Effect of arm number of poly(acrylic acid) on cloud point temperature of poly(2-ethyl-2-oxazoline)

The phase transition behavior of poly(2-ethyl-2-oxazoline) (PEtOx) under complexation with star-shaped poly(acrylic acid) (PAA) having various arm numbers (two, three, four, and six) has been studied by turbidity and laser light scattering measurements. The change in cloud point temperature (T-cp) of PEtOx was monitored as a function of pH, ionic strength, and arm number of the star polyelectrolyte. The shift in T-cp to a lower value than that of pure PEtOx was more pronounced at pH 4.2 (pH<pK(a)), when the carboxylic acid groups are protonated as compared to pH 7.0 (pH>pK(a)), when the acid moieties are partially ionized. Dynamic light scattering showed that these complexes may have micellar core-shell type structure with a mean hydrodynamic radius (R-h) ranging from 12 nm to similar to 200 nm depending upon the temperature. Significant shift in T-cp was observed for six-arm star poly(acrylic acid) complexes at both pH values. This change in the T-cp is accredited to the differences in the driving forces of phase transition, including hydrogen bonding between carboxylic acid groups of PAA and the carbonyl moiety of PEtOx as well as the hydrophobic interactions.