Star-Like Poly(N-isopropylacrylamide) and Poly(ethylene glycol) Copolymers Self-Arranged in Newfound Single Crystals and Associated Novel Class of Polymer Brush Regimes with V-Type Tethers

摘要

Linear poly(ethylene glycol) (PEG)-block-poly(N-isopropylacrylamide) (PNIPAM) and star-like three-arm PEG-star-(PNIPAM)2 copolymers having one PEG and two PNIPAM blocks are synthesized by atom transfer radical polymerization (ATRP). Single crystals of these block copolymers are grown from amyl acetate and toluene dilute solutions. To recognize PNIPAM and PEG thicknesses, small angle X-ray scattering (SAXS) is applied. V-type brushes behave differently from linear brushes because doubly grafted PNIPAM blocks from a common point onto PEG substrate exert a higher osmotic pressure, leading to a thinner crystal. In addition to three ordinary regimes, a fourth or extremely stretched regime is detected for V-type PNIPAM brushes. Although in PEG_(5000)-star-(PNIPAM)2 single crystals with overall PNIPAM molecular weight of 37 000 g/mol, each PNIPAM arm is shorter than PNIPAM grafts in linear PEG_(5000)-block-PNIPAM_(26000) single crystals, their switching point from second to third regime is significantly lower (22 vs 33 °C). The V-type configuration of PNIPAM brushes cause them to be entered into the extremely stretched or fourth regime, which has not been previously detected for coily brushes. The boundary between third and fourth regimes for PEG_(5000)-star-(PNIPAM)_2 single crystals is verified at 31.5 and 23.5 °C in amyl acetate and toluene, respectively.