Cross-linked Polymeric Materials Research Articles

Relative Humidity and Temperature Sensing Characteristics of an Optical Fiber Probe Based on Hollow-Core Fiber and Calcium Alginate Hydrogel Film

Hydrogel is a kind of functional cross-linked polymer material with 3-D network structure. When hydrogel is exposed to water environment, a mass of hydrophobic groups and hydrophilic residues existing in the cross linked netlike nano-structure is willing to combine with water molecules and expand, respectively. In this paper, a novel optical fiber probe based on hollow-core fiber and calcium alginate (CaAlg) hydrogel film is proposed and experimentally demonstrated. The experimental results of relative humidity (RH) and temperature sensing characteristics show that the proposed probe has a binomial response to RH in the range of 20%~90% RH and a linear response to temperature with the sensitivity of 121pm/°C. CaAlg is considered as an effective and potential as the humidity-sensitive material to optical fiber RH sensors.

Eco-friendly cross-linked polymeric dielectric material based on natural tannic acid

Until now, development of conventional electronic devices has mainly focused on performance, and e-waste disposal has been neglected. To alleviate e-waste disposal problems, the use of naturally degradable materials as electronic components has been studied. While most research on such materials has focused on the substrate, other components such as dielectric insulators have not been widely studied. Here, we developed a poly-methacrylated tannic acid (PMTA) naturally degradable dielectric material with good electrical breakdown strength (5.4 MV cm−1), good long-term stability in ambient, and a stable surface. Pentacene thin-film transistors (TFTs) with a PMTA dielectric insulator layer showed reasonable field effect mobility of 0.2 cm2 V−1 s−1 at −20 V of gate voltage (Vg). In addition, the pentacene TFTs with a PMTA dielectric insulator layer and a polyvinyl alcohol substrate show a good degradability in natural; the TFTs were fully decomposed within 8 days in phosphate buffered saline (PBS buffer solution). Also, the TFTs with a PMTA dielectric insulator layer were fully decomposed within 19 days in 3.5 wt% sodium chloride solution, similar to seawater, which has a good potential for reduction of e-waste.

Dynamic Cross-Linking of Polyethylene via Sextuple Hydrogen Bonding Array

Multiple hydrogen bonding motifs are promising tools for polymer functionalization to obtain adaptable networks combining advantages of permanently cross-linked systems with processability of thermoplastics. Here we describe the use of a new multiple hydrogen bonding motif to impart increased tensile strength, stiffness, barrier properties, and a plateau modulus after melting to functional polyolefins, while retaining adaptability of the polymer network. The cross-linked nature of these polymers was elucidated by thermal and mechanical analysis, revealing a raised glass transition and rheology similar to permanently cross-linked polymer matrices. The apolar polymer matrix was found to stabilize the new hydrogen bonding motif at elevated temperatures. The resulting polymer showed thermal resistance superior to ureidopyrimidone (UPy) motif functionalized materials, the most commonly employed synthetic multiple hydrogen bonding motif to date.

β-Cyclodextrin-containing pseudorotaxanes as building blocks for cross-linked polymers

Rotaxanes and pseudorotaxanes are the supramolecular objects that attract much attention due to their low toxicity, sliding, dethreading and easy modification. Thus, polyrotaxanes and polypseudorotaxanes can be considered as components in drug delivery systems, sensor devices, implants, contrasting agents, fluorescent probes in other diagnostic systems. Therefore, we have prepared the pseudorotaxanes based on the β-cyclodextrin (β-CD) and molecule-“guest”—polyoxypropylenedimethacrylate (POPDMA) as carrier of end-capped methacrylate groups. The presence of such groups allows pseudorotaxane to be co-polymerized with acrylamide and methylene-bis-acrylamide and to develop cross-linked polymer matrices, which implies their further investigation as systems for a drug release. The structure of these substances was confirmed by FTIR- and NMR-spectroscopy, differential scanning calorimetry (DSC), X-ray analysis [wide-angle X-ray scattering (WAXS)], pyrolysis mass spectrometry. The ratio of POPDMA to β-CD was found to be 1:3, according to NMR data. The interactions between β-CD and POPDMA in the aqueous solution and in the dry mechanical mixture are entirely different, due to formation of pseudorotaxane. The results obtained by pyrolysis mass-spectrometry, WAXS and DSC well correlate with mechanism of formation of inclusion complexes, involving a linear molecule and cyclodextrins as described earlier.

Fluorinated Vitrimer Elastomers with a Dual Temperature Response.

Vitrimers are an emerging new class of permanently cross-linked polymeric materials that show a liquid behavior upon heating wherein the macroscopic deformation is controlled by the rate of internal chemical bond exchange reactions. Thus, quite uniquely among polymeric materials, flow rates and material viscosities can be enhanced or controlled by the addition of catalysts and additives. We now report a catalyst-free vitrimer system, prepared from mixing two simple components, wherein two competing bond exchange mechanisms coexist, each showing a strikingly different temperature dependence, related to the large difference in activation energy for the different exchange pathways (60 vs 130-170 kJ/mol). The low barrier process is predominant at lower temperatures, but is outcompeted by the high barrier process that becomes dominant at higher temperatures because of its much more pronounced temperature dependence. The result is an interesting and highly unusual dual viscosity profile for this new class of vitrimer materials: a very gradual decrease in viscosity at lower temperatures, intercepted by a much sharper drop in viscosity at higher temperatures. The highly counterintuitive effect where a higher barrier pathway is dominant over a much lower barrier process can be rationalized by the exchange mechanisms that involve different reactive species, but lead to the overall same exchange. We observed this unusual but highly promising behavior first for fluorinated vitrimer elastomers, aimed at high performance materials, but the effect was also shown to hold in related nonfluorinated elastomers. A new way to control and design the rheological behavior of vitrimers toward finely tuned and precisely controlled processing applications has thus been provided.

Cyclodextrin-Based Polymer-Supported Bacterium for the Adsorption and in-situ Biodegradation of Phenolic Compounds.

Dual function polymer materials with immobilized Sphingobium Chorophenolicum (SpC) bacterium cells are reported herein that undergo tandem adsorption and biodegradation of phenolic compounds. The cross-linked polymer materials contain β-cyclodextrin (β-CD) with incremental hexamethylene diisocyanate (HDI) cross-linker at variable mole ratios (X = 1, 3, or 6), denoted as HDI-X systems. The adsorptive uptake properties of the insoluble HDI-X polymers (X = 3 and 6) with various phenolic compounds [pentachlorophenol (PCP), 2,4,6-trichlorophenol (TCP), and 2,4,6-trimethylphenol (TMP)] were studied using batch adsorption isotherms. The molecular selective phenol removal (SR) capacity of the HDI-3 and HDI-6 materials was evaluated by electrospray ionization mass spectrometry (ESI-MS). The results were compared against granular activated carbon (GAC) and native β-CD, where 1D/2D 1H NMR spectral characterization of the complexes formed between phenolic guests and a soluble polymer (HDI-1) in aqueous solution provide insight on the intermolecular interactions and the role of cross-linking effects. Immobilization of SpC onto HDI-3 was shown to form a composite polymer/bacterium material. The composite system displays synergistic removal effects due to tandem PCP adsorption and SpC biodegradation to yield by-products such as 2,6-dichloro-1,4-hydroquinone (DCHQ). Apoptosis and cytotoxicity of DCHQ were evaluated using three breast cancer cell lines.

Approaches to Sustainable and Continually Recyclable Cross-Linked Polymers

Cross-linked polymers are ubiquitous in daily life, finding applications as tires, insulation, adhesives, automotive parts, and countless other products. The covalent cross-links in these materials render them mechanically robust, chemically resistant, and thermally stable, but they also prevent recycling of these materials into similar-value goods. Furthermore, cross-linked polymers are typically produced from petroleum-based feedstocks, and their hydrocarbon backbones render them nondegradable, making them unsustainable in the long term. In recent years, much effort has focused on the development of recycling strategies for cross-linked polymeric materials. In the following Perspective, we discuss many of these approaches, and highlight efforts to sustainably produce recyclable cross-linked polymers. We present our thoughts on future challenges that must be overcome to enable widespread, viable, and more sustainable and practical implementation of these materials, including the sustainable sourcing of f...

Monolithic 3D Cross-Linked Polymeric Graphene Materials and the Likes: Preparation and Their Redox Catalytic Applications

While there have been tremendous studies about graphene and its applications in the past decade, so far the proposed huge potential of this material has not been materialized. One of the prerequisites to overcome these challenges is maintaining the nature and intrinsic properties of individual graphene sheets while in the state of bulk material. Thus, in this Perspective contribution, the fabrication/synthesis of the monolithic polymeric and three-dimensional (3D) cross-linked bulk materials (3DGraphene) with (doped) 2D graphene sheets as the building block will first be briefly summarized. Then, the second part will cover the redox catalytic application of these bulk materials including doped 3DGraphene, the graphene-like material polymeric C3N4 and their hybrid materials. These will include mainly oxygen reduction reactions (ORR), hydrogen evolution reactions (HER), and CO2 reduction for their latest development. Finally, challenges and outlook related to the design of 3D cross-linked graphene based materials and their catalytic applications will be briefly discussed.

Dynamic and Static Mechanical Properties of Crosslinked Polymer Matrices: Multiscale Simulations and Experiments.

We studied the static and dynamic mechanical properties of crosslinked polymer matrices using multiscale simulations and experiments. We continued to develop the multiscale methodology for generating atomistic polymer networks, and applied it to the case of phthalonitrile resin. The mechanical properties of the resulting networks were analyzed using atomistic molecular dynamics (MD) and dissipative particle dynamics (DPD). The Young’s and storage moduli increased with conversion, due both to the appearance of a network of covalent bonds, and to freezing of degrees of freedom and lowering of the glass transition temperature during crosslinking. The simulations’ data showed good quantitative agreement with experimental dynamic mechanical analysis measurements at temperatures below the glass transition. The data obtained in MD and DPD simulations at elevated temperatures were conformable. This makes it possible to use the suggested approach for the prediction of mechanical properties of a broad range of polymer matrices, including ones with high structural heterogeneity.

A Self-Healable High Glass Transition Temperature Bioepoxy Material Based on Vitrimer Chemistry

The design of high glass transition temperature (Tg) thermoset materials with considerable reparability is a challenge. In this study, a novel biobased triepoxy (TEP) is synthesized and cured with an anhydride monomer in the presence of zinc catalyst. The cured TEP exhibits a high Tg (187 °C) and comparable strength and modulus to the cured bisphenol A epoxy. By adopting the vitrimer chemistry, the cross-linked polymer materials are imparted significant stress relaxation and reparability via dynamic transesterification. It is noted that the reparability is closely related to the repairing temperature, external force, catalyst content, and the magnitude of rubbery modulus of the sample. The width of the crack from the cured TEP can be efficiently repaired within 10 min. This work introduces the first high-Tg biobased epoxy material with excellent reparability and provides a valuable method for the design of high-Tg self-healing materials suitable for high service temperature.

(Plenary) Slide-Ring Materials: Novel Molecular Concept for SHINAYAKA Polymers

We have recently developed a novel type of polymer network called slide-ring materials by cross-linking polyrotaxane, the supramolecular architecture with topological characteristics. In the network, polymer chains are topologically interlocked by figure-of-eight cross-links. Hence, these cross-links can pass along the polymer chains freely to equalize the tension of the threading polymer chains similarly to pulleys. The structure and physical properties of the polymeric materials are drastically different from conventional cross-linked or noncross-linked materials. For instance, the slide-ring gel or elastomer shows quite small Young’s modulus, which is not proportional to the cross-linking density and much lower than those of chemical gels with the same density. This arises from the difference in the molecular mechanism of the entropic elasticity: While the conformational entropy is mainly responsible for the elasticity in usual chemical gels or rubbers, the mechanical properties of the slide-ring gel should be inherently governed by the alignment entropy of free cyclic molecules in polyrotaxane as well as the conformational entropy of backbone polymer. This means that the softness in the slide-ring gel is due to the novel entropic elasticity, which is also expected to yield sliding state and sliding transition. Consequently, the slide-ring materials have the dynamic coupling between two kinds of entropy: the conformational entropy of strings and alignment entropy of rings. 2,3) The concept of the slide-ring gel is not limited to cross-linked gels but also includes elastomer, cross-linked polymeric materials without solvent. Accordingly it can be applied to wide area such as soft contact lens, paints, rubbers, soft actuator and so on. As a typical example, the scratch-resist properties of the self-restoring slide-ring elastomer were adopted into the top coating on the mobile phone.4) Finally, I would like to introduce ImPACT (Impulsing Paradigm Change through Disruptive Technologies) program, which is promoting to create flexible and tough polymers by using the slide-ring materials as a key technology.

Self-Healing Phase Change Salogels with Tunable Gelation Temperature.

Chemically cross-linked polymer matrices have demonstrated strong potential for shape stabilization of molten phase change materials (PCM). However, they are not designed to be fillable and removable from a heat exchange module for an easy replacement with new PCM matrices and lack self-healing capability. Here, a new category of shapeable, self-healing gels, "salogels", is introduced. The salogels reversibly disassemble in a high-salinity environment of a fluid inorganic PCM [lithium nitrate trihydrate (LNH)], at a preprogrammed temperature. LNH was employed as a high latent heat PCM and simultaneously as a solvent, which supported the formation of a network of polyvinyl alcohol (PVA) chains via physical cross-linking through poly(amidoamine) dendrimers of various generations. The existence of hydrogen bonding and the importance of low-hydration state of PVA for the efficient gelation were experimentally confirmed. The thermal behavior of PCM salogels was highly reversible and repeatable during multiple heating/cooling cycles. Importantly, the gel-sol transition temperature could be precisely controlled within a range of temperature above LNH's melting point by the choice of dendrimer generation and their concentration. Shape stabilization and self-healing properties of the salogels, taken together with tunability of their temperature-induced fluidization make these materials attractive for thermal energy storage applications that require on-demand removal and replacement of used inorganic PCM salt hydrates.

Quadruple H-Bonding Cross-Linked Supramolecular Polymeric Materials as Substrates for Stretchable, Antitearing, and Self-Healable Thin Film Electrodes.

Herein, we report a de novo chemical design of supramolecular polymer materials (SPMs-1-3) by condensation polymerization, consisting of (i) soft polymeric chains (polytetramethylene glycol and tetraethylene glycol) and (ii) strong and reversible quadruple H-bonding cross-linkers (from 0 to 30 mol %). The former contributes to the formation of the soft domain of the SPMs, and the latter furnishes the SPMs with desirable mechanical properties, thereby producing soft, stretchable, yet tough elastomers. The resulting SPM-2 was observed to be highly stretchable (up to 17 000% strain), tough (fracture energy ∼30 000 J/m2), and self-healing, which are highly desirable properties and are superior to previously reported elastomers and tough hydrogels. Furthermore, a gold, thin film electrode deposited on this SPM substrate retains its conductivity and combines high stretchability (∼400%), fracture/notch insensitivity, self-healing, and good interfacial adhesion with the gold film. Again, these properties are all highly complementary to commonly used polydimethylsiloxane-based thin film metal electrodes. Last, we proceed to demonstrate the practical utility of our fabricated electrode via both in vivo and in vitro measurements of electromyography signals. This fundamental understanding obtained from the investigation of these SPMs will facilitate the progress of intelligent soft materials and flexible electronics.

Vinylogous Urea Vitrimers and Their Application in Fiber Reinforced Composites

Vitrimers are covalently cross-linked polymeric materials that can be thermally processed in a liquid state without losing their network integrity. In this work, novel vitrimers based on the dynamic amine exchange reaction of vinylogous urea moieties are introduced. Following a systematic exploration of different vinylogous acyl compounds (urethanes, amides, and urea), vinylogous urea clearly emerges as showing the fastest intrinsic exchange kinetics. The combination of these networks with a simple acid catalyst (0.5 mol % pTsOH) resulted in materials with good mechanical properties (Tg ∼ 110 °C, E ∼ 2.2 GPa) and remarkably short relaxation times above Tg, in the order of a few seconds when heated. This attractive combination of properties made it possible to prepare vinylogous urea based composites as enduring prepregs. We demonstrate that the dynamic material properties give fully cured composites that still allow an efficient thermal fusion of multiple layers as well as thermoforming. Finally, we also ...

Chemical treatment for dissolution of amine-cured epoxies at atmospheric pressure

Carbon fiber/epoxy composites have largely resisted attempts to recycle because the crosslinked polymer matrices cannot be easily separated from the fiber reinforcements. In this study, two chemical treatment methods - depolymerization and acid digestion (both at atmospheric pressure) - were employed to dissolve amine-cured epoxy formulations. Both depolymerization and acid digestion were shown to be effective dissolution processes for all amine/epoxy samples that encompassed variations in amine/epoxy stoichiometric ratio (A/E ratio), epoxy monomer functionality, and amine curing agent type. The relationship between epoxy properties and dissolution rate was determined, and the key parameters affecting thermoset matrix dissolution were identified. The dissolution rate was controlled by both the chemical reaction and diffusion rates. The components of the chemical solutions after epoxy dissolution were analyzed and identified, and protocols to quantitatively track the products after dissolution were developed. The two major cleavable sites during epoxy dissolution were the C-N and C-O bonds, and the aromatic structures of the epoxies were preserved.

Ionic liquid compatibility in polyethylene oxide/siloxane ion gel membranes

Ion gel films were prepared by incorporating eight commercially available ionic liquids in two different cross-linked polymer matrices to evaluate their phase miscibility, gas permeability and ionic conductivity for potential applications as gas separation membranes and solid electrolyte materials. The ionic liquids cations were 1-ethyl-3-methylimidazolium, 1-ethyl-3-methylpyridinium, 1-butyl-1-methylpyrrolidinium, tributylmethylphosphonium, and butyltrimethylammonium with a common anion (bis(trifluoromethylsulfonyl)imide). In addition, ionic liquids with 1-ethyl-3-methylimidazolium cation with acetate, dicyanamide and tetrafluoroborate counterions were evaluated. The two polymers were cross-linked poly(ethylene oxide) and cross-linked poly(ethylene oxide)/siloxane copolymer. Differential scanning calorimetry, X-ray diffractometry and visual observations were performed to evaluate the ion gels’ miscibility, thermal stability and homogeneity. Ionic liquids with the least basic anion (bis(trifluoromethylsulfonyl)imide) and aromatic cations containing acidic proton (e.g. imidazolium and pyridinium) gave the most stable and miscible ion gels. Phase stability was shown to be a function of both ionic liquid content and temperature, with phase separation observed at elevated temperatures. Gas permeability testing with carbon dioxide and nitrogen and ionic conductivity measurements confirmed that these ionic liquids increased the gas permeability and ionic conductivity of the polymers.

Movable Cross-Linked Polymeric Materials from Bulk Polymerization of Reactive Polyrotaxane Cross-Linker with Acrylate Monomers

Topological cross-linked polymers attract much attention from their unique mechanical properties derived from their cross-linking structure. Here, we have fabricated a polymeric material (Acryl-AcO-PRx) incorporating topological cross-links into various universal acrylate polymers by using a modified polyrotaxane as a polymerizable cross-linking agent. Acryl-AcO-PRx obtained by a simple photocuring technique under bulk conditions showed a distinctly higher extensibility compared with the chemical cross-linked polymers. In addition, Acryl-AcO-PRx materials showed high stress relaxation and deformation hysteresis. These results indicate that the topologically cross-linked structure composed of polyrotaxane plays an important role in mechanical properties even in bulk state.

Interrogating Charge Storage on Redox Active Colloids via Combined Raman Spectroscopy and Scanning Electrochemical Microscopy.

Redox active colloids (RACs) are dispersible, cross-linked polymeric materials that incorporate a high concentration of redox-active motifs, making them attractive for next-generation size-exclusion redox flow batteries. In order to tap into their full potential for energy storage, it is essential to understand their internal charge mobility, capacity, and cyclability. Here we focus on using a combined suite of Raman spectroscopy and scanning electrochemical microscopy (SECM) tools for evaluating three important parameters that govern charge storage in viologen-RACs: their intraparticle redox active concentration, their reduction/oxidation mechanism, and their charge transfer rate. We addressed RACs using SECM imaging and single-particle experiments, from which the intraparticle diffusion and concentration parameters were elucidated. By using Raman spectroscopy coupled to surface interrogation SECM, we further evaluated their reversible redox properties within monolayer films of 80- and 135-nm-sized RACs. Most notably we have confirmed that the concentration and redox mechanisms are essentially unchanged when varying the RAC size. As expected, we see that larger particles inherently require longer times for electrolysis independent of the methodology used for their study. Our simulations further verify the internal concentration of RACs and suggest that their porosity enables solution redox active mediators to penetrate and titrate charge in their interior. The combined methodology presented here sets an important analytical precedent in decoupling the charge storage properties of new bulk materials for polymer batteries starting from probing low-dimensional assemblies and single particles using nano- and spectroelectrochemical approaches.

Spectroscopic Studies of the Interactions between a Cationic Cyanine Dye and a Synthetic Phyllosilicate: From Photophysics to Hybrid Materials.

The interaction of the cationic organic dye Astrazon orange R (AO-R) with the synthetic phyllosilicate Laponite leads to very interesting hybrid materials. Indeed, the Laponite nanoparticles modify the photophysical properties of AO-R, inducing a stabilization of its excited emissive state by preventing ultrafast isomerization. The long-lived emissive clay-dye hybrid complex can be used to develop efficient photoinitiating systems, leading to organic-inorganic hybrid crosslinked polymer materials.

Semibatch monomer addition as a general method to tune and enhance the mechanics of polymer networks via loop-defect control

Controlling the molecular structure of amorphous cross-linked polymeric materials is a longstanding challenge. Herein, we disclose a general strategy for precise tuning of loop defects in covalent polymer gel networks. This "loop control" is achieved through a simple semibatch monomer addition protocol that can be applied to a broad range of network-forming reactions. By controlling loop defects, we demonstrate that with the same set of material precursors it is possible to tune and in several cases substantially improve network connectivity and mechanical properties (e.g., ∼600% increase in shear storage modulus). We believe that the concept of loop control via continuous reagent addition could find broad application in the synthesis of academically and industrially important cross-linked polymeric materials, such as resins and gels.