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A hydrogel is a biphasic material, a mixture of porous and permeable solids and at least 10% of water or other interstitial fluid. [1] [2] The solid phase is a water insoluble three dimensional network of polymers, having absorbed a large amount of water or biological fluids.
Experimentally, stress relaxation is determined by step strain experiments, i.e. by applying a sudden one-time strain and measuring the build-up and subsequent relaxation of stress in the material (see figure), in either extensional or shear rheology. a) Applied step strain and b) induced stress as functions of time for a viscoelastic material.
Poly(2-hydroxyethyl methacrylate) (pHEMA) is a polymer that forms a hydrogel in water. Poly (hydroxyethyl methacrylate) (PHEMA) hydrogel for intraocular lens (IOL) materials was synthesized by solution polymerization using 2-hydroxyethyl methacrylate as raw material, ammonium persulfate and sodium pyrosulfite (APS/SMBS) as catalyst, and triethyleneglycol dimethacrylate (TEGDMA) as cross ...
Hydrogels have the remarkable ability to swell in water and aqueous solvents. During the process of swelling, surface instability can occur. This instability depends on the thickness of the hydrogel layers and the surface tension. [7] A higher surface tension stabilizes the flat surface of the hydrogel, which is the outer-most layer.
AMPS is made by the Ritter reaction of acrylonitrile and isobutylene in the presence of sulfuric acid and water. [2] The recent patent literature [3] describes batch and continuous processes that produce AMPS in high purity (to 99.7%) and improved yield (up to 89%, based on isobutene) with the addition of liquid isobutene to an acrylonitrile / sulfuric acid / phosphoric acid mixture at 40°C.
Specifically, hydrogels can be designed to release drugs or other agents in response to physical characteristics of the environment like temperature and pH. [12] The responsiveness of hydrogels is a result of their molecular structure and polymer networks. [12] Hydrogel nanoparticles have a promising future in the drug delivery field.
There are also fluids whose strain rate is a function of time. Fluids that require a gradually increasing shear stress to maintain a constant strain rate are referred to as rheopectic. An opposite case of this is a fluid that thins out with time and requires a decreasing stress to maintain a constant strain rate (thixotropic).
In rheology, shear thinning is the non-Newtonian behavior of fluids whose viscosity decreases under shear strain. It is sometimes considered synonymous for pseudo- plastic behaviour, [ 1 ] [ 2 ] and is usually defined as excluding time-dependent effects, such as thixotropy .