Reversible Melting of Thermally Fractionated Polyethylene

Different grades of linear low density polyethylenes (LLDPEs) have been quenched cooled step-wise and crystallised isothermally at (a series of increasing) temperatures in a DSC (thermal fractionated samples). These samples have been investigated by temperature modulated DSC (MDSC). The heat flow curves of the thermal fractionated materials were compared with those obtained from samples crystallised at a relatively slow cooling rate of 2 K min^-1(standard samples). The melting enthalpy obtained from the total heat flow of the thermal fractionated samples was 0-10 J g^-1higher than those of standard samples. The melting enthalpy obtained from the reversing heat flows was 13-31 J g^-1lower in the thermal fractionated samples than in the standard samples. The ratio of the reversing melting enthalpy to the total melting enthalpy increased with decreasing density of the PE. The melting temperature of the endotherms formed by the step-wise cooling was 9 K higher than the crystallisation temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

Morphology and melting behavior of nascent ultra-high molecular weight polyethylene

The nascent morphology of UHMW PE exhibits high melting point, high crystallinity, and increased WAXS line breadth relative to samples formed by melt crystallization. Different empirical relationships between crystal size and melting point are observed for nascent and molded samples. This differentiation is removed following nitric acid treatment of the nascent flake. Solid-state annealing behavior is differentiated by several regimes. Regime I is characterized by increasing crystallite dimensions and crystallinity at low annealing temperatures. Regime II[a] and II[b] is identified by double melting in DSC scans of moldings and nascent flake, respectively. The double melting is due to partial melting with incomplete recrystallization. Regime II[a] of moldings is differentiated from Regime II[b] of flake by an increase in melting point of the higher melting endotherm. Within Regime II[b], the partial melting of the nascent structure is sensitive to the distribution of morphological stability. Regime III is initiated at annealing temperatures approaching the zero heating rate melting point, and shows melting kinetics by DSC or time-resolved WAXS using synchrotron x-ray radiation. The superheat, partially associated with Regime III behavior, is sensitive to morphological heterogeneity and annealing history. Morphological models are discussed which highlight the role of noncrystalline regions and melting kinetics on the melting behavior of nascent form crystallinity. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 495–517, 1998     

Isothermal crystallisation kinetics of poly(3-hydroxybutyrate) using step-scan DSC

The crystallisation kinetics, melting behaviour and morphology, of bacterial poly(3-hydoxybutyrate) (PHB) have been investigated by using differential scanning calorimetry (DSC), step-scan DSC (SDSC), wide angle X-ray diffraction (WAXRD) and hot stage polarised optical microscopy (HSPOM). DSC imparted isothermal crystallisation thermal history. The subsequent melting behaviour revealed that all PHB materials experienced secondary crystallisation during heating and the extent of secondary crystallisation varied depending on the crystallisation temperature. PHB samples were found to exhibit double melting behaviour due to melting of SDSC scan-induced secondary crystals, while considerable secondary crystallisation or annealing took place under the modulated heating conditions. The overall melting behaviour was rationalised in terms of recrystallisation and/or annealing of crystals. Interestingly, the PHB materials analysed by SDSC showed a broad exotherm before the melting peak in the non-reversing curve and a multiple melting peak reversing curve, verifying that the melting-recrystallisation and remelting process was operative. HSOM studies supported the conclusions from DSC that the radial growth rate of the PHB spherulites was significantly varied upon the crystallisation conditions. One form of crystals was shown by WAXRD from isothermally crystallised PHB.     

DSC ofγ-irradiated ultra-high molecular weight polyethylene and high density polyethylene of normal molecular weight

The melting and the crystallization of-irradiated (doses: 0–6Mrad) ultra-high molecular weight nascent polyethylene (UHMWPE) and high density nascent polyethylene with normal molecular weight (NMWPE) were investigated by DSC. The heat of melting of the nascent UHMWPE (DSC degree of crystallinity, respectively) increases up to a dose of 3 Mrad, after which it slightly decreases. The heat of the second melting of UHMWPE and of the first and second melting of NMWPE increases slightly up to a dose of 3 Mrad, after which it does not change. The X-ray degree of crystallinity of the nascent non-irradiated and irradiated polymers was 0.62±0.02. The calorimetric crystallinity was compared to the X-ray one. The results show that radiation does not affect the polymer crystallinity, but influences the thermodynamic heat of melting. The increase ofH _m vs. dose in UHMWPE is explained in terms of processes of tie molecule scission within the amorphous regions and on the surface of the crystals, which predominate over crosslinking up to a dose of 3 Mrad. That leads to an increase in the conformational mobility of the molecules and to an increase in the enthalpy, according to Peterlin's formula. The scission of the chains at the points of entangling of the tie molecules leads to a decrease in the temperature and to an increase in the enthalpy of crystallization of UHMWPE vs. dose. In NMWPE these effects are considerably weaker.     

Thermophysical theory of DSC melting peak

Melting peak for metals is described with expressions derived from thermophysical consideration of DSC operation. Three parameters govern the shape of the peak: thermophysical coefficient derived from the DSC design, enthalpy of fusion of a sample, and heating rate. Rigorous evaluation yields rather complex expressions, but simplified expressions can be used in common practice. The peak shape is described by two different expressions for two separate stages in the process of metal melting (1) the melting itself and (2) heat relaxation after the melting completion. The validity of the expressions was demonstrated after the experiments on gallium melting. The thermophysical coefficient is shown be affected to small variations by the changes in sample preparation or experimental conditions (melting Ga, In, Zn).     

DSC by the TGA/SDTA851e Considering Mass Changes

DSC measurements in open pans are often disturbed by mass losses such as sublimation during melting or release of water during chemical reactions. By simultaneous DSC and TG measurements the DSC signal can be corrected. For this purpose, a temperature dependent calibration function has to be determined by which the SDTA signal from the TGA/SDTA851^e measuring cell can be converted into a heat flow curve (DSC). By this procedure, accurate heat of melting can be determined despite ongoing sublimation in open pans. This method is illustrated with reference of the melting of anthracene. Additionally, condensation reactions were investigated and analyzed by DSC/TG even under ambient pressure, knowing the heat of evaporation. Using phenol formaldehyde resins the influence of the presence or the release of volatile reaction products on the reaction rate and kinetic parameters were studied. In general, the method can be used to correct DSC curves for thermal effects related to mass change. This revised version was published online in July 2006 with corrections to the Cover Date.     

Crystallization and melting of a branched polyethylene with precisely controlled chemical structure

The heat capacity of a linear polyethylene with dimethyl branches, at every 21st backbone atom was analyzed by differential scanning calorimetry (DSC) and quasi-isothermal temperature-modulated DSC. This novel copolyethylene (PE2M) is relatively difficult to crystallize from the melt. On subsequent heating, a first, sharp melting peak is followed by a sharp cold-crystallization and crystal perfection and a smaller endotherm, before reaching the main melting at 315–320 K, close to the melting temperatures of eicosane and tetracontane. The low-temperature melting is sensitive to the cooling rate and disappears below 1.0 K min⁻¹. The cold crystallization can be avoided by heating with rates faster than 80 K min⁻¹. The PE2M exhibits some reversing and reversible melting, which is typical for chain-folded polymers. The glass transition of semicrystalline PE2M is broadened and reaches its upper limit at about 260 K (midpoint at about 0.355 K). Above this temperature, the crystals seem to have a heat capacity similar to that of the liquid. A hypothesis is that the melting transition can be explained by changes in crystal perfection without major alteration of the crystal structure and the lamellar morphology. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3461–3474, 2006     

Annealing of Oriented PP/PE Double-layer Film within the Melting Range of PE: the Role of Partial Melting and Self-nucleation

Due to the mechanical stability of the PP layer, the oriented PP/PE double-layer film with a row-nucleated crystalline structure can be annealed at a higher temperature than the PE monolayer film. In this work, the effects of annealing temperature within the melting range of PE on the crystalline structure and properties of PP/PE double-layer films were studied. When the annealing temperature is between 100 and 130 °C, below the melting point of PE, the crystallinity, the long period, lateral dimension and orientation of the lamellae in the PE layer increase with the annealing temperature due to the melting of thin lamellae and the self-nucleated effect of partially-melted melts during annealing. With the annealing temperature further increasing to 138 °C, near the melting ending point of PE, since the lamellae melt completely and the melt memory becomes weak during annealing, some spherulite structures are formed in the annealed sample, resulting in a decrease of orientation. In contrast, the annealing only causes the appearance of a low-temperature endothermic plateau in the PP layer. The improved size and orientation of lamellar structure in the PE layer increase the pore arrangement and porosity of the stretched PP/PE microporous membrane. This study successfully applies the self-nucleation effect of partially-melted polymer melt into the practical annealing process, which is helpful to guide the production of high-performance PP/PE/PP lithium batteries separator and the annealing process of other multilayer products.

     

A Detailed Comparison of First Order Transitions by DSC and TMC

Indium was analyzed with both, standard differential scanning calorimetry (DSC) and temperature-modulated DSC (TMDSC) using sinusoidal and saw-tooth modulation. Instrument and sample effects were separated during nucleated, reversible melting and crystallization transitions, and irreversible crystallization with supercooling. The changes in heat flow, time, and sample and reference temperatures were correlated as functions of heating rate, mass, and modulation parameters. The transitions involve three regions of steady state (an initial and a final region before and after melting/crystallization, a region while melting/crystallization is in progress) and one region of approach to steady state (melting peak to final steady state region). Analyses in the time domain show promise when instrument lags, known from DSC, are used for correction of TMDSC. A new method of integral analysis is introduced for quantitative analysis even when irreversible processes occur in addition to reversible transitions. The information was derived from heat-flux calorimeters with control at the heater block or at the reference temperature sensor. This revised version was published online in July 2006 with corrections to the Cover Date.     

Melting of nascent and thermally treated super-high molecular weight polyethylene

The melting of nascent and thermally treated super-high molecular weight polyethylene (SHMWPE) is investigated by means of differential scanning calorimetry (DSC). Higher melting temperatures and enthalpies of the nascent and annealed samples are observed. The melting temperatures and enthalpies of melt crystallized SHMWPE is lower and depends on the temperature of thermal treatment. All melting properties are explained by presuming that the lamellar structure contains a high concentration of entangled tie macromolecules in amorphous regions formed during polymerization. It was supposed that the concentration of the entanglements and the stressed tie molecules are changed with the temperature of the thermal pretreatment.     

差示扫描量热法测定聚乙烯结晶度的不确定度评定

采用差示扫描量热法(DSC)测定聚乙烯(PE)的结晶度，讨论了测定PE熔融焓过程中由测量重复性、称量过程及PE本身结构的不均匀性等因素所带来的不确定度分量，计算了测定PE熔融焓的合成标准不确定度及扩展不确定度和测定PE结晶度的扩展不确定度。采用Dsc法测定高密度聚乙烯(HDPE)和低密度聚乙烯(LDPE)结晶度的扩展不确定度均小于l％。     

Study of CCl3F hydrate formation and dissociation in W/O emulsion by differential scanning calorimetry and X-ray diffraction

Differential scanning calorimetry (DSC) and time-resolved synchrotron X-ray diffraction as a function of temperature (XRDT) were combined in a novel way in order to study conditions of formation and the amount of gas clathrate formed in dispersed systems. The formation and dissociation of trichlorofluoromethane hydrate CCl₃F·(H₂O)₁₇ in a water-in-oil emulsion were followed by using these combined techniques. An emulsion containing 3 wt.% NaCl was submitted to a cooling and heating cycle between 20 and −50 °C. During cooling, a single exothermic peak at −43 °C, found in DCS thermograms was assigned to the freezing of under-cooled water droplets; however, no noticeable signal related to hydrate crystallisation was detected. Conversely, during subsequent heating, the progressive melting of ice was followed by an endothermic signal indicative of hydrate decomposition. From X-ray diffraction performed on an emulsion sample, it was possible to identify the exact condition of CCl₃F·(H₂O)₁₇ formation. XRDT diffraction patterns clearly demonstrated that only ice crystallised in the aqueous droplets during cooling and that the hydrate only formed during heating simultaneously with melting of ice. From the solid–liquid phase diagrams of systems H₂ONaCl and CCl₃FH₂ONaCl and from the DSC and XRDT experiments, the composition of the droplets was deduced. The upper limit of the amount of hydrate that could form in the system was calculated.     

Correlation between GPC,DSC, and Analytical TREF for Linear Polyethylene

Analytical temperature rising elution fractionation (TREF) of linear polyethylene (PE) samples with different densities was done in 1-chloronaphthalene using a gel permeation chromatograph (GPC) coupled with a gas chromatograph. The corrected peak elution temperatures completed the previously obtained data in trichlorobenzene, xylene, and dibutoxymethane. A mathematical correlation was found for diluted linear PE samples between the α parameter of the Mark-Houwink-Sakurada equation governing the retention time in GPC, the bulk melting temperature measured by differential scanning calorimetry (DSC), and the TREF peak elution temperature. The extrapolation to the melting temperature measured by DSC gives α = 0.5, thus confirming the hypothesis that polymer conformations in the melt are similar to those in a theta solvent.     

Nanoscale morphology of melt crystallized polyethylene/graphite (HOPG) interphase

High density polyethylene (PE) was crystallised from the melt on freshly cleaved surface of highly oriented pyrolitic graphite (HOPG) or mica. Atomic force microscopy (AFM) studies of structure of the polymer surface adjacent to the graphite or mica were performed after peeling of from the substrate. Significant differences of crystalline structure on the interface were found between PE crystallised on graphite and mica. The surface of polyethylene crystallised on graphite shows large areas with regularly arranged rectangular structures. These objects (ca 20‐80 nm big) probably represent the nucleation centres of the lamellar growth. The surface of polyethylene crystallised at mica surface shows some dot‐like structures showing no particular arrangement.     

Phase and state transition effects on dielectric, mechanical, and thermal properties of polyols

The present study investigated the glass transition, crystallisation and melting behaviour of erythritol, xylitol, and glucitol (sorbitol) using dielectric analysis (DEA), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA). Sorbitol and xylitol were plasticised by water and their glass transition temperatures decreased when water content was increased. Erythritol crystallised rapidly, and its water plasticisation behaviour could not be determined. Melting of the crystalline polyols occurred at their specific melting temperatures. Melts of erythritol and xylitol crystallised on recooling and no glass transition was apparent on reheating. Quench cooled sorbitol melt remained amorphous and showed a glass transition on reheating. Glass transition and crystallisation were apparent in the DSC thermogram and the dielectric and the dynamic mechanical spectra of mixtures of amorphous and crystalline xylitol.     

固态缩聚尼龙66的熔化行为

固态缩聚尼龙６６的熔化行为沈百拴，张喜亮，李新法（郑州大学材料工程系郑州４５００５２）关键词尼龙６６，固态缩聚物，熔化行为，差示扫描量热注关于固态缩聚反应的研究国内外已有报道［１～４］，叵未见固态缩聚尼龙６６（ＳＳＰＰＡ６６）熔化行为的报道。本文用Ｄ...     

Calorimetry in nonstandard conditions: The noncrystalline phases of linear polyethylene

A linear Union Carbide PE (UC) has been analyzed by nonstandard calorimetry with a common DSC calorimeter and a Setaram C80 calorimeter. Nonstandard calorimetry entails using a low rate of heating (0.5–1 K/min), a small mass (0.5–1.5 mg), and an open cell (O‐cell) instead of the standard C‐cell. Events in O‐cells overlap less and occur with a faster kinetics than in C‐cells. PE crystals are nascent, solution‐grown (S‐grown), press‐grown (P‐grown), and strained by extrusion. In Part A, the traces show that the phase‐changes in the melt, previously observed in a C80 calorimeter (slow T‐ramp) and characterized by ΔH_network, can be observed with a common DSC in nonstandard conditions. In Part B, the difference between the C‐ and O‐cells and the changes in the main peak enthalpy (ΔH_ortho) are of interest. The main result is that, in O‐cells, the value of ΔH_ortho around T_ortho, exceeds unambiguously in certain conditions ΔH_ortho found for perfect orthorhombic crystals. The main endotherm contains then another contribution, namely ΔH_network. Crystal reorganization during the slow T‐ramp is followed in the C‐ and O‐cells on S‐grown crystals. In O‐cells, lamellar thickening observed in the slow‐ramp is more extensive. The ease of phase‐change depends on the sample history. It is as follows: strained‐part extruded > nascent > S‐grown > P‐grown. Co‐operative chain motions are more hindered in the standard C‐cells than in the O‐cells. In Part C, lower values of m succeed in bringing phase‐changes in P‐grown (O‐cells) samples. The origin of the events is discussed: three processes are thought to contribute to the phase‐changes namely, melting of strained short‐range order, activation of vibrations in the CH₂ groups, and fast decay of chain orientation which occurs simultaneously with melting. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1932–1949, 2007     

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、...     

Morphological Changes of Linear,Branched Polyethylenes and their Blends during Crystallization and Subsequent Melting by Synchrotron SAXS and DSC

Summary: The crystalline structure and phase morphology of linear, branched polyethylenes and their blends during crystallization and subsequent melting were investigated, using a combination of differential scanning calorimetry (DSC), and synchrotron small angle X-ray scattering (SAXS). A linear polyethylene (PE1) with weight-average molecular weight (M_w) of 114 000 g/mol, and two branched polyethylene copolymers, containing 4.8 mol% (PE4) and 15.3 mol% (PE10) hexane, with molecular weights of 93 000 g/mol and 46 000 g/mol were used as pure samples. Two blends, PE1-4 and PE1-10, each with a weight ratio of 50/50, were prepared by solution blending. Our results indicate that in PE4 a phase separation within the branched component itself occurred, forming a broad distribution of lamellar thicknesses during the crystallization process. PE10 on the other hand did hardly crystallize because of the high degree of branching. Co-crystallization of both components took place in blend PE1-4 and liquid-liquid phase separation occurred in the melt of PE1-10. Morphological parameters were determined by using Bragg's law and the correlation function, respectively. The detected semicrystalline morphology can be well described by the lamellar insertion mode where thin lamellae develop between thicker primary lamellae. During subsequent heating, lamellae melted in the reversed sequence of their formation. The evolution of the structural parameters as a function of temperature revealed that surface melting began at first, and then the complete melting of stacks occurred until the final melting temperature was reached.     

Origin of double melting peaks in drawn nylon 6 yarns

The differential scanning calorimetry (DSC) melting curves of drawn nylon 6 were studied from the standpoint of reorganization of the crystals during the heating process. A new method was presented to obtain the DSC curve associated with the growth and melting of the original crystals, and that with the recrystallization and final melting process, separately. The results obtained show that, in the case of a heating rate of 10°C/min, the original crystals in the sample start perfecting themselves at temperatures far below their initial melting temperature and melt out below 222°C, recrystallization starts at about 210°C, and the newly emerged crystals melt out at 228°C. The superposition of two such constructed DSC curves reproduces the observed DSC curve well. Therefore, the double melting peaks of the sample are considered to be the result of superposition of three processes which occur successively during heating; perfection of the original crystals, melting of the perfected crystals concurrently with recrystallization, and melting of the recrystallized crystals.