Orientation Of Chain Segments Research Articles

“Like binding a book”: Interfacial improvement of pre-melted ultrahigh molecular weight polyethylene by pulse vibration molding

Ultrahigh molecular weight polyethylene (UHMWPE) is widely used in industrial, military, and civilian applications owing to its excellent properties. However, the high entanglement degree of UHMWPE melt leads to great difficulties in diffusion and crystallization, which causes poor interfacial strength and mechanical properties of pre-melted UHMWPE, restricting its high-quality processing and high-value recycling. To this end, we propose a rapid interface stitching technology with a pulsed vibration force driving free volume homogenization and chain segment orientation. For the first time, pulse vibration molding (PVM) is applied for the hot-pressing of pre-melted UHMWPE, successfully realizing the interface stitching and product strengthening. Compared with those of the convention molding (CM) samples at 180 °C, the interfacial adhesive fracture toughness of the adhesive samples processed using PVM is increased by 105.9 % and the work of tension of the recycled samples processed using PVM is also increased by 49.09 %, respectively. The fracture strength of the PVM recycled samples at 180 °C reaches 91.77 % of that of the pure samples and the yield strength is also higher, which significantly enhances the recycling of UHMWPE. The characterization results of the interfacial molecular chain entanglement and crystallization show that PVM strengthens the molecular chain diffusion and induces crystallization by promoting the orientation of chain segments, especially at the interface. Consequently, we believe that the proposed technology can play a significant role in the UHMWPE processing methods that involve surface healing.

NeuroSuitUp: System Architecture and Validation of a Motor Rehabilitation Wearable Robotics and Serious Game Platform

This article presents the system architecture and validation of the NeuroSuitUp body-machine interface (BMI). The platform consists of wearable robotics jacket and gloves in combination with a serious game application for self-paced neurorehabilitation in spinal cord injury and chronic stroke. The wearable robotics implement a sensor layer, to approximate kinematic chain segment orientation, and an actuation layer. Sensors consist of commercial magnetic, angular rate and gravity (MARG), surface electromyography (sEMG), and flex sensors, while actuation is achieved through electrical muscle stimulation (EMS) and pneumatic actuators. On-board electronics connect to a Robot Operating System environment-based parser/controller and to a Unity-based live avatar representation game. BMI subsystems validation was performed using exercises through a Stereoscopic camera Computer Vision approach for the jacket and through multiple grip activities for the glove. Ten healthy subjects participated in system validation trials, performing three arm and three hand exercises (each 10 motor task trials) and completing user experience questionnaires. Acceptable correlation was observed in 23/30 arm exercises performed with the jacket. No significant differences in glove sensor data during actuation state were observed. No difficulty to use, discomfort, or negative robotics perception were reported. Subsequent design improvements will implement additional absolute orientation sensors, MARG/EMG based biofeedback to the game, improved immersion through Augmented Reality and improvements towards system robustness.

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.

The Effect of Annealing on the Structure and Electric Performance of Polypropylene Films

Biaxially oriented polypropylene (BOPP) and uniaxially oriented polypropylene (UOPP) films were annealed. The effect of annealing temperature (Ta) on dielectric strength was studied. The electric breakdown strength (Eb) of BOPP and UOPP films changes in a quite different trend with the annealing process. Eb of BOPP films decreases with the increase in Ta, whereas Eb of UOPP films increases first and then decreases with Ta. The structural changes during annealing were investigated. The crystallinity rises with Ta, while the orientation degree and Eb show a similar trend with Ta. Although the crystallinity and crystal structure can affect Eb of polypropylene films, the orientation of chain segments has a much larger correlation with Eb. Our results indicate that the deterioration of the metallized BOPP film capacitor may originate from the orientation degree decrease of chain segments after experiencing high temperature.

Investigation of Crystallization and Relaxation Effects in Coarse-Grained Polyethylene Systems after Uniaxial Stretching.

In this study, we investigate the thermo-mechanical relaxation and crystallization behavior of polyethylene using mesoscale molecular dynamics simulations. Our models specifically mimic constraints that occur in real-life polymer processing: After strong uniaxial stretching of the melt, we quench and release the polymer chains at different loading conditions. These conditions allow for free or hindered shrinkage, respectively. We present the shrinkage and swelling behavior as well as the crystallization kinetics over up to 600 ns simulation time. We are able to precisely evaluate how the interplay of chain length, temperature, local entanglements and orientation of chain segments influences crystallization and relaxation behavior. From our models, we determine the temperature dependent crystallization rate of polyethylene, including crystallization onset temperature.

Stress Relaxation and Underlying Structure Evolution in Tough and Self-Healing Hydrogels.

The tough and self-healing hydrogels composed of polyampholytes (PA gels) are drawing great attention due to their multiscale structures and the resultant multiple mechanical properties. This work studies the stress relaxation behavior of PA gels and reveals the underlying multiscale structure evolutions by combining birefringence and small-angle X-ray scattering measurements. The PA gels show a fast and strong stress relaxation that obeys the stress-optical rule, which could be associated with relaxation of chain segment orientation by the breaking of ionic bonds. A slow and weak relaxation of phase structure (∼100 nm) is also observed, which tells that the stress redistributes and local strain amplification gradually builds in the phase network at long relaxation times as a result of synergetic breaking of multiple ionic bonds. This work gives insight into exploring the formation of the crack precursor that is important in the fracture and fatigue of self-healing hydrogels.

Interfacial regions and network dynamics in epoxy/POSS nanocomposites unravelling through their effects on the motion of molecular chains

Interfacial regions formed by incorporating a certain percentage of nanofillers into a polymer matrix can impact on the motion of molecular groups, chain units or chain segments, and alter molecular chain network dynamics, which is regarded as a key factor to achieve a finer degree control of dielectric properties of polymer nanocomposites. It is of interest to investigate the molecular chain motion behavior at interfacial regions with the incorporation of nanofillers. Epoxy resin/polyhedral oligomeric silsesquioxane (EP/POSS) nanocomposites are fabricated and the observations of morphology structure and chemical elements indicate that POSS nanofillers are well dispersed in the matrix. Differential scanning calorimeter, dynamic thermomechanical analysis, and dielectric relaxation spectroscopy are measured to analyze the effect of nanodoping on the motion properties of molecular chain units and chain segments that satisfy the Arrhenius and Vogel-Fulcher-Tammann equations respectively. The thicknesses of interfacial regions between POSS nanofillers and EP matrix are estimated from dielectric spectra and effective relative permittivity simulations, which are around several nanometers and decrease with the increment in the nanofiller content. In addition, the dielectric spectra scaling analysis indicates that the motion of small chain units can be hindered in interfacial regions and be tuned simultaneously by the variations of molecular chain network structure and dynamics. Nevertheless, the thin interfacial regions cannot effectively restrict the orientation of long molecular chain segments during the glass transition process. The variation of chain segment orientation behavior is ascribed to the interactions among molecular chains tuned by the binding effect of POSS nanofillers.

Atomistic simulations of long-chain polyethylene melts flowing past gold surfaces: structure and wall-slip

ABSTRACT The current article presents results from MD simulations of high molar mass polyethylene melts with the scope to investigate the structure and dynamics at the polymer/solid interphase, and to assess their dependence on strong Couette flows. The density profiles are decomposed into the contributions of specific types of segments such as tails, loops and trains in order to arrive at a detailed description of the structure under various flow conditions. The size and orientation of chain segments is quantified to assess the reorganisation of the chains at the interface leading to possible shear-thinning effects. The segmental velocity profiles and the layer- and direction-resolved mean square displacement of the chain segments are extracted from the simulations so as to compute the effective shear rate, slippage, and the emergence of possible interfacial failure mechanisms. Through representative snapshots of chain trajectories we unveil the dominant mechanisms dictating the chain reorganisation in the interfaces and the overall adsorption–desorption processes. Our findings suggest the manifestation of a hybrid boundary condition attributed mainly to interfacial failure and partly to cohesive failure.

Reinforcement and Toughening of Rubber by Bridging Graphene and Nanosilica

Nowadays, there are considerable researches focusing on using different types of nanofillers to reinforce rubber materials. Here, we described a method of bridging graphene and nanosilica by covalent bonding, and rubber nanocomposites were prepared by emulsion mixing of nanofillers and natural rubber. The presence of the covalent bridging could be confirmed by the experimental results of Fourier transform infrared spectrometry, thermo-gravimetric analysis and transmission electron microscopy. The results of tensile test showed that the addition of a small amount of the hybrid nanofillers can lead to the significant improvement of tensile strength without the decrease in extensibility of the filled rubbers. Based on equilibrium swelling method, it was found that the covalent bridging is beneficial for the increase of interaction between rubber and fillers and the force transfer between different chains. Besides, dynamic Monte Carlo simulations demonstrated that the hybrid nanofillers enhance the orientations of chain segments in interfacial regions.

The Effect of Epoxidation on Strain-Induced Crystallization of Epoxidized Natural Rubber.

The effect of epoxidation on strain-induced crystallization (SIC) of epoxidized natural rubber (ENR) and mechanism are studied with synchrotron radiation wide-angle X-ray diffraction (SR-WAXD) and polarized infrared spectroscopy (P-IR). WAXD results reveal that appropriate epoxidation, for example, ENR-25 epoxidized with ≈25% isoprene units, can unexpectedly enhance the SIC of natural rubber (NR), resulting in the improvement of tear resistance. On the other hand, exorbitant epoxidation, for example, ENR-40 epoxidized with ≈40% isoprene units, depresses the SIC and weakens the mechanical properties of NR remarkably. P-IR studies reveal that epoxidation can promote the orientation of chain segments along the stretching direction, which plays a determining role on SIC of NR. Accordingly, hierarchical multiscale schematic models are proposed. This insight into epoxidation on SIC of ENR strongly suggests that ENR with appropriate epoxidation degree is a promising candidate material for the fabrication of high-performance engineering rubber products.

Quadruple-shape hydrogels

The capability of directed movements by two subsequent shape changes could be implemented in shape-memory hydrogels by incorporation of two types of crystallizable side chains. While in non-swollen polymer networks even more directed movements could be realized, the creation of multi-shape hydrogels is still a challenge. We hypothesize that a quadruple-shape effect in hydrogels can be realized, when a swelling capacity almost independent of temperature is generated, whereby directed movements could be enabled, which are not related to swelling. In this case, entropy elastic recovery could be realized by hydrophilic segments and the fixation of different macroscopic shapes by means of three semi-crystalline side chains generating temporary crosslinks. Monomethacrylated semi-crystalline oligomers were connected as side chains in a hydrophilic polymer network via radical copolymerization. Computer assisted modelling was utilized to design a demonstrator capable of complex shape shifts by creating a casting mold via 3D printing from polyvinyl alcohol. The demonstrator was obtained after copolymerization of polymer network forming components within the mold, which was subsequently dissolved in water. A thermally-induced quadruple-shape effect was realized after equilibrium swelling of the polymer network in water. Three directed movements were successfully obtained when the temperature was continuously increased from 5 °C to 90 °C with a recovery ratio of the original shape above 90%. Hence, a thermally-induced quadruple-shape effect as new record for hydrogels was realized. Here, the temperature range for the multi-shape effect was limited by water as swelling media (0 °C–100 °C), simultaneously distinctly separated thermal transitions were required, and the overall elasticity indispensable for successive deformations was reduced as result of partially chain segment orientation induced by swelling in water. Conclusively the challenges for penta- or hexa-shape gels are the design of systems enabling higher elastic deformability and covering a larger temperature range by switching to a different solvent.

Correlation anisotropy and stiffness of DNA molecules confined in nanochannels.

The anisotropy of orientational correlations in DNA molecules confined in cylindrical channels is explored by Monte Carlo simulations using a coarse-grained model of double-stranded (ds) DNA. We find that the correlation function ⟨C(s)⟩⊥ in the transverse (confined) dimension exhibits a region of negative values in the whole range of channel sizes. Such a clear-cut sign of the opposite orientation of chain segments represents a microscopic validation of the Odijk deflection mechanism in narrow channels. At moderate-to-weak confinement, the negative ⟨C(s)⟩⊥ correlations imply a preference of DNA segments for transverse looping. The inclination for looping can explain a reduction of stiffness as well as the enhanced knotting of confined DNA relative to that detected earlier in bulk at some channel sizes. Furthermore, it is shown that the orientational persistence length Por fails to convey the apparent stiffness of DNA molecules in channels. Instead, correlation lengths P∥ and P⊥ in the axial and transverse directions, respectively, encompass the channel-induced modifications of DNA stiffness.

Prediction of Flow Effect on Crystal Growth of Semi-Crystalline Polymers Using a Multi-Scale Phase-Field Approach.

A multi-scale phase-field approach, which couples the mesoscopic crystallization with the microscopic orientation of chain segments and macroscopic viscoelastic melt flow, is proposed to study how the crystal growth of semi-crystalline polymers is affected by flows. To make the simulation feasible, we divide the problem into three parts. In the first part, a finitely extensible nonlinear elastic (FENE) dumbbell model is used to simulate the flow induced molecular structure. In the second part, formulas for estimating the density, orientation and aspect ratio of nuclei upon the oriented molecular structure are derived. Finally, in the third part, a massive mathematical model that couples the phase-field, temperature field, flow field and orientation field is established to model the crystal growth with melt flow. Two-dimensional simulations are carried out for predicting the flow effect on the crystal growth of isotactic polystyrene under a plane Poiseuille flow. In solving the model, a semi-analytical method is adopted to avoid the numerical difficult of a “high Weissenberg number problem” in the first part, and an efficient fractional step method is used to reduce the computing complexity in the third part. The simulation results demonstrate that flow strongly affects the morphology of single crystal but does not bring a significant influence on the holistic morphology of bulk crystallization.

The combination of fluctuation-assisted crystallization and interface-assisted crystallization in a crystalline/crystalline blend of poly(ethylene oxide) and poly(ε-caprolactone)

The nucleation and crystallization of poly(ethylene oxide) (PEO) and poly(e-caprolactone) (PCL) in the PEO/PCL blends have been investigated by means of optical microscopy (OM) and differential scanning calorimetry (DSC). During the isothermal or nonisothermal crystallization process, when the adjacent PEO is in the molten state, PCL nucleation preferentially occurs at the PEO and PCL interface; after the crystallization of the adjacent PEO, much more PCL nuclei form on the surface of the PEO crystal. However, PEO crystallizes normally and no interfacial nucleation occurs in the blend. The concentration fluctuation caused by liquid–liquid phase separation (LLPS) induces the motion of PEO and PCL chains through interdiffusion and possible orientation of chain segments. The oriented PEO chain segments can assist PCL nucleation, and the heterogeneous nucleation ability of PEO increases with the orientation of PEO chains. Oriented PCL chain segments have no heterogeneous nucleation ability on PEO. It is postulated that the interfacial nucleation of PCL in the PEO/PCL blend follows the combination of “fluctuation-assisted crystallization” and “interface-assisted crystallization” mechanisms.

Reinforcement in Natural Rubber Elastomer Nanocomposites: Breakdown of Entropic Elasticity

We propose an approach based on the combination of different techniques in order to discriminate various reinforcement effects in vulcanized natural rubber elastomers with various cross-link densities, filled with carbon black or silica: mechanical response, independent measurements of the cross-link density by proton multiple-quantum NMR, and measurements of chain segment orientation under stretching by X-ray scattering. We show that, while the modulus measured in dynamical mechanical measurements decreases as the strain amplitude increases (Payne effect), the response of the elastomer matrix in terms of average chain segment orientation under stretching measured by X-ray scattering stays constant. The amplification of average chain segment orientation is comparable to the amplification of modulus measured at medium/large strain amplitude. By analyzing the deviations with respect to the behavior of the pure unfilled elastomer matrix, we show that the contribution due to strain amplification effects in the elastomer matrix can be selectively distinguished. We show that the mechanical response at medium/large strains is essentially driven by strain amplification effects, while, in the linear regime, there is a strong additional reinforcement which is not related to the properties of the elastomer matrix. Hypothesis on the origin of this additional reinforcement are suggested and discussed.

2H NMR and Simulation Studies of Chain Segment Orientation in PDMS Bimodal Networks

We present a systematic study through 2H NMR and molecular simulations of chain segment orientation in model PDMS bimodal networks. Estimates of the average segment orientation order parameter in model PDMS bimodal networks compare well to those of equivalent simulated networks. We find that line shapes of short and long chains are different due to the dissimilar degrees of motional restrictions. Short-chain conformations are perturbed from random coiled states when incorporated into bimodal networks whereas long-chain conformations are unaffected. Although experiments and simulations both indicate that the overall segment orientation normalized by the elastic modulus is fairly constant across all bimodal network compositions, short-chain normalized segment orientation displays a nonmonotonic trend with the network composition (related to changes in network microstructure and mechanical properties). Finally, we show that the unusual outer splittings observed in spectra of highly extended unimodal and bimodal samples correspond to segments in long chains able to be exceptionally stretched and oriented along the strain axis.

Stratification in Bimodal Polymer Brushes

The density profiles in a mixture of bimodal polymer brushes and solvent were simulated with the aid of a self-consistent field theory (SCFT). The results showed that the bimodal brushes in good solvent form two sub-layers, which were in good agreement with those from both experiments and Monte Carlo simulations. And the increase of solubility not only makes the density profiles obtained by SCFT close to the results calculated with strong segregation theory (SST) asymptotically, but also leads to further stretching of polymer chains and sufficient orientation of chain segments. Moreover, the enlargement of grafting densities facilitates the stratification of brushes. In good solvent condition, density profiles with different total grafting densities could fall onto the same master curve. Additionally, polymerization index of long chains plays an important role in stretching of long chains, but it has little effect on density profiles of short chains.

IRRAS Studies on Chain Orientation in the Monolayers of Amino Acid Amphiphiles at the Air−Water Interface Depending on Metal Complex and Hydrogen Bond Formation with the Headgroups

Monolayers of N-octadecanoyl-L-alanine at the air-water interface on pure water and metal ion containing subphases have been studied using polarized infrared reflection-absorption spectroscopy (IRRAS). The metal complex and hydrogen bond formation with the headgroups give rise to a change in chain order depending on metal ion in the subphase. On pure water and Ag(+)-/Pb(2+)-containing subphase, the antisymmetric CH(2) stretching band intensity [nu(a)(CH(2))] undergoes a slower increase than the symmetric one [nu(s)(CH(2))] below the Brewster angle, so the intensity ratios of nu(a)(CH(2))/nu(s)(CH(2)) are less than 1 in the cases of Ag(+) and Pb(2+). Beyond the Brewster angle, the nu(a)(CH(2)) band intensities are substantially reduced in comparison with the nu(s)(CH(2)) ones in the cases of pure water and Ag(+), but the nu(a)(CH(2)) bands still remain negative-oriented in the presence of Pb(2+). These unusual spectral features indicate that the alkyl chains take a preferential orientation with their C-C-C planes parallel to the water surface. The parallel packing of the alkyl chains results from the intermolecular hydrogen bonds C=O...H-N between the neighboring amide groups, strengthened by the metal complex of covalent interaction. On the Ca(2+)-/Cu(2+)-containing subphase, the corresponding polarized spectra display a usual behavior. The alkyl chains are roughly estimated to be inclined around 35-40 degrees from the surface normal on the assumption of chain segment orientation for the monolayers in the liquid-expanded phase. The chain conformation and tilt are closely related to the formation of intramolecular hydrogen bonds and the ionic interaction of the metal complex in the cases of Ca(2+) and Cu(2+).

Dynamic mechanical properties of crystalline polymer blends. The influence of interface and orientation

AbstractModification of the interface changes the viscoelastic response of crystalline polymer blends. For incompatible systems, usually one or more additional tan δ (mechanical loss) peaks appear which reflect the embrittlement of the material. Upon proper compatibilization, the additional peaks disappear and the material becomes tougher. The associated phenomena are explained and illustrated by a number of examples including polypropylene filled with calcium carbonate, polyethylene (PE) and polystyrene compatibilized blends, compatibilized blends of poly(ethylene terephthalate) (PET) with polyethylene as well as polyamide 6/polypropylene- graft-acrylic acid blends. Modes of deformation producing cavitation and cavity free deformation were applied to polymer blends in order to study the influence of molecular orientation and the presence of cavities. It is concluded that interfaces between blend components are weak elements of the blends even in the presence of a compatibilizer. Dynamic mechanical analysis (DMTA) evidenced the occurrence of interactions of the compatibilizer with blend components through temperature shift and intensity change of α, β and γ relaxation processes of polymer components. In oriented films of PET/high-density PE compatibilized blends, the chain segments in the amorphous phase of PET are oriented along machine direction. However, there is a significant anisotropy of chain packing in the plane perpendicular to the drawing direction - the pseudohexagonal packing of chain fragments being in register over the whole film. The PET amorphous phase exhibits anisotropy: DMTA studies show that the glass transition occurs at different temperatures while bending the film in different directions. The source of the ‘anisotropy’ in glass transition is the anisotropy of the amorphous phase: the pseudohexagonal packing and the orientation of chain segments. It is evident that the free volume and frozen fraction of the amorphous phase exhibit some degree of anisotropy. In highly compressed samples free of cavities, the glass transition temperature is shifted to higher temperature as compared to undeformed or drawn and cavitated blends. The shift is the result of high molecular orientation of the amorphous phase of matrix polymer and of the stabilization effect of unbroken interfaces in compressed samples.

Structure of polymers in the vicinity of convex impenetrable surfaces: the athermal case

The influence of the presence of a curved (convex) solid wall on the conformations of long, flexible polymer chains is studied in a dense polymer system and in the athermal limit by means of lattice Monte Carlo simulations. It is found that the chain conformation entropy drives a reduction of the density at the wall, similar to the flat wall case. The chain end density is higher next to the interface compared to the bulk polymer (segregation), with the difference increasing with chain length. The wall curvature does not significantly affect the segregation. The bonds are preferentially oriented in the direction tangential to the wall. The distance from the interface over which this effect is observed is about two bond lengths. Similar results are obtained when probing the preferential orientation of chain segments. In this case, the perturbed region has a thickness on the order of the considered probing chain segment length. This suggests that experimental results on the thickness of the ‘bonded layer’ next to a wall depend on the wavelength of the radiation employed for probing. The chains are ellipsoidal in the bulk and rotate close to the surface with the large semi-axis of the ellipsoid normal to the line connecting their center of mass with the filler center. Since there is no energetic interaction with the filler, no adsorption transition is observed, but the chains tend to wrap around the filler once the gyration radius becomes comparable to the filler radius.