Multiple Melting Behavior of High-Speed Melt Spun Polylactide Fibers

High-speed melt spinning of polylactide (PLA) was conducted and the structure and multiple melting behavior of the as-spun fibers were investigated. In the analysis of temperature modulated differential scanning calorimetry (TMDSC) thermograms for the as-spun PLA fibers taken-up at 1 and 6 km/min, the peaks around the melting temperature region in the reversing heat flow (RHF) and nonreversing heat flow (NRHF) curves were mainly separated into (1) a pair of an endothermic peak (Peak L) in RHF and an exothermic peak (Peak R) in NRHF in a low temperature region, (2) an endothermic peak (Peak M) both in RHF and NRHF (only in RHF for PLA fiber spun at the low-speed) in an intermediate temperature region, and (3) an endothermic peak (Peak H) both in RHF and NRHF in a higher temperature region. Wide-angle X-ray diffraction (WAXD) measurements were conducted during the heating process of the as-spun fibers cut into powders. In the case of fibers obtained at 1 km/min, disordered crystals, i.e. α′-form crystals, were formed through cold crystallization followed by a disorder-to-order phase transition, i.e. α′ to α crystalline modification, with partial melting of the α′ crystals around 148.5°C in the temperature range of Peaks R and L. Finally, the α form crystals melted above 169.4°C, in the temperature range of Peak H. On the other hand, the PLA crystals generated by the orientation-induced crystallization during the spinning process at a spinning velocity of 6 km/min did not show a WAXD profile of perfect α form crystals but showed an intermediate structure having lattice spacings between the α′ and α forms. Such intermediate crystals did not transformed into α form crystals during the heating process.     

Crystallization Behavior of Amorphous Poly(l-Lactide)

Amorphous poly(l-lactide) (PLLA) was annealed in two different ways: amorphous samples were heated at a given temperature to induce crystallization (one-step annealing); and amorphous samples were first crystallized at a low temperature and subsequently annealed at a higher temperature than the crystallization temperature. Samples thus prepared were measured by DSC. The original amorphous sample exhibited an exothermic peak at about 100°C (exothermic peak I), an exothermic peak just below the melting point (exothermic peak II), and an endothermic peak when it was melted. Exothermic peak I was caused by cold crystallization. When the melting points of PLLA samples, heat-treated in various ways, were plotted as a function of annealing temperature, there was discontinuity at about 120°C. From analyses of wide-angle X-ray diffraction patterns, it was found that when amorphous PLLA was crystallized at a temperature below 120°C, crystallites of the β-form formed, and when annealed at a temperature above 120°C, crystallites of the α-form grew. Thus, exothermic peak I was attributed to cold crystallization of the β-form, and peak II was caused by the phase transition of the β-form to a more stable form.     

Fiber Structure,Tensile Behavior and Antibacterial Activity of Polylactide/Poly(butylene terephthalate) Bicomponent Fibers Produced by High-Speed Melt-Spinning

Abstract

Various types of bicomponent fibers composed of polylactide (PLA) and poly(butylene terephthalate) (PBT) with different molecular weights, arranging the polymers separately in the skin or core, were produced by high-speed melt-spinning. The bicomponent spinning, arranging the PLA with high molecular weight (melt flow rate =1.9?g/10?min, L-lactide content = 98.7%) in the skin and the PBT with low molecular weight (IV = 0.835–0.865 dL/g) in the core, resulted in orientation-induced crystallization in the PLA component at the spinning speed of 2?km/min. This crystallization effect was ascribed to a chain-extending treatment applied to the original PLA (MFR = 4.0?g/10?min) to increase its molecular weight. By the treatment the PLA could crystallize when spun even at 1?km/min in its single-component spinning. On the other hand, the bicomponent spinning system interfered with the orientation-induced crystallization of PBT in the core. As a result, the critical spinning speed needed to generate the orientation-induced crystallization in the core PBT was elevated to 4?km/min. The inferior tensile behavior of the bicomponent fibers, as compared to the single-component PLA or PBT fibers, suggested poor compatibility between PLA and PBT. Transesterification reactions rarely occurred at the interface of the two polymers. The bicomponent fibers prepared from high molecular weight PLA and low molecular weight PBT, however, showed sufficient antibacterial activity and physical properties to be suitable for designing medical clothing materials.     

Stacked-Lamellar Structure in High-Speed Spun PET Fibers as Revealed by TEM

Morphological study of high-speed spun poly(ethylene terephthalate) (HSS-PET) fibers spun at 6 km/min was performed by a combination of alkaline etching, surface replica method, and transmission electron microscopy. In the two-stage replica of the alkaline-etched fiber, the stacked-lamella-like structure was observed, in which lamella-like striations were stacked in a direction leaning at about 40° away from the fiber axis. The spacing between the adjacent striations, namely the lamellar periodicity, was measured to be 10–30 nm. We have proposed that the stacked-lamellar structure, which is mainly composed of mosaic lamellar crystals and has the extended-chain crystals connecting the adjacently stacked lamellae, is an appropriate model for the fiber structure of HSS-PET fibers.     

Experimental Investigations on Latent Heat Storage Unit using Paraffin Wax as Phase Change Material

Thermal performance of a latent heat storage unit is evaluated experimentally. The latent heat thermal energy storage system analyzed in this work is a shell-and-tube type of heat exchanger using paraffin wax (melting point between 58°C and 60°C) as the phase change material. The temperature distribution in the phase change material is measured with time. The influence of mass flow rate and inlet temperature of the heat transfer fluid on heat fraction is examined for both the melting and solidification processes. The mass flow rate of heat transfer fluid (water) is varied in the range of 0.0167 kg/s to 0.0833 kg/s (1 kg/min to 5 kg/min), and the fluid inlet temperature is varied between 75°C and 85°C. The experimental results indicate that the total melting time of the phase change material increases as the mass flow rate and inlet temperature of heat transfer fluid decrease. The fluid inlet temperature influences the heat fraction considerably as compared to the mass flow rate of heat transfer fluid during the melting process of the phase change material.     

Development of Stereocomplex Crystal of Polylactide in High-Speed Melt Spinning and Subsequent Drawing and Annealing Processes

High-speed melt spinning of racemate polylactide (r-PLA), which is a blend of equal amounts of poly(l-lactide) and poly(d-lactide) molecules, was performed up to the take-up velocity of 7.5 km/min. In the fiber structure analysis, particular attention was paid to the formation of stereocomplex crystals, because this crystal form has a melting temperature about 60° higher than the homocrystals. It was found that highly oriented and highly crystallized fibers containing the α-form and stereocomplex crystals were obtained when the take-up velocity exceeded about 4 km/min. The amount of stereocomplex crystal was higher under the spinning conditions of higher take-up velocity, lower throughput rate, and lower extrusion temperature. Under these conditions, higher tensile stress can be applied to the spinning line, and therefore, the orientation-induced crystallization is promoted. Annealing of the fibers obtained at high-take-up velocities, such as 6 km/min, which already have the crystalline structure with a certain amount of stereocomplex crystal, at a temperature between the melting temperatures of α-form and stereocomplex crystals, yielded the fiber structure mainly consisting of highly oriented stereocomplex crystal. The annealed fibers showed fairly high mechanical properties and good thermal stability.     

Amorphous structure heat: Temperature dependence of heats of solution for polystyrene in toluene and ethylbenzene

When a polymer is dissolved in a solvent, the heat measured is a sum of a polymer-solvent interaction term and a term related to the structure that existed in the solid polymer relative to its amorphous liquid state. This latter contribution, termed the “residual” heat, can have an endothermic contribution due to the fusion of crystalline regions and an exothermic contribution due to the disruption of structure in noncrystalline amorphous regions. For atactic polystyrene between 30 and 110°C, it is shown that the “residual” heat is exothermic, decreases linearly with temperature differences below T_g, and extrapolates to zero in the vicinity of T_g. The existence of an exothermic heat above T_g is probably related to a 160°C transition in polystyrene. This “residual” heat was further observed to be independent of the pressure at which the polystyrene was glassified.     

Melting and Recrystallization of Nylon-6,10 Thin Films

The melting and recrystallization of nylon-6,10 thin films immersed in an aqueous solution of calcium chloride were investigated by DSC measurements. The crystal length, ζ, was determined as a function of the melting peak temperature, T _m . The end surface free energy of nylon-6,10 crystals used for the ζ–T _m conversion was derived thermodynamically. For films of 0.01 mm thickness, the original length of ζ (=7.6 structural units) at T _m decreased step by step with increasing immersion time by the length near the structural unit (2.24 nm) per step. However, the suppression of the recrystallization after melting of the original crystals formed during the first cooling by the adsorbed calcium ions did not occur completely, even for films immersed for 30～60 min at 50°C.     

Multiordered Phase Structures Of Nematic Pbo/ppa Solution

Liquid crystalline solutions of poly (p-phenylene benzoxazole) (PBO) in polyphosphoric acid (PPA) were prepared and studied. The typical banded texture and disclination of sheared PBO/PPA solution were observed by polarizing optical microscopy (POM), and the microstructure of the periodic aligned polymer chain that induced the texture was investigated by scanning electron microscopy (SEM). Spherulite structure was found in the PBO/PPA system with little shearing. Differential scanning calorimeter (DSC) analysis on the system revealed the existence and melting behavior of two ordered phases. An exothermic peak at 85°C was interpreted as the enhancement of the interaction between PBO molecules and PPA solvent, and the endothermic peaks at 140 to 150°C and 180 to 200°C are explained as the melting of the ordered phases. The changes of small-angle light scattering (SALS) patterns of PBO/PPA solution at different temperature further supported these results.     

Thermal analysis of precipitation reactions in a Ti–25Nb–3Mo–3Zr–2Sn alloy

A study was undertaken on a Ti–25Nb–3Mo–3Zr–2Sn alloy using differential scanning calorimetry (DSC) in order to improve understanding of the precipitation reactions occurring during aging heat treatments. The investigation showed that isothermal ω phase can be formed in the cast and solution treated alloy at low aging temperatures. An exothermic peak in the temperature range of 300 to 400°C was detected for precipitation of the ω phase, with approximate activation energy of 176 kJ/mol. The ω phase begins to dissolve at temperatures around 400°C and precipitation of the α phase is favoured at higher temperatures between 400°C and 600°C. An exothermic peak with activation energy of 197 kJ/mol was measured for precipitation of the α phase. Deformation resulting in the formation of the stress induced α″ phase altered the DSC heating profile for the solution treated alloy. The exothermic peak associated with precipitation of the ω phase was not detected during heating of the deformed material and increased endothermic heating associated with recovery and recrystallisation was observed.     

High-Speed Melt Spinning of Polylactide/Poly(Butyleneterephthalate) Bicomponent Fibers: Mechanism of Fiber Structure Development and Dyeing Behavior

Bicomponent fibers consisting of polylactide (PLA) as the sheath and poly(butylene terephthalate) (PBT) as the core were produced by high-speed spinning to obtain materials suitable for medical clothing. The higher-order structure of the PLA fiber component appeared to exhibit simple, alternately stacked, uniaxially oriented amorphous and crystalline regions. Therefore, fairly large tanδ peaks were observed for single-component PLA fibers, even when the orientation-induced crystallization was achieved by high-speed spinning. By conjugating PLA with PBT, although limited mutual interference with the crystallization of each component occurred, both the PLA (Mw?=?170,000, L-lactide content?=?98.7%) and PBT (intrinsic viscosity?=?0.835-0.865 dL/g) could crystallize on a high-speed spinning line, and the proposed formation of a shish-kebab-like structure in the PBT component enhanced the thermal stability of the bicomponent fibers, particularly resulting in shrink-proof properties. The bicomponent fibers developed herein could be deeply dyed at 98?°C, with results comparable to those of industrial polyester, and peeling of the PLA skin layer was rarely observed, even when the dyed fibers were flattened by a rubbing force.     

Magnetic properties and microstructure of melt-spun Sm(Co,Fe,Cu,Zr)8 magnets

We report on the microstructural and magnetic properties of rapidly quenched Sm(Co_balFe_0.1Cu_xZr_0.04)₈ ribbons. Samples spun at velocities above 40 m/s are nanocrystalline and magnetically hard, even in the as-spun state. The coercivity and its temperature coefficient can be improved by annealing at 750°C. Samples spun at low speeds have low coercivity in the as-spun state and are characterized by larger grain sizes. By annealing a cellular precipitation structure within the grains, similar to that of bulk Sm(Co,Fe,Cu,Zr)_z magnets, can be obtained in Cu-containing samples, resulting in an increase of the coercivity, since the precipitates act as pinning centers. Some precipitation occurs even in as-spun Cu-free samples, indicating that their formation is not directly related to the existence of Cu, in contrast to the cellular structure. Compared to the bulk materials, lower annealing temperatures and shorter heat treatment times are required and the slow cooling process is not needed. The activation volumes for the magnetic reversal process are estimated by magnetic relaxation measurements.     

Higher-Order Structure of Poly(Ethylene 2,6-Naphthalene Dicarboxylate) Fibers Produced by Direct High-Speed Spin-Drawing

In order to efficiently produce poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) fibers comprising α-phase crystallites with a high melting temperature, a direct high-speed spin-drawing (HSSD) system was implemented. The higher-order structure of the HSSD PEN fibers was investigated by focusing on the radial anisotropy of the fibers. A nucleating agent was added to the PEN fibers, and its effect on the formation of the higher-order structure was also studied. Highly crystalline PEN fibers predominantly composed of α-phase crystallites were produced by operating the HSSD system at 4?km/min. The PEN fibers exhibited a melting temperature of 288.8?°C and a storage modulus of 31.4?GPa. The excellent thermal stability of the HSSD PEN fibers was presumably brought about by the formation of a shish-kebab-like structure. The generation of the shish-kebab-like structure and radial anisotropy seemed to be more dependent on the effect of tensile stress during the drawing process rather than from the nucleating additive. The nucleating agent used in this study effectively increased both the α- and β-phase crystallite sizes. Such structural modification, however, did not appear to contribute to the thermal stability enhancement of the PEN fibers.     

An isothermal titration calorimetry study of the interactions between an oxyphenylethylene/oxyethylene diblock copolymer and sodium dodecyl sulfate

Interactions between the diblock copolymer S₁₅E₆₃ and the surfactant sodium dodecyl sulfate (SDS) have been investigated by isothermal titration calorimetry (ITC) in the temperature range 10–40°C. At 20°C, the block copolymer is associated into micelles with a hydrodynamic radius of 11.6?nm, which is composed of a hydrophobic styrene oxide (S) core and a water-swollen oxypolyethylene (PEO) corona. The copolymer/surfactant system has been studied at a constant copolymer concentration of 0.25?wt% and over a wide range of surfactant concentration, from 7.5?×?10^?6 up to 0.3?M. The titration calorimetric data for SDS in the temperature range 10–20°C presents a first endothermic increase indicating the formation of mixed copolymer rich-surfactant micelles. From that point, important differences in the ITC plots for surfactant titrations in the presence and in the absence of the copolymer are present. A shallow second endothermic peak is assigned to the interaction between SDS molecules and copolymer molecules resulting from the beginning of micelle disruption. An exothermic peak indicates the end of this disruption where only SDS micelles attached to single copolymer monomers are present, as shown by DLS in a previous paper. At higher temperatures in the range 25–40°C, the first endothermic maximum is not totally shown because interactions between surfactant and block copolymer start at very low SDS concentrations. Moreover, the second endothermic peak is absent and the exothermic minimum is less pronounced as a consequence of the increased micellization of the block copolymer.     

Surface morphology study of recrystallization dynamics of amorphous ZnO layers prepared on different substrates

Amorphous oxides-based devices are exposed, during fabrication, to different processing conditions affecting their properties. Zinc oxide is a prospective candidate for transparent amorphous oxides, but its structure is changing under the influence of temperature. We investigated surface recrystallization of amorphous zinc oxide layers deposited onto fused silica, sapphire and Si substrates by pulsed laser deposition. The prepared three series of layers had highly nonequilibrium phase structures. Using atomic force microscopy and scanning electron microscopy, the effect was studied of subsequent annealing at 200, 400, 600, 800 °C for 60 min upon the surface structural properties of the layers. The following parameters were analyzed: average roughness, RMS roughness and size of formed grains on selected places with 1 × 1 μm² area. Surface structural analysis revealed that annealing led to recrystallization of the prepared layers and roughening of the structural features on the surface. With increasing annealing temperature, the calculated parameters were increasing. The average surface roughness of zinc oxide layers annealed at 800 °C is three times higher than that of the layers annealed at lower temperatures for all substrates used. The process dynamics of thermally caused recrystallization of the layers was different for each of the substrates used.     

CHANGES IN MELTING BEHAVIOR AND MORPHOLOGY ON ANNEALING OF LINEAR AND BRANCHED POLYETHYLENE

Changes in morphology and melting behavior of various types of commercial polyethylenes as a result of annealing were studied using differential scanning calorimetry, transmission electron microscopy, and density measurements. The range of polyethylenes whose densities varied between 0.96 and 0.90 g/cm³ included linear polyethylene (LPE), high-density polyethylene, a 1-octene copolymer traditional linear low density polyethylene, low density polyethylene, and a 1-octene copolymer prepared by Dow's INSITE constrained geometry catalysts technology. Two sets of samples were initially prepared by fast cooling and slow cooling from the melt. Despite an initial lower crystalline content and crystal thickness for the fast cooled (FC) branched polyethylene samples, a higher melting temperature than for the slow cooled (SC) samples was found using a 10°C/min heating rate. In concordance with a recent work, melting–recrystallization processes are held responsible for the anomalous behavior. The annealing treatment consisted of heating the two sets of samples at 1°C/min from room temperature to a temperature located at the start of the endotherm. The thermal treatment stabilizes the crystals through an increase in their thickness, which prevents melting–recrystallization processes from taking place on subsequent heating. A lower melting temperature after annealing was observed for the FC branched polyethylene samples. No such behavior was found for the SC samples and even for the FC LPE sample.     

Melting and Crystallization Behaviors of Poly(Lactic Acid) Modified with Graphene Acting as a Nucleating Agent

Graphene (GN)-filled polylactic acid (PLA) nanocomposites were prepared through a solution blending method with GN weight percent ranging from 0.5 to 2?wt%. Rheological, melting and crystallization behaviors of the prepared PLA/GN nanocomposites were investigated by means of dynamic rheological measurements and differential scanning calorimetry (DSC). The shear viscosities of the PLA/GN nanocomposites decreased with increasing GN content, which was remarkably different from previous reports on the modifications using traditional nanofillers (e.g., clay, carbon nanotubes, etc.). The nonisothermal melt crystallization kinetic analysis suggested that GN served as a nucleating agent and could considerably promote the PLA’s crystallization through heterogeneous nucleation. Our findings suggested that at relatively low cooling rates (??≤?10?°C/min) even a small amount of GN promoted the nucleation and considerably increased the crystallization rate. However, the crystallinity began to decrease at higher cooling rates (e.g., ??≥?20?°C/min), especially when the GN content was high (e.g., 2?wt%), possibly owing to the GN aggregation effect considering PLA is a slowly crystallizing polymer.     

Isothermal Crystallization and Melting Behavior of Composites Composed of Poly(L-lactic Acid) and Poly(glycolic Acid) Fibers

Several composites of poly (L-lactic acid) (PLLA) with poly (glycolic acid) (PGA) fibers were prepared. The isothermal crystallization kinetics and melting behavior of PLLA and all of the composites were characterized by using differential scanning calorimetry. The experimental data were processed by using the Avrami equation. The relative parameters, such as the Avrami exponent and half-time crystallization, revealed that PGA fibers had positive effects on the crystallization of PLLA, but these effects had only a minimal dependence on the PGA fiber content. Moreover, at low isothermal crystallization temperatures (85°C～110°C), recrystallization during the heating scan was observed, which could lower the melting point of the samples to a certain extent.     

High-Pressure Analysis of the Multiple Melting Endotherms of Poly(ethylene succinate) and Poly(butylene succinate)

The origin of the multiple melting peaks in two linear polyesters, poly(ethylene succinate) (PES) and poly(butylene succinate) (PBS), of isothermally crystallized samples was investigated by differential scanning calorimetry (DSC) at atmospheric pressure and high-pressure differential thermal analysis (HP-DTA) at elevated pressures. In PES, the DSC melting curves showed three endothermic peaks at slow heating rates, which decreased to two with increasing heating rates. The HP-DTA curves showed that the area (qualitative) and peak height of the high-temperature peak decreased with increasing pressure and merged with the low-temperature peak at pressures above 450 MPa. This behavior supported the melting, recrystallization, and remelting model for the observed multiple melting endotherms. In PBS, the DSC melting curves were similar to those seen in PES. The HP-DTA curves were also similar to PES up to 400 MPa, but above this pressure the area and the peak height of the high-temperature peak and the temperature difference between the high- and low-temperature peaks remained unchanged. This observation suggested that the two peaks in PBS were due to the melting of two populations of crystals with different lamellar thickness originally present in the sample. The multiple melting behavior in isothermally crystallized PBS is proposed to incorporate both the melting of two populations of crystals and melting, recrystallization, and remelting.     

Double yield in tensile deformation of high-density polyethylene fiber

Fiber-grade high-density polyethylene (HOPE) was melt spun into a multifilament yam that, after quenching in air, was wound at speeds in the range 450 m/min to 1000 m/min. Tensile tests on these anisotropic yarns showed two yield points of varying intensity, similar to those reported in the literature for compression molded, isotropic HOPE sheets and films. However, unlike the sheets, the second yield in the case of fibers always occurred at a stress higher than that for the first yield. More detailed studies on the yam spun at 450 m/ min showed that these characteristics persisted at different strain rates and test temperatures and were also present in the heat-set as-spun samples. The applicability of the existing models to explain the double-yield phenomenon has been critically examined in the light of the results obtained. For this, the necessary structural and morphological characterization of the samples was also done.