Polymer Network Structure Research Articles

Smart Hydrogel Reactor of Poly(N-isopropylacrylamide)/Polyethylene Glycol Interpenetrating Polymer Networks for Oxidative Coupling of 2-Naphthol

AbstractHydrogels with an interpenetrating polymer network (IPN) structure composed of poly(N-isopropylacrylamide) (poly-NIPAM) gel and a gel containing polyethylene glycol (PEG) chains were synthesized. They showed a typical temperature-responsive volume change in water owing to the constructed poly-NIPAM gel component. Oxidative coupling of 2-naphthol with IPN cryogels and a conventional catalyst, the CuCl2 complex of N,N,N′,N′-tetramethylenediamine, was conducted in water under an O2 atmosphere; the IPN gel prepared from PEG with a larger molecular weight of 11000 afforded a product with a good yield of 73% (91% conv.) during the reaction in basic media. The hydrogel effectively promoted the reaction but hardly produced any product without the catalyst, acting as a reactor vessel in the water. Owing to the low durability of the PEG gel component for hydrolysis, a limitation was also suggested during experiments on the recyclability of the hydrogel.

An interpenetrating polymer networks structure formed by in situ crosslinking of flame retardant for improvement in mechanical properties and flame retardancy of epoxy resins

AbstractEpoxy resins (EPs) possess flammable characteristics inherently, and various methods have been utilized to enhance their flame retardancy. However, the improvement of flame retardancy may reduce its mechanical properties at the same time. Hence, this work synthesized a novel interpenetrating polymer network structure by introducing double bonds into flame retardants based on crosslinking strategy. When the flame retardant was crosslinked inside the EPs, the increased crosslink density improved the mechanical properties and the fire retardancy of the EP thermosets. With the addition of just 3 wt% of a crosslinked flame retardant, its vertical combustion achieved grade V‐0 and had a limited oxygen index of 31.8%. In the cone calorimeter test, the heat release rate was significantly decreased by 56.7% compared with pure EP when the crosslinked flame retardant reached 5 wt%. The study on the pyrolysis behavior of the material and the residual char further revealed that the flame retardant has a biphasic flame‐retardant effect.

Efficient self-repairing high permittivity cyanosilicone dielectric elastomers

Dielectric elastomers (DEs) always suffer from easy damage, cracks, tearing and breakdowns after detrimental events in practical applications. Endowing the DEs materials with self-repairing ability is an effective method to enhance the product reliability and extend the service lifetime. However, the preparation of DEs with ultra-high dielectric permittivity and efficient self-repairing ability is still challenges. Herein, we prepared silicone rubber-based DEs by regulating the carboxyl/amino group ratio and cyano group content in polydimethylsiloxanes and tuning the polymer network structure by introducing the reversible ionically and hydrogen bonds cross-linking. The obtained elastomer materials possess an ultra-high dielectric permittivity (14.38 at 0.1 Hz), low Young's modulus (100–350 kPa), and their mechanical properties can be recovered to over 90% compared to the materials before cutting, exhibit high self-repairing efficiency. Furthermore, the influence mechanism of the network structure on the dielectric properties and efficient self-repairing performance of the silicone elastomers were detailed disclosed. The introduction of polar cyano group enhance the dielectric permittivity and facilitate the self-repairing performance by synergistic effect of the ionic and hydrogen bonds. This work paves the way for preparation of efficient self-repairing dielectric elastomer materials with prominent comprehensive performance.

The preparation of double network hydrogel with high mechanical properties by photopolymerization under the green LED irradiation and enhancement of wet adhesion by tannic acid

Double network hydrogel with excellent mechanical properties is a polymer material composed of interpenetrating polymer network structure. In this study, poly(acrylamide-co-acrylic acid)/sodium alginate (P(AAM-co-AAC)/SA) hydrogels were rapidly prepared by photopolymerization under green LED irradiation. Subsequently, poly(acrylamide-co-acrylic acid)/sodium alginate/tannic acid (P(AAM-co-AAC)/SA/TA) hydrogels were prepared by immersion method. The morphology, mechanical properties, the wet adhension ability and swelling behaviour morphology of P(AAM-co-AAC)/SA/TA hydrogels were studied in detail. The double network hydrogel has excellent mechanical properties, with the elongation at break and tensile strength reaching 862% and 0.77 MPa respectively. The hydrogel with tannic acid showed excellent adhesion, wet adhesion and anti-swelling capability. Our research provides a more efficient and environmentally friendly preparation method for hydrogels used in wet environment.

The influence of crystalline component with varied chain sequence structure and crystallinity on properties of NR based nanocomposites

The influences of polymer network structures on the properties of natural rubber (NR) based products were investigated in this work through blending NR with trans-1,4-poly(butadiene-co-isoprene) (TBIR) copolymers with varied crystallinity. Series of TBIR copolymers with varied chain sequence structure and crystallinity were synthesized. Blending 10 phr TBIR-1 (crystallinity 9.4%) with NR, the vulcanized NR/TBIR-1 composite presented the highest fatigue resistance, reduced rolling resistance and abrasion, reduced tear strength when compared with NR composite. While NR/TBIR-3 (crystallinity 4.1%) vulcanizate presented satisfied dynamic properties, like much lower rolling resistance and abrasion, excellent fatigue resistance and tear strength. The chain sequence structures of TBIR had significant effects on the dynamic properties of NR composites by constructing polymer ternary-network structures. This work is dedicated to understand the key parameters determining the dynamic properties of rubber composites and proposed the ideal polymer network structure for high performance rubber composites.

Module-Assembled Elastomer Showing Large Strain Stiffening Capability and High Stretchability.

Elastomers are indispensable materials due to their flexible, stretchable, and elastic nature. However, the polymer network structure constituting an elastomer is generally inhomogeneous, limiting the performance of the material. Here, a highly stretchable elastomer with unprecedented strain-stiffening capability is developed based on a highly homogeneous network structure enabled by a module assembly strategy. The elastomer is synthesized by efficient end-linking of a star-shaped aliphatic polyester precursor with a narrow molecular-weight distribution. The resulting product shows high strength (≈26MPa) and remarkable stretchability (stretch ratio at break ≈1900%), as well as good fatigue resistance and notch insensitivity. Moreover, it shows extraordinary strain-stiffening capability (>2000-fold increase in the apparent stiffness) that exceeds the performance of any existing soft material. These unique properties are due to strain-induced ordering of the polymer chains in a uniformly stretched network, as revealed by in situ X-ray scattering analyses. The utility of this great strain-stiffening capability is demonstrated by realizing a simple variable stiffness actuator for soft robotics.

Single-Ion Gel Polymer Electrolyte based on In-Situ UV Irradiation Cross-Linked Polyimide Complexed with PEO for Lithium-Ion Batteries.

Lithium-ion batteries (LIBs) have become the research focus of energy storage products. Due to the combination of Li+ and the Lewis basic sites of polymer chains, anions move more than five times faster, which do not participate in the electrode reaction during the discharge cycles, leading to concentration polarization, voltage losses, and high internal resistance. To solve this phenomenon, in this work, a polymer network structure of single-ion polymer electrolyte-based polyimide (DPI-SIGPE) with plasticizer ethylene carbonate (EC)/dimethyl carbonate (DMC) is formed by in-situ cross-linking double bond polyimide, 4-styrene sulfonyl (benzenesulfonyl) imide, and cross-linking agent pentaerythritol tetra(2-thiol acetate) under UV irradiation. By incorporating the anion as a part of the polymer chain, DPI-SIGPE exhibits high lithium-ion conductivity of 2.7 × 10-4 S cm-1 at 30 °C and transference number of 0.87. Typical lithium stripping/plating cycling of 900h demonstrates uniform lithium deposition impacted by DPI-SIGPE. Meanwhile, it has good dimensional thermal stability with no obvious shrinkage at 200 °C for 0.5h and wide electrochemical window of 4.6V. Thus, the polyimide-based cross-linked single-ion gel polymer electrolyte has the promising potential for application in LIBs.

Effect of Electrospinning Network Instead of Polymer Network on the Properties of PDLCs.

In this study, polymer-dispersed liquid crystal (PDLC) membranes were prepared by combining prepolymer, liquid crystal, and nanofiber mesh membranes under UV irradiation. EM, POM, and electro-optic curves were then used to examine the modified polymer network structure and the electro-optical properties of these samples. As a result, the PDLCs with a specific amount of reticular nanofiber films had considerably improved electro-optical characteristics and antiaging capabilities. The advancement of PDLC incorporated with reticulated nanofiber films, which exhibited a faster response time and superior electro-optical properties, would greatly enhance the technological application prospects of PDLC-based smart windows, displays, power storage, and flexible gadgets.

3D printed thermo-responsive electroconductive hydrogel and its application for motion sensor

Stimulus-responsive hydrogels with excellent conductivity have been widely used in electrical, electrochemical, biomedical, and other fields. It is still a challenge to prepare gels with high conductivity. In this paper, poly (N-isopropyl acrylamide) is 3D printed by changing the rheological properties of the printing solution with clay. By forming phytic acid cross-linked polyaniline conductive polymer network in situ on the poly (N-isopropyl acrylamide) matrix, 3D printing of thermally responsive conductive hybrid hydrogels was realized. The interpenetrating polymer network structure provides an electron transport path for hydrogels. The hydrogels have high porosity, strong interaction, high electrical conductivity, high thermal response sensitivity, and significant mechanical enhancement. The results show that the swelling and mechanical properties of the gel are influenced by soaking in different concentrations. The application scenario of the hydrogel was confirmed by a temperature-sensitive switch and finger motion detection.

Electrostatically Complexed Natural Polysaccharides as Aqueous Barrier Coatings for Sustainable and Recyclable Fiber-Based Packaging.

Driven by the ever-growing awareness of sustainability and circular economy, renewable, biodegradable, and recyclable fiber-based packaging materials are emerging as alternatives to fossil-derived, nonbiodegradable single-use plastics for the packaging industry. However, without functional barrier coatings, the water/moisture vulnerability and high permeability of fiber-based packaging significantly restrain its broader application as primary packaging for food, beverages, and drugs. Herein, we develop waterborne complex dispersion barrier coatings consisting of natural, biodegradable polysaccharides (i.e., chitosan and carboxymethyl cellulose) through a scalable, one-pot mechanochemical pathway. By tailoring the electrostatic complexation, the key element to form a highly crosslinked and interpenetrated polymer network structure, we formulate complex dispersion barrier coatings with excellent film-forming property and adaptable solid-viscosity profiles suitable for paperboard and molded pulp substrates. Our complex dispersions enable the formation of a uniform, defect-free, and integrated coating layer, leading to a remarkable oil and grease barrier and efficient water/moisture sensitivity reduction while still exhibiting excellent recyclability profile of the resulting fiber-based substrates. This natural, biorenewable, and repulpable barrier coating is a promising candidate to serve as a sustainable option for fiber-based packaging intended for the food and food service packaging industry.

TiO2-Embedded Biocompatible Hydrogel Production Assisted with Alginate and Polyoxometalate Polyelectrolytes for Photocatalytic Application

The hybrid hydrogel materials meet important social challenges, including the photocatalytic purification of water and bio-medical applications. Here, we demonstrate two scenarios of polyacrylamide-TiO2 (PAAm@TiO2) composite hydrogel design using calcium alginate (Alg-Ca) or Keplerate-type polyoxometalates (POMs) {Mo132} tuning the polymer network structure. Calcium alginate molding allowed us to produce polyacrylamide-based beads with an interpenetrating network filled with TiO2 nanoparticles Alg-Ca@PAAm@TiO2, demonstrating the photocatalytic activity towards the methyl orange dye bleaching. Contrastingly, in the presence of the POM, the biocompatible PAAm@TiO2@Mo132 composite hydrogel was produced through the photo-polymerization approach (under 365 nm UV light) using vitamin B2 as initiator. For both types of the synthesized hydrogels, the thermodynamic compatibility, swelling and photocatalytic behavior were studied. The influence of the hydrogel composition on its structure and the mesh size of its network were evaluated using the Flory–Rehner equation. The proposed synthetic strategies for the composite hydrogel production can be easily scaled up to the industrial manufacturing of the photocatalytic hydrogel beads suitable for the water treatment purposes or the biocompatible hydrogel patch for medical application.

Improved Strength and Heat Distortion Temperature of Emi-Aromatic Polyamide 10T-co-1012 (PA10T/1012)/GO Composites via In Situ Polymerization.

In this paper, an effective method for preparing poly (p-phenylene terephthalamide) -co- poly (dodecanedioyl) decylamine (PA10T/1012)/graphene oxide (GO) composites by pre-dispersion and one-step in situ polymerization was proposed for the first time. During the process of polycondensation, the condensation between the terminal amino groups of PA10T/1012 chains and the oxygen-containing functional groups of GO allowed nylon to be grafted onto graphene sheets. The effects of polymer grafting on the thermal and mechanical properties of (PA10T/1012)/GO composites were studied in detail. Due to the interaction between PA10T/1012 grafted graphene sheets and its matrix, GO is well dispersed in the PA10T/1012 matrix and physically entangled with it, forming a cross-linked network structure of polymer bridged graphene, thus obtaining enhanced tensile strength, tensile modulus and impact strength. More importantly, benefiting from the cross-linked network structure, the heat distortion temperature (HDT) of the composite is greatly increased from 77.3 °C to 144.2 °C. This in situ polycondensation method opens a new avenue to prepare polycondensate graphene-based composites with high strength and high heat distortion temperatures.

Structure and Dynamics of Temperature‐Responsive Microgels and Hydrogels by NMR Spectroscopy, Relaxometry, and Diffusometry

AbstractStimuli‐responsive polymer networks like microgels and hydrogels possess a variety of properties that need to be analyzed for their rational design and successful application in the targeted field of interest. Nanoscale characterization of gels can be achieved by highly selective and sensitive techniques including high‐resolution NMR spectroscopy, relaxometry, and diffusometry. This review is focusing on recent results using 1H and 13C 1D and 2D NMR techniques, which give chemically site‐selective information about the polymer network revealing the interplay between structure and dynamics of polymer networks at the nanoscale level. By that, NMR can allow to obtain information about i) internal structure, ii) stimuli‐induced phase transition, iii) mesh size, and iv) aggregation of microgels and hydrogels. The rational design of responsive microgels and hydrogels with application in drug delivery, cell carrier systems, catalysis, actuators, and as antibacterial agent can be made, if crucial polymer network properties can be correlated with cross‐linking density, type of cross‐link interactions, electrical charges, hydrophobic nano‐structuring, and dispersion concentration.

Ultra‐Stretchable Multicolor Fluorescent Hydrogels Based on Polymer Networks Involving both Strong and Weak Crosslinks

AbstractOwing to their inherent soft wet nature and the capacity to offer tunable emission color/intensity responses, multicolor fluorescent polymeric hydrogels (MFPHs) are attracting tremendous research interests. However, most of the reported MFPHs suffer from poor stretchability, thus restricting their versatile uses in artificial muscles, soft robotics, and so on. In this study, a robust type of lanthanide coordinated MFPHs with an ultra‐stretchability of over 1200% is designed and prepared. It is found that the ultra‐stretchability originates from the unique polymer network structure, in which a small quantity of strong crosslinking interactions (polymer chain entanglement/fixation around the nano‐clay and lanthanide coordination) are utilized to ensure the polymer network integrity, and a large quantity of hierarchical (single, double, and quadruple) hydrogen‐bonding interactions are designed to efficiently dissipate energy. Furthermore, a braiding strategy is proposed to tightly twist several hydrogel stripes into mechanical strong hydrogel ropes that still keep the ultra‐stretchability (>1000%) but can lift heavy weights >350 times their own weight. This study provides new insights into the design of ultra‐stretchable fluorescent hydrogels, and is expected to inspire the future development of multifunctional artificial hydrogel muscles.

Nonaqueous Interfacial Polymerization-Derived Polyphosphazene Films for Sieving or Blocking Hydrogen Gas.

A series of cyclomatrix polyphosphazene films have been prepared by nonaqueous interfacial polymerization (IP) of small aromatic hydroxyl compounds in a potassium hydroxide dimethylsulfoxide solution and hexachlorocyclotriphosphazene in cyclohexane on top of ceramic supports. Via the amount of dissolved potassium hydroxide, the extent of deprotonation of the aromatic hydroxyl compounds can be changed, in turn affecting the molecular structure and permselective properties of the thin polymer networks ranging from hydrogen/oxygen barriers to membranes with persisting hydrogen permselectivities at high temperatures. Barrier films are obtained with a high potassium hydroxide concentration, revealing permeabilities as low as 9.4 × 10-17 cm3 cm cm-2 s-1 Pa-1 for hydrogen and 1.1 × 10-16 cm3 cm cm-2 s-1 Pa-1 for oxygen. For films obtained with a lower concentration of potassium hydroxide, single gas permeation experiments reveal a molecular sieving behavior, with a hydrogen permeance of around 10-8 mol m-2 s-1 Pa-1 and permselectivities of H2/N2 (52.8), H2/CH4 (100), and H2/CO2 (10.1) at 200 °C.

In situ nanofibrillar composite fiber: A model system for understanding the structural evolution of crosslinked nanofibrils

AbstractIn situ formed nanofibrils, which are formed from the deformation of dispersed phase polymer during the shearing/stretching process, offer exciting opportunities for large scale manufacture of materials with high strength and toughness. However, how the topological structure, especially crosslinking network structure of dispersed phase polymer, affects the structural evolution of nano/microfibrils is remaining unclear. Here, the nanofibrillar morphology of nanofibrillar composite fibers is manipulated via controlling the crosslinking network structure of dispersed phase. By changing the length of crosslinker and mol ratio of crosslinkers, the diameter of nanofibrils can be controlled. The rheological property of the crosslinked microspheres and corresponding morphology of in situ formed nanofibrils confirm that the inhomogeneous crosslinking network shows lower storage modulus and consistency coefficient, leading to higher deformation adaptability of crosslinking network and smaller diameter of the nanofibrils. The distribution of nanofibrils along the axial direction of composite fiber greatly improved the tensile strength and toughness of composite fiber to 74 MPa and 719 MJ/m3, which are 45% and 180% increasement compared with the counterpart pure PMMA fiber. The findings in current study provide a new strategy to control the nanofibrillar morphology by increasing the heterogeneity of crosslinking network structure of the dispersed phase polymer.

Proton exchange membrane based on interpenetrating polymer network structure for excellent cell performance and chemical stability

Chemical stability and proton conductivity are the most critical factors for proton exchange membranes (PEMs). Here, we designed and prepared a co-polymer containing radical scavenger groups and imidazole groups. The membrane's acid-base crosslinking and interpenetrating network structure offer the PEM excellent dimensional stability and gas permeation inhibition properties. The continuous acid-base pairs and imidazole groups, which have high water retention, effectively improve the proton conductivity and reduce the PEM's gas permeability. Hence, a membrane electrode assembly based on as-prepared composite membranes exhibits higher single-cell performance and lower impedance than other membranes used for comparison. In addition, the introduction of hindered amine scavenger groups effectively eliminates the free radicals generated in the process of the electrochemical reaction. Both in-situ and ex-situ stability experiments show that the prepared composite membrane has excellent chemical stability. Morphological changes according to degradation level are also systematically analyzed. This paper shows that the introduction of imidazole and HA groups in the PFSA matrix membrane is effective for improving fuel cell performance and lifetime and for constructing continuous proton transport networks and interpenetrating network structures, making this a promising approach for achieving excellent proton conductivity and chemical stability in PEMs.

A physically-based nonlocal strain gradient theory for crosslinked polymers

When the length scale of external stimulations is comparable to most chain lengths within a polymeric solid, the nonlocal and microstructure-dependent strain-gradient effects become significant. The present work proposes a physically-based nonlocal strain gradient theory of polymer networks, where the kernel functions and intrinsic length scales have unambiguous physical meanings. The main contribution lies in the establishment of the general framework that takes in an arbitrary complete set of microscopic descriptions (chain energetics, chain-length distribution, structure of interpenetrating network) and outputs a corresponding nonlocal strain gradient constitutive relation. Based on the hypotheses of interpolatory correlation, homogenization, and superposition, the physically-based construction of constitutive relation is specified to a degree to uniquely determine two linear functionals, which are used to evaluate the nonlocal stress and nonlocal hyper-stress. Application to eight-chain interpenetrating networks yields a specific theory of nonlocal strain gradient elasticity in which the explicit functional form of the kernel is analytically derived from the chain length distribution function. It is shown that the physically derived kernel for the strain gradient field results from the superposition of a spectrum of length scales. The proposed physically-based nonlocal strain gradient theory can be beneficial to clarify the relationship between the microstructure-dependent intrinsic lengths and properties (or responses) of crosslinked polymer network structures.

Circularity in polymers: addressing performance and sustainability challenges using dynamic covalent chemistries.

The circularity of current and future polymeric materials is a major focus of fundamental and applied research, as undesirable end-of-life outcomes and waste accumulation are global problems that impact our society. The recycling or repurposing of thermoplastics and thermosets is an attractive solution to these issues, yet both options are encumbered by poor property retention upon reuse, along with heterogeneities in common waste streams that limit property optimization. Dynamic covalent chemistry, when applied to polymeric materials, enables the targeted design of reversible bonds that can be tailored to specific reprocessing conditions to help address conventional recycling challenges. In this review, we highlight the key features of several dynamic covalent chemistries that can promote closed-loop recyclability and we discuss recent synthetic progress towards incorporating these chemistries into new polymers and existing commodity plastics. Next, we outline how dynamic covalent bonds and polymer network structure influence thermomechanical properties related to application and recyclability, with a focus on predictive physical models that describe network rearrangement. Finally, we examine the potential economic and environmental impacts of dynamic covalent polymeric materials in closed-loop processing using elements derived from techno-economic analysis and life-cycle assessment, including minimum selling prices and greenhouse gas emissions. Throughout each section, we discuss interdisciplinary obstacles that hinder the widespread adoption of dynamic polymers and present opportunities and new directions toward the realization of circularity in polymeric materials.

High Water Resistance and Enhanced Mechanical Properties of Bio-Based Waterborne Polyurethane Enabled by in-situ Construction of Interpenetrating Polymer Network

In this study, acrylic acid was used as a neutralizer to prepare bio-based WPU with an interpenetrating polymer network structure by thermally induced free radical emulsion polymerization. The effects of the content of acrylic acid on the properties of the resulting waterborne polyurethane-poly (acrylic acid) (WPU-PAA) dispersion and the films were systematically investigated. The results showed that the cross-linking density of the interpenetrating network polymers was increased and the interlocking structure of the soft and hard phase dislocations in the molecular segments of the double networks was tailored with increasing the content of acrylic acid, leading to enhancement of the mechanical properties and water resistance of WPU-PAA films. Notably, with the increase in content of acrylic acid, the tensile strength, Young’s modulus, and toughness of the WPU-PAA-110 film increased by 3 times, and 8 times, and 2.4 times compared with WPU-PAA-80, respectively. The WPU-PAA-100 film showed the best water resistance, and the water absorption rate at 96 h was only 3.27%. This work provided a new design scheme for constructing bio-based WPU materials with excellent properties.