Using Copolymers to Design Tunable Stimuli-Reponsive Brushes

Unsynchronous conformational transitions induced by the asymmetric adsorption-response of an active diblock copolymer in an inert brush

To exploit the chemical asymmetry of diblock copolymer chains on the design of high-performance switch sensors, we propose an analytically tractable model system which contains an adsorption-responsive diblock copolymer in an otherwise inert brush, and study its phase transitions by using both analytical theory and self-consistent field calculations. The copolymer chain is chemically asymmetric in the sense that the two blocks assume different adsorption strengths, which is characterized by the defined adsorption ratio. We found that the conformation states, the number of stable phases, and transition types are mainly controlled by the length of each block and the adsorption ratio. In particular, when the length of the ungrafted block is longer than the brush chains, and the adsorption ratio is smaller than a critical value, the copolymer chain shows three thermodynamically stable states, and undergoes two unsynchronous transitions, where the two blocks respond to the adsorption in a different manner, when the adsorption changes from weak to sufficiently strong. For this kind of three-state transition, the transition point, transition barrier, and transition width are evaluated by using the self-consistent field method, and their scaling relationship with respect to the system parameters is extracted, which matches reasonably well with the predictions from the analytical theory. The self-consistent field calculations also indicate that the conformational transitions involved in the three-state transition process are sharp with a low energy barrier, and interestingly, barrier-free transitions are observed. Our finding shows that the three-state transitions not only specify a region where high performance unsynchronous switch sensors can be exploited, but may also provide a useful model understanding the unsynchronous biological processes.

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.

Fluoropolymer: A Review on Its Emulsion Preparation and Wettability to Solid-Liquid Interface

In the preparation of a superamphiphobic surface, the most basic method is to reduce the surface free energy of the interface. The C-F bond has a very low surface free energy, which can significantly change the wettability of the solid-liquid interface and make it a hydrophobic or oleophobic, or even superamphiphobic surface. Based on the analysis of a large number of research articles, the preparation and application progress in fluoropolymer emulsion were summarized. After that, some corresponding thoughts were put forward combined with our professional characteristics. According to recent research, the status of the fluoropolymer emulsion preparation system was analyzed. In addition, all related aspects of fluoropolymer emulsion were systematically classified in varying degrees. Furthermore, the interaction between fluoropolymer structure and properties, especially the interaction with nanomaterials, was also explored. The aim of this review is to try to attract more scholars' attention to fluorocarbon interfacial materials. It is expected that it will make a certain theoretical and practical significance in the preparation and application of fluoropolymer.

Glass transition and dynamics of semiflexible polymer brushes

The glass transition and dynamics of densely grafted semiflexible polymer brushes are studied by molecular dynamics simulation. The glass transition temperature (T_g) increases with the polymer rigidity. The local glass transition temperature (T_g,local) is estimated from the temperature-dependent dynamics of individual segments including the lateral position fluctuation and lateral mean square displacement. Different from the flexible polymer brush, T_g,local of semiflexible polymer brushes is roughly independent of the segment height. Our simulation reveals that the glass transition is in synchronism with an abrupt change of the chain conformation in semiflexible polymer brushes. When the temperature drops to near T_g, the semiflexible polymer chains elongate, tilt, and become more ordered. Moreover, enhanced segmental dynamics is observed at temperatures just above T_g for the semiflexible polymer brushes.

Conformational transitions of adsorption-responsive single diblock copolymers in homopolymer brushes

Using self-consistent field calculations, we examine the effect of brush polydispersity on conformational transitions of single adsorption-active diblock copolymer chains embedded in inert polydisperse brushes. To represent the polydispersity, we adopt the continuous Schulz-Zimm chain length distributions, and three typical distributions are chosen such that a wide range of polydispersity is covered. A phase diagram of the diblock copolymer switches has been constructed showing that the first order phase transitions occupy a larger space in the case of polydisperse brushes. We further characterize these first order phase transitions by specifying their transition points, transition widths and transition barriers, where the latter two are particularly important as they determine the performance of the polymer switches. Our calculation indicates that polydispersity has different effect on the switching behavior depending on the lengths of both the active block and the inert block of the copolymer switch chain. In general, polydispersity improves the switching performance in the case of short active blocks, i.e. shorter or not very longer than the average length of the brush chains, and the corresponding energy barrier is smaller than a few kBT. In contrast, monodisperse brushes have the advantages when these two blocks are particularly long, i.e., lower transition barriers and fast switching. Notably, when the inert block approaches the average length of the brush chains, the transition barrier becomes almost zero in any case for monodisperse brushes, while a large finite value is still observed for that in polydisperse brushes. The complex interplay between the brush polydispersity and the switch behavior is attributed to the wide-range repulsions generated by the polydisperse brushes.