Non-isothermal crystallization of polyethylene blends with bimodal molecular weight distribution

Non-isothermal crystallization of polyethylene (PE) blends with bimodal molecular weight distribution (MWD) was investigated by differential scanning calorimetry (DSC) at various scanning rates. The bimodal PE blends were prepared by blending two unimodal polyethylenes with large difference in molecular weigh in different ratio in xylene solution. Different kinetic parameters such as the half-time of crystallization (t_1/2), crystallization rate constant (Z_c), F(T) and the effective activation energy were determined. Some complicated relationships between these parameters and the average molecular weight were found. The crystallization rate first increased and reached a maximum then decreased, and also correlated with the MWD. The Avrami index under non-isothermal conditions was analyzed with a method developed by Harnisch and Muschik; the results indicated that homogeneous nucleation and spherulitic growth regimes were present in all samples studied.     

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.     

Crystallization and melting behavior of poly(ethylene oxide)/poly(n-butyl methacrylate) blends

The phase diagram, crystallization and melting behavior of poly(ethylene oxide) (PEO)/poly(n-butyl methacrylate) (PnBMA) blends have been investigated using differential scanning calorimetry and optical microscopy. The results show that the blends are miscible up to 85 °C and show an lower critical solution temperature-type demixing at a higher temperature. The isothermal crystallization studies of the blends indicate a reduction in the overall rate of crystallization. Analysis of isothermal crystallization data by means of Avrami equation leads to average values of the Avrami index of 2.5 for pure PEO and 3.0 for the different blend compositions. The melting behavior of the blends reveals double endotherms, which is ascribed to both secondary crystallization and recrystallization. The melting point depression study yielded χ₁₂=0, indicating a relatively low interaction strength.     

Nonisothermal crystallization and melting behavior of poly(β‐hydroxybutyrate)–Poly(vinyl‐acetate) blends

Nonisothermal crystallization and melting behavior of poly(β‐hydroxybutyrate) (PHB)–poly(vinyl acetate) (PVAc) blends from the melt were investigated by differential scanning calorimetry using various cooling rates. The results show that crystallization of PHB from the melt in the PHB–PVAc blends depends greatly upon cooling rates and blend compositions. For a given composition, the crystallization process begins at higher temperatures when slower scanning rates are used. At a given cooling rate, the presence of PVAc reduces the overall PHB crystallization rate. The Avrami analysis modified by Jeziorny and a new method were used to describe the nonisothermal crystallization process of PHB–PVAc blends very well. The double‐melting phenomenon is found to be caused by crystallization during heating in DSC. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 443–450, 1999     

Effect of PEG on the crystallization of PPDO/PEG blends

A new biodegradable polymer system, poly(p-dioxanone) (PPDO)/poly(ethylene glycol) (PEG) blend was prepared by a solvent casting method using chloroform as a co-solvent. The PPDO/PEG blends have different weight ratios of 95/5, 90/10, 80/20 and 70/30. Crystallization of homopolymers and blends were investigated by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). When 5% of PEG was blended, the crystallization exothermal peaks (T_c) of PPDO increased sharply and the crystallization exothermal peaks (T_c) of PEG decreased slightly compared with the homopolymers. The crystallization rates of both components increased, and caused greater relative crystallization degree (X_t%). But when the content of PEG was more than 5%, the crystalline behaviors of blends had no more significant changes accordingly. The melting points of each sample varied little over the entire composition range in this study. The nonisothermal crystallization of PPDO homopolymer and blend (PPDO/PEG = 70/30) were also studied by DSC. The crystallization began at a higher temperature when the cooling rates were slower. The nonisothermal crystallization kinetics of blends was analyzed by Ozawa equation. The results showed that the Ozawa equation failed to describe the whole crystallization of the blend, but Mo equation could depict the nonisothermal crystallization perfectly.     

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     

Implications of LDPE Branching and Mw on Thermal and Mechanical Properties of PP/LDPE Blends

Summary: The influences of short chain branching (SCB) and molecular (M_w) weight of low density polyethylene (LDPE) on the solid state properties of polypropylene (PP)-LDPE blends were investigated by mechanical and thermal techniques. DSC analysis of all blends exhibit a double melting peak at all compositions studied thus suggesting that both PP and LDPE crystals exist separately in the solid state. It was found that the SCB and Mw of LDPE influenced the modulus and ultimate tensile strength of the blends. However, elongation at break seems to be independent of the molecular characteristics of the pure homopolymer especially at PP blend composition greater than 50%. LDPE with high SCB showed broader melting peaks. Addition of a small amount of a low Mw LDPE (10%) resulted in a higher elongation at break than a high Mw LDPE. There is likely a correlation between the presence of a new peak in the thermograms of PP-rich blends and the observed poor elongation at break.     

Kinetics of nonisothermal crystallization and melting of normal high density and ultra-high molecular weight polyethylene blends

The kinetics of nonisothermal crystallization and melting of blends of ultra-high molecular weight polyethylene (UHMWPE) and polyethylene high density with normal molecular weight (NMWPE) are investigated by means of differential scanning calorimetry (DSC). Mixing the components at a temperature below the flow temperature of UHMWPE (215 °C) results in increased crystallization/melting rates of the individual components in the blends above the corresponding additive values. The morphological observations of the blends, carried out by means of polarization microscopy, show that a strong boundary of both types of structures (UHMWPE non-flowing aggregates and NMWPE spherulite structures) does not exist. The NMWPE spherulites' dimensions decrease on increasing the UHMWPE concentration in the blends, but their number increases. The facilitation of the crystallization/melting of the components in the blends is explained in terms of mutual influence exhibited by the components with respect to each other. It is due to the inner stresses in nonflowing UHMWPE characterized with a lot of entangled tie molecules and to the partial co-crystallization of NMWPE molecules with the flowing part of UHMWPE. At mixing temperatures above 215 °C the melting/crystallization integral kinetic curves have only one linear part in contrast to these of the same blend (11 ratio of components), prepared at 190 °C. The rates of melting/crystallization remain almost constant with the increase of the mixing temperatures.     

Isothermal crystallization kinetics of AB2 hyper-branched polymer (HBP)-filled polypropylene (PP)

In this paper, the isothermal crystallization kinetics of pure polypropylene (PP) and AB₂ hyper-branched polymer (HBP)/PP blends have been investigated by differential scanning calorimetry (DSC). During isothermal crystallization, the crystallization rates of the blends are higher than those of PP. Furthermore, in the blends with different HBP contents, the value of t _1/2 became smaller with increasing HBP content; however, the crystallization rate of the blend decreased slightly when the content of HBP is 5 %. An increase in the Avrami exponent means the addition of HBP influences the mechanism of nucleation and the growth of PP crystallites. In addition, the crystallization activation energy of pure PP and HBP/PP blends were also discussed, and the result showed that the crystallization activation energy has decreased remarkably in HBP/PP blends.     

Crystallisation behaviour of high density polyethylene blends with bimodal molar mass distribution: 2. Non-isothermal crystallisation

Crystallisation of high density polyethylene (HDPE) blends with broad bimodal molar mass distribution was investigated by differential scanning calorimetry (DSC) under non-isothermal conditions. The blends were prepared by blending a high molar mass PE (M_w=330 kg/mol, M_w/M_n=4.8) and a low molar mass linear PE (M_w=34 kg/mol, M_w/M_n=10) in different ratios in xylene solution. The samples were analysed by the normal DSC at different crystallisation rates and by a thermal fractionation technique.The blends and their parent polymers behaved according to general expectations i.e., crystallinity and density decreased when the molar mass of the samples increased. Additionally, non-linear relationships between MM and different analysed parameters were found. Small addition of the high molar mass parent polymer to the low molar mass parent polymer increased crystallisation temperature, although the general trend was decreasing. Furthermore, a complicated relationship between the reciprocal of crystallisation half-time and sample composition was found. The value increased first with increasing molar mass, reached a maximum when the average molar mass of the blend was between 150 and 200 kg/mol and then decreased. The detected maximum correlated with the broadest molar mass distribution of the blends. The crystallinities and densities of the blends with the broadest molar mass distribution also deviated from the linear correlation between them and molar mass. The Avrami index under non-isothermal conditions was analysed with a method developed by Harnisch and Muschik. The results indicated that thermal nucleation and spherical growth regimes are present in all studied materials.     

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   

Thermal analysis of the double-melting behavior of poly(L-lactic acid)

The melting and crystallization behavior of poly(L -lactic acid) (PLLA; weight-average molecular weight = 3 × 10⁵) was studied with differential scanning calorimetry (DSC). DSC curves for PLLA samples were obtained at various cooling rates (CRs) from the melt (210 °C). The peak crystallization temperature and the exothermic heat of crystallization determined from the DSC curve decreased almost linearly with increasing log(CR). DSC melting curves for the melt-crystallized samples were obtained at various heating rates (HRs). The double-melting behavior was confirmed by the double endothermic peaks, a high-temperature peak (H) and a low-temperature peak (L), that appeared in the DSC curves at slow HRs for the samples prepared with a slow CR. Peak L increased with increasing HR, whereas peak H decreased. The peak melting temperatures of L and H [T_m(L) and T_m(H)] decreased linearly with log(HR). The appearance region of the double-melting peaks (L and H) was illustrated in a CR–HR map. Peak L decreased with increasing CR, whereas peak H increased. T_m(L) and T_m(H) decreased almost linearly with log(CR). The characteristics of the crystallization and double-melting behavior were explained by the slow rates of crystallization and recrystallization, respectively. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 25–32, 2004     

Crystallisation behaviour of high density polyethylene blends with bimodal molar mass distribution 1. Basic characteristics and isothermal crystallisation

Isothermal crystallisation of high density polyethylene (HDPE) blends and their parent polymers was investigated. The blends having broad bimodal molar mass distributions and various compositions were prepared by blending a high molar mass (M_w=330 kg/mol, M_w/M_n=4.8) and a low molar mass HDPE (M_w=34 kg/mol, M_w/M_n=10) in different ratios in xylene solution. The blends and their parent components were characterised by size-exclusion chromatography, dynamic rheological and density measurements. Crystallisation kinetics were studied using a polarised light microscope equipped with an in-house built hot stage and by differential scanning calorimetry. The Avrami theory was applied for crystallisation kinetics analysis. Such crystallisation kinetics parameters as nucleation rate, nucleation density, the Avrami index and cystallisation rate contant were determined for the blends and their parent polymers.According to the results obtained an increasing polydispersity of the sample had a slight increasing effect on the Avrami index, indicating gain in prevalence of the thermal nucleation over the athermal one. In all samples nucleation density increased continuously during crystallisation verifying that the presence of a certain thermal nucleation was typical for all the materials studied. Both the crystallisation rate constant and the nucleation rate decreased with increasing molar mass of the sample. The nucleation density increased proportionally to the increase in average molar mass and the values were larger at lower crystallisation temperatures.The formed supermolecular structure was found to be sensitive to the blend composition and crystallisation temperature. Irregular banded or non-banded spherulites were observed in the materials. Banding of spherulites was typical for the samples having higher average molar mass. The superstructures observed in this work were smaller and vaguer than the superstructures reported in the earlier studies of polyethylene materials having similar average molar mass but narrow molar mass distribution.     

PC/PET/EPDM共混体系的结晶与熔融行为

本文用解偏振光法与DSC法分别测定并研究了PC/PET/EPDM共混体系的结晶速度、结晶度、Avrami指数(n)和熔融温度及其影响因素,共混物中PET的结晶速度、结晶度均随PC含量增加而下降;EPDM用量不超过10%时,可提高PET的结晶速度,但不影响结晶度和成核与增长方式,n值不变。当EPDM为5%时,结晶速度呈现极大值。经退火处理的共混物呈现熔融双峰,PC量增加,高温熔融峰略移向高温方向;热处理温度升高或时间延长,则低温熔融峰移向高温方向。     

Non-isothermal crystallization kinetics of poly(trimethylene terephthalate)/poly(ethylene 2,6-naphthalate) blends

The glass-transition temperature and non-isothermal crystallization of poly(trimethylene terephthalate)/poly(ethylene 2,6-naphthalate) (PTT/PEN) blends were investigated by using differential scanning calorimeter (DSC). The results suggested that the binary blends showed different crystallization and melting behaviors due to their different component of PTT and PEN. All of the samples exhibited a single glass-transition temperature, indicating that the component PTT and PEN were miscible in amorphous phase. The value of T_g predicted well by Gordon-Taylor equation decreased gradually with increasing of PTT content. The commonly used Avrami equation modified by Jeziorny, Ozawa theory and the method developed by Mo were used, respectively, to fit the primary stage of non-isothermal crystallization. The kinetic parameters suggested that the PTT content improved the crystallization of PEN in the binary blend. The crystallization growth dimension, crystallization rate and the degree of crystallinity of the blends were increased with the increasing content of PTT. The effective activation energy calculated by the advanced iso-conversional method developed by Vyazovkin also concluded that the value of E_a depended not only on the system but also on temperature, that is, the binary blend with more PTT component had higher crystallization ability and the crystallization ability is increased with increasing temperature. The kinetic parameters U^* and K_g were also determined, respectively, by the Hoffman-Lauritzen theory.     

A calorimetric study of normal high density and ultra-high molecular weight polyethylene blends

The melting and the crystallization of blends of ultra-high molecular weight polyethylene (UHMWPE) and polyethylene high density with normal molecular weight (NMWPE) are investigated by means of differential scanning calorimetry (DSC). Mixing the components at a temperature below the flow temperature of UHMWPE (215 °C) results in segregated melting and crystallization. The segregated melting and crystallization temperatures of both components do not depend on composition of the blend. The extreme enthalpy dependence on blend composition is explained in terms of mutual influence exhibited by the components with respect to each other. It is due to the inner stresses in nonflowing UHMWPE characterized with a lot of entangled tie molecules. Mixing the components above the flow temperature of UHMWPE results in only one peak of melting and crystallization respectively. Complete mixing and probably co-crystallization between the components takes place on mixing NMWPE with flowing UHMWPE.     

Effect of nanoscale fully vulcanized acrylic rubber powders on crystallization of poly(butylene terephthalate): Isothermal crystallization

Poly(butylene terephthalate) (PBT) was blended with nanoscale fully vulcanized acrylic rubber (FVAR) powders in a twin extruder, and the FVAR powders were dispersed well in PBT from scanning electron microscopy (SEM) and transmission electron microscope (TEM) investigation. The isothermal crystallization kinetics of PBT/FVAR blends were investigated by differential scanning calorimeter (DSC) and simulated by Avrami model. Equilibrium melting temperature was estimated by the nonlinear Hoffman-Weeks relation. The active energy (ΔE) and nucleation parameters (K_g) increased with the addition of FVAR, suggesting that FVAR particles hindered the crystallization; however more content FVAR had a lower ΔE and K_g because FVAR powders acted as heterogeneous nuclei in the nucleation of crystallization and facilitated the crystallization of PBT. The crystallization ability followed the order: PBT > PBT/FVAR6 > PBT/FVAR3 > PBT/FVAR1 when undercooling was considered.     

Effect of electron-irradiation on cross-link density and crystalline structure of low- and high-density polyethylene

Low- and high-density polyethylenes (LDPE and HDPE) were cross-linked in solid state by electron beam irradiation. Molar mass between cross-link joints, M_c, and cross-link density,ν, were calculated using rubber elasticity theory and hot set data. The results showed that the ν and creep modulus increased and creep strain and M_c decreased with increasing irradiation dose. As compared to HDPE, the LDPE had higher ν and lower M_c values at a similar irradiation dose. X-ray analysis and differential scanning calorimetry investigation of first heating cycle revealed no changes in crystalline structure of the irradiated samples. This was attributed to immobilization of radicals frozen in the crystalline phase. As a result of hindered mobility of the polymeric chains, these radicals were not able to cross-link the chains in the crystalline region. However, after melting of the crystals and during subsequent re-solidification process, different levels of crystallinity were developed depending on the applied irradiation dose. The irradiated samples with higher dose had lower crystallization and melting temperatures with reduced crystallinities. These confined crystallization behaviors, observed after a series of cooling and heating cycles, could be attributed to the decrease in the M_c values. The length of chain segment needed for usual crystallization by chain folding is decreased due to formation of cross-link joints and hence the crystallization process was hindered.     

Blends of low-density polyethylene with nylon compatibilized with a sodium-neutralized carboxylate ionomer

An ethylene-methacrylic acid copolymer partially neutralized with sodium (Na-EMAA) was successfully used to compatibilize Nylon 6 (Ny6) and low-density polyethylene (LDPE) blends. The phase morphology and thermal behavior of these blends were investigated over a range of compositions using a variety of analytical techniques. The addition of small amounts (0.5 phr) of Na-EMAA improved the compatibility of Ny6/LDPE blends as evidenced by a significant reduction in dispersed phase sizes. TGA measurements demonstrated an improvement in thermal stability when Na-EMAA was added to either LDPE or Ny6. DSC results of Ny6/Na-EMAA binary blends showed that with increasing Na-EMAA content, the crystallization temperature of Ny6 phase decreased indicating that Na-EMAA retarded crystallization of Ny6. TGA and DSC results indicate that chemical reactions might have taken place between Ny6 and Na-EMAA, a hypothesis confirmed by the Molau test.     

低密度聚乙烯/乙丙烯三元共聚(LDPE/EPO)共混体系的结晶动力学

用DSC方法研究了LDPE/EPO共混体系的等温及非等温结晶动力学,对LDPE/EPO共混体系的等温结晶动力学研究表明,共混物是三维生长的异相成核,共混物在各个结晶温度下的结晶过程都是以方式K_g(Ⅱ)进行的.采用联系Avrami方程和Ozawa方程导出的新非等温结晶动力学方程,处理了LDPE/EPO共混体系,得到了非等温结晶过程的一些基本参数,新方程很好地描述了此共混体系的非等温结晶动力学过程.