BEH.462/3. 962J Molecular Principles of Biomaterials Spring 2003 o Observed swelling as a function of ph o Data for poly(2-hydroxyethyl methacrylate-co-acrylic acid)gels cross-linked with ethylene glycol dimethacrylate Data for poly (HE MA-Co-AA) covalent hydrogel CH -CH-CH-Im-CH2-C C-O CH2 CH, OH Cl- OH Na+ (1) H2O (4)II OH Physical chemistry of swelling at high pH (example for anionic gels) o Stepwise process in basic solutions 1. lonization of carboxyl groups, releasing H a. At high ionic group density, carboxylate anions repel one another, driving swelling- but this is not the main driving force for swelling in typical conditions . Electrostatic force decays as 1/, too weak at typical charged group separation to have a significant effect i. In water: F=g12 4er=-e214ter=2.04x10-39/(r in m) e=1602×1019c ⅲi.F1nmF0.2nm=0.04! 2. H recombines with OH to give water 3. Charge is compensated by diffusion of cations (e.g. Na)and OH into gel 4. Influx of new ions creates osmotic pressure that drives swelling Lecture 9-polyelectrolyte hydrogels 20f17BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 o Observed swelling as a function of pH: o Data1 for poly(2-hydroxyethyl methacrylate-co-acrylic acid) gels cross-linked with ethylene glycol dimethacrylate o Physical chemistry of swelling at high pH (example for anionic gels): o Stepwise process in basic solutions:1 1. Ionization of carboxyl groups, releasing H+ a. At high ionic group density, carboxylate anions repel one another, driving swelling- but this is not the main driving force for swelling in typical conditions i. Electrostatic force decays as 1/r2 , too weak at typical charged group separation to have a significant effect ii. In water: F = q1q2/4πεr 2 = -e2 /4τεr 2 = 2.04x10-39/r2 (r in m) 1. ε = 80 in water 2. e = 1.602x10-19 C iii. F1 nm/F0.2 nm = 0.04! 2. H+ recombines with OH￾to give water 3. Charge is compensated by diffusion of cations (e.g. Na+ ) and OH￾into gel 4. Influx of new ions creates osmotic pressure that drives swelling2 Lecture 9 – polyelectrolyte hydrogels 2 of 17