cool-heat method lead to an increase in the peak melting temperature and the final traces of melting. For instance, iPP melting peak shifted by up to 3.5°C with temperature amplitude of 1.0°C while the crystallinity was increased from 0.45 (linearly cooled) to 0.53. Multiple melting endotherms were also observed in some cases, but this was sensitive to the temperature changes experienced on cooling. Even with a slower underlying cooling rate and small temperature amplitudes, some recrystallization and reorganization occurred during the subsequent heating scan. The crystallinity was increased significantly and this was attributed to the crystal perfection that occurred at the crystal growth surface. In addition, temperature modulated differential scanning calorimetry (TMDSC) has been used to study the melting of iPP for various crystallization treatments. The reversing and non-reversing contribution under the experimental time scale was modified by the relative crystal stability formed during crystallization. Much of the melting of iPP was found to be irreversible.This revised version was published online in November 2005 with corrections to the Cover Date.     

Reversible melting of polyethylene extended‐chain crystals detected by temperature‐modulated calorimetry

The melting and crystallization of extended‐chain crystals of polyethylene are analyzed with standard differential scanning calorimetry and temperature‐modulated differential scanning calorimetry. For short‐chain, flexible paraffins and polyethylene fractions up to 10 nm length, fully reversible melting was possible for extended‐chain crystals, as is expected for small molecules in the presence of crystal nuclei. Up to 100 nm length, full eutectic separation occurs with decreasingly reversible melting. The higher‐molar‐mass polymers form solid solution crystals and retain a rapidly decreasing reversible component during their melting that decreases to zero about 1.5 K before the end of melting. An attempt is made to link this reversible melting to the known, detailed morphology and phase diagram of the analyzed sample that was pressure‐crystallized to reach chain extension and practically complete crystallization. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2219–2227, 2002     

Melt-crystallization, glass transition and morphology of a (R)-3-hydroxybutyrate pentamer

The melt-crystallization of an oligo[(R)-3-hydroxybutyrate] with five repeating units has been analyzed using standard and temperature-modulated calorimetry, optical microscopy, and atomic force microscopy. Specimens of different crystallinity and supermolecular structure were generated by variation of the rate of cooling of a quiescent melt, or by variation of the temperature of isothermal crystallization. Completely amorphous samples can be obtained by cooling of the melt at a rate of 40 K min⁻¹, or faster, to a temperature lower than the glass transition. The crystallinity depends on the crystallization temperature. The maximum enthalpy-based crystallinity of about 40-45% is obtained by crystallization at temperatures lower than the temperature of the maximum crystallization rate, which is between 310 and 320 K. Analysis of the apparent heat capacity in metastable structural equilibrium reveals reversible melting at temperatures between 320 and 370 K by observation of an excess heat capacity above the level of the vibrational heat capacity, i.e., in the temperature range of irreversible reorganization and melting. The reversible melting is discussed in the context of coupling of the crystalline and amorphous phases, and compared to earlier studies on oligoethylene and oligo(oxyethylene). The presence of crystals causes formation of a rigid amorphous fraction of about 30% at a crystallinity of 40%. Optical and atomic force microscopy reveal spherulitic crystallization. At relatively high crystallization temperature, and in the early stage of the crystallization process, dendrites are observed which finally yield spherulites of decreased perfection. Larger spherulites of higher perfection grow at relatively low crystallization temperature, as deduced from the appearance of the Maltese cross, and the regularity of banding. The band spacing is less than 5 μm, as is accurately determined by atomic force microscopy. The temperature dependence of the spherulitic growth rate is in accord with the calorimetric analysis of the crystallization rate.     

Etudes comparatives et identification des polyethylenes par analyses thermique differentielle et de diffraction des rayons X

In thermal analysis polyethylenes can be characterized by their melting temperature. With the polyethylene mixtures studied, we obtained the best results during solidification. Crystallization temperature decreased in the order: high density PE, low density PE linear, radical low density PE.Calorimetric measurement of crystallization enthalpies allowed the determination of the composition of each of the polyethylenes, in regenerated and recycled mixtures in relation with the frame of the plastic waste valorisation process.The rate of crystallization obtained from X-ray diffraction spectra of these polymers is function of their volumic mass.A good agreement has been observed between these two techniques.

     

Time-resolved SAXS studies of morphological changes in cold crystallized poly(ethylene terephthalate) during annealing and heating

Structural changes occurring during crystallization of quenched amorphous poly(ethylene terephthalate) (PET) and subsequent cooling/heating cycles have been studied by real-time small-angle x-ray scattering (SAXS), using synchrotron radiation. Initial crystallization is found to occur by insertion of new lamellae between the existing ones, while rapid continuous melting/recrystallization happens when the cold-crystallized PET samples are heated above the previous highest annealing temperature. Such melting/recrystalization results in irreversible increases in the lamellar long period, the crystal thickness and the density difference between the crystalline and amorphous regions; in contrast, at temperatures below the prior highest crystallization temperature, the structural changes are dominated by reversible effects such as thermal expansion. However, throughout the entire temperature range up to the melting point around 250 °C, the crystal core thickness remains quite small, less than ca. 50 Å, and the linear crystallinity of lamellar stacks remains nearly constant around 0.3. Such a low crystallinity indicates the presence of thick order-disorder interfacial layers on the lamellar surface, whose thickness increases with temperature.Dedicated to Prof. E. W. Fischer on the occasion of his 65th birthday.     

茂金属聚乙烯的非等温结晶行为及其动力学研究

为探索分子量和支链含量对聚乙烯非等温结晶过程的影响,选用3组样品:(1)不同分子量的无支链线形聚乙烯;(2)低分子量的支链含量不同的试样;(3)高分子量的支链含量不同的试样.用DSC研究了这3组样品的非等温结晶动力学.结果表明:(1)与支链含量相比,分子量大小对结晶的影响是次要的,但高分子量样品的结晶度比低分子量样品低;(2)支链对聚乙烯的非等温结晶有重要影响,在支化聚乙烯中起决定作用;(3)无论是高分子量试样还是低分子量试样,支化含量增加,聚乙烯的结晶温度、结晶度、结晶动力学以及晶体的熔点等显著降低.     

Deliberately designed processes to physically tether the carboxyl groups of poly(pentacosadiynoic acid) to a poly(vinyl alcohol) glassy matrix to make poly(pentacosadiynoic acid) thermochromically reversible in the matrix

In this article, we demonstrate that by tethering carboxyl groups of poly(10,12-pentacosadiynoic acid) (PDA) to a poly(vinyl alcohol) (PVA) matrix, PDA, which is irreversible in its pure form, becomes reversible in the thermochromism. The tethering is realized by simple but deliberately designed processes: (1) Disperse the commercially available monomer 10,12-pentacosadiynoic acid (DA) nanocrystals in a PVA aqueous solution by the "NCCM" method invented in our laboratory. (2) Anneal and dry the mixture solution at a temperature higher than the melting point of pure DA crystal. (3) Polymerize the as-annealed DA/PVA blend films by UV irradiation. After the polymerization, PDA/PVA films with completely reversible thermochromism are obtained. The reversible PDA/PVA films can be easily dissolved in water, leading to water-dispersible nanoaggregates with the reversibility. Blends of PDA with other water-soluble polymers such as poly(ethylene oxide) (PEO), poly(acrylic acid) (PAA) and poly(allyamine) (PAM), were prepared respectively, by the same processes and under the same conditions. It is found that all these nanocomposites are irreversible or partially reversible in the thermochromism; either the relatively low glassy transition temperature of the polymer matrix (in the case of PEO) or the partial ionization nature of the polymer (in the cases of PAA and PAM) is responsible for the irreversibility or the partial reversibility.     

A theory of the stress-induced crystallization of crosslinked polymeric networks

A non-Gaussian theory of the stress-induced crystallization of polymeric networks is presented. It is predicted that for uniaxial extension of crosslinked polyethylene, a perfectly oriented, extended-chain crystal is formed initially, changing to one-fold crystal oriented perpendicular to the stretch direction at low extension and to a two-fold crystal having nearly perfect orientation at high extension. The stress is predicted to decay initially and then to rise as the network chains switch from an extended- to a folded-chain morphology, the rise being delayed and finally suppressed by additional crosslinking. The final, equilibrium birefringence is calculated and found to be negative at low extension and positive at high extension. The initial rate of crystallization is calculated using irreversible thermodynamics and is found to increase with extension and decrease with increasing crosslinking and temperature. All of the theoretical predictions are in qualitative agreement with experiment.     

Multiple melting behavior of isotactic polypropylene and poly(propylene-co-ethylene) after stepwise isothermal crystallization

The multiple melting behavior of several commercial resins of isotactic polypropylene (iPP) and random copolymer, poly(propylene-co-ethylene) (PPE), after stepwise isothermal crystallization (SIC) were studied by differential scanning calorimeter and wide-angle X-ray diffraction (WAXD). For iPP samples, three typical melting endotherms appeared after SIC process when heating rate was lower than 10 °C/min. The WAXD experiments proved that only α-form crystal was formed during SIC process and no transition from α1- to α2-form occurred during heating process. Heating rate dependence for each endotherm was discussed and it was concluded that there were only two major crystals with different thermal stability. For the PPE sample, more melting endotherms appeared after stepwise isothermal crystallization. The introduction of ethylene comonomer in isotactic propylene backbone further decreased the regularity of molecular chain, and the short isotactic propylene sequences could crystallize into γ-form crystal having a low melting temperature whereas the long sequences crystallized into α-form crystal having high melting temperature.     

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.     

含间位取代苯基聚醚酮酮的结晶与晶体结构研究

通过差示扫描法（ＤＳＣ）及广角Ｘ 射线衍射（ＷＡＸＤ）技术研究了含间位取代苯基聚醚酮酮（ＰＥＫｍＫ）的结晶行为与晶体结构．Ｘ 射线结果表明，从熔融态及玻璃态结晶时，ＰＥＫｍＫ只有一种晶型，其晶胞参数为：ａ＝０７６７２ｎｍ，ｂ＝０６１４９ｎｍ，ｃ＝１５９９ｎｍ．ＤＳＣ结果表明，ＰＥＫｍＫ热分析曲线都出现了熔融双峰，低熔融峰（ＤＯＷｎ）热焓占总热焓４～７％，它源于初始结晶形成的同一晶型不同厚度片晶．低熔融峰在２５０℃以上结晶转化成高熔融峰（Ｉ），ＰＥＫｍＫ平衡熔点为２９５℃