Bad solvents

Koutsos V, van der Vegte E W, Pelletier E, Stamouli A and Hadziioannou G 1997 Structure of chemically end-grafted polymer chains studied by scanning force microscopy in bad-solvent conditions Macromolecules 30 4719-26... 

In summary, water is clearly an extremely bad solvent for coordination of a hard Lewis acid to a hard Lewis base. Hence, catalysis of Diels-Alder reactions in water is expected to be difficult due to the relative inefficiency of the interactions between the Diels-Alder reactants and the Lewis-acid catalyst in this medium. 

The structure of the chain, i.e., whether it is a helix or a random coil, might influence not only the rate but also the stereospecificity of the growing polymer. For example, it is plausible to expect that in normal vinyl polymerization helix formation might favor specific placement, say isotactic, while either placement would be approximately equally probable in a growing random coil. Formation of a helix requires interaction between polymer segments, and this intramolecular interaction is enhanced by bad solvents particularly those which precipitate the polymer. 

In a bad solvent collapse of the expanded conformation can occur in two principally different ways (Fig. 21) ... 

The presence of a folded state was evaluated by studying the spectroscopic properties of 1-9 in both chloroform and acetonitrile, which are good and bad solvents, respectively [23,25,26]. The molar absorptivity of 1-9 in chloroform increased in a linear fashion with respect to oligomer length (Fig. 2). However, when UV spectra were taken in acetonitrile two linear trends were identified. Oligomers 1-5 displayed a slope similar to that observed in chloroform, while 4-9 displayed a much smaller slope. The decrease in slope is... 

As for direct emulsions, the presence of excess surfactant induces depletion interaction followed by phase separation. Such a mechanism was proposed by Binks et al. [ 12] to explain the flocculation of inverse emulsion droplets in the presence of microemulsion-swollen micelles. The microscopic origin of the interaction driven by the presence of the bad solvent is more speculative. From empirical considerations, it can be deduced that surfactant chains mix more easily with alkanes than with vegetable, silicone, and some functionalized oils. The size dependence of such a mechanism, reflected by the shifts in the phase transition thresholds, is... 

Since the degree of expansion of the polymer coils is directly dependent on the solvating power of the solvent, under otherwise comparable conditions, both a and [q] provide a measure of the goodness of a solvent high values of a and [q] (at constant molecular weight and temperature) indicate remarkable coil expansion and therefore a good solvent. Low values of a and [q] indicate a bad solvent. For example, the values a for poly(vinyl acetate) in methanol and acetone are 0.60 and 0.72, respectively. 

Realization of this phase transition in networks requires high q0 and sufficiently high v and a bad solvent as a swelling agent. Under these conditions the dependence of A fxY on g-1 may exhibit two maxima (as is shown in Fig. 18) corresponding to two minima on the A /x2 vs. q-1 plot [Af 2 ist he chemical potential of the network polymer per equivalent segment). 

A more serious factor that has direct regard to initial steps of mesophase formation in polymer solutions has to be assessed. That is the interaction between the molecules of LC polymer and of the solvent. The conformation of the macromolecule appears to be sensitive to the thermodynamic quality of the solvent, and this has a very pronounced effect on the mode of intramolecular structure formation. For instance, the folding of the chain in a bad solvent leads to a sharp rise in intramolecular orientational ordering of the side branches. This is manifest as an increase of optical... 

The fragments of macromolecules with ordered cholesterol group sequences, that are formed in bad solvents, may serve as nuclei of supermolecular order in films, obtained from these solvents. Structural and optical studies have shown that PChMA-11 films produced by solvent evaporation display different properties those obtained from chloroform and toluene solutions (small relaxation times, see Table 17) are optically isotropic, and those obtained from heptane solutions (large relaxation times, see Table 17) are optically anisotropic, what reflects the differences in conformational state of polymeric chains in these films. Contrary to the optically isotropic films, a high degree of side branch ordering characterizes optically anisotropic films, which is confirmed by X-ray studies. The observed difference of LC polymer structure in the bulk is thus the consequence of their different conformational state in solution this reveals some possibilities for the control of LC polymer structure at the initial steps of mesophase nucleation in solutions. 

The polymer assumes this average radius only if the individual links can move freely. This is the case if we neglect the excluded volume effect due to the other segments and if we assume an ideal solvent. In an ideal solvent, the interaction between subunits is equal to the interaction of a subunit with the solvent. In a real solvent the actual radius of gyration can be larger or smaller. In a good solvent a repulsive force acts between the monomers. The polymer swells and Rg increases. In a bad solvent the monomers attract each other, the polymer shrinks and Rg decreases. Often a bad solvent becomes a good solvent if the temperature is increased. The temperature, at which the polymer behaves ideally, is called the theta temperature, T0. The ideal solvent is called the theta solvent. 

The force between surfaces which are coated with polymers is mainly determined by two factors. The first one is the quality of the solvent. In good solvents the force tends to be repulsive, in bad solvents attractive. Moreover, in good solvents polymer tends to remain in solution rather than adsorbing to surfaces. 

Other inorganic polymers, such as silicones, have excellent optical and mechanical properties for optical membranes. A great number of easily handled commercial silicone prepolymers are available but they have some disadvantages towards other materials. The surface is not easily modifiable to covalently immobilize indicators, they are not suited to combine with the customarily used support structures, owing to hard adhesion, and they are bad solvents for most of the indicators. 

Leiva et al. [65] have reported for poly(itaconates) monolayers the surface behavior at the air - water interface at different surface concentrations. They have found that for these type of polymers, the air - water interface at 298 K, is a bad solvent, very close to the theta solvent. At the semidilute region concentration, the surface pressure variation was expressed in terms of the scaling laws as a power function of the surface concentration. According to equation (3.3), the log it vs log T plot shows a linear variation with slope 2 v/(2 u-1). 

There is aggregate formation if an amphiphilic block copolymer dissolves in a liquid that acts as a good solvent for one of the blocks and a bad solvent for another. Their macromolecules can be associate to form aggregates similar to those surfactants obtained with low molecular mass. 

From the energy point of view polymer solvent contacts as compared with polymer-polymer contacts are preferred for some solvents called good solvents in this situation. A macromolecular coil swells and enlarges its dimension in a good solvent. On the contrary in a bad solvent, a macromolecular coil decreases in its dimension and can collapse, turning into a condensed globule (Flory 1953 Grossberg and Khokhlov 1994). 

FIGURE 3 7. A pressure trace as a diagnostic aid. (a) A properly performing solvent delivery system, (b) A pressure dip probably due to a bubble being pumped. (c) An improperly pulsing solvent delivery system which probably has bad check valves. Note that the relative pulsation for a good and bad solvent delivery system must be determined for each system. Each solvent delivery system will be somewhat different. 

Usually in physical chemistry of polymers they consider three states of flexible polymer chain Gauss ball, swollen ball and globule. The state of swollen ball is characteristic for macromolecule in "good" solvent in which interaction polymer-solvent prevails over interaction polymer-polymer. Macromolecule has configuration of Gauss ball in 9-solvent in which interaction between units of polymer chain doesn t differ from interaction polymer-solvent. State of globule is realized in "bad" solvent in which intramolecular interaction of polymer units significantly exceeds interaction of macromolecule s units with solvent [4-6],... 

Fig. 2,12. Properties of network polymers prepared in good solvents and bad solvents for the growing polymer chain.

In a slightly modified approach, the MIPs can be rendered super-porous by the use of 1-25% iso-octane as a porogenic agent [40,41]. The porogen is composed of a mixture of solvents in which the porogenic agent (iso-octane) acts as a bad solvent for the growing polymer chains, while the other solvent (toluene for... 

In the perturbed chain the characteristic ratio C(q) is to be effectively multiplied by 5 (q), so that in the good-solvent case we have longer relaxation times [see Eqn. (3.1.11)] than in the unperturbed state, unlike the bad-solvent case. It must be remembered, though, that under collapse the Fourier modes that are significantly contracted are relatively few, being comprised between n, = 1 and n, N/nJ [see Eqn. (2.2.12), q = n nlN see also Figure 6]. 

---