Properties Of Polymer Networks Research Articles

Recent Advances in Environmentally Friendly Dual‐crosslinking Polymer Networks

AbstractEnvironmentally friendly crosslinked polymer networks feature degradable covalent or non‐covalent bonds, with many of them manifesting dynamic characteristics. These attributes enable convenient degradation, facile reprocessibility, and self‐healing capabilities. However, the inherent instability of these crosslinking bonds often compromises the mechanical properties of polymer networks, limiting their practical applications. In this context, environmentally friendly dual‐crosslinking polymer networks (denoted EF‐DCPNs) have emerged as promising alternatives to address this challenge. These materials effectively balance the need for high mechanical properties with the ability to degrade, recycle, and/or self‐heal. Despite their promising potential, investigations into EF‐DCPNs remain in their nascent stages, and several gaps and limitations persist. This Review provides a comprehensive overview of the synthesis, properties, and applications of recent progress in EF‐DCPNs. Firstly, synthetic routes to a rich variety of EF‐DCPNs possessing two distinct types of dynamic bonds (i.e., imine, disulfide, ester, hydrogen bond, coordination bond, and other bonds) are introduced. Subsequently, complex structure‐ and dynamic nature‐dependent mechanical, thermal, and electrical properties of EF‐DCPNs are discussed, followed by their exemplary applications in electronics and biotechnology. Finally, future research directions in this rapidly evolving field are outlined.

Effects of Operating Mechanical Conditions and Polymer Networks of Nematic Elastomers on Photo-Induced Mechanical Performances.

Photoresponsive liquid-crystalline elastomers (LCEs) are promising candidates for light-controlled soft actuators. Photoinduced stress/strain originates from the changes in mechanical properties after light irradiation. However, the correlation between the photoinduced mechanical performance and in-use conditions such as stress/strain states and polymer network properties (such as effective crosslink density and dangling chain density) remains unexplored for practical applications. Here, isometric photo-induced stress or isotonic strain is investigated at different operating strains or stresses, respectively, on LCEs with polymer network variations, produced by different amounts of solvent during polymerization. As the solvent volume increases, the moduli and photoinduced stresses decrease. However, the photo-induced strain, fracture strain, fracture stress, and viscosity increase. The optical response performance initially increases with the operating strain/stress, peaks at a higher actuation strain/stress, and then, decreases depending on the polymer network. The maximum work densities, which also depend on the operating stress, are in the range of ≈200-300 kJm-3. These findings, highlighting the significant variations in the mechanical performance with the operating stress/strain ranges and amount of solvent used in the synthesis, are critical for designing LCE-based mechanical devices.

Effect of penetrant-polymer interactions and shape on the motion of molecular penetrants in dense polymer networks.

The diffusion of dilute molecular penetrants within polymers plays a crucial role in the advancement of material engineering for applications such as coatings and membrane separations. The potential of highly cross-linked polymer networks in these applications stems from their capacity to adjust the size and shape selectivity through subtle changes in network structures. In this paper, we use molecular dynamics simulation to understand the role of penetrant shape (aspect ratios) and its interaction with polymer networks on its diffusivity. We characterize both local penetrant hopping and the long-time diffusive motion for penetrants and consider different aspect ratios and penetrant-network interaction strengths at a variety of cross-link densities and temperatures. The shape affects the coupling of penetrant motion to the cross-link density- and temperature-dependent structural relaxation of networks and also affects the way a penetrant experiences the confinement from the network meshes. The attractive interaction between the penetrant and network primarily affects the former since only the system of dilute limit is of present interest. These results offer fundamental insights into the intricate interplay between penetrant characteristics and polymer network properties and also suggest future directions for manipulating polymer design to enhance the separation efficiency.

Thermal, mechanical, and self-healing properties of polymer networks produced by photo-polymerizing α-cyclodextrin-glycidyl methacrylate adduct and poly(ethylene glycol) methacrylate

Thermal, mechanical, and self-healing properties of polymer networks produced by photo-polymerizing α-cyclodextrin-glycidyl methacrylate adduct and poly(ethylene glycol) methacrylate

Mechanical properties of polymer networks with polyrotaxane crosslinkers with different molecular structures based on polyethylene glycol and α-cyclodextrin

The modification of vinyl groups on polyrotaxane derivatives, obtained with polyethylene glycol as the axial molecule and α-cyclodextrin as the cyclic molecule, allows the production of crosslinkers that react with various monomers, facilitating the formation of polymer networks. In contrast to conventional polymer crosslinked networks using typical crosslinking agents, polymer networks employing polyrotaxane crosslinking agents allow for mobile crosslinking points. Consequently, the resulting polymeric crosslinked network may exhibit increased toughness, as external stress is uniformly distributed throughout the polymeric network. This distribution helps to avoid excessive stress concentration in specific areas. To optimize the mechanical properties of polymer networks formed using polyrotaxane crosslinkers, we investigated the impact of the molecular weight of the axial molecules and the number of crosslinking points of the cyclic molecules in the polyrotaxane crosslinkers on the mechanical properties of the resulting polymer networks. Our results show that the fracture strain, fracture stress, and fracture energy of the resultant polymer networks increase with increasing molecular weight of polyethylene glycol. We also observed that the fracture stress and toughness of the polymer networks increased when there was less than one modified vinyl group on α-cyclodextrin, compared to the case with multiple vinyl groups.

Physico‐mechanical properties of poly(ethylene glycol)‐based polymer networks

AbstractPoly(ethylene glycol) (PEG) based networks have been used extensively for biomedical applications and as solid state electrolytes. In this work, a series of PEG networks was prepared using bifunctional PEG of molar mass 400, 1000, 2000, 4000, and 6000 g mol−1 and star shaped PEG (Mn = 1000 g mol−1) cross‐linker using copper‐catalyzed Huisgen 1,3‐dipolar cycloaddition or “click” chemistry. The end‐group modification of the bifunctional polymers and the star shaped cross‐linker with alkyne and azide groups, respectively, was confirmed by 1H NMR spectroscopy. The coupling reaction between azide and alkyne functionalities for the network formation was confirmed by Fourier transform infrared (FTIR) spectroscopy. The thermal properties of polymer network were determined by differential scanning calorimetry. Swelling studies of the networks were performed to correlate the network structure with the physical properties. The molar mass between the cross‐links was determined using the Bray‐Merill modified Flory‐Rehner equation. The effect of molar mass between the cross‐linking points on the strength of the networks was compared. Additionally, the effect of the stoichiometry of the precursors on the network strength was also studied. Finally, thickness‐dependent tensile testing was performed to investigate the stress oscillation phenomenon.

Reversible Change in Performances of Polymer Networks via Invertible Architecture-Transformation of Cross-Links.

A polymer nanoparticle network using single-chain nanoparticles (SCNPs) as cross-links is designed. The experimental and theoretical study shows that incorporating SCNPs in polymer networks leads to smaller mesh size, faster terminal relaxation time, and reduced fluctuation among cross-links, resulting in a significant increase in shear storage modulus, and enhancement in tensile stress. Notably, the reversible single-chain collapse of SCNPs under thermal stimulation enables the polymer network to undergo coherent changes between two topological states, thereby exhibiting reversible transformations between soft and stiff states. This approach and finding can effectively tailor the mechanical properties of polymer networks, potentially leading to the development of intelligent, responsive materials.

Multifunctional deoxybenzoins: Preparation of low heat release polymer networks by orthogonal crosslinking

Multifunctional epoxide monomers present opportunities to design cross-linked networks with conveniently tunable thermal and mechanical properties. This paper describes the synthesis of a novel, multifunctional deoxybenzoin monomer containing both epoxide and allyl groups, and its use in the preparation of epoxide networks. The orthogonal reactivity of these functional groups, operating via epoxide-amine and thiol-ene chemistry, facilitated the formation of networks with tunable glass transition temperatures and crosslink densities. Thermogravimetric analysis (TGA) and microscale combustion calorimetry (MCC) characterization revealed these networks to possess impressively low heat release properties (∼50% lower than conventional bisphenol A (BPA)-based networks) and very high char residues that reached ∼60%. Overall, introducing functional deoxybenzoin epoxide monomers into these types of network architectures imparts a method for combining low heat release structures with the advantageously tunable physical and mechanical properties of polymer networks.

Connectivity Defects in Metallo-Supramolecular Polymer Networks at Different Self-Sorting Regimes

The properties of polymer networks depend on their connectivity, specifically on the presence of connectivity defects like loops and dangling ends. While chemically crosslinked networks have a frozen distribution of defects on multiple length scales, equivalent transient ones are capable of self-repairing. To study this potential, we employ spatial hindrance as an effective self-sorting mechanism to develop metallo-supramolecular polymer networks based on heteroleptic complexes. We functionalize tetra-arm poly(ethylene glycol) (tetraPEG) chains with the mesitylene-substituted phenanthroline ligand, which is incapable of homoleptic complexation, yet it can form heteroleptic complexes with tetraPEG precursors functionalized with unsubstituted phenanthroline or terpyridine ligands. This way, the formation of primary loops can be avoided upon selective formation of heteroleptic complexes. Nevertheless, the coordination geometry preference of the utilized metal ion can tune the extent of self-sorting and consequently the network structure and defect composition. The latter is pursued via MQ-NMR, and periodic density functional theory simulations are used to visualize the structure and explain its relationship with the dynamics as measured by rheology. Our results demonstrate that Cu(I) can inherently adopt the geometry requirements of both desired heteroleptic complexes by benefiting from the π-stabilization effect, thereby creating a network with the least primary loops and dangling ends.

Modular Synthesis and Patterning of High-Stiffness Networks by Postpolymerization Functionalization with Iron-Catechol Complexes.

Bioinspired iron-catechol cross-links have shown remarkable success in increasing the mechanical properties of polymer networks, in part due to clustering of Fe3+-catechol domains which act as secondary network reinforcing sites. We report a versatile synthetic procedure to prepare modular PEG-acrylate networks with independently tunable covalent bis(acrylate) and supramolecular Fe3+-catechol cross-linking. Initial control of network structure is achieved through radical polymerization and cross-linking, followed by postpolymerization incorporation of catechol units via quantitative active ester chemistry and subsequent complexation with iron salts. By tuning the ratio of each building block, dual cross-linked networks reinforced by clustered iron-catechol domains are prepared and exhibit a wide range of properties (Young's moduli up to ∼245 MPa), well beyond the values achieved through purely covalent cross-linking. This stepwise approach to mixed covalent and metal-ligand cross-linked networks also permits local patterning of PEG-based films through masking techniques forming distinct hard, soft, and gradient regions.

Phantom Chain Simulations for the Fracture of Energy-Minimized Tetra- and Tri-Branched Networks

Effects of branch functionality on mechanical properties of polymer networks are yet to be fully elucidated, although multi-functional approaches have been mainly attempted. For instance, Fujiyabu et al. [Sci. Adv. 2022, 8, abk0010] recently reported that polymer networks made from tri-branch prepolymers exhibit superior mechanical properties to tetra-branch analogues. Although they attribute the difference to stretch-induced crystallization observed in tri-branch poly (ethylene glycol) networks, the mechanism still needs to be clarified. In this study, we performed coarse-grained molecular simulations to extract the effect of branch functionality. The prepolymers were replaced by bead-spring phantom chains, and gelation was simulated by a Brownian dynamics scheme. We subjected the resultant networks to energy minimization and uniaxial stretch by introducing breakage for elongated segments. In the stress–strain relation thus obtained, stress and strain at the break were larger for tri-branch networks than that for tetra-branch analogues, consistent with the experiment. The superiority of tri-branch networks is observed in a wide range of the conversion ratio in gelation, molecular weights of prepolymers, and polymer concentrations. The result implies that the mechanical superiority of tri-branch networks to tetra-branch ones is due to a fundamental structural difference generated during gelation.

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.

The Impact of Polymerization Chemistry on the Mechanical Properties of Poly(dimethylsiloxane) Bottlebrush Elastomers.

We compare the low-strain mechanical properties of bottlebrush elastomers (BBEs) synthesized using ring-opening metathesis and free radical polymerization. Through comparison of experimentally measured elastic moduli and those predicted by an ideal, affine model, we evaluate the efficiency of our networks in forming stress-supporting strands. This comparison allowed us to develop a structural efficiency ratio that facilitates the prediction of mechanical properties relative to polymerization chemistry (e.g., softer BBEs when polymerizing under dilute conditions). This work highlights the impact that polymerization chemistry has on the structural efficiency ratio and the resultant mechanical properties of BBEs with identical side chains, providing another "knob" by which to control polymer network properties.

Derivation of the chain length distribution in crosslinked elastomers from thermoporometric literature data

Thermoporometry appears to be an under-exploited technique to obtain an insight of polymer network properties, and chain length distribution in particular. In this paper, the derivation of a relationship between raw differential scanning calorimetry (DSC) data of a solvent-swollen natural rubber (i.e. basic data for thermoporometry applied to polymers) and chain length distribution is conducted, based on the literature relating first DSC data to pore size distribution. Thanks to these results, a description of the crosslinking process is proposed, highlighting successive stages of network organization. Results are in agreement with experimental mean chain length, and responses observed in mechanical and swelling experiments. The thermoporometry data analysis and the breakdown of crosslinking process into different stages are expected to introduce a rational description of network transformation at different crosslinking degrees.

Surface-Enforced Alignment of Reprogrammable Liquid Crystalline Elastomers.

Liquid crystalline elastomers (LCEs) are stimuli‐responsive materials capable of undergoing large deformations. The thermomechanical response of LCEs is attributable to the coupling of polymer network properties and disruption of order between liquid crystalline mesogens. Complex deformations have been realized in LCEs by either programming the nematic director via surface‐enforced alignment or localized mechanical deformation in materials incorporating dynamic covalent chemistries. Here, the preparation of LCEs via thiol‐Michael addition reaction is reported that are amenable to surface‐enforced alignment. Afforded by the thiol‐Michael addition reaction, dynamic covalent bonds are uniquely incorporated in chemistries subject to surface‐enforce alignment. Accordingly, LCEs prepared with complex director profiles are able to be programmed and reprogrammed by (re)activating the dynamic covalent chemistry to realize distinctive shape transformations.

Impact of Chain Conformation on Structural Heterogeneity in Polymer Network.

Polymer networks generally consist of an ensemble of single chains. However, understanding how chain conformation affects the structure and properties of polymer networks remains a challenge for optimizing their functionality. Here, we present the fabrication and comparative study of a polymer network composed of collapsed self-entangled chains (intrachain entangled network) and a standard polymer network in which random-coil chains are entangled with each other (interchain entangled network). For poly(methyl methacrylate) thin films composed of these networks, we coupled solvent vapor swelling and single-molecule tracking techniques to examine the anomalies in the dynamics of a small-molecular probe included in the system. We demonstrate that when compared to the interchain entangled network the intrachain one exhibits a more substantial structural heterogeneity, particularly under highly crowded conditions. This network also exhibits physical compactness, which keeps the heterogeneous network structure frozen over time and impedes network plasticization through solvent uptake by the film.

Photopolymerizable Ionogel with Healable Properties Based on Dioxaborolane Vitrimer Chemistry.

Ionogels are solid polymer gel networks loaded with ionic liquid (IL) percolating throughout each other, giving rise to ionically conducting solid electrolytes. They combine the mechanical properties of polymer networks with the ionic conductivity, non-volatility, and non-flammability of ILs. In the frame of their applications in electrochemical-based flexible electronics, ionogels are usually subjected to repeated deformation, making them susceptible to damage. It appears critical to devise a simple and effective strategy to improve their durability and lifespan by imparting them with healing ability through vitrimer chemistry. In this work, we report the original in situ synthesis of polythioether (PTE)-based vitrimer ionogels using fast photopolymerization through thiol-acrylate Michael addition. PTE-based vitrimer was prepared with a constant amount of the trithiol crosslinker and varied proportions of static dithiol spacers and dynamic chain extender BDB containing dynamic exchangeable boronic ester groups. The dynamic ionogels were prepared using 50 wt% of either 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide or 1-Ethyl-3-methylimidazolium trifluoromethanesulfonate, both of which were selected for their high ionic conductivity. They are completely amorphous (Tg below −30 °C), suggesting they can be used at low temperatures. They are stretchable with an elongation at break around 60%, soft with Young’s modulus between 0.4 and 0.6 MPa, and they have high ionic conductivities for solid state electrolytes in the order of 10−4 S·cm−1 at room temperature. They display dynamic properties typical of the vitrimer network, such as stress relaxation and healing, retained despite the large quantity of IL. The design concept illustrated in this work further enlarges the library of vitrimer ionogels and could potentially open a new path for the development of more sustainable, flexible electrochemical-based electronics with extended service life through repair or reprocessing.

Mechanical Properties of Homogeneous Polymer Networks Prepared by Star Polymer Synthesis Methods

To investigate the effect of the network structure on the mechanical properties of a polymer network formed from a certain polymer, we should prepare polymer networks with clearly different network...

Thermal and mechanical properties of polymer networks prepared by the thiol-ene reaction of a vanillin/acetone condensate and its related compounds

• Vanillin and benzaldehyde condensates with acetone were cured with a tetrathiol, S4P. • A diallylated vanillin/acetone condensate (DADVAT) was cured with S4P. • Radical-initiated photo/thermal cure and thermal cure using Et 3 N were investigated. • Photo/thermal dual cured DADVAT/S4P 1/1 exhibited excellent physical properties. • The thiol-ene reaction of the enone/thiol groups was promoted by the basic catalyst. A new bio-based diallyl compound, DADVAT, was synthesized by the allylation of 2,5-bis(4-hydroxy-3-methoxybenzylidene)acetone (DVAT), which had been prepared by the cross-aldol condensation of vanillin and acetone. The radical-initiated photothermal thiol–ene reactions of DADVAT/pentaerythritol tetrakis(3-mercaptopropionate) (S4P) at feed molar ratios of 1/1 and 2/1, DVAT/S4P, and trans,trans -dibenzylidene acetone (DBAT)/S4P at a molar ratio of 2/1 produced bio-based polymer networks (ptDADVAT-S4P-11, ptDADVAT-S4P-21, ptDVAT-S4P-21, and ptDBAT-S4P-21). The thermal and mechanical properties of the dual cured polymer networks were compared with those of thermally cured products (tbDADVAT-S4P-21 and tbDBAT-S4P-21) at a molar ratio of 2/1 using triethylamine as a basic catalyst for the nucleophilic thiol–ene reaction. ptDADVAT-S4P-11 displayed excellent thermal and mechanical properties compared to the other cured products. tbDADVAT-S4P-21 and tbDBAT-S4P-21 exhibited better thermal and mechanical properties than ptDADVAT-S4P-21 and ptDBAT-S4P-21, respectively, because the thiol–ene reaction of the enone and thiol groups is promoted by the basic catalyst rather than the radical initiators.

Control Viscoelasticity of Polymer Networks with Crosslinks of Superposed Fast and Slow Dynamics.

Depending on the dynamics of the crosslinks, polymer networks can have distinct bulk mechanical behaviors, from viscous liquids to tough solids. Here, by means of designing a crosslink with variable molecular dynamics, we show the control of viscoelasticity of polymer networks in a broad range quantitatively. The hexanoate-isoquinoline@cucurbit[7]uril (HIQ@CB[7]) crosslink exhibits in a combination of protonated and deprotonated states of similar association affinity but distinct molecular dynamics. The molecular property of this crosslink is contributed by linear combination of the parameters at the two states, which is precisely tuned by pH. Using this crosslink, we achieve the quantitative control of viscoelasticity of quasi-ideal networks in 5 orders of magnitude, and we show the reversible control of mechanical response, such as stiffness, strength and extensibility, of tough random polymer networks. This strategy offers a way to tailor the mechanical properties of polymer networks at the molecular level and paves the way for engineering "smart" responsive materials.