Free-standing Polymer Research Articles

Water vapor responsiveness of chitosan: An experimental and simulation analysis.

Stimuli-responsive polymers have gained significant research interest in recent years owing to their potential applications in diverse areas. Here, we present a study on the actuation characteristics of chitosan-based free-standing films that exhibit full reversibility and repeatability in response to water vapor exposure. The effect of pH of the water and the degree of cross-linking of the chitosan films on the actuation performance is studied. In the case of free-standing polymer film-based actuators, the primary driving force behind actuation is understood to be the differential strain induced by the gradient in volume changes across the thickness of the film. To understand it further, we conducted full atomistic molecular dynamics simulation studies to explore water absorption and adsorption into the chitosan matrix. Our simulations revealed an accumulation of water molecules in the surface layer that rapidly desorb when shielded from water vapor. Furthermore, estimates of the energy gain resulting from the adsorption of water on the surface suggest that it is adequate to drive the shape change of the actuator when subjected to asymmetric exposure to water vapor. This finding supports the fact that the adsorbed layer of water on the surface of the chitosan film plays a role in actuation.

Free-standing millimeter-range 3D waveguides for on-chip optical interconnects

Next-generation energy-efficient photonic integrated systems, such as neuromorphic computational chips require efficient heterogeneous integration of ultracompact light sources and photodetectors through highly dense waveguide circuits. However, interconnecting these devices in emitter-receiver communication circuits remains a challenge and an obstacle towards upscaling heterogeneous photonic chips. Here we report on versatile air-cladded free-standing 3D polymer waveguides (OrmoCore, n≈1.5) spanning up to 900 µm in length without intermediate mechanical support structures, microprinted via two-photon polymerization. The presented waveguides are suitable for on-chip out-of-plane light coupling as well as non-connected 3D crossings, needed for high density optical circuits. The waveguides show optical transmission losses of 1.93 dB mm−1 at λ = 635 nm, and of 3.71 dB mm−1 at λ = 830 nm in the wavelength range of GaAs-based microLEDs spectral emission. On-chip imaging for high-precision alignment and TPP microfabrication are performed seamlessly by utilizing the same laser source for both steps, allowing accurate 3D printing on microstructured substrates. As proof of concept, we interconnect two GaAs-based microLEDs via an on-chip microprinted 3D waveguide. Such combined systems can serve as building blocks of future complex integrated heterogeneous photonic networks.

Single crystals of purely organic free-standing two-dimensional woven polymer networks

The aesthetic and practicality of macroscopic fabrics continue to encourage chemists to weave molecules into interlaced patterns with the aim of providing emergent physical and chemical properties when compared with their starting materials. Weaving purely organic molecular threads into flawless two-dimensional patterns remains a formidable challenge, even though its feasibility has been proposed on several occasions. Herein we describe the synthesis of a flawless, purely organic, free-standing two-dimensional woven polymer network driven by dative B−N bonds. Single crystals of this woven polymer network were obtained and its well-defined woven topology was revealed by X-ray diffraction analysis. Free-standing two-dimensional monolayer nanosheets of the woven polymer network were exfoliated from the layered crystals using Scotch Magic Tape. The surface features of the nanosheets were investigated by integrated low-dose and cryogenic electron microscopy imaging techniques. These findings demonstrate the precise construction of purely organic woven polymer networks and highlight the unique opportunities for the application of woven topologies in two-dimensional organic materials.

Effect of crosslinker/porogen ratio on sponge-nested polymer monoliths for solid-phase extraction

Polymer monoliths can be polymerised within different molds, but limited options are available for the preparation of free-standing polymer monoliths for analytical sample preparation, and in particular, solid-phase extraction (SPE). Commercial melamine-formaldehyde sponges can be used as supports for the preparation of polymer monoliths, due its flexibility, giving various shapes to monoliths. Herein, the crosslinker/porogen ratio of highly porous sponge-nested divinylbenzene (DVB) polymer monoliths has been evaluated. Monoliths prepared using different crosslinker/porogen ratios were applied to the extraction of bisphenol F, bisphenol A, bisphenol AF, and bisphenol B. Monoliths containing 50 wt % DVB and 50 wt % porogens presented the highest recovery of bisphenols. Under the optimised conditions, the developed method showed a linear range between 2.5 µg L−1 and 150 µg L−1 for BPA and BPAF, and between 5 µg L−1 and 150 µg L−1 for BPB and BPF. The limits of detection (LOD, S/N = 3) and limits of quantification (LOQ, S/N = 10) ranged from 0.36 µg L−1 to 1.09 µg L−1, and from 1.20 µg L−1 to 3.65 µg L−1, respectively. The recoveries for spiked bisphenols (10 µg L−1) in tap water and water contained in a polycarbonate containers were between 82 % and 114 %.

In situ formation of polymer electrolytes using a phosphazene cross-linker for high-performance lithium-ion batteries

Polymer electrolytes (PEs) have been intensively studied in lithium-ion batteries and the studies have recently focused on improving their electrochemical performances. In this study, an in situ thermal curing method is proposed to prepare polyethylene glycol-based electrolytes using a cyclic phosphazene cross-linker directly onto the lithium anode, endowing the preparation of PEs with improved ion transport across the interfaces and better electrochemical performances than those using free-standing polymer electrolytes. The buildup of the cross-linked electrolyte network by incorporating the phosphazene cross-linker endows the as-prepared PEs exhibiting excellent deformation and thermal stability, a wide electrochemical window, high ionic conductivity (3.07 × 10−3 S cm−1) at room temperature. The in situ-formed, cross-linked electrolyte network with good elasticity and conformal attachment at the electrolyte/electrode interface achieves a high capacity (>160 mAh g−1 at 0.1C rate) and a long cycle life at room temperature (0.2C rate, 93.8% at 250 cycles) in the equipped Li|LiFePO4 cell, making this strategy a promising approach for developing the next-generation lithium-ion batteries.

Large-Scale Fabrication of Freestanding Polymer Ultrathin Porous Membranes for Transparent Transwell Coculture Systems.

Advancements in cell coculture systems with porous membranes have facilitated the simulation of human-like in vitro microenvironments for diverse biomedical applications. However, conventional Transwell membranes face limitations in low porosity (ca. 6%) and optical opacity due to their large thickness (ca. 10 μm). In this study, we demonstrated a one-step, large-scale fabrication of freestanding polymer ultrathin porous (PUP) membranes with thicknesses of hundreds of nanometers. PUP membranes were produced by using a gap-controlled bar-coating process combined with polymer blend phase separation. They are 20 times thinner than Transwell membranes, possessing 3-fold higher porosity and exhibiting high transparency. These membranes demonstrate outstanding molecular permeability and significantly reduce the cell-cell distance, thereby facilitating efficient signal exchange pathways between cells. This research enables the establishment of a cutting-edge in vitro cell coculture system, enhancing optical transparency, and streamlining the large-scale manufacturing of porous membranes.

Rapid and facile preparation of freestanding dense polymer membranes from high-boiling-point solvents for anion exchange membrane water electrolyzers

Transparent, dense, and freestanding polymer membranes are typically prepared by solution casting (SC). However, SC is a time-consuming (>24 h) multi-step process that requires high-temperature vacuum treatment. Here, we report a rapid and facile method—nonsolvent-induced film formation (NIFF)—for preparing dense, freestanding membranes by immersing a polymer solution in a nonsolvent, using two types of poly(arylene piperidinium) (PAP) and their quaternized versions. Thermodynamic, kinetic, and molecular dynamics analyses of polymer/solvent/nonsolvent ternary systems reveal that the PAP polymers have high affinities for the nonsolvent (water), yielding a gel between homogeneous and heterogeneous regions. The protective gel layer at the polymer/solvent–nonsolvent interface averts penetration of the nonsolvent and permits withdrawal of the solvent from the polymer solution, forming a nonporous dense membrane. The physicochemical, mechanical, morphological, and electrochemical characteristics of NIFF- and conventional-SC-derived membranes are comparable. Moreover, the alkaline water electrolysis cell performance is evaluated using the membranes and platinum group metal-free catalyst. Both NIFF- and SC-derived membrane-based cells achieve an equivalent current density of 910 mA cm−2 at 2.0 V, which is 20 % higher than commercial FAA-3-50 membrane-based cells. This study establishes an innovative strategy to facilely fabricate freestanding polymer films with a time- and energy-saving process for ion-conducting membranes.

Enhancement in Capacitance of Ionic Type of EAP-Based Strain Sensors

This paper aims to enhance the capacitance of electroactive polymer (EAP)-based strain sensors. The enhancement in capacitance was achieved by using a free-standing stretchable polymer film while introducing conducting polymer to fabricate a hybrid dielectric film with controlled conductivity. In this work, styrene-ethylene-butylene-styrene (SEBS) rubber was used as the base material, and dodecyl benzene sulfonate anion (DBSA)-doped polyaniline (PANI) was used as filler to fabricate a hybrid composite conducting film. The maleic anhydride group of the SEBS Rubber and DBSA, the anion of the polyaniline dopant, make a very stable dispersion in Toluene and form a free-standing stretchable film by solution casting. DBSA-doped polyaniline increased the conductivity and dielectric constant of the dielectric film, resulting in a significant enhancement in the capacitance of the EAP-based strain sensor. The sensor presented in this article exhibits capacitance values ranging from 24.7 to 100 µF for strain levels ranging from 0 to 100%, and sensitivity was measured 3 at 100% strain level.

A flexible PEO-based polymer electrolyte with cross-linked network for high-voltage all solid-state lithium-ion battery

Solid state lithium-ion batteries (SLIBs) have been considered as one of the most promising sustainable next-generation technologies for energy storage. However, the poor interfacial compatibility and low ion conductivity of solid electrolytes still remain a major challenge for SLIBs. Herein, a free-standing flexible solid polymer LA-PAM-PEO electrolyte is constructed through the electrospinning technology featuring with high Li+ conductivity (6.1 × 10−4 S cm−1), strong mechanical strength and high Li+ migration number (0.32), which breaks the restriction between ionic conductivity and mechanical strength in polymer solid electrolyte. The cross-linking between LA, PAM and PEO is verified to decrease the crystalline of PEO, thus increasing the Li+ conductivity. Moreover, benefiting from the 3D network composed of interconnected nanofibers and the covalent bonds between LA, PAM and PEO, the mechanical strength of LA-PAM-PEO SPE was also effectively improved. The LA-PAM-PEO SPE also delivers a high electrochemical window (4.95 V), and low interface resistance (243.8 Ω). As a result, the Li/Li symmetrical cell with the LA-PAM-PEO displayed outstanding stability after 1000 h with the uniform Li deposition on the interface of Li electrode, in sharp contrast to the PEO SPE. In addition, the Li/LA-PAM-PEO SPE/LFP displays a discharge capacity of 135 mA h g−1 after 1000 cycles at the rate of 1 C, with a capacity retention of 93.5%. The proposed LA-PAM-PEO SPE thus opens new possibilities for the fabrication and engineering of solid-state Li-ion batteries.

Bi-enzymatic chemo-mechanical feedback loop for continuous self-sustained actuation of conducting polymers

Artificial actuators have been extensively studied due to their wide range of applications from soft robotics to biomedicine. Herein we introduce an autonomous bi-enzymatic system where reversible motion is triggered by the spontaneous oxidation and reduction of glucose and oxygen, respectively. This chemo-mechanical actuation is completely autonomous and does not require any external trigger to induce self-sustained motion. The device takes advantage of the asymmetric uptake and release of ions on the anisotropic surface of a conducting polymer strip, occurring during the operation of the enzymes glucose oxidase and bilirubin oxidase immobilized on its surface. Both enzymes are connected via a redox polymer at each extremity of the strip, but at the opposite faces of the polymer film. The time-asymmetric consumption of both fuels by the enzymatic reactions produces a double break of symmetry of the film, leading to autonomous actuation. An additional break of symmetry, introduced by the irreversible overoxidation of one extremity of the polymer film, leads to a crawling-type motion of the free-standing polymer film. These reactions occur in a virtually unlimited continuous loop, causing long-term autonomous actuation of the device.

Free-Standing Polymer Covalent Organic Framework Membrane with High Proton Conductivity and Structure Stability

Free-Standing Polymer Covalent Organic Framework Membrane with High Proton Conductivity and Structure Stability

Glass transition temperature of (ultra-)thin polymer films.

The glass transition temperature of confined and free-standing polymer films of varying thickness is studied by extended molecular dynamics simulations of bead-spring chains. The results are connected to the statistical properties of the polymers in the films, where the chain lengths range from short, unentangled to highly entangled. For confined films, perfect scaling of the thickness-dependent end-to-end distance and radius of gyrations normalized to their bulk values in the directions parallel and perpendicular to the surfaces is obtained. In particular, the reduced end-to-end distance in the perpendicular direction is very well described by an extended Silberberg model. For bulk polymer melts, the relation between the chain length and Tg follows the Fox-Flory equation. For films, no further confinement induced chain length effect is observed. Tg decreases and is well described by Keddie's formula, where the reduction is more pronounced for free-standing films. It is shown that Tg begins to deviate from bulk Tg at the characteristic film thickness, where the average bond orientation becomes anisotropic and the entanglement density decreases.

Measuring the ionic conductivity of solid polymer electrolyte powders

Ex-situ characterization of solid polymer electrolytes plays an important role in their development as materials for energy applications, with ionic conductivity being a crucial parameter to quantify. Conventional measurements of ionic conductivity often require the formation of a free-standing polymer film which in many instances is difficult to fabricate, thus there may be a need to quantify their ionic conductivity in powder form. In this work, we present a practical and reproducible method for measuring the ionic conductivity of solid polymer electrolytes (SPEs) in their powder form. By using a modified configuration of a through-plane cell, demonstrated with both a proton conducting- and an anion conducting-solid polymer electrolyte powder (SPEP), we are able to obtain ionic conductivity values under variable conditions in order to explore the influence of external parameters on the ionic conductivity of powders. Two types of SPEs in insoluble powder form were employed in this work: (1) a proton-exchange material (SPEP-H+) based on a hyperbranched, sulfo-phenylated poly(phenylene) SPEP (HB-sPPT-H+), with measured ionic conductivity of ⁓ 210 mS cm−1 at 80 °C and 95 % of relative humidity (RH); (2) an anion-exchange conducting polymer in its chloride form (SPEP-Cl-), consisting of a radiation-grafted ultra-high density polyethylene insoluble SPEP containing covalently-bonded benzyltrimethylammonium (BTMA) head-groups, with measured ionic conductivity of ⁓ 53 mS cm−1 at 80 °C and 95 % RH.

Characterization and application of flexible, highly conductive freestanding films of SWCNT and PMMA nanocomposite prepared by facile solution method

The aim of this work was to prepare single-walled carbon nanotube (SWCNT)-embedded polymethyl methacrylate (PMMA) freestanding polymer films and investigate their mechanical and electrical properties. PMMA is widely used in various applications; however, its poor mechanical and electrical properties limit its use. The electrical and mechanical performance of PMMA can be effectively improved by producing nanoparticle-reinforced PMMA composites. Carbon nanotubes (CNTs) are highly attractive nanoparticles used to develop polymer nanocomposite films with novel properties useful for diverse technological applications. The superior electrical and mechanical properties of CNTs can enhance the corresponding properties of the CNT/PMMA nanocomposite. The SWCNT/PMMA nanocomposite films were prepared with a simple sonication technique by employing ethanol and water as solvents. The prepared films were characterized by X-ray diffraction, photoluminescence, Fourier transform infrared spectroscopy, ultraviolet–visible spectroscopy, and scanning electron microscopy analysis to examine the structural, morphological, and optical properties as well as chemical composition of the films. In addition, nanoindentation and current vs. voltage studies were also conducted. Further, the hydrogen evaluation reaction effect of the prepared electrode was evaluated to investigate the catalytic activity of the films. This technologically attractive SWCNT/PMMA nanocomposite freestanding film prepared first time via a simple low-cost technique showed good conductivity, photosensitivity and flexibility with catalytic property.

Novel poly(ionic liquid)s reinforced with three-dimensional electrospun fiber network as electrolyte for all solid state lithium batteries

Solid polymer electrolytes have been identified lately as the most pragmatic research area towards the development of safe and reliable lithium batteries with zero electrolyte leakage. In this work, ionene oligomers based on 1,4-Diazabicyclo [2.2.2] octane (DABCO) bearing oxyethylene spacers with varying –CH2CH2O- units and having bis (trifluoromethanesulfonyl) imide (TFSI−) counter anion were synthesized. The ionene oligomers exhibited excellent thermal stability up to 300 °C with glass transition below room temperature. Interestingly, the presence of oxyethylene linkers between the cationic moieties in the oligomer backbone significantly affected the Li+ ion coordination and ionic conductivity as explained by Raman spectroscopy and electrochemical impedance spectroscopy (EIS) studies carried out on binary mixtures of POnTFSI/LiTFSI (where ‘P’ indicates the oligomer and ‘On’ denotes the number ‘n’ of oxyethylene units in the spacer). The oxygen atoms present in the spacer backbone of the ionene can interact with the Li+ ions and prevent the formation of aggregates with the anions while the mobility of the TFSI−anions is restricted by the polar nature of the ionene backbone. Such binary mixtures were incorporated in an electrospun nanofibrous framework, to form a free-standing solid polymer electrolyte and the electrochemical performance was evaluated in a Li/LiFePO4 (LFP) battery.

Novel nanoarchitecture of 3D ion transfer channel containing nanocomposite solid polymer electrolyte membrane based on holey graphene oxide and chitosan biopolymer

In the arena of three-dimensional (3D) nanoarchitecture, holey graphene oxide (HGO)—a class of two-dimensional (2D) graphene oxide porous nanosheet, has emerged as a promising choice of additive material to design advanced 3D nanocomposite for energy and environmental applications. With a facile and cost-effective solution-casting technique, we fabricated here a flexible, wearable, free-standing, and super-thin (∼0.08 mm) solid polymer electrolyte membrane (SPEM) consisting of 2D-HGO, lithium bis(trifluromethanesulfonyl)imide (LiTFSI) salt, polyvinylpyrrolidone (PVP) polymer binder, and chitosan (CH) biopolymer. SPEM exhibited impressive ionic conductivity of 2.76 × 10-3 S⋅cm−1 at room temperature (RT = 23 °C) which is comparable to liquid electrolytes. Robust mechanical property (5.87 MPa) and easy lithium-ion diffusion capability of SPEM were identified by the ultra-low activation energy (Ea) of 0.089 eV, which is one of the best values among the reported solid polymer electrolytes. Good lithium-ion transference number (tLi+= 0.76) and wide electrochemical stability window (ESW = 4.4) indicated single ion conduction and stable battery operation voltage capability of SPEM. Moreover, fast ion transfer mechanism of SPEM was proposed according to the comprehensive characterizations; mostly relating to uniform and strong interconnecting novel 3D ion transferring routes. Temperature and frequency responsive charge carrier mobility trend were also investigated with an in-depth dielectric study. Promising RT galvanostatic Li plating-stripping performance was observed at 5 mA⋅cm−2 with a hybrid symmetric cell. Using Li metal as anode, SPEM, and LiCoO2 as cathode in the full cell, a good RT specific discharge capacity of 142.8 mA⋅h⋅g−1 was achieved at 0.1 C rate.

In Situ Synthesis of AIEgen-based Porous Organic Polymer Films by Interfacial Amino-yne Click Polymerization for Efficient Light-Harvesting.

We developed a catalyst-free, atom-economical interfacial amino-yne click polymerization to in situ synthesize new aggregation-induced emission luminogen (AIEgen)-based free-standing porous organic polymer films at room temperature. The crystalline properties of POP films were confirmed by powder X-ray diffraction and high-resolution transmission electron microscopy. The good porosity of these POP films was proved by their N2 uptake experiments. The thickness of POP films can be easily regulated from 16 nm to ≈1 μm by adjusting monomer concentration. More importantly, these AIEgen-based POP films show bright luminescence with high absolute photoluminescent quantum yields up to 37.8 % and good chemical and thermal stability. The AIEgen-based POP film can encapsulate an organic dye (e.g., Nile red) to further form an artificial light-harvesting system with a large red-shift (Δλ=141 nm), highly efficient energy-transfer ability (ΦET =91 %), and high antenna effect (11.3).

A single-ion conducting quasi-solid polymer electrolyte made from synthetic rubber for lithium metal batteries

A lithium-ion conducting triblock copolymer bearing pendant lithium sulfonates, termed as SSEBS-Li, is derived from commercial synthetic rubber poly(styrene-block-(ethylene-ran-butylene)-block-styrene); and free-standing quasi-solid polymer electrolyte (QSPE) membranes made of demonstrate high efficiency and high safety in the application for lithium metal batteries (LMBs). SSEBS-Li possesses a hybrid molecular structure containing “soft” segments that enable seamless contact with the lithium metal; and “hard” segments possessing anchored anions that offer continuous pathways to benefit lithium ion conduction and homogeneous lithium deposition. The unique molecular architecture features the QSPE membranes interpenetrated network microstructures to provide good mechanical strengths and ductility at a thickness of 25 µm. A high lithium ion conductivity of up to 5 × 10-4 S cm−1 at ∼ 35 wt% liquid electrolyte uptake is achieved, along with a high lithium transference number of 0.92 at room temperature. Galvanostatic cycling studies prove the excellent capability of SSEBS-Li QSPE in suppressing lithium dendrite growth. The Li//QSPE//LFP cell exhibits a high discharge capacity and excellent long-term stability after 300 cycles at 0.5C and room temperature without deleterious decay, elucidating a feasible and economical approach to improve the stability and safety of LMBs.

On the bridge hypothesis in the glass transition of freestanding polymer films.

Freestanding thin polymer films with high molecular weights exhibit an anomalous decrease in the glass-transition temperature with film thickness. Specifically, in such materials, the measured glass-transition temperature evolves in an affine way with the film thickness, with a slope that weakly depends on the molecular weight. De Gennes proposed a sliding mechanism as the hypothetical dominant relaxation process in these systems, where stress kinks could propagate in a reptation-like fashion through so-called bridges, i.e. from one free interface to the other along the backbones of polymer macromolecules. Here, by considering the exact statistics of finite-sized random walks within a confined box, we investigate in details the bridge hypothesis. We show that the sliding mechanism cannot reproduce the basic features appearing in the experiments, and we exhibit the fundamental reasons behind such a fact.

A Flexible Solid-State Ionic Polymer Electrolyte for Application in Aluminum Batteries

Rechargeable aluminum batteries are promising candidates for post-lithium energy storage systems. The electrolyte system of rechargeable aluminum batteries is an urgent research topic hindering the deployment in large-scale applications. To solve the critical problems of current ionic liquid electrolytes, such as leakage, corrosivity, and the need for using separators, we developed a freestanding ionic polymer electrolyte using the direct complexation of polyamide 6 (PA6) with aluminum chloride (AlCl3) and the organic salt triethylamine hydrochloride (Et3NHCl). This complexation allows a dissolution of up to 20 wt % of PA6 in the sample to achieve outstanding mechanical strength and stable electrode–electrolyte interfaces without loss of electrolyte properties. The mechanical and electrochemical properties of the polymer were studied and correlated to the complexation mechanism, which was investigated by FTIR spectroscopy. The ionic polymer electrolyte allows the elimination of unfavorable separators and achieves a high ionic conductivity up to 0.3 mS cm–1 and is capable of electrochemical stripping/plating of aluminum. An Al/graphite battery using this electrolyte provides excellent cycling performance. In addition, the nonflowing ionic polymer electrolyte eliminates the corrosion caused by electrolyte leakage in aluminum batteries and enables the use of conventional stainless steel housings. Our results indicate that this type of solid-state ionic polymer is a promising strategy for achieving stable and safe, yet flexible aluminum batteries.