Carbon nitride-coated transparent glass vials as photoinitiators for radical polymerization

Benign polymerization routes offer new perspectives in current polymer technology. Especially for automated or continuous flow synthesis of polymers, new devices and principles have to be considered by the means of minimizing addition or separation sequences as well as the type of a polymer initiation. Near-UV and visible light-induced polymerization utilizing metal-free semiconductor polymeric carbon nitride (pCN) as heterogeneous photocatalyst was a first step into this direction. Moving from heterogeneous powder catalysis (which still requests catalyst separation) to surface photocatalysis via coating glass tubes or vials with pCN thin films is presented. Performance and effectivity of those photoactive reactors are proven by free radical photopolymerization of variety of monomers. Reusability of vials is demonstrated via reversible addition-fragmentation chain-transfer polymerization-assisted block copolymer synthesis. This strategy eliminates the necessity of adding or removing initiators, works at room temperature, and offers a platform for cheap and effective polymer synthesis at the age of automated synthesis

A numerical investigation on the use of the virtual element method for topology optimization on polygonal meshes

A classical formulation of topology optimization addresses the problem of finding the best distribution of an assigned amount of isotropic material that minimizes the work of the external forces at equilibrium. In general, the discretization of the volume-constrained minimum compliance problem resorts to the adoption of four node displacement-based finite elements, coupled with element-wise density unknowns. When regular meshes made of square elements are used, well-known numerical instabilities arise, see in particular the so-called checkerboarded patterns. On the other hand, when unstructured meshes are needed to cope with geometry of any shape, additional instabilities can steer the optimizer towards local minima instead of the expected global one. Unstructured meshes approximate the strain energy of the members of the arising optimal design with an accuracy that is strictly related to the geometrical features of the discretization, thus remarkably affecting the achieved layouts. In light of the above remarks, in this contribution we consider polygonal meshes and implement the virtual element method (VEM) to solve two classes of topology optimization problems. The robustness of the adopted discretization is exploited to address problems governed by (nearly incompressible and compressible) linear elasticity and problems governed by Stokes equations. Numerical results show the capabilities of the proposed polygonal VEM-based approach with respect to more conventional discretizations

A Unified Analysis of Balancing Domain Decomposition by Constraints for Discontinuous Galerkin Discretizations

The BDDC algorithm is extended to a large class of discontinuous Galerkin (DG) discretizations of second order elliptic problems. An estimate of C(1 + log(H/h))2 is obtained for the condition number of the preconditioned system where C is a constant independent of h or H or large jumps in the coefficient of the problem. Numerical simulations are presented which confirm the theoretical results. A key component for the development and analysis of the BDDC algorithm is a novel perspective presenting the DG discretization as the sum of element-wise “local” bilinear forms. The element-wise perspective allows for a simple unified analysis of a variety of DG methods and leads naturally to the appropriate choice for the subdomain-wise local bilinear forms. Additionally, this new perspective enables a connection to be drawn between the DG discretization and a related continuous finite element discretization to simplify the analysis of the BDDC algorithm.Boeing CompanyMassachusetts Institute of Technology (Zakhartchenko Fellowship

Optical Anisotropy of Carbon Nitride Thin Films and Photografted Polystyrene Brushes

Polymer brushes on surfaces enable advanced material design. In the present contribution, transparent and flat photoactive polymeric carbon nitride (pCN) thin films are employed as a photoactive substrate and primer layer to grow polystyrene (PS) brushes. These films are then characterized by ellipsometry. For the first time herein is reported on the optical anisotropy of pCN thin films revealing a high positive birefringence up to 0.71 with an in-plane nD of 2.54 making this material of high interest for photonic devices. Furthermore and rather surprising, the photografted polystyrene brushes exhibit an unusual high negative birefringence, too. This negative birefringence can be attributed to a practically complete stretching of the polymer chains throughout growth in the radical chain process. As the stretched PS brushes grafted from the pCN surfaces also provide unusual surface properties, the overall system can be of great interest for photonics, but also as mechanical coating and membranes for gas separation

Kontinuierliche heterogene Photokatalyse in seriellen Mikro‐Batch‐Reaktoren

Abstract Solid reagents, leaching catalysts, and heterogeneous photocatalysts are commonly employed in batch processes but are ill-suited for continuous-flow chemistry. Heterogeneous catalysts for thermal reactions are typically used in packed-bed reactors, which cannot be penetrated by light and thus are not suitable for photocatalytic reactions involving solids. We demonstrate that serial micro-batch reactors (SMBRs) allow for the continuous utilization of solid materials together with liquids and gases in flow. This technology was utilized to develop selective and efficient fluorination reactions using a modified graphitic carbon nitride heterogeneous catalyst instead of costly homogeneous metal polypyridyl complexes. The merger of this inexpensive, recyclable catalyst and the SMBR approach enables sustainable and scalable photocatalysis

Multigrid algorithms for hp-version interior penalty discontinuous Galerkin methods on polygonal and polyhedral meshes

In this paper we analyze the convergence properties of two-level and W-cycle multigrid solvers for the numerical solution of the linear system of equations arising from hp-version symmetric interior penalty discontinuous Galerkin discretizations of second-order elliptic partial differential equations on polygonal/polyhedral meshes. We prove that the two-level method converges uniformly with respect to the granularity of the grid and the polynomial approximation degree p, provided that the number of smoothing steps, which depends on p, is chosen sufficiently large. An analogous result is obtained for the W-cycle multigrid algorithm, which is proved to be uniformly convergent with respect to the mesh size, the polynomial approximation degree, and the number of levels, provided the latter remains bounded and the number of smoothing steps is chosen sufficiently large. Numerical experiments are presented which underpin the theoretical predictions; moreover, the proposed multilevel solvers are shown to be convergent in practice, even when some of the theoretical assumptions are not fully satisfied