Orientation-induced Crystallization Research Articles

Competition effect of solid-state stretching induced orientation and phase separation on stereocomplex crystallization of PLLA/PDLA during annealing

Benefiting from its superior thermal and mechanical properties, the stereocomplex crystal (SC) of PLLA/PDLA is catching increasing interest. In this work, a systematic investigation was performed to reveal the effect of solid-state stretching on the formation of SC during annealing. Results show that as the stretching temperature was close to the glass transition temperature (Ts = 60 °C), the polymer chains orientation was increased gradually with draw ratio (λ). As a result, the crystallization induction time (t0) was shortened, the growth rate (Gr) of SC was enlarged, and the crystallinity of SC (XSC) was elevated as a result of orientation-induced crystallization. Whereas, the saturation of t0, Gr, and XSC appeared quickly when λ reached 2.7, although the orientation of polymer chains continued to increase. The reason is that in addition to orientation, strong phase separation between PLLA and PDLA was also triggered therein. The phase separation prohibited greatly the crystallization of SC due to a long time would be needed for PLLA or PDLA chains to diffuse to a distance close enough to activate the hydrogen-bonding interaction, which is the prerequisite of SC crystallization. When the stretching temperature was elevated to 75 °C, the phase separation became much weaker. So XSC was higher compared to that of the blend stretched at 60 °C to the same λ. Moreover, in the annealing temperature from 180 °C–190 °C, t0 was much smaller for the blend stretched at 75 °C since the crystallization of SC in this temperature range is a diffusion-controlled process.

Fiber structure development of poly(ethylene terephthalate-co-isophthalate) copolymer

Isophthalic acid copolymerization is generally used to control the properties of polyethylene terephthalate fiber. To investigate the controlling mechanism, the effects of copolymerization on orientation-induced crystallization behavior and smectic phase formation after necking were analyzed by acquiring X-ray diffraction images during the continuous drawing process. Isophthalic acid copolymerization suppressed orientation-induced crystallization during the spinning and drawing processes. It also decreased the d-spacing and apparent thickness of the smectic phase, while the integrated intensity and length of the smectic phase were not affected. Furthermore, the melting point and crystallinity decreased, and the thermal shrinkage of drawn fibers increased, while the birefringence and tensile strength were unchanged. Almost no change in the molecular orientation and microfibril length presumably resulted in the fiber strength being retained.

Melt-Spun Fibers from Bio-Based Polyester-Fiber Structure Development in High-Speed Melt Spinning of Poly(ethylene 2,5-furandicarboxylate) (PEF).

Poly(ethylene 2,5-furandicarboxylate) (PEF) is regarded as a bio-based alternative or complementary polyester for the widely used fossil resource-based polyester, poly(ethylene terephthalate) (PET). High-speed melt spinning of PEF of low and high molecular weight (L-PEF, H-PEF) was conducted, and the structure and properties of the resultant as-spun fibers were investigated. The occurrence of orientation-induced crystallization was confirmed for the H-PEF at the take-up velocity of 6.0 km/min, the highest speed for producing PET fibers in the industry. Molecular orientation and crystallinity of the as-spun fibers increased with the increase of take-up velocity, where the H-PEF fibers always showed a higher degree of structural development than the L-PEF fibers. The tensile modulus of the high-speed spun H-PEF fibers was relatively low at 5 GPa, whereas a sufficiently high tensile strength of approximately 500 MPa was measured. These values are adequately high for the application in the general semi-engineering fiber field.

Dyeing Behavior and Multi-Functionalities of Polylactide/Poly (Ethylene Terephthalate) Conjugate Fibers Produced by High-Speed Melt-Spinning

Various types of conjugate fibers comprised of polylactide (PLA) and polyethylene terephthalate (PET), with their blend in the sheath, PET as the core, were produced by high-speed melt-spinning. Due to the higher glass transition temperature of PET, the orientation-induced crystallization only occurred in the PET core. The crystalline and continuous PET core of the conjugate fibers effectively suppressed the shrinkage during dyeing, even in a pressurized water bath at 130 °C comparable to the commercial level, to less than 5% while achieving a color strength higher than 15. Post-annealing in the restrained mode could effectively cause the fine PET fibrils embedded in the sheath to rise out from the surface of the fibers during dyeing by the sliding friction force between the fibers. The PET fibrils rising out from the sheath were strong enough and showed an entangling behavior which would be useful to fabricate non-woven fabrics. The amorphous PLA domains exposed on the fiber surface showed the bacteriostatic activity valuable for the inhibition of the formation of biofilms.

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

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.

Effect of Polycondensation Catalyst on Fiber Structure Development in High-Speed Melt Spinning of Poly (Ethylene Terephthalate).

We conducted a preliminary study on fiber structural development in the high-speed melt spinning of environmentally friendly polyethylene terephthalate (Ti-PET) synthesized with 25 ppm of titanium-based catalyst, which was compared with conventional PET (Sb-PET) synthesized with 260 ppm of antimony-based catalyst. Gel permeation chromatography of Ti- and Sb-PET resins of intrinsic viscosity 0.63 confirmed that both resins have similar molecular weights and distributions. However, differential scanning calorimetry revealed that the Ti-PET resin exhibited a lower melt–crystallization peak and isothermal melt-crystallization rate than the Sb-PET resin. High-speed melt spinning of the Ti- and Sb-PET was possible up to a spinning velocity of 6 km/min. Two-dimensional wide-angle X-ray diffraction analyses showed that the molecular orientation of the obtained as-spun Ti- and Sb-PET fibers increased with spinning velocity, and a highly oriented, crystalline structure by orientation-induced crystallization started to appear from 5 km/min. Notably, Ti-PET fibers showed a lower degree of crystalline structural development and lower tensile strength compared with Sb-PET fibers under the high-speed spinning conditions. Our results suggest that the catalyst in PET resins can act as nucleating agents in thermal- and orientation-induced crystallization, and that differences in catalyst content can influence PET fiber structure development under extreme conditions in high-speed melt spinning.

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.

Tracking the origins of size dependency in the mechanical properties of polymeric nanofibers at the atomistic scale

Mechanical properties of ultra-fine polymeric nanofibers are highly size-dependent and the mechanisms for causality of the size dependency is still not quite clear yet. In this work, we investigate the origins of this size dependency at the atomistic level. By using molecular dynamics (MD) framework, two fabrication methods are utilized in this study to prepare non-drawn and hot-drawn fibers, in order to investigate the two distinguished mechanisms for nanofiber size dependency. One is rooted in the effect of surface chains which is intrinsic to low dimensional materials and the other is the chain alignment induced by drawing during fiber fabrication process. Our results show that the size dependency is not chiefly attributed to the effect of surface polymeric chains, but rather strongly relates to the chain alignment in nanofiber's microstructure. The atomistic study shows that non-drawn nanofibers have a dense core covered by a less dense shell layer, but interestingly, our investigations reveal that the core-shell structure itself will not result in the remarkable increase of modulus and strength when diameter of the fiber drops down. By testing the hot-drawn fibers, we found the size dependency mainly originates from the chain alignment. When fiber is formed by hot-drawing, higher drawing ratio leads to thinner fiber and more aligned chains in the microstructure. A three-fold increase of modulus and strength is observed when the chain orientation parameter of the hot-drawn nanofiber increases from 0.1491 to 0.3375 while the diameter of the fiber drops from 19 to 10 nm. By examining the energy evolution associated with the bond lengths, bond angles and dihedral angles at the atomistic scale, our results show that the dihedral angles play a vital role in the size-effect. Our results also show the mechanical response of fibers are affected by changing the temperature, additionally, the orientation-induced crystallization is monitored on hot-drawn fibers at 300 K, which makes the fibers become stiffer but less ductile.

Effects of dimensions and regularity on the mechanical properties of the smectic phase formed during orientation-induced crystallization of poly(ethylene terephthalate)

Abstracts A fibrillar-shaped metastable smectic phase forms during orientation-induced crystallization of poly(ethylene terephthalate). We investigated the effects of dimensions and regularity of the smectic phase on the mechanical properties by high-precision X-ray measurements. The drawing stress dependence was 76–168 MPa and the spinning speed dependence of 250–2000 m/min. The proportion, persistence length, and thickness of the smectic phase achieved their maximum values at 0.3–0.4 ms after necking. At this time gII also reached a minimum value of 3.7%–4.3%. The persistence length increased linearly with the natural draw ratio of as-spun fibers at less than 1500 m/min. The maximum values of the proportion, persistence length, and thickness showed no dependence on the drawing stress; however, the proportion and persistence length increased more rapidly. The increase of d-spacing with drawing stress led to an apparent elastic modulus of approximately 40 GPa for the oriented molecular bundle.

Lattice-matched carbon dots induced the oriented self-assembly of Cu nanoparticles

Orientation-induced crystallization is a widely applied method of forming crystal with highly oriented structure. Here, we reported a promising matrix for orientation-induced crystallization–carbon dots (CDs), who exhibited high lattice-matching with Cu crystal. First, CDs induce self-assembly of Cu nanoparticles through organics-inorganics interface interaction, forming self-assembled mesocrystal and single crystal with large size. Second, the CDs selectively deposit on Cu {111} plane via lattice-matching, endowing Cu self-assemblies with tunable morphology including sphere, truncated octahedron, octahedron, concave octahedron and hexpods. Third, CDs also protect Cu self-assemblies from oxidation and act as efficient interface for optical conversion and electron transfer. So using as surface-enhanced Raman scattering (SERS) substrate for the first time, the CDs-coating hexpods Cu mesocrystal require no post processing and exhibit high stability and surface-enhanced Raman scattering (SERS) activity which ordinary Cu nanostructure cannot achieve.

Multiple Melting Behavior of High-Speed Melt Spun Polylactide Fibers

ABSTRACTHigh-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.

High-time Resolution Measurement of Orientation-induced Crystallization by Using Laser-heated-drawing and Synchrotron X-ray Source

High-time Resolution Measurement of Orientation-induced Crystallization by Using Laser-heated-drawing and Synchrotron X-ray Source

Preparation, mechanical properties and microstructure of polyoxymethylene fiber through melt spinning and hot drawing by using injection-molding grade resins

The polyoxymethylene (POM) fiber was melt spun by use of different commercial grades of POM resin, and the effect of post-drawing on mechanical properties and microstructures was investigated extensively. The fiber obtained from the POM resin with a higher melt flow index (MFI) exhibits a better hot-drawing capability and also achieves a greater ultimate draw ratio. The mechanical evaluation reveals that the tensile strength and elastic modulus of POM fiber are improved significantly after post-drawing compared to the as-spun fibers. Although the greater draw ratios result in higher mechanical strength and modulus for the POM fiber, the fiber obtained from the POM resin with an MFI of 13.0 g/10 min achieves the optimal mechanical performance at the ultimate draw ratio. The morphologic and structural developments of POM fiber were studied by scanning electronic microscopy and X-ray powder diffraction. The results indicate that the POM fiber spun by the resin with an MFI of 13.0 g/10 min has a smooth lateral surface and a compact cross section after post-drawing. The fiber samples spun by the POM resins with low MFIs show some hollow disfigurements as well as a rough surface at the ultimate draw ratio, whereas the fiber obtained from the resin with a high MFI of 27.0 g/10 min presents the ununiformity of diameter after post-drawing. The POM fibers achieve a crystalline orientation during the hot-drawing process, which results in a transformation from the spherulitic crystals to the lamellar structure in the drawing direction. The level of crystalline orientation can be improved with an increase of draw ratio and thus results in a high modulus and strength for the resulting POM fiber samples. In addition, the thermal analysis indicates that the crystallinity of the as-spun fibers can be enhanced by post-drawing due to the orientation-induced crystallization.

Microstructural evolution and mechanics of hot-drawn CNT-reinforced polymeric nanofibers

To unravel reinforcing effect of CNTs in polymeric nanofibers, polyacrylonitrile (PAN)/SWNTs nanofibers with various CNTs content (0.1–0.5 wt%) were electrospun. Nanofibers were hot-drawn to enhance PAN chains and SWNT alignment. The microstructure of composite nanofibers was studied via polarized Raman and IR spectroscopy, x-ray diffraction, and TEM imaging. The study pointed to a marked contribution of SWNTs at low SWNT contents (0.1 wt%) to chain mobility and alignment during electrospinning. Evidence of volume changes was reported during hot-drawing, depending on CNT content, alluding to arrangement of CNTs in nanofibers. Indirect evidence of SWNT agglomeration, adversely affecting reinforcement efficiency at higher SWNT contents, was presented. Hot-drawing above Tg of PAN led to massive chain and SWNT alignment and orientation-induced crystallization. The most promising mechanical performance was achieved in hot-drawn ribbons with low SWNT content, where formation of an interphase –evidenced in TEM images – led to significant enhancement of effective reinforcement modulus of ∼2.15 ± 1.97 TPa, with the average value considerably higher than modulus of CNTs. This study demonstrates a clear path to prepare PAN nanofibers with high chain alignment as precursor for the next generation of carbon nanofibers with unprecedented strength and modulus.

On the Orientation-Induced Crystallization of Polymers.

In order to understand orientation-induced crystallization of polymers, we introduced an intermolecular interaction between polymer chains based on quantum mechanics. We therefore considered a pair of perfectly extended chains where the intermolecular interaction is assumed to be based on the hydrogen interaction with a single chain. When two protons of each extended chain become closer together under tension, the attractive force between the extended chains is caused by the interaction between hydrogen atoms surrounding the main chains based on the hydrogen molecule ion H. The energy is split into the ground and excited states, and the spontaneous process leading to the ground state is the origin for orientation-induced crystallization.

Hierarchical microstructure changes and the molecular mechanism of polypropylene under a critical failure strain during creep

The hierarchical microstructure evolution of polypropylene during creep was explored via various methods, such as differential scanning calorimetry (DSC), scanning electron microscope (SEM), two-dimensional small-angle X-ray scattering (2D-SAXS), two-dimensional wide angle X-ray diffraction (2D-WAXD) and positron annihilation lifetime spectroscopy (PALS). The results revealed a correlation among the changes of micron-scale spherulites, nano-scale lamellae, crystalline blocks, atomic scale free volume and the deformation of polypropylene during creep. The elongation of micron-scale spherulites along the creep direction, accompanying with the increase of nano-scale lamellar long spacing, as well as the enlargement and amalgamation of atomic scale free volume were observed at ε below 17%; the imperfect fibrillar crystallites with polymer chains preferentially oriented along the creep direction, formed in the stress-induced crystalline block disaggregation–recrystallization process, were proved by SEM and 2D-SAXS results when ε was between 17% and 55%; the further orientation of polypropylene chains led to a higher degree of orientation and crystallinity. The molecular deformation mechanism of polypropylene during creep included three stages: the intralamellar slipping of crystalline blocks, accompanying with the enlargement and amalgamation of free volume, was activated at small strain (ε ≤ 17%); whereas the stress-induced crystalline block disaggregation–recrystallization process as well as the rearrangement and orientation of chains were proceeded at medium strain (17% < ε ≤ 55%); at last, orientation-induced crystallization occurred at larger strain (ε > 55%).

Evolution of the microstructure and morphology of polyimide fibers during heat-drawing process

The development of crystalline structure and morphology for polyimide (PI) fibers in the heat-drawing process was investigated by simultaneous synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). WAXD results indicated that the drawing process resulted in a high crystal orientation and ordered crystal structure. Especially, as the drawing ratio increases to 2.0, a well-defined crystalline structure forms in the fibers. We propose that the highly oriented molecular chains induce the formation of crystalline regions. Namely, an orientation-induced crystallization occur with stretching in the case of the heat-drawing polyimide fibers. The meridional scattering streaks in the SAXS patterns for the as-spun fibers suggest the presence of periodic lamellar structure in the fibers. These crystalline lamellae may evolve to more complete crystalline regions. The size of microvoids in the cross-section of the PI fibers is analyzed by SAXS. As a result, the drawing process leads to the orientation of microvoids along the fiber, and to reduced diameter of the microvoids in the fiber. Dynamic thermomechanical analysis indicates that the activation energy Ea of α relaxation increases with the increase in the crystallinity and orientation in the fibers.

PET高速溶融紡糸過程のオンライン広角X線散乱測定

We report results of on-line wide-angle X-ray scattering (WAXS) measurements for melt spinning process of poly(ethylene terephthalate) (PET). In order to obtain good results, we have conducted experiments by paying attention as much as possible. First of all, a special experimental set-up has been developed by employing a purge box which contains the spin line and the X-ray detector (imaging plate), as well, with helium gas purge, in order to minimize noise level due to air scattering. Furthermore, a high-molecular weight PET sample prepared by solid-state polymerization was used in order to increase the spin line stress, which accommodates us with starting of orientation-induced crystallization at low take-up velocities. A special care to minimize the swaying of the spin line has been taken as well by employing a perforated metal guard inside the spinning tube which covers the spin line from the spinneret to the winder with insertion of the helium gas purge box in the middle. This worked well for suppression of the air turbulence. By such special experimental treatments, we have succeeded in conducting on-line two-dimensional WAXS measurements during the high speed melt spinning of PET fibers with the take-up velocity of 1600m/min. As a high brilliant X-ray source, we utilized SPring-8 (Japan Synchrotron Radiation Research Institute, Harima, Japan). The combination of above mentioned techniques enabled us to obtain crystalline reflections from oriented crystals with only 2min exposure. Nevertheless, we observed no appreciable change in crystallinity and orientation of crystallites along the spin line. It is because the control of positioning the neck point where the crystallization sets in was sensitively influenced by the ambient gas condition. Further special care to overcome this difficulty should be taken in our future study.

Effect of drawing stress on mesophase structure formation of poly(ethylene 2,6-naphthalene dicarboxylate) fiber just after the neck-drawing point

The structural development of poly(ethylene 2,6-naphthalene dicarboxylate) (PEN) fibers was analyzed by in situ X-ray diffraction and fiber temperature measurements. The PEN fiber was drawn continuously under three drawing stresses, where the neck-drawing point was fixed accurately by CO2 laser irradiation heating. The developed crystal structures of the drawn fibers depended on the drawing stresses, that is, only the α-crystal was obtained under a drawing stress of 148 MPa, an α-rich mixed crystal was obtained for 54 MPa, and a β-rich mixed crystal was obtained under 23 MPa stress. Fiber containing over 70% β-crystal was obtained in the third case. Orientation-induced crystallization rates (K) and crystallization induction times (t0) were estimated for the three drawing stresses: K = 2210 s−1 and t0 = 0.5 ms for 148 MPa, K = 940 s−1 and t0 = 1.0 ms for 54 MPa, and K = 655 s−1 and t0 = 4.0 ms for 23 MPa. In addition, the drawing stress acted as a definitive influence not only on the resulting crystal form but also on the chain conformation of the mesophase structure. The d-spacing of the (001′) diffraction increased with drawing stress, and the longer (001′) spacing generated the α-crystal while the comparatively shorter (001′) spacing yielded the β-crystal. The d-spacings of 1.27 and 1.23 nm for the drawing stresses of 148 and 23 MPa, respectively, were somewhat shorter than the c-axis lengths of the α- and β-crystals of 1.32 and 1.27 nm, respectively.

Fiber Structure Development and Mechanical Properties in Continuous Drawing and Annealing of Poly (phenylene sulfide) Filament

Poly(phenylene sulfide) (PPS) multi-filaments were continuously drawn and annealed. The maximum values of birefringence, crystallinity, tensile strength and Young′s modulus in the drawn and annealed PPS filament were 249×10−3 ‚ 27%, 5.3 cN/dtex and 64 cN/dtex‚ respectively. The orientation-induced crystallization was observed at 150×10-3 of birefringence. The crystal orientation factor f′ was estimated by wide angle X-ray diffraction. The λnet was estimated by the strain shifting of true stress - true strain curve to the master curve. The orientation estimated by the λnet differed from that estimated by birefringence. The fiber structure development and the mechanical properties of resultant fibers were discussed in terms of orientation parameters, which were birefringence‚ network draw ratio (λnet) and crystal orientation factor (f′). The birefringence shows a good correlation with the Young’s modulus. On the other hand, λnet shows a good correlation with the tensile strength. The f′ shows good correlations with both Young′s modulus and tensile strength‚ however‚ they tend to saturate at high f′. The melting temperature of PPS changed from 280.1oC to 288.7oC with the increase of birefringence and λnet.