Glycine elicited self-assembly of amphiphilic star block copolymers with contradistinct hydrophobicities

Abstract

We report here calorimetric and scattering studies on two pharmaceutically important, commercially available nonlinear ethylene oxide-propylene oxide (EO-PO) octablock copolymers, Tetronic®1304 (T1304) and Tetronic®1307 (T1307) (with almost same molecular weight of polypropylene oxide (PPO) and varying % polyethylene oxide (PEO)), in the presence of amino acid, glycine. Added glycine in aqueous solution exhibits water structure-making behaviour and thus induces micellization by altering the polarity of water. Consequently, lowering of cloud point (CP), critical micelle concentration (CMC) and critical micellization temperature (CMT) of copolymer solutions with a corresponding change in solution viscosity is observed. The microstructural changes as evidenced by small angle neutron scattering (SANS) and dynamic light scattering (DLS), for Tetronic® micelles strongly depend upon their hydrophobicity, glycine concentration, temperature and pH of solution. In the case of T1304 with lesser % PEO, behaves as moderately hydrophobic and displays pronounced micellar growth as a function of glycine concentration and at alkaline pH. Such a transition in micelle size with pH is explained in terms of sticking parameter. T1307 with higher % PEO, is highly hydrophilic and does not show micellar growth even at higher concentration of glycine/alkaline pH. The measured CMTs obtained from high sensitivity differential scanning calorimetry (HSDSC) and calculated thermodynamic parameters clearly signify glycine induced micellization. The results were elucidated in terms of structure making properties of glycine. The information attained from this contribution will be potentially commodious for an efficient exploitation of Tetronic® micelles in diverse pharmaceutical applications utilizing a common excipient, glycine.