Molecular Dynamic Simulation Study of the Volume Transition of PNIPAAm Hydrogels

Transcription

1 Molecular Dynamic Simulation Study of the Volume Transition of PNIPAAm Hydrogels Jonathan Walter 1, Jadran Vrabec 2, Hans Hasse 1 1 Laboratory of Engineering, University of Kaiserslautern, Germany 2 and Energy Technology, University of Paderborn, Germany

2 Hydrogels What is a Hydrogel? Three dimensional hydrophilic polymer network Most important property: Swelling in aqueous solutions Degree of Swelling depends on: Monomer Co-monomer(s) Cross linker amount Solvent Temperature 2

3 Poly(N-isopropylacrylamide) (PNIPAAm) Cross-linker: N,N -Methylenbisacrylamid (MBA) MBA PNIPAAm Atom Colors: Carbon Hydrogen Oxygen Nitrogen Jonathan Walter 3

4 Degree of swelling of PNIPAAm as a function of the temperature Laboratory of Engineering LCST ca. 305 K PNIPAAm, MBA, water Jonathan Walter 4

5 Degree of swelling of PNIPAAm as a function of the methanol concentration Laboratory of Engineering PNIPAAm, MBA Jonathan Walter 5

6 Degree of swelling of PNIPAAm as a function of the methanol concentration Laboratory of Engineering PNIPAAm, MBA Jonathan Walter 6

7 Microscopic structure of PNIPAAm hydrogel REM of freeze-dried hydrogel 10 µm 1 µm

8 Why atomistic molecular simulations? Quantitative predictions Processes on atomistic level Vision: Screening of Hydrogels for new applications

9 Molecular modeling Geometry

10 Molecular modeling Geometry Electrostatics: Point charges Ur ij qq i r ij j

11 Molecular modeling Geometry Electrostatics: Point charges Despersion and repulsion: Lennard-Jones 12-6 potential U r ij 4 r ij r ij 12 6

12 Molecular modeling Geometry Electrostatics: Point charges Despersion and repulsion: Lennard-Jones 12-6 potential Inner degrees of freedom: Bond 1 U 2 b rij kb rijr0 Angel 2 Torsion 1 U k 2 ijk 3 2 ijk U k cos n 1 t ijkl n ijkl n n1 0

13 Molecular simulation of PNIPAAm Modeling: Force fields and models from literature: PNIPAAm Water Methanol Simulation: Molecular dynamics (MD) NpT-ensemble Software: GROMACS VMD MegaMol Jonathan Walter 13

14 Simulation of the single PNIPAAm chain Laboratory of Engineering T = 360 K T = 280 K Radius of gyration Jonathan Walter 14

15 Preliminary study: chain length PNIPAAm, water, 360 K

16 Force field comparison PNIPAAm water: SPC/E water: TIP4P Gromos87 UA Gromos96 53a6 UA OPLS AA 16

17 Preliminary study: force fields PNIPAAm, water, OPLS-AA + SPC/E 17

18 Preliminary study: force fields PNIPAAm, water, OPLS-AA + SPC/E 18

19 Preliminary study: force fields PNIPAAm, water, OPLS-AA + SPC/E 19

20 Force field comparison - results PNIPAAm water: SPC/E water: TIP4P Gromos87 UA - - Gromos96 53a6 UA - + OPLS AA ++ + Legend - Force field does not predict effect + Force field predicts effect 20

21 Radius of gyration of PNIPAAm as a function of the temperature LCST ca. 320 K OPLS-AA + SPC/E Jonathan Walter 21

22 Radius of gyration of PNIPAAm as a function of the methanol concentration Laboratory of Engineering Simulation single chain OPLS-AA + SPC/E Experiment hydrogel 0.5 mol/mol 0.5 mol/mol PNIPAAm, MBA Jonathan Walter 22

23 Solvent structure around PNIPAAm in the mixture Atom Colors: Carbon Hydrogen Oxygen Nitrogen Methyl Jonathan Walter 23

24 Hydrogen-bonding in molecular simulations Geometric criterion Distance donor-acceptor < 0,35 nm Angle hydrogen-donor-acceptor < 30 H D A

25 Hydrogen-bonds between PNIPAAm and the solvents C=O N-H 0.1 mol/mol Methanol Methanol OPLS-AA + SPC/E, Water 298 K C=O N-H Jonathan Walter 25

26 Solvent concentration at PNIPAAm surface in water-methanol mixture Solvent Colors: Water Methanol Jonathan Walter 26

27 Summary Volume transition of hydrogels: PNIPAAm Temperature and methanol concentration dependency Molecular modeling and simulation Preliminary studies: Chain length Force fields Qualitative predictions with force fields from the literature Phenomenon of co-nonsolvency explained by molecular simulation and visualization Jonathan Walter 27

28 Thank you for your attention Laboratory of Engineering