Swelling-deswelling process

A great number of researches have so far been carried on the incorporation of poly(IPAAm) and its copolymers in various biomedical devices, utilizing soluble/insoluble or swelling/deswelling processes in the temperature range of LCST. As overviewed by Okano et al. [44] these include drug delivery system (DDS) solute separation concentration of dilute solutions immobilization of enzymes detachment of cultured cells coupling to biomolecules, and other aspects. 

Yu, K., Han, Y. A stable PEO-tethered PDMS surface having controllable wetting property by a swelling-deswelling process. Soft Matter 2006, 2, 705. 

The swelling/deswelling process is time-consuming for gels in the millimeter and centimeter range. 

The third possible reason for the extremely slow deswelling relates to the efficiency of ion transport into and out of the gel. As will be seen later, the buffer systems we have used can significantly accelerate the charging process during swelling, but are probably ineffective in the discharging and deswelling process. 

Since buffers can be utilized to speed up swelling and deswelling processes, they may be useful even in physiochemical studies in which simple electrolytes are the desired medium. Thus, one might consider using a buffer to accelerate the ionization or deionization of the gel. When ionization equilibrium is established, the buffer can then be exchanged out of the gel by the simple electrolyte. This two step process may be much faster than the single step process using the simple salt by itself. 

A variety of techniques are known to monitor the swelUng/deswelling process. The simplest are based on the determination of degree of swelling in dependence on time. It is possible to measure the size change of a gel piece by optical methods or the increase in mass by weighing them. 

In any case, the displacement of cations, anions, and solvent molecules through the film/ electrolytic solution interface is likely to provoke a volume change in the ECP films [128]. Nevertheless, other reasons may explain such swelling/deswelling (contraction/expan-sion) processes. For example, it may also result from changes in conjugation length... 

Cyclic voltammetric behaviour of redox polymers including PVF has been studied previously in acetonitrile and in water solutions [18]. In acetonitrile, PVF exhibits stable, symmetrically shaped cyclic voltammetry peaks at potentials characteristic of oxidation and re-reduction of ferrocene sites in the polymer film. In aqueous electrolyte solutions, non-symmetrical peaks are evident in both anodic and cathodic branches. Differences in PVF behaviour in the two solvents have been attributed to solvent uptake in the polymer film (lower for aqueous solutions), changes in site-site interaction parameters for the polymer film (attractive for aqueous electrolytes and repulsive for acetonitrile electrolytes), and differences in deswelling processes in aqueous solution in the reduction half of the cycle as compared with the oxidation half (Figure 2.4). Acetonitrile is a better swelling solvent for PVF than water [18] and break in of the spin-coated films usually requires more cycling in water than in acetonitrile. 

The swelling and the deswelling processes follow apparent first-order kinetics and the rate constants are of almost the same orders whether obtained by microscopic observation or calorimetric measurement. By the addition of sodium chloride or acids, the rate constants of the swelling were depressed and those of the deswelling were increased. 

Solvent-sensitive hydrogels show a dependence of the swelling degree on the eomposition of the solvent. The swelhng/deswelling processes are usually too... 

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