In this work is reported the use of stimuli-responsive materials on the study of self-assembly systems, exploring their surface anisotropy, for example, as patchy micelles and more specifically, Janus particles, in which responsive properties allow colloidal self-assembly of higher order.
The self-assembly enables the formation of a hierarchical aggregates through the association of anionic block copolymers with cationic surfactants.[1,2] In this case a combination of two different diblocks was used: poly(N-isopropylacrylamide)-b-poly(acrylic acid) and poly(ethylene oxide)-b-poly(acrylic acid) with cetyltrimethylammonium hydroxide (C16TAOH) or dodecyltrimethylammonium hydroxide (C12TAOH). The aggregates were prepared with each diblock as a control and the mixture of diblocks as the main system of study. The surfactant/polymer aggregates are formed by a diffuse shell and a dense core which exhibits a liquid crystalline structure of Pm3n cubic symmetry (C12TA+) or hexagonal structure (C16TA+). Size and aggregation numbers were estimated from a combination of light scattering and small-angle X-ray scattering experiments. It is found that the size of the aggregates decreases by 80% with an increase in temperature (25-45°C, above PNIPAM´s Tc), although the lattice parameter a showed little change (1%). This possibly indicates that the attraction and repulsion forces remain the same. As for the thermo-responsive character of the system, when only poly(N-isopropylacrylamide)-b-poly(acrylic acid) is used with C12TA+, after one cycle of temperature the system is not reversible, however the presence of another diblock allows reversibility.
These core-shell aggregates also respond to addition of electrolytes and it was possible to disorder the surfactant mesophase upon addition of salts, which could be reverted upon dialysis.
[1] Vitorazi, L; Berret, J.; Loh, W. Langmuir, 2013, 29, 14024-14033.
[2] Piculell, L. Langmuir, 2013, 29, 10313-10329.
