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     

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     

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     

Reversible and irreversible crystallization in high-density polyethylene at low temperature

Reversible and irreversible crystallization and melting of high-density polyethylene at low temperature has been re-evaluated and is discussed in terms of the concept of the specific reversibility of a crystal. The concept of the specific reversibility links reversible and irreversible melting of a specific crystal such that reversible melting occurs only at slightly lower temperature than irreversible melting. In this study evidence for irreversible crystallization at low temperature in high-density polyethylene is provided, non-avoidable by primary crystallization and extended annealing at high temperature. The simultaneously observed reversible crystallization and melting at low temperature can be attributed to lateral-crystal-surface activity in addition to the well-established reversible fold-surface melting, dominant at high temperature, and evidenced by small-angle X-ray data available in the literature. This revised version was published online in August 2006 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     

Investigation of the deformation of homogeneous poly(ethylene‐co‐1‐octene) by wide‐ and small‐angle X‐ray scattering using synchrotron radiation

The deformation behavior of homogeneous ethylene‐1‐octene copolymers was investigated as a function of the crystallinity and the crystal size and perfection, respectively, by wide‐ and small‐angle X‐ray scattering using synchrotron radiation. The crystallinity and the crystal size and perfection, respectively, are controlled by the copolymer composition and the condition of melt crystallization. The deformation includes rotation of crystals, followed by plastic deformation and complete melting of the initial crystal population, and final formation of microfibrils. The process of rotation, plastic deformation, and melting of crystals of the initial structure is completed at lower strain if the size and perfection of the crystals, respectively, decrease, that is, if crystals thermally melt at lower temperature. The kinetics of the fibrillation of the initial structure seems independent of the crystal symmetry, that is, rotation and melting of pseudohexagonal and orthorhombic polyethylene crystals (as evident in low‐crystalline specimens) are similar. The structure of the microfibrils, before and after stress release, is almost independent of the condition of prior melt crystallization, which supports the notion of complete melting of the initial crystal population. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1919–1930, 2002     

Polydispersity of ethylene sequence length in metallocene ethylene/α‐olefin copolymers. II. Influence on crystallization and melting behavior

In this work, crystallization and melting behavior of metallocene ethylene/α‐olefin copolymers were investigated by differential scanning calorimetry (DSC) and atomic force microscopy (AFM). The results indicated that the crystallization and melting temperatures for all the samples were directly related to the long ethylene sequences instead of the average sequence length (ASL), whereas the crystallization enthalpy and crystallinity were directly related to ASL, that is, both parameters decreased with a decreasing ASL. Multiple melting peaks were analyzed by thermal analysis. Three phenomena contributed to the multiple melting behaviors after isothermal crystallization, that is, the melting of crystals formed during quenching, the melting‐recrystallization process, and the coexistence of different crystal morphologies. Two types of crystal morphologies could coexist in samples having a high comonomer content after isothermal crystallization. They were the chain‐folded lamellae formed by long ethylene sequences and the bundlelike crystals formed by short ethylene sequences. The coexistence phenomenon was further proved by the AFM morphological observation. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 822–830, 2002     

Reversible melting and crystallization of high‐molar‐mass poly(oxyethylene)

The heat capacity of poly(oxyethylene) (POE) with a molar mass of 900,000 Da has been analyzed with differential scanning calorimetry and quasi‐isothermal, temperature‐modulated differential scanning calorimetry. The crystal structure, lattice parameters, and coherently scattering domain sizes have been measured with wide‐angle X‐ray diffraction as a function of temperature. 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 reversible crystallinity depends on the modulation amplitude and has been varied in the melting range from ±0.2 to ±3.0 K. Before melting, there is neither a change in the crystal structure nor a change in the domain size, but the expansivity of the crystals increases at about 320 K. These observations support the interpretation that the monoclinic POE crystals possess a glass transition temperature with a midpoint at about 324 K, whereas the maximum melting temperature is 341 K. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 475–489, 2007     

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.     

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.     

Thermal,crystallization, mechanical,and rheological characteristics of poly(trimethylene terephthalate)/poly(ethylene terephthalate) blends

Blends of poly(trimethylene terephthalate) (PTT) and poly(ethylene terephthalate) in the amorphous state were miscible in all of the blend compositions studied, as evidenced by a single, composition‐dependent glass‐transition temperature observed for each blend composition. The variation in the glass‐transition temperature with the blend composition was well predicted by the Gordon–Taylor equation, with the fitting parameter being 0.91. The cold‐crystallization (peak) temperature decreased with an increasing PTT content, whereas the melt‐crystallization (peak) temperature decreased with an increasing amount of the minor component. The subsequent melting behavior after both cold and melt crystallizations exhibited melting point depression behavior in which the observed melting temperatures decreased with an increasing amount of the minor component of the blends. During crystallization, the pure components crystallized simultaneously just to form their own crystals. The blend having 50 wt % of PTT showed the lowest apparent degree of crystallinity and the lowest tensile‐strength values. The steady shear viscosity values for the pure components and the blends decreased slightly with an increasing shear rate (within the shear rate range of 0.25–25 s^?1); those of the blends were lower than those of the pure components. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 676–686, 2004     

Correlation of the melting behavior and copolymer composition distribution of Ziegler–Natta‐catalyst and single‐site‐catalyst polyethylene copolymers

The multimodal differential scanning calorimetry melting endotherms observed for commercial linear low‐density polyethylenes are due to broad and multimodal short‐chain‐branching distributions. Multiple peaks, observed in melting endotherms of isothermally melt‐crystallized and compositionally homogeneous polyethylene copolymers are due to intrachain heterogeneity. This intrachain heterogeneity is quantified by the distribution of ethylene sequence lengths within the chains. These compositionally homogeneous copolymers undergo a primary crystallization, which produces a population of thicker lamellae, creating a network that places severe restrictions on segment transport in subsequent secondary crystallization, which produces a population of thinner crystals. The restrictions on segment transport imposed by the initial network created by the primary crystallization of thicker lamellae severely limits the total crystallinity achieved in the random copolymers studied. The solution crystallization of such copolymers produces a continuous distribution due to more facile segment transport in a dilute solution, in contradistinction to the multimodal distribution produced in the melt crystallization. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2800–2818, 2001     

Simultaneous wide‐angle X‐ray diffraction and differential scanning calorimetry analysis of the melting and recrystallization behavior of polyethylene and isotactic polypropylene containing cyclopentane units in the main chain

The melting and crystallization behavior of polyethylene and isotactic polypropylene containing 1,2‐ or 1,3‐disubstituted cyclopentane units in the main chain has been studied with simultaneous wide‐angle X‐ray diffraction (WAXD) and differential scanning calorimetry. For the ethylene‐based copolymers, the position of a reflection peak in the WAXD patterns shifts to a low angle with the increasing acquired temperature. The temperature dependence on the axial length of the crystal lattice is more marked in the copolymers forming orthorhombic crystals (containing 1,2‐cyclopentane or 5.6 mol % 1,3‐cyclopentane units) than in those forming hexagonal crystals (containing 8.1 mol % 1,3‐cyclopentane units). For the isotactic propylene‐based copolymers, the position of the reflection peaks in the WAXD patterns is independent of the acquired temperature. The proportion of the γ form in the copolymer containing the 1,2‐cyclopentane units is higher than that in the copolymers containing the 1,3‐cyclopentane units. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1457–1465, 2004     

Isotactic polypropylene reversible crystallization investigated by modulated temperature and quasi‐isothermal FTIR

Modulated temperature techniques allow to separate the reversing and non‐reversing contributions of material transitions. To investigate reversible crystallization and melting of isotactic polypropylene (iPP) at microstructural level, in this research, modulated temperature Fourier transform infrared (MTFTIR) and quasi‐isothermal FTIR (QIFTIR) analyses are used. By following the intensity variation of iPP regularity bands, associated with 3₁ helix structures of different lengths (n repeating units), MTFTIR evidences that, independently from helix length, a reversing coil–helix transition takes place few degrees below the non‐reversing crystallization onset. By comparing spectroscopic and differential scanning calorimetry experiments performed in quasi‐isothermal conditions, the reversing transition was found to be associated with the reversible melting‐crystallization phenomenon. Moreover, QIFTIR evidences that helices of different lengths contribute differently to the reversible transition: the helices composed of n = 10 and n = 12 are active into all the explored temperature range (30–130 °C) whereas the shortest (n = 6) and the longest (n > 15) helices contribute to reversibility at T > 100 °C. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 922–931     

Recent developments in thermal analysis of polymers: Calorimetry in the limit of slow and fast heating rates

This review focuses on new insights into the crystal melting transition and the amorphous glass transition of polymers that have been gained through recent advances in thermoanalytical methods. The specific heat capacity can now be studied under two extreme limits, that is, under quasi‐isothermal conditions (limit of zero heating rate) and, at the other end of the scale, under rapid heating conditions (heating rates on the order of thousands of degrees per second), made possible through nanocalorimetry. The reversible melting, and multiple reversible melting, of semicrystalline polymers is explored using quasi‐isothermal temperature modulated differential scanning calorimetry, TMDSC. The excess reversing heat capacity, above the baseline, measured under nearly isothermal conditions is attributed to locally reversible surface melting and crystallization processes that do not require molecular nucleation. Observations of double reversible melting endotherms in isotactic polystyrene suggest existence of two distinct populations of crystals, each showing locally reversible surface melting. The second subject of the review, nanocalorimetry, is utilized to study samples of small mass under conditions of very fast heating and cooling. The glass transition properties of thin amorphous polymer films are observed under adiabatic conditions. The glass transition temperature appears to be independent of film thickness, and is observed even in ultra‐thin films. Recrystallization and reorganization during rapid heating are studied by nanocalorimetry of semicrystalline polymers. The uppermost endotherm seen under normal DSC scanning of poly(ethylene terephthalate) is caused by reorganization, and vanishes under the rapid heating conditions (3000K/s) provided by nanocalorimetry. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 629–636, 2005     

Reversible and irreversible heat capacity of poly(trimethylene terephthalate) analyzed by temperature‐modulated differential scanning calorimetry

The heat capacity of poly(trimethylene terephthalate) (PTT) has been analyzed using temperature‐modulated differential scanning calorimetry (TMDSC) and compared with results obtained earlier from adiabatic calorimetry and standard differential scanning calorimetry (DSC). Using quasi‐isothermal TMDSC, the apparent reversing and nonreversing heat capacities were determined from 220 to 540 K, including glass and melting transitions. Truly reversible and time‐dependent irreversible heat effects were separated. The extrapolated vibrational heat capacity of the solid and the total heat capacity of the liquid served as baselines for the analysis. As one approaches the melting region from lower temperature, semicrystalline PTT shows a reversing heat capacity, which is larger than that of the liquid, an observation that is common also for other polymers. This higher heat capacity is interpreted as a reversible surface or bulk melting and crystallization, which does not need to undergo molecular nucleation. Additional time‐dependent, reversing contributions, dominating at temperatures even closer to the melting peak, are linked to reorganization and recrystallization (annealing), while the major melting is fully irreversible (nonreversing contribution). © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 622–631, 2000     

Compressed‐gas‐induced crystallization in tert‐butyl poly(ether ether ketone)

tert‐Butyl‐substituted poly(ether ether ketone) (tBuPEEK), which does not undergo crystallization with thermal annealing, crystallizes readily when treated with compressed CO₂. The dissolved CO₂ causes a reduction in the glass‐transition temperature of the polymer–gas system and enhances the chain mobility of the macromolecules, thereby bringing about crystallization. In the presence of CO₂, crystallization is increasingly favored with increasing CO₂ pressure and treatment temperature. The melting point of tBuPEEK crystals increases linearly with the CO₂ pressure applied in the treatment, indicating an increase in the order and/or size of the crystals. The extent of crystallinity increases when small amounts of methanol or dichloromethane are used as a cosolute with CO₂. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1505–1512, 2001     

Crystallization and melting of a branched polyethylene with precisely controlled chemical structure

The heat capacity of a linear polyethylene with dimethyl branches, at every 21st backbone atom was analyzed by differential scanning calorimetry (DSC) and quasi-isothermal temperature-modulated DSC. This novel copolyethylene (PE2M) is relatively difficult to crystallize from the melt. On subsequent heating, a first, sharp melting peak is followed by a sharp cold-crystallization and crystal perfection and a smaller endotherm, before reaching the main melting at 315–320 K, close to the melting temperatures of eicosane and tetracontane. The low-temperature melting is sensitive to the cooling rate and disappears below 1.0 K min⁻¹. The cold crystallization can be avoided by heating with rates faster than 80 K min⁻¹. The PE2M exhibits some reversing and reversible melting, which is typical for chain-folded polymers. The glass transition of semicrystalline PE2M is broadened and reaches its upper limit at about 260 K (midpoint at about 0.355 K). Above this temperature, the crystals seem to have a heat capacity similar to that of the liquid. A hypothesis is that the melting transition can be explained by changes in crystal perfection without major alteration of the crystal structure and the lamellar morphology. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3461–3474, 2006     

Analysis of the complex thermal behavior of poly(L‐lactic acid) film. I. Samples crystallized from the glassy state

The melting behavior of poly(L ‐lactic acid) film crystallized from the glassy state, either isothermally or nonisothermally, was studied by wide angle X‐ray diffraction (WAXD), small angle X‐ray scattering (SAXS), differential scanning calorimetry (DSC), and temperature‐modulated differential scanning calorimetry (TMDSC). Up to three crystallization and two melting peaks were observed. It was concluded that these effects could largely be accounted for on the basis of a “melt‐recrystallization” mechanism. When molecular weight is low, two melting endotherms are readily observed. But, without TMDSC, the double melting phenomena of high molecular weight PLLA is often masked by an exotherm just prior to the final melting, as metastable crystals undergo melt‐recrystallization during heating in the DSC. The appearance of a double cold‐crystallization peak during the DSC heating scan of amorphous PLLA film is the net effect of cold crystallization and melt‐recrystallization of metastable crystals formed during the initial cold crystallization. Samples cold‐crystallized at 80 and 90 °C did not exhibit a long period, although substantial crystallinity developed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3200–3214, 2006     

Reversible and irreversible heat capacity of poly[carbonyl(ethylene‐co‐propylene)] by temperature‐modulated calorimetry

The heat capacity of poly[carbonyl(ethylene‐co‐propylene)] with 95 mol % C₂H₄? CO? (Carilon EP®) was measured with standard differential scanning calorimetry (DSC) and temperature‐modulated DSC (TMDSC). The integral functions of enthalpy, entropy, and free enthalpy were derived. With quasi‐isothermal TMDSC, the apparent reversing heat capacity was determined from 220 to 570 K, including the glass‐ and melting‐transition regions. The vibrational heat capacity of the solid and the heat capacity of the liquid served as baselines for the quantitative analysis. A small amount of apparent reversing latent heat was found in the melting range, just as for other polymers similarly analyzed. With an analysis of the heat‐flow rates in the time domain, information was collected about latent heat contributions due to annealing, melting, and crystallization. The latent heat decreased with time to an even smaller but truly reversible latent heat contribution. The main melting was fully irreversible. All contributions are discussed in the framework of a suggested scheme of six physical contributions to the apparent heat capacity. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1565–1577, 2001