Javier Rojo-GonzΓ‘lez, Adrian Arenas Gullo
Hydrogels are solids that consist on a crosslinked polymer network filled with water. Within these materials, responsive hydrogels are very interesting since they change their behaviour when some external parameter like temperature, pH β¦ is changed. Pnipam hydrogels change their behaviour with temperature, changing from hydrophilic to hydrophobic when the temperature is risen above ~32C, expelling the water they have inside and thus shrinking.
For a homogeneously crosslinked Pnipam hydrogel, when heated above the transition temperature in a quasistatic way, independently of its shape, it will shrink in an affine way. But, when heated fast enough (βquenchedβ) water from the surface of the gel will be expelled fast forming an impermeable polymer layer that prevents the water inside the gel from escaping. The volume of the gel has to remain constant but the polymer wants to separate from the water. This way, a constant volume phase transition induces phase separation into a shrunken and a swollen phase.
When dealing with shaped gel with circular cross-section, like a torus or an S, the shrunk phase would tend to occupy the region with negative curvature/negatively curved region, while the swollen the swollen the region with positive curvature [1]. The deformation induced by the phase separation results in a change of shape of gel. This way, through a thermodynamic instability, big deformations of gels can be achieved. By similarity to extreme mechanics where small forces can lead to big deformation through mechanical instabilities, this phenomenon is called extreme thermodynamics.
When working with toroidal Pnipam hydrogels, quenching the tori leads to its deformation into a Pringle shape [2] (figure 1). The main goal of this project is to keep exploring experimentally the effects of extreme thermodynamics in gels with various shapes. Some of the shapes of interest are a cylinder, or an S-shaped gel where some twist is predicted to appear.
Figure 1: Left: Initial Pnipam torus. Right: Pnipam torus after quenching. It can be seen that the shrunken region appears in the region of negative curvature of the torus.
[1] M. S. Dimitriyev, Y-W. Chang, P. M. Goldbart, A. Fernandez-Nieves, Swelling thermodynamics and phase transitions of polymer gels, Nano Futures 3, 042001 (2019)
[2] Y-W. Chang, M. S. Dimitriyev, A. Souslov, S. V. Nikolov, S. M. Marquez, A. Alexeev, P. M. Goldbart, A. Fernandez-Nieves, Extreme thermodynamics with polymer gel tori: Harnessing thermodynamic instabilities to induce large-scale deformations, Physical Review E 98, 020501(R) (2018)
Skin Formation
Boyang Zhou
Shape actuation is often realized with polymeric materials, such as polymer gels, which are crosslinked polymer networks immersed in a solvent. The gel network equilibrates when the total free energy consisting of a polymer-solvent mixing contribution and an elasticity term of the polymer network is minimized. The polymer gel can also reach to a thermodynamically unstable phase coexistence. Thermo-responsive gels with a lower critical solution temperature, Tc, are in an equilibrium swollen state when T < Tc. In contrast, if the temperature rises above Tc rapidly, the gel can reach a phase coexistence and can be characterized by a free energy with two minima, corresponding to a polymer dense skin that is formed at the gel surface, preventing solvent from leaving and a solvent-rich gel interior. It is also reported that the solvent-poor skin grows inward from the boundary, and the continuous growth of the skin comes at the cost of solvent addition to the core region, Fig. 2(1-3).
Figure 2: Fully swollen gel in π»2π at 20β (β ~1.7cm). B: deswollen gel at 40β after progressive heating from 20β. C: π»2>π at 40β was dumped onto the gel to achieve rapid heating. 1-3: Polymer skin growth after rapid heating. D: Fully swollen gel. E: 10s after applying heat rapidly. F: 20s after. G: 30s after.
Further, despite the transition between swollen and deswollen gels is like the gas-liquid transition, the shear rigidity of the gel also has an impact on the phase-coexistence equilibrium due to the anisotropic stress at the interface of the solvent-poor and the solvent-rich. Macroscopically, this stress between a polymer-rich skin and a solvent-rich interior may cause a deformation of the gel, such as buckling or transverse deflection. As a result, the phase coexistence equilibrium of the gel is also believed to be shape dependent. So far, details of how the shear rigidity and shape of the gel influences the volume phase behavior remain unclear. However, this information is essential to understand the thermodynamic instability of the gel under rapid heating. The neutron scattering intensity will be compared altogether with the neutron imaging data and rheology data taken at the same conditions. The goal is to study the connection between the microscopic structure obtained with SANS and the mechanical properties such as the bulk modulus and skin thickness, to understand the phase coexistence that is far from the thermodynamically stable regime. We have used pNIPAM gel with 1π€π€t.% of crosslinker that has a phase transition temperature of π = 32β (Fig.A). We slowly increased the temperature from 20β to 40β and the gel diameter decreased by 50%. Further, we directly dumped H2>O at 40β onto the gel, which results in deswelling of only 5% and the size is unchanged over the course of hours. The restricted deswelling clearly indicates the presence of a polymer-rich skin that traps the solvent. In addition, we prepared cylindrically shaped gels with both the diameter and the height equal to 2 mm in D2O for measurements on BOA (neutron imaging), SINQ, PSI. The sample was kept in a quartz glass capillary and a heat gun was used to rapidly increase the temperature, which made the gel change to a donut-like shape with a more transparent center and a polymer-dense peripheral region. The width of the periphery was estimated to be 30πm, (Fig.2D-G). In addition, we press the gels on the circular surface by 50% in height using a rheometer and recorded the normal force at different stage of gel (swollen, de-swollen and swollen with skin), the goal is to treat the gel with skin as a composite material and extract the thickness and mechanical properties of the polymer dense skin. If we press the gel for a long time (20 hours), the force stabilizes which means that the polymer network has rearranged and has reached the equilibrium state in the new geometry. This is clear evidence that beside temperature, the gel microscopic structure is also connected to the stress.