Collapsed state

Other interesting Langmuir monolayer systems include spread thermotropic liquid crystals where a foam structure forms on expansion from a collapsed state [23]. Spread monolayers of clay dispersions form a layer of overlapping clay platelets that can be subsequently deposited onto solid substrates [24]. 

In the collapse phase the monomer density p = N/R is constant (for large N). Thus, the only confonnation dependent tenn in (C2.5.A1) comes from the random two-body tenn. Because this tenn is a linear combination of Gaussian variables we expect that its distribution is also Gaussian and, hence, can be specified by the two moments. Let us calculate the correlation i,) / between the energies and E2 of two confonnations rj ]and ry jof the chain in the collapsed state. The mean square of E is... 

For (Ar) interactions the collapsed state of the polymer is a tight globule from which solvent is excluded. Figure 15 shows the polymer bead and solvent radial distribution functions relative to the center of mass of the globule,... 

This, however, is not the only way multilayers of polymers can be formed. If the macromolecule is a compactly folded copolymer, such as a protein, its collapsed state will be determined by internal compensation of energetically favorable interactions. 

LCST) of 32°C. At temperatures lower than 32°C, the gel is swollen, whereas at temperatures higher than 32°C, the gel dehydrates to the collapsed state due to the breakdown of the delicate hydrophilic/hydrophobic balance in the network structure. 

The brash layer thickness (dry collapsed state) obtained after seven days of polymerization time and successive soxhlet extraction was found to be approx. 10 nm and very uniform ( 0.3 nm). The uniform thickness values are provided by the homogeneous initiation, polymerization and termination reaction. Meanwhile poly(2-oxazoline) homopolymers brushes with layer thicknesses of 20 to 30 nm can be obtained [275]. 

The properties of both organic matter and clay minerals may affect the release of contaminants from adsorbed surfaces. Zhang et al. (1990) report that desorption (in aqueous solution) of acetonitrille solvent from homoionic montmorillonite clays is reversible, and hysteresis appears to exist except for K+-montmorillonite. This behavior suggests that desorption may be affected by the fundamental difference in the swelling of the various homoionic montmorillonites, when acetonitrile is present in the water solution. During adsorption, it was observed that the presence of acetonitrile affects the swelling of different homoionic clays. At a concentration of 0.5 M acetonitrile in solution, the layers of K+-montmorillonite do not expand as they would in pure water, while the layers of Ca +- and Mg +-montmorillonite expand beyond a partially collapsed state. The behaviors of K+-, Ca +-, and Mg +-montmorillonite are different from the behavior of the these clays in pure water. Na+-montmorillonite is not affected by acetonitrile presence in an aqueous solution. 

FIGURE 4.6 II versus A isotherms for different types of states (a) gas film (b) liquid-expanded (Lex) and liquid-condensed (Lco) solid films collapse state. 

The measurements of n versus A isotherms generally exhibit, when compressed, a sharp break in the isotherms that has been connected to the collapse of the mono-layer under given experimental conditions. The monolayer of some lipids, such as cholesterol, is found to exhibit an unusual isotherm (Figure 4.7). The magnitude of FI increases very little as compression takes place. In fact, the collapse state or point is the most useful molecular information from such studies. It has been found that this is the only method that can provide information about the structure and orientation of amphiphile molecules at the surface of water (Birdi, 1989). 

Grazing incidence x-ray diffraction (GID) measurements have indicated that both precollapse and collapsed state monolayers at the air-water interface can be crystalline (Birdi, 1989). A general procedure was delineated that could provide near-atomic resolution of two-dimensional crystal structures of -triacontanoic acid (C29H59COOH). A monolayer composed of rod-like molecules would generally pack in such a way that each molecule has six nearest neighbors, that is, hexagonal cell. 

Molecular domains have been observed using the AFM analyses of LB films of a collapsed state (Birdi, 1997, 2003). For example, cholesterol films showed half-but-terfly-shaped domains (each domain consisting of 107 molecules Figure 10.4). This quantity was estimated from the following data the height of the domains was 90 A, which corresponds with six layers of the cholesterol molecule (length of the molecule is found to be 15 A from molecular models). AFM image analysis is capable... 

Some gels were synthesized which underwent the phase transition twice as the solvent composition was monotonically varied from 0% to 100%. Figure 30 shows the swelling behaviors of NIPA (open circles) and acrylamide (solid circles) gels in a mixture of dimethylsulfoxide (DMSO) and water [24]. When the DMSO concentration was lower than 33%, the NIPA gel was slightly swollen. Above 33% DMSO, a discrete transition to a collapsed state occurred. However, the gel showed a discontinuous re-swelling at 90%. Such reentrant behavior has also been observed when temperature or pH were used as variables. 

Fig. 31. Temperature dependence for equilibrated volumes of NIPA gel including the Con A-DDS complex (DSS-gel, open circles), MP (MP-gel, filled circles), and free of both DSS and MP squares). The latter was prepared as a control sample. Hysteresis was observed in the volume changes of DSS-gel and the free-Con A gel on heating and cooling, indicating a discontinuous phase transition. The diameter of each gel in the collapsed state, determined at 50 °C, was do = 0.074 mm the volume of this gel is denoted by V0. The concentration of dry matter in the collapsed state was estimated from the preparation recipe to be 90wt%.

It is possible to include real chain microstructure into consideration for the case when Gy, 1 both in the swollen and the collapsed state. Then, it becomes possible to use virial expansion for Fin, [23-25] ... 

The simplest way to consider the formation of ion pairs was discussed in Ref. [29]. It was shown that the concentration of ion pairs exponentially increases with the decrease of the dielectric constant of the solvent . This effect should be taken into account in the theory of collapse of polyelectrolyte networks, because effective values of in the collapsed state are usually much less than in the swollen state (e depends mainly on the water content which is much larger in swollen networks). This effect has not been taken into account in the theories developed so far. 

The systematic investigation of the influence of the topological structure of weakly charged networks on the type of occurrence of the transition in the collapsed state was performed by Ilavsky and coworkers [42, 43]. In these papers, the authors studied the swelling and the elastic characteristics of PAA gels, containing a small number of anionic SMA groups. It has been shown that ... 

When our work started, the phase transitions was observed only for weakly charged networks of PAA gels swollen in the mixtures of water (good solvent) with acetone (precipitant) of different compositions. The first stage of our work was the investigation of the nature and polarity of a precipitant on the position and the amplitude of the phase transition. According to the results of theoretical consideration of Refs. [7,18,20], the transition point and the value of the jump of the volume are primarily determined by the network structure and by the parameter of polymer-solvent interaction Xns- By smoothly changing the composition of the binary solvent, it is possible to vary effective value of the Xns parameter and to convert the network to a collapsed state. In this case, the amplitude of phase transition should not depend on the nature of precipitant. 

The experimental result obtained was explained by the formation of ion pairs between the charges of a network and counter ions. The theoretical analysis of this problem has shown that the degree of ion pairs formation very strongly (exponentially) depends on e (cf. Sect. 2.2). Thus, if the precipitant has a small dielectric constant e (e.g. dioxane) the degree of dissociation of ion pairs is sufficiently small and this fact leads to the decrease of the osmotic pressure of counter ions which defines the swelling of the gel and the point of the transition in the collapsed state. As a result, in this case the degree of swelling of the gel near the transition point is less pronounced than for other solvents and only a relatively small amount of the precipitant is required to reach this point. In... 

In order to find the influence of compression on collapse of the polymer networks, the experiments on the swelling of the deformed gels of AA-SMA in water-methanol and water-dioxane mixtures were performed [29]. It was shown that uniaxial compression of the gel really affects the swelling curves and that, in a good agreement with the theory, the region of stability of the collapsed state increases and the sharpness of collapse decreases under compression. 

In our experiments, we also noticed that for neutral and for polyampholyte networks near the isoelectric point the transition in the collapsed state is essentially smoother than for polyelectrolyte gels with charges of one sign, in good agreement with the theory. 

---