Height-Switching Dynamics of Mixed Polymer Brushes with Polymers of Different Stiffnesses.

Mixed brushes consisting of flexible and semiflexible polymers of the same chain length exhibit a height-switching phenomenon because of rigidity-dependent critical adsorption [Yang et al. Macromolecules 2020, 53, 7369]. Semiflexible polymers stand higher at weak surface attraction (high temperature), but they close to the attractive surface at strong attraction (low temperature). In this work, the height-switching dynamics of the mixed polymer brushes is studied by Metropolis Monte Carlo simulation. The height-switching time is calculated by a sudden change in the surface attraction. Two surface attraction change modes, i.e., the weak-to-strong mode where the attraction is changed from weak to strong and the strong-to-weak mode where it is changed from strong to weak, are investigated. Simulation results show that the height-switching time is related to the grafting density, the polymer stiffness, and surface attraction change mode. We find that the height-switching time is significantly decreased for the strong-to-weak mode. And our results also show that the height switching in the mixed polymer brushes is reversible.

Simulation study on the critical adsorption and diffusion of polymer chains on heterogeneous surfaces with random adsorption sites.

The critical adsorption and diffusion of a linear polymer chain on a heterogeneous surface with randomly distributed adsorption sites are studied using dynamic Monte Carlo simulations. Results show that the critical fraction of the adsorption sites at which critical adsorption takes place decreases exponentially with the increasing polymer-surface attraction strength and, at the same time, decreases with the increasing intra-polymer attraction strength. For adsorbed polymers with large intra-polymer attraction strength, we also find an adsorption-induced structural transition from a three-dimensional compact globule to a two-dimensional compacted pancake with an increasing fraction of adsorption sites. Anomalous sub-diffusion is observed for the adsorbed polymer diffusion on heterogeneous surfaces, in contrast to the normal diffusion on a homogeneous surface. The polymer on heterogeneous surfaces shows larger fluctuation in the total surface attraction energy and a longer waiting time.

A simulation study on the effect of nanoparticle size on the glass transition temperature of polymer nanocomposites.

The effect of the size of nanoparticles, σNP, on the glass transition temperature, Tg, of polymer nanocomposites is studied by using molecular dynamics simulations. The variation of Tg with σNP shows two distinct behaviours for polymer nanocomposites at low and high volume fractions of nanoparticles (fNP). At a low fNP, Tg decays almost exponentially with σNP, whereas at a high fNPTg shows a complex behaviour: it initially increases and then decreases with increasing σNP. The decrease in Tg with σNP is due to the significant decrease of adsorbed polymer monomers, while the increase in Tg with σNP is attributed to the slower diffusion of larger nanoparticles. We have also investigated the diffusion and relaxation of polymer chains at a temperature above Tg for both low and high fNPs. The diffusion constant and relaxation time of polymer chains are highly consistent with the behaviour of Tg.

A novel shift in the glass transition temperature of polymer nanocomposites: a molecular dynamics simulation study.

The effect of the loading of nanoparticles on the glass transition temperature, Tg, of polymer nanocomposites is studied by using molecular dynamics simulations. Tg is estimated from the variation of system volume with temperature and the temperature-dependent diffusion of the polymer described by the Vogel-Fulcher-Tammann law. The estimated values of Tg from the two methods are consistent with each other. Results show that Tg can be regulated by changing the volume fraction of nanoparticles, fNP. A novel shift in Tg is observed, that is, Tg increases with fNP at fNP < , while it decreases with increasing fNP at fNP > . The basic mechanism behind the novel shift in Tg is the competition between the attraction of nanoparticles towards polymer chains and the fast diffusion of nanoparticles. The increase in Tg at low fNP is due to the attraction of nanoparticles, whereas the decrease in Tg at high fNP is attributed to the fast diffusion of nanoparticles. The diffusion of the polymer above Tg is also investigated. The diffusion of the polymer decreases with increasing fNP below and increases with fNP above , in agreement with the variation of Tg.

Controllable Synthesis of Cyclopenta[b]indolines via Photocatalytic Fluoroalkylative Radical Cyclization Cascade of Ynamides.

A de novo method for direct construction of cyclopenta[b]indolines via a photocatalytic fluoroalkylative radical cyclization cascade of ynamides has been established, which proceeds via a sequence of radical addition, 1,5-HAT, 5-endo-trig cyclization, intramolecular arylation, and oxidative deprotonation. This protocol allows for the controllable assembly of a tricyclic architecture with three contiguous stereocenters, showcasing its high efficiency, compatibility, and regio- and diastereoselectivity for accessing pharmacologically significant fluoroalkylated cyclopenta[b]indolines. It represents one of the very few examples of tetrafunctionalization of alkynes.