智能软-水凝胶答题.ppt

11 Soft Active Materials Zhigang Suo Harvard University III. Polyelectrolytes 2 Polyelectrolyte: polymer with electrolyte group In a solvent, the electrolyte dissociates. solvent fixed charge counterion ?? Ca ? Cl co-ion counterion covalent bond ionic bond polymer chain 3 Polyelectrolyte gel - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - Gel External solution Fixed charges Counter ions Co-ions -+- work of polymers Solvent (., water) Crosslinks 4 Many tissues are polyelectrolyte gels Structure and function: ? Cartilage is polyelectrolyte. ? Does this fact have anything to do with low friction of a joint? 5 Plant cell walls are polyelectrolytes Pectin 6 Gels regulate flow in plants Zwieniecki, Melcher, Holbrook, Hydrogel control of xylem hydraulic resistance in plants Science 291, 1095 (2001) Missy Holbrook 7 Osmosis in a polyelectrolyte gel membrane solution solvent Osmosis balanced by gravity ? Network tatters fixed charges. ? To maintain electroneutrality, fixed charges are balanced by counterions. ? Counterions cause osmosis crosslink -- ---- - - - -+++ ++ +++ + fixed charge counterion work Osmosis balanced by elasticity 8 Super absorbent diaper Sodium polyacrylate: polyelectrolyte Masuda, Trends in the development of superabsorbent polymers for diapers, pp. 88-89, Superabsorbent polymers: science and technology (1994). 9 Ion exchanger ?????? 2Na XCa Ca XNa 2 10 pH-sensitive gel ????H RCOO RCOOH fixed mobile ?????? K??? RCOOH H RCOO