Stimuli-responsive behavior

Polymeric materials are unique owing to the presence of a glass-transition temperature. At the glass-transition temperatures, the specific volume of the material and its rate of change changes, thus, affecting a multitude of physical properties. Numerous types of devices could be developed based on this type of stimuli—response behavior however, this technology is beyond the scope of this article. 

FIGURE 10.4 Illustration of stimuli-responsive behavior applied in the controlled drug delivery systems. 

In spite of significant experimental efforts made in the last decade, it remains difficult to provide an unambiguous proof of the dynamic (equilibrium) nature of polymeric micelles. A review of selected experimental results on the stimuli-responsive behavior of PE micelles is presented at the end of this chapter. 

In summary, the surface wetting shows reversible stimuli-responsive behavior when applying external stimuli (e.g., temperature and solvent), as observed via contact angle measurements. A contact angle shift of up to 23° from 61° to 84° and vice versa was observed after heating and DMF treatment, respectively (Fig. 61b). 

The Stimuli-Responsive Behavior and Self-Assembly Properties of ELRs. 150... 

The importance of ELRs resides in the fact that these polymers show a versatile and broad range of interesting properties above and beyond their simple mechanical performance that are not easily found together in other materials, including stimuli-responsive behavior or the ability to self-assemble. These properties arise due to a molecular transition of the polymer chain in the presence of water at temperatures above a certain level. This transition, known as the Inverse Temperature Transition (ITT) [7, 8], has become the key issue in the development of peptide-based polymers for use as molecular machines and materials. 

Yu, K., Han, Y. (2009). Effect of block sequence and block length on the stimuli-responsive behavior of polyampholyte brushes hydrogen bonding and electrostatic interaction as the driving force for surface rearrangement. Soft Matter, 5,759-768. 

Another interesting design of mixed polymer brushes was obtained by sequential grafting of asymmetric nonsticky/sticky diblock copolymers of poly(styrene-b-3-(trimethoxysilyl)propylmethacrylate) and monomethoxypoly(ethylene glycol)-trimethoxysilane onto silicon wafers (Han et al., 2013). The nanoscopic morphology of these mixed polymer brushes exhibited stimuli responsive behavior to various temperatures and solvents. 

Han, M., Ryu, J.-S., Park, J.-W. (2013). Homo- and mixed polymer brushes prepared by surface-grafting of asymmetric non-sticky/sticky diblock copolymers and their stimuli-responsive behaviors. Reactive and Functional Polymers, 73, 66-72. 

Surfaces tethered with such controlled polymer brushes and their stimuli-responsive behaviors have been characterized using a large number of techniques that have been recently reviewed. Among them, electrochemical techniques are particularly appealing as they could be used both for the characterization of the materials and for their actuation. For instance, polymer brushes are appealing materials for the design of electrochemical biosensors. In this context, polymer brushes are commonly characterized by... 

The goal of SGA is to monitor not only the important characteristics of the reactions, but also the onset and evolution of StimuU responsiveness of the polymers during synthesis. This will allow determination of what conditions are needed to produce specific stimuli-responsive behavior, which will help both in the fundamental understanding of these phenomena and materials, and in the practical task of optimizing their synthesis conditions. 

There are many natural hydrogels such as konnyaku, agar, and kamaboko. As polymer chemistry develops, synthetic polymer gels are increasingly used for separation, and as medical materials and sealants. However, these gels do not respond to external stimuli. Since the discovery and theoretical development of the volumetric phase transition by Tanaka (MIT) [9] many stimuli-responsive gels have been synthesized and their stimuli responsive behavior has been studied. 

Sensitivity, reversibility, accuracy, and self-assembly of such polymers are the keys to constructing intelligent stimuli-responsive systems and would be affected by the polymer primary structures. However, systematic investigations of the relationships between polymer structures and stimuli-responsive behavior were limited until several years ago, as the living polymerizations of related monomers involved in these syntheses were difficult to carry out. For example, the structure and molecular weight of polymers could not be freely controlled either for the conventionally investigated thermoresponsive polymers such as cellulose derivatives [8], poly(ethylene oxide) (PEO) derivatives [9], poly(methyl vinyl ether) [10], partly hydrolyzed poly(vinyl acetate) [11], and poly(M-alkylacrylamide)s [12,13], or for poly(J -isopropylacrylamide) (PNI-PAM), on which there have been advanced studies [14-18]. 

Living cationic polymerization has led to precision synthesis of functionalized polymers from species such as vinyl ethers, isobutene, and styrene derivatives [215]. Among those, vinyl ether polymers allow a wide choice of functional groups, which, along with the possibility of precision synthesis, has led to the development of a variety of stimuli-responsive polymers with well-defined structures [216]. This chapter describes our recent study on the synthesis and stimuli-responsive behavior of various poly(vinyl ether)s obtained by living cationic polymerization in the presence of an added base. 

Figure 20 Typical examples of stimuli-responsive behavior of poly(VE)s in water.

Xiong, Z., Peng, B., Han, X., Peng, C., Liu, H., Hu, Y. (2011). Dual-stimuli responsive behaviors of diblock polyampholyte PDMAEMA-b-PAA in aqueous solution. Journal of Colloid and Interface Science, 356, 557-565. 

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