Allyl sulfides and α‐substituted acrylates as addition–fragmentation chain transfer agents for methacrylate polymer networks

The rapid and uncontrolled nature of network formation from di(meth)acrylate monomers produces high shrinkage stress and results in polymers with oftentimes brittle mechanical properties. Methods for regulating polymerization and network formation are sought. One option is the use of addition–fragmentation chain transfer (AFCT) agents, which are well known to control molecular weight and molecular weight distribution of monofunctional (meth)acrylates. A series of novel and previously described AFCT reagents were synthesized and screened with laser flash photolysis to determine reactivity. Well‐performing AFCT reagents were then tested in polymerizations with monofunctional and difunctional methacrylates. With monofunctional monomers, the molecular weight and polydispersity of the resultant linear polymers tend to decrease with the addition of AFCT agent. In copolymerization with dimethacrylate monomers, the AFCT agents were found to substantially lower and sharpen the glass transition. Sharpness of the glass transition is here indicative of a more regular and homogenous network. After coupling of the instruments, photorheology was performed simultaneously with real‐time IR to show an increase in monomer conversion at the time of gelation, which appears to have a positive effect on reducing shrinkage stress. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 394–406     

Between Aromatic and Quinoid Structure: A Symmetrical UV to Vis/NIR Benzothiadiazole Redox Switch

Reversibly switching the light absorption of organic molecules by redox processes is of interest for applications in sensors, light harvesting, smart materials, and medical diagnostics. This work presents a symmetrical benzothiadiazole (BTD) derivative with a high fluorescence quantum yield in solution and in the crystalline state and shows by spectroelectrochemical analysis that reversible switching of UV absorption in the neutral state, to broadband Vis/NIR absorption in the 1st oxidized state, to sharp band Vis absorption in the 2nd oxidized state, is possible. For the one-electron oxidized species, formation of a delocalized radical is confirmed by electron paramagnetic resonance spectroelectrochemistry. Furthermore, our results reveal an increasing quinoidal distortion upon the 1st and 2nd oxidation, which can be used as the leitmotif for the development of BTD based redox switches.     

The Intersection of Opposed Big Cells in The Real Flag Variety of Type G2

We compute the Euler characteristics of the individual connectedcomponents of the intersection of two opposed big cells in thereal flag variety of type G₂, verifying a conjecture of Rietsch[6]. 2000 Mathematical Subject Classification: primary 14M15; secondary20G20.     

Terahertz spectroscopy of enantiopure and racemic polycrystalline valine

Experimental and computational THz (or far-infrared) spectra of polycrystalline valine samples are reported. The experimental spectra have been measured using THz time-domain spectroscopy. Spectra of the pure enantiomers, both D and L, as well as the dl racemate have been taken at room temperature and low temperature (78 K). The spectra of the pure D and L enantiomers are essentially identical, and they are markedly different from the DL racemate. In addition, a temperature-dependent study of L-valine was undertaken in which the absorption maxima were found to red shift as a function of increasing temperature. The vibrational absorption spectra (frequencies and intensities) were calculated using the harmonic approximation with the Perdew-Burke-Ernzerhof (PBE) functional, localized atomic orbital basis sets, and periodic boundary conditions. The calculated and experimental spectra are in good qualitative agreement. A general method of quantifying the degree to which a calculated mode is intermolecular versus intramolecular is demonstrated, with the intermolecular motions further separated into translational versus rotational/librational motion. This allows straightforward comparison of spectra calculated using different basis sets or other constraints.     

Propriétés viscoélastiques du poly(éthylène téréphtalate) en déformations homogène et plastique

The viscoelastic behaviour of poly(ethylene terephthalate) drawn at two temperatures, viz. 80 (homogenous drawing) and 20° (cold drawing), has been examined in order to characterize the influence of the thermomechanical history in the vicinity of the glass transition temperature T_g (transition α). Moreover, the shrinkage behaviour which is produced near T_g allows characterization of the elastic and viscous components which are developed through these two drawing processes.     

Discrete Morse theory for totally non-negative flag varieties

In a seminal 1994 paper Lusztig (1994) [26], Lusztig extended the theory of total positivity by introducing the totally non-negative part (G/P)_?0 of an arbitrary (generalized, partial) flag variety G/P. He referred to this space as a “remarkable polyhedral subspace”, and conjectured a decomposition into cells, which was subsequently proven by the first author Rietsch (1998) [33]. In Williams (2007) [40] the second author made the concrete conjecture that this cell decomposed space is the next best thing to a polyhedron, by conjecturing it to be a regular CW complex that is homeomorphic to a closed ball. In this article we use discrete Morse theory to prove this conjecture up to homotopy-equivalence. Explicitly, we prove that the boundaries of the cells are homotopic to spheres, and the closures of cells are contractible. The latter part generalizes a result of Lusztig's (1998) [28], that (G/P)_?0 - the closure of the top-dimensional cell - is contractible. Concerning our result on the boundaries of cells, even the special case that the boundary of the top-dimensional cell (G/P)_>0 is homotopic to a sphere, is new for all G/P other than projective space.     

Vinyl Sulfonate Esters: Efficient Chain Transfer Agents for the 3D Printing of Tough Photopolymers without Retardation

The formation of networks through light‐initiated radical polymerization allows little freedom for tailored network design. The resulting inhomogeneous network architectures and brittle material behavior of such glassy‐type networks limit the commercial application of photopolymers in 3D printing, biomedicine, and microelectronics. An ester‐activated vinyl sulfonate ester (EVS) is presented for the rapid formation of tailored methacrylate‐based networks. The chain transfer step induced by EVS reduces the kinetic chain length of the photopolymer, thus shifting the gel point to higher conversion, which results in reduced shrinkage stress and higher overall conversion. The resulting, more homogeneous network is responsible for the high toughness of the material. The unique property of EVS to promote nearly retardation‐free polymerization can be attributed to the fact that after the transfer step no polymerizable double bond is formed, as is usually seen in classical chain transfer agents. Laser flash photolysis, theoretical calculations, and photoreactor studies were used to elucidate the fast chain transfer reaction and exceptional regulating ability of EVS. Final photopolymer networks exhibit improved mechanical performance making EVS an outstanding candidate for the 3D printing of tough photopolymers.