Effects of Temperature and Polymer Composition upon the Aqueous Solution Properties of Comblike Linear Poly(ethylene imine)/Poly(2-ethyl-2-oxazoline)-Based Polymers

A range of well-defined, sparsely grafted, comblike linear poly(ethylene imine)/poly(2-ethyl-2-oxazoline) (LPEI-comb-PEtOx) and comblike linear poly(ethylene imine)/linear poly(ethylene imine) (LPEI-comb-LPEI) polymers with various degrees of polymerization of both main and side chains were prepared. Their aqueous solution properties were investigated by means of dynamic light scattering (DLS) and small-angle neutron scattering (SANS) over a temperature range of 25-65 °C. For LPEI-comb-LPEI polymers the particle distributions were mono-, bi-, or trimodal depending upon the temperature and/or polymer composition. The particles tended to decrease in size upon heating due to the weakening of intra- and interchain hydrogen bonds between -NH groups of the polymers and also with water molecules. However, the aggregates formed from the LPEI-comb-LPEI polymers featuring longer main chains, shorter branches, and lower grafting densities were more resistant to heat-induced disassociation. LPEI-comb-PEtOx particles featured smoother temperature variations of hydrodynamic radius and typically bimodal distributions. The shape and structure of the small LPEI-comb-PEtOx aggregates (average radius ∼6 nm) as well as their temperature evolution were studied by SANS. A variety of structures were observed, depending upon the polymer composition. The LPEI-comb-PEtOx polymers with low grafting densities and short branches formed elongated aggregates, whereas particles of spherical core-shell structure were observed for the more densely grafted polymer with longer branches. Spherical or rodlike aggregates were monitored in LPEI-comb-LPEI aqueous solutions.