Effect of Molecular Architecture on Associating Behavior of Star-Like Amphiphilic Polymers Consisting of Plural Poly(ethylene oxide) and One Alkyl Chain

Associating behavior of star-like amphiphilic polymers consisting of two or three poly(ethylene oxide) (PEO) chains and one stearyl chain (C18) was investigated. Although the aggregation number (N(agg)) of linear analogue of amphiphilic polymers monotonically decreased with increasing number-average molecular weight of PEO (M(n)(,PEO)), the N(agg) of micelles of star-like amphiphilic polymers with M(n)(,PEO) = 550 g/mol was smaller than that with M(n)(,PEO) = 750 g/mol, whereas that with M(n)(,PEO) ≥ 750 g/mol showed general M(n)(,PEO) dependence. Small-angle X-ray scattering analyses revealed that the occupied area of one PEO chain on the interface between hydrophobic core and corona layer in the micelles of star-like polymers was much narrower than that in the linear amphiphilic polymers. This result indica ted the PEO chains of star-like polymers partially took unfavorable conformation near the core–corona interface in polymer micelles. The effect of local conformation of PEO chains near the interface on the associating behavior became significant as M(n)(,PEO) decreased. Therefore, in polymer micelles of star-like amphiphilic polymers containing PEO with M(n)(,PEO) = 550 g/mol, the enlargement of occupied area of PEO on the core–corona interface should be caused to avoid the formation of unfavorable conformations of partial PEO chains, resulting in a decrease in N(agg)s.