Phase Separation Dynamics of Aqueous Thermo-responsive Polymer Solutions Studied by a Laser-induced Temperature Jump Method Combined with Transient Photometry

Poly(N-isopropylacrylamide) (PNIPAM) is a well-known thermoresponsive polymer. In aqueous solution, PNIPAM shows drastically change of its conformation upon temperature elevation above a critical solution temperature. Such conformational changes in solution directly lead to changes in the solubility of the polymer, resulting in phase separation of the polymer solution. Although there has been wealth of observations of the structural changes upon thermal phase separation of an aqueous PNIPAM solution, little is known about the dynamic behaviors of phase separation such as the time scales of the phase separation processes. For precise understanding of fundamental mechanisms of phase separation of aqueous thermoresponsive polymer solutions, it is indispensable to reveal the phase separation dynamics quantitatively. Also, such dynamic study will promote applications of the polymers to smart materials such as drug delivery systems. In this thesis, the phase separation rates in aqueous PNIPAM solutions and the factors regulating the phase separation rate are investigated by means of a laser-induced temperature jump technique combined with transient transmittance-photometry techniques. The phase separation rate of an aqueous PNIPAM solution was investigated as a function of the following three important factors: the PNIPAM concentration, the molecular weight of PNIPAM, and the stereoregularity of PNIPAM. Three main conclusions in the present study are summarized:1)Phase separation of an aqueous PNIPAM solution is accelerated with increasing the polymer concentration (chapter 3).2)There is an optimum molecular weight of PNIPAM for rapid phase separation in an aqueous solution (chapter 3).3)Phase separation in an aqueous solution is accelerated by setting a high stereoregularity of PNIPAM (chapter 4 and 5). The dynamic aspects of the phenomena are well correlated with the satic aspects. Entanglements of the polymer chains formed at a high polymer concentration play an important role in the phase separation dynamics: the phase separation time constant levels-off at a certain concentration value. Phase separation of aqueous highly syndiotactic-rich PNIPAM solutions becomes considerably fast when the polymer chains are entangled below the critical temperature. Cooperative dehydration of PNIPAM in an aqueous solution, defined by simultaneous dissociation of the bound water molecules from the sequential polymer chains, and subsequent formation of intrapolymer hydrogen bondings also result in rapid phase separation of an aqueous highly syndiotactic-rich PNIPAM solution. Furthermore, it has been demonstrated that formation of microscopic inter-chain networks even below the critical temperature in an aqueous isotactic-rich PNIPAM solution is the origin of rapid phase separation in the solution.By accumulating the knowledge about the phase separation dynamics as a function of the primary structures of a polymer in solution, it would be possible to develop a new model predicting the phase separation rate based on the fundamental structural and solution properties of a thermo-responsive polymer. The present study and further investigation of the phase separation dynamics as a function of various primary factors controlling phase separation will open new channels toward the design and development of stimuli-responsive-polymer-based smart materials.