Functional Polymer Materials

Switchable Polymer (Hydrogel) Films


Responsive hydrogels are water-based cross-linked polymer networks that can be designed to expand and contract in response to various external stimuli such as temperature, pH, liquid composition, electric stimulation. Thermoresponsive aqueous polymer systems are known to exhibit large, reversible conformational changes in response to small thermal stimuli. Thermosensitive polymer materials, often called intelligent, can be used as actuators, artificial muscles, solute separators, drug delivery systems, thermoresponsive surfaces, light modulation systems, optical switching devices and molecular recognition agents. Depending on the type of cross-links, physical and chemical gels are distinguished. Physical gels are held together by polymer chain entanglement and/or by attractive forces present between the network polymers, whereas in chemical gels the polymers are covalently cross-linked. Poly(N-isopropylacrylamide) (PNIPAM) is probably among the most studied thermosensitive polymers. PNIPAM in water exhibits a phase transition at a lower critical solution temperature (LCST), which has been investigated by a variety of experimental techniques in the dilute and concentrated regimes. The LCST of PNIPAM in water is approximately at 32 °C and thus slightly less than body temperature, which makes PNIPAM a representative of environmental-sensitive polymers studied for biomedical applications. The temperature-driven change in the conformation of single PNIPAM chains and the macroscopic phase separation reflects rather subtle changes in polymer/water interactions, primarily the release of water molecules from a polymer hydration layer into bulk water. Several models have been described to account for the coil-to-globule collapse of PNIPAM in water and the complex water/PNIPAM phase diagram. At low temperatures, intermolecular hydrogen bonds between water and polar groups of PNIPAM solubilize the polymer. Above LCST the hydrogen bonds break and hydrophobic associations between the collapsed polymer chains take place.

Because pure PNIPAM homopolymer films miss the possibility of internal cross-links to build up a gel, the addition of hydrophobic chain ends (a triblock copolymer in the extreme case) is a simple way to overcome this problem. However, with increasing glassy, hydrophobic block length the ability to swell decreases. In case of the triblock copolymer P(S-b-NIPAM-b-S) bulk compositions with spherical PS domains and a PNIPAM continuous phase swelled very well in water, whereas composition resulting in cylindrical PS domains or in the bicontinous gyroid structure in bulk swelled only by a factor of 3 to 5, respectively. Finally, lamellar compositions did not show any swelling.

Recently, we have shown that thin diblock copolymer films, can incorporate water molecules from a surrounding water vapor atmosphere, without a significant swelling of the films. Thus these films will avoid problems related to strains which are caused by swelling. The basic key for the preparation of such thin films is the installation of a glassy network which has sufficient space to incorporate water molecules but cannot increase in size. This is achieved by a thin film preparation using an asymmetric diblock copolymer poly(styrene-block-N-isopropyl acrylamide) (P(S-b-NIPAM)) with a long PS and short PNIPAM block in combination with a solvent which is more equal in interaction with the both blocks as compared to water. Such films allow the total water storage of 17.4% with a total film thickness increase of only 2.5%. In subsequent storage and removal cycles, the aging of these P(S-b-NIPAM) films is investigated.

Because pure PNIPAM homopolymer films miss the possibility of internal cross-links to build up a gel, the addition of hydrophobic chain ends (a triblock copolymer in the extreme case) is a simple way to overcome this problem. However, with increasing glassy, hydrophobic block length the ability to swell decreases. In case of the triblock copolymer P(S-b-NIPAM-b-S) bulk compositions with spherical PS domains and a PNIPAM continuous phase swelled very well in water, whereas composition resulting in cylindrical PS domains or in the bicontinous gyroid structure in bulk swelled only by a factor of 3 to 5, respectively. Finally, lamellar compositions did not show any swelling.

To address the LCST behavior in thin films as a function of he degree of swelling the films are exposed to saturated water vapor. For this purpose a temperature-controlled swelling chamber with a water reservoir inside was operated. The thickness of dry films and of swollen films is measured as a function of temperature with optical interference.


Selected Publications:

  • 1. W.Wang, K.Troll, G.Kaune, E.Metwalli, M.Ruderer, K.Skrabania, A.Laschewsky, S.V.Roth, C.M.Papadakis, P.Müller-Buschbaum
    Thin films of poly(N-isopropylacrylamide) end-capped with n-butyltrithiocarbonate;
    Macromolecules 41, 3209-3218 (2008) link
  • 2. K.Troll, A.Kulkarni, W.Wang, C.Darko, A.M.B.Koumba, A.Laschewsky, P.Müller-Buschbaum, C.M.Papadakis
    The collapse transition of poly(styrene-b-(N-isopropylacrylamide) diblock copolymers in aqueous solution and in thin films;
    Colloid Polym. Sci. 286, 1079-1092 (2008) link
  • 3. W.Wang, E.Metwalli, J.Perlich, K.Troll, C.Papadakis, R.Cubitt, P.Müller-Buschbaum
    Water storage in novel undeformable hydrogel thin films as probed with in-situ neutron refelctometry;
    Macromol. Rap. Com. 30, 114-119 (2009) link
  • 4. A.M.Bivigou-Koumba, J.Kristen, A.Laschewsky, P.Müller-Buschbaum, C.M.Papadakis
    Synthesis of symmetrical triblock copolymers of styrene and N-isopropylacrylamide using bifunctional bis(trithiocarbonate)s as RAFT agents;
    Macromol. Chem. Phys. 210, 565-578 (2009) link
  • 5. W.Wang, E.Metwalli, J.Perlich, C.M.Papadakis, R.Cubitt, P.Müller-Buschbaum
    Cyclic switching of water storage in thin block copolymer films containing poly(N-isopropylacrylamide);
    Macromolecules 42, 9041-9051 (2009) link

Last change: June 5, 2012