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Single-Chain Mechanics of Poly(N,N-diethylacrylamide) and Poly(N-isopropylacrylamide): Comparative Study Reveals the Effect of Hydrogen Bond Donors
Single-Chain Mechanics of Poly(N,N-diethylacrylamide) and Poly(N-isopropylacrylamide): Comparative Study Reveals the Effect of Hydrogen Bond Donors
The
single-chain mechanics of two similar thermosensitive polymers, poly(N,N-diethyl-acrylamide) (PDEAM) and poly(N-isopropyl-acrylamide) (PNIPAM), have been studied by atomic
force microscopy-based single-molecule
force spectroscopy (SMFS). In a typical nonpolar organic
solvent, octane, both the two polymers show the same
inherent elasticity, although they have different substitutional groups. However, the mechanics of the
two polymers present large differences in water. The energies needed for the changes of the
bound water during elongation at room temperature are
estimated by the SMFS method at the single-chain level, which is ~1.13 ± 0.10 and ~5.19 ± 0.10 kJ/mol for PDEAM and
PNIPAM, respectively. In addition, PNIPAM shows a temperature-dependent single-chain mechanics
when the temperature is increased across the lower critical solution
temperature (LCST), while PDEAM does not. These differences observed in aqueous
solution originate from the different structures of the two polymers. With a hydrogen bond donor in the amide group, PNIPAM will be more hydrated when T < LCST. When T > LCST, PNIPAM will have a larger change in both conformation
and hydration. These findings also suggest that PNIPAM is a good candidate for thermo-driven
single-molecule motor, while PDEAM is not.
