مشخصات پژوهش

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 PEG5000-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 PEG5000-block-PNIPAM26000 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 PEG5000-star-(PNIPAM)2 single crystals is verified at 31.5 and 23.5 °C in amyl acetate and toluene, respectively.