You have created a fragile hydrogel that is easily broken when exposed to shear

Question

You have created a fragile hydrogel that is easily broken when exposed to shear stress. However, when left undisturbed for a period of time after being broken, it can self-heal and regain its original gel structure. You want to further characterize this hydrogel. (a) (5) Describe what experiment(s) you could perform to calculate the maximum amount of shear stress the hydrogel can withstand before breaking. b) (6) Describe what experiment(s) you could perform to calculate the time required for the hydrogel to self-heal after being broken. (c) (5) You notice that after exposing a new sample to very high shear stresses, the hydrogel rebuilds into an even stronger gel than it was before. Deseribe what might be happening to your sample at the microstructural level to explain this phenomenon.

Explanation / Answer

(a) You can perform speckle pattern interferometry (spi) experiment in which first the hydrogel is deformed and then a monochromatic light source illuminates the diffusive surface and a speckle pattern is recorded.The speckle pattern is used to measure displacements of the surface in the plane.Furthermore this displacement gives the strain value from then after we can find the stress.

(b) For this tensile-heal-tensile-test can be done.By cyclic loading -unloading the hydrogel a time-dependent tensile- recover - tensile -test can be conducted which will give time dependent recoverability of the hydrogel from which time can be estimated,normally it takes 2 hours.

(c) hydrogel generally composed of PDA and PMA molecules .PDA has catechol groups which interacts with amino group of PMA .PDA chains interwined to recoverable non-covalent bond .Thus PMA natwork is hybrid cross linked by reversible non covalent bond of PDA .when we cut hydrogel these catechol groups combined with recoverable non covalent bond seperates .These freshly cut surfaces have active catechol groups .when these freshly cut surfaces brought together they again form recoverable non covalent bonds.But this time cross linked bonds are formed due to highly active catechol groups then previous one.Hence hydrogel becomes more stronger than before.

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