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     

Crystallization and structure formation of block copolymers containing a glassy amorphous component

Formation of higher‐order structure in crystallization from microphase‐separated melts was studied for polystyrene–polyethylene (PS–PE) diblock copolymers and PS–PE–PS triblock copolymers with time‐resolved synchrotron small‐angle X‐ray scattering (SR–SAXS) techniques. The PE block was crystallized at temperatures when the PS block was in the glassy state. In both crystallization and melting processes, only the peak intensity in the SR–SAXS curve changed, however, the peak positions including higher‐order peaks did not change. This means that the microphase‐structure in the crystalline state was completely the same as that in the molten state. These behaviors were observed regardless of any melt microphase structure. Also, once a stable microphase structure was formed in the molten state, the structure was not changed even if crystallization and melting were repeated. Behavior of crystallization from such microphase‐separated melts was also studied. Apparent activation energies of crystallization were high for all block copolymers, compared with that for the PE homopolymer. In particular, the triblock copolymers showed higher apparent activation energies than the diblock copolymers. Both degrees of crystallinity and Avrami indices were greatly suppressed in crystallization from the cylindrical domain. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4199–4206, 2004     

嵌段共聚物PVCH-b-PE-b-PVCH的多重结晶行为

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

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     

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     

New insights into the multiple melting behaviors of the semicrystalline ethylene‐hexene copolymer: Origins of quintuple melting peaks

A semicrystalline ethylene‐hexene copolymer (PEH) was subjected to a simple thermal treatment procedure as follows: the sample was isothermally crystallized at a certain isothermal crystallization temperature from melt, and then was quenched in liquid nitrogen. Quintuple melting peaks could be observed in heating scan of the sample by using differential scanning calorimeter (DSC). Particularly, an intriguing endothermic peak (termed as Peak 0) was found to locate at about 45 °C. The multiple melting behaviors for this semicrystalline ethylene‐hexene copolymer were investigated in details by using DSC. Wide‐angle X‐ray diffraction (WAXD) technique was applied to examine the crystal forms to provide complementary information for interpreting the multiple melting behaviors. Convincing results indicated that Peak 0 was due to the melting of crystals formed at room temperature from the much highly branched ethylene sequences. Direct heating scans from isothermal crystallization temperature (T_c, 104–118 °C) were examined for comparison, which indicated that the multiple melting behaviors depended on isothermal crystallization temperature and time. A triple melting behavior could be observed after a relatively short isothermal crystallization time at a low T_c (104–112 °C), which could be attributed to a combination of melting of two coexistent lamellar stack populations with different lamellar thicknesses and the melting‐recrystallization‐remelting (mrr) event. A dual melting behavior could be observed for isothermal crystallization with both a long enough time at a low T_c and a short or long time at an intermediate T_c (114 °C), which was ascribed to two different crystal populations. At a high T_c (116–118 °C), crystallizable ethylene sequences were so few that only one single broad melting peak could be observed. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2100–2115, 2008     

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     

ABA triblock copolymers with a ring‐opening metathesis polymerization/macromolecular chain‐transfer agent approach

Block copolymers containing polystyrene and polycyclooctene were synthesized with a ring‐opening metathesis polymerization/chain‐transfer approach. Polystyrene, containing appropriately placed olefins, was prepared by anionic polymerization and served as a macromolecular chain‐transfer agent for the ring‐opening metathesis polymerization of cyclooctene. These unsaturated polymers were subsequently converted to the corresponding saturated triblock copolymers with a simple heterogeneous catalytic hydrogenation step. The molecular and morphological characterization of the block copolymers was consistent with the absence of significant branching in the central polycyclooctene and polyethylene blocks [high melting temperatures (114–127 °C) and levels of crystallinity (17–42%)]. A dramatic improvement in both the long‐range order and the mechanical properties of a microphase‐separated, symmetric polystyrene–polycyclooctene–polystyrene block copolymer sample was observed after fractionation. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 361–373, 2007     

Crystallization and Melting of Model Polyethylenes with Different Chain Structures

The crystallization and melting of three model polyethylenes of different chain structures have been studied. The polymers studied were a linear copolymer, hydrogenated poly(butadiene); a hydrogenated poly(butadiene)-atactic poly(propylene) diblock copolymer; and a three-arm star hydrogenated poly(butadiene). An important feature of this work was that the crystallizing portions of the copolymers all have the same molecular lengths.It was found that the overall crystallization rate decreases steadily from a linear to a diblock to the star copolymer. The differences in crystallization rates are related primarily to the activation energy for segmental transport. The non-crystallizable structure affects the segmental mobility to different degrees. An estimation of this effect is presented from the analysis of the overall crystallization rates using classical nucleation theory. In spite of thedifferences in their molecular structure, there are no major differences in the supermolecular structure of samples crystallized rapidly or slowly cooled.The melting process followed by DSC of the isothermally crystallized linear and star copolymers shows two endothermic peaks at intermediate undercoolings. The double melting is associated with a partitioning of crystallizable ethylene sequences during crystallization. The longest sequences are preferentially selected in the early stages of the crystallization. Single melting peaks are obtained for high and very low undercoolings for the linear and the star copolymers as well as for the diblock in the whole range of temperatures. The lack of the second, lower melting endotherm in the diblock could be associated with the influence in the crystallization process of the amorphous block in the microphase segregated melt.This revised version was published online in November 2005 with corrections to the Cover Date.     

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     

Block copolymers of poly(ethylene terephthalate–polybutylene terephthalate). I. Preparation and crystallization behavior

Block copolymers of two crystallizable compounds, poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT), were developed with PET as the major component and the amount of PBT varying from 1.0 to 20.0 wt %. These block copolymers were prepared by end-group coupling of preformed oligomers. All polymers prepared were of equivalent molecular weight as determined by the intrinsic viscosity method. Thermal properties were determined by differential thermal analysis (DTA), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). With increasing PBT content, the block copolymers showed a general decrease in the values of glass transition temperature, melting temperature, initial decomposition temperature, and maximum decomposition temperature. The heat of fusion and heat of crystallization first increased and then decreased slightly. Rates of crystallization were determined by measuring density as a function of time of isothermal crystallization carried out at 95°C. It was found that small amounts of PBT increased the crystallization rate considerably over that of PET. Random copolymers did not show this phenomenon and behaved more like pure PET. The crystallization behavior of block copolymers was analyzed by the Avrami equation and Avrami exponents were determined. Results were explained on the basis that the faster-crystallizing PBT blocks crystallized first and provided built-in nucleation sites for the subsequent crystallization of PET, thus resulting in a relatively fast-crystallizing copolyester.     

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,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     

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     

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     

Structure of ethylene sulphide crystallites in block copolymers

Block copolymers, having the configuration ethylene sulphide-isoprene-styrene or ethylene sulphide-isoprene-ethylene sulphide, have been characterized to define the structure of the ethylene sulphide moiety. It is concluded that ethylene sulphide-isoprene-styrene block copolymers exist in solution and in bulk (above the styrene softening point) as radial aggregates held together by polyethylene sulphide crystallites. During polymerization, these crystallites are formed with a linear extended chain morphology which is retained in bulk in ethylene sulphide-isoprene-styrene copolymers: in the copolymers which require processing above the melting point of the ethylene sulphide crystallites, the linear morphology is destroyed during moulding.     

Study on Melting and Recrystallization of Poly(butylene succinate) Lamellar Crystals via Step Heating Differential Scanning Calorimetry

Differential scanning calorimetry (DSC) has been widely applied to study crystallization and melting of materials.However,for polymeric lamellar crystals,the melting thermogram during heating process usually exhibits a broad endothermic peak or even multiple endotherms,which may result from changes of metastability via recrystallization process.Sometimes,the recrystallization exotherm cannot be observed due to its overlapping with the melting endotherm.In this work,we employed a step heating procedure consisting of successive heating and temperature holding stages to measure the metastability of isothermally crystallized poly(butylene succinate) (PBS) crystals.With this approach we could gain the fraction of crystals melted at different temperature ranges and quantitatively detect the melting-recrystallization behavior.The melting-recrystallization behavior depends on the polymer chain structure and the crystallization temperature.For instance,PBS block copolymer hardly shows recrystallization behavior while PBS oligomer and high molecular weight PBS homopolymer demonstrate remarkable melting-recrystallization phenomenon.High molecular weight PBS isothermally crystallized in the low temperature range shows multiple melting-recrystallization while those isothermally crystallized at elevated temperatures do not exhibit observable recrystallization behavior.Furthermore,the melting endotherms were fitted via the melting kinetics equations.The original isothermally crystallized lamellae demonstrate quite different melting kinetics from the recrystallized lamellar crystals that melt at the highest temperature range,which is attributed to the different degrees of stabilization.Finally,the mechanism of melting-recrystallization is briefly discussed.We propose that apparent meltrecrystallization phenomenon be observed when melting of preformed lamellar crystals and recrystallization of thicker lamellae have similar free energy barrier.     

Melting and crystallization of poly(ethylene terephthalate) under pressure

A piston-cylinder-type high-pressure dilatometer has been built and the effect of pressure on melting behavior of poly(ethylene terephthalate) (PET) has been studied. The melting temperature increases but the rate of change of the melting temperature decreases with increasing pressure. Poly(ethylene terephthalate) crystallized from the melt at elevated pressure and temperature was studied by thermal analysis, and wide-angle and small-angle x-ray diffraction and electron microscopy. Chain-extended PET crystals were observed for the first time and some of their properties are described. A similarity to extended-chain polyethylene is suggested.     

Characterization of polyethylene with partially random chlorine substitution

A series of chlorine‐containing polymers were prepared by ring‐opening metathesis polymerization (ROMP) followed by hydrogenation. This synthesis route was chosen specifically so that chain microstructures would be obtained that resembled copolymers of ethylene and vinyl chloride. The chlorine content was varied by the copolymerization of 5‐chlorocyclooctene and cyclooctene. Differential scanning calorimetry, light microscopy, tapping‐mode atomic force microscopy, wide‐angle X‐ray diffraction (WAXD), and density were employed to characterize the polymers. The copolymers had certain restrictions on the length of the methylene sequence between substituted carbons, however, ROMP copolymerization introduced enough variation in the methylene sequence length that model copolymers with the equivalent of 14 mol % vinyl chloride or less closely resembled random copolymers of ethylene and vinyl chloride. These materials organized as spherulites and exhibited the orthorhombic crystal form. Constraints on the placement of chlorine atoms strongly affected the crystallization of polymers with more than the equivalent of 14 mol % vinyl chloride. More regular chlorine substitution along the polyethylene chain translated into better ordered crystal structures with sharp melting peaks. The granular morphology of these materials at ambient temperature was interpreted as fringed micellar crystals. The WAXD patterns provided definitive evidence that chains in the fringed micelle took the hexagonal crystal form. The lower density hexagonal form facilitated the crystallization of short ethylene sequences and accommodated chlorine atoms more easily than the orthorhombic form. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2062–2070, 2003