Phase Transitions and Pattern Formation in Chemo‐Responsive Gels and Composites

Abstract

Design of multifunctional materials often requires the chemical functionalization enabling the material to respond to a variety of stimuli. Herein, we focus on modeling of functionalized chemo‐responsive gels and composites; functionalization alters the volume phase transitions allowing system to undergo structural reconfiguration. For instance, we consider the volume phase transitions in photo‐sensitive hydrogels functionalized with two different types of chromophores that either alter the hydrophobicity of the illuminated sample or cause these samples to heat up. We then focus on gels that are driven by the oscillatory Belousov‐Zhabotinsky reaction and show that chemically mediated interactions among these gels drive them to self‐aggregate in a well‐defined manner. Finally, we focus on the self‐oscillating composite that does not contain oscillatory chemically reactions. In all these examples, effective control over the volume phase transitions is achieved through the combination of chemical functionalization and external stimuli, extending the potential utility of the host materials.

Mechanical Adaptability of Patterns in Constrained Hydrogel Membranes

“…We take the polymer–solvent interaction parameter as χ­(ϕ, T ) = χ 0 ( T ) + χ 1 ϕ, where χ 0 ( T ) = δ h – T δ s / k B T , χ 1 = 0.518, and δ h = 1.246 × 10 –13 erg and δ s = −4.717 × 10 –16 erg/K are the changes in the enthalpy and entropy during the mixing, respectively. With the above choice, the 3D gLSM was shown to accurately reproduce an analytical solution for both continuous and discontinuous volume phase transitions observed experimentally in gels with corresponding physical properties. , …”

“…We take the polymer−solvent interaction parameter as 52 χ(ϕ,T) = χ 0 (T) + χ 1 ϕ, where χ 0 (T) = δh − Tδs/k B T, χ 1 = 0.518, and δh = 1.246 × 10 −13 erg and δs = −4.717 × 10 −16 erg/K are the changes in the enthalpy and entropy during the mixing, respectively. With the above choice, the 3D gLSM was shown 94 to accurately reproduce an analytical solution for both continuous and discontinuous volume phase transitions observed experimentally in gels with corresponding physical properties. 52,94 In the reference case scenario in the simulations below, we set the polymer volume fraction of PNIPAAm gels in preparation at 52 ϕ 0 = 0.114, the dimensional crosslink density at c 0 = 4 × 10 −3 , and choose the sample size of 105 × 15 × 3 nodes.…”

“…With the above choice, the 3D gLSM was shown 94 to accurately reproduce an analytical solution for both continuous and discontinuous volume phase transitions observed experimentally in gels with corresponding physical properties. 52,94 In the reference case scenario in the simulations below, we set the polymer volume fraction of PNIPAAm gels in preparation at 52 ϕ 0 = 0.114, the dimensional crosslink density at c 0 = 4 × 10 −3 , and choose the sample size of 105 × 15 × 3 nodes. These reference parameters are used unless specified otherwise.…”

Mechanics Underpinning Phase Separation of Hydrogels

“…A hydrogel is composed of a polymer network dispersed in a solvent, and its volume can significantly change through the transport of solvent molecules. In response to small changes of external stimuli, such as the temperature, solvent composition, electric field, and light, certain hydrogels can undergo a discontinuous volume transition from a swollen state to a shrunk state, which is called a volume phase transition. − Hydrogels capable of a volume phase transition have diverse applications, including sensors, actuators, soft robots, drug delivery, and so on. During the process of a volume phase transition, which is governed by the kinetics of solvent migration, phase separation occurs, and the swollen and shrunk states coexist.…”