From raspberries to brains, the (in)stable swelling of hydrogels
Citation for published version (APA):
Aangenendt, F. J., & Wyss, H. M. (2016). From raspberries to brains, the (in)stable swelling of hydrogels.
Document status and date: Published: 11/11/2016 Document Version:
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F.J. Aangenendt*, D. Staal, H.M. Wyss*
*DPI affiliation: ‘Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX Eindhoven, the Netherlands.
Soft matter matters
Methods and Results
To understand these morphology changes we need to determine the physical conditions that govern their occurrence. We study this by imaging the swelling particle as a function of time. To get a better understanding of the influence of the initial volume, we stop the swelling process, let the particle equilibrate (become smooth) and then perform new swelling experiment with these lower density particles, as shown in Fig. 1M. We also measure the time it takes to become smooth, see fig. 2D. Hereafter we want to determine the polymer concentration inside the gel as function of time and position, therefor we cut gel slices as can been seen in Fig. 2(A-C). There is difference in polymer concentration for the raspberry and the brain phases, although both have high concentration in the centre. More work on this is needed.
Conclusion and future work
We have experimentally studied the swelling behaviour of hydrogels at different starting volumes as well as the associated morphology changes. We are developing a simple physical model to predict these morphology changes, taking into account the polymer density profile over time.
that takes into account the polymer density profile over time.
Hydrogels
Hydrogels are cross-linked polymer networks, swollen in water. They can be responsive to stimuli like pH, salt concentration and temperature. Hydrogels are known for absorbing enormous quantities of water while maintaining structural integrity. Fully swollen, the polymer weight fraction can be as low as 0.01 percent of the total hydrogel.
Swelling behaviour
When a completely dry hydrogel is submerged in water it can exhibit surprisingly complex 3D patterns. Depending on the swelling rate the surface of the hydrogel stays smooth or interesting surface patterns start to appear, as shown in Fig. 1(A-L). Previous studies of similar swelling behaviour have mainly focused on time-independent 2D systems [Tanaka et al. 1987, Wang and Zhao 2015], even though the different hydrogel morphologies are clearly time-dependent.
Goal
We want to understand the time-depended morphology changes in 3D and exploit this for creating complex structures and surfaces.
From raspberries to brains,
the (in)stable swelling of hydrogels
Contact information: f.j.aangenendt@tue.nl
This research forms part of the research program of the Dutch Polymer Institute (DPI), project 738
\ Department of Mechanical Engineering
Fig. 2. Hydrogel slices and relaxation time. A slice of a fully
swollen particle (A), slice of particle in the raspberry phase (B) and a particle in the brain phase (C). The polymer density is higher in the middle for the brain and raspberry phases but homogenous for the fully swollen particle. The relaxation time 𝑇2 of particles for which the swelling process was stopped after 𝑇1 (D).
C
B
A
D
Fig. 1. Typical swelling behaviour of a hydrogel sphere. The
particle starts dry and smooth (A) and after being immersed in water for 5 min it start to show small bumps similar to a raspberry (B). Then is start to look like a brain (H). When the volume increase is over 40 the particle becomes smooth. An overview of all these different steps is given in (M) where red indicate the smooth phase, light blue and green indicate the raspberry and brain phases, respectively.
