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     

Analysis of the complex thermal behavior of poly(L‐lactic acid) film. II. Samples crystallized from the melt

The complex thermal behavior of poly(l ‐lactic acid) films crystallized from the melt, either isothermally or nonisothermally, was studied by differential scanning calorimetry (DSC), wide angle X‐ray diffraction, and small angle X‐ray scattering. The variation of the thermal behavior with crystallization temperature, time, and cooling rate was documented and analyzed. After nonisothermal crystallization at low cooling rates that develop high crystallinity, an obvious double melting peak appears at modest heating rates (e.g., 10 °C/min). At higher heating rates, these samples exhibit only single melting. However, an unusual form of double melting occurs under the majority of the conditions studied under either isothermal or nonisothermal conditions. In this case, double melting is marked by the appearance of a recrystallization exotherm just prior to the final melting that obscures the observation of the melting of the crystals formed during the initial crystallization process. The occurrence of double melting in melt‐crystallized samples was concluded to be the result of a melt‐recrystallization process occurring during the subsequent DSC heating scan; it is a function of crystalline perfection, not the initial crystallinity, nor whether or not the crystallization reached completion at the crystallization temperature. Many other very interesting observations are also discussed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3378–3391, 2006     

Oxygen‐barrier properties of cold‐crystallized and melt‐crystallized poly(ethylene terephthalate‐co‐4,4′‐bibenzoate)

This study examined the oxygen‐transport properties of poly(ethylene terephthalate‐co‐bibenzoate) (PETBB55) crystallized from the melt (melt crystallization) or quenched to glass and subsequently isothermally crystallized by heating above the glass‐transition temperature (cold crystallization). The gauche–trans conformation of the glycol linkage was determined by infrared analysis, and the crystalline morphology was examined by atomic force microscopy. Oxygen solubility decreased linearly with volume fraction crystallinity. For melt‐crystallized PETBB55, extrapolation to zero solubility corresponded to an impermeable crystal with 100% trans glycol conformations, a density of 1.396 g cm^?3, and a heat of melting of 83 J g^?1. From the melt, PETBB55 crystallized as space‐filling spherulites with loosely organized lamellae and pronounced secondary crystallization. The morphological observations provided a structural model for permeability consisting of impermeable platelets randomly dispersed in a permeable matrix. In contrast, cold‐crystallized PETBB55 retained the granular texture of the quenched polymer despite the high level of crystallinity, as measured by the density and heat of melting. Oxygen solubility decreased linearly with volume fraction crystallinity, but zero solubility corresponded to an impermeable defective crystal with a trans fraction of 0.83 and a density of 1.381 g cm^?3. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2489–2503, 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     

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     

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     

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     

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     

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     

Preparation of oriented β‐form poly(L‐lactic acid) by solid‐state extrusion

Melt‐crystallized, low molecular weight poly(L ‐lactic acid) (PLLA) consisting of α crystals was uniaxially drawn by solid‐state extrusion at an extrusion temperature (T_ext) of 130–170 °C. A series of extrusion‐drawn samples were prepared at an optimum T_ext value of 170 °C, slightly below the melting temperature (T_m) of α crystals (～180 °C). The drawn products were characterized by deformation flow profiles, differential scanning calorimetry (DSC) melting thermograms, wide‐angle X‐ray scattering (WAXD), and small‐angle X‐ray scattering as a function of the extrusion draw ratio (EDR). The deformation mode in the solid‐state extrusion of semicrystalline PLLA was more variable and complex than that in the extensional deformation expected in tensile drawing, which generally gave a mixture of α and β crystals. The deformation profile was extensional at a low EDR and transformed to a parabolic shear pattern at a higher EDR. At a given EDR, the central portion of an extrudate showed extensional deformation and the shear component became progressively more significant, moving from the center to the surface region. The WAXD intensities of the (0010)_α and (003)_β reflections on the meridian as well as the DSC melting thermograms showed that the crystal transformation from the initial α form to the oriented β form proceeded rapidly with increasing EDR at an EDR greater than 4. Furthermore, WAXD showed that the crystal transformation proceeded slightly more rapidly at the sheath region than at the core region. This fact, combined with the deformation profiles (shear at the sheath and extensional at the core), indicated that the crystal transformation was promoted by shear deformation under a high pressure rather than by extensional deformation. Thus, a highly oriented rod consisting of only β crystals was obtained by solid‐state extrusion of melt‐crystallized, low molecular weight PLLA slightly below T_m. The structure and properties of the α‐ and β‐form crystals were also studied. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 95–104, 2002     

Multivariable structural characterization of semicrystalline polymer blends by small‐angle light scattering

A new multi‐variable‐measurement approach for characterizing and correlating the nanoscale and microscale morphology of crystal‐amorphous polymer blends with melt‐phase behavior is described. A vertical small‐angle light scattering (SALS) instrument optimized for examining the scattering and light transmitted from structures ranging from 0.5 to 50 μm, thereby spanning the size range characteristic of the initial‐to‐late stages of thermal‐phase transitions (e.g., melt‐phase separation and crystallization) in crystal‐amorphous polymer blends, was constructed. The SALS instrument was interfaced with differential scanning calorimetry (DSC), and simultaneous SALS/DSC/transmission measurements were performed. We show that the measurement of transmitted light and SALS under H_V (cross‐polarized) optical alignments during melting can be used to reliably measure the thermodynamic (e.g., crystal melting and melt‐phase separation temperatures) and structural variables (e.g., crystalline fraction within the superstructures and volume fraction of superstructures) necessary for describing the multiphase behavior of crystal‐amorphous blends in one combined measurement. We also evaluate the orientation correlations of crystalline volume elements within the superstructures. Our results indicate that simultaneous measurement of transmitted light can provide a reliable estimate of the total scattering from density and orientation fluctuations and the melt‐phase separation temperature of polymer blends. For solution‐cast poly(?‐caprolactone)/poly(D,L‐lactic acid) blends, our multivariable measurements during melting provide the parameters necessary to generate a crystal–liquid and liquid–liquid phase diagram and characterize the solid‐state morphology. This opens up the challenge to explore use of our vertical SALS instrument as a rapid and convenient method for developing structure–property relationships for crystal‐amorphous polymer blends. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2714–2727, 2002     

Morphology of homogeneous copolymers of ethylene and 1‐octene. III. Structural changes during heating as revealed by time‐resolved SAXS and WAXD

The structural changes of two linear polyethylenes, LPEs, with different molar mass and of two homogeneous copolymers of ethylene and 1‐octene with comparable comonomer content but different molar mass were monitored during heating at 10 °C per minute using synchrotron radiation SAXS. Two sets of samples, cooled at 0.1 °C per minute and quenched in liquid nitrogen, respectively, were studied. All LPEs display surface melting between room temperature and the end melting temperature, whereas complete melting, according to lamellar thickness, only occurs at the highest temperatures where DSC displays a pronounced melting peak. There is recrystallization followed by isothermal lamellar thickening if annealing steps are inserted. The lamellar crystals of slowly cooled homogeneous copolymers melt in the reverse order of their formation, that is, crystals melt according to their thickness. Quenching creates unstable crystals through the cocrystallization of ethylene sequences with different length. These crystals repeatedly melt and co‐recrystallize during heating. The exothermic heat due to recrystallization partially compensates the endothermic heat due to melting resulting in a narrow overall DSC melting peak with its maximum at a higher temperature than the melting peak of slowly cooled copolymers. With increasing temperature, the crystallinity of quenched copolymers overtakes the one of slowly cooled samples due to co‐recrystallization by which an overcrowding of leaving chains at the crystal surfaces is avoided. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1975–1991, 2000     

Thermal and X‐ray analysis of polymorphic crystals,melting, and crystalline transformation in poly(butylene adipate)

Polymorphic crystals and complex multiple melting behavior in an aliphatic biodegradable polyester, poly(butylene adipate) (PBA), were thoroughly examined by wide‐angle X‐ray diffraction (WAXD) and differential scanning calorimetry (DSC). Further clarification on mechanisms of multiple melting peaks related to polymorphic crystal forms in PBA was attempted. More stable α‐form crystal is normally favored for crystallization from melt at higher temperatures (31–35 °C), or upon slow cooling from the melt; while the β‐form is the favored species for crystallization at low temperatures (25–28 °C). We further proved that PBA crystallization could also result in all α‐form even at low temperatures (25–28 °C) if it crystallized with the presence of prior α‐form nuclei. PBA packed with both crystal forms could display as many as four melting peaks (P1 ? P4, in ascending temperature order). However, PBA initially containing only the α‐crystal exhibited dual melting peaks of P1 and P3, which are attributed to dual lamellar distributions of the α‐crystal. By contrast, PBA initially containing only the β‐crystal could also exhibit dual melting peaks (P2 and P4) upon scanning. While P2 is clearly associated with melting of the initial β‐crystal, the fourth melting peak (P4), appearing rather broad, was determined to be associated with superimposed thermal events of crystal transformation from β‐ to α‐crystal and final re‐melting of the new re‐organized α‐crystal. Crystal transformation from one to the other or vice versa, lamellae thickening, annealing at molten state, and influence on crystal polymorphism in PBA were analyzed. Relationships and mechanisms of dual peaks for isolate α‐ or β‐crystals in PBA are discussed. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1662–1672, 2005     

Effect of silver nanoparticles on the dynamic crystallization behavior of nylon‐6

The effect of introducing silver nanoparticles on the rheological properties and dynamic crystallization behavior of nylon‐6 was investigated. The nanocomposites showed slightly higher viscosity than pure nylon‐6 in the low‐frequency range even at an extremely low loading level of the silver particles (0.5–1.0 wt %). The nanoparticles had a more noticeable effect on the storage modulus than on the loss modulus of a nylon‐6 melt and reduced its loss tangent. They increased the crystallization temperature of nylon‐6 by about 14 °C and produced a sharper crystalline peak. The silver nanoparticles promoted the crystallization of nylon‐6, and their effect on the dynamic crystallization of nylon‐6 at 200 °C was more notable at a lower shear rate and at 190 °C at a higher frequency. Nylon‐6 produced large spherulitic crystals, but the nanocomposites showed a grainy structure. In addition, the silver nanoparticles reduced the fraction of the α‐form crystal but increased that of the γ‐form crystal. The nanocomposites crystallized at 190 °C showed a lower melting temperature than nylon‐6 by about 3 °C, whereas the nanocomposites crystallized at 200 °C showed almost the same melting temperature. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 790–799, 2004     

Direct formation of form I poly(1‐butene) single crystals from melt crystallization in ultrathin films

The morphologies and crystalline structures of melt‐crystallized ultrathin isotactic poly(1‐butene) films have been studied with transmission electron microscopy and electron diffraction. It is demonstrated that a bypass of form II crystallization can be achieved with an increase in its crystallization temperature. Electron microscopy observations show that melt‐grown isotactic poly(1‐butene) single crystals have a well‐shaped hexagonal form, whereas form I crystals converted from form II display the morphologies of their tetragonal precursors. Electron diffraction results indicate that, instead of the twinned hexagonal pattern of the converted form I crystal, the directly formed form I single crystals exhibit an untwinned hexagonal pattern. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2641–2645, 2002     

Calorimetry of nanophase‐separated poly(oligoamide‐alt‐oligoether)s

The miscibility, crystallization, vitrification, and melting behavior of multiblock copolymers consisting of alternating components of oligo[imino(1‐oxododecamethylene)] and oligo(oxytetramethylene) were investigated in their dependence of composition and molecular mass of the blocks. In all compositions studied, the copolymers present two separate glass transitions at temperatures not far from those of the corresponding homopolymers. The immiscible components are linked by chemical bonds to nanophase‐separated layers. In such a situation, the structure and mobility of each phase affect the other, causing small shifts of the glass‐transition temperatures. The enthalpy‐based crystallinity was calculated separately for each component, using the known information on heat capacities and latent heats. The crystallinity is influenced by the block lengths. The oligoamide segments crystallize from an isotropic melt with a relatively high crystallinity, whereas the oligoether blocks solidify in the presence of crystals and glass of the second component in the adjacent solid nanophases, which greatly reduces their overall long‐range chain mobility, and as a consequence, their crystallinity is rather small. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1594–1604, 2001     

Reversible melting in nanophase‐separated poly(oligoamide‐alt‐oligoether)s and its dependence on sequence length,crystal perfection,and molecular mobility

The degree of reversibility of the melting of multiblock copolymers of alternating oligoamides and oligoethers was investigated with respect to the composition and molecular mass of the blocks. The analysis was conducted with temperature‐modulated calorimetry, and it revealed different degrees of reversibility of the melting process that depended on the block length, crystal perfection, and molecular mobility. For the oligoamide blocks, the amount of crystal that melts and crystallizes reversibly during quasi‐isothermal analysis increases with decreasing molar mass, and shorter amide sequences form poorer crystals that have a higher tendency toward reorganization. Reorganization of the oligoamides is also favored by the presence of the more mobile oligoether units. Reversible melting of the oligoether segments is influenced by the presence of glassy and crystalline oligoamide blocks in the adjacent nanophases. Because of the segmented nature of the copolymers, the oligoether segments are not free to flow as in an isotropic melt but are anchored to the oligoamide surfaces with different degrees of restriction that change the local equilibrium of melting and recrystallization. A comparison of the copolymers with the corresponding homopolymers provides information about the role of molecular nucleation and mobility in the reversibility of melting. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2969–2981, 2001     

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     

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     

Extraordinary transport behavior of gases in isothermally annealed poly(4‐methyl‐1‐pentene) membranes

Poly(4‐methyl‐1‐pentene) (PMP) membranes were modified through isothermal annealing to investigate the change of their crystalline structure and rigid and mobile amorphous fractions (RAF and MAF), assuming a three‐phase model, affected the gas transport behavior. The crystalline structure was characterized by wide‐angle X‐ray diffraction (WAXD) and small‐angle X‐ray scattering (SAXS) techniques, and the free volume properties were analyzed by positron annihilation lifetime spectroscopy. Compared with the pristine membrane, the annealed membranes show higher crystallinity; the crystals undergo partial structural change from form III to form I. The lamellar crystal thickness, rigid amorphous fraction thickness, and long period in the lamellar stacks increase with crystallinity. The annealed PMP membranes exhibit higher permeability due to the increase in larger size free volumes in MAF and higher selectivity due to the increase in smaller size free volumes in RAF, respectively. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2368–2376