During the last decades, thermo-sensitive polymer networks have been the subject of many investigations owed to their potential use as drug delivery systems, chemical sepa-ration media,4 nanoactuators or sensors.53, 54 In particular, polyacrylamides are potential candidates for this purpose because they exhibit thermoresponsive behaviour. In this group, poly(N-isopropylacrylamide) (poly(NIPAM)) is the most well known member of the class of thermoresponsive ”intelligent” polymers. The following section briefly reviews the unique temperature responsive properties of poly(NIPAM) systems.
Pure poly(NIPAM) exhibits a sharp transition from a hydrophilic to a more hydrophobic structure at the temperature known as the volume phase transition temperature (VPTT), which is related to the lower critical solution temperature (LCST) of poly(NIPAM). The normal range of the VPTT for poly(NIPAM) is typically found to be around 32oC in aqueous media.15–17 The change in the hydrophilic character of the polymer is due to the presence of the hydrophilic amide groups and the hydrophobic isopropyl group on its side chain (see figure 2.1). If the temperature is below the VPTT, the hydrophilic chains are hydrated and the hydrogel is in the swollen state. With an increase in temperature above the VPTT, the hydrophobic interactions become stronger and the equilibrium between the hydrophilic/hydrophobic interactions is disturbed. Therefore, the solvent (in the case of hydrogels the solvent is water) inside the network is expelled and the poly(NIPAM) network collapses.
The swelling capacity of a gel network strongly depends on the crosslinking density of
Figure 2.1: Schematic illustration of the structural rearrangement of water molecules around poly(NIPAM) during the VPT
the network.25, 31, 55, 56 In the case of poly(NIPAM) often N,N’-methylenebisacrylamide (BIS) is used as crosslinking agent due to the similarity in the chemical structure between BIS and NIPAM. This is related to the fact, that for the formation of a homogeneous gel the reactivity ratio between monomer and crosslinking agent is very important. If the reactivity of the crosslinker is too high or too small in comparison to the monomer, the rate of the crosslinking reaction at the beginning or at the end of the polymerization is higher compared to the chain growth, resulting in an inhomogeneous network structure.
Colloidal microgel particles and macroscopic gels, based on the same chemical compo-sition, differ in a number of aspects. These differences mainly rely on the fact that the gels are prepared using different synthetical methods. In the case of BIS-crosslinked poly(NIPAM) gels, a macrogel is usually synthesized using a bulk solution polymerization at room temperature. Due to this gel preparation at temperatures below the VPTT of poly(NIPAM), which means good solvent conditions, a macrogel with a nearly homoge-neous crosslinker distribution is obtained. In contrast to this, microgel particles are pre-pared using methods such as emulsion polymerization with or without surfactant. There
is evidence for a non-homogeneous crosslinker distribution within the microgel particles caused by a faster polymerization of the crosslinker BIS than the monomer NIPAM.22, 23 Like macroscopic gels, colloidal microgel particles are generally characterized by the de-gree of swelling, the average crosslink density and by a characteristic response time for swelling and deswelling. Since macrogels have dimensions several orders of magnitude higher than microgel particles, the driving force of swelling should be the same, but the time scale for the swelling process is very sensitive to the size of the gels. For colloidal microgel solutions the swelling/deswelling process is fast and the particles achieve their equilibrium state after a temperature change in less than a second. In contrast to this, macroscopic gels need a very long time (minutes to hours) to respond upon a change of an external parameter, because the collapse of the outer parts of the gel prevents the water transport from the inner part of the gel to the outside.9, 20
Experimentally, the phase transition of gels can be described by the order of transition (first or second order) and whether this transition is continuous or discontinuous. For macroscopic poly(NIPAM) gels the degree of discontinuity depends strongly on the used components for the gel preparation.54 It was confirmed that with an increase of the crosslinker density inside the macrogel network a change from a discontinuous to a con-tinuous phase transition can be observed.21 Furthermore, by incorporation of charged groups into the gel network, it was on one hand possible to shift the phase transition to higher temperatures and on the other hand, the degree of discontinuity increased with an increasing content of charged groups.57 In the case of linear poly(NIPAM) homopolymers it is expected that the volume phase transition is discontinuous if the polymer chains exhibit a totally monodisperse molecular weight distribution.8, 58 Since it is experimen-tally not possible to prepare such monodisperse linear polymers, the chains with different lengths will collapse at different transition temperatures and hence, the phase transition changes to a continuous one. This relation between the molecular weight (MW) and the phase transition temperature of poly(NIPAM) can be extended to microgels. Due to the fact, that the length of the chains between the crosslinking points inside the gel network is randomly distributed, larger chains will collapse at lower temperatures and shorter chains
at higher temperatures.8 Thus, the phase transition of microgel solutions is generally continuous and depends strongly on the homogeneity of the crosslinker distribution. A theoretical description of the volume phase transition of gels is given by the Flory-Rehner theory and will be described in detail in section 2.3.2.