Extended-chain Crystals Research Articles

Oriented crystalline structure in melt-drawn ultrahigh-molecular-weight polyethylene induced by entanglement networks

In this study, we focused on the unique structure with oblique periodicity formed by the melt-drawing of metallocene-catalyzed ultrahigh-molecular-weight polyethylene (UHMW-PE) to clarify the relationship between tight entanglements that cannot be disentangled during melt-drawing and the formation of the periodic structure. In situ X-ray measurements during the heating of the melt-drawn UHMW-PE film revealed that the disappearance of the characteristic oblique streaks that appeared tilted from the drawing direction and the orthorhombic-to-hexagonal phase transition occurred simultaneously, suggesting that the change in the tension state of extended-chain crystals due to the orthorhombic-to-hexagonal phase transition and the disappearance of order in the structure resulting in oblique streaks are related behaviors. This indicates the presence of the network structure including oriented amorphous chains in which molecular chains are fixed by tight entanglements in metallocene-catalyzed UHMW-PE melt-drawn film.

Boosting solvent resistance and structure stability of degradable polyester through extended-chain crystal structure: A preliminary case of oil/water separation membrane

Developing polymer-based oil/water separation membranes has emerged as a promising approach for wastewater, and fabrication of environmental-friendly separation membranes is becoming a vital issue for sustainable development. Currently, the application of degradable polyesters for oil/water separation is severely limited due to their inferior resistance to various solvents. Herein, we report a new kind of separation membrane of degradable aliphatic polyester based on extended-chain crystals. Though significant improvements in the crystalline structure, including crystallinity and thermodynamic stability, the polyester membranes gain outstanding solvent resistance, maintaining their structural integrity even after prolonged immersion in diverse solvents for up to 100 days. Meanwhile, the hydrophobic and lipophilic characteristic of aliphatic polyester is well inherited in the separation membrane, imparting exceptional wettability to various solvents. Consequently, the extended-chain crystal porous membranes exhibit excellent flux performance and good separation efficiency for solvent and water mixtures and emulsions. In addition, the extended-chain crystal structure gives the membrane outstanding mechanical properties. This work provides the first membrane application report of extended-chain crystal in degradable aliphatic polyester and reveals the potential of extended-chain crystal materials to deliver superior performance and extensive application.

Challenges and opportunities in piezoelectric polymers: Effect of oriented amorphous fraction in ferroelectric semicrystalline polymers

AbstractDespite extensive research on piezoelectric polymers since the discovery of piezoelectric poly(vinylidene fluoride) (PVDF) in 1969, the fundamental physics of polymer piezoelectricity has remained elusive. Based on the classic principle of piezoelectricity, polymer piezoelectricity should originate from the polar crystalline phase. Surprisingly, the crystal contribution to the piezoelectric strain coefficient d31 is determined to be less than 10%, primarily owing to the difficulty in changing the molecular bond lengths and bond angles. Instead, >85% contribution is from Poisson's ratio, which is closely related to the oriented amorphous fraction (OAF) in uniaxially stretched films of semicrystalline ferroelectric (FE) polymers. In this perspective, the semicrystalline structure–piezoelectric property relationship is revealed using PVDF‐based FE polymers as a model system. In melt‐processed FE polymers, the OAF is often present and links the crystalline lamellae to the isotropic amorphous fraction. Molecular dynamics simulations demonstrate that the electrostrictive conformation transformation of the OAF chains induces a polarization change upon the application of either a stress (the direct piezoelectric effect) or an electric field (the converse piezoelectric effect). Meanwhile, relaxor‐like secondary crystals in OAF (SCOAF), which are favored to grow in the extended‐chain crystal (ECC) structure, can further enhance the piezoelectricity. However, the ECC structure is difficult to achieve in PVDF homopolymers without high‐pressure crystallization. We have discovered that high‐power ultrasonication can effectively induce SCOAF in PVDF homopolymers to improve its piezoelectric performance. Finally, we envision that the electrostrictive OAF mechanism should also be applicable for other FE polymers such as odd‐numbered nylons and piezoelectric biopolymers.

Macromolecular structure development of dry-spinning UHMWPE fibers through the industrial high-ratio hot-drawing process

Ultra-high molecular weight polyethylene (UHMWPE) and its decalin suspension were subjected to several industrial processes including swelling, dissolving, spinning, drying, and multi-stage high-ratio hot drawing to fabricate UHMWPE fibers (UPE fibers). The morphologies and architectures of the UPE fibers at different drawing stages were characterized by polarizing optical microscope (POM), scanning electron microscope (SEM) and atomic force microscopy (AFM). The aggregation structure evolution of UHMWPE macromolecules during the high-ratio hot-drawing process was studied by differential scanning calorimeter (DSC), wide-angle X-ray diffraction (WAXD), small angle X-ray scattering (SAXS) and other testing methods. The experimental results demonstrated that the UHMWPE/decalin spinning suspension was extruded from the spinneret hole to form a fine spinning flow, followed by the formation of the transitional-state fibers at different processing stages, and finally the ready-made fibers. The total drafting ratio of the UPE fibers after fabrication reached more than 600 times. The diameter of the single UPE filament was reduced to 5.66 μm with elevated drawing ratios. The crystal structure of the prepared UPE fibers was progressively improved, and the folding-chain crystals of fibers gradually transformed into extended-chain crystals. The mechanical properties of the UPE fibers were significantly improved. The fracture strength of the as-spun UPE fibers reaches 48.1 cN/dtex, which endows the fibers with specific characteristics such as ultra-fine denier and ultra-high strength.

Structural and morphological changes at initial state under uniaxial elongation of rolled polytetrafluoroethylene

Polytetrafluoroethylene (PTFE) membranes, which are prepared by a biaxial stretching process of rolled sheets, have a porosity structure consisting of nodes and fibrils, and exhibits a unique mechanical behavior. We investigated the differences in deformation behavior of rolled PTFE sheets between the rolling and the cross directions using synchrotron radiation in situ WAXD and in situ SAXS measurements. The mechanical anisotropy for rolled PTFE sheets results from a planar or strip-biaxial deformation mode. The rolled PTFE sheets showed an extended chain crystal (ECC) structure. According to in situ WAXD profiles and 2D-SAXS patterns during deformation, the stretching in the rolling direction (RD) causes the fibrillation accompanied with the mechanical yielding, at which the stretching in the RD resulted in pore expansion and lattice distortion rather than obvious chain orientation. The direction of the pore expansion changed to the cross direction (CD) when stretching perpendicular to the RD. We found a positive dependence between the pore size in CD and the average molecular weight of PTFE sheets.

Double-lamella phenomenon of once-folding polymer crystallization raised by chain-end hydrogen-bonding interactions

Poly (ethylene glycol) (PEG) holding molecular weights around 7000 Da exhibits a double-lamella phenomenon on the surface replicas of bulk once-folding crystallization, whose mechanism is still unclear. Concerning to its specific hydrogen-bonding interactions at chain-ends, we performed dynamic Monte Carlo simulations to investigate the effects of specific chain-end interactions on once-folding crystallization of 16-mers. The results showed that the chain-end hydrogen-bonding interactions bring little effects to both kinetics of primary nucleation and lateral growth of chain-folded lamellar crystals but make a more significant terrace at the lateral growth front with a step of metastable once-folding towards the next step of stable chain-extending, raising the double-lamella phenomenon of their surface replicas. We found that the chain-end hydrogen-bonding interactions are rich at the surfaces of once-folded lamellar crystals, which thus enhance their metastability and retard their thickening into the chain-extended crystals. In addition, intra- and inter-molecular secondary nucleation at corresponding chain-folded and extended growth fronts show different growth rate dependences on the specific interaction strengths. Our results revealed the role of specific chain-end interactions in PEG crystallization.

Structural characters of biaxially stretched polypropylene films and the relevant electrical insulating properties

Abstract Capacitor films from biaxially oriented polypropylene (BOPP) involve intensive external stress field, resulting in special crystallization and orientation characters. However, it still remains ambiguous on the relationship between crystallized morphology in BOPP bulk film and electric properties. In this work, two stretching modes, simultaneously biaxial stretching and sequentially biaxial stretching, were chosen to adjust film thickness, extended chain crystal content, fibrillar morphology, and orientation texture. Meanwhile, the working rules of these structural issues on electrical insulating properties were inspected. It reveals that extended chain crystals with thermal stability and isotropous fibrillated network favor to improved breakdown strength and lowered dielectric loss. These results offer good understanding on the processing-structure-property relation of polymer film dielectrics.

Thermal Conductivity of Polyvinylidene Fluoride Films with a Multi-Scale Framework.

The orientation of amorphous regions in pure polymers has been noted to be critical to the enhancement of thermal conductivity (TC), but the available reports are still rather few. Here, we propose to prepare a polyvinylidene fluoride (PVDF) film with a multi-scale framework by introducing anisotropic amorphous nanophases in the form of cross-planar alignments among the in-planar oriented extended-chain crystals (ECCs) lamellae, which show an enhanced TC of 1.99 Wm-1 K-1 in the through-plane direction (K⟂) and 4.35 Wm-1 K-1 in the in-plane direction (K∥). Structural characterization determination using scanning electron microscopy and high-resolution synchrotron X-ray scattering showed that shrinking the dimension of the amorphous nanophases can effectively reduce entanglement and lead to alignments formation. Moreover, the thermal anisotropy of the amorphous region is quantitatively discussed with the aid of the two-phase model. Superior thermal dissipation performances are intuitively displayed by means of finite element numerical analysis and heat exchanger applications. Moreover, such unique multi-scale architecture also results in significant benefit in the improvement of dimensional stability and thermal stability. This paper provides a reasonable solution for fabricating inexpensive thermal conducting polymer films from the perspective of practical applications.

Co-Crystallization between Aliphatic Polyesters through Co-Inclusion Complexation with Small Molecule.

Crystalline/crystalline blends of polymer have shown advantages in the preparation of new polymeric materials. However, the regulation of co-crystallization in a blend is still full of challenges due to the preferential self-crystallization driven by thermodynamics. Here, an inclusion complex approach is proposed to facilitate the co-crystallization between crystalline polymers, because the crystallization process displays a prominent kinetics advantage when polymer chains are released from the inclusion complex. Poly(butylene succinate) (PBS), poly(butylene adipate) (PBA) and urea are chosen to form co-inclusion complexes, where PBS and PBA chains play as isolated guest molecules and urea molecules construct the host channel framework. The coalesced PBS/PBA blends are obtained by fast removing the urea framework and systematically investigated by differential scanning calorimetry, X-ray diffraction, proton nuclear magnetic resonance and Fourier transformation infrared spectrometry. It is demonstrated that PBA chains are co-crystallized into PBS extended-chain crystals in the coalesced blends, while such a phenomenon has not been detected in simply co-solution-blended samples. Though PBA chains could not be totally accommodated in the PBS extended-chain crystals, their co-crystallized content increases with the initial feeding ratio of PBA. Consequently, the melting point of the PBS extended-chain crystal gradually declines from 134.3 °C to 124.2 °C with an increasing PBA content. The PBA chains playing as defects mainly induce lattice expansion along the a-axis. In addition, when the co-crystals are soaked in tetrahydrofuran, some of the PBA chains are extracted out, leading to damage to the correlative PBS extended-chain crystals. This study shows that co-inclusion complexation with small molecules could be an effective way to promote co-crystallization behavior in polymer blends.

Crystallization of Star-Shaped Poly(l-lactide)s with Arm Chains Aligned in the Same Direction in Two-Dimensional Crystals in a Langmuir Monolayer

Polylactide (PLA) crystallizes to form extended-chain crystals in a Langmuir monolayer because crystallization is accelerated on the water surface. This is a unique situation where chain packing can be analyzed by simply measuring the lamellar thickness. Herein, star-shaped poly(l-lactide)s (PLLAs) with 2-12 arms were synthesized through the polymerization of l-lactide with various polyols as initiators, and their crystallization behavior in a monolayer was studied via atomic force microscopy. The PLLAs comprising 2-4 arms crystallized with all arms aligned in the same direction and being folded at the central polyol unit. Meanwhile, the PLLAs comprising 6 and 12 arms crystallized with both halves of the arms extended from the center to the opposite directions, most likely due to the steric hindrance of the crowded arms. Considering that the PLLAs crystallized from a once-formed condensed amorphous state during compression, they have a strong tendency to crystallize with the arms aligned in the same direction. The crystallization rate of star-shaped PLAs is known to reduce compared with that of a linear PLA even if the number of arms is as few as 2. This should be closely related to the unique crystallization behavior of the star-shaped PLLAs with the arms aligned in the same direction.

Role of Shear Flow on Structure Development during Post-Processing Annealing for Poly(lactic acid).

The effect of shear history on structure development during post-processing annealing was studied using poly(lactic acid) PLA. Since PLA shows a low crystallization rate, quenched films had no crystallinity. Moreover, molecular orientation was not detected in the films. During the annealing procedure beyond its glass transition temperature, however, molecular orientation to the flow direction occurred with the crystallization growth in the films having an appropriate shear history. This peculiar crystal growth during the annealing was most probably attributed to the crystallization from extended chain crystals generated during the applied shear history, although the amount of extended chain crystals was low. The results obtained in this study should be noted because the molecular orientation proceeded due to the annealing history applied. Furthermore, this phenomenon will be used to suppress dimensional change and increase product rigidity.

Optimum processing conditions for the maximum crystallization rate of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)

The effect of thermal and shear histories on the crystallization rate of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) was studied. As with other crystalline polymers, the shear history greatly affected the crystallization rate when the shear rate was beyond a critical value, i.e., the inverse of the Rouse relaxation time. Even after the formation of extended chain crystals, spherulite texture was clearly discernable. It grew from certain points on the extended chain crystals. Consequently, a row of spherulites appeared along the flow direction. The resin temperature in the molten state was also significant. When the sample was heated to 170 °C, which is beyond the main melting peak in the differential scanning calorimetry curve, unmolten crystals did not affect the linear viscoelastic properties. They acted as effective nucleating agents for the rest of the polymer during cooling. Therefore, the shear history hardly affected the crystallization rate and the number of spherulites.

Thermodynamic Features of Extended-Chain Crystals of Poly(butylene succinate) and Its Random Copolymers with Adipic Acid

Thermodynamic Features of Extended-Chain Crystals of Poly(butylene succinate) and Its Random Copolymers with Adipic Acid

Thermal Analysis of PE Extended Chain Crystal of PE Extended Chain Crystal from Gel Spinning

Abstract It has been recognized that ultra-high strength and high modulus ultra-highly drawn Polyethylene (PE) fiber is produced from the formation of extended chain crystal. Nevertheless, whether extended chain crystal exists in gel spinning ultra-highly drawn PE fiber is still an unsettled question. In this work, decalin was adopted as a solvent for PE gel spinning. By means of thermal analysis, X-ray diffraction methods etc, the existence of extended chain crystal was explored. Reasons for confusion in previous studies of this problem were discussed. In PE fiber from 3 wt-°/o dope, extended chain crystal starts to form during draw ratios over 20. When draw ratio equals 42, the content of extended chain crystal roughly equals that of folded chain crystal. The melting point of extended chain crystal is 6°C higher than that of folded chain crystal. They are 145.5° C and 150° C to 152.5° C respectively. The latter would slightly rise with arise of draw ratio. Multi-crystallization behavior of annealed PE fiber was determined and discussed. It was also confirmed that there is a linear logarithm relationship between thermal shrinkage (internal stresses) and fiber moduli.

Understanding the Extraordinary Flexibility of Polydimethylsiloxane through Single-Molecule Mechanics

Linear (un-cross-linked) polydimethylsiloxane (PDMS) exists as a viscous fluid at room temperature even though its molecular weight reaches a high level (105 Da). Accordingly, one can infer the extraordinary flexibility of PDMS. However, the mechanism of the extraordinary flexibility is still unclear at present. Herein the single-chain mechanics of PDMS is measured by single-molecule atomic force microscopy, which is consistent with the results from quantum mechanical calculations. Then, the effects of several factors of microscopic structures of PDMS on the macroscopic mechanical properties are qualitatively analyzed. With the extremely low elasticity of the individual chain, the large effective cross-sectional area, and the very weak intra/interchain noncovalent interactions and their synergistic effect, the relatively small modulus of PDMS is reasonable. We further propose a method to estimate the modulus of an extended chain crystal of a polymer from the single-chain force–extension curve. For the polymer crystals in which only van der Waals forces are involved in the intra/interchain noncovalent interactions, the estimated moduli are very close to the measured values, which indicates that this method should be valid for PDMS. It is found that the estimated modulus of PDMS is smaller than those of most polymers, which directly shows its extraordinary flexibility at the macroscopic level. This work may be helpful for bridging the gap between the single-molecule and macroscopic properties of polymer materials.

Effect of blending small amount of high-density polyethylene on molecular entanglements during melt-drawing of ultrahigh-molecular-weight polyethylene

The effect of blending a small amount of high-density polyethylene (HDPE) on molecular entanglements during melt-drawing of ultrahigh-molecular-weight polyethylene (UHMW-PE) was investigated in this study. UHMW-PE powders with a small amount of HDPE added and a bimodal MW distribution were synthesized by reactor blending. In situ wide-angle X-ray diffraction and small-angle X-ray scattering measurements during melt-drawing of UHMW-PE and UHMW-PE/HDPE films were performed to evaluate the state of molecular entanglements on the basis of the behavior of oriented crystallization and extended-chain crystal formation. The addition of a small amount of HDPE reduced the amount of tight molecular entanglements originating from UHMW-PE chains, and further addition reduced the amount of molecular entanglements that transmitted the applied stress. These results demonstrated that the addition of a small amount of HDPE significantly affected the amount of molecular entanglements of UHMW-PE chains during melt-drawing.

Electrostriction-enhanced giant piezoelectricity via relaxor-like secondary crystals in extended-chain ferroelectric polymers

<h2>Summary</h2> Piezoelectricity in ferroelectric polymers originates from the electrostrictive effect coupled with a remanent polarization. However, its structural origin remains controversial, and it is not clear how modifying the electrostriction can further improve piezoelectricity for polymers. Here, we report that electrostriction can be significantly enhanced in poled poly(vinylidene fluoride-<i>co</i>-trifluoroethylene) [P(VDF-TrFE)] random copolymers containing extended-chain primary crystals and relaxor-like secondary crystals in the oriented amorphous fraction (SC_OAF). As a result of the high polarizability of dipoles and ferroelectric nanodomains in the SC_OAF, the inverse piezoelectric coefficient d₃₁ reaches as high as 77 ± 5 pm/V for the P(VDF-TrFE) 55/45 copolymer at 55°C. This finding not only extends our understanding of piezoelectricity in polymers but also provides guidance for further enhancing the piezoelectricity of ferroelectric polymers in the future.

Influence of early thermal-oxidative ageing on the structure and properties of polyoxymethylene copolymer.

Thermal-oxidative ageing of polyoxymethylene (POM) copolymer in the oven at 100°C for 1, 2, 3, 5, 7, 10, 14 and 21 days and the influence of early thermal-oxidative ageing on POM structure and properties were studied by means of wide-angle X-ray diffraction, Fourier transform infrared spectroscopy, differential scanning calorimetry and tensile test. Based on the results, we found that the early thermal-oxidative ageing of POM copolymer can be divided into three regions. The region I is the initial 3 days. In this region, some molecular chains rearranged, resulting in internal stress relaxation, increase of crystallinity degree and grain size due to the perfection of crystal structure; both extended chain crystal (ECC) and folded chain crystal (FCC) increased and ECC grew faster than FCC. The region II is from 3 days to 10 days, and in this region, chain scission took place in amorphous region and led to chemi-crystallization. The region III is after 10 days. In this region, the structure and performance of POM copolymer reached a stable situation at this stage. In this work, the difference between skin and core were also analysed.

Melting and Annealing Peak Temperatures of Poly(butylene succinate) on the Same Hoffman-Weeks Plot Parallel to Tm=Tc Line

The crystallization and melting behavior of polymers is of theoretical importance. In this work, poly(butylene succinate) (PBS) was selected as an example to study such behavior at low supercooling via introduction of the extended-chain crystal (ECC) of the same polymer as nucleating agent. The crystallization of PBS with its ECC as nucleating agent in a wide temperature range (90–127 °C) and the following melting behavior were studied. It is revealed that the melting point (Tm', for Tc≥113 °C) and the annealing peak temperature (Ta', for Tc=90–100 °C) show similar asymptotic behavior. Both Tm and Ta approach to a value of ca. 3.3 °C higher than the corresponding Tc when the crystallization time tc approaches the starting point. That is to say, the Hoffman-Weeks plot is parallel to Tm=Tc line. The crystallization line became parallel to the melting line when PBS was crystallized at Tc higher than 102 °C. Based on these results, we propose that the parallel relationship and the intrinsic similarity between the Ta and the Tm observed at the two ends of the Tc range could be attributed to the metastable crystals formed at the beginning of crystallization.

New insights into the memory effect on the crystallization behavior of poly(phenylene sulfide)

In this article, the melt and crystallization kinetics of poly(phenylene sulfide) (PPS) powder was studied by differential scanning calorimetry (DSC), fourier transform infrared spectroscopy (FTIR) and polarized optical microscopy (POM). It is found that the folded-chain crystals of PPS (melting point at 281 °C) could be erased at about 350 °C, while the equilibrium melting point of extended-chain crystals (melting point at 295 °C) is 370 °C. Although the residual nuclei could be eliminated completely at 370 °C, the spherulite growth rate of PPS during crystallization keeps accelerating with the increasing melting relaxation time, resulting from the transformation of the lamellae and the amorphous cluster to an ordered structure with low entanglement density. Furthermore, due to the restricted chain mobility for the semi-flexible PPS, this unique memory effect would survive in the whole manufacturing process, resulting in the varying results from different researches and the significant influence on the processability and performance of PPS products.