pH-responsive nanoaggregation of diblock phosphorylcholine copolymers.

ABSTRACT

We have characterized three diblock copolymers bearing zwitterionic phosphorylcholine and weak tertiary amine groups, namely, poly[((2-(methacryloyloxy)ethyl)phosphorylcholine)30- block-(2-(dimethylamino)ethyl methacrylate)60] (denoted as MPC30-DMA60, Mn=18,000), poly[((2-(methacryloyloxy)ethyl)phosphorylcholine)30- block-(2-(diethylamino)ethyl methacrylate)60) (denoted as MPC30-DEA60, Mn=20,000), and poly[((2-(methacryloyloxy)ethyl)phosphorylcholine)30- block-(2-(diisopropylamino)ethyl methacrylate)60) (denoted as MPC30-DPA60, Mn=21,000), by studying their surface tension and solution aggregation through a combined approach of surface tension measurement, dynamic light scattering, and small-angle neutron scattering. Our results show that larger tertiary amine substituents lead to an increasing tendency to form micellar aggregates, which is consistent with the increasing copolymer hydrophobicity. Thus, MPC30-DMA60 did not aggregate under the experimental conditions studied. The free chains exist in the form of thin cylinders, whose length decreases with copolymer concentration and solution temperature but increases with solution pH. The diameters of the MPC30-DMA60 cylinders remained almost constant at around 30 A under all the conditions studied. At the lower copolymer concentration of 0.5 wt %, the cylindrical lengths correspond to the persistence length of the copolymer backbone and are close to its full length, indicating a rather high rigidity. Further data analysis showed that, at the two higher concentrations of 2 and 4 wt %, the phosphorylcholine and amine blocks associate, inducing bending of the copolymer backbone. One backbone kink was required to satisfy all the constraints, including the dry volume of the copolymer. MPC30-DEA60 showed a similar trend of pH- and concentration-dependent conformational responses for the free copolymer, but in addition micellar aggregation occurred at pH 9. In contrast, MPC30-DPA60 exhibited significantly reduced solubility associated with strong aggregation, which is consistent with it being the most hydrophobic copolymer in the series.