Phase separation kinetics of aqueous polymer systems by means of modulated temperature DSC
01.10.2000 - 01.04.2005
The phase behaviour of thermo-responsive aqueous polymer systems was studied by means of Modulated Temperature DSC (MTDSC). The systems investigated in this work display an LCST-type of demixing behaviour around physiological temperatures, which makes them appropriate for biomedical or pharmaceutical applications, like controlled drug delivery.
A new analysis methodology was developed using MTDSC, based on the fact that upon phase separation an excess heat capacity is superimposed onto the baseline heat capacity, originating from heat effects coupled with fast reversible demixing and remixing processes at the polymer-water interphase. In this way, state diagrams covering the entire composition range were established, including among others the demixing curves, the crystallization and melting curves of water and the glass transition-composition curve.
Moreover, the kinetic properties of these aqueous polymer systems could be studied quantitatively by examining the time-dependence of the excess heat capacity, related to changes in specific interactions of water molecules surrounding the polymer chains. The accompanying morphology evolution can take up to several days before achieving an equilibrium state, which is too slow for most applications. In this respect, the kinetic response of well-chosen molecular architectures, like block- or graft-copolymers and hydrogels, containing hydrophilic components, was evaluated. Using the newly developed methodology it was shown that by incorporation of hydrophilic components an instantaneous thermo-responsiveness could be achieved, explained by a higher diffusion rate of water molecules through the thermo-responsive polymer matrix. The possibility of fine-tuning the kinetic response of these aqueous polymer systems broadens the range of potential applications.