Microphase separation in polyelectrolytic diblock copolymer melt: Weak segregation limit

Abstract

We present a generalized theory of microphase separation for charged-neutral diblock copolymer melt. Stability limit of the disordered phase for salt-free melt has been calculated us-

Thermodynamics of Ion-Containing Polymer Blends and Block Copolymers

“…Since the Li þ ions are strongly bound to the EO groups [11], one may consider the PEO with its bound Li þ ions as an effective polyelectrolyte, with the anions acting as the counterions. However, existing theories for diblock copolymers with a charged block and a neutral block [12,13] predict enhanced miscibility between the blocks relative to the uncharged system, opposite to experimental observations; there is also no dependence on the radius of the counterions.The strong binding of Li þ to the EO groups clearly will affect the thermodynamics of PEO-PS diblock copolymers. However, as suggested in Ref.…”

“…However, unlike the salt-free systems for which k à is independent of in the disordered phase, here k à changes slightly with temperature [3] and salt concentration; the changes in k à with the salt ions are primarily due to the Born energy. More importantly, in the absence of the Born energy effect, the peak value Sðk Ã Þ would decrease upon the addition of salt, relative to the salt-free system [13], implying enhanced miscibility between the two blocks. Without the solvation energy effect, the translational entropy of the anions always makes the two blocks more miscible, consistent with our analysis of the entropic contribution for the case of the binary blend.…”

Salt-doped block copolymers: ion distribution, domain spacing and effective χ parameter

“…A key effect in this system is the preferential solvation energy of the anions, which provides a driving force for microphase separation of the two blocks, counteracting the tendency for increased miscibility due to the translational entropy of anions. The latter effect has been wellknown in polyelectrolyte systems, 37,38,41 whereas immiscibility induced by the solvation energy of salt ions is a new feature for the ion-containing polymers. Furthermore, our theory predicts that, unlike the salt-free system, the peak position of the structure factor of the disordered phase decreases with temperature and increases with salt loading; these shifts are consistent with an earlier observation in the experiment by Ruzette et al 15 An issue of particular interest is the definition of an effective c parameter to characterize the increased tendency for microphase separation of the two blocks upon salt doping.…”

“…In this case, the domain spacing decreases with increasing salt concentration. As mentioned in Section I and III, in the absence of the preferential solvation energy for the anions, their translational entropy leads to enhanced miscibility between the two blocks, 37,38,41 which correlates with decreased domain spacing. However, as we have shown in Section III, the addition of salts not only gives rise to an effective c eff , but also leads to a shift in the peak position of the structure factor in the disordered phase.…”

“…17,20 In order to get a sense of the relative contribution due to the Born energy to other contributions, we note that in ref. 38 it was shown that the Coulomb interactions and translational entropy of the ions make comparable contributions to the effective interaction between the two blocks. Furthermore, in ref.…”