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Liquid crystalline elastomers (LCEs) are shape-changing materials that exhibit large deformations in response to applied stimuli. Local control of the orientation of LCEs spatially directs the deformation of these materials to realize spontaneous shape change in response to stimuli. Prior approaches to shape programming in LCEs utilize patterning techniques that involve the detailed inscription of spatially varying nematic fields to produce sheets. These patterned sheets deform into elaborate geometries with complex Gaussian curvatures. Here, we present an alternative approach to realize shape-morphing in LCEs where spatial patterning of the crosslink density locally regulates the material deformation magnitude on either side of a prescribed interface curve. We also present a simple mathematical model describing the behavior of these materials. Further experiments coupled with the mathematical model demonstrate the control of the sign of Gaussian curvature, which is used in combination with heat transfer effects to design LCEs that self-clean as a result of temperature-dependent actuation properties.

The influence of radical-disulfide exchange on the polymerization and relaxation dynamics of thiol–ene–disulfide networks is investigated.

Snap-through mechanisms are pervasive in everyday life in biological systems, engineered devices, and consumer products. Snap-through transitions can be realized in responsive materials via stimuli-induced mechanical instability. Here, we demonstrate a rapid and powerful snap-through response in liquid crystalline elastomers (LCEs). While LCEs have been extensively examined as material actuators, their deformation rate is limited by the second-order character of their phase transition. In this work, we locally pattern the director orientation of LCEs and fabricate mechanical elements with through-thickness (functionally graded) modulus gradients to realize stimuli-induced responses as fast as 6 ms. The rapid acceleration and associated force output of the LCE elements cause the elements to leap to heights over 200 times the material thickness. The experimental examination in functionally graded LCE elements is complemented with computational evaluation of the underlying mechanics. The experimentally validated model is then exercised as a design tool to guide functional implementation, visualized as directional leaping.

The macroscopic alignment of the nematic director within liquid crystalline elastomers (LCEs) amplifies the magnitude of the directional strain response. Theory predicts that the stimuli response o...

A scalable synthesis of high refractive index, optically transparent photopolymers from a family of low-viscosity multifunctional thiol and alkyne monomers via thiol-yne "click" is described herein. The monomers designed to incorporate high refractive index cores consisting of aryl and sulfide groups with high intrinsic molar refraction were synthesized starting from commercially available low-cost raw materials. The low-viscosity (

The kinetic effects of the substitution and functionality of the thiol in thiol-Michael reactions were investigated using model monofunctional thiols and multifunctional thiols used in various cros...

Microparticle-mediated nucleic acid delivery is a popular strategy to achieve therapeutic outcomes via antisense gene therapy. However, current methods used to fabricate polymeric microparticles suffer from suboptimal properties such as particle polydispersity and low encapsulation efficiency. Here, a new particulate delivery system based on step-growth thiol-Michael dispersion polymerization, is reported in which a low polydispersity microparticle is functionalized with a synthetic nucleic acid mimic, namely click nucleic acids (CNA). CNA oligomers, exhibiting an average length of approximately four nucleic acid repeat units per chain for both adenine and thymine bases, were successfully conjugated to excess thiols present in the microparticles. Effective DNA loading was obtained by simple mixing; and, up to 6 ± 2 pmol of complementary DNA/mg of particle was achieved, depending on the length of DNA used. In addition, DNA loading was orders of magnitude less for noncomplementary sequences and sequences containing an alternating base mismatch. The DNA release properties were evaluated, and it was found that release could be triggered by sudden changes in temperature but was unaffected over a range of pH. Finally, phagocytosis of loaded microparticles was observed by confocal microscopy and corroborated by an increase in cellular metabolic activity up to 90%. Overall, this work suggests that CNA functionalized microparticles could be a promising platform for controlled DNA delivery.

Storage and transportation of protein therapeutics using refrigeration is a costly process; a reliable electrical supply is vital, expensive equipment is needed, and unique transportation is requir...

This article describes an efficient enamine organocatalyzed thiol-Michael click reaction and its broad application in cross-linking polymerizations. A series of enamines was shown to catalyze the t...

Thermoplastics encompass the majority of commercial plastics but are limited to manufacturing techniques that require heat and/or solvent to enable material reprocessing and reshaping. Photopolymer...

Photomechanical effects in liquid crystal elastomers (LCEs) functionalized with photochromic moieties, such as azobenzene, have been widely studied. This prior work has demonstrated that isothermal, photoinduced (e.g. phototropic) disruption of order via isomerization of azobenzene affects the anisotropic chain configurations of the polymer network. Here, we examine the contribution of the strength of intermolecular interactions between polymer chains in the LCE to both the thermotropic and phototropic response. By incorporating liquid crystalline monomers with reduced aromatic content, both the temperature and irradiation conditions to induce mechanical response are reduced.

A photoprintable dynamic thiol-ene resin was developed based on commercially available anhydride, thiol, and ene monomers. The dynamic chemistry chosen for this study relied on the thermal reversibility of the in situ generated thioester-anhydride links. The resin's rheological and curing properties were optimized to enable 3D printing using the masked stereolithography (MSLA) technique. To achieve a desirable depth of cure of 200 μm, a combination of radical photoinitiator (BAPO) and inhibitor (pyrogallol) were used at a weight ratio of 0.5 to 0.05, resulting in more than 90% thiol-ene conversion within 12 s curing time. In a series of stress relaxation and creep experiments, the dynamic reversible exchange was characterized and yielded rapid exchange rates ranging from minutes to seconds at temperatures of 80-140 °C. Little to no exchange was observed at temperatures below 60 °C. Various 3D geometries were 3D printed, and the printed objects were shown to be reconfigurable above 80 °C and depolymerizable at or above 120 °C. By deactivation of the exchange catalyst (DMAP), the stimuli responsiveness was demonstrated to be erasable, allowing for a significant shift in the actuation threshold. These highly enabling features of the dynamic chemistry open up new possibilities in the field of shape memory and 4D printable functional materials.

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.

Click nucleic acids (CNAs) are a new, low-cost class of xeno nucleic acid (XNA) oligonucleotides synthesized by an efficient and scalable thiol-ene polymerization. In this work, a thorough characterization of oligo(thymine) CNA-oligo(adenine) DNA ((dA)20) hybridization was performed to guide the future implementation of CNAs in applications that rely on sequence-specific interactions. Microscale thermophoresis provided a convenient platform to rapidly and systematically investigate the effects of several factors (i.e., sequence, length, and salt concentration) on the CNA-DNA dissociation constant (Kapp). Because CNAs have limited water solubility, all studies were performed in aqueous-DMSO mixtures. CNA-DNA hybrids between oligo(thymine) CNA (average length of 16 bases) and (dA)20 DNA have good stability despite the high organic content, a favorable attribute for many emerging applications of XNAs. In particular, the Kapp of CNA-DNA hybrids in 65 vol % DMSO with 10 mM sodium chloride (NaCl) was 0.74 ± 0.1 μM, whereas the Kapp for (dT)20-(dA)20 DNA-DNA was found to be 45 ± 2 μM in a buffer without DMSO but at the same NaCl concentration. CNA hybridized with DNA following Watson-Crick base pairing with excellent sequence specificity, discriminating even a single-base-pair mismatch, with Kapp values of 0.74 ± 0.1 and 3.7 ± 0.6 μM for complementary and single-base-pair mismatch sequences, respectively. As with dsDNA, increasing CNA length led to more stable hybrids as a result of increased base pairing, where Kapp decreased from 5.6 ± 0.8 to 0.27 ± 0.1 μM as the CNA average length increased from 7 to 21 bases. However, unlike DNA-DNA duplexes, which are largely unstable at low salt concentrations, the CNA-DNA stability does not depend on salt concentration, with Kapp remaining consistent between 1.0 and1.9 μM over a NaCl concentration range of 1.25-30 mM.

A high-performance holographic recording medium was developed based on a unique combination of photoinitiated thiol–ene click chemistry and functional, linear polymers used as binders. Allyl reacti...

In the present work, the photocatalytic activity was investigated toward a thiol-Michael reaction of different combinations of 9-fluorenylmethyl carbamate (Fmoc) derivatives and photocaged amines (PCA) as base amplifiers and the catalytic base, respectively. This phototriggering approach was systematically studied for evaluating its effect on kinetics between thiol and Michael acceptors like acrylate or sulfone wherein, butyl 3-mercaptopropionate (BMP), 1-hexyl acrylate (HA), and ethyl vinyl sulfone (EVS) were used as model reactants. Interestingly, PCAs exhibited low quantum yields by themselves; NPPOC-Hex (2.5 mol %) which when used with BMP and HA, resulted in only 25% thiol conversion; however, when used along with Fmoc-Hex or Fmoc-TMG (2.5 or 5 mol %), it resulted in a higher thiol conversion of 50–60%. Furthermore, use of NPPOC-DEA (5 mol %) with 1 mol % Fmoc-TMG resulted in >70% thiol conversion for the same system. Upon using BMP and EVS nearly complete conversion of functional groups with 5 mol % NPPOC-DEA and 5 mol % Fmoc-DEA was obtained. This enhancement in reaction kinetics and conversion upon addition of an Fmoc derivative to a monofunctional thiol-Michael system was extended to multifunctional derivatives for polymerizing cross-linked polymer networks. Moreover, the kinetic study on model reactants also demonstrated efficient dark curing, resulting in 50–75% thiol conversion with only 30 s irradiation time, leading to validation of the efficacy of Fmoc derivatives and PCAs as photocatalysts for dark cure. Upon precise characterization in cross-linked systems using Raman spectroscopy for TMPTA/PETMP in the presence of 20 mol % NPPOC-DEA and 1 mol % Fmoc-TMG, the extent of dark cure was evaluated for a distance of 16.5 mm, which was observed to undergo maximum conversion and high dark cure propagation upon heating to 70 °C. Therefore, Fmoc-PCA catalysis is a practically useful approach for improving the photoinitiated efficiency of the thiol-Michael reaction and enabling photopolymerization in the dark with a marked improvement in photosensitivity.

The effect of thiol substitution in radical thiol–ene reactions has been studied by using model, monofunctional thiols as well as multifunctional thiol monomers along with the assessment of their subsequent polymerization reactions and polymer mechanical behavior. FT-IR was used to monitor the polymerization rate and quantify the overall conversion. While the total conversion was observed to range from 70% to 100%, the polymerization rate was found to decrease by as much as 10-fold as the thiol substitution was changed from primary to tertiary. Analogous multi-thiol monomers of similar structure but varying substitution were synthesized to observe the effect of substitution type on polymerization kinetics and polymer behavior. Methylation at the α-carbon was varied from primary to tertiary to observe these differences. Mechanical properties were assessed by using dynamic mechanical analysis and water sorption experiments, where the glass transition temperatures were found to be within 1–2 °C as thiol substitution varied. Furthermore, primary thiol films absorbed 1–3% more water than secondary thiol films. Resin shelf stability experiments were performed by using rheometry to measure storage time-dependent viscosity changes, and it was found that secondary thiol films remained relatively stable for up to 100 times longer than their primary counterparts. It was concluded that while there are differences under relatively slow initiation conditions, at typical initiation rates all three thiol substitutions may be made to react at similar rates for both monofunctional and polymeric systems.

The unique photodynamics of aromatic thiols (relative to alkyl thiols) allowed their employment as effective ultraviolet and visible light photoinitiators (PIs) for acrylate photopolymerizations, r...

The reaction of thiols and anhydrides to form ring opened thioester/acids is shown to be highly reversible and it is accordingly employed in the fabrication of covalent adaptable networks (CANs) that possess tunable dynamic covalent chemistry. Maleic, succinic, and phthalic anhydride derivatives were used as bifunctional reactants in systems with varied stoichiometries, catalyst, and loadings. Dynamic characteristics such as temperature-dependent stress relaxation, direct reprocessing and recycling abilities of a range of thiol-anhydride elastomers, glasses, composites and photopolymers are discussed. Depending on the catalyst strength, 100 % of externally imposed stresses were relaxed in the order of minutes to 2 hours at mild temperatures (80-120 °C). Pristine properties of the original materials were recovered following up to five cycles of a hot-press reprocessing technique (1 h/100 °C).

This document is the unedited Author's version of a Submitted Work that was subsequently accepted for publication in ACS Applied Materials & Interfaces, copyright © American Chemical Society after peer review. To access the final edited and published work see https://doi.org/10.1021/acsami.2c05113. Deposited by shareyourpaper.org and openaccessbutton.org. We've taken reasonable steps to ensure this content doesn't violate copyright. However, if you think it does you can request a takedown by emailing help@openaccessbutton.org.

Functional nanogels are attractive additives for use in polymer composites. In this study, nanogels with internal allyl sulfide moieties throughout their network structure were prepared via a thiol-Michael addition reaction. The excess thiol-functionalized nanogels were less than 60 nm as discrete particles but act as room-temperature liquids in the bulk state. The reactive nanogels can be dispersed in and swollen by a thiol-ene matrix resin, which upon photopolymerization yields dramatically decreased levels of polymerization shrinkage stress. Furthermore, the postcured nanogel-modified polymers effectively relaxed applied stresses as well as enhanced toughness during exposure to a UV light source that activated the addition-fragmentation as a means for dynamic bond exchange. These nanogels provide a generic approach to introduce adaptable network performance that significantly improves a number of key properties of glassy cross-linked polymer.

Because of facile implementation, quantitative conversions, and an insensitivity to oxygen, water, and most organic functional groups, radical-mediated thiol-ene coupling (TEC) reactions have emerged as a valuable tool in macromolecule synthesis. It was recently demonstrated that the kinetics and conversions of thiyl radical-mediated reactions are adversely affected in the presence of basic amines by the formation of retardive thiolate anions. Herein, the performance of TEC polymerizations is evaluated under a variety of reaction environments with the intention to aid in the optimal formulation design of TEC reactions in the presence of amines. Results from both bulk and aqueous-phase network photopolymerizations established that sensitivity to amine basicity and pH is dependent on the thiol acidity, although norbornene-type alkenes exhibit a unique ability to achieve high conversions, where allyl ethers, vinyl ether, and vinyl siloxanes are highly inhibited. Additionally, the protic solvents such as alcohols and acetic acid are established as ideal solvents or additives to suppress or eliminate amine-induced retardation.

Motivated by the various reported potential applications of poly(phosphine oxide) materials, a visible light photoinitiated iodo-ene reaction was successfully employed in network polymerization between the phosphorus-containing multifunctional monomer, tris(allyloxymethyl)phosphine oxide (TAOPO), and diiodoperfluorobutane. The cross-linked poly(phosphine oxide) network exhibited a higher glass transition temperature than a similarly cross-linked polymer formulated with trimethylolpropane triallyl ether (TMPTAE). Interestingly, the TMPTAE/DIPFB cross-linked polymer, changed color from clear to yellow within 10 min of exposure to air, whereas the cross-linked poly(phosphine oxide) underwent a similar change only upon heating. Upon investigation, it was determined that alkenes were generated within the polymer network, presumably via elimination, accounting for the observed color. These double bonds, formed in the polymer matrix, permitted surface modification via radical thiol-ene reaction. The successful surface functionalization with PEG-SH resulted in increasing the surface wettability. Additionally, the phosphorus-containing network polymer with double bonds in the polymer matrix showed shape memory capability, this representing an exciting and versatile materials platform.

A new group of nitrogen-centered nucleophilic catalysts for the thiol-Michael addition “click” reactions is examined. These nucleophiles showed efficient catalytic activities as compared with traditional base catalysts, such as triethylamine, and are demonstrated to be a viable strategy for cross-linking polymerization reactions. Additionally, an experimental and computational mechanistic study was performed, suggesting a pathway for the nitrogen-centered catalyst to undergo the nucleophilic addition mechanism.

A visible-light base generating system was successfully employed in catalyzing the thiol-Michael addition reaction to yield cross-linked polymers from a stoichiometric mixture of model thiol and vinyl monomers. Implementation of the radical inhibitor TEMPO with a combination of a photosensitizer (isopropylthioxanthone, ITX) and a photobase generator (triazabicyclodecene tetraphenylborate, TBD·HBPh4) resulted in suppression of radical mediated side reactions and provided stoichiometric and complete conversion of both thiol and vinyl functional groups. The new initiating system acts as an efficient visible-light photobase generator that improves the orthogonality of the thiol-Michael addition with respect to off-stoichiometric radical thiol-vinyl addition/vinyl chain reactions. This approach opens up a variety of possibilities for base-catalyzed reactions in multiple applications such as coatings and biomaterials that require biocompatible, environmentally friendly, and low-energy visible-light initiation.

A photoinitiation system that utilizes phosphonium tetraphenylborate as the key component was developed for the visible light-triggered nucleophile-catalyzed thiol-Michael addition reaction. This highly reactive catalyst was composed of a photocaged phosphine (methyldiphenylphosphonium tetraphenylborate, MDPP·HBPh

Diarylethene-functionalized liquid-crystalline elastomers (DAE-LCEs) containing thiol-anhydride bonds were prepared and shown to undergo reversible, reprogrammable photoinduced actuation. Upon exposure to UV light, a monodomain DAE-LCE generated 5.5 % strain. This photogenerated strain was demonstrated to be optically reversible over five cycles of alternating UV/Visible light exposure with minimal photochrome fatigue. The incorporation of thiol-anhydride dynamic bonds allowed for retention of actuated states. Further, re-programming of the nematic director was achieved by heating above the temperature for bond exchange to occur (70 °C) yet below the nematic-to-isotropic transition temperature (100 °C) such that order was maintained between mesogens. The observed thermal stability of each of the diarylethene isomers of over 72 h allowed for decoupling of photo-induced processes and polymer network effects, showing that both polymer relaxation and back-isomerization of the diarylethene contributed to LCE relaxation over a period of 12 hours after actuation unless bond exchange occurred.

An approach to efficient and scalable production of oligonucleotide-based gel networks is presented. Specifically, a new class of xenonucleic acid (XNA) synthesized through a scalable and efficient thiol–ene polymerization mechanism, “clickable” nucleic acids (CNAs), were conjugated to a multifunctional poly(ethylene glycol), PEG. In the presence of complementary single stranded DNA (ssDNA), the macromolecular conjugate assembled into a crosslinked 3D gel capable of achieving storage moduli on the order of 1 kPa. Binding studies between the PEG-CNA macromolecule and complementary ssDNA indicate that crosslinking is due to the CNA/DNA interaction. Gel formation was specific to the base sequence and length of the ssDNA crosslinker. The gels were fully thermoreversible, completely melting at temperatures above 60 °C and re-forming upon cooling over multiple cycles and with no apparent hysteresis. Shear stress relaxation experiments revealed that relaxation dynamics are dependent on crosslinker length, which is hypothesized to be an effect of the polydisperse CNA chains. Arrhenius analysis of characteristic relaxation times was only possible for shorter crosslinker lengths, and the activation energy for these gels was determined to be 110 ± 20 kJ mol−1. Overall, the present work demonstrates that CNA is capable of participating in stimuli-responsive interactions that would be expected from XNAs, and that these interactions support 3D gels that have potential uses in biological and materials science applications.

An athermal approach to mRNA enrichment from total RNA using a self-immolative thioester linked nucleic acids (TENA) is described. Oligo(thymine) (oT) TENA has a six-atom spacing between bases which allowed TENA to selectively base-pair with polyadenine RNA. As a result of the neutral backbone of TENA and the hydrophobicity of the octanethiol end group, oT TENA is water insoluble and efficiently pulled down 93±2 % of EGFP mRNA at a concentration of 10 ng μL-1 . Self-immolative degradation of TENA upon ambient temperature exposure to nucleophilic buffer components (Tris, DTT) allowed recovery of 55±27 ng of mRNA from 3.1 μg of total RNA, which was not statistically different from the amount recovered using Dynabeads® mRNA DIRECT Kit (89±24 ng). Gene expression as measured by RT-qPCR was comparable for both enrichment methods, suggesting that the mild conditions required for enrichment of mRNA using oT TENA are compatible with RT-qPCR and other downstream molecular biology applications.

Beyond their central role in storing and transmitting genetic information, nucleic acids are renowned for their high-specificity, high-affinity hybridization. In the past several decades, scientists have become increasingly interested in emulating this unique property of nucleic acids using synthetic mimics. Many creative strategies for the synthesis of xenonucleic acids (XNAs) have been developed, with the field ultimately striving for high-efficiency, scalable routes to achieving sequence-controlled XNA synthesis. Emerging strategies in both biology (e.g., directed evolution) and chemistry (e.g., dynamic covalent reactions) are leading to new breakthroughs in XNA synthesis that will make applications of nucleic acids, such as gene therapy, agricultural disease management, and electronics, more accessible.

Semicrystalline polymeric materials possessing extraordinary mechanical properties were rapidly fabricated using light from low viscosity liquids at room temperature. Unlike conventional photopolymerizations which form crosslinked networks, this material is a noncrosslinked polymer which renders it meltable and intrinsically reprocessable. This introduces a new class of high performance, recyclable materials applicable for light-based additive manufacturing.

Dynamic photopolymer networks that take advantage of the thermodynamically controlled reversibility of thiol–succinic anhydride adducts were synthesized from commercial substrates and investigated as a new class of covalent adaptable networks (CANs). Through systematic studies of the catalyst and stoichiometry effects on the exchange dynamics two distinctive exchange mechanisms were found, and then demonstrated to contribute to the overall dynamic characteristics. By varying the catalyst activity, i.e. basicity and/or nucleophilicity, control over the dynamic responsiveness through changes in the type of dynamic covalent chemistry mode (reversible addition vs. reversible exchange) was achieved in otherwise compositionally analogous materials. More specifically, the participation of the associative mechanism (thiol–thioester exchange) in the otherwise dissociative networks, and its relevance on materials properties was demonstrated by dielectric analysis (DEA) and dynamic mechanical analysis (DMA). The activation energies (Ea) for viscous flow obtained from DMA stress relaxation experiments and from dielectric modulus and loss crossover points were shown to match well between the two techniques. The Ea in stoichiometric systems was found to be 110–120 kJ mol−1, whereas 50% excess thiol systems were characterized by Ea ranging 95–105 kJ mol−1. The thermodynamic equilibrium conversion, estimated in the temperature controlled FTIR, for a stoichiometric 3-mercaptopropionate-succinic anhydride combination was determined at 92 ± 1% at ambient temperature, and decreased to 67 ± 1% at 120 °C within one hour of equilibration time (ΔH° = −46 ± 5 kJ mol−1). Such high potential for reversibility of the thioester anhydride linkages resembles maleimide-furan Diels–Alder networks but has many other attributes that make these CANs of unprecedented value in fundamental research on dynamic materials.