Water absorption, hydrophilic polymers

Poly(ethylene oxide)/Poly(propylene oxide)/ Poly(ethylene oxide) Triblock Copolymer Pluronic is a trade name for poloxamers, which are nonionic triblock copolymers of PEO chains on the two ends of the molecules with a poly(propylene) oxide (PPO) chain in the middle. The PEO chain is hydrophilic, while the PPO chain is hydrophobic. The polymer properties can be tailored depending on the content ratio of the constituents. Due to the combined hydro-phobic and hydrophilic properties in the structure (amphiphilic), it can be used as a stabilizer and a surfactant. It also can enhance miscibility and water absorption of polymers, and it has low toxicity itself. Pluronic has been used in drug delivery devices for controlled release purposes [220]. 

In previous models of the interaction of water with hydrophilic polymers, it is generally hypothesized that the water molecules are either bonded to specific polymer chain sites or are freely dispersed homogeneously throughout the amorphous polymer matrix (Huang and Yang, 2005). However, based on the FT-IR analysis of the BIN-SMPU, the moisture absorption mechanism can be explained by the theory of dynamic combinatorial chemistry or constitutional dynamic chemistry proposed by Lehn (2007), which relies on the selection of the thermodynamically most stable product from an equilibrating mixture. 

Acrylonitrile has been grafted onto many polymeric systems. In particular, acrylonitrile grafting has been used to impart hydrophilic behavior to starch (143—145) and polymer fibers (146). Exceptional water absorption capabiUty results from the grafting of acrylonitrile to starch, and the use of 2-acrylamido-2-methylpropanesulfonic acid [15214-89-8] along with acrylonitrile for grafting results in copolymers that can absorb over 5000 times their weight of deionized water (147). 

It 1s well known that water absorption bears a direct relation with the number of polar groups in the polymer and that Ionic diffusion occur via "hopping" along hydrophilic sites. Therefore, the hydrophilic/ hydrophobic characteristics of the Inhibitor exert a profound effect on dissolution rate. 

The aryloxy substituent serves a specific purpose. For drug release applications, it is normally preferred that the device should erode steadily from the outside only. Polymers such as 3.86 are so hydrophilic that water absorption followed by catastrophic breakdown of the capsule may occur, with the result that the drug is released precipitously in a massive dose. This is a situation to be avoided. The hydrophobic groups in 3.87 prevent rapid water penetration and, in so doing, limit the hydrolysis to the surface layers. 

The structure has primary and secondary alcohol groups uniformly interspersed throughout the length of the polymer chain. These hydroxy units impart high water absorption characteristics to the fiber and can act as reactive sites. The cotton fibers are hydrophilic and swell in water. It is hydrolyzed by hot acid and swollen by concentrated alkali. The cotton is treated with caustic soda solution (12 to 25 percent) under tension to develop a silk-luster and stop longitudinal shrinkage. This process is called mercerization. Mercerized cotton exhibits increased moisture and dye absorption. 

It is known that papers, cotton, and cellulose are natural hydrophilic polymers, which can absorb water, but are insoluble in water. Therefore, they have been used as disposable diapers, feminine napkins, etc. They absorb water by capillary action. Therefore, they absorb only a small amount of water and the water absorbed is easily removed by applying low pressure. Synthetic crosslinked poly(vinyl alcohol) and crosslinked poly(oxyethylene) are also hydrophilic polymers and they have almost the same water-absorption capacity of about 1-20 g water per gram of polymer as natural polymers. Those natural polymers and synthetic polymers do not essentially have ionic groups. 

As mentioned before, superabsobents should be essentially low crosslinked hydrophilic polymers and have ionic groups such as anionic groups, cationic groups, or betaine in the polymers. They usually have a high water-absorption capacity of more than 100 g-water/g-polymer even in dilute salt solution. In Fig. 2, swollen crosslinked poly (sodium acrylates) are illustrated. 

Where A molecules are distributed over B sites and C represents the partitioning of bound and liquid water. This "BET" like absorption model has been modified to more simplified forms for saturable sorption models. For low water activity, this sorption model has been applied to several hydrophilic polymers including epoxies (13) and nylons (]L4). 

Salame (35 ) has considered the scalping of flavor constituents into polymers by dilute solution absorption. He found sorption in hydrophilic polymers to be dependent on surrounding water activity (Table III). In the most hydrophilic polymers such as nylon and EVOH, there was an increase both in non-polar organics and polar organics in these two polymers in the presence of water. The increase in non-polar organics could be attributed to increased free volume of the polymer in the sorbed polymer state, while the increase in polar organics could be attributed to additional interactions available for the polymer molecule. 

Free-radical and reagent absorption. Free radicals, monomers, and other reagents are transported into the polymer particles. The free radicals are likely to be oligomers because a hydrophilic ion-radical would remain in the aqueous phase. Monomers and other reagents can diffuse from the monomer droplets to the particles if they possess adequate water solubility. Dissolved polymer and other water-insoluble ingredients would remain in the droplets. 

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