PNIPAAm gels

Table la. Properties of PNIPAAm gels at 25 CC when the polymerization reaction was quenched shortly after the gel point. Reprinted from Polymer International (1992) 28 29 by permission of the copyright holder, the Society of Chemical Industry [45]... 

Fig. 7. Heat transfer occurs much faster than mass transfer in temperature-sensitive gels. This example compares the rate of temperature change with the rate of shrinking for a 16 x 1 PNIPAAm gel taken from 27 °C to 35 °C [59]...

Fig. 9. Plot of normalized approach to equilibrium mass against the square root of time for a temperature-sensitive 10 x 4 PNIPAAm gel sheet swelling and shrinking between 10 and 25 °C-Shown are the curve fits to the kinetic data of theory developed from Fick s law of diffusion in a polymer-fixed reference frame [149]. The equilibrium degree of swelling is 17.0 at 10 °C and 11.1 at 25 °C the diffusion coefficients obtained from the curve fits are 2.3 x 10 7 cm2/s for swelling and 3.6 x 10 7 cm2/s for shrinking [121]...

Fig. 11. Diffusion coefficients for PNIPAAm gels obtained from swelling and shrinking experiments over small temperature intervals. The gels were initially equilibrated at 10 °C, then wanned in increments to 32 °C, where the process was reversed and the gels were cooled until they returned to 10 °C. The temperature axis is the temperature at which the volume change occurred (bath temperature). The trends are interpreted using polymer scaling concepts [121]. The lines drawn are to guide the eye...

Table 4. Scaling exponents from the scaling law D cc 4>" for PNIPAAm gels of various compositions caused to swell and shrink over a series of temperature intervals, as shown in Fig. 11 [121]...

Fig. 13. A swelling/shrinking cycle of PNIPAAm gel crosslinkcd in a phase-separated state, across the volume transition temperature of 33 °C. The time to complete one cycle is several thousand times less than required for a homogeneous PNIPAAm gel of identical chemical composition and dimensions. The lines are to guide the eye. Reprinted from Polymer Communications (1991) 32 322, by permission of the publishers, Butterworth Heinemann [65]...

The diffusion coefficients of acetaminophen in PNIPAAm gel at different temperatures were measured by a simple sorption experiment. PNIPAAm gel with a 10 x 4 composition (see Fig. 1) was synthesized in cylindrical form by the procedure described in Sect 4.1.1. After equilibration of a gel sample in water at... 

Fig. 15. Diffusion of dilute, aqueous acetaminophen into a long, swollen cylinder of 10x4 PNIPAAm gel at 25 °C. The diffusion coefficient is extracted from a nonlinear least squares curve fit of the exact solution for diffusion into a cylinder of infinite length immersed in a well-stirred solution of finite volume to the data [123, 149]...

Fig. 16. The diffusion coefficient of acetaminophen in 10 x 4 PNIPAAm gels falls as the swelling degree (Q) of the gel decreases due to increasing temperature. Below the transition temperature of the gel, the linear relationship between log D and (Q — 1) 1 predicted by the free volume theory of Yasuda et al. [10] is observed. Above the transition temperature, the theory underestimates D by 35 times. Reprinted from the Journal of Controlled Release (1992) 18 1, by permission of the publishers, Elsevier Science Publishers BV [70]...

The experiments which yielded the diffusion coefficients for acetaminophen in PNIPAAm gel in Fig. 16 also yielded the corresponding partition coefficients. While the diffusion coefficients fit theory, the partition coefficients as plotted in Fig. 18 do not at all. In fact, a trend opposite to theory is observed as the partition coefficients are seen to increase as the gel swelling decreases. In fact, above the transition temperature of the gel, at 35 °C, the partition coefficient is seven times the maximum possible size exclusion coefficient, 1. This implies the dominance of hydrophobic effects over steric effects, since acetaminophen is a relatively small, nonionic but hydrophobic solute, and while the gel mesh size shrinks with increasing temperature, its level of hydrophobicity increases with temperature. 

Drug Octanol/water partition coefficient E-o/w Partition coefficient between water and 10x4 PNIPAAm gel at 25 °C Partition coefficient between water and 10x4 PNIPAAm gel at 40 C... 

The slurry dewatering experiments were simple in principle but required great care to complete with accuracy. 10 x 4 PNIPAAm gels were synthesized as... 

Fig. 21. Coal slurry dewatering capability by gel extraction using thermally responsive 10x4 PNIPAAm gel [117], The final solids contents rises as the ratio of gel to slurry increases, but plateaus due to reduced sorption capacities of gels in high solids content slurries. The maximum extent of dewatering declines as the particles become finer...

For use in the gel electrophoresis system, the PNIPAAm gel must be formed in a buffer solution containing sodium dodecyl sulfate (SDS). These additional solutes present in the reaction mixture altered the polymerization of the gel most significantly, the initiator levels required to form the gel successfully in... 

Thus electrophoretic separation of proteins on PNIPAAm gel can be achieved, and the proteins can be recovered from the gel. However, the substantial technical difficulties we encountered made our vision of a simple swap of PNIPAAm gel for polyacrylamide gel in standard procedures impossible to realize. Thus, we discontinued further work on this project. However, the concept might still find use in cases of routine preparative gel electrophoresis where the problems and expense of solute recovery as described earlier were significant enough to warrant the developmental work required to make the PNIPAAm process practical. 

Many recent studies on polymer gels are related to volume phase transition phenomena of poly(acrylamide) PAAm gel [7] and poly(N-isopropyl acrylamide) PNIPAAm gel [8, 9]. The volume phase transition in gels was extensively studied by Tanaka and his coworkers [10, 11]. 

The lower critical solution temperature (LCST) of the pure, weakly cross-linked PNIPAAm gel is 34°C determined by the swelling method (Shibayama et al. 1994). In order to increase the mechanical properties hydrogels with higher cross-linking densities can be prepared. In Fig. 2 the temperature-dependent swelling behaviour... 

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