Varying Crosslinking Densities Research Articles

Network homogeneity on linear and nonlinear viscoelasticity of polyethylene terephthalate vitrimers

Designing network topology with tunable dynamics and rheology behaviors is the key for vitrimers applications. In this study, PET vitrimers with varying crosslinking density and network homogeneity were created by using different Mw of precursors and branching agents (catalysts). Our results show that the dynamic network formed by propane-type branching agent such as 1,3-bis[tris(hydroxymethyl) methylamino] propane (BIS-TRIS propane), is more uniform due to steric hindrance effect compared to 2,2-Bis(hydroxymethyl)-2,2′,2″-nitrilotriethanol (BIS-TRIS). The thermal behavior of PET vitrimers was investigated by non-isothermal experiments, including glass transition and crystallization process. In particular, rheology behavior is sensitive to network structure and bond exchange process. The linear viscoelasticity (LVE) region of PET vitrimers is characterized by constructed pseudo-master curves that are guided by two distinct relaxation mechanisms: Rouse-type relaxation of strands and terminal relaxation of the network. Each relaxation mechanism is associated with a unique activation energy. The mechanical and processing properties of the PET vitrimers are reflected in their plateau modulus and characteristic relaxation time, which varies depending on the network topology. Based on the LVE analysis, we further investigated nonlinear rheology of PET vitrimers under extensional flow. Our results indicate that PET vitrimers exhibit significant strain hardening behavior, but display distinct stretching trends, corresponding to polymer melts' ductility. The ductility of materials decreases with a decrease in precursor's molecular weight (Mw), but improved with enhanced network homogeneity. Therefore, the crosslinking density and network homogeneity of dynamic network can be selectively tailored using appropriate precursors and branching agents to meet various industrial applications.

Modifying pickering polymerized high internal phase emulsion morphology by adjusting particle hydrophilicity

This study investigates the use of submicron polymeric particles with varying crosslinking densities as the sole stabilizer for producing Polymerized High Internal Phase Emulsions (PolyHIPE). We establish a direct correlation between the crosslinking density and the hydrophilicity of the polymer particles. The hydrophilicity of these particles significantly influences the morphology and rheology of HIPEs. These differences manifest as various morphological variations in the resulting PolyHIPE templates. It was discovered that by increasing the crosslinker weight percentage in the particles from 0 % to 100 %, PolyHIPEs with semi-open, open, and closed porous structures can be obtained. Furthermore, non-crosslinked particles were observed to dissolve in the continuous phase, acting as macromolecular surfactants that generate small pores akin to surfactant-stabilized structures in PolyHIPE. These findings offer fresh insights into the relationship between particle localization at the interface, HIPE rheology, and the formation of pore throats in Pickering PolyHIPEs, leading to the creation of either closed or open porous networks. Additionally, interfacial rheological results demonstrate that particles synthesized with varying monomer-to-crosslinker ratios exhibit different interfacial elasticities, which are linked to PolyHIPE morphology.

Polymeric ultralong organic phosphorescence with excellent humidity and temperature resistance via hydrophobic effect

AbstractPolymeric ultralong organic phosphorescence (UOP) with persistent emission is of great importance in practical applications. However, achieving good water‐resistance for long‐term environmental stability is a formidable challenge. In this contribution, through tailoring the alkyl‐chain length of the hardeners and emitters, polymeric UOPs with varying crosslinking density and hydrophobic effect were obtained. Notably, all the polymers show no obvious decrease in UOP emission after high temperature‐humidity test (85°C/85% relative humidity for 7 days). Detailed investigations demonstrate that the rigid covalent crosslinking networks suppress the quenching of triplet excitons while the hydrophobic microenvironment affords good water/moisture‐resistance ability. Moreover, the polymers with superior processability are successfully applied as optical coatings, prepreg, and afterglow displays. With this work, we provide a new strategy to promote the long‐term stability of polymeric UOP materials in high‐temperature‐humidity conditions.

Mechanism of Pressure-Sensitive Adhesion in Nematic Elastomers.

Nematic liquid crystal elastomers (LCEs) have anomalously high vibration damping, and it has been assumed that this is the cause of their anomalously high-pressure-sensitive adhesion (PSA). Here, we investigate the mechanism behind this enhanced PSA by first preparing thin adhesive tapes with LCE of varying cross-linking densities, characterizing their material and surface properties, and then studying the adhesion characteristics with a standard set of 90° peel, lap shear, and probe tack tests. The study confirms that the enhanced PSA is only present in (and due to) the nematic phase of the elastomer, and the strength of bonding takes over 24 h to fully reach its maximum value. Such a long saturation time is caused by the slow relaxation of local stress and director orientation in the nematic domains after pressing against the surface. We confirm this mechanism by showing that freshly pressed and annealed tape reaches the same maximum bonding strength on cooling, when the returning nematic order is forming in its optimal configuration in the pressed film.

Tuning the static and dynamic properties of epoxy vitrimers through modulation of cross-link density

Vitrimers, a polymer class that combines the benefits of thermosetting plastics and thermoplastics, have extensive potential applications in our daily lives and industry. Effectively controlling the dynamic properties of vitrimer materials has received considerable attention. This research prepared a series of dynamic epoxy crosslinking network with varying crosslinking densities by introducing reactive monofunctional small-molecule epoxy monomers through a transesterification reaction. Stress relaxation behavior confirmed the impact of small molecular epoxy on the rearrangement of the transesterification topological network. The findings indicated that decreasing the crosslinking density of the resin resulted in shorter stress relaxation time. Adding small molecular epoxy from 0% to 5% increased the tensile strength of the material from 13.2 MPa to 18.1 MPa. The findings demonstrate that it is efficient to adjust the dynamic properties of glass-like polymers by changing the cross-linking density. Furthermore, the mechanical properties of glass-like polymers can be enhanced by appropriately reducing the cross-linking density, which offers a new method for preparing vitrimer materials with desirable properties.

Bio-Based Polymer Developments from Tall Oil Fatty Acids by Exploiting Michael Addition.

In this study, previously developed acetoacetates of two tall-oil-based and two commercial polyols were used to obtain polymers by the Michael reaction. The development of polymer formulations with varying cross-link density was enabled by different bio-based monomers in combination with different acrylates—bisphenol A ethoxylate diacrylate, trimethylolpropane triacrylate, and pentaerythritol tetraacrylate. New polymer materials are based on the same polyols that are suitable for polyurethanes. The new polymers have qualities comparable to polyurethanes and are obtained without the drawbacks that come with polyurethane extractions, such as the use of hazardous isocyanates or reactions under harsh conditions in the case of non-isocyanate polyurethanes. Dynamic mechanical analysis, differential scanning calorimetry, thermal gravimetric analysis, and universal strength testing equipment were used to investigate the physical and thermal characteristics of the created polymers. Polymers with a wide range of thermal and mechanical properties were obtained (glass transition temperature from 21 to 63 °C; tensile modulus (Young’s) from 8 MPa to 2710 MPa and tensile strength from 4 to 52 MPa). The synthesized polymers are thermally stable up to 300 °C. The suggested method may be used to make two-component polymer foams, coatings, resins, and composite matrices.

Electrical Conductivity of Rubber Composites with Varying Crosslink Density under Cyclic Deformation.

Studies addressing electroconductive composites based on rubber have attracted great interest for many engineering applications. To contribute to obtaining useful materials with reproducible behavior, this study focused on understanding the mechanism of conductivity changes during mechanical deformation for rubber composites based on styrene-butadiene rubber (SBR) or ethylene-propylene-diene terpolymer (EPDM) vulcanized for various times. The composites were characterized by static electrical conductivity, tensile testing, dynamic mechanical thermal analysis (DMTA), and crosslink density measurements. The tensile strength and Young’s modulus were found to increase significantly with rising vulcanization time. Higher static conductivity values of the composites were observed with the increase in vulcanization time. The most important aspect of this investigation consisted in the electrical current measurement online with recording the stress-strain curves, revealing the details of the uniaxial cyclic deformation effect on changes in the structure of conductive pathways indirectly. The electrical conductivity during five runs of repeated cyclic mechanical deformations for SBR composites increased permanently, although not linearly, whereas EPDM composites showed a slight increase or at least a nearly constant current, indicating healing of minor defects in the conductive pathways or the formation of new conductive pathways.

Limitations of Diels–Alder Dynamic Covalent Networks as Thermal Conductivity Switches

Creating polymers that exhibit a switchable thermal conductivity requires a fundamental understanding of how polymer structure and inter- and intramolecular interactions influence thermal transport. To develop this fundamental understanding, we synthesize a series of precursor un-cross-linked statistical copolymers with varying mole fractions of furan functional groups via reversible-addition fragmentation chain transfer (RAFT) polymerization. These precursor polymers, containing the furan diene, were then cross-linked using a reversible Diels–Alder cycloaddition to form dynamic covalent networks of varying cross-link densities. Furthermore, two separate dienophile cross-linkers were used to investigate how the molecular weight and length of the cross-linker influence the mechanical and thermal properties of the network. Counter to some recent theoretical studies, we found that both the elastic modulus and thermal conductivity were largely unaffected by the extent of cross-linking and the structure of the cross-linker. For this system, the Diels–Alder reaction was also found to be slow in the solid state, further limiting its utility as a thermal conductivity switch.

Tuning Superfast Curing Thiol-Norbornene-Functionalized Gelatin Hydrogels for 3D Bioprinting.

Photocurable gelatin-based hydrogels have established themselves as powerful bioinks in tissue engineering due to their excellent biocompatibility, biodegradability, light responsiveness, thermosensitivityand bioprinting properties. While gelatin methacryloyl(GelMA) has been the gold standard for many years, thiol-ene hydrogel systems based on norbornene-functionalized gelatin (GelNB) and a thiolated crosslinker have recently gained increasing importance. In this paper, a highly reproducible water-based synthesis of GelNB is presented, avoiding the use of dimethyl sulfoxide (DMSO) as organic solvent and covering a broad range of degrees of functionalization (DoF: 20% to 97%). Mixing with thiolated gelatin (GelS) results in the superfast curing photoclick hydrogel GelNB/GelS. Its superior properties over GelMA, such as substantially reduced amounts of photoinitiator (0.03%(w/v)), superfast curing (1-2s), higher network homogeneity, post-polymerizationfunctionalization ability, minimal cross-reactivity with cellular components, and improved biocompatibility of hydrogel precursors and degradation products lead to increased survival of primary cells in 3D bioprinting. Post-printing viability analysis revealed excellent survival rates of >84% for GelNB/GelS bioinks of varying crosslinking density, while cell survival for GelMA bioinks is strongly dependent on the DoF. Hence, the semisynthetic and easily accessible GelNB/GelS hydrogel is a highly promising bioink for future medical applications and other light-based biofabrication techniques.

Dynamic multimodal holograms of conjugated organogels via dithering mask lithography.

Polymeric materials have been used to realize optical systems that, through periodic variations of their structural or optical properties, interact with light-generating holographic signals. Complex holographic systems can also be dynamically controlled through exposure to external stimuli, yet they usually contain only a single type of holographic mode. Here, we report a conjugated organogel that reversibly displays three modes of holograms in a single architecture. Using dithering mask lithography, we realized two-dimensional patterns with varying cross-linking densities on a conjugated polydiacetylene. In protic solvents, the organogel contracts anisotropically to develop optical and structural heterogeneities along the third dimension, displaying holograms in the form of three-dimensional full parallax signals, both in fluorescence and bright-field microscopy imaging. In aprotic solvents, these heterogeneities diminish as organogels expand, recovering the two-dimensional periodicity to display a third hologram mode based on iridescent structural colours. Our study presents a next-generation hologram manufacturing method for multilevel encryption technologies.

Biodegradable zwitterionic poly(carboxybetaine) microgel for sustained delivery of antibodies with extended stability and preserved function.

Many recent innovative treatments are based on monoclonal antibodies (mAbs) and other protein therapies. Nevertheless, sustained subcutaneous, oral or pulmonary delivery of such therapeutics is limited by the poor stability, short half-life, and non-specific interactions between the antibody (Ab) and delivery vehicle. Protein stabilizers (osmolytes) such as carboxybetaine can prevent non-specific interactions within proteins. In this work, a biodegradable zwitterionic poly(carboxybetaine), pCB, based microgel covalently crosslinked with tetra(ethylene glycol) diacrylate (TTEGDA) was synthesized for Ab encapsulation. The resulting microgels were characterized via FTIR, diffusion NMR, small-angle neutron scattering (SANS), and cell culture studies. The microgels were found to contain up to 97.5% water content and showed excellent degradability that can be tuned with crosslinking density. Cell compatibility of the microgel was studied by assessing the toxicity and immunogenicity in vitro. Cells exposed to microgel showed complete viability and no pro-inflammatory secretion of interleukin 6 (IL6) or tumor necrosis factor-alpha (TNFα). Microgel was loaded with Immunoglobulin G (as a model Ab), using a post-fabrication loading technique, and Ab sustained release from microgels of varying crosslinking densities was studied. The released Abs (especially from the high crosslinked microgels) proved to be completely active and able to bind with Ab receptors. This study opens a new horizon for scientists to use such a platform for local delivery of Abs to the desired target with minimized non-specific interactions.

Spatially Controlled Permeability and Stiffness in Photopatterned Two-Stage Reactive Polymer Films for Enhanced CO2 Barrier and Mechanical Toughness

Controlling the microstructure of heterogeneous, polymer membranes used in gas barrier and gas separation technologies is challenging. Being able to control composite structures is beneficial to achieve an optimum combination of gas permeation and mechanical performance. In addition, unique properties such as anisotropy and confined transport can be controlled by tailoring the size and position of constituent materials. Two-stage reactive polymer (TSRP) networks are an emerging dual-cure polymer material for spatially varying cross-linking density via photopatterning. In this work a thiol–acrylate-based TSRP was used to investigate the effects of pattern geometry on CO₂ permeability and mechanical properties. Line and square patterns of alternating high and low cross-linking density, with characteristic dimension between 1 mm and 10 μm, were generated in TSRP membranes. Notably, synergistic enhanced barrier properties were observed for 10 μm square patterns of lower cross-linking density (or higher permeability) material exhibiting two confined dimensions compared to line gratings with only one confined dimension.

Shining Light on Poly(ethylene glycol): From Polymer Modification to 3D Laser Printing of Water Erasable Microstructures.

The implementation of stimuli-responsive bonds into 3D network assemblies is a key concept to design adaptive materials that can reshape and degrade. Here, a straightforward but unique photoresist is introduced for the tailored fabrication of poly(ethylene glycol) (PEG) materials that can be readily erased by water, even without the need for acidic or basic additives. Specifically, a new class of photoresist is developed that operates through the backbone crosslinking of PEG when irradiated in the presence of a bivalent triazolinedione. Hence, macroscopic gels are obtained upon visible light-emitting diode irradiation (λ > 515 nm) that are stable in organic media but rapidly degrade upon the addition of water. Photoinduced curing is also applicable to multiphoton laser lithography (λ > 700 nm), hence providing access to 3D printed microstructures that vanish when immersed in water at 37 °C. Materials with varying crosslinking densities are accessed by adapting the applied laser writing power, thereby allowing for tunable hydrolytic erasing timescales. A new platform technology is thus presented that enables the crosslinking and 3D laser printing of PEG-based materials, which can be cleaved and erased in water, and additionally holds potential for the facile modification and backbone degradation of polyether-containing materials in general.

Comprehensive characterization of tense and relaxed quaternary state glutaraldehyde polymerized bovine hemoglobin as a function of cross‐link density

Previously, our lab developed high molecular weight (MW) tense (T) quaternary state glutaraldehyde polymerized bovine hemoglobins (PolybHbs) that exhibited reduced vasoactivity in several small animal models. In this study, we prepared PolybHb in the T and relaxed (R) quaternary state with ultrahigh MW (>500 kDa) with varying cross-link densities, and investigated the effect of MW on key biophysical properties (i.e., O2 affinity, cooperativity (Hill) coefficient, hydrodynamic diameter, polydispersity, polymer composition, viscosity, gaseous ligand-binding kinetics, auto-oxidation, and haptoglobin [Hp]-binding kinetics). To further optimize current PolybHb synthesis and purification protocols, we performed a comprehensive meta-data analysis to evaluate correlations between procedural parameters (i.e., cross-linker:bovine hemoglobin (bHb) molar ratio, gas-liquid exchange time, temperature during sodium dithionite addition, and number of diafiltration cycles) and the biophysical properties of both T- and R-state PolybHbs. Our results showed that, the duration of the fast-step auto-oxidation phase of R-state PolybHb increased with decreasing glutaraldehyde:bHb molar ratio. Additionally, T-state PolybHbs exhibited significantly higher bimolecular rate constants for binding to Hp and unimolecular O2 offloading rate constants compared to R-state PolybHbs. The methemoglobin (metHb) level in the final product was insensitive to the molar ratio of glutaraldehyde to bHb for all PolybHbs. During tangential flow filtration processing of the final product, 14 diafiltration cycles was found to yield the lowest metHb level.

Tuning corneal epithelial cell adhesive strength with varying crosslinker content in silicone hydrogel materials.

PurposeTo quantify the effect of silicone hydrogel crosslink density on the adhesion at corneal epithelial cells/silicone hydrogel contact lens interface.MethodsA custom-built rheometer, referred to as the live cell monolayer rheometer, was used to measure the adhesive strengths between corneal epithelial cell monolayers and silicone hydrogel lens surfaces. The resulting stress relaxations of senofilcon A–derived silicone hydrogel materials with varying crosslinking densities and delefilcon A were tested. Senofilcon A–like materials labeled L1, L2, L3, L4, and L5 contained crosslinker concentrations of 1.2, 1.35, 1.5, 1.65, and 1.8 wt%, respectively. The residual modulus measured from the live cell monolayer rheometer provided a direct indication of adhesive attachment.ResultsWithin the senofilcon-derived series, the adhesive strength shows a surprising minimum with respect to crosslink density. Specifically, L1 (1.20%) has the highest adhesive strength of 39.5 ± 11.2 Pa. The adhesive strength diminishes to a minimum of 11.2 ± 2.1 Pa for L3, whereafter it increases to 14.5 ± 2.5 Pa and 18.1 ± 5.1 Pa for L4 and L5, respectively. The delefilcon A lens exhibits a comparable adhesive strength of 27.8 ± 6.3 Pa to L1.ConclusionsThese results demonstrated that increasing the crosslink density has a nonmonotonic influence on the adherence of lenses to mucin-expressing corneal epithelial cells, which suggests a competition mechanism at the cell/lens interface.Translational RelevanceBecause the adhesiveness of contact lenses to ocular tissues may impact the comfort level for lens wearers and affect ease of removal, this study suggests that lens adhesion can be optimized through the control of crosslink density.

Ion-Conducting Dynamic Solid Polymer Electrolyte Adhesives.

Cross-linked polymer electrolytes containing structurally dynamic disulfide bonds have been synthesized to investigate their combined ion transport and adhesive properties. Dynamic network polymers of varying cross-link densities are synthesized via thiol oxidation of a bisthiol monomer, 2,2'-(ethylenedioxy)diethanethiol, and tetrathiol cross-linker, pentaerythritol tetrakis(3-mercaptopropionate). At optimal loading of lithium bis(trifluoromethane-sulfonyl-imide) (LiTFSI) salt, the ionic conductivities (σ) at 90 °C are about 1 × 10-4 and 1 × 10-5 S/cm at the lowest and highest cross-linking, respectively. Notably, in comparison to the equivalent nondynamic network, the dynamic network shows a positive deviation in σ above 90 °C, which suggests the enhancement of ion transport occurs from the difference in structural relaxation on account of the dissociation of disulfide bonds. Lap shear adhesion and conductivity tests on ITO-coated glass substrates reveal the dynamic network exhibits a higher adhesive shear strength of 0.2 MPa (vs 0.03 MPa for the nondynamic network) and higher σ after the application of external stimulus (UV light or heat). The adhesive strength and σ are stable over multiple debonding/rebonding cycles and, thus, demonstrating the utility of these structurally dynamic networks as solid polymer electrolyte adhesives.

Synthesis and characterization of thin film polyelectrolytes for solid-state lithium microbatteries

A critical challenge associated with the efficient fabrication of solid-state, three-dimensional microbatteries is the synthesis of conformal, solid-state thin film electrolytes. This study presents the synthesis of lithium ion conducting thin film polyelectrolytes based on poly(methacrylic acid-co-ethylene glycol diacrylate) using initiated chemical vapor deposition. These films are transformed to lithium-bearing polyelectrolytes through an H+/Li+ ion exchange reaction in a 1M LiOCH3 solution in methanol. Infrared spectroscopy of the vibrational stretching modes associated with the carbonyl bond in methacrylic acid confirmed that the ion exchange was successful, and quantification by x-ray photoelectron spectroscopy showed that ion exchange was nearly 100% complete. Incorporation of the crosslinking comonomer ethylene glycol diacrylate (EGDA) was necessary for physical stability of the film during the ion exchange process, and four polyelectrolyte compositions with varying crosslinking densities (ethylene glycol diacrylate content) were prepared to understand its role on ionic conductivity. The highest ionic conductivity was achieved in the polymer film with the lowest crosslinking density (9 mol. % EGDA); its conductivity was determined to be 6 × 10−9 S cm−1 at 20 °C and 1 × 10−5 S cm−1 at 100 °C. The activation energy for ionic conductivity was 0.88 eV. Using atomic force microscopy, the surfaces of thin film polyelectrolytes were shown to be continuous, smooth, and pinhole-free, suggesting that these films are suitable for integration into 3D microbatteries.

From Molecular Electrostatic Interactions and Hydrogel Architecture to Macroscopic Underwater Adherence

We investigate the macroscopic adhesion energy (Wa) in pure water between a positively charged hydrogel of varying cross-link densities made from (methacryloyloxyethyl)trimethylammonium chloride an...

Redox-responsive degradable prodrug nanogels for intracellular drug delivery by crosslinking of amine-functionalized poly(N-vinylpyrrolidone) copolymers

HypothesisFacile approaches for the development of new tailored drug carriers are of high importance for the controlled administration of drugs. Herein we report a method for the synthesis of water-soluble reactive copolymers with well-defined architectures for fabrication of redox-sensitive degradable prodrug nanogels for intracellular drug release. ExperimentsPrimary amine-functionalized statistical copolymers were obtained by hydrolysis of poly(N-vinylpyrrolidone-co-N-vinylformamide) copolymers which were synthesized via Reversible Addition–Fragmentation chain-Transfer (RAFT) polymerization. Redox-sensitive degradable nanogels with varying crosslinking densities were synthesized with a redox-sensitive cross-linker. Doxorubicin (DOX) was loaded to form prodrug nanogels (DNG) with hydrodynamic radius from 142 nm to 240 nm. FindingsThe nanogels demonstrated slower degradation and retarded drug release rate with increased crosslinking density in the presence of 10 mM reduced glutathione (GSH) at 37 °C. The in vitro release studies revealed that maximum 85% DOX was released in 24 h under a reductive environment. Intracellular drug release profiles in HeLa cells indicated that the DOX delivery rate was tunable via varying crosslinking density of the nanogels. Cell viability assay demonstrated that the blank nanogels were biocompatible in wide concentrations up to 0.5 mg/mL while the DOX-loaded nanogels displayed medium antitumor activity with IC50 (half-maximal inhibitory concentration) of 1.80 μg/mL, 2.57 μg/mL, 3.01 μg/mL for DNG5, DNG10 and DNG15 respectively.

AB070. 125. Evaluation of a hydrogel scaffold seeded with mature adipocytes for breast reconstruction post-mastectomy

Background: Hydrogels are suitable for adipose tissue engineering for breast regeneration due to their ability to mimic native extracellular matrix. The aim of this study was to assess the suitability of adipose tissue seeded hyaluronic acid (HA) hydrogels at varying crosslinking densities and cell loading densities as adjuncts to implant breast reconstruction post-mastectomy.