Solvent dependency, polymer brushes

A spiropyran derivative with an amine linker was synthesized and coupled to grafted brushes of PGMA and of PMAA activated with perfluorophenyl trifluoroacetate. Exposure of the modified brushes to UV light, which converts the attached moieties from the uncharged transparent spiropyran to the zwitterionic merocyanine form, rendered the brushes deep purple in color, fluorescent, and hydrophilic. The properties switched back by thermal or visible light-induced relaxation. The kinetics of the switching was found to be dependent on the chemical environment provided by the polymer brushes as well as on the polarity of added solvent. 

The first theories that implemented a proper balance of intramolecular interactions and conformational elasticity of the branches were developed by Daoud and Cotton [21] and by Zhulina and Birshtein [22-24]. These theories use scaling concepts (the blob model), originally developed by de Gennes and Alexander to describe the structure of semidilute polymer solutions [64] and planar polymer brushes [65, 66]. Here, the monomer-monomer interactions were incorporated on the level of binary or ternary contacts (corresponding to good and theta-solvent conditions, respectively), and both dilute and semidilute solutions of star polymers were considered. Depending on the solvent quality and the intrinsic stiffness of the arms, the branches of a star could be locally swollen, or exhibit Gaussian statistics [22-24]. 

The melt brush (see Fig. la), on the other hand, is free of solvent. Thus, its properties depend significantly less on the presence of a solvent. The melt brush has its main importance in the context of phase separated block copolymers and the often complex morphologies of these systems (for example, lamellar [see Fig. Ic], hexagonal, cubic) are a result of the balance between the interfacial tension and the interactions of the closely packed polymer chains [4-6]. While there is considerable interest in these bulk systems, it has been very advantageous to study polymer brushes as monolayers at flat surfaces. In such a monolayer it is relatively easy to determine and tune the surface concentration of the head groups and to give the system a preferred orientation in space. 

Samanta, S. Locklin, J. Formation of photochromic spiropyran polymer brushes via surface-initiated, ring-opening metathesis polymerization reversible photocontrol of wetting behavior and solvent dependent morphology changes. Langmuir 2008,24,9558-9565. 

We have stndied the macroscopic frictional properties of high-density polymer brushes prepared by surface-initialed ATRP of methyl methacrylate (MM A) [31] and hydrophilic methacrylates [32, 33] from silicon substrates. Friction tests were carried out using a stainless steel or glass ball as the sliding probe under a normal load of 100 MPa from the viewpoint of practical engineering applications. This chapter reviews the macroscopic frictional properties of polymer brushes under a high normal load, the dependence of solvent qnaUty, the effect of humidity on hydrophilic brnsh, and wear resistance, and we compare these with alkylsilane monolayers. 

Wang J, Mueller M Microphase separation of mixed polymer brushes dependence of the morphology on grafting density, composition, chain-length asymmetry, solvent quality, and selectivity, J Phys Chem B 113 11384—11402, 2009. 

However, in a series of publications Ito et al. [7,23,24,26] have shown that polymer brush-decorated membranes offer a much faster response while depending on the mechanical properties of the porous membrane, they can be more mechanically robust. In these publications it was also demonstrated that different types of external stimuli, such as variation of pH, temperature, solvent quality, and ultraviolet irradiation, might be used for the control of solvent permeation through the membranes. 

In block-copolymer brushes (Fig. 18.6) two or more chemically different polymers (typically two or three different blocks) constitute a polymer brush with block-copolymer architecture. Responsiveness of these brushes is determined by phase segregation of unlike polymer blocks however, the structure of the brush layer depends on whether the AB block copolymer is tethered by the more (A) or the less (B) soluble block. In poor solvents... 

---