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.     

Phase change materials based on low-density polyethylene/paraffin wax blends

Phase change materials, based on low-density polyethylene blended with soft and hard paraffin waxes respectively, were studied in this paper. DSC, DMA, TGA and SEM were employed to determine the structure and properties of the blends. The blends were able to absorb large amounts of heat energy due to melting of paraffin wax, whereas the LDPE matrix kept the material in a compact shape on macroscopic level. The hard paraffin wax was, however, much more miscible with LDPE because of co-crystallization than the soft paraffin wax. LDPE blended with hard paraffin wax degrades in just one step, while blends containing soft paraffin wax degrade in two distinguishable steps. SEM showed completely different morphology for the two paraffin waxes and confirmed the lower miscibility of LDPE and soft paraffin wax. DMA analyses demonstrated the toughening effect of the waxes on the polymer matrix. This technique was also used to follow the thermal expansion as well as the dimensional stability of the samples during thermal cycling. The most visible expansion could be seen in the first cycle, probably due to a totally different thermal history of the sample. With further cycling the dimensions stabilized after two and four cycles for soft and hard paraffin wax, respectively. Controlled force ramp testing on DMA confirmed poor material strength of the blends containing soft wax, especially at temperatures above wax melting.     

Melting behaviour and evolved gas analysis of xylose

Two enantiomeric forms of xylose were identified as α-D-xylopyranose and α-L-xylopyranose by powder diffraction. Their melting behaviour was studied with conventional DSC and StepScan DSC method, the decomposition was studied with TG and evolved gases were analyzed with combined TG-FTIR technique. The measurements were performed at different heating rates. The decomposition of xylose samples took place in four steps and the main evolved gases were H₂O, CO₂ and furans. The initial temperature of TG measurements and the onset and peak temperatures of DSC measurements were moved to higher temperatures as heating rates were increased. The decomposition of L-xylose started at slightly higher temperatures than that of D-xylose and L-xylose melted at higher temperatures than D-xylose. The differences were more obvious at low heating rates. There were also differences in the melting temperatures among different samples of the same sugar. The StepScan measurements showed that the kinetic part of melting was considerable. The melting of xylose was anomalous because, besides the melting, also partial thermal decomposition and mutarotation occurred. The melting points are affected by both the method of determination and the origin and quality of samples. Melting point analysis with a standardized method appears to be a good measure of the quality of crystalline xylose. However, the melting point alone cannot be used for the identification of xylose samples in all cases.     

Thermal, mechanical and electrical properties of copper powder filled low-density and linear low-density polyethylene composites

Low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE) with different copper contents were prepared by melt mixing. The copper powder particle distributions were found to be relatively uniform at both low and high copper contents. There was cluster formation of copper particles at higher Cu contents, as well as the formation of percolation paths of copper in the PE matrices. The DSC results show that Cu content has little influence on the melting temperatures of LDPE and LLDPE in these composites. From melting enthalpy results it seems as if copper particles act as nucleating agents, giving rise to increased crystallinities of the polyethylene. The thermal stability of the LDPE filled with Cu powder is better than that for the unfilled polymer. The LLDPE composites show better stability only at lower Cu contents. Generally, the composites show poorer mechanical properties (except Young's modulus) compared to the unfilled polymers. The thermal and electrical conductivities of the composites were higher than that of the pure polyethylene matrix for both the LDPE and LLDPE. From these results the percolation concentration was determined as 18.7 vol.% copper for both polymers.     

Comparative study of DSC pattern, colour and texture of shrimps during heating

Fish oil which is characterised by important amounts of poly-unsaturated ω-3 fatty acids attach increasing importance within functional foods. Recently attention is directed on physical methods that allow fast and relatively easy the identification and discrimination of oils. DSC measurements yield in information on thermal effects, characterised by changes in enthalpy and their temperature range such as melting and crystallisation. The aim of the investigation presented here was to take DSC curves in the temperature range +20 to −40°C on several fish oils and fish oil capsules to visualise the crystallisation and melting behaviour and to compare transition temperatures and enthalpies.     

DSC Studies on Polytetrahydrofurans with Various Molecular Masses, Their Blends and Their Cured Samples

DSC studies are given for polytetrahydrofurans with molecular masses equal to 650, 1400, and 2900, for their blends, and for their cured samples. The samples were stored, annealed, and quenched to obtain the samples with different thermal histories. Two or more endothermic peaks appear in the DSC curves for the stored samples, even for the non-blended samples. A hyperbolic curve forced the plot of the highest melting temperature vs. the molecular mass to asymptote to about 50°C. The relationship between the highest melting temperature and the composition for the blended samples is suitable to linear or Fox’s relation. A peak and a shoulder appear in the DSC curves of the cured samples. As the samples are cooled at the faster rates in the thermal treatment, the shoulder appears at the lower temperatures. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal behavior and polymorphism in medium–high temperature range of the sulfur containing amino acids <Emphasis Type="SmallCaps">l</Emphasis>-cysteine and <Emphasis Type="SmallCaps">l</Emphasis>-cystine

A thermophysical study of the sulfur containing amino acids l-cysteine and l-cystine has been carried out by differential scanning calorimetry (DSC). Heat capacities of both compounds were measured in the temperature interval from T = 268 K to near their respective melting temperatures. DSC and variable temperature powder X-ray diffraction analysis (PXRD) gave evidence for a solid–solid phase transition close to the melting point only in the l-cysteine sample. DSC experiments show that this solid–solid transition is not reversible in the temperature interval T = 235–485 K and presents a behavior depending on heating temperature, time, and rate. This behavior is also supported by variable-temperature PXRD. The patterns for the commercial samples, at room temperature, are consistent with those simulated for the orthorhombic and hexagonal polymorphic forms from the single-crystal X-ray analysis.     

Die ermittlung des schmelzpunktes von kristallinen polymeren mittels wärmeflusskalorimetrie (DSC)

Small and imperfect crystals in polymers reorganize during slow heating. This leads to an increase of their melting point T_m. In order to measure the melting point of the original crystals, high heating rates are needed. This is possible with the modern heat-flow-calorimeters, which work with very small samples. The thermal lag of a DSC cell causes a shift of the melting peak by ΔT to higher temperature. From the theory of a heat-flow-calorimeter, it follows that the error ΔT is proportional to the square root of heating rate. heat of fusion and sample mass. Measurements with sharp melting low molecular weight compounds confirm that this square root relation is quantitatively followed. In order to measure the true melting point of the crystals present in a polymer sample, one has to use different high heating rates and constant sample mass. By plotting the melting peak temperatures as a function of the square root of heating rate and linear extrapolation to zero heating rate, the true melting point is found. This method is applied to HDPE, LDPE and some polyamides.     

Solid-state relaxations in linear low-density (1-octene comonomer), low-density,and high-density polyethylene blends

Extensive thermal and relaxational behavior in the blends of linear low-density polyethylene (LLDPE) (1-octene comonomer) with low-density polyethylene (LDPE) and high-density polyethylene (HDPE) have been investigated to elucidate miscibility and molecular relaxations in the crystalline and amorphous phases by using a differential scanning calorimeter (DSC) and a dynamic mechanical thermal analyzer (DMTA). In the LLDPE/LDPE blends, two distinct endotherms during melting and crystallization by DSC were observed supporting the belief that LLDPE and LDPE exclude one another during crystallization. However, the dynamic mechanical β and γ relaxations of the blends indicate that the two constituents are miscible in the amorphous phase, while LLDPE dominates α relaxation. In the LLDPE/HDPE system, there was a single composition-dependent peak during melting and crystallization, and the heat of fusion varied linearly with composition supporting the incorporation of HDPE into the LLDPE crystals. The dynamic mechanical α, β, and γ relaxations of the blends display an intermediate behavior that indicates miscibility in both the crystalline and amorphous phases. In the LDPE/HDPE blend, the melting or crystallization peaks of LDPE were strongly influenced by HDPE. The behavior of the α relaxation was dominated by HDPE, while those of β and γ relaxations were intermediate of the constituents, which were similar to those of the LLDPE/HDPE blends. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1633–1642, 1997     

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.     

Probing multiple melting behaviors in poly(ethylene naphthalene 2,6‐dicarboxylate) with different thermal histories by simultaneous wide‐angle and small‐angle X‐ray scattering

A simultaneous wide‐angle and small‐angle X‐ray scattering study of two poly(ethylene naphthalene 2,6‐dicarboxylate) samples crystallized from the glassy state at different annealing temperatures for different annealing times was carried out with synchrotron radiation. Either single or dual melting was induced in the samples, as confirmed by differential scanning calorimetry (DSC). The correlation function and interface distribution function were calculated to evaluate microstructural parameters such as the long spacing, the thickness of the amorphous and crystalline phases, and the width of the size distributions. The sample with dual melting behavior exhibited an abrupt increase of all microstructural parameters at temperatures above the melting of the lowest endotherm, whereas the sample revealing a single melting endotherm did not show such a sudden change. This finding agrees with the concept that the appearance of two melting peaks in DSC traces can be explained by the dual lamellar stacking model. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 881–894, 2001     

Crystallization and melting behaviour of poly(m-xylene adipamide)

The melting and crystallisation behaviour of poly(m-xylene adipamide) (MXD6) are investigated by using the conventional DSC, X-ray diffraction and polarised light microscopy. Triple, double or single melting endotherms are obtained in subsequent heating scan for the samples after isothermal crystallisation from the melt state at different temperatures. The lowest melting peak can be ascribed to the melting of secondary crystals. The melting of primary crystals causes the medium melting peak and the highest melting peak is attributed to the melting of recrystallised species formed during heating. Following the Hoffman–Weeks theory, the equilibrium melting temperature is equal to 250°C and the equilibrium melting enthalpy ΔH _m ⁰ to 175 J g^–1. Then, using the Lauritzen–Hoffmann theory of secondary crystallisation, the analyse of the spherulitic growth shows that the temperature of transition between the growing regimes II and III is equal to 176°C. Finally the Gibbs-Thomson relationship allows the determination of the distribution function of crystalline lamellae.     

Quantitative small-angle light-scattering studies of the melting and crystallization of LLDPE/LDPE blends

Small-angle light-scattering (SALS) patterns were obtained during melting and crystallization of blends of linear low-density polyethylene (LLDPE) with conventional low-density polyethylene (LDPE). Quantitative measurements of these SALS patterns using a two-dimensional optical multichannel analyzer apparatus (OMA2) indicate that the LLDPE which is miscible with the LDPE component in the molten state crystallizes first, forming volume-filling spherulites. The LDPE then crystallizes within the preformed spherulites. These findings are supported by optical microscopy studies showing that the blend samples were volume filled with one kind of the spherulites having a radius comparable to that of the pure LLDPE. The SALS intensity curve changes with composition of the blends in a manner that may be interpreted by considering the orientation of crystals within spherulites. It has been observed that the spherulites in the blend have more diffuse boundaries as the LDPE content increases. The lattice spacing and long spacings in blends were obtained by wide-angle and small-angle x-ray scattering, respectively. The SALS technique along with differential scanning calorimetry (DSC) is shown to be useful for determining the crystallization behavior of a crystallizable polymer blend system.     

Effect of sample size on isothermal crystallization measurements performed in a differential scanning calorimeter: A method to determine avrami parameters without sample thickness effects

Isothermal crystallization studies of low-density polyethylene (LDPE) and high-density polyethylene (HDPE) using differential scanning calorimetry (DSC) were performed using different sample thicknesses to determine the effect of non-ideal heat-transfer. Polyethylene was chosen because of its importance, its extensive coverage in the literature, and its fast crystallization kinetics. Thermal gradients were found to significantly affect the measured crystallization exotherm; slower crystallization rates were observed for thicker samples measured at lower temperatures (greater supercoolings). Differences between different sample thicknesses disappeared at higher temperatures, consistent with finite heat-transfer rates being responsible for the effect. A power-compensation and a heat-flux DSC were used; these experiments also enabled the determination that the performance of the latter was acceptable for this study. Finally, thickness-independent Avrami parameters have been calculated.     

Behavior of polyethylene in a variable temperature density gradient column

Polymer morphology (phase size and phase density) of slow cooled and quenched polyethylene (HDPE, LDPE, and LLDPE) has been characterized over a range of temperatures. The characterization methodology includes variable-temperature density gradient column (VT-DGC), small-angle x-ray scattering (SAXS), wide-angle x-ray diffraction (WAXD) and differential scanning calorimetry (DSC). Using a novel technique, a VT-DGC was prepared and cycled over a range of service temperatures (20-60 °C) for 5 cycles to investigate the changes of slow cooled and quenched HDPE, LDPE and LLDPE. A significant change in bulk density was present in each sample between the first cycle and subsequent cycles. Morphological analysis was performed using both the two-phase and three-phase models. The two-phase model showed that, for a particular sample, the thickness of the crystalline and amorphous phases varied very little within the experimental temperature range. Using the three-phase model, differences in the interfacial layer thickness were measured and observed to be significant compared to the amorphous and crystalline phase changes. The amorphous and crystalline densities of all samples varied less than 2%. Overall, significant difference in crystalline density was observed between HDPE, LDPE and LLDPE due to molecular structure.     

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.     

Thermostimulated currents in low-density polyethylene and its blends with various components after high-pressure plastic deformation

It is established that the plastic deformation of low-density polyethylene (LDPE) under a pressure 0.5–2.0 GPa on a high-pressure apparatus of the Bridgman anvil type leads to the appearance of thermostimulated currents in samples, indicating that the samples contain trapped electrons. It is shown that two peaks are present on the temperature dependences of the currents; one of these is most probably related to the cold crystallization of the polymer, its structure being destroyed upon high-pressure deformation, while the other is related to the melting of the polymer. It is noted that the peaks were absent on temperature dependences of the currents for LDPE blends with some components; this could be due to the formation of a conducting state at interfaces. It is found that the electroconductivity of some blends after processing under pressure was higher than that in LDPE itself by a factor of 25.     

Improvement of the thermo-oxidative stability of low-density polyethylene films by organic-inorganic hybrid coatings

LDPE films have been coated with single or bi-layer hybrid coatings formed through sol-gel reactions in order to improve their thermo-oxidative resistance. Different chemical compositions of the coating were investigated which differ either in the amount of the inorganic phase (silica deriving from tetraethoxysilane) or in the organic component (either alkoxy silane functionalized polyethylene-poly(ethylene glycol) diblock copolymers or poly(vinyl alcohol)). The thermo-oxidative stability of the coated films thus obtained has been assessed by means of isothermal differential scanning calorimetry (DSC) and isothermal thermo-gravimetric analysis (TGA) under accelerated conditions, i.e. at high temperatures in pure oxygen flow. Conventional ageing in air at lower temperature, slightly above the in-service one, has also been carried out. The obtained data show: a) a general improvement of the thermal-resistance for the coated LDPE samples; b) a particularly high thermal-resistance for LDPE coated with a bi-layer coating with pure silica in the top layer; c) the effectiveness of the accelerated techniques in qualitatively assessing the thermo-oxidative resistance of the coated polymeric systems.     

Crystallisation and melting behaviour of fish oil measured by DSC

Fish oil which is characterised by important amounts of poly-unsaturated ω-3 fatty acids attach increasing importance within functional foods. Recently attention is directed on physical methods that allow fast and relatively easy the identification and discrimination of oils. DSC measurements yield in information on thermal effects, characterised by changes in enthalpy and their temperature range such as melting and crystallisation. The aim of the investigation presented here was to take DSC curves in the temperature range +20 to ?40°C on several fish oils and fish oil capsules to visualise the crystallisation and melting behaviour and to compare transition temperatures and enthalpies.     

High-temperature creep resistant ternary blends based on polyethylene and polypropylene for thermoplastic power cable insulation

The impact of a small amount of polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS) on the thermomechanical and electrical properties of blends comprising low-density polyethylene (LDPE) and isotactic polypropylene (PP) is investigated. SEBS is found to assemble at the PP:LDPE interface as well as within isolated PP domains. The addition of 10 wt% SEBS significantly increases the storage modulus between the melting temperatures of the two polyolefins, 110 and 160°C, and results in improved resistance to creep during both tensile deformation as well as compression. Furthermore, the ternary blends display a very low direct-current (DC) conductivity as low as 3.4 × 10⁻¹⁵ S m⁻¹ at 70°C and 30 kV mm⁻¹, which is considerably lower than values measured for neat LDPE. The here presented type of ternary blend shows potential as an insulation material for high-voltage direct current power cables.