Cononsolvency in Self-Assembled Thermo-Responsive Polymers

Temperature is one of the stimuli that can be controlled in various applications and most importantly in the field of biomedicine as the human body is thermo-regulated. For this reason thermo sensitive hydrogels that can respond to temperature changes have been developed and widely explored in recent years. Block copolymers with various topologies have been designed for this purpose, the common feature of which is that they bear blocks exhibiting thermosensitivity in the temperature range of interest. This thermosensitivity usually relies on the reversible thermal phase transition of polymeric aqueous solutions characterized by a critical solution temperature. There are two cases. The polymer becomes insoluble by increasing temperature above the so called lower critical solution temperature (LCST) which in fact is a coil to globule transition, or in terms of interactions with water, a hydrophilic to hydrophobic transition. Poly(N-isopropylacrylamide) (PNIPAN) is soluble in water, as long as the solution is kept below the cloud point. It is also soluble in organic solvents, such as chloroform, acetone, methanol, and various other alcohols. Heating an aqueous PNIPAM solution above 32 °C (the cloud point (CP) or LCST) instantaneously converts the clear solution into a milky suspension. However, in water/methanol mixtures, the cloud point is decreased, and PNIPAM collapses and precipitates. This is the so-called cononsolvency phenomenon. Only few theories were proposed in the past two decades; however, none of them can fully describe this phenomenon. In order to study cononsolvency, we first carried out the turbidimetry experiments for P(S14-b-NIPAM270-b-S14) (PS: polystyrene) triblock copolymer in different cononsolvent systems, such as methanol/water, ethanol/water and acetone/water. Time-resolved in situ small-angle neutron scattering (SANS) experiments of P((S-d8)11-b-NIPAM431) diblock copolymers in water/methanol mixture were measured at beamline D22, Institut Laue-Langevin (ILL).