'Schizophrenic' behavior of poly (N-isopropylacrylamide)-b-poly (2-vinylpyridine) in aqueous solutions

A series of well-defined double hydrophilic block copolymers Poly (N-isopropylacrylamide)-b-poly (2-vinylpyridine) (PNIPAM38-b-P2VP43) were synthesized. The 'schizophrenic' behavior and the aggregation process of PNIPAM-b-P2VP in response to temperature and pH in dilute aqueous solution were tested via transmittance measurements, dynamic light scattering (DLS), and small-angle neutron scattering (SANS). The results revealed that: i) both PNIPAM-core and P2VP-core micelles can be induced from an intermediate unimers state via the adjustment of temperature and pH; ii) The lower critical solution temperature (LCST) of PNIPAM homopolymers is narrowly in the range of 31-34 °C, while PNIPAM-b-P2VP diblock copolymers at pH = 7 and 2 have broad LCSTs of 25-40 °C and 41-50 °C, respectively; iii) SANS results show that only the PNIPAM-b-P2VP solution at pH = 7 and 50 °C (both blocks are hydrophobic) has a sphere form factor with diameter of about 110 nm.