Responsive polymer solutions

Several attempts have been made to construct stimuli-responsive polymer solution and gel systems which undergo isothermal phase transitions by external stimulation, such as photons or chemicals. Aqueous solutions of poly(A -isopropylacrylamide) having photoisomerizable chromophores or host molecules in the pendant groups showed reversible phase separations by photoirradiation or by the addition of specific metal or ammonium ions. The gels made of the polymers also underwent photostimulated or chemical-induced volume phase transitions. 

We hope that this brief review has given the reader a general feeling of the development and application of CE in the separation of nucleic acids. With the advent of capillary array electrophoresis and microchip electrophoresis, as well as remarkable improvements in separation matrices, CE has become a standardized and cost-effective technique in the separation of nucleic acids. Novel thermo-responsive polymer solutions combine the merits of different monomers, and offer the possibility to fine-tune the desirable properties of polymer molecular architecture and chemical composition. Artificial entropic trapping systems obviate the use of viscous polymer solutions, and even offer fast, unattended, miniaturized, and multiplexed platforms. Optimizing the geometry of these electrophoretic systems to both increase the separation and reduce the diffusion (band broadening) is the main topic for future research. 

Abstract This chapter describes polymers that undergo a temperature-induced phase transition in aqueous solution providing an important basis for smart materials. Different types of temperature-responsive polymers, including shape-memory materials, hquid crystalline materials and responsive polymer solutions are briefly introduced. Subsequently this chapter will focus on thermoresponsive polymer solutions. At first, the basic principles of the upper and lower critical temperature polymer phase transitions will be discussed, followed by an overview and discussion of important aspects of various key types of such temperature-responsive polymers. Finally, selected potential apphcations of thermoresponsive polymer solutions will be described. 

So far we have considered linear transport phenomena, in which the response is directly proportional to the circumstance causing the response. Polymer solutions, however, are fundamentally nonlinear, and show a wide variety of additional behaviors not expected from simple linear descriptions. These behaviors may be divided, somewhat crudely, into unusual flow behaviors arising from nonzero normal stress differences, time-dependent phenomena in which the system shows memory, so that the response to a series of forces depends on when they were applied, and several modern discoveries not discussed in more classical references. 

By comparing impedance results for polypyrrole in electrolyte-polymer-electrolyte and electrode-polymer-electrolyte systems, Des-louis et alm have shown that the charge-transfer resistance in the latter case can contain contributions from both interfaces. Charge-transfer resistances at the polymer/electrode interface were about five times higher than those at the polymer/solution interface. Thus the assignments made by Albery and Mount,203 and by Ren and Pickup145 are supported, with the caveat that only the primary source of the high-frequency semicircle was identified. Contributions from the polymer/solution interface, and possibly from the bulk, are probably responsible for the deviations from the theoretical expressions/45... 

Poly[(4-carboxylatophenoxy)(methoxyethoxyethoxy)phosphazene] copolymers of variable compositions were synthesized by Allcock [645] in 1996. These polymers were found to be soluble in alkaline solutions. When crosslinked (by y-rays or by addition of CaCl2 to the polymer solution) the resulting hydrogels were found able to contract or expand as a function of the pH of the solution and their utilization as pH-responsive materials for drug delivery systems could be envisaged. 

The recent interest in substituted silane polymers has resulted in a number of theoretical (15-19) and spectroscopic (19-21) studies. Most of the theoretical studies have assumed an all-trans planar zig-zag backbone conformation for computational simplicity. However, early PES studies of a number of short chain silicon catenates strongly suggested that the electronic properties may also depend on the conformation of the silicon backbone (22). This was recently confirmed by spectroscopic studies of poly(di-n-hexylsilane) in the solid state (23-26). Complementary studies in solution have suggested that conformational changes in the polysilane backbone may also be responsible for the unusual thermochromic behavior of many derivatives (27,28). In order to avoid the additional complexities associated with this thermochromism and possible aggregation effects at low temperatures, we have limited this report to polymer solutions at room temperature. 

Factorizability has also been found to apply to polymer solutions and melts in that both constant rate of shear and dynamic shear results can be analyzed in terms of the linear viscoelastic response and a strain function. The latter has been called a damping function (67,68). 

In the previous sections, we described the overall features of the heat-induced phase transition of neutral polymers in water and placed the phenomenon within the context of the general understanding of the temperature dependence of polymer solutions. We emphasised one of the characteristic features of thermally responsive polymers in water, namely their increased hydropho-bicity at elevated temperature, which can, in turn, cause coagulation and macroscopic phase separation. We noted also, that in order to circumvent this macroscopic event, polymer chemists have devised a number of routes to enhance the colloidal stability of neutral globules at elevated temperature by adjusting the properties of the particle-water interface. 

The term y(t,t ) is the shear strain at time t relative to the strain at time t. The use of a memory function has been adopted in polymer modelling. For example this approach is used by Doi and Edwards11 to describe linear responses of solution polymers which they extended to non-linear viscoelastic responses in both shear and extension. 

Fluorescence is also a powerful tool for investigating the structure and dynamics of matter or living systems at a molecular or supramolecular level. Polymers, solutions of surfactants, solid surfaces, biological membranes, proteins, nucleic acids and living cells are well-known examples of systems in which estimates of local parameters such as polarity, fluidity, order, molecular mobility and electrical potential is possible by means of fluorescent molecules playing the role of probes. The latter can be intrinsic or introduced on purpose. The high sensitivity of fluo-rimetric methods in conjunction with the specificity of the response of probes to their microenvironment contribute towards the success of this approach. Another factor is the ability of probes to provide information on dynamics of fast phenomena and/or the structural parameters of the system under study. 

AS the solvent in a polymer solution becomes poorer, e.g., through a temperature change, a phase transition will eventually take place. There have been a number of reports on the phase transition polymers in response to various external stimuli such as pH [1-5], temperature [6-10], light [11-14], and chemical substances [15-20], These polymer systems have been model systems for understanding the fundamental and classic problems in polymer physics. 

It has been shown (, , 2.) that a membrane casting dope is a strongly structurlzed polymer solution, and that the morphology of the membrane surface layer can be correlated to the structure of the casting solution. The latter parameter affects the nature and details of the phase inversion process occuring in the upper part of the cast solution, in an incipient skin. Thus the solution structure is one of the factors responsible for the skin properties, and consequently for the performance of the ultimately formed asymmetric membrane. 

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