Formation of Stable Mesoglobules by a Thermosensitive Dendronized Polymer

We present a study on the formation of stable mesoglobules by a thermosensitive dendronized polymer in aqueous solution. The polymer consists of a polymethacrylate backbone ( M n = 0. 34 × 10 6, M w = 1.1 × 10 6 g/mol) with ethoxy-terminated oligoethylene oxide (OEO) dendrons of second generation appended onto each repeat unit. The collapse of the dendronized chains as well as their aggregation with increasing temperature is followed by monitoring the hydrodynamic radius R h as a function of temperature with dynamic light scattering. The aggregation of the polymer stops at a certain size that depends on the heating rate leading to the formation of stable mesoglobules. Cryogenic transmission electron microscopy demonstrates that these mesoglobules adopt a spherical shape and are rather monodisperse. It is found that for dilute concentrations the size of the mesoglobules at 50 °C only weakly depends on concentration. Moreover, no hysteresis of the formation of mesoglobules is found; that is, R h is solely a function of temperature for both heating and cooling runs. The kinetics of the early stage of mesoglobule formation can be monitored by measurements of R h as the function of time. It can be modeled in terms of diffusion-limited colloid aggregation (DLCA). The transition from the DLCA to the stable mesoglobules is found to be well-defined. Different possible origins of the stability that prevents further aggregation are discussed. A weak electrostatic stabilization caused by charges that are complexed by the OEO dendrons is the primary reason for the marked transition from the DLCA to the stable mesoglobules. Most probably, an additional slowing down of the coalescence of the mesoglobules is effected by vitrification.