Polymer Chain Density Research Articles

High-strength conductive hydrogels based on the Hofmeister effect for friction nanogenerators

Hydrogels have received much attention in the field of flexible electronics as materials with flexibility and multifunctionality. The mechanical strength of conventional hydrogels is usually difficult to meet the requirements of practical applications in electronic devices. How to fabricate a high-strength hydrogel should remain a challenge. Here, a strategy to enhance the mechanical properties of conductive hydrogels based on the Hofmeister effect is reported. The mechanical properties of hydrogels were enhanced by increasing the polymer chain density, enhancing the hydrophobicity and increasing the crystallinity, the high-strength and high-toughness polyvinyl alcohol/carbon nanotubes/polyethyleneimine (PVA/MWCNTs/PEI) conductive hydrogel was successfully produced. The ultimate stress of the hydrogel was as high as 3.5–6.3 MPa, the elongation at break was between 500 and 1200 %, and the toughness was up to 23.62 MJ/m3. The conductivity of high-strength, high-toughness hydrogel is 0.05–0.45 S/m. Hydrogel was manufactured into a single-electrode friction nanogenerator (TENG), and it can easily light up to 100 LEDs. Therefore, this high-strength and high-toughness conductive hydrogel has great potential for TENG applications, offering the possibility of extending the working life of TENG in harsh environments.

Stimulating Mesoporous Characteristics of Activated Carbon through Pyrolysis of Compacted Hydroxyethyl Cellulose—A Showcase for H2S Removal

Stimulating Mesoporous Characteristics of Activated Carbon through Pyrolysis of Compacted Hydroxyethyl Cellulose—A Showcase for H2S Removal

Revealing the Role of Chain Conformations on the Origin of the Mechanical Reinforcement in Glassy Polymer Nanocomposites.

Understanding the mechanism of mechanical reinforcement in glassy polymer nanocomposites is of paramount importance for their tailored design. Here, we present a detailed investigation, via atomistic simulation, of the coupling between density, structure, and conformations of polymer chains with respect to their role in mechanical reinforcement. Probing the properties at the molecular level reveals that the effective mass density as well as the rigidity of the matrix region changes with filler volume fraction, while that of the interphase remains constant. The origin of the mechanical reinforcement is attributed to the heterogeneous chain conformations in the vicinity of the nanoparticles, involving a 2-fold mechanism. In the low-loading regime, the reinforcement comes mainly from a thin, single-molecule, 2D-like layer of adsorbed polymer segments on the nanoparticle, whereas in the high-loading regime, the reinforcement is dominated by the coupling between train and bridge conformations; the latter involves segments connecting neighboring nanoparticles.

Effect of uniaxial stretching on gas separation performance of polyimide membranes

Fine-tuning of the physical stacking structure for microporous polyimide (PI) membranes via the stretching processing is a practical way of achieving advanced membrane materials with improved aging resistance as well as high gas separation performance. Here, a fluorinated polyimide 6FDA:BPDA-TFDB(1:1:2) is used as a prototype and a series of stretched membranes X%-PI are fabricated by uniaxially stretching the membrane to different elongation ratio and then annealing. Based on birefringence and wide-angle X-ray diffraction measurements, it's found that regular orientation of polymer chains along the stretching direction and fine-tuning of d-spaces from interchain packing and π-π stacking happen during the uniaxial stretching process. As such, compared to pristine PI membrane, the stretched membranes show better mechanical and thermal stability, especially for the one with higher stretching ratio. In addition, owing to the increased packing density of polymer chains, the highly stretched membrane of 40%-PI shows significantly improved gas perm-selectivity by 32.8%, 24.3%, and 12.6% for He/CH4, H2/CH4, and CO2/CH4 gas pairs, respectively, in comparison to pristine membrane. Furthermore, 40%-PI exhibits much higher gas permeability retention rate than that of other membranes after three months' aging. We expect that this work can offer polymer materials processing guidance for the preparation of high-performance gas separation membrane in future.

Rational fabrication of completely amorphous chitosan-formyl-sucrose sorbents with excellent durability and regenerability for high-throughput dye removal

We have developed chitosan-formyl-sucrose sorbents with densely charged groups and ultra-low polymer chain density via a room-temperature one-step approach for industrial adaptations. This sorbent reaches theoretic sorption and maximum desorption capacities with extremely high concentration factors suitable for fast continuous dye removal. We focus on the development of affordable cleaning technology. Such a technology must have high cleaning capacity and durable reusability. Via sorption of three reactive dyes, Reactive Blue 19, Reactive Black 5, and Reactive Red 120, the sorbent we report provided a dyestuff sorption capacity of as high as 7 g/g, 99% of its theoretical sorption value, due to its densely charged groups and complete accessibility. This sorbent also exhibited a 99% desorption efficiency even when the eluate concentrated 19 times the initial dye solution due to the adjustable charged groups and ultra-low polymer chain density, therefore, minimal van der Waals attractions to the dyes. Formyl-sucrose endows the sorbents with desirable mechanical properties and excellent water stability for high-speed continuous wastewater cleaning and sorbent regeneration readily adaptable to cost-effective industrializations.

Delocalization of interacting directed polymers on a periodic substrate: Localization length and critical exponents from non-Hermitian spectra.

We study a classical model of thermally fluctuating polymers confined to two dimensions, experiencing a grooved periodic potential, and subject to pulling forces both along and transverse to the grooves. The equilibrium polymer conformations are described by a mapping to a quantum system with a non-Hermitian Hamiltonian and with fermionic statistics generated by noncrossing interactions among polymers. Using molecular dynamics simulations and analytical calculations, we identify a localized and a delocalized phase of the polymer conformations, separated by a delocalization transition which corresponds (in the quantum description) to the breakdown of a band insulator when driven by an imaginary vector potential. We calculate the average tilt of the many-body system, at arbitrary shear values and filling density of polymer chains, in terms of the complex-valued non-Hermitian band structure. We find the critical shear value, the localization length, and the critical exponent by which the shear modulus diverges in terms of the branch points (exceptional points) in the band structure at which the bandgap closes. We also investigate the combined effects of non-Hermitian delocalization and localization due to both periodicity and disorder, uncovering preliminary evidence that while disorder favors localization at high values, it encourages delocalization at lower values.

Energetics of cytoskeletal gel contraction.

Cytoskeletal gels are prototyped to reproduce the mechanical contraction of the cytoskeleton in vitro. They are composed of a polymer network (backbone), swollen by the presence of a liquid solvent, and active molecules (molecular motors, MMs) that transduce chemical energy into the mechanical work of contraction. These motors attach to the polymer chains to shorten them and/or act as dynamic crosslinks, thereby constraining the thermal fluctuations of the chains. We describe both mechanisms thermodynamically as a microstructural reconfiguration, where the backbone stiffens to motivate solvent (out)flow and accommodate contraction. Via simple steady-state energetic analysis, under the simplest case of isotropic deformation, we quantify the mechanical energy required to achieve contraction as a function of polymer chain density and molecular motor density. We identify two limit regimes, namely, fast MM activation (FM), and slow MM activation (SM). FM assumes that MMs provide all the available mechanical energy 'instantaneously' and leave the polymer in a stiffened state, i.e. the MM activity occurs at a time scale that is much smaller than that of solvent diffusion. SM assumes that the timescale for MM activation is much longer than that of solvent diffusion. To achieve the same final contracted state, FM requires the largest amount of work per unit reference volume, while SM requires the least. For all intermediate cases where the timescale of MM activation is comparable with that of solvent diffusion, the required work ranges between these two limits. We provide all these quantities as a function of chain density and MM density. Finally, we compare our results on contraction energetics with experiments and observe good agreement.

Phase separation enabled silver nano-array

The surface-supported silver nanoparticles have been studied and applied in various applications. Many unique nanostructures have been introduced into this field to improve the functionalities of the surfaces depending on application purposes. We created featured silver nano-array surfaces by utilizing the solvent-mediated phase transition on the surface grafted with poly (acrylic) acids polymer chains and taking advantage of the low temperature of argon gas discharged plasma as a reducing agent. The applied solvents and grafted polymer chain densities affected the phase transition and thus determined the outcome of surface nano-array patterns. However, the total loaded silver ions on the surface affected silver nano-array structures at the sub-micron levels. The featured silver patterned surfaces made in the optimal conditions present a favorable surface-enhanced Raman spectroscopy enhancement as well as recyclability for detection re-usage. This novel method prepares tunable silver nanopatterned surfaces and provides a new approach to various potential applications.

Effect of melt shearing on d-mannitol crystal twisting in the presence of small molecule and macromolecular additives

Many molecular crystals grow as twisted lamellae from the melt when the driving force is high, but the mechanisms governing the spontaneous formation of helicoidal crystallites are varied and those operative in particular cases are difficult to identify. Polyvinylpyrrolidone (PVP) induces the crystallization of d-mannitol as banded spherulites comprised of twisted fibrils emanating radially from spherulite nucleation centers. The molecular weight of the PVP phase affects the twisting pitch, P, (rotation of the crystallite by 180°), in d-mannitol thin films crystallized from the melt. At a relatively high crystallization temperature (Tc = 130 °C), P was sensitive to the PVP molecular weight, ranging from 390 ± 20 μm for d-mannitol films incorporating 15 wt% 10 kDa PVP to 20 ± 3 μm for the same weight of 1300 kDa PVP. Magnitudes of complex viscosities, η∗, of d-mannitol/PVP melts measured via small-amplitude oscillatory shearing were strongly dependent on the PVP molecular weight, but were not correlated to P(T). Instead, P was sensitive to the dynamic PVP chain conformation during d-mannitol crystallization. Under steady torsional shear, P decreased from ∼ 30 μm to ∼ 8 μm with increasing shear rates from 0.01 to 100 s−1 for d-mannitol films crystallized at 130 °C in the presence of 15 wt% 10 or 1300 kDa PVP. Shear forces decrease the entanglement density of polymer chains while orienting the chains along the stream lines of the viscometric flow, indicating that the conformations of macromolecular additives can affect the pitch of banded spherulites. By contrast, P in d-mannitol twisted by d-sorbitol was independent of shear rate.

Progress in the mechanical enhancement of hydrogels: Fabrication strategies and underlying mechanisms

AbstractHydrogels have become ideal materials in the nascent applications of tissue engineering, soft robots, drug delivery, and so forth. However, compared with biological tissues, the inherent heterogeneous microstructure and low density of polymer chains make hydrogels mechanically weak, severely limiting their use as structural materials. In recent decades, in order to meet the mechanical requirements of load‐bearing biomaterials, significant research effort have been devoted to improving the mechanical parameters of hydrogels. To achieve this goal, fiber/fabric reinforced hydrogels, double network hydrogels, supramolecular‐interaction‐based hydrogels, hydrogels with well‐aligned microstructures, and solvent induced robust hydrogels have been investigated. In this review, the fabrication strategies, the relationships between the structure and the mechanical properties of the resulting hydrogel, and the underlying enhancement mechanisms in various classes of hydrogel have been summarized. Here, the mechanisms behind these strategies rely on creating the mechanically effective networks, which are achieved by introducing a rigidly reinforced phase, synergistic network with distinctive features, sacrificial bonds, oriented hierarchical structures, or increased supramolecular interactions. Despite significant achievements toward strong and tough hydrogels, considerable important challenges remain, such as simultaneously achieving high strength, toughness, and high water content of hydrogels. It is believed that the already proposed strategies will push the development of hydrogels.

Temperature- and strain-dependent transient microstructure and rheological responses of endblock-associated triblock gels of different block lengths in a midblock selective solvent.

Endblock associative ABA gels in midblock selective solvents are attractive due to their easily tunable mechanical properties. Here, we present the effects of A- and B-block lengths on the rheological properties and microstructure of ABA gels by considering three low and one high polymer concentrations. The triblock polymer considered is poly(methyl methacrylate)-poly(n-butyl acrylate)-poly(methyl methacrylate) [PMMA-PnBA-PMMA] and the midblock solvent is 2-ethyl-1-hexanol. The gelation temperature has been found to be strongly dependent on the B-block (PnBA) length, as longer B-blocks facilitate network formation resulting in higher gelation temperature even with lower polymer chain density. Longer A-blocks (PMMA chains) make the endblock association stronger and significantly increase the relaxation time of gels. Temperature-dependent microstructure evolution for the gels with high polymer concentration reveals that the gel microstructure does not change significantly after the gel formation takes place. The dynamic change of microstructure in an applied strain cycle was captured using RheoSAXS experiments. The microstructure orients with the applied strain and the process is reversible in nature, indicating no significant A-block pullout. Our results provide new understandings regarding the temperature and strain-dependent microstructural change of ABA gels in midblock selective solvents.

Flexible and Robust Ultrablack Elastomers with a Dual‐Gradient Design for Broadband and Efficient Light Management

AbstractEngineering flexible broadband light absorbers is crucial to various applications ranging from dark‐level reduction to light harvesting. However, developing solvent free, ultrablack elastomers with high light‐absorption performance and broad absorption band remains a challenge. Here, a class of composite elastomers is fabricated for ultrablack sheet materials. The fabrication process involves a fast photo‐polymerization of acrylates in the presence of gradient deposited polypyrrole (PPy) nanoparticles. The resultant film shows a dual‐gradient structure (i.e., lower side features lower polymer chain density while higher concentration of deposited PPy). Such structure design critically contributes to the formation of bio‐mimetic, hierarchical surface microstructures for efficient light trapping. As a result, lower side of the film exhibits incident‐light‐angle independent, high light absorptivity of 98.43% at 250–2400 nm. Moreover, flexible and mechanically robust absorbers have been demonstrated to offer versatile solar energy related applications, such as sunlight heater, photoactuator, pattern display, and anticounterfeiting. Rather than tedious processing ways generally used in carbon nanotube forests or plasmonic involved ultrablack materials, dual‐gradient design principle is concise and compatible with polymer thin‐film fabrication process, and applicable to various elastic substrates, thus will provide guidance for fabricating various flexible ultrablack sheet materials and devices in a cost‐effective and large scalable way.

Dextran-based matrix functionalization to promote WJ-MSCs amplification: synthesis and characterization

Several clinical studies have reported the benefit of the administration of Mesenchymal Stem Cells (MSCs) in cell therapies. However, their routine applications need new substrates to amplify MSCs in vitro according to Good Manufacturing Practices (GMP) conditions and microcarriers are particularly suited for these purposes. In order to optimize the surface properties of Cytodex I microcarriers (Cyt), poly N-isopropylacrylamide (pNIPAM) has been grafted on their surface to promote MSCs adhesion, proliferation, but also to control their detachment by a decrease in temperature. The polymer coating generated on the microcarriers was analyzed using Time-of-Flight, Nanoscale Secondary Ion Mass Spectrometry, and Atomic Force Microscopy. We have confirmed the success of the pNIPAM grafting on Cyt with a two-steps reaction and correlated the influence on matrix functionalization in the function of the organic solvent used to disperse the microcarriers. The effects of pNIPAM functionalization have been explored on Wharton’s jelly-MSCs (WJ-MSCs) culture and cell thermal detachment was monitored with fluorescent microscopy. The in vitro results have indicated that WJ-MSCs have a better growth on Cyt-pNIPAM. However, pNIPAM thermal cell detachment was lower than trypsinization, implying that the minimum effective molecular weight and surface density of polymer chains have still to be future optimized.

Study of the effect of modification with small additives of phenylenediamines

The studies of the mechanical electrophysical properties of polymers and their modifications under the simultaneous influence of electrical discharges and mechanical force are presented. The paper presents the results of the experimental studies of the electrophysical and mechanical strength, the temperature dependence of the tangent of the dielectric loss angle, the specific volume electrical resistance and the relative deformation at break of new film polymer materials obtained on the basis of high-pressure polyethylene (LDPE) with a mixture of phenylenediamine additives OPD and PhI. The content of additives varied in the range of (0.03-0.5) wt%. The established increase in the electrophysical characteristics (increase in resistance to high electric field) of the LDPE film with the introduction of 0.1 wt% of chemical additives of PhI and with subsequent three-fold orientation extraction can be associated with a decrease in the concentration of charge carriers and their mobility, as well as an increase in the degree of crystallinity and packing density of polymer chains, which makes it difficult for ionization processes to occur at a given high electric field strength. It is shown that a decrease in the electrophysical characteristics of the studied polymer materials in the presence of tensile mechanical forces leads to the accumulation of bulk charges in them and the formation of submicrocracks.

Evaluation of renewable pH-responsive starch-based flocculant on treating and recycling of highly saline textile effluents

Herein, we report a novel renewable pH-responsive starch-based flocculant (CIAT-ST) via etherifying 2-chloro-4,6-isopropylamino-[1,3,5]-triazine (CIAT) onto the starch backbones for decontamination and reuse of highly saline effluents. The obtained CIAT-ST shows a unique pH-sensibility and reversibility in response to a subtle pH change due to a pH-controllable surface charge density of polymer chains. The level of residual CIAT-ST in the solution can be facilely monitored by using UV–vis detection. The dye flocculation performance of CIAT-ST was evaluated by using a batch experiment. The results exhibited that the dye removal was highly dependent on the solution pH (optimal pH was 2), the flocculation equilibrium can be achieved within 5 min, and the maximum flocculation capacity of CIAT-ST for K–2BP and KN–B5 were calculated to be 2452.6 ± 23.9 and 792.7 ± 14.1 mg/g, respectively. The multiple flocculation mechanisms, including charge neutralization, bridging and charge patching, may participate in the flocculation process. Adjustment in pH-mediated hydrophilicity-hydrophobicity switch of flocculant facilitates readily recovery and then sequentially reused three times while retaining satisfying flocculation efficiency. A significant contribution was also confirmed that the highly saline effluents after flocculation and sedimentation were reused in three successive dyeing processes without sacrificing fabric quality (ΔE* < 1) due to relatively low polymer residuals, and the efficiency of salt reuse for consecutive regeneration processes could be achieved above 85%. The present work could provide alternative thoughts for the reutilization of spent flocculant and clarified saline wastewater, which is also an efficient and sustainable strategy for textile wastewater management.

Bottlebrush Polymer Excipients Enhance Drug Solubility: Influence of End-Group Hydrophilicity and Thermoresponsiveness.

Bottlebrush polymers have great potential as vehicles to noncovalently sequester, stabilize, and deliver hydrophobic small molecule actives. To this end, we synthesized a poly(N-isopropylacrylamide-stat-N,N-dimethylacrylamide) bottlebrush copolymer using ring-opening metathesis polymerization and developed a facile method to control the thermoresponsive properties using postpolymerization modification. Six increasingly hydrophilic end-groups were installed, yielding cloud point temperature control over a range of 22-42 °C. Solubility enhancement of the antiseizure medication, phenytoin, increased significantly with the hydrophilicity of the end-group moiety. Notably, carboxylated bottlebrush copolymers solubilized formulations with higher drug loadings than linear copolymers because they exist as unimolecular nanoparticles with a synthetically defined density of polymer chains that are more stable in solution. This work provides the first investigation of bottlebrush polymers for hydrophobic noncovalent sequestration and solubilization of pharmaceuticals.

Carbon fiber plastics for vehicles manufactured by resource-saving formation technology

The advantages of the technology of molding carbon plastics based on pre-pregs with separate application of components (SAC-technology) of epoxy binders cured by the mechanism of migration polymerization of epoxy cycles are presented. Using the methods of linear dilatometry and dynamic mechanical tests, the molecular mobility of the segmental and local types in polymer matrices of carbon fiber reinforced plastics based on traditional mixed pre-pregs and binary pre-pregs with separate application of components has been investigated. The question of the influence of the activation ability of an acid catalyst during the curing of an epoxy binder on the degree of decrease in internal stresses and the realization of an increased packing density of polymer chains in the polymer matrix of the composite is considered. The importance of choosing an acid catalyst for the curing of a polymer binder is shown, which for CFRP composites provides the necessary combination of elastic and dissipative properties. The use of the considered binders makes it possible to improve a number of important operational characteristics, especially in the case of the production of large-sized products: solidity, crack resistance, impact resistance, fracture toughness. These properties are very significant during the composite materials applying in transport engineering.

Precise surface modification of porous membranes with well-defined zwitterionic polymer for antifouling applications

Fouling is a major problem in water treatment membranes. Although surface modification with a zwitterionic polymer is effective for antifouling, precisely controlling the surface state of the modified polymer, considering both polymer chain length and density has not been demonstrated. Herein, we propose a precise surface modification method for water treatment membranes through grafting to approach and evaluate the influence of the graft length and the density of the modifying polymer on the fouling behavior of the membrane. The polymers used to modify the membrane surface were synthesized via atom transfer radical polymerization. Furthermore, the polymer density was controlled during immobilization reaction by varying the solvent type; high-density surface modification was achieved using poor solvents, whereas, low-density surface modification was realized using good solvents. Via this method, poly (2-methacryloyloxyethyl phosphorylcholine) (polyMPC) was successfully modified with controlled densities on porous membrane surfaces with different morphologies. The static and dynamic antifouling properties of the polyMPC-modified membranes were evaluated using the bovine serum albumin solution, and the membrane modified using a higher molecular-weight polyMPC with a higher density was more effective for antifouling. This precise surface control of the membrane can help to rationalize ideal surface characteristics for antifouling applications.

EMI shielding and dynamic mechanical analysis of graphene oxide-carbon nanotube-acrylonitrile butadiene styrene hybrid composites

Carbon nanomaterials such as carbon nanotubes (CNTs), graphene and their hybrid have been studied extensively. Despite having excellent properties of CNTs and graphene have not yet been fully realized in the polymer composites. During fabrication agglomeration of CNTs and restacking of graphene is a serious concern that results in the degradation of properties of nanomaterials into the final composites. To improve the dispersion of CNTs and restacking graphene, in the present research work, we focused on the hybridization of graphene oxide and CNTs. Multiwalled carbon nanotubes (MWCNTs), functionalized carbon nanotubes (FCNTs), and graphene oxide-carbon nanotubes (GCNTs) reinforced acrylonitrile butadiene styrene (ABS) composites were prepared separately by vacuum filtration followed by hot compression molding. Further, dynamic mechanical analysis (DMA), and electromagnetic interference (EMI) shielding properties of ABS composites reinforced carbon nanofillers were investigated. The dynamic mechanical properties of polymers strongly depend on the adhesion of fillers and polymer, entanglement density of polymer chains in the presence of carbon fillers. The dynamic mechanical characteristics such as storage, loss modulus, and damping factor of prepared composites were significantly affected by the incorporation of MWCNTs, FCNTs, and GCNTs. Maximum EMI shielding effectiveness of −49.6 dB was achieved for GCNT-ABS composites which were highest compared to MWCNTs-ABS composites (−38.6 dB) and FCNTs-ABS composites (−36.7 dB) in the Ku band (12.4–18 GHz). These results depict the great potential of GCNTs-ABS composites to be used in various applications of efficient heat dissipative EMI shielding materials for electronic devices.

Tailoring and Remotely Switching Performance of Ultrafiltration Membranes by Magnetically Responsive Polymer Chains.

Magnetically responsive ultrafiltration membranes were prepared by grafting poly(2-hydroxyethyl methacrylate) chains from the outer surface of 100-kDa regenerated cellulose ultrafiltration membranes. Surface-initiated atom transfer radical polymerization was used to graft the polymer chains. Grafting from the internal pore surface was suppressed by using glycerol as a pore-filling solvent during initiator immobilization at varied densities. Glycerol suppresses the initiator attachment to the pore surface. Polymerization times of up to four hours were investigated. Superparamagnetic nanoparticles were covalently attached to the chain end. Membrane performance was determined using bovine serum albumin and dextran as model solutes. Increasing the grafted polymer chain density and length led to a decrease in the permeate flux and an increase in the apparent rejection coefficient. In an oscillating magnetic field, movement of the grafted polymer chains led to a decrease in the permeate flux, as well as an increase in the apparent rejection coefficient of the model solutes.