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     

Real‐time SAXS and WAXS study of the multiple melting behavior of poly(ε‐caprolactone)

The multiple melting behavior of poly(ε‐caprolactone) (PCL) was investigated by real‐time small angle X‐ray scattering (SAXS) and wide angle X‐ray scattering (WAXS) measurements coupling with differential scanning calorimetry (DSC). Semicrystalline specimens prepared by a continuous cooling process showed lengthening of the Bragg period during the progress of double melting. A model of variable thickness of lamella was proposed to fit to the SAXS patterns and revealed that both the crystalline lamella and the amorphous layer contributed to the increase in Bragg period while the later dominated the contribution. The model of variable thickness although satisfied the SAXS data was unable to compromise the data from other probing tools. A modification of the model proposed that each lamella piling up to construct the stacks in the crystallites was itself nonuniform in thickness. The modification with the parallel occurrence of the mechanism of surface melting and crystallization successfully compromised the observations from SAXS, DSC, and optical microscopy and provided a new perspective for the explanation to lengthening of the Bragg period related to multiple melting behavior. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1777–1785, 2010     

Plasticization and cocrystallization in LLDPE/wax blends

The structure and thermal properties of linear low‐density polyethylene (LLDPE)/medium soft paraffin wax blends, prepared by melt mixing, were investigated by differential scanning calorimetry (DSC) and small‐ and wide‐angle X‐ray scattering (SAXS and WAXS). The blends form a single phase in the melt as determined by SAXS. Upon cooling from the melt, two crystalline phases develop for blends with more than 10 wt % wax characterized by widely different melting points. The wax acts as an effective plasticizer for LLDPE, decreasing both its crystallization and melting temperature. The higher melting point crystalline phase is formed by less branched LLDPE fractions. On the other hand, the lower melting point crystalline phase is a wax‐rich phase constituted by cocrystals of extended chain wax and short linear sequences of highly branched LLDPE chains. The presence of cocrystals was evidenced by standard DSC results, successive self‐nucleation and annealing (SSA) thermal fractionation and by the detection of a new SAXS signal attributed to the lamellar long period of the cocrystals. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1469–1482     

Molecular dynamics simulations of monodisperse/bidisperse polymer melt crystallization

We use large scale coarse‐grained molecular dynamics simulations to study the kinetics of polymer melt crystallization. For monodisperse polymer melts of several chain lengths under various cooling protocols, we show that short chains have a higher terminal crystallinity value compared to longer ones. They align at the early stages and then cease evolving. Long chains, however, align, fold into lamella structures and then slowly optimize their dangling ends for the remaining simulation time. We then identify the mechanism behind bidisperse blend crystallization. To this end, we introduce a new algorithm (called Individual Chain Crystallinity) that allows the calculation of the crystallinity separately for short and long chains in the blend. We find that, in general, bidispersity hinders crystallization significantly. At first the crystallinity of the long chain components exceeds that of the monodisperse melt, but subsequently falls below the corresponding monodisperse melt curve after a certain “crossover time.” The time of the crossover can be attributed to the time required for the full crystallization of the short chains. This indicates that at the early stages the short chains are helping long chains to crystallize. However, after all short chains have crystallized they start to hinder the crystallization of the long chains by obstructing their motion. Lastly, polymer crystallization upon various thermodynamic protocols is studied. Slower cooling is found to increase the crystallinity value. Upon an instantaneous deep quench and subsequent isothermal relaxation, the crystallinity grows rapidly with time at early stages and subsequently saturates. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2318–2326     

Microhardness studies of chain-extended PE. I. Correlations to microstructure

The hardness–microstructure correlation of various polyethylene (PE) samples crystallized at high pressure from the melt (chain-extended), with different molecular weights, has been investigated and compared to melt crystallized samples at atmospheric pressure (chain-folded). The hardness, H, of melt crystallized PE is confirmed to increase linearly with the logarithm of the annealing time, t_a, at a constant annealing temperature. The H increase with t_a is discussed in terms of the crystallinity and crystalline lamellar thickness variation. Unusually high hardness values are obtained for samples crystallized or annealed at high pressure as a consequence of the resulting high degree of crystallinity and large crystalline lamellar thickness values. However, it is shown that the high surface free energy value of the chain-extended crystals considerably lowers the hardness values from that of an ideal infinitely thick PE crystal. Analysis of the crystal hardness and the melting temperature data of different polymeric materials emphasizes the close existing relationship between both quantities. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3151–3158, 1999     

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.     

Morphology and melting behavior of semicrystalline poly(ethylene terephthalate). II. Annealed PET

Annealing of poly(ethylene terephthalate) samples crystallized under isothermal conditions above the crystallization temperature has a marked influence on their morphology and results in increased thermal stability of the crystalline structure as indicated by the melting point increase of the samples. The morphological transformation processes induced by annealing are very complex and depend on the thermal history of the samples, i.e., crystallization temperature and heating procedure. Depending on the nature of the processes occurring during annealing, various parameters characterizing the semicrystalline state of the samples can be affected such as the degree of crystallinity, the long spacing, the thickness of amorphous and crystalline layers, the crystal perfection, the fold-surface structure, and the mosaic structure of the crystalline lamellae. Annealing involves a solid-state transformation of the original crystalline structure including crystal perfection without thickening or a melting followed by recrystallization with crystal perfection and crystal thickening. The combination of differential scanning calorimetric (DSC) measurements and small-angle x-ray scattering is a powerful analytical tool to detect morphological changes and helps in deciding on the processes which are involved in the transformation of the microstructure upon annealing.     

Thermal fractionation of vinyl acetate-vinyl alcohol copolymers

Thermal fractionation via the method of successive self-nucleation and annealing was used for the first time to study the crystallinity of vinyl acetate-vinyl alcohol copolymers with different random distributions of chain units. The lamella-thickness distribution was calculated through the Gibbs-Thomson equation. It was shown that, for all samples, the minimum lamella thickness is the same and corresponds to a block of no less than 15 vinyl alcohol units. On the basis of these data and with the use of the computer simulation of the polymer-analogous reaction via the Monte Carlo method, the block-length distribution in the crystalline phase was found. It was shown through a comparison of the lamella-thickness and block-length distributions that the maximum lamella thickness increases with the block length and vinyl alcohol content in the copolymer. In crystallites, blocks with lengths exceeding the maximum lamella thickness comprise a significant fraction. Thus, it is probable that these blocks form folds. The dependences of melting temperatures of crystalline lamellas on their thicknesses, as well as the dependences of the melting temperatures of copolymers not subjected to thermal fractionation on the chain-structure parameters, are adequately described by the Flory crystallization theory.     

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.     

Crystallinity of poly(3‐hexylthiophene) in thin films determined by fast scanning calorimetry

Using fast scanning calorimetry, we determined the crystallinity of thin films of poly(3‐hexylthiophene) crystallized from the melt from measurements of the specific melting enthalpy. A broad range of film thicknesses from 10 µm down to 26 nm was covered. The sample mass was determined from measurements of the specific heat capacity in the molten state allowing a quantitative analysis of the heat flow data. Films with a thickness 400 nm slowly cooled from the melt showed the same crystallinity as bulk samples measured with conventional DSC. Below 350 nm the melting enthalpy decreased strongly. We assign this strongly reduced crystallinity to the restricted crystallization kinetics originating from hindered spherulitic growth under thin film confinement. A higher crystallinity could be partially regained by extended isothermal crystallization at elevated temperatures. Much faster cooling, with rates above about 100 Ks^?1 led to a partial suppression of crystallization even for thick films. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1791–1801     

Crystallization and melting behavior of liquid crystalline copolyesters based on poly(ethylene terephthalate)

A series of copolyesters were prepared by the incorporation of p‐hydroxybenzoic acid (HBA), hydroquinone (HQ), and terephthalic acid (TA) into poly(ethylene terephthalate) (PET). On the basis of viscosity measurements, high molar mass copolyesters were obtained in the syntheses, and ¹H‐NMR analyses indicated the total insertion of comonomers. They exhibit nematic phase above melting temperature, as observed by polarized light microscope (PLM). Their crystallization and melting behaviors were also studied by differential scanning calorimetry (DSC) and wide angle X‐ray diffraction (WAXD). It was found that these copolyesters are more crystalline than copolyesters prepared from PET and HBA. Introduction of HQ/TA disrupts longer rigid‐rod sequences formed by HBA, and thus enhances molecular motion and increases crystallization rate and crystallinity. Isothermal crystallization at solid phase polymerization conditions (up to 24 h at 200°C) resulted in increased copolymer randomness (by NMR) and higher melting point, the latter attributed to structural annealing. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 369–377, 1999     

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     

An equilibrium study on the distribution of structural defects between the lamellar and amorphous portions of poly(vinylidene fluoride) and (vinylidene fluoride‐tetra fluoro ethylene) copolymer crystals

Samples of poly(vinylidene fluoride) (PVF₂) and (vinylidene fluoride‐tetra fluoroethylene) (VF₂‐VF₄) copolymer were etched with a chromium‐based etching reagent. The etching rate was lower for the VF₂‐VF₄ copolymer samples than for the PVF₂ samples. The melting point and enthalpy of fusion increased with increased etching time of the etched specimen. This was also true for the melt‐quenched (etched) samples, whose values were always lower than those obtained from the direct run of the etched samples. The scanning electron micrographs of specimens etched for 24 h indicated that only the amorphous portion was etched without affecting the crystalline lamella. The sequence distribution of the PVF₂ and VF₂‐VF₄ copolymer crystals were determined by ¹⁹F NMR measurements of the samples and their etched species. The observed probabilities (P_obs), calculated from the integrated area of the NMR peaks, indicated that the crystalline lamella had a more oriented chain structure than that of the amorphous overlayer portion. The head‐to‐head defects calculated from the aforementioned sequence analysis indicated a greater propensity in the amorphous portion than in the crystalline lamella. The equilibrium constant (K) for the distribution of defects between the lamella and amorphous portion of the crystal varied from 0.7 to 0.9. It was higher at a higher quenching rate of the crystallization, and in the isothermal crystallization, it also had a substantially high value, indicating the equilibrium inclusion of defects in the crystal. The distribution constant increased with an increase in the defect content in the chain and decreased with an increase in the defect size. The sequence distribution data, analyzed through a suitable melting‐point depression equation, indicated a defect energy of 2.25 kcal/mol for the α‐phase PVF₂ crystals and 0.68 kcal/mol for the β‐phase VF₂‐VF₄ copolymer. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 297–308, 2000     

Poly(L‐lactide) crystallization induced by multiwall carbon nanotubes at very low loading

Poly(L ‐lactide) (PLLA)/multiwall carbon nanotube (MWNT) composites were prepared by the solvent‐ultrasonic‐casting method. Only very low concentrations of MWNTs (<0.08 wt %) were added in the composites. Isothermal and nonisothermal crystalline measurements were carried out on PLLA/MWNT composites to investigate the effect of MWNTs on PLLA crystalline behavior. DSC results showed that the incorporation of MWNTs significantly shortened the crystalline induction time and increased the final crystallinity of the composite, which indicated MWNTs have a strong nucleation effect on PLLA even at very low concentrations. The nonisothermal crystallization measurements showed that the MWNTs greatly speed up crystallization during cooling. From isothermal crystallization results, both PLLA and PLLA/MWNT composites samples closely followed a relationship based on Lauritzen‐Hoffman theory, with the regime II to III transition shifting to lower temperature with increasing MWNT concentration. A double melting peak appeared in both nonisothermal and isothermal measurements. The conditions under which it appeared were found to closely depend on the regime II‐III transition temperature obtained from Lauritzen‐Hoffman theory. From the magnitude and position of melting peaks, it is proposed that the double melting peak is caused by a disorder‐order crystal phase transition. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2341–2352, 2009     

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     

Non-isothermal crystallization behaviors of poly(4-methyl-pentene-1)

Non-isothermal crystallization of isotactic poly(4-methyl-pentene-1) (P4MP1) is studied by differential scanning calorimeter (DSC), and kinetic parameters such as the Avrami exponent and the kinetic crystallization rate (Z _c) are determined. From the cooling and melting curves of P4MP1 at different cooling rates, the crystalline enthalpy increases with the increasing cooling rate, but the degree of crystalline by DSC measurement shows not much variation. Degree of crystalline of P4MP1 calculated by wide angle X-ray diffraction pattern shows the same tendency with crystalline enthalpy, indicating that re-crystallization occurs when samples heated above the second glass transition temperature of P4MP1. By Jeziorny analysis, n ₁ value suggests that mainly spherulites’ growth at 2.5 K min⁻¹ transforms into a mixture mode of three-dimensional and two-dimensional space extensions with further increasing cooling rate. In the secondary crystallization process, n ₂ values indicate that the secondary crystallization is mainly the two-dimensional extension of the lamellar crystals formed during the primary crystallization process. The rates of the crystallization, Z _c and t _1/2 both increase obviously with the increase of cooling rate, especially at the primary crystallization stage. By Mo’s method, higher cooling rate should be required in order to obtain a higher degree of crystallinity at unit crystallization time.     

Effect of shear on the crystallization of the poly(ether ether ketone)

The effect of shear on the crystallization behavior of the poly(ether ether ketone) (PEEK) has been investigated by means of ex situ wide‐angle X‐ray diffraction (WAXD), small‐angle X‐ray scattering, and differential scanning calorimetry (DSC). The changes of the intensity of WAXD patterns along shear direction of the PEEK induced by short‐term shear were observed when the samples crystallized at 330 °C. The results showed that the dimensions of the crystallites perpendicular to the (110) and (111) planes reduced with the increase of shear rate, whereas the dimensions of the crystallites perpendicular to (200) plane increased with the increase of shear rate. Moreover, increasing shear rate can lead to the increase of the crystallinity as well as the average thickness of the crystalline layers. Correspondingly, a new melting peak at higher temperature was found during the subsequent DSC scanning when the shear rate was increased to 30 s^?1. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 220–225, 2010     

The influence of wood flour and compatibilizer (<Emphasis Type="Italic">m</Emphasis>-TMI-<Emphasis Type="Italic">g</Emphasis>-PP) on crystallization and melting behavior of polypropylene

Wood flour/polypropylene composites (WPC) were prepared by melt extruding with different wood flour (WF) loadings. The non-isothermal crystallization and melting was studied with different WF loadings, for W40P60 and W40P60M6, the melting was investigated after non-isothermal and isothermal crystalline. Comparing with neat polypropylene, the melting behavior of the composites, both non-isothermally and isothermally, was investigated by differential scanning calorimetry (DSC). The results showed that WF was an effective heterogeneous nucleating agent, as evidenced by an increase in the crystallization temperature and the crystallinity for melt crystallization of PP with increasing WF content. For the non-isothermal samples, the origins of the double melting behaviors were discussed, based on the DSC results of PP. The XRD measurements confirmed that no crystalline transition existed during the non-isothermal crystallization process. With m-TMI-g-PP adding, due to compatibilization phenomenon were probably responsible for decreasing T _m, X _c. In the DSC scan after isothermal crystallization process, the single melting behaviors were found and each melting endotherm had a different origin.     

Crystallization and melting of isotactic polypropylene crystallized from quiescent melt and stress‐induced localized melt

Temperature dependency of crystalline lamellar thickness during crystallization and subsequent melting in isotactic polypropylene crystallized from both quiescent molten state and stress‐induced localized melt was investigated using small angle X‐ray scattering technique. Both cases yield well‐defined crystallization lines where inverse lamellar thickness is linearly dependent on crystallization temperature with the stretching‐induced crystallization line shifted slightly to smaller thickness direction than the isothermal crystallization one indicating both crystallization processes being mediated a mesomorphic phase. However, crystallites obtained via different routes (quiescent melt or stress‐induced localized melt) show different melting behaviors. The one from isothermal crystallization melted directly without significant changing in lamellar thickness yielding well‐defined melting line whereas stress‐induced crystallites followed a recrystallization line. Such results can be associated with the different extent of stabilization of crystallites obtained through different crystallization routes. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 957–963     

Crystalline structure and morphology of microphases in compatible mixtures of poly(tetrahydrofuran‐methyl methacrylate) diblock copolymer and polytetrahydrofuran

The crystalline structure and morphology of compatible mixtures of poly(tetrahydrofuran‐methyl methacrylate) diblock copolymers (PTHF‐b‐PMMA) with a polytetrahydrofuran homopolymer (PTHF) were studied with synchrotron X‐rays. Wide‐angle diffraction was used to study the crystalline structures in a confined lamellar region with a PTHF thickness ranging from 12.2 to 19.5 nm, and in a PTHF matrix with an interface distance between the PMMA cylinders ranging from 17 to 22 nm. As the above thickness values are around the long period (ca. 17 nm) of PTHF homopolymer under the crystallization condition used, the crystalline structure has been found to be very sensitive to the average thickness of the PTHF phase. The changes in the diffraction patterns with changing PTHF homopolymer content suggested a chain folding model in confined PTHF lamellae with the PTHF fiber axes being perpendicular to the thick PTHF lamella. In the case of hexagonally packed cylindrical PMMA microdomains with an interface distance ranging from 12 to 16 nm, the effects of PMMA cylinders on the crystallization morphology of PTHF in the PTHF matrix, and the effects of the PTHF crystallization on the hexagonally packed structure of PMMA cylinders were also studied. It is shown that only when the interdistance of two neighboring PMMA cylinders is comparable with the long period of the pure PTHF homopolymer, ordered PTHF stacks can be formed in the PTHF matrix. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 779–792, 1999