Aggregation Phenomena of Model PS/PI Super-H-Shaped Block Copolymers. Influence of the Architecture

Abstract

The super-H-shaped block copolymers belong to a new class of polymeric materials, the miktoarm stars (mikto from the Greek word μικτος meaning mixed), which includes star-shaped polymers in which from one or more junction points radiate different arms. Schematically, the super-H structure can be described as two three-arm stars which are bridged with a common fourth arm. Model super-H-shaped block copolymers of the PI3(PS-d8)PI3 type, where PI is protonated polyisoprene and PS-d8 is deuterated polystyrene, were synthesized by anionic polymerization using the chlorosilane approach. These nonlinear block copolymers were characterized by size exclusion chromatography (SEC), ultraviolet (UV) spectroscopy, low-angle laser light scattering (LALLS), and membrane (MO) and vapor pressure osmometry (VPO). The micellar behavior of these stars was investigated in n-decane, which is a selective solvent for the PI part. Small angle neutron scattering (SANS), LALLS, and viscometry were used in order to characterize the micelles. SANS measurements were performed at two different contrasts, where in each case one of the monomeric units was matched to the solvent. The coherent scattering cross sections of the single cores and coronas were determined by extrapolating low-concentration data to φ = 0. A theoretically calculated cross section of an unswollen hard sphere of PS-d8 was found to fit fairly well the experimental curves for the core. A calculated cross section of a hollow sphere with a constant density profile for the PI chains was found to describe the cross sections of the corona. Experimentally, it was found that the super-H structure increases the solubility and drastically reduces the aggregation number in comparison with linear PS/PI diblocks. A scaling approach is presented to take into account the specific influence of the SH structure on the micellar properties. It was found that the derived scaling laws of an intermediate regime between the starlike and the crewcut regime are in fair agreement with the experimental observations.