In this review we intend to provide a relatively comprehensive summary of the work of supramolecular hydrogelators after 2004 and to put emphasis particularly on the applications of supramolecular hydrogels/hydrogelators as molecular biomaterials. After a brief introduction of methods for generating supramolecular hydrogels, we discuss supramolecular hydrogelators on the basis of their categories, such as small organic molecules, coordination complexes, peptides, nucleobases, and saccharides. Following molecular design, we focus on various potential applications of supramolecular hydrogels as molecular biomaterials, classified by their applications in cell cultures, tissue engineering, cell behavior, imaging, and unique applications of hydrogelators. Particularly, we discuss the applications of supramolecular hydrogelators after they form supramolecular assemblies but prior to reaching the critical gelation concentration because this subject is less explored but may hold equally great promise for helping ...

Supramolecular Hydrogelators and Hydrogels: From Soft Matter to Molecular Biomaterials

Novel injectable neutral solutions of chitosan form biodegradable gels in situ

1. Introduction to colloid science and rheology 2. Hydrodynamic effects 3. Brownian hard spheres 4. Stable colloidal suspensions 5. Non-spherical particles 6. Weakly flocculated suspensions 7. Thixotropy 8. Shear thickening 9. Rheometry of suspensions 10. Suspensions in viscoelastic media 11. Advanced topics.

Colloidal Suspension Rheology

Synthesis and physicochemical characterization of end-linked poly(ethylene glycol)-co-peptide hydrogels formed by Michael-type addition.

Dynamic mechanical measurements allow direct determination of the instant at which a network polymer gels. In such an experiment, the evolution of G′(t,ω0) and G″ (t,ω0) is measured in small amplitude oscillatory shear as a function of cross-linking time t. The frequency ω0 is kept constant throughout. At the beginning of the experiment, G″ is orders of magnitude larger than G′, and at completion of reaction, this order is reversed. It recently has been suggested by Tung and Dynes that the gel point (GP) might occur at the time at which G′ and G″ cross each other. However, there is much dispute whether GP occurs exactly at the crossover or just somewhere in its vicinity. This study resolves the dispute by modeling the rheological behavior at GP: There is only one class of network polymers for which GP coincides with the crossover. This class of polymers exhibits, when reaching GP, power law relaxation G(t) ∼ t−n with a specific exponent value n = 1/2. Examples are stoichiometrically balanced network polymers and networks with excess cross-linker, however, only at temperatures much above the glass transition. Otherwise, the power law behavior would be masked by vitrification. Power law relaxation seems to be property of polymers at GP in general. However, some polymers have a different exponent value, n ≠ 1/2, in which case the crossover occurs before GP (for n 1/2); i.e. the crossover cannot be used for detecting GP. While there are no networks known to us with n 1/2. A new method is suggested for measuring GP of these imbalanced networks.

Can the gel point of a cross-linking polymer be detected by the G′ – G″ crossover?

We suggest a very simple memory integral constitutive equation for the stress in crosslinking polymers at their transition from liquid to solid state (gel point). The equation allows for only a single (!) material parameter, the strength S[Pas1/2], and it is able to describe every known viscoelastic phenomenon at the gel point. Measurements were performed on polydimethylsiloxane model networks with balanced stoichiometry for which the crosslinking reaction has been stopped at different degrees of conversion. At the gel point, the loss and storage moduli were found to be congruent and proportional to ω1/2 over a wide range of temperature (−50°C to +180°C) and five decades of frequency ω. The hypothesis is made that this behavior is valid in the entire range 0<ω<∞. This congruence hypothesis is consistent with the Kramers‐Kronig relation and leads to a constitutive equation which shows that, for our polymer, congruent functions G′(ω)=G″(ω) are as much a rheological property at the gel point as are infinite ...

Analysis of Linear Viscoelasticity of a Crosslinking Polymer at the Gel Point

The evolution of linear viscoelasticity during cross‐linking of a stoichiometrically imbalanced polydimethylsiloxane (PDMS) was measured by small amplitude oscillatory shear. At the gel point (GP), stress relaxation was found to follow a power law, St−n, as described by the previously suggested gel equation. However, while stoichiometrically balanced gels (PDMS, polyurethanes) gave the specific exponent value of n=1/2, a higher exponent value, 1/2 Gv′(ω), and a higher rate of stress relaxation. GP was found to occur before the crossover point of the loss and storage moduli, G″(ω0,t), and G′(ω0,t), as measured during the cross‐linking reaction (reaction time, t) at constant frequency, ω0. This suggests new met...

Linear Viscoelasticity at the Gel Point of a Crosslinking PDMS with Imbalanced Stoichiometry

A method has been developed to stop the crosslinking reaction of a polydimethylsiloxane system without disturbing the state of the sample. Oscillatory shear experiments on samples just before and just beyond the gelation point demonstrated the transition of the material from a viscoelastic liquid to a viscoelastic solid. At the gel point the loss modulus and the storage modulus were found to be identical over several decades of frequency and for temperatures ranging between -50°C and +180°C. Both moduli were proportional to the square root of the frequency.

Stopping of crosslinking reaction in a PDMS polymer at the gel point

The universality of the gel equation, a recently suggested rheological equation for polymers at the gel point, was tested on three well-defined cross-linking polyurethanes (PU). Stoichiometric amounts of triisocyanate cross-linker (DRF) were mixed with a,w-dihydroxypoly(propy1ene oxides) (PPO) of nominal molecular weights 425, 1000, and 2000. As the cross-linking reaction progressed at 30 "C the evolution of the viscoelastic properties was measured. At the gel point, the storage modulus G'and the loss modulus G" were found to be congruent and proportional to d2, where w is the frequency of the oscillatory shear experiment. The same behavior was previously observed with an end-linking poly(dimethylsiloxane), however, with tetrafunctional cross-linking points.*** This suggests universal validity of the rheological equation for stoi- chiometrically balanced end-linking polymers. (3) where + is the rate of deformation of the sample at GP. The only material parameter in the equation is the gel strength S. The apparent simplicity and clarity of the gel equation suggests its universal validity for end-linking polymers with balanced stoichiometry. We therefore selected three po- lyurethanes (PU) and studied their rheology in the vicinity of GP. Similarly to the PDMS system which was used in the earlier experiments, these PU networks are also formed by an end-linking reaction of stoichiometrically balanced chemical species. However, the cross-linking process occurs by a different reaction and the cross-link functionality was chosen to be 3 instead of 4 for the PDMS. Nevertheless they are found to exhibit the same rheological behavior at GP as PDMS. This will be shown in the following.

/pdf/rheology-of-model-polyurethanes-at-the-gel-point-152ytsvkch.pdf

Rheology of model polyurethanes at the gel point

Reseaux prepares par reaction d'α,ω-dihydroxypolypropene oxyde avec le tris(isocyanato-4 phenyl thiophosphate)

Francois Chambon

Papers

Analysis of Linear Viscoelasticity of a Crosslinking Polymer at the Gel Point

Linear Viscoelasticity at the Gel Point of a Crosslinking PDMS with Imbalanced Stoichiometry

Stopping of crosslinking reaction in a PDMS polymer at the gel point

Rheology of model polyurethanes at the gel point

Stoichiometry effects on rheology of model polyurethanes at the gel point