Stimulus-responsive hydrogels
(SRHs) are composed of a network of cross-linked polymer chains in water.
Many different polymers can be used to make hydrogels, in this case the
polymer was N-isopropylacrylamide (NIPAM). SRHs are unique in that they
change states when exposed to extreme stimuli, such as heat or pH. The
gels are formed in a swollen state, composed almost completely of water.
In this state they are transparent and not very strong. When they are
put in an oven or in contact with a basic solution, such as methanol,
they will convert to their collapsed state as seen in the image. This
state change involves a decrease in volume and an increase in strength.
The collapsed samples are opaque. They will then change quickly back to
the swollen state once the stimulus is removed.
As a result of the large volume changes that
can be induced in the gels, they are being looked at for various applications
and have already been used in many different capacities. These include
drug carriers and pumps, optical switches, and microfluidic actuators.
The gels are made by cross-linking NIPAM and
methylene-bis-acrylamide (MBAAm). Various amounts of cross-linker can
be used in order to create different cross-link densities. These have
a large range and can have a substantial impact on the properties of the
SRH. The higher the cross-link density, the stronger and less uniform
the gel will be. The strength found in high density SRHs is favorable,
as it means they have less chance of failing under stress. However, as
the density increases, the volume ratio of the swollen to collapsed states
decreases. This is less than ideal, for instance, in the actuator application,
as the higher the volume ratio, the more displacement is possible in the
piston. Therefore it is necessary to find the cross-link density with
the optimal combination of strength and volume ratio.
One of the most standard ways to characterize a material is using its fracture toughness value, KIC. Fracture toughness is the measure of a materials resistance to the propagation of a pre-existing crack. When a flaw is initiated in a material and a tensile stress is applied, the materials fracture toughness can be related to its strength. Another variable measured from fracture tests is the energy released as the crack propagates. Both of these values can be used to better describe the material's behavior. There are ASTM standards for the fracture testing of metals and some polymers, but none for materials similar to the hydrogels. Therefore the test method being used is one of the standards for metals, the single-edge notched tension test. The gels are formed with a notch and dimensions of ASTM standards and are then subjected to a tensile load, as shown in the figure below.
It was conlcuded that the best way to quantify the failure characteristics of the notched gels in tension was to measure what is known as the "crack opening displacement". By recording the gels failure against a grid, the growing displacment at the opening of the crack and also the crack's profile can be measured and related to the surface energy of the sample. The force and displacement of the sample as the flaw propagates are also recorded by computer data acquisition. Based on knowledge of the SRHs behavior and fracture toughness trends, it is believed that the fracture toughness will decrease as the cross-link density of the SRH decreases. In order to test this, several questions must be asked.
1. What is, roughly, the fracture toughness of a gel? And is fracture
toughness the correct way to quantify a gels failure?
2. How does the fracture toughness compare for a swollen sample versus
a collapsed one of the same cross-link density?
3. How does the fracture toughness vary with cross-link density?
4. How does the fracture toughness vary with the bulk moduli of the gels
(i.e. Young’s modulus, Poisson’s ratio, and volume ratio)?
Gel Sample in Swollen State
Gel Sample in Collapsed State