Physical gels from PVC: A light scattering investigation of dilute solutions

Direct observation by laser scanning confocal microscopy of microstructure and phase migration of PVC gels in an applied electric field

The fluorescent probe lucigenin was incorporated in poly(vinyl chloride) (PVC) gels, and laser scanning confocal microscopy (LSCM) was used to clarify the internal structures of the gels. From the two-dimensional and three-dimensional information by LSCM, we first observed the internal structure of the PVC gel at a wet status, where the PVC gels comprised a polymer-rich phase and a polymer-poor phase uniformly with a three-dimensional network structure. After an electric field was applied, an effect of the electric field resulted in the change of internal structure in the gels. The polymer-poor phase moved from the cathode to the anode and the polymer-rich phase formed linelike arrangement between electrodes due to the attraction force. On the other hand, the freeze-dried PVC gels with/without in-situ dc voltage casting were particularly fabricated to confirm above results by the field emission scanning electron microscopy (FE-SEM). It was found that many craters remained on the surface of the gel near the anode due to sublimation in freeze-drying. This phenomenon did not appear on the surface near the cathode. The results of in-situ dc voltage casting also suggested that a substantial amount of polymer-poor phase was moved and fixed at the anode. Thus, results of both LSCM and in-situ dc voltage casting corresponded to the effect of electric field on PVC gels and provided a convincing evidence for the interpretation of the deformation mechanism of PVC gel actuators by an applied electric field.

Thermoreversible lysozyme hydrogels: properties and an insight into the gelation pathway

The gelation behaviour of aqueous solutions of hen egg white lysozyme (HEWL) in the presence of 20 mM DTT in the concentration range 0.7 to 4.0 mM has been investigated using microDSC, FTIR, cryoTEM, SANS and oscillatory rheology. The macroscopic critical gelation concentration, Cgel, was found to be ∼ 3.0 mM. The disruption of the disulfide bonds by the DTT and the destabilisation of the protein were found to be a prerequisite for the formation of β-sheet rich fibrils under the mild conditions used in this work. Using our methodology the hydrogels obtained have a pH of 7, hence are suitable for cell culture, and are also thermoreversible. The hydrogel melting temperature was found to increase with increasing concentration and a similar structure was observed across the concentration range investigated. Our results suggest these systems are composed of a well defined regular network where single β-sheet rich fibrils (∼ 3 nm diameter) form initially, then two of these fibrils associate two-by-two to form junctions (∼ 6 nm diameter) and then on cooling further aggregate to form larger bundles of fibres. The network mesh size was found to decrease with increasing concentration. Our results suggest that below Cgel small unconnected gel-like aggregates exist that have a similar structure to the hydrogels obtained above Cgel. Using our data we propose a model for the denaturation and gelation behaviour of our system. During the first heating an α-helix to β-sheet molecular transition for the protein conformation occurs resulting in β-sheet rich fibrils forming through the self-assembly of β-sheet rich denaturated proteins. At high temperature the solution contains β-sheet rich fibrils with dissolved protein. On cooling an increase in the amount of β-sheet was observed via FTIR suggesting that as the temperature is decreased more and more protein forms β-sheet rich fibrils. At the gelation temperature these fibrils associate two-by-two to form the network junctions resulting in the macroscopic gelation of the sample. Our results suggest the network junctions are formed via specific hydrophobic interactions. The hydrogels elastic modulus was found to scale as C2.45 for C > Cgel.