Synthesis and Characterization of Model Acrylic-Based Polymer Gels

Materials made from polymer gel networks are important to many everyday applications in health care to building materials. These gel materials can be easily synthesized in various ways, but characterizing the overall material mechanics and properties are challenging due to the soft nature of their respective bulk structures. Model acrylic-based copolymer gels were investigated to understand the fundamental characteristics and mechanical properties from different crosslinking and gelation processes. First, model hydrogels with fracture-healing characteristics similar to materials needed for injectable drug delivery systems were studied using shear rheology to determine timescales of fully healed networks. The industrially available gel was a thermoreversible triblock copolymer composed of poly(methyl methacrylate)-poly(n-butyl acrylate)-poly(methyl methacrylate) in 2-ethyl hexanol to form a physical gel from polymer-solvent interactions. A methodology of quantifying healing was developed from an applied constant shear rate and monitoring the shear stress response of the samples. The maximum shear stress responses observed during fracture and re-fracture after allowing the sample to rest indicated gel healing. Given sufficient time, gel healing was determined to be dependent on testing temperature and polymer volume fraction. The time for fully healed networks was achieved on the order of minutes for the lowest volume fraction of 5 vol. % at temperatures of 28 and 25°C to several hours for the highest volume fraction of 6 vol. % at lower temperatures of 23 and 20°C. Lastly, spherical superabsorbent polymer (SAP) gels with silica nanoparticles (SiO2) were synthesized from inverse suspension polymerization to form chemically crosslinked composite hydrogels of polyacrylamide and poly(acrylic acid). The hydrogels were studied for understanding the interaction of SiO2 nanoparticles within polyelectrolyte networks for use as a chemical admixture for internal curing of high performance concrete. The composite SAP hydrogels were produced with bare or silane-functionalized SiO2 particles to investigate the effects on swelling performance, shape, and cement paste microstructure

Rheo-PIV Investigation of Fracture and Self-Healing in a Triblock Copolymer Gel

Physically associating polymer gels have shown the ability to heal after failure, making them promising candidates for various medical applications or consumer products. However, the processes by which these materials self-heal is not well-understood. This study seeks to explain the self-healing behavior of the triblock copolymer poly(methyl methacrylate)-poly(n-butyl acrylate)-poly(methyl methacrylate), or PMMA-PnBA-PMMA, by probing the material’s post-fracture behavior with rheometry and particle image velocimetry (PIV). The self-healing behavior was studied by deforming each gel in shear until failure multiple times with “recovery” periods in-between. PIV was used to verify the occurrence of each fracture in both time and space. Stress relaxation experiments were also performed on the gels to give greater context to the results of the investigation into fracture recovery. Using these data, it was possible to determine the activation energy required for the network chain dissociation and re-association that transpires during the deformation and self-healing of the gel. Stress relaxation experiments yielded an activation energy of 359 kJ/mole for chain dissociation, while fracture-recovery experiments produced an activation energy of 439 kJ/mole for chain re-association. Building upon these insights could lead to a better understanding of the microscopic mechanisms that govern the behavior of intrinsic self-healing materials so that they can be used to their full potential

Toward Bioinspired Polymer Adhesives: Activation Assisted Via HOBt For Grafting of Dopamine Onto Poly(Acrylic Acid)

The design of bioinspired polymers has long been an area of intense study, however, applications to the design of concrete admixtures for improved materials performance have been relatively unexplored. In this work, we functionalized poly(acrylic acid) (PAA), a simple analogue to polycarboxylate ether admixtures in concrete, with dopamine to form a catechol-bearing polymer (PAA-g-DA). Synthetic routes using hydroxybenzotriazole (HOBt) as an activating agent were examined for their ability in grafting dopamine to the PAA backbone. Previous literature using the traditional coupling reagent 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) to graft dopamine to PAA were found to be inconsistent and the sensitivity of EDC coupling reactions necessitated a search for an alternative. Additionally, HOBt allowed for greater control over per cent functionalization of the backbone, is a simple, robust reaction, and showed potential for scalability. This finding also represents a novel synthetic pathway for amide bond formation between dopamine and PAA. Finally, we performed preliminary adhesion studies of our polymer on rose granite specimens and demonstrated a 56% improvement in the mean adhesion strength over unfunctionalized PAA. These results demonstrate an early study on the potential of PAA-g-DA to be used for improving the bonds within concrete

The Stability of Aliphatic Azo Linkages Influences the Controlled Scission of Degradable Polyurethanes

A better understanding of polymer degradation and post-degradation processes are essential for the development of novel degradable polymers. Herein, we present the synthesis of a new aliphatic azo-containing polyurethane and its degradation behavior toward external stimuli like heat and UV light. A relatively stable radical forming azo-monomer present in the current polyurethane is readily undergoing thermal degradation, whereas the azo-group is less susceptible to optical degradation. A comparison of the stimuli-responsive properties of the new azo-polymer with a previously known, relatively active radical forming monomer incorporated azo-polymer reveals the dependencies of the monomer and radical stability in the controlled degradation process. Our results point toward the importance of radical activity in azo-containing polymers

Investigation of Pre-Reaction and Cure Temperature on Multiscale Dispersion in POSS-Epoxy Nanocomposites

Dispersion of monoamine-functionalized polyhedral oligomeric silsesquioxane (POSS) in an epoxy network was improved by pre-reacting the POSS with excess epoxide and employing a high-temperature cure. DGEBA/DDS networks were formulated with 2.5 and 10 wt% POSS. In some samples, POSS was pre-reacted with DGEBA. The hybrid materials were characterized via SEM, TEM, and DMA. The microscopy and DMA results evinced a multiscale morphology with POSS-rich glassy domains, nano- and microcrystallites, and crystallite agglomerations. For a loading level of 2.5 wt% POSS, the sample with unmodified POSS cured at 125 °C had inorganic crystallites on the order of 1–5 μm and agglomerations on the order of 10–20 μm, whereas the sample with pre-reacted POSS cured at 180 °C had near-perfect dispersion with no agglomerations and very few POSS crystallites. The 10 wt% POSS epoxies also showed improved dispersion with pre-reaction and increasing cure temperature. The dispersion improvements were attributed to the enhanced miscibility of the pre-reacted POSS and the increased rate of POSS reaction into the growing epoxy network at a higher cure temperature

Reversible Hetero-Diels–Alder Amine Hardener As Drop-In Replacement For Healable Epoxy Coatings

We describe a new amine-hardener that enables intrinsic thermally activated healing in epoxy-amine coatings. A hetero-Diels Alder (HDA) adduct bridges biosourced fatty acids and serves as the thermoreversible linker. We validated the thermal reversion of the adduct in solution and the HDA-containing diamine served as a drop-in replacement for commercial hardeners

Diketoenamine‐Based Vitrimers via Thiol‐ene Photopolymerization

Likened to both thermosets and thermoplastics, vitrimers are a unique class of materials that combine remarkable stability, healability, and reprocessability. Herein, this work describes a photopolymerized thiol-ene-based vitrimer that undergoes dynamic covalent exchanges through uncatalyzed transamination of enamines derived from cyclic β-triketones, whereby the low energy barrier for exchange facilitates reprocessing and enables rapid depolymerization. Accordingly, an alkene-functionalized β-triketone, 5,5-dimethyl-2-(pent-4-enoyl)cyclohexane-1,3-dione, is devised which is then reacted with 1,6-diaminohexane in a stoichiometrically imbalanced fashion (≈1:0.85 primary amine:triketone). The resulting networks exhibit subambient glass transition temperature (Tg = 5.66 °C) by differential scanning calorimetry. Using a Maxwell stress-relaxation fit, the topology-freezing temperature (Tv) is calculated to be −32 °C. Small-amplitude oscillatory shear rheological analysis enables to identify a practical critical temperature above which the vitrimer can be successfully reprocessed (Tv,eff). Via the introduction of excess primary amines, this work can readily degrade the networks into monomeric precursors, which are in turn reacted with diamines to regenerate reprocessable networks. Photopolymerization provides unique spatiotemporal control over the network topology, thereby opening the path for further investigation of vitrimer properties. As such, this work expands the toolbox of chemical upcycling of networks and enables their wider implementation