Shape Permanence in Diarylethene-Functionalized Liquid-Crystal Elastomers Facilitated by Thiol-Anhydride Dynamic Chemistry.

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.

Obstructive Sleep Apnea as a Predictor of Arrhythmias in 24-h ECG Holter Monitoring.

A relationship between obstructive sleep apnea (OSA) and abnormalities in 24-h electrocardiogram (ECG) Holter monitoring has not been sufficiently documented. The aim of this study was to analyze the relation between the occurrence and the severity of OSA and the parameters of ECG Holter monitoring in a group of patients with diagnosed OSA. Ninety-four patients with clinical suspicion of OSA were qualified for the study (mean age 53.7). All participants underwent a medical history, laboratory tests, 24-h ECG Holter monitoring, and single-night video-polysomnography (vPSG) using the American Academy of Sleep Medicine recommendations. A group of patients with diagnosed OSA was characterized by more frequent occurrence of supraventricular tachycardia (SVT) and ventricular arrhythmias (VPC). A statistically significant positive correlation was shown between the apnea-hypopnea index (AHI) and average heart rate, supraventricular arrhythmias (SVPC) pairs, SVT, and pauses >2.5 s. In regression analysis, higher AHI constituted an independent predicator for the increased number of pauses >2.5 s, SVT, and SVPC pairs in 24-h ECG Holter monitoring. In summary, patients with OSA are characterized by the increased number of abnormalities in 24-h ECG Holter monitoring.

High Refractive Index Photopolymers by Thiol-Yne "Click" Polymerization.

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 (<500 cP) thiol-yne resins formulated with these new multifunctional monomers and a phosphine oxide photoinitiator underwent efficient thiol-yne polymerizations upon exposure to 405 nm light at 30 mW/cm2. In contrast to the previously reported thiol-ene systems, the kinetic profile of these photopolymerizations showed significant dependence on the nature of the thiol and alkyne monomers. However, the ability of the thiol-yne reaction to introduce a large number of sulfide linkages compared to that of thiol-ene systems yielded cross-linked high optical quality photopolymers with a polymer refractive index that exceeds 1.68 (nD/20 °C). Interestingly, the photopolymer formed from the least sterically hindered alkynyl thioether monomer 2b with flexible thioether core and the dithiol 1a exhibited unprecedented difference in the polymer refractive index as compared to that of the resin with polymerization-induced changes reaching up to 0.08. Furthermore, the implementation of these low-viscosity thiol-yne resins was demonstrated by preparing two-stage photopolymeric holographic materials with a dynamic range of ∼0.02 and haze < 1.5% in two-dimensional high refractive index structures.

Additive Manufacture of Dynamic Thiol-ene Networks Incorporating Anhydride-Derived Reversible Thioester Links.

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.

Holographic Photopolymer Material with High Dynamic Range (Δn) via Thiol-Ene Click Chemistry.

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 reactive sites were incorporated along the backbone of the linear polymer binder to enable facile film casting and to facilitate cross-linking by photopolymerization of the thiol-ene monomers that also serve as the writing monomers in this distinctive approach to holographic materials. The allyl content and the ratio of the linear polymer to the writing monomers were varied to maximize and control the refractive index contrast. A blade-coating-based film preparation method was developed to form films from the mixture of linear polymer and the thiol-ene monomers. This approach results in a holographic material with a peak to mean index contrast (Δn) that reaches 0.04. The refractive index contrast was stable for at least two weeks. Haze in holograms with a high writing monomer loading was significantly reduced when a higher allyl content was incorporated into the binder, resulting in the lowest haze around 0.2%. Finally, the media exhibit high resolution as demonstrated by the ability to record reflection holograms with 140 nm pitch and diffraction efficiency in excess of 90%.

Cross-Linked Polythiomethacrylate Esters Based on Naphthalene-Synthesis, Properties and Reprocessing.

Two structurally different aromatic dithioesters were synthesized from two dithiols and methacryloyl chloride. The polymer networks based on methyl methacrylate and/or styrene and the new dimethacrylates were subsequently prepared. The polymerization yields of copolymers were in the range of 95-99%. The thermal and mechanical properties of the copolymers were determined by means of differential scanning calorimetry (DSC), thermogravimetric analysis (TG/DTG), and Shore D hardness. The addition of dithioesters-1,5-NAF-S-Met (or 1,4(1,5)-NAF-CH2S-Met) (from 0.5% to 5%) to MMA- or ST-based polymers results in lowering the glass transition temperature (Tg) by about 8 °C. The thioester-containing polymers based on MMA exhibit lower thermal stability than those with ST. The polythioesters are stable up to 250 °C. The UV/vis spectra and refractive indexes of prepared liquid compositions were also measured. The 1,5-NAF-S-Met (and 1,4(1,5)-NAF-CH2S-Met) improved the refractive index values of ST and MMA compositions. The double bond conversion was also determined for all synthesized materials. The swelling studies of polymers with 20% addition of thioester crosslinkers were investigated. For all polymeric materials with 20% addition of thioesters, depolymerization of the network was carried out by thiol-thioester exchange. The depolymerization products were re-reacted in a thiol-ene reaction with 2-hydroxyethyl methacrylate by thermal initiation. The thiol-ene procedure enabled reprocessing of starting polymers and obtaining new materials characterized by distinctly different thermal, mechanical, and swelling properties. The thiol-ene materials exhibit a lower Shore hardness in the range of 20-50 °Sh, as well as decreased Tg values when compared to starting copolymers. Due to these possible exchange reactions, one can facilely manipulate the properties of the polymers which could lead to the manufacturing of the new products with the desired features. Degradation of the cross-linked structure and recycling of copolymers were also discussed.

Thiol-Anhydride Dynamic Reversible Networks.

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).

Toward Stimuli-Responsive Dynamic Thermosets through Continuous Development and Improvements in Covalent Adaptable Networks (CANs).

Covalent adaptable networks (CANs), unlike typical thermosets or other covalently crosslinked networks, possess a unique, often dormant ability to activate one or more forms of stimuli-responsive, dynamic covalent chemistries as a means to transition their behavior from that of a viscoelastic solid to a material with fluid-like plastic flow. Upon application of a stimulus, such as light or other irradiation, temperature, or even a distinct chemical signal, the CAN responds by transforming to a state of temporal plasticity through activation of either reversible addition or reversible bond exchange, either of which allows the material to essentially re-equilibrate to an altered set of conditions that are distinct from those in which the original covalently crosslinked network is formed, often simultaneously enabling a new and distinct shape, function, and characteristics. As such, CANs span the divide between thermosets and thermoplastics, thus offering unprecedented possibilities for innovation in polymer and materials science. Without attempting to comprehensively review the literature, recent developments in CANs are discussed here with an emphasis on the most effective dynamic chemistries that render these materials to be stimuli responsive, enabling features that make CANs more broadly applicable.

Dynamic covalent chemistry (DCC) in dental restorative materials: Implementation of a DCC-based adaptive interface (AI) at the resin-filler interface for improved performance.

OBJECTIVE: Dental restorative composites have been extensively studied with a goal to improve material performance. However, stress induced microcracks from polymerization shrinkage, thermal and other stresses along with the low fracture toughness of methacrylate-based composites remain significant problems. Herein, the study focuses on applying a dynamic covalent chemistry (DCC)-based adaptive interface to conventional BisGMA/TEGDMA (70:30) dental resins by coupling moieties capable of thiol-thioester (TTE) DCC to the resin-filler interface as a means to induce interfacial stress relaxation and promote interfacial healing. METHODS: Silica nanoparticles (SNP) are functionalized with TTE-functionalized silanes to covalently bond the interface to the network while simultaneously facilitating relaxation of the filler-matrix interface via DCC. The functionalized particles were incorporated into the otherwise static conventional BisGMA/TEGDMA (70:30) dental resins. The role of interfacial bond exchange to enhance dental composite performance in response to shrinkage and other stresses, flexural modulus and toughness was investigated. Shrinkage stress was monitored with a tensometer coupled with FTIR spectroscopy. Flexural modulus/strength and flexural toughness were characterized in three-point bending on a universal testing machine. RESULTS: A reduction of 30% in shrinkage stress was achieved when interfacial TTE bond exchange was activated while not only maintaining but also enhancing mechanical properties of the composite. These enhancements include a 60% increase in Young's modulus, 33% increase in flexural strength and 35% increase in the toughness, relative to composites unable to undergo DCC but otherwise identical in composition. Furthermore, by combining interfacial DCC with resin-based DCC, an 80% reduction of shrinkage-induced stress is observed in a thiol-ene system "equipped" with both types of DCC mechanisms relative to the composite without DCC in either the resin or at the resin-filler interface. SIGNIFICANCE: This behavior highlights the advantages of utilizing the DCC at the resin-filler interface as a stress-relieving mechanism that is compatible with current and future developments in the field of dental restorative materials, nearly independent of the type of resin improvements and types that will be used, as it can dramatically enhance their mechanical performance by reducing both polymerization and mechanically applied stresses throughout the composite lifetime.

Vinyl sulfonamide based thermosetting composites via thiol-Michael polymerization.

OBJECTIVE: To assess the performance of thiol Michael photocurable composites based on ester-free thiols and vinyl sulfonamides of varying monomer structures and varied filler loadings and to contrast the properties of the prototype composites with conventional BisGMA-TEGDMA methacrylate composite. METHODS: Synthetic divinyl sulfonamides and ester-free tetrafunctional thiol monomers were utilized for thiol-Michael composite development with the incorporation of thiolated microfiller. Polymerization kinetics was investigated using FTIR spectroscopy. Resin viscosities were assessed with rheometry. Water uptake properties were assessed according to standardized methods. Thermomechanical properties were analyzed by dynamic mechanical analysis. Flexural modulus/strength and flexural toughness were measured on a universal testing machine in three-point bending testing mode. RESULTS: The vinyl sulfonamide-based thiol-Michael resin formulation demonstrated a wide range of viscosities with a significant increase in the functional group conversion when compared to the BisGMA-TEGDMA system. The two different types of vinyl sulfonamide under investigation demonstrated significant differences towards the water sorption. Tertiary vinyl sulfonamide did not undergo visible swelling whereas the secondary vinyl sulfonamide composite swelled extensively in water. With the introduction of rigid monomer into the polymer matrix the glass transition temperature increased and so increased the toughness. Glassy thiol-Michael composites were obtained by ambient curing. SIGNIFICANCE: Employing the newly developed step-growth thiol-Michael resins in dental composites will provide structural uniformity, improved stability and lower water sorption.

Obstructive Sleep Apnea as a Predictor of Abnormal Heart Rate Turbulence.

Obstructive sleep apnea (OSA) causes dysfunction of the autonomic nervous system, but the exact mechanism has not been fully understood. The aim of this study was to analyse the relationship between the incidence and severity of OSA and heart rate turbulence (HRT). Seventy one patients with clinical suspicion of OSA were qualified to participate in the study. All participants took part in a survey and were subjected to laboratory tests, 24-hour electrocardiogram (ECG) Holter monitoring with HRT analysis and polysomnography. The group with OSA manifested significantly higher turbulence onset (TO) and lower turbulence slope (TS) as compared to the group without OSA. Older age, diabetes, hypertension and higher apnea/hypopnea index (AHI) were found to be independent risk factors for increased TO, whereas older age, higher body mass index (BMI), higher blood glucose levels, hypertension and higher AHI were independent risk factors for TS reduction. The AHI ≥65 criterion indicates abnormal HRT in patients with OSA with 94.9% sensitivity and 50.0% specificity, which gives a prediction accuracy of 85.7%. In summary, OSA should be considered as a predictor of abnormal HRT.

Enabling Applications of Covalent Adaptable Networks.

The ability to behave in a fluidlike manner fundamentally separates thermoset and thermoplastic polymers. Bridging this divide, covalent adaptable networks (CANs) structurally resemble thermosets with permanent covalent crosslinks but are able to flow in a manner that resembles thermoplastic behavior only when a dynamic chemical reaction is active. As a consequence, the rheological behavior of CANs becomes intrinsically tied to the dynamic reaction kinetics and the stimuli that are used to trigger those, including temperature, light, and chemical stimuli, providing unprecedented control over viscoelastic properties. CANs represent a highly capable material that serves as a powerful tool to improve mechanical properties and processing in a wide variety of polymer applications, including composites, hydrogels, and shape-memory polymers. This review aims to highlight the enabling material properties of CANs and the applied fields where the CAN concept has been embraced.

Obstructive sleep apnea as a predictor of reduced heart rate variability.

PURPOSE: This study aimed to analyze the relationship between the occurrence of obstructive sleep apnea (OSA) and heart rate variability (HRV) in a group of patients with clinical suggestion of OSA. METHODS: 104 patients with clinical suspicion of OSA were qualified to participate in the study (age: 53.15 ± 13.43 years). All participants took part in a survey and were subjected to laboratory tests, 24-hour ECG Holter monitoring, and polysomnography. The participants were divided into groups depending on the criterion of the presence of OSA. RESULTS: The analysis of time HRV demonstrated lower parameters of SDNN for the entire recording and a 15-minute fragment of daily activity, as well as a lower pNN50 for the entire recording in those patients with diagnosed OSA. A statistically significant difference was observed for the spectral analysis of the LF/HF which was higher in the participants with OSA during the 15-minute fragment of N3 sleep. A negative correlation was observed between AHI and the following parameters: SDSD from the entire Holter recording (r = -0.21, p < 0.05) and from the 15-minute fragment of daily activity (r = -0.19, p < 0.05), mRR from the fragment of N3 sleep (r = -0.19, p < 0.05) and VLF from the entire Holter recording (r = -0.26, p < 0.05). A statistically significant positive correlation between AHI and LF/HF in 15-minute fragments of N3 sleep was found (r = 0.26, p < 0.05). CONCLUSIONS: The study group of patients with OSA is characterized by reduced HRV. The higher AHI constitutes an independent predictor of reduced HRV, both in the sympathetic and parasympathetic components, and the sympathetic-parasympathetic balance.

A readily programmable, fully reversible shape-switching material.

Liquid crystalline (LC) elastomers (LCEs) enable large-scale reversible shape changes in polymeric materials; however, they require intensive, irreversible programming approaches in order to facilitate controllable actuation. We have implemented photoinduced dynamic covalent chemistry (DCC) that chemically anneals the LCE toward an applied equilibrium only when and where the light-activated DCC is on. By using light as the stimulus that enables programming, the dynamic bond exchange is orthogonal to LC phase behavior, enabling the LCE to be annealed in any LC phase or in the isotropic phase with various manifestations of this capability explored here. In a photopolymerizable LCE network, we report the synthesis, characterization, and exploitation of readily shape-programmable DCC-functional LCEs to create predictable, complex, and fully reversible shape changes, thus enabling the literal square peg to fit into a round hole.

Thermoreversible Folding as a Route to the Unique Shape-Memory Character in Ductile Polymer Networks.

Ductile, cross-linked films were folded as a means to program temporary shapes without the need for complex heating cycles or specialized equipment. Certain cross-linked polymer networks, formed here with the thiol-isocyanate reaction, possessed the ability to be pseudoplastically deformed below the glass transition, and the original shape was recovered during heating through the glass transition. To circumvent the large forces required to plastically deform a glassy polymer network, we have utilized folding, which localizes the deformation in small creases, and achieved large dimensional changes with simple programming procedures. In addition to dimension changes, three-dimensional objects such as swans and airplanes were developed to demonstrate applying origami principles to shape memory. We explored the fundamental mechanical properties that are required to fold polymer sheets and observed that a yield point that does not correspond to catastrophic failure is required. Unfolding occurred during heating through the glass transition, indicating the vitrification of the network that maintained the temporary, folded shape. Folding was demonstrated as a powerful tool to simply and effectively program ductile shape-memory polymers without the need for thermal cycling.

Multifunctional monomers based on vinyl sulfonates and vinyl sulfonamides for crosslinking thiol-Michael polymerizations: monomer reactivity and mechanical behavior.

Multifunctional vinyl sulfonates and vinyl sulfonamides were conveniently synthesized and assessed in thiol-Michael crosslinking polymerizations. The monomer reactivities, mechanical behavior and hydrolytic properties were analyzed and compared with those of analogous thiol-acrylate polymerizations. Materials with a broad range of mechanical properties and diverse hydrolytic stabilities were obtained.

Assessment of TEMPO as a Thermally Activatable Base Generator and Its Use in Initiation of Thermally-Triggered Thiol-Michael Addition Polymerizations.

We present a thermally initiated thiol-Michael reaction based on initiation via the temperature-dependent thiol-TEMPO oxidation-reduction reaction. In the presence of a thiol, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO, pK a = 5.5) is reduced to produce a much stronger base, i.e., tetramethylpiperidine (TMP, pK a = 11.4) in a temperature dependent process. This oxidation-reduction process is dramatically accelerated at elevated temperature, which allows for thermally controlled initiation of the base-catalyzed thiol-Michael addition reaction and potentially other base-catalyzed reaction systems. Several critical factors that affect base generation from TEMPO reduction were investigated via systematic variation of reaction conditions including the solvent, temperature, and the thiol type and concentration. The highly temperature-dependent attributes of this redox reaction were demonstrated in various thiol-TEMPO based systems and were further utilized to thermally control thiol-Michael polymerizations under different heating conditions. The strong amine species, TMP, formed at elevated temperatures from the TEMPO-thiol interaction combined with high temperature, enables rapid formation of thiol-Michael-based polymer networks and large scale material preparation without any detrimental effects often associated with highly exothermic polymerizations. This novel approach to develop thermally-initiated thiol-Michael polymer networks is unique, versatile and robust, resulting in wide utility in applications such as facile handling of highly reactive resins, bulk material preparation, pH sensitive materials construction, and composite/macro-particle synthesis.

Effect of electromagnetic field accompanying the magnetic resonance imaging on human heart rate variability - a pilot study.

The effect of electromagnetic field on cardiovascular system in the literature is defined in ambiguous way. The aim of this study was to evaluate the effect of electromagnetic field on the heart rate variability (HRV) during the examination with magnetic resonance. Forty-two patients underwent Holter ECG heart monitoring for 30 minutes twice: immediately before and after the examination with magnetic resonance imaging (MRI). HRV was analysed by assessing a few selected time and spectral parameters. Is has been shown that standard deviation of NN intervals (SDNN) and very low frequency rates increased, whereas the low frequency:high frequency parameter significantly decreased following the MRI examination. These results show that MRI may affect the HRV most likely by changing the sympathetic-parasympathetic balance.

Visible-Light-Initiated Thiol-Michael Addition Polymerizations with Coumarin-Based Photobase Generators: Another Photoclick Reaction Strategy.

An efficient visible-light-sensitive photobase generator for thiol-Michael addition reactions was synthesized and evaluated. This highly reactive catalyst was designed by protecting a strong base (tetramethyl guanidine, TMG) with a visible-light-responsive group which was a coumarin derivative. The coumarin-coupled TMG was shown to exhibit extraordinary catalytic activity toward initiation of the thiol-Michael reaction, including thiol-Michael addition-based polymerization, upon visible-light irradiation, leading to a stoichiometric reaction of both thiol and vinyl functional groups. Owing to its features, this visible-light photobase generator enables homogeneous network formation in thiol-Michael polymerizations and also has the potential to be exploited in other visible-light-induced, base-catalyzed thiol-click processes such as thiol-isocynate and thiol-epoxy network-forming reactions.