Polymer solutions coil-globule transition

Taken together, the experimental observations reported in the previous sections suggest that the formation of colloidally stable particles heated above their phase-transition temperature may be a universal phenomenon, taking place not only in aqueous polymer solutions, but also in solutions of polymers that can undergo a coil-globule transition in organic solvents. 

From a comparison of Eqs. 25 and 26 one could conclude that the osmotic pressure actually leads to a much higher swelling than electrostatic interaction. Since in poor solvent the value of the subchain size does not depend on the value of fi, the amplitude of the collapse transition in the case of j8 = 0 is higher than in the case of fi = 1. It was also shown that the transition point between the swollen and collapsed states of the chain and the character of this transition depends essentially on whether the counterions are inside the polymer coil or whether they have moved for the outer solution region. The coil-globule transition for fi = 0 in most cases is the first-order phase transition. The sharpness of this transition decreases with an increase in fi. In some cases the character of this transition for fi 0 becomes continuous in contrast to a jump-like first-order phase transition for fi = 0 (Figure 5). 

In dilute solutions, demixtion is preceded by change of the polymer shape which becomes more compact this is the coil-globule transition. Consequently, though both effects are related, two different types of studies appeared. 

The distinction between poor and good solvent was introduced in the 1950s by Fox and Flory after experimental studies of the intrinsic viscosity of polymer solutions. These authors recognized that the viscosity varies in relation to the dependence of the chain sizes on temperature the poor solvent state is the state of a solution in which the chains have quasi-Brownian configurations. Systematic experiments have been made in this domain, for instance to determine the Flory temperature, but they have never given very precise results. Physicists are just now beginning to overcome the experimental and theoretical difficulties. Experiments have been made to show the existence of a collapse of the polymer chain, and certain authors have been prone to compare it with the coil-globule transition in proteins. 

Some other methods have also been employed to observe the coil-globule transition for isolated polymer molecules (e.g., ordinary light scattering, viscosity and osmotic pressure measurements, and elastic neutron scattering off polymer solutions). However, the two techniques we mentioned before are the best for this purpose. They are very sensitive and allow measurements of solutions at extremely low concentrations. 

The coil—globule transition of a linear flexible polymer in dilute solution has been formulated by Orofino and Floty [13], de Gennes [14], Moore [15], Sanchez [16], and others. The results obtained give substantially similar predictions except for one point which will be explained below. Here we refer to an elementary theory due to de Gennes [14], though it is virtually identical to the earlier theory of Orofino and Flory. 

Polymer samples used for coil—globule transition studies must be as close to monodispersity as possible. When a polydisperse solution is cooled to a certain temperature below 9 its part containing high-molecular-weight fractions enters two-phase regions while the rest still maintains homogeneity. Behavior of such a system cannot be analyzed by the theory of uniform solutions. 

In all the above cases, the coil-globule transition proceeds in a very dilute polymer solution below the 0 point when attraction predominates with pair contacts of segments, but repulsion remains al ternary contacts 71 < 0, G > 0 (Equations 17, 18). 

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