Isochoric swelling

A gel can be viewed as a container of solvent made of a three dimensional mesh [9,10]. In a dried state, a gel is a solid material. However, a gel swells until it reaches the swelling equilibrium when a solvent is added. The solvent molecules are kept in the three dimensional mesh and the combination of the mesh and the solvent molecules creates a world having characteristic properties which will be described later. This world can be either isolated from (isochore) or linked to (isobar) its surrounding world by changing the population, i.e. the solvent molecules. 

The swelling behavior of poly(N-isopropylacrylamide) has been studied extensively [18,19]. It has been shown that this gel has a lower critical point due to the hydrophobic interaction. Such a swelling curve is schematically illustrated in Fig. 9. The gel is swollen at a lower temperature and collapses at a higher temperature if the sample gel is allowed to swell freely in water. The volume of the gel changes discontinuously at 33.6°C. The swelling curves obtained in this way correspond to the isobar at zero osmotic pressure. On the other hand, the friction coefficient is measured along the isochore, which is given in Fig. 9,... 

During the friction measurement, a test tube containing a small piece of gel is placed in the same water bath and the appearance of the gel is continuously monitored. The decrease of the diameter of the free gel from the isochore diameter is less than 10% at the highest temperature of the present friction experiment. This observation confirms that the gel swelling or shrinking as a whole should not have a significant effect on the friction measurement. A slight opacity of the gel develops in the gel in the vicinity of the temperature at which the friction of the gel diminishes and is presumably due to the dynamic density fluctuations. 

The mechanical properties of a gel are influenced by the degree of swelling. The volume phase transition and therefore the switching between two temperatures (above and below the temperature for volume phase transition) can be performed as a process under isobar or under isochoric conditions. For PNIPAAm, the bulk modulus shows a strong increase at 33 °C during heating. The abmpt change in the... 

The volume phase transition can be triggered by different quantities which are able to change the interaaions between polymer and solvent drastically. The macroscopic consequence is a swelling or deswelling (Figure 24). Other properties are changed drastically too. Therefore, it is possible to monitor the transition process by various analytical methods. An understanding of the volume phase transition needs a microscopic model of processes. A two-step model is discussed for the volume phase transition under isochore conditions. ... 

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