Developments in Dynamic Covalent Chemistries from the Reaction of Thiols with Hexahydrotriazines.

Dynamic covalent chemistries have garnered significant attention for their potential to revolutionize technologies in the material fields (engineering, biomedical, and sensors) and synthetic design strategies as they provide access to stimuli responsiveness and adaptive behaviors. However, only a limited number of molecular motifs have been known to display this dynamic behavior under mild conditions. Here, we identified a dynamic covalent motif-thioaminals-that is produced from the reaction of hexahydrotriazines (HTs) with thiols. Furthermore, we report on the synthesis of a new family of step-growth polymers based on this motif. The condensation efficiently proceeds to quantitative yields within a short time frame and offers versatility in functional group tolerance; thus, it can be exploited to synthesize both small molecule thioaminals as well as high molecular weight polymers from the step-growth polymerization of HTs with dithiols. Careful evaluation of substituted HTs and organic thiols supported by DFT calculations led to a chemically diverse library of polymers based on this motif. Finally, dynamic substitution reactions were employed toward the facile preparation of functional oligomers and macromolecules. This dynamic covalent motif is particularly attractive for a range of applications that include material design and drug delivery due to the economic feasibility of synthesis.

Development of a method for detecting trace metals in aqueous solutions based on the coordination chemistry of hexahydrotriazines.

The detection of trace amounts (<10 ppb) of heavy metals in aqueous solutions is described using 1,3,5-hexahydro-1,3,5-triazines (HTs) as chemical indicators and a low cost fluorimeter-based detection system. This method takes advantage of the inherent properties of HTs to coordinate strongly with metal ions in solution, a fundamental property that was studied using a combination of analytical tools (UV-Vis titrations, (1)H-NMR titrations and computational modeling). Based on these fundamental studies that show significant changes in the HT UV signature when a metal ion is present, HT compounds were used to prepare indicator strips that resulted in significant fluorescence changes when a metal was present. A portable and economical approach was adopted to test the concept of utilizing HTs to detect heavy metals using a fluorimeter system that consisted of a low-pressure mercury lamp, a photo-detector, a monolithic photodiode and an amplifier, which produces a voltage proportional to the magnitude of the visible fluorescence emission. Readings of the prepared HT test strips were evaluated by exposure to two different heavy metals at the safe threshold concentration described by the U.S. Environmental Protection Agency (EPA) for Cr(3+) and Ag(2+) (100 µg L(-1) and 6.25, respectively). This method of detection could be used to the presence of either metal at these threshold concentrations.

Thermal conductivity in nanostructured films: from single cellulose nanocrystals to bulk films.

We achieved a multiscale description of the thermal conductivity of cellulose nanocrystals (CNCs) from single CNCs (∼0.72-5.7 W m(-1) K(-1)) to their organized nanostructured films (∼0.22-0.53 W m(-1) K(-1)) using experimental evidence and molecular dynamics (MD) simulation. The ratio of the approximate phonon mean free path (∼1.7-5.3 nm) to the lateral dimension of a single CNC (∼5-20 nm) suggested a contribution of crystal-crystal interfaces to polydisperse CNC film's heat transport. Based on this, we modeled the thermal conductivity of CNC films using MD-predicted single crystal and interface properties along with the degree of CNC alignment in the bulk films using Hermans order parameter. Film thermal conductivities were strongly correlated to the degree of CNC alignment and the direction of heat flow relative to the CNC chain axis. The low interfacial barrier to heat transport found for CNCs (∼9.4 to 12.6 m(2) K GW(-1)), and their versatile alignment capabilities offer unique opportunities in thermal conductivity control.

Acrylic Platform from Renewable Resources via a Paradigm Shift in Lactide Polymerization.

A new polyacrylate, poly(methylidenelactide), with high thermal stability and derived from biobased resources is reported. This polymer is formed from the radical polymerization of a modified lactide derivative and represents one of the few examples of an acrylic from which the entire mass is bioderived and is made from a simplistic synthesis. Furthermore, poly(methylidenelactide) serves as a foundation for a platform of new acrylic structures, owing to pendant cyclic diesters that are susceptible to postpolymerization modification via simple transesterification chemistry. Several examples of unique acrylics made from poly(methylidenelactide) are synthesized and characterized.

Recyclable, strong thermosets and organogels via paraformaldehyde condensation with diamines.

Nitrogen-based thermoset polymers have many industrial applications (for example, in composites), but are difficult to recycle or rework. We report a simple one-pot, low-temperature polycondensation between paraformaldehyde and 4,4'-oxydianiline (ODA) that forms hemiaminal dynamic covalent networks (HDCNs), which can further cyclize at high temperatures, producing poly(hexahydrotriazine)s (PHTs). Both materials are strong thermosetting polymers, and the PHTs exhibited very high Young's moduli (up to ~14.0 gigapascals and up to 20 gigapascals when reinforced with surface-treated carbon nanotubes), excellent solvent resistance, and resistance to environmental stress cracking. However, both HDCNs and PHTs could be digested at low pH (<2) to recover the bisaniline monomers. By simply using different diamine monomers, the HDCN- and PHT-forming reactions afford extremely versatile materials platforms. For example, when poly(ethylene glycol) (PEG) diamine monomers were used to form HDCNs, elastic organogels formed that exhibited self-healing properties.

Analysis of deprotonated acids with silicon nanoparticle-assisted laser desorption/ ionization mass spectrometry.

Chemically modified silicon nanoparticles were applied for the laser desorption/negative ionization of small acids. A series of substituted sulfonic acids and fatty acids was studied. Compared to desorption ionization on porous silicon (DIOS) and other matrix-less laser desorption/ionization techniques, silicon nanoparticle-assisted laser desorption/ionization (SPALDI) mass spectrometry allows for the analysis of acids in the negative ion mode without the observation of multimers or cation adducts. Using SPALDI, detection limits of many acids reached levels down to 50 pmol/µl. SPALDI of fatty acids with unmodified silicon nanoparticles was compared to SPALDI using the fluoroalkyl silylated silicon powder, with the unmodified particles showing better sensitivity for fatty acids, but with more low-mass background due to impurities and surfactants in the untreated silicon powder. The fatty acids exhibited a size-dependent response in both SPALDI and unmodified SPALDI, showing a signal intensity increase with the chain length of the fatty acids (C12-C18), leveling off at chain lengths of C18-C22. The size effect may be due to the crystallization of long chain fatty acids on the silicon. This hypothesis was further explored and supported by SPALDI of several, similar sized, unsaturated fatty acids with various crystallinities. Fatty acids in milk lipids and tick nymph samples were directly detected and their concentration ratios were determined by SPALDI mass spectrometry without complicated and time-consuming purification and esterification required in the traditional analysis of fatty acids by gas chromatography (GC). These results suggest that SPALDI mass spectrometry has the potential application in fast screening for small acids in crude samples with minimal sample preparation.

Strong, low-density nanocomposites by chemical vapor deposition and polymerization of cyanoacrylates on aminated silica aerogels.

Strong polymer-silica aerogel composites were prepared by chemical vapor deposition of cyanoacrylate monomers onto amine-modified aerogels. Amine-modified silica aerogels were prepared by copolymerizing small amounts of (aminopropyl)triethoxysilane with tetraethoxysilane. After silation of the aminated gels with hexamethyldisilazane, they were dried as aerogels using supercritical carbon dioxide processing. The resulting aerogels had only the amine groups as initiators for the cyanoacrylate polymerizations, resulting in cyanoacrylate macromolecules that were higher in molecular weight than those observed with unmodified silica and that were covalently attached to the silica surface. Starting with aminated silica aerogels that were 0.075 g/cm(3) density, composite aerogels were made with densities up to 0.220 g/cm(3) and up to 31 times stronger (flexural strength) than the precursor aerogel and about 2.3 times stronger than an unmodified silica aerogel of the same density.