The branching angle effect on the properties of rigid dendrimers studied by Monte Carlo simulation

We studied the properties of rigid dendrimers with different branching angles by means of Monte Carlo simulations on a coarse-grained level. It was found that the terminal groups of dendrimers with both rigid and flexible spacers could locate near the center of the molecule. In flexible dendrimers, the wide distribution is attributed to the back folding of flexible spacers, while in rigid dendrimers, it is caused by the branching angle effect that a branch will grow laterally due to the restriction of a non-zero branching angle. It has been established that the branching angle is a key parameter for rigid dendrimers, which can be applied to tune the properties of rigid dendrimers: decreasing branching angle is helpful to obtain dendrimers with a larger size, lower density, and more terminal groups locating at periphery.This work was supported by the Natural Science Foundation of China (No. 21464004); the State Key Laboratory Cultivation Base for the Chemistry and Molecular Engineering of Medicinal Resources (Nos. CMEMR2013-A08, CMEMR2013-C11 and CMEMR2018-C9); Guangxi Natural Science Foundation of China (Nos. 2014GXNSFAA118038 and 2015GXNSFCB139005) and the Program for Key Scientific Researchof Guangxi Normal University (No. 2013ZD004)

Non-volatile tuning of narrowband guided-mode resonance with strong absorption modulation

Dynamic resonance tuning enables the development of active optical devices with reconfigurable functionalities, which are of importance for many technological applications. In this work, we demonstrate non-volatile tuning of a narrowband guided-mode resonance (GMR) with strong light absorption modulation. By fabricating a waveguide grating integrated with a phase-change Ge2Sb2Te5 (GST) thin film, a tunable GMR is measured in the mid-infrared region. The GMR is shifted from 10.24 to 10.99 μm with a wavelength tuning range of 0.75 μm when the GST film is changed from the amorphous state to the crystalline phase. At the resonance wavelength of 10.24 μm, a modulation depth of 94% in absorption is measured. Continuous tuning of the GMR is also obtained by controlling the intermediate phases of the GST film. These results manifest the great potential of active GMRs, which are suitable for applications in infrared modulation, thermal emissivity control, camouflaging, etc

A Simple Stochastic Reaction Model for Heterogeneous Polymerizations

The stochastic reaction model (SRM) treats polymerization as a pure probability‐based issue, which is widely applied to simulate various polymerization processes. However, in many studies, active centers were assumed to react with the same probability, which cannot reflect the heterogeneous reaction microenvironment in heterogeneous polymerizations. Recently, we have proposed a simple SRM, in which the reaction probability of an active center is directly determined by the local reaction microenvironment. In this paper, we compared this simple SRM with other SRMs by examining living polymerizations with randomly dispersed and spatially localized initiators. The results confirmed that the reaction microenvironment plays an important role in heterogeneous polymerizations. This simple SRM provides a good choice to simulate various polymerizations

Surface-Initiated Polymerization with an Initiator Gradient: A Monte Carlo Simulation

Due to the difficulty of accurately characterizing properties such as the molecular weight (Mn) and grafting density (σ) of gradient brushes (GBs), these properties are traditionally assumed to be uniform in space to simplify analysis. Applying a stochastic reaction model (SRM) developed for heterogeneous polymerizations, we explored surface-initiated polymerizations (SIPs) with initiator gradients in lattice Monte Carlo simulations to examine this assumption. An initial exploration of SIPs with ‘homogeneously’ distributed initiators revealed that increasing σ slows down the polymerization process, resulting in polymers with lower molecular weight and larger dispersity (Đ) for a given reaction time. In SIPs with an initiator gradient, we observed that the properties of the polymers are position-dependent, with lower Mn and larger Đ in regions of higher σ, indicating the non-uniform properties of polymers in GBs. The results reveal a significant deviation in the scaling behavior of brush height with σ compared to experimental data and theoretical predictions, and this deviation is attributed to the non-uniform Mn and Đ