Thermothickening modification of the poly(ethylene glycol) and amino acid ester grafted polyphosphazenes by monomethyl end-capped poly(ethylene glycol) addition

To control physical properties of the temperature-responsive biomedical materials for targeted applications in easy and safe way biocompatible additives are tried. Systematic controls are studied to obtain effectively controlled results in molecular interaction viewpoint. Thermothickening behaviors of the polyphosphazenes grafted with poly(ethylene glycol) (PEG) and amino acid ester are uniquely modified by physical mixing with another PEG molecules. Depending on the length of the added PEGs and the type of the subtituents of the polyphosphazenes, there are characteristic changes in the maximum viscosity (ηmax) and the phase transition temperature (Tmax) reflecting peculiar molecular interactions. For the short chain PEG addition, hydrophobic co-association of the end-capped methyl group of the added PEGs seems to play an important role while for the longer chain PEG additions the interaction between the added and the substituted PEGs seems to be dominating, which changes the ηmax and Tmax in a unique way. Longer PEG substituted polyphosphazene is not much affected by the type of the PEGs added probably because of more effective intra-molecular self-interaction than interaction with added ones. For the less hydrophobic amino acid ester substituted polyphosphazene the interaction between the substituted and the added PEGs dominates the de-hydration, which leads to the changes in the phase transition temperature sensitive to the length of the added PEGs. This unique thermothickening modification of the thermosenstitive and biocompatible polyphosphazenes is proposed to be contributed to the special and competitive hydrogen bonding between PEG–water and PEG–PEG molecules, which changes under different conditions like concentration, molecular weight and temperature.