Browsing by Subject "viscoelastic"

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Eﬀect of pH on the Properties of Hydrogels Cross-Linked via Dynamic Thia-Michael Addition Bonds
(ACS Publications, 2021-12-28) FitzSimons, Thomas M.; Anslyn, Eric V.; Rosales, Adrienne M.
Hydrogels cross-linked with dynamic covalent bonds exhibit time-dependent properties, making them an advantageous platform for applications ranging from biomaterials to self-healing networks. However, the relationship between the cross-link exchange kinetics, material properties, and stability of these platforms is not fully understood, especially upon addition of external stimuli. In this work, pH was used as a handle to manipulate cross-link exchange kinetics and control the resulting hydrogel mechanics and stability in a physiologically relevant window. Poly(ethylene glycol)-based hydrogels were cross-linked with a reversible thia-Michael addition reaction in aqueous buﬀer between pH 3 and pH 7. The rate constants of bond exchange and equilibrium constants were determined for each pH value, and these data were correlated with the resulting mechanical proﬁles of the bulk hydrogels. With increasing pH, both the forward and the reverse rate constants increased, while the equilibrium constant decreased. These changes led to faster stress relaxation and less stiﬀ hydrogels at more basic pH values. The elevated pH values also led to an increased mass loss and a faster rate of release of an encapsulated model bovine serum albumin ﬂuorescent protein. The connection between the kinetics, mechanics, and molecular release proﬁles provides important insight into the structure−property relationships of dynamic covalent hydrogels, and this system oﬀers a promising platform for controlled release between physiologically relevant pH values.
Eﬀect of pH on the Properties of Hydrogels Cross-Linked via Dynamic Thia-Michael Addition Bonds
(ACS Publications, 2021-12-28) FitzSimons, Thomas M.; Anslyn, Eric V.; Rosales, Adrienne M.
Hydrogels cross-linked with dynamic covalent bonds exhibit time-dependent properties, making them an advantageous platform for applications ranging from biomaterials to self-healing networks. However, the relationship between the cross-link exchange kinetics, material properties, and stability of these platforms is not fully understood, especially upon addition of external stimuli. In this work, pH was used as a handle to manipulate cross-link exchange kinetics and control the resulting hydrogel mechanics and stability in a physiologically relevant window. Poly(ethylene glycol)-based hydrogels were cross-linked with a reversible thia-Michael addition reaction in aqueous buﬀer between pH 3 and pH 7. The rate constants of bond exchange and equilibrium constants were determined for each pH value, and these data were correlated with the resulting mechanical proﬁles of the bulk hydrogels. With increasing pH, both the forward and the reverse rate constants increased, while the equilibrium constant decreased. These changes led to faster stress relaxation and less stiﬀ hydrogels at more basic pH values. The elevated pH values also led to an increased mass loss and a faster rate of release of an encapsulated model bovine serum albumin ﬂuorescent protein. The connection between the kinetics, mechanics, and molecular release proﬁles provides important insight into the structure−property relationships of dynamic covalent hydrogels, and this system oﬀers a promising platform for controlled release between physiologically relevant pH values.
Linear viscoelastic properties of adhesive soft particle glasses
(AIP Publishing, 2021-05-01) Shahmohammadi, Ali; Bonnecaze, Roger T.
A model is presented to predict the linear viscoelastic rheology of hydrophobically modified adhesive soft particle glasses in an aqueous solution. The hydrophobes on the surfaces of particles in contact preferentially associate with each other, creating an adhesive force between particles. The extent of this adhesive force depends on the number of associating or physically bonded hydrophobes and the strain on the bonds. The model is first presented for two horizontal surfaces with hydrophobes attached to them. The force required for oscillatory movement between these adhesive surfaces exhibits a Maxwellian behavior with a single relaxation time that is about the time for hydrophobe dissociation. The model is extended to predict the storage and loss moduli of adhesive soft particle glasses in ordered cubic lattices. In addition to the adhesive force, the particles also exhibit repulsive elastic and elastohydrodynamic interparticle forces. For situations where there is no adhesive force between particles, the storage modulus is independent of frequency, and the loss modulus is a linear function of frequency. The storage and loss moduli as functions of frequency are richer with adhesive forces. The storage modulus exhibits two plateaus, one at low and one at high frequency. The loss modulus exhibits a local maximum in frequency that occurs at approximately the dissociation rate of the hydrophobes.