Effects of Concentration and Interactions Between Polymer Stars

2 Effects of Concentration and Interactions Between Polymer Stars 

There are relatively few experimental studies on the interactions between star polymers. Therefore, to date, MD simulations have mostly been used to validate theoretical models. A comprehensive comparison between theoretical and experimental results has recently been reviewed by Jusufi and Likos [84], 

Beyond the overlap concentration threshold, c c = pN/lP, star polymers form a semidilute solution. Because of the fact that the arms in a star are stretched, the scaling theory [24] predicts that the properties of semidilute solutions of star polymers are distinctively different from those of linear polymers. When the polymer concentration c c, a semidilute solution is envisioned as a system of closely packed and virtually non-interpenetrating (segregated) polymer stars. A further increase in polymer concentration leads to a progressive contraction of the coronae of the individual stars. This contraction results in an increase in the conformational entropy of the partially stretched star arms. 

The blob picture of a semidilute solution of polymer stars is schematically presented in Fig. lb. The peripheral (contracted) regions of the star coronae are envisioned as a sea of blobs of constant size, which corresponds to a constant polymer concentration in this region. In contrast to this, within radius p(c) R, the structure of the corona of individual stars is preserved (a system of growing 

Only at a sufficiently high polymer concentration, when the dimensions of the arms approach those of individual linear chains in a semidilute solution at the same concentration, do the star coronae become fully interpenetrated and the correlation peak in the scattering curves disappears. In this concentration regime, the thermodynamic properties of the solution of branched macromolecules become similar to those of linear chains. 

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