Michael R. Kessler

Waterborne polyurethane dispersions (PUDs) derived from castor oil were synthesized and converted into grafted hybrid latexes through emulsion polymerization of styrene and butyl acrylate. 2‐Hydroxyethyl acrylate (HEA) was incorporated during polyurethane synthesis to introduce pendant vinyl functionality for graft‐mediated hybrid formation. Because castor oil has a relatively low average hydroxyl functionality (≈2.7 per triglyceride), variation of HEA content alters the balance between polyurethane network growth and graft‐site incorporation. Thermomechanical and mechanical behavior were evaluated using tensile testing, Mooney–Rivlin analysis, dynamic mechanical analysis, differential scanning calorimetry, thermogravimetric analysis, and transmission electron microscopy. Moderate incorporation of HEA enhances hybrid integration and improves thermomechanical performance relative to the vinyl‐containing polyurethane. However, at higher HEA concentrations, a compositional threshold is reached at which effective polyurethane strand length is reduced, resulting in measurable changes in particle morphology, cross‐link density, and mechanical behavior. These results demonstrate that competition between castor oil‐derived hydroxyl functionality and HEA‐mediated graft‐site formation governs hybrid network architecture and provides tunable control over grafted hybrid latex performance.

Liquid crystal elastomers (LCEs) are stimuli-responsive materials capable of reversible and programmable shape change in response to an environmental stimulus. Despite the highly responsive nature of these materials, the modest elastic modulus and blocking stress exhibited by these actuating materials can be limiting in some engineering applications. Here, we engineer a semicrystalline LCE, where the incorporation of semicrystallinity in a lightly cross-linked liquid crystalline network yields tough and highly responsive materials. Directed self-assembly can be employed to program director profiles through the thickness of the semicrystalline LCE. In short, we use the alignment of a liquid crystal monomer phase to pattern the anisotropy of a semicrystalline polymer network. Both the semicrystalline-liquid crystalline and liquid crystalline-isotropic phase transition temperatures provide controllable shape transformations. A planarly aligned sample's normalized dimension parallel to the nematic director decreases from 1 at room temperature to 0.42 at 250 °C. The introduction of the semicrystalline nature also enhances the mechanical properties exhibited by the semicrystalline LCE. Semicrystalline LCEs have a storage modulus of 390 MPa at room temperature, and monodomain samples are capable of generating a contractile stress of 2.7 MPa on heating from 25 to 50 °C, far below the nematic to isotropic transition temperature. The robust mechanical properties of this material combined with the high actuation strain can be leveraged for applications such as soft robotics and actuators capable of doing significant work.

Rapid advances in the biomedical field increasingly often demand soft materials that can be processed into complex 3D shapes while being able to reliably bear significant loads. Granular hydrogels have the potential to serve as artificial tissues because they can be 3D printed into complex shapes and their composition can be tuned over short length scales. Unfortunately, granular hydrogels are typically soft such that they cannot be used for load‐bearing applications. To address this shortcoming, individual microgels can be connected through a percolating network, such that they introduce the double network toughening mechanism into granular hydrogels. However, the influence of the microgel size and concentration on the processing and toughness of microgel‐reinforced hydrogels (MRHs) remains to be elucidated. Here, it is demonstrated that processing and toughness depend on the inter‐microgel connectivity, while the stress at break is solely dependent on the microgel size. These findings offer an in‐depth understanding of how liquid‐ and paste‐like precursors containing soft, deformable microgels can be processed into bulk microstructured soft materials and how the size and concentration of these microgels influence the mechanical properties of microgel‐reinforced hydrogels.

Eugenol‐based epoxy monomer (Eu‐EP) is prepared here, followed by curing with a novel eugenol‐derived diamine (Eu‐DA) curing agent for fully eugenol‐based epoxy thermosets. The cure behavior, thermal stability, mechanical properties, and thermomechanical properties of these eugenol‐based epoxy resins are investigated and compared with a partially biobased epoxy thermoset prepared from diglycidyl ether of bisphenol A (DGEBA) and Eu‐DA. Both epoxy materials exhibit storage moduli higher than 2.1 GPa at room temperature. The tensile strength of Eu‐EP/Eu‐DA (21.9 MPa) is comparable to that of the DGEBA/Eu‐DA (24.3 MPa) thermosets and the epoxy thermoset cured from DGEBA and Eu‐DA shows a higher glass transition temperature than those from Eu‐EP and Eu‐DA. The synthesis strategy of Eu‐DA and Eu‐EP shows great potential to be applied in other biobased building blocks with hydroxyl groups. Overall, eugenol is a renewable resource to develop high value‐added chemicals and sustainable polymers with comparable properties to petroleum‐based counterparts.

The development of multi-stimuli-responsive shape memory polymers has received increasing attention because of its scientific and technological significance. In this work, epoxy elastomers with reversible crosslinks are synthesized by polymerizing an anthracene-functionalized epoxy monomer, a diepoxy comonomer, and a dicarboxylic acid curing agent. The synthesized elastomers exhibit active responses to both light and heat enabled by the incorporated anthracene groups. When exposed to 365 nm UV light, additional crosslinking points are created by the photo-induced dimerization of pendant anthracene groups. The formation of the crosslinking points increases modulus and glass transition temperature of the elastomers, allowing for the fixation of a temporary shape at room temperature. The temporary shape remains stable until an external heat stimulus is applied to trigger the scission of the dimerized anthracene, which reduces the modulus and glass transition temperature and allows the elastomers to recover their original shapes. The effects of external stimuli on the thermal and dynamic mechanical properties of the elastomers are investigated experimentally and are correlated with molecular dynamics simulations that reveal the changes of structure and dynamics of the anthracene molecules and flexible chains.

Environmental concerns and the need for sustainable industrial practices have sparked the search for alternative materials that offer a lower impact on the environment. Over the course of the past few decades, numerous bio‐based polymers have been developed from natural resources. Among the materials, bio‐based thermosets are of particular interest due to their increased thermal stability in comparison to thermoplastics. This review covers recent advancements made in the field of bio‐based thermosets, with a special focus devoted to resins prepared from vegetable oils, natural phenolic compounds and a selection of other thermosetting systems developed recently based on either the polymerization of multifunctional monomers or the crosslinking of linear systems. This review article concludes with a critical perspective on the environmental implications of bio‐based thermosets. © 2020 Society of Industrial Chemistry