BEH.462/3. 962J Molecular Principles of Biomaterials Spring 2003 Lecture 9: Polyelectrolyte Hydrogels Last Day: Physical hydrogels Structure and chemistry Toda polyelectrolyte hydrogels, complexes, and coacervates rolyte multilayer theory of swelling in ionic hydrogels Reading S.K. De et aL., 'Equilibrium swelling and kinetics of pH-responsive hydrogels: Models experiments, and simulations, J. Microelectromech. Sys. 11(5)544 (2002) Supplementary Reading L. Brannon-Peppas and N.A. Peppas, 'Equilibrium swelling behavior of pH-sensitive hydrogels, Chem Eng. Sci. 46(3)715-722(1991) USE DEMO OF AMINOETHYL METHACRYLATE HYDROGEL TO SHOW PH-DEPENDENT SWELLING? Covalent polyelectrolyte hydrogels Response of polyelectrolyte gels to pH of environment o Reminder of the response of ionizable groups to pH changes ionization of charged groups 14 o Presence of ionizable groups makes polyelectrolyte hydrogels sensitive to p o lonic strength o Electric fields o D Lecture 9-polyelectrolyte hydrogels 1of17
BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Lecture 9: Polyelectrolyte Hydrogels Last Day: Physical hydrogels Structure and chemistry Today: polyelectrolyte hydrogels, complexes, and coacervates Polyelectrolyte multilayers theory of swelling in ionic hydrogels Reading: S.K. De et al., ‘Equilibrium swelling and kinetics of pH-responsive hydrogels: Models, experiments, and simulations,’ J. Microelectromech. Sys. 11(5) 544 (2002). Supplementary Reading: L. Brannon-Peppas and N.A. Peppas, ‘Equilibrium swelling behavior of pH-sensitive hydrogels,’ Chem. Eng. Sci. 46(3) 715-722 (1991). USE DEMO OF AMINOETHYL METHACRYLATE HYDROGEL TO SHOW PH-DEPENDENT SWELLING? Covalent polyelectrolyte hydrogels Response of polyelectrolyte gels to pH of environment o Reminder of the response of ionizable groups to pH changes: ionization of charged groups 1.2 1 0.8 0.6 0.4 0.2 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 pH o Presence of ionizable groups makes polyelectrolyte hydrogels sensitive to: o pH o Ionic strength o Electric fields o (T) Lecture 9 – polyelectrolyte hydrogels 1 of 17
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 20f17
BEH.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 OHto give water 3. Charge is compensated by diffusion of cations (e.g. Na+ ) and OHinto gel 4. Influx of new ions creates osmotic pressure that drives swelling2 Lecture 9 – polyelectrolyte hydrogels 2 of 17
BEH.462/3. 962J Molecular Principles of Biomaterials Spring 2003 Kinetcs for ph change from 3 to 6: Kinetics for pH change from 6 to 3 pe ad AIn 2002 o Kinetics: deswelling faster(-10X) than swelling Swelling in -166 mi De-swelling in-16 min (300 Hm thick gels) Theory based on diffusion of ions into and out of gel semi-quantitatively predicts observed swelling behavior Swelling rate inversely proportional to square of gel size the size of the gel o Implies that response time of gels will scale inversely with th o Swelling rate can also be increased by creating greater porosity in gel-increase surface/volume ratio allows solute to diffuse into gel more rapidly Rapid swelling/deswelling of superporous gels Low pH and shrinking kinetics of I )in a ph=11.77 NaOH solution and a 1. 92 buffer with ionic strength of 0. 2 M. Three cycles of swelling inking were shown for gel 1; two cycles were shown fo o hydrogels containing basic groups show opposite pH sensitivity o swelling in acidic solutions o e.g. Peppas papers Lecture 9-polyelectrolyte hydrogels 3of17
BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 o Kinetics: deswelling faster (~10X) than swelling • Swelling in ~166 min. • De-swelling in ~16 min. • (300 µm thick gels) • Theory based on diffusion of ions into and out of gel semi-quantitatively predicts observed swelling behavior o Implies that response time of gels will scale inversely with the size of the gel o Swelling rate inversely proportional to square of gel size3 o Swelling rate can also be increased by creating greater porosity in gel- increase surface/volume ratio allows solute to diffuse into gel more rapidly Rapid swelling/deswelling of superporous gels: Low pH High pH (Zhao and Moore, 2001) o hydrogels containing basic groups show opposite pH sensitivity o swelling in acidic solutions o e.g. Peppas papers Lecture 9 – polyelectrolyte hydrogels 3 of 17
BEH.462/3. 962J Molecular Principles of Biomaterials Spring 2003 Polyion complex hydrogels Coacervates o complexation between two oppositely charged polyelectrolytes can lead to precipitation(insoluble solid phase driven by charge neutralization on hydrophobic polymers driven by macro-aggregate formation 2. coacervate formation(dense liquid phase) 3. soluble comple o mechanisms of formation Polyanion Polycation Random primary Ordered secondary 解 Complex aggregates 1. initial rapid Coulombic bonding 2. formation of new bonds/restructuring of chain distortions gregation of secondary complexes o mixing of two polyions can lead to 90% complex formation Polyelectrolytes studied as coacervates for biomaterials: Polyanions o Carboxymethylcellulose o Dextran sulfate o Carboxymethyl dextran o Pectin o Polycau chitosan(derived from crab shells) o Polyethyleneimine o Poly (4-vinyl-N-butylpyridinium) bromide o Quarternized polycations o Poly(vinylbenzyltrimethyl)ammonium hydroxide Lecture 9-polyelectrolyte hydrogels 4of17
BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 Polyion complex hydrogels4 Coacervates o complexation between two oppositely charged polyelectrolytes can lead to: 1. precipitation (insoluble solid phase) driven by charge neutralization on hydrophobic polymers driven by macro-aggregate formation 2. coacervate formation (dense liquid phase) 3. soluble complexes o mechanisms of formation 1. initial rapid Coulombic bonding 2. formation of new bonds/restructuring of chain distortions 3. aggregation of secondary complexes o mixing of two polyions can lead to 90% complex formation o Polyelectrolytes studied as coacervates for biomaterials:4 o Polyanions o Carboxymethylcellulose o Alginate o Dextran sulfate o Carboxymethyl dextran o Heparin o Carrageenan o Pectin o xanthan o Polycations o Chitosan (derived from crab shells) o Polyethyleneimine o Poly(4-vinyl-N-butylpyridinium) bromide o Quarternized polycations o Poly(vinylbenzyltrimethyl)ammonium hydroxide Lecture 9 – polyelectrolyte hydrogels 4 of 17
BEH.462/3. 962J Molecular Principles of Biomaterials Spring 2003 o Microstructure of coacervate hydrogels o Example structures: xanthan/chitosan coacervates(Dumitriu et al. 1998) Xanthan Chitosan omani poly o-H NH2 CH2OH SEM o Pore sizes formed 0. 1-1 um; fiber diameters -100 nm Polyelectrolyte multilayers(PEMs Structure of pems Assembly ayer-by-layer deposition o How is it done o Surface properties change in digital fashion with adsorption of sequential layers Lecture 9-polyelectrolyte hydrogels 5of17
BEH.462/3.962J Molecular Principles of Biomaterials Spring 2003 o Microstructure of coacervate hydrogels o Example structures: xanthan/chitosan coacervates (Dumitriu et al. 1998) o Pore sizes formed 0.1-1 µm; fiber diameters ~100 nm Polyelectrolyte multilayers (PEMs) Structure of PEMs Assembly • Layer-by-layer deposition o How is it done o Surface properties change in digital fashion with adsorption of sequential layers5 Lecture 9 – polyelectrolyte hydrogels 5 of 17