Influence of mechanical history on the crystallization behaviour and morphology of low density polyethylene

Summary If a virginal LDPE melt is sheared a memory is frozen during crystallization inasmuch as after remelting the nucleation density and the rate of crystallization are strongly increased as compared with the original material. This is not caused by the introduction of impurities into the melt during shearing.Part of this memory is recovered by annealing or solvent treatment. This reversible contribution is discussed in terms of a shear-induced increase of trans conformations in the melt, which is partly frozen on pelletizing and which becomes active again on remelting.The non-recoverable part of the memory must be due to changes of molecular structure. Since other parameters do not change it is supposed that a change of the long-chain branching frequency during shearing is the reason for the irreversible effects.Blown films produced from LDPE pellets, which originate from a strongly sheared melt and consequently have an increased crystallization rate, are known to have improved processing and end-use properties (maximum draw-down speed, optical quality). The film blowing process itself is shown to introduce an additional mechanical history into the LDPE melt such that differences of the crystallization kinetics of the starting pellets are completely equalized. A different crystallization behaviour of pellets, therefore, cannot be responsible for the differing film properties, which are more likely influenced by the different viscoelastic response of the melt.Dedicated to Professor Dr. Werner Reif on the occasion of his 60th birthday.     

Nonisothermal crystallization kinetics and melting behavior of bimodal medium density polyethylene/low density polyethylene blends

Nonisothermal crystallization kinetics and melting behavior of bimodal-medium-density- polyethylene (BMDPE) and the blends of BMDPE/LDPE were studied using differential scanning calorimetry (DSC) at various scanning rates. The Avrami analysis modified by Jeziorny and a method developed by Mo were employed to describe the nonisothermal crystallization process of BMDPE. The BMDPE DSC data were analyzed by the theory of Ozawa. Kinetic parameters such as the Avrami exponent (n), the kinetic crystallization rate constant (Z_c), the peak temperatures (T_p) and the half-time of crystallization (t_1/2) etc. were determined at various scanning rates. The appearance of double melting peaks and the double crystallization peaks in the heating and cooling DSC curves of BMDPE/LDPE blends indicated that the BMDPE and LDPE could crystallize respectively.     

Nonisothermal crystallization and melting behavior of mPE/LLDPE/LDPE ternary blends

Nonisothermal crystallization kinetics of ternary blends of the metallocence polyethylene (mPE), low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE) were studied using DSC at various scanning rates. The Ozawa theory and a method developed by Mo were employed to describe the nonisothermal crystallization process of the two selected ternary blends. The results speak that Mo method is successful in describing the nonisothermal crystallization process of mPE/LLDPE/LDPE ternary blends, while Ozawa theory is not accurate to interpret the whole process of nonisothermal crystallization. Each ternary blend in this study shows different crystallization and melting behavior due to its different mPE content. The crystallinity of the ternary blends rises with increasing mPE content, and mPE improve the crystallization of the blends at low temperature. The crystallization activation energy of the five ternary blends that had been calculated from Vyazovkin method was increased with mPE content, indicating that the more mPE in the blends, the easier the nucleus or microcrystallites form at the primary stage of nonisothermal crystallization. LLDPE and mPE may form mixed crystals due to none separated-peaks were observed around the main melting or crystallization peak when the ternary blends were heating or cooling. The fixed small content of LDPE made little influence on the main crystallization behavior of the ternary blends and the crystallization behavior was mainly determined by the content of mPE and LLDPE.     

Studies of non-isothermal crystallization and melting of ultra high molecular weight polyethylene

The non-isothermal crystallization and melting of ultra high molecular weight polyethylene (UHMWPE) were observed by means of differential scanning calorimetry and compared with those of ordinary high-density polyethylene (HDPE). The crystallization temperature (T _c ) and melting point (T _m ) of UHMWPE were found to be higher thanT _c andT _m of HDPE, and the latent heat of crystallization (δH _c ) and fusion (δH _m ) of UHMWPE are smaller thanδH _c andδH _m of HDPE. The results were explained in terms of the theory of polymer crystallization and the structure characteristics of UHMWPE. The relationships between the parameters (T _c ,T _T ,δH _c andδH _m ) and the molecular weight (M) of UHMWPE are discussed. Processing of the experimental data led to the establishment of four expressions describing the above relationships.     

The effects of natural zeolite and silane coupling agents on melting and crystallization behaviour of polypropylene

The thermal characterization of polypropylene (PP) composites containing untreated and treated zeolite with different silane coupling agents was carried out using thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) to investigate the effects of natural zeolite and surface modifiers on melting, crystallization and degradation behaviour of PP. 3-aminopropyltriethoxysilane (AMPTES), methyltriethoxysilane (MTES) and 3-mercaptopropyltrimethoxysilane (MPTMS) were used as surface modifiers at four different concentrations (0.5–2.0 mass%). Thermal analyses indicated that silane treatment and 2–6 mass% zeolite addition have no significant effect on the melting and degradation temperatures of the composites. The crystallization temperatures of the composites were increased due to the nucleating effect of the zeolite. The influence of the modifiers on the interactions between PP and zeolite was determined by the activities of untreated and treated zeolite. The maximum interactions leading to good adhesion were observed in the AMPTES treated composites. Also, non-isothermal crystallization kinetics of the composites was analyzed using Avrami and Kissinger models.     

Morphology and crystallization of blends of linear low density polyethylene with a semiflexible liquid crystalline polymer

The morphology and the crystallization behavior of blends of linear low density polyethylene (LLDPE) with an experimental sample of a semiflexible liquid crystalline polymer (SBH 112 by Eniricerche, Italy) have been studied by differential scanning calorimetry (DSC), polarized optical microscopy (POM) and scanning electron microscopy (SEM). The blends possess a two-phase morphology, due to immiscibility of the two components. SEM observations show that dispersion of the minor SBH phase is favored at low (相似文献   

Effect of thermal history on the melting behavior and the crystal-crystal transitions of tetrafluoroethylene-perfluoroalkylvinylether copolymers

By differential scanning calorimetry (DSC), the effect of thermal treatments on the melting of tetrafluoroethylene-perfluoroalkylvinylether copolymers (PFA) with different contents of perfluoromethylvinylether as a comonomer has been investigated. Two melting peaks can be identified for all copolymers, whose presence, value, and extent depend upon the thermal treatments. The higher temperature one, scarcely influenced by the crystallization conditions (i.e., cooling rate and annealing), can be attributed to more perfect crystals present in the original samples. The lower temperature one is produced only by annealing (annealing peak) and can be interpreted as resulting from much poorer crystals grown among the larger ones. For the copolymer with the lowest content of comonomer, the effect of thermal treatments on the low-temperature (crystal-crystal) transitions has also been studied. The rejection of the counits from the crystals, at least in equilibrium conditions, is confirmed. © 1996 John Wiley & Sons, Inc.     

Mathematical modeling of crystallization analysis fractionation of ethylene/1‐hexene copolymers

Crystallization analysis fractionation (Crystaf) is a polymer characterization technique for estimating the chemical composition distributions of semicrystalline copolymers. Although Crystaf has been widely used during the recent years, it is still a relatively new polymer characterization technique. More quantitative understanding of its fractionation mechanism is essential for further developments. In this work, three ethylene/1‐hexene copolymers with different 1‐hexene fractions, but similar number‐average molecular weights, were analyzed by Crystaf at several cooling rates. A mathematical model was proposed to describe the effect of comonomer fraction and cooling rate on Crystaf fractionation from a fundamental point of view. The model describes the experimental Crystaf profiles of ethylene/1‐hexene copolymers with different 1‐hexene fractions measured at distinct cooling rates very well. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1010–1017, 2007     

The deformation behaviour of polyethylene shish-kebabs produced by stirring-induced crystallization

Polyethylene mats of shish-kebab fibrils were prepared from solution by stirring-induced crystallization, and subjected to deformation. A morphological study by scanning electron microscopy showed that the elementary shish-kebabs are elongated during drawing. For low draw ratios, the average distance between the lamellae on the fibrils increases proportionally to the draw ratio. The invariance of the fibril diameter upon drawing indicates a transformation of lamellar into fibrillar material. The molecular topology which underlies this deformation mode is discussed and related to the crystallization process.     

Polyolefin analysis by single‐step crystallization fractionation

Temperature rising elution fractionation (TREF) fractionates polymer chains with respect to their crystallizability, independently of molecular weight effects. In order to achieve a good fractionation, TREF requires a time‐consuming polymer deposition step over an inert support before the elution step. A single‐step crystallization fractionation method has been developed recently,^1,2 Crystallization Analysis Fractionation (CRYSTAF), in which the chemical composition (or short chain branching) distribution of olefin copolymers can be measured by monitoring on‐line polymer concentration in solution at decreasing temperatures. For the present experimental investigation, a CRYSTAF‐prototype has been assembled and used to fractionate several linear low‐density polyethylene (LLDPE) samples. These results were compared to the ones measured by the commercial CRYSTAF apparatus from Polymer ChAR. Additionally, CRYSTAF results from Polymer ChAR were compared to analytical TREF results. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 539–552, 1999     

Random copolymers of poly(butylene terephthalate): Effect of thiodiethylene terephthalate units on melting behaviour and crystallization kinetics

The melting behavior and the crystallization kinetics of poly(butylene terephthalate/thiodiethylene terephthalate) copolymers were investigated by DSC technique. The multiple endotherms were influenced both by T _c and composition. By applying the Hoffman—Weeks' method, T _m ⁰ the of the copolymers was derived. The isothermal crystallization kinetics was analyzed according to the Avrami's treatment. Values of the exponent n close to 3 were obtained, independently of T _c and composition. The introduction of thiodiethylene terephthalate units decreased the PBT crystallization rate. H _m was correlated to c _p for samples with different degree of crystallinity and the results were interpreted on the basis of the existence of an interphase.This revised version was published online in November 2005 with corrections to the Cover Date.     

LLDPE／EAA共混体系结晶行为及相容性

通过ＤＭＡ、ＤＳＣ、偏光显微镜（ＰＬＭ）、ＷＡＸＤ及力学性能测试等方法，对线性低密度聚乙烯（ＬＬＤＰＥ）／乙烯－丙烯酸共聚物（ＥＡＡ）共混体系的研究表明，ＬＬＤＰＥ与ＥＡＡ的非晶相可部分相容，结晶相不能形成共晶；共混物结晶时，两组分相互影响，ＬＬＤＰＥ的结晶速度高于ＥＡＡ，两者结晶没有进入对方晶胞中．还发现ＬＬＤＰＥ与ＥＡＡ力学性能上相容．低含量ＥＡＡ共混体系显示出较佳的力学性能．     

Effect of molecular architecture on quiescent and shear‐induced crystallization of polyethylene

The aim of this work is to investigate the effect of the molecular structure of polyethylene on the crystallization kinetics. In static conditions, the increase of the degree of branching leads to the decrease of the crystallization temperature, the melting temperature, and the crystallinity. Indeed, the crystal thickness is controlled by the length of PE segments between branching. The effect of preshear on crystallization kinetics was studied by following the dynamic modulus along the time after a treatment of constant shear rate. Particularly, the effect of the shear rate was investigated. The enhancement of crystallization kinetics appears directly linked to the relaxation time of the melt polymer. Expressed by the Weissenberg number, a “master curve” is obtained independent of the amount and length of branching, leading to the conclusion that the nucleation due to shear is conditioned by the molecular architecture mainly via its effect on the relaxation time. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1597–1607, 2006     

Effect of the presence of silica nanoparticles in the coefficient of thermal expansion of LDPE

The effect of the presence of alumina microparticles and silica nanoparticles on the coefficient of thermal expansion (CTE) of films of low density polyethylene (LDPE) based composites was investigated. A new method based on the use of an atomic force microscope (AFM) is proposed for measuring nano-thermal expansion of films to finally obtain the CTE in polymer based materials. Nanocomposites based on silica nanoparticles and LDPE were prepared by mixing those constituents by high energy ball milling (HEBM). Pure alumina microparticles come from the milling tools used to mix the components of the composites. When silica nanoparticles are used as nanofiller of LDPE the effectiveness on reducing the CTE (about a 40% of CTE reduction) is higher than that obtained when high amount of alumina microparticles are present in the LDPE. Only when high amount of silica nanoparticles and low amount of alumina microparticles are present, the reduction of CTE expected from the Levin model is in accordance with the experimental results. This effect was associated to the high surface to volume ratio of nanoparticles considering uniform dispersions of them within the polymer. The region of polymer between particles must be so thin (few nanometers) that constraint effects must play an important role on reducing the chain mobility and therefore the thermal expansion.     

Nonisothermal crystallization and melting behavior of syndiotactic polypropylenes of different microstructure

Syndiotactic polypropylenes and their copolymers with 1‐olefins were synthesized using two metallocene/MAO catalytic systems, and the effect of the different microstructures on nonisothermal crystallization and subsequent melting was studied. Using differential scanning calorimetry (DSC) it was observed that samples with lower content of defects showed crystallization on cooling from the melt, and a double melting peak in the subsequent heating scan, the latter associated with melt, recrystallization and remelt processes that it was confirmed by its nonreversing exothermic process found by means of temperature modulated DSC (MDSC). However, polymers with high amount of defects showed cold crystallization on heating followed by a melting process, that it was observed by MDSC. Wide angle X‐ray diffraction was used for characterizing the changes of crystalline forms in relationship with crystallization process. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 798–806, 2008     

固相法氯化聚乙烯对PVC／LLDPE共混体系性能和形态的影响

采用固相法氯化聚乙烯（ＣＰＥ）对聚氯乙烯／线型低密度聚乙烯（ＰＶＣ／ＬＬＤＰＥ）共混体系进行增容改性。扫描电子显微镜、透射电子显微镜、动态力学分析和力学性能测试结果表明，ＣＰＥ对ＰＶＣ／ＬＬＤＰＥ共混体系具有很好的增容作用。     

Effects of thermal stability on the crystallization behavior of poly(vinylidene chloride)

Thermal analysis based on TGA (thermal gravimetric analysis) and DSC (differential scanning calorimeter) shows no significant degradation for PVDC which has been annealed at 210°C for less than 2 min. And the following recrystallization behavior at lower temperature (120°C) is also independent of the thermal treatment and is not affected by the difference of molecular weight. The degradation which includes dehydrochlorination at lower temperature and intramolecular cyclization or intermolecular crosslinking of the polyenes at higher temperature starts when the melting time at 210°C is more than 2 min, which also causes weight loss and heat exchange in the TGA and DSC thermograms. The recrystallization behavior of the degraded PVDC (staying at 210°C for more than 2 min) shows a strong dependence on the molecular weight. The crystallinity is decreased with the melting time at 210°C due to the increase of the degree of crosslinking. However, the POM (polarized optical microscopy) pictures and IR spectra show a favorable nucleation effect is present due to the formation of trichlorobenzene from the cyclization of the polyenes as nuclei. The crystallinity of the PVDC recrystallized at 120°C after staying at 210°C for more than 2 min is actually dependent on the molecular weight, melting time at 210°C, and cyclized or crosslinking types of degradation. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3269–3276, 1999     

Isothermal crystallization kinetics and melting behaviour of PET/ATO nanocomposites prepared by in situ polymerization

Neat poly(ethylene terephthalate) (PET) and PET/antimony doped tin oxide (ATO) nanocomposites were prepared by in situ polymerization. The study of the isothermal crystallization behaviors of neat PET and PET/ATO nanocomposites was carried out using differential scanning calorimetry (DSC). The crystallization kinetics under isothermal conditions could be described by the Avrami equation. For neat PET and PET/ATO nanocomposites, the Avrami exponent n both decreased with increasing crystallization temperature. In addition, for the same crystallization temperature, the value of n increased with increasing ATO content. These suggested that the crystallization types related to the values of n in the Avrami theory could not be suitable for the crystallization of PET and its nanocomposites. The change of the n values indicated that the addition of ATO resulted in the increase of the crystallizing growth points. That is a heterogeneous nucleating effect of ATO on crystallization of PET. In the DSC scan after isothermal crystallization process, multiple melting behavior was found. And the multiple endotherms could be attributed to melting of the recrystallized materials or the secondary lamellae produced during different crystallization processes. The equilibrium melting temperature of PET in the nanocomposites increased with increasing the ATO content. Surface free energy of PET chain folding for crystallization of PET/ATO nanocomposites was lower than that of neat PET, confirming the heterogeneous nucleation effect of ATO.     

Compatibilizing effect of isocyanate functional group on polyethylene terephthalate/low density polyethylene blends

To evaluate the compatibilizing effects of isocyanate (NCO) functional group on the polyethylene terephthalate/low density polyethylene (PET/LDPE) blends, LDPE grafted with 2-hydroxyethyl methacrylate-isophorone diisocyanate (LDPE-g-HI) was prepared and blended with PET. The chemical reaction occurred during the melt blending in the PET/LDPE-g-HI blends was confirmed by the result of IR spectra. In the light of the blend morphology, the dispersions of the PET/LDPE-g-HI blends were very fine over the PET/LDPE blends. DSC thermograms indicated that PET microdispersions produced by the slow cooling of the PET/LDPE-g-HI blends were largely amorphous, with low crystallinity, due to the chemical bonding. The tensile strengths of the PET/LDPE-g-HI blends were higher than those of the PET/LDPE blends having a poor adhesion. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 447–453, 1998