Chain Orientation Research Articles

Conducting Electrospun Nanofibres: Monitoring of Iodine Doping of P3HT through Infrared (IRAV) and Raman (RaAV) Polaron Spectroscopic Features

Aligned polymer nanofibres are prepared by means of the electrospinning of a chlorobenzene solution containing regioregular poly(3-hexyltiophene-2,5-diyl), P3HT, and poly(ethylene oxide), PEO. The PEO scaffold is easily dissolved with acetonitrile, leaving pure P3HT fibres, which do not show structural modification. Polymer fibres, either with or without the PEO supporting polymer, are effectively doped by exposure to iodine vapours. Doping is monitored following the changes in the doping-induced vibrational bands (IRAVs) observed in the infrared spectra and by means of Raman spectroscopy. Molecular orientation inside the fibres has been assessed by means of IR experiments in polarised light, clearly demonstrating that electrospinning induces the orientation of the polymer chains along the fibre axis as well as of the defects introduced by doping. This work illustrates a case study that contributes to the fundamental knowledge of the vibrational properties of the doping-induced defects-charged polarons-of P3HT. Moreover, it provides experimental protocols for a thorough spectroscopic characterisation of the P3HT nanofibres, and of doped conjugated polymers in general, opening the way for the control of the material structure when the doped polymer is confined in a one-dimensional architecture.

A Sustainable Hydroxypropyl Cellulose-Nanodiamond Composite for Flexible Electronic Applications.

Designing fully green materials for flexible electronics is an urgent need due to the growing awareness of an environmental crisis. With the aim of developing a sustainable, printable, and biocompatible material to be exploited in flexible electronics, the rheological, structural and charge transport properties of water-based hydroxypropyl cellulose (HPC)-detonation nanodiamond (DND) viscous dispersions are investigated. A rheological investigation disclosed that the presence of the DND affects the orientation and entanglement of cellulose chains in the aqueous medium. In line with rheological analyses, the NMR diffusion experiments pointed out that the presence of DND modifies the hydrodynamic behavior of the cellulose molecules. Despite the increased rigidity of the system, the presence of DND slightly enhances the ionic conductivity of the dispersion, suggesting a modification in the charge transport properties of the material. The electrochemical analyses, performed through Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS), revealed that the HPC-DND system is remarkably stable in the explored voltage range (-0.1 to +0.4 V) and characterized by a lowered bulk resistance with respect to HPC. Such features, coupled with the printability and filmability of the material, represent good requirements for the exploitation of such systems in flexible electronic applications.

Crystal structure of Sphingobacterium multivorum serine palmitoyltransferase complexed with tris(hydroxymethyl)aminomethane.

Serine palmitoyltransferase (SPT) catalyses the first reaction in sphingolipid biosynthesis: the decarboxylative condensation of L-serine (L-Ser) and palmitoyl-CoA to form 3-ketodihydrosphingosine. SPT from Sphingobacterium multivorum has been isolated and its crystal structure in complex with L-Ser has been determined at 2.3 Å resolution (PDB entry 3a2b). However, the quality of the crystal was not good enough to judge the conformation of the cofactor molecule and the orientations of the side chains of the amino-acid residues in the enzyme active site. The crystal quality was improved by revision of the purification procedure and by optimization of both the crystallization procedure and the post-crystallization treatment conditions. Here, the crystal structure ofSPT complexed with tris(hydroxymethyl)aminomethane (Tris), a buffer component, was determined at 1.65 Å resolution. The protein crystallized at 20°C and diffraction data were collected from the crystals to a resolution of 1.65 Å. The crystal belonged to the tetragonal space group P41212, with unit-cell parameters a = b = 61.32, c = 208.57 Å. Analysis of the crystal structure revealed C4-C5-C5A-O4P (77°) and C5-C5A-O4P-P (-143°) torsion angles in the phosphate-group moiety of the cofactor pyridoxal 5'-phosphate (PLP) that are more reasonable than those observed in the previously reported crystal structure (14° and 151°, respectively). Furthermore, the clear electron density showing a Schiff-base linkage between PLP and the bulky artificial ligand Tris indicated exceptional flexibility of the active-site cavity of this enzyme. These findings open up the possibility for further study of the detailed mechanisms of substrate recognition and catalysis by this enzyme.

Initial Crystallization Effects in Coarse-Grained Polyethylene Systems After Uni- and Biaxial Stretching in Blow-Molding Cooling Scenarios

This study investigates the initial stage of the thermo-mechanical crystallization behavior for uni- and biaxially stretched polyethylene. The models are based on a mesoscale molecular dynamics approach. We take constraints that occur in real-life polymer processing into account, especially with respect to the blowing stage of the extrusion blow-molding process. For this purpose, we deform our systems using a wide range of stretching levels before they are quenched. We discuss the effects of the stretching procedures on the micro-mechanical state of the systems, characterized by entanglement behavior and nematic ordering of chain segments. For the cooling stage, we use two different approaches which allow for free or hindered shrinkage, respectively. During cooling, crystallization kinetics are monitored: We precisely evaluate how the interplay of chain length, temperature, local entanglements and orientation of chain segments influence crystallization behavior. Our models reveal that the main stretching direction dominates microscopic states of the different systems. We are able to show that crystallization mainly depends on the (dis-)entanglement behavior. Nematic ordering plays a secondary role.

Overcoming CD19-Negative Relapses in Patients with B-Cell Lymphomas Treated with Tisagenlecleucel

Anti-CD19 chimeric antigen receptor T cell (CART) immunotherapy has revolutionized the treatment of large B-cell lymphomas (LBCL). However, up to 70% of LBCL patients (pts) ultimately fail CART treatment. Several mechanisms of relapse have been described, such as T cell exhaustion, an immunosuppressive tumor microenvironment, and antigen-negative escape. While antigen-negative relapse after anti-CD19 CART (CART19) is well studied in acute B lymphoblastic leukemia, less data are available for LBCL, especially in pts treated with the 4-1BB costimulated CART19, tisagenlecleucel (tisa-cel, formerly CTL019). In this study, we evaluated the frequency of CD19 loss in LBCL pts treated with tisa-cel and identified CD79 as an antigen highly expressed by immunohistochemistry (IHC) in CD19-negative biopsies. We then generated and optimized an anti-CD79 CART (CART79) and subsequently cloned a dual anti-CD79b/anti-CD19 CART that showed high potency against CD19-neg models. We first studied an exploratory cohort of 38 pts treated at the University of Pennsylvania on the first single-center clinical trial of CTL019 for B-cell lymphomas (NCT02030834 and Chong, NEJM, 2021). Among relapsed or refractory pts (n=23, 60.5%), 17 pts had a biopsy at disease progression (PD). We analyzed CD19 and CD79a expression in these biopsies by IHC. Four biopsies (23.5%) showed no CD19 expression by neoplastic cells. Also, 12 pts had available matched biopsies pre-CTL019 infusion (performed within 1 year before infusion) and at PD. We observed that 3/12 (25%) pts converted from a CD19-positive to a CD19-negative tumor after CTL019. Notably, 100% of biopsies at PD showed high CD79a expression. We then studied a confirmatory cohort including 110 LCBL pts treated with commercial tisa-cel. In this retrospective cohort, 63 pts had PD, and 31 had an available biopsy at PD. In line with previous observations, 10/26 (38%) showed loss of CD19 at PD, and, of note, 5/23 (22%) loss of CD20, a B-cell marker targeted by previous therapies. Again, 100% of pts maintained CD79a expression. (Figure A). These data confirm that loss of detection of CD19 at the level of sensitivity of IHC is frequently observed at PD and may be a cause of resistance in LBCL pts treated with tisa-cel. In addition, persistence of CD79 expression suggests that it may be an optimal therapeutic target for potentially preventing CD19-negative PD. Therefore, we aimed to generate and optimize a CART79 product. We cloned and screened a large panel of anti-CD79 CAR constructs (n=32) generated using 6 different scFvs (targeting either CD79a or CD79b), two scFv chain orientations (H to L vs. L to H), three hinges (derived from CD8a, IgG4, or truncated IgG4). We used the CD8 transmembrane and 4-1BB co-stimulatory domains of tisa-cel, given its established clinical safety and efficacy profile. The constructs were initially screened in-vitro against LBCL cells (OCI-Ly18), evaluating cytotoxicity, proliferation, and cytokine production. The six top candidates were then tested in-vivo using human xenograft models (NSG mice subcutaneously engrafted with OCI-Ly18 cells). When the tumor was palpable, mice were randomized to receive one dose (2.5x106/mouse) of CART79 candidates, CART19, or untransduced T cells (UTD) intravenously. An anti-CD79b CAR construct (CART79#3: H-L orientation, truncated IgG4 hinge), demonstrated the strongest anti-tumor effect. This construct was incorporated into a bicistronic vector also containing anti-CD19 CAR (FMC63-based). We tested the activity of this novel anti-CD19/CD79b dual CART in-vitro, using CRISPR/Cas9 gene editing to knock out (KO) either CD19 or CD79b in HBL-1 LBCL cells. CD19 KO and CD79b KO HBL-1 cells were mixed 1:1 to simulate the heterogeneity of lymphomas and tumor targeting was tested for CART19, CART79b, dual CART79b/19, and UTD. In both in-vitro and in-vivo models, dual CART19/79b demonstrated a strong anti-tumor effect, possibly preventing or treating CD19-negative escape (Figure B). In conclusion, our study demonstrates that CD19 loss by IHC is a marker of treatment failure in tisa cel-treated LBCL pts. We also demonstrate that CD79, as opposed to CD20, is expressed in virtually all cases of PD and is an ideal target for next-generation CART immunotherapy. Finally, we developed a dual CART19/79b that shows high activity against CD19-negative disease progression. This construct will be taken to phase I clinical studies. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

Synergy of Fiber Surface Chemistry and Flow: Multi-Phase Transcrystallization in Fiber-Reinforced Thermoplastics.

Fiber-reinforced polymer composites are largely employed for their improved strength with respect to unfilled matrices. Considering semi-crystalline materials under relevant processing conditions, the applied pressure and flow induce shear stresses at the fiber-polymer interface. These stresses may strongly enhance the nucleation ability of the fiber surface with respect to the quiescent case. It is thus possible to assume that the fiber features are no longer of importance and that crystallization is dominated by the effect of flow. However, by making use of an advanced experimental technique, i.e., polarization-modulated synchrotron infrared microspectroscopy (PM-SIRMS), we are able to show that the opposite is true for the industrially relevant case of isotactic polypropylene (iPP). With PM-SIRMS, the local chain orientation is measured with micron-size spatial resolution. This orientation can be related to the polymer nucleation density along the fiber surface. For various combinations of an iPP matrix and fiber, the degree of orientation in the cylindrical layer that develops during flow correlates well with the differences in nucleation density found in quiescent conditions. This result shows that the morphological development during processing of polymer composites is not solely determined by the flow field, nor by the nucleating ability of the fiber surface alone, but rather by a synergistic combination of the two. In addition, using finite element modeling, it is demonstrated that, under the experimentally applied flow conditions, the interphase structure formation is mostly dominated by the rheological characteristics of the material rather than perturbations in experimental conditions, such as shear rate, layer thickness, and temperature. This once again highlights the importance of matrix-filler interplay during flow and, thus, of material selection in the design of hybrid and lightweight composite technologies.

Material Extrusion Additive Manufacturing of Poly(Lactic Acid): Influence of infill orientation angle

The effect that the infill orientation angle has on the strain-rate dependence of the yield stress for material extrusion additive manufactured (ME-AM) PolyLactic Acid (PLA) material was investigated. Symmetric angle-ply stacking sequences were used to produce ME-AM tensile test samples. Measured yield stresses were compensated for the voided structure, typical of ME-AM components. Furthermore, molecular orientation and stretch was macroscopically assessed by a thermal shrinkage procedure. Additionally, hot-press compression molded ( CM ) samples were manufactured and mechanically characterized in uniaxial tensile and compression in order to determine the material’s isotropic bulk properties. Initial model parameters for the Ree–Eyring modification of the Eyring flow rule were determined using CM data. According to SEM fractography, all samples showed microscopically brittle fracture behavior. Notwithstanding, contrary to CM samples, ME-AM specimens showed macroscopically ductile stress–strain behavior and a transition from a regime with only a primary α -deformation process, at low strain rates, to a regime with 2 deformation processes ( α + β ), at high strain rates. These effects are an influence of the processing step and are attributed to the molecular orientation and stretch of the polymer chains, provoking anisotropic mechanical properties. As a consequence, a deformation-induced change of the Eyring rate constants is needed to adequately describe the strain-rate dependence of the ME-AM yield stress behavior, leaving the initial activation volumes unchanged. Taking this deformation-dependence of the rate constants into account, yield stresses as a function of infill orientation angle can be appropriately predicted. • ME-AM samples with different symmetric angle-ply stacking sequences were produced. • PLA shows anisotropy in yield stress and strain-rate dependent behavior. • ME-AM samples show ductile stress–strain behavior, due to molecular orientation. • ME-AM samples show transition in strain-rate dependence, due to molecular orientation. • PLA demonstrates deformation-dependent Eyring rate constants.

Determinant Role of Solution‐State Supramolecular Assembly in Molecular Orientation of Conjugated Polymer Films

AbstractThe solid‐state molecular orientation of conjugated polymers is of vital importance for their charge transport properties, where the edge‐on orientation with π‐stacking direction parallel to the surface is generally preferable to achieving high‐mobility planar field‐effect transistors. However, so far, little is known about the origin of packing‐orientation formation in thin films. Here, it is shown that the solution‐state supramolecular structure of widely studied PffBT4T‐based polymers can be reversibly tuned between 1D worm‐like and 2D lamellar structures for the same polymer/solvent system through solution temperature. Such dimensionality in solution determines the solid‐state packing orientation of the polymer chains, where edge‐on and face‐on textures are generated from solutions with 1D and 2D structures, respectively. More importantly, the transition temperature of solution‐state supramolecular dimensionality is in excellent agreement with that of solid‐state packing orientation. These experimental observations unambiguously demonstrate the predominant roles of solution‐state supramolecular assembly in solid‐state molecular orientation, which is further verified using different molecular weight batches and other two representative polymers. The findings provide new insights into the growth mechanism of polymer semiconductors during transistor fabrication, and open prospective pathways for boosting device performance of solution‐processable plastic electronics.

Isotropic-to-Nematic Transition in Salt-Free Polyelectrolyte Coacervates from Coarse-Grained Simulations

Recent interest in complex coacervation between oppositely charged polyelectrolytes (PEs) has been fueled by its relevance to biology in the context of membraneless organelle formation within living cells. For PEs with limited flexibility (such as double-stranded DNA), theoretical treatments and recent experiments have reported the emergence of liquid crystalline order (LCO) within the resulting coacervate phases. In this work, we study the underlying physics of this phenomenon using coarse-grained molecular dynamics simulations of symmetric semiflexible–semiflexible and asymmetric semiflexible–flexible coacervates. By comparing coacervates with the corresponding semidilute solutions of neutral polymers, we demonstrate that the presence of Coulomb interactions in coacervates facilitates orientational ordering, in agreement with theoretical predictions. Quantitative comparisons between our simulations and theory indicate that, for asymmetric nematic coacervates, the strong orientational ordering of stiff polyanions induces a weak ordering of the flexible polycations─an effect that was not anticipated by available theoretical studies. Simulations reveal that, for nematic coacervates, the preferred orientation of the PE chains at the liquid–liquid coacervate–supernatant interface is parallel, and the alignment of semiflexible PEs is homogeneous. The results presented here provide new molecular-level insights into the intra-coacervate LCO and will help motivate further experimental and theoretical activities in this area.

Influence of melt stretching process on POK dielectric properties and relaxation behavior

The dielectric properties and relaxation behaviors of POK films with different melt draw ratios were investigated. The dielectric constant of the sample increases to 11.1 (1 kHz) after melt stretching. The molecular chain orientation induced by melt stretching reduces the thermodynamic relaxation temperature, but the well-aligned lamellae restrict the movement of amorphous chains. The dielectric relaxation of the crystalline region and the amorphous region is inconsistent, and the dipole relaxation in crystalline region is obviously weaker than that of the amorphous region. After melt stretching, the activation energy of dipole motion perpendicular to the oriented molecular chains decreases, resulting in an increase in the dielectric constant of POK films with increasing melt stretching ratio.

Microscopic Dynamics in the Strain Hardening Regime of Glassy Polymers

We study by dielectric spectroscopy the molecular dynamics of relaxation processes during plastic flow of glassy polymers up to the strain hardening regime for three different protocols of deformation. The measured dielectric spectra cover 4 decades in frequencies and allow us to measure the evolution as a function of the applied strain of the dominant relaxation time τα and of the width wτ of the distribution of relaxation times. The first protocol is performed at constant strain rate λ̇. We confirm that for increasing strain both τα and wτ first decrease, reaching a minimum in the stress softening regime before increasing in the strain hardening regime. In the second protocol we stop the deformation at some point λw in the strain hardening regime, and we let the sample age for a waiting time tw, during which the applied stress remains high. Upon resuming the deformation at constant λ̇, stress–strain displays a yield stress and a stress softening regime comparable in magnitude to that of the reference protocol before rejoining the reference curve. In contrast, the dielectric spectrum measured during the second protocol recovers the one measured during the reference curve much later than strain–stress. In the third protocol the stress is canceled during tw. In this case, after recovering the constant λ̇ the dielectric spectrum and the stress–strain curve rejoin almost immediately the reference curve. We interpret these different behaviors as the consequence of changes in the free energy barriers for α-relaxation induced by the stress applied to the sample. These changes are the sum of two contributions: (a) The first one, which allows for plastic flow, is due to the applied stress σ and, according to a recently published theory, scales as −σ2. (b) The second contribution κ(λ), which is a function of the chain orientation at the monomer level, is positive and is responsible for the stress hardening regime. The first one evolves immediately upon varying the stress, whereas the second relaxes very slowly upon cessation of the applied stress. Our interpretation for the results of the third protocol is that aging dynamics is frozen when the stress is removed, as it is known for polycarbonate at room temperature. Our experiments set precise conditions for a theory of strain hardening.

Influence of drawing and annealing on the structure and properties of bio‐based polyamide 56 fibers

AbstractThe influences of drawing and annealing parameters on the structure and mechanical properties of melt spinning polyamide 56 (PA56) fibers were investigated and discussed in details. The as‐spun PA56 fibers have the relatively low crystallinity and rather low orientation. The crystalline phase in as‐spun PA56 fibers is mainly in γ phase. PA56 α‐like crystalline phase develops from the amorphous phase with the increase in chain orientation upon drawing of the as‐spun PA56 fibers. The crystallinity of PA56 fiber increases with rising draw ratio, which accompanies the increase in thickness of the crystalline lamella. The increase in draw ratio leads to the increase in tensile strength and the decrease in elongation at break of the resultant fibers. Tension annealing of the full‐drawn PA56 fibers at specific temperatures has obvious influences on the structure and properties of the fibers. The crystallinity and orientation increase with rising annealing temperature in the range of 150–210°C, during which the content of PA56 α‐like phase increases with the rising annealing temperature. Annealed the fibers at 230°C results in the increase in the content of PA56 γ phase with a tremendous decrease in the content of α‐like phase due to Brill transition. The optimum annealing temperature is around 170°C, at which the tenacity of the PA56 fibers is the highest. These results can provide valuable information for the production of PA56 fibers.

Preparation of graphene oxide-silica nanohybrid/poly(lactic acid) biaxially oriented films with enhanced mechanical properties

Poly(lactic acid) (PLA) biaxially stretched films and graphene oxide-silica nanohybrids (GO-SiO2)/PLA nanocomposite biaxially stretched films (PGSi) with different stretching ratios were prepared by simultaneous biaxial stretching. The structural changes during biaxial stretching have a direct impact on the properties of the biaxially stretched films. Therefore, this work mainly studies the effect of the presence of filler (i.e., GO-SiO2) on the structural evolution of PLA during biaxial stretching, and establishes a bridge between structural changes and property changes. Through wide-angle X-ray diffraction (WAXD) and atomic force microscopy (AFM), it was found that GO-SiO2 facilitates the isotropic orientation of molecular chains during biaxial stretching and attenuates the degree of preferred orientation. As a result, the mechanical properties of PGSi biaxially oriented films are inferior to those of PLA biaxially oriented films. However, the synergistic effect of the excellent barrier property of GO-SiO2 and biaxial stretching make the barrier property of PGSi biaxially stretched films even better. In addition, when the tensile force is stronger than the interfacial force between GO-SiO2 and PLA, GO-SiO2 will precipitate out of PLA under the condition of a large stretching ratio, resulting in the generation of defects, which is not conducive to the improvement of PLA performance.

A General Strategy to Electrospin Nanofibers with Ultrahigh Molecular Chain Orientation

The degree of polymer chain orientation is a key structural parameter that determines the mechanical and physical properties of fibers. However, understanding and significantly tuning the orientation of fiber macromolecular chains remain elusive. Herein, we propose a novel electrospinning technique that can efficiently modulate molecular chain orientation by controlling the electric field. In contrast to the typical electrospinning method, this technique can piecewise control the electric field by applying high voltage to the metal ring instead of the needle. Benefiting from this change, a new electric field distribution can be realized, leading to a non-monotonic change in the drafting force. As a result, the macromolecular chain orientation of polyethylene oxide (PEO) nanofibers was significantly improved with a record-high infrared dichroic ratio. This was further confirmed by the sharp decrease in the PEO jet fineness of approximately 80% and the nanofiber diameter from 298 to 114 nm. Interestingly, the crystallinity can also be adjusted, with an obvious drop from 74.9% to 31.7%, which is different from the high crystallinity caused by oriented chains in common materials. This work guides a new perspective for the preparation of advanced electrospun nanofibers with optimal orientation–crystallinity properties, a merited feature for various applications.

Structural design of poly(vinyl alcohol)/carbon fiber composite film used for antistatic packaging via asynchronous biaxial stretching

AbstractAntistatic packaging film plays an important role in protecting sensitive electronic components from the damage of electrostatic discharge (ESD) during transportation and application. Considering the antistatic property and environmental friendliness of polyvinyl alcohol (PVA), PVA was chosen as matrix to prepare PVA/carbon fiber (CF) composite film via melt casting and asynchronous biaxial stretching. The addition of CF made more perfect conductive pathway formed in system, thus improving the electric property of the composite precursor sheet. And the subsequent biaxial stretching further promoted the cross arrangement of CF as well as the orientation of PVA molecular chains, so endowed the stretched composite film with excellent mechanical, antistatic and water‐resistant properties. When CF content was 5 wt% and the stretching ratio was 3.0 × 3.0, the tensile strength, volume resistivity and water contact angle of the composite film reached 69.4 MPa, 1.07 × 105 Ω·cm, and 74.7° respectively, which met the requirements of electrostatic dissipation packaging. This work highlights the structural evolution of PVA/CF composites under biaxial stretching and spurs the devising of antistatic packaging materials.

Investigating the origin of the core-shell structure of polymeric nanofibers during fabrication process at the atomistic scale

Polymeric nanofibers demonstrate core-shell structure when their diameter drops below the nanoscale. Understanding the corresponding mechanism at the atomistic scale paves the way for achieving the long-term goal of adjusting their microstructure and controlling their properties purposefully. To explain the current contradictory views on the origins of the nanofiber core-shell structure, a molecular dynamics (MD) study is performed. In the MD simulation, cold-drawing and hot-drawing methods are implemented to investigate the role of evaporation rate, stretching force and external temperature on the formation of polymeric nanofiber’s core-shell structure. The results show that the distribution of the solvent atoms inside the as-formed nanofiber is strongly associated with the evaporation rate. The increase of the stretching force during the hot-drawing or cold-drawing process has a significant influence on the polymeric chain orientation inside the nanofiber. The external temperature has little influence on nanofiber formation. The final microstructure of nanofibers relies on the interplay between the stretching force and the evaporation rate. When the evaporation is dominant, fiber tends to form a tabulated structure with a less dense core embedded in a denser shell layer. On the other hand, when the stretching force is dominant, the fiber may form a denser structure with chains more aligned.

Role of Draw Rate and Molecular Weight when Electrospun Nanofibers are Post‐Drawn with Residual Solvent

AbstractThe postdrawing process is poorly understood for polymer nanofibers due to the difficulty of manipulating nanofiber structures. Here, an angled track system facilitates postdrawing of individual nanofibers with control of parameters including molecular weight, draw rate, draw ratio, and solvent evaporation time. In this study, the effects of molecular weight, draw rate, and relative residual solvent content on final nanofiber properties are investigated. Molecular weight is first investigated to clarify any influence polymer chain length can have on drawing in facilitating or hindering chain extensibility. Polyacrylonitrile nanofibers with 50 and 150 kDa molecular weights behave similarly with postdrawing resulting in reduced diameters and enhanced mechanics. Since solvent quantity during drawing is a time sensitive component it is meaningful to assess the impact of draw rate on the chemical and structural makeup of postdrawn fibers. Chemical bond vibrations and chain orientation are sensitive to draw rate when polycaprolactone nanofibers are dried for 3 minutes prior to postdrawing, but this dependency to draw rate is not observed when fibers are postdrawn immediately upon collection. These findings demonstrate that the amount of retained solvent at collection is relevant to this postprocessing approach, and highlights the dynamics of solvent evaporation during postdrawing.

VISCOELASTICITY AND DYNAMIC FATIGUE CRACK GROWTH BEHAVIOR OF NATURAL RUBBER/CIS-POLYBUTADIENE RUBBER COMPOSITES

ABSTRACT Because cis-polybutadiene rubber (BR) can improve the fatigue crack growth (FCG) resistance of natural rubber (NR) in the low-tearing-energy (G) range, a blending system of NR/BR is often used in tire materials. In this study, based on relationship of (FCG rate (dc/dN) − G) established from the perspective of fracture mechanics and then inspired by Persson's theory and starting from the perspective of viscoelasticity, the FCG behavior and mechanism of NR/BR were investigated. NR/BR with different blending ratios were prepared, and dc/dN under different G inputs (500/1500 J/m2) was measured. According to the viscoelastic parameters (storage modulus E′, loss factor tan δ, and loss compliance modulus J″) recorded in situ, energy dissipation distribution at the crack tip (energy consumption for FCG inside crack tip: G0 and energy loss in linear viscoelastic zone near crack tip: G0f [v, T]) was determined, and the relationship of (viscoelasticity − dc/dN) was finally set up. When G = 500 J/m2, blending BR can reduce dc/dN as compared with pure NR. On one hand, with a higher BR fraction, an increased cross-linking density and enhanced filler network provided greater rigidity, which increased E′; on the other hand, a low glass transition temperature and flexible chain of BR reduced hysteresis, which decreased tan δ. The joint action led to a decrease in J″, which caused more G0f (v, T) and less G0, resulting in the ultimate reduction of dc/dN. In contrast, for G = 1500 J/m2, when the BR content was >50 phr, dc/dN showed a significant increase. Although more BR evidently decreased J″ and then led to a large amount of G0f (v, T), due to absence of strain-induced crystallization, the chain orientation of BR was hard to resist FCG when G increased. Finally, the morphology of the crack tip propagation path was captured to corroborate the different orientation characteristics of NR and BR and their effects on FCG behavior.

Structure and dynamics of a 1,4-polybutadiene melt in an alumina nanopore: A molecular dynamics simulation.

We present results of molecular dynamics simulations of a chemically realistic model of 1,4-polybutadiene confined in a cylindrical alumina nanopore of diameter 10nm. The simulations are done at three different temperatures above the glass transition temperature Tg. We investigate the density layering across the nanopore as well as the orientational ordering in the polymer melt, brought about by the confinement, on both the segmental and chain scales. For the chain scale ordering, the magnitude and orientation of the axes of the gyration tensor ellipsoid of single chains are studied and are found to prefer to align parallel to the pore axis. Even though double bonds near the wall are preferentially oriented along the pore walls, studying the nematic order parameter indicates that there is no nematic ordering at the melt-wall interface. As for the dynamics in the melt, we focus here on the mean-square-displacement of the monomers for several layers across the nanopore as well as the movement of the chain center of mass both of which display a slowing down of the dynamics in the layer at the wall. We also show the strong adsorption of the monomers to the pore wall at lower temperatures.

Incremental shear strain chain: a mesoscale concept for slip lines in 2D granular materials

Constitutive behaviors of granular materials are driven by both particle interactions and geometric arrangements of contact network. To bridge the gap between the grain scale and the sample scale, the mesoscale is of great importance as it corresponds to the smallest scale at which geometrical effect can be accounted for. Meso shear structures (sometimes called microbands) have been observed frequently on incremental strain maps in granular materials under shearing, while the Rudnicki and Rice localization criterion for shear band is not fulfilled yet. These meso structures are thin, quasi-linear and they involve a few grains as well as their surrounding voids. This paper introduces the concept of “incremental shear strain chain” (simply called “shear chain”) to provide a specific quantitative definition of such mesostructures. “Shear chains” are defined based on incremental deviatoric strain fields in 2D biaxial simulations. Particular attention is paid to demonstrate that the shear chain orientation is a material scale property, insensitive to boundary conditions, loading paths and sample densities. Since shear chains are shown to be closely related to sliding mechanisms, they can stand for a mesoscale definition of the concept of slip lines as defined in the standard elasto-plasticity theory.