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1. Highly stretchable ionically crosslinked acrylate elastomers inspired by polyelectrolyte complexes

   https://doi.org/10.1039/D2SM00755J  

   Cai, Hongyi ; Wang, Zhongtong ; Utomo, Nyalaliska W. ; Vidavsky, Yuval ; Silberstein, Meredith N. ( October 2022 , Soft Matter)

   Dynamic bonds are a powerful approach to tailor the mechanical properties of elastomers and introduce shape-memory, self-healing, and recyclability. Among the library of dynamic crosslinks, electrostatic interactions among oppositely charged ions have been shown to enable tough and resilient elastomers and hydrogels. In this work, we investigate the mechanical properties of ionically crosslinked ethyl acrylate-based elastomers assembled from oppositely charged copolymers. Using both infrared and Raman spectroscopy, we confirm that ionic interactions are established among polymer chains. We find that the glass transition temperature of the complex is in between the two individual copolymers, while the complex demonstrates higher stiffness and more recovery, indicating that ionic bonds can strengthen and enhance recovery of these elastomers. We compare cycles to increasing strain levels at different strain rates, and hypothesize that at fast strain rates ionic bonds dynamically break and reform while entanglements do not have time to slip, and at slow strain rates ionic interactions are disrupted and these entanglements slip significantly. Further, we show that a higher ionic to neutral monomer ratio can increase the stiffness, but its effect on recovery is minimal. Finally, taking advantage of the versatility of acrylates, ethyl acrylate is replaced with the more hydrophilic 2-hydroxyethylmore »acrylate, and the latter is shown to exhibit better recovery and self-healing at a cost of stiffness and strength. The design principles uncovered for these easy-to-manufacture polyelectrolyte complex-inspired bulk materials can be broadly applied to tailor elastomer stiffness, strength, inelastic recovery, and self-healing for various applications.« less
2. Photoinduced SET to access olefin-acrylate copolymers

   https://doi.org/10.1039/d1py01643a  

   Garrison, John B. ; Hughes, Rhys W. ; Young, James B. ; Sumerlin, Brent S. ( February 2022 , Polymer Chemistry)

   The advantageous material properties that arise from combining non-polar olefin monomers with activated vinyl monomers have led to considerable progress in the development of viable copolymerization strategies. However, unfavorable reactivity ratios during radical copolymerization of the two result in low levels of olefin incorporation, and an abundance of deleterious side reactions arise when attempting to incorporate many polar vinyl monomers via the coordination–insertion pathway typically applied to olefins. We reasoned that design of an activated monomer that is not only well-suited for radical copolymerization with polar vinyl monomers ( e.g. , acrylates) but is also capable of undergoing post-polymerization modification to unveil an olefin repeat unit would allow for the preparation of statistical olefin-acrylate copolymers. Herein, we report monomers fitting these criteria and introduce a post-polymerization modification strategy based on single-electron transfer (SET)-induced decarboxylative radical generation directly on the polymer backbone. Specifically, SET from an organic photocatalyst (eosin Y) to a polymer containing redox-active phthalimide ester units under green light leads to the generation of reactive carbon-centered radicals on the polymer backbone. We utilized this approach to generate statistical olefin-acrylate copolymers by performing the decarboxylation in the presence of a hydrogen atom donor such that the backbone radical is cappedmore »by a hydrogen atom to yield an ethylene or propylene repeat unit. This method allows for the preparation of copolymers with previously inaccessible comonomer distributions and demonstrates the promise of applying SET-based transformations to address long-standing challenges in polymer chemistry.« less
3. Nanostructure and Fracture Behavior of Carbon Nanofiber-Reinforced Cement Using Nanoscale Depth-Sensing Methods

   https://doi.org/10.3390/ma13173837  

   Akono, Ange-Therese ( September 2020 , Materials)

   In recent years, carbon nanofibers have been investigated as a suitable reinforcement for cementitious composites to yield novel multifunctional materials with improved mechanical, electrical, magnetic, and self-sensing behavior. Despite several studies, the interactions between carbon nanofibers and Portland cement hydration products are not fully understood, with significant implications for the mechanical response and the durability at the macroscopic lengthscale. Thus, the research objective is to investigate the influence of carbon nanofibers on the nanostructure and on the distribution of hydration products within Portland cement paste. Portland cement w/c = 0.44 specimens reinforced with 0.0 wt%, 0.1 wt%, and 0.5 wt% CNF by mass fraction of cement are cast using a novel synthesis procedure. A uniform dispersion of carbon nanofibers (CNF) via a multi-step approach: after pre-dispersing carbon nanofibers using ultrasonic energy, the carbon nanofibers are further dispersed using un-hydrated cement particles in high shear mixing and mechanical stirring steps. High-resolution scanning electron microscopy analysis shows that carbon nanofibers fill nanopores and connect calcium–silicate hydrates (C–S–H) grains. Grid nano-indentation testing shows that Carbon nanofibers influence the probability distribution function of the local packing density by inducing a shift towards higher values, η = 0.76–0.93. Statistical deconvolution analysis shows that carbon nanofibersmore »result in an increase in the fraction of high-density C–S–H by 6.7% from plain cement to cement + 0.1 wt% CNF and by 10.7% from plain cement to cement + 0.5 wt% CNF. Moreover, CNF lead to an increase in the C–S–H gel porosity and a decrease in both the capillary porosity and the total porosity. Based on scratch testing, adding 0.1 wt% CNF yields a 4.5% increase in fracture toughness and adding 0.5 wt% CNF yields a 7.6% increase in fracture toughness. Finally, micromechanical modelling predicts an increase of respectively 5.97% and 21.78% in the average Young’s modulus following CNF modification at 0.1 wt% CNF and 0.5 wt% CNF levels.« less
4. Zwitterionic copolymer additive architecture affects membrane performance: fouling resistance and surface rearrangement in saline solutions

   https://doi.org/10.1039/C8TA11553B  

   Kaner, Papatya ; Dudchenko, Alexander V. ; Mauter, Meagan S. ; Asatekin, Ayse ( February 2019 , Journal of Materials Chemistry A)

   Membrane separations are simple to operate, scalable, versatile, and energy efficient, but their broader use is curtailed by fouling or performance decline due to feed component depositing on the membrane surface. Surface functionalization with groups such as zwitterions can mitigate the adsorption of organic compounds, thus limiting fouling. This can be achieved by using surface-segregating copolymer additives during membrane manufacture, but there is a need for better understanding of how the polymer structure and architecture affect the effectiveness of these additives in improving membrane performance. In this study, we aim to explore the impact of the architecture of zwitterionic copolymer additives for polyvinylidene fluoride (PVDF)-based membranes in fouling mitigation and ionic strength response. We prepared membranes from blends of PVDF with zwitterionic (ZI) copolymers with two different architectures, random and comb-shaped. As the random copolymer, we used poly(methyl methacrylate- random- sulfobetaine-2-vinyl pyridine) (PMMA- r -SB2VP) synthesized by free radical polymerization. The comb-shaped copolymer was synthesized by grafting SB2VP side-chains from a PVDF backbone by controlled radical polymerization. Membranes were fabricated from PVDF-copolymer blends containing up to 5 wt% ZI copolymer. Compared to the additive-free PVDF membrane, water permeance increased five-fold with 5 wt% addition of either copolymer. The comb copolymermore »additive led to better resistance to fouling by a saline oil-in-water emulsion and to simulated protein adsorption in Atomic Force Microscopy (AFM) force measurements. The additive architecture had a significant influence on how membranes respond to changes in feed salinity, which is known to affect intra- and inter-molecular interactions in zwitterionic polymers. The random copolymer containing membrane showed a small and mostly reversible decrease in its permeance with salinity. In contrast, the comb copolymer-containing membrane underwent a conformational reorganization in saline solutions that leads to an irreversible permeance decrease, increased zwitterionic group content on the membrane surface, and smoother surface topography. The higher mobility of the zwitterionic groups in the comb-shaped architecture facilitates reorganization of the zwitterionic side-chains in response to ionic strength. Overall, this study establishes a new approach for developing highly fouling resistant membranes and defines how the architecture of a zwitterionic copolymer additive impacts the ionic strength response and fouling resistance of the membrane.« less
5. A green solvent-to-polymer upgrading approach to water-soluble LCST poly( N -substituted lactamide acrylate)s

   https://doi.org/10.1039/D2GC02780A  

   Palà, Marc ; El Khannaji, Hafssa ; Garay-Sarmiento, Manuela ; Ronda, Juan Carlos ; Cádiz, Virginia ; Galià, Marina ; Percec, Virgil ; Rodriguez-Emmenegger, César ; Lligadas, Gerard ( October 2022 , Green Chemistry)

   We report a green solvent-to-polymer upgrading transformation of chemicals of the lactic acid portfolio into water-soluble lower critical solution temperature (LCST)-type acrylic polymers. Aqueous Cu(0)-mediated living radical polymerization (SET-LRP) was utilized for the rapid synthesis of N -substituted lactamide-type homo and random acrylic copolymers under mild conditions. A particularly unique aspect of this work is that the water-soluble monomers and the SET-LRP initiator used to produce the corresponding polymers were synthesized from biorenewable and non-toxic solvents, namely natural ethyl lactate and BASF's Agnique® AMD 3L ( N , N -dimethyl lactamide, DML). The pre-disproportionation of Cu( i )Br in the presence of tris[2-(dimethylamino)ethyl]amine (Me 6 TREN) in water generated nascent Cu(0) and Cu( ii ) complexes that facilitated the fast polymerization of N -tetrahydrofurfuryl lactamide and N , N -dimethyl lactamide acrylate monomers (THFLA and DMLA, respectively) up to near-quantitative conversion with excellent control over molecular weight (5000 < M n < 83 000) and dispersity (1.05 < Đ < 1.16). Interestingly, poly(THFLA) showed a degree of polymerization and concentration dependent LCST behavior, which can be fine-tuned ( T cp = 12–62 °C) through random copolymerization with the more hydrophilic DMLA monomer. Finally, covalent cross-linking of these polymers resulted in amore »new family of thermo-responsive hydrogels with excellent biocompatibility and tunable swelling and LCST transition. These illustrate the versatility of these neoteric green polymers in the preparation of smart and biocompatible soft materials.« less