Gelation Studies, 4

Polymeric Sol-Gel Transition with the Diverging Correlation Length Verified by Small-Angle X-ray Scattering

Sol-gel transitions of polymers are pivotal phenomena in material science, yet the critical phenomenon of structure during gelation has remained unclear. Here, we investigated the sol-gel transition of a fluorous polymer, poly(vinylidene fluoride-co-hexafluoropropylene), in a blend of two ionic liquids. This system features a quite high amount of cross-linker and binding sites with ion-dipole interactions between the cation and C-F dipoles, thereby facilitating easy exchange of the cross-links. Changing the mixing ratio of the two ionic liquids enabled tuning the ion-dipole interactions and inducing sol-gel transition. Notably, the correlation length and molar mass, obtained by small-angle X-ray scattering, diverged at the gelation point. Moreover, the derived critical exponents (ν = 0.85 ± 0.05) aligns remarkably well with the prediction from percolation theory (ν = 0.88). To our knowledge, this is the first report on the evident divergence during polymeric gelation by small-angle scattering and the verification of the critical exponents of the percolation theory.

Hopping Behavior Mediates the Anomalous Confined Diffusion of Nanoparticles in Porous Hydrogels

Diffusion is an essential means of mass transport in porous materials such as hydrogels, which are appealing in various biomedical applications. Herein, we investigate the diffusive motion of nanoparticles (NPs) in porous hydrogels to provide a microscopic view of confined diffusion. Based on the mean square displacement from particle tracking experiments, we elucidate the anomalous diffusion dynamics of the embedded NPs and reveal the heterogeneous pore structures in hydrogels. The results demonstrate that diffusive NPs can intermittently escape from single pores through void connective pathways and exhibit non-Gaussian displacement probability distribution. We simulate this scenario using the Monte Carlo method and clarify the existence of hopping events in porous diffusion. The resultant anomalous diffusion can be fully depicted by combining the hopping mechanism and the hydrodynamic effect. Our results highlight the hopping behavior through the connective pathways and establish a hybrid model to predict NP transport in porous environments.

Real-Space, in Situ Maps of Hydrogel Pores

We characterize the porosity of hydrogels by imaging the displacement trajectories of embedded tracer particles. This offers the possibility of characterizing the size and projected shape of individual pores as well as direct, real-space maps of heterogeneous porosity and its distribution. The scheme shows that when fluorescent spherical particles treated to avoid specific adsorption are loaded into the gel, their displacement trajectories from Brownian motion report on the size and projected shape in which the pore resides, convoluted by the particle size. Of special interest is how pores and their distribution respond to stimuli. These ideas are validated in agarose gels loaded with latex particles stabilized by adsorbed bovine serum albumin. Gels heated from room temperature produced an increasingly more monodisperse pore size distribution because increasing temperature preferentially enlarges smaller pores, but this was irreversible upon cooling, and shearing agarose gels beyond the yield point destroyed larger pores preferably. The method is considered to be generalizable beyond the agarose system presented here as proof of concept.

Role of microscopic phase separation in gelation of aqueous gelatin solutions

Using a unique home-made cell for four-contact impedance spectroscopy of conductive liquid samples, we establish the existence of two low frequency conductivity relaxations in aqueous solutions of gelatin, in both liquid and gel states. A comparison with diffusion measurements using pulsed field gradient NMR, and circular dichroism spectroscopy, shows that the faster relaxation process is due to gelatin macromolecule self-diffusion. This single molecule diffusion is mostly insensitive to the macroscopic state of the sample, implying that we have a clear separation of gelatin molecules into a free and network-bound phase. Scaling relationships for the self-diffusion indicate that the gelation process is not a percolative phenomenon, but is caused by aggregation of triple helices into a system-spanning fibre network.

Direct Observation of Early-Time Hydrogelation in beta-Hairpin Peptide Self-Assembly

Triggered hydrogelation of MAX1 peptide, (VK)(4)-V(D)PPT-(KV)(4)-NH(2), proceeds through peptide intramolecular folding into beta-hairpins and concomitant self-assembly into branched clusters of well-defined (uniform, 3 nm cross section), semiflexible, beta-sheet-rich nanofibrils. Cryogenic transmission electron microscopy indicates that dangling fibrils extend from one growing cluster to another and lead to early, intercluster communication in solution. At the apparent percolation threshold, the dynamic shear modulus measured by oscillatory rheology (G'(omega), G''(omega) proportional, variant omega(n)) and the field-intensity autocorrelation function measured by dynamic light scattering (g(1)(tau) proportional, variant tau(-beta')) show power-law behavior with comparable critical dynamic exponents (n approximately 0.47 and beta' approximately 0.45). Finite interpenetration of percolating clusters with smaller clusters, along with permanent intercluster entanglements, increase the network rigidity. The self-assembly of MAX1 peptide was compared and contrasted with the assembly of other biopolymeric networks in literature.

Gel point determination of gelatin hydrogels by dynamic light scattering and rheological measurements

The sol-gel transition of gelatin aqueous solutions has been investigated in terms of time-resolved dynamic light scattering (DLS) and rheological measurements during the cooling process. A drastic increase in the scattering intensity, ergodic-nonergodic transition, and a power-law behavior in the scattering intensity-time correlation function were observed at the gelation temperature Tgel. Thus obtained "microscopic" Tgel was confirmed to be in good agreement with a "macroscopic" Tgel obtained by rheological measurements irrespective of gelatin concentration C . The fractal exponent Dp evaluated by DLS was found to be q and C independent and was also in good agreement with that obtained by rheology (n), i.e., Dp congruent with n congruent with 0.73, where q is the magnitude of the scattering vector.

Diffusion NMR as a New Method for the Determination of the Gel Point of Gelatin

The gelation of gelatin has been investigated using pulsed field gradient (PFG) NMR. For the first time, diffusion results have been used to determine the gelation point, which is indicated by a minimum in the self-diffusion coefficient of the free polymer fraction vs temperature. Biexponential analysis of the diffusion decay data allowing the diffusion of free and network-bound gelatin to be determined separately has been applied to provide an extended insight into the gelation process. Low-amplitude oscillatory shear rheology and time-resolved dynamic light scattering (DLS) as classical polymer characterization methods were applied as control experiments. All three methods yielded a gelation temperature of 24-25 degrees C for the cooling regime. Hysteresis effects could also be observed.