Poly water absorbency

The water content of a hydrogel depends on the hydrophilicity of the monomer, eg, cured poly(HEMA) absorbs 60% of its weight of water and thus forms a hydrogel with about 38% water content. Other hydrophilic monomers, such as A/-vinylpyrrohdinone [88-12-0] (NVP) (6), and glycerol methacrylate [100-92-5] (GM) (7), and acrylamide monomers, such as diacetone acrylamide [2873-94-9] (DAA), have also been used to form hydrogels with higher water content. 

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. 

In another study poly(acrylic acid-g-styrene) copolymers were also shown to have good emulsifying ability and high water absorbency [105]. Membranes produced from intermolecular complexes of the above materials with poly(ethylacry-late-g-ethylene oxide) copolymers behaved like chemical valves, whose perme-ativity could be controlled reversibly by changing the pH of the surrounding medium, since both graft copolymers behave as polyelectrolytes in aqueous solution. 

Johnson et al. [27] report that polysulfone and poly(vinyl acetate) show enhanced low-temperature -loss transitions in proportion to the unclustered water. Clustered water in poly(vinyl acetate) has no effect on Tg, although shifts with increasing amount of unclustered water. Fuzek [3T] found that water absorbed by synthetic fibers and silk at room temperature and 65% RH substantially lowers Tq s, the effect being reflected in several different fiber properties. Wet soaking has an additional effect. 

Water absorbed in a polymer can exist in an unassociated state or as a separate phase (cluster). In this investigation the DSC technique of water cluster analysis was used in conjunction with coulometric water content measurements to characterize the water sorption behavior of polysulfone and poly(vinyl acetate) The polysulfone had to be saturated above its Tg (190°C) and quenched to 23°C for cluster formation to occur while cluster formation occurred isothermally at 23°C in the poly(vinyl acetate) Both polymers showed an enchancement of their low temperature 3-loss transitions in proportion to the amount of unclustered water present. Frozen clustered water produced an additional low-temperature dielectric loss maximum in PVAc and polysulfone common to polyethylene and polycarbonate as well. Dielectric data obtained on a thin film of water between polyethylene sheets was in quantitative agreement with the clustered water data. 

Clay and mineral fillers have been used for reducing production costs and improving the comprehensive water absorbing properties of superabsorbent materials For example, a poly(acrylic acid)/mica superabsorbent has been synthesized with water absorbency higher than 1100 g H20/g In a typical method of preparation, acrybc acid monomer is neutralized at ambient temperature with an amount of aqueous sodium hydroxide solution to achieve 65% neutralization (optimum) Dry ultrafine (<0.2 tm) mica powder (10 wt%) is added, followed by cross-linker N,N-methylene-bisacrylamide (0.10 wt%) and radical initiator, potassium persulfate The mixture is heated to 60-70°C in a water bath for 4 h. The product is washed, dried under vacuum at 50°C, and screened. 

Diffusion Process Analysis. - The diffusion of water into cylinders of poly-HEMA and copolymers of HEMA with THFMA, BMA and CHMA were studied. The diffusion of water into the polymers was found to follow a Fickian, or case I mechanism. The diffusion coefficients of water were determined from mass measurements and NMR imaging studies. They were found to vary from 1.7 0.2 X 10 " m s for polyHEMA at 37 °C to lower values for the copolymers. The mass of water absorbed at equilibrium relative to the mass of dry polymer varied from 52-58 wt% for polyHEMA to lower values for the copolymers. 

From the research of Zhang [88], a novel flax yarn waste-g-poly(acrylic acid-co-acrylamide) [FYW/PAA] superabsorbent composite has been synthesized (Fig. 3.6). In order to develop an eco-friendly superabsorbent composite, flax yarn waste (FYW) was used as raw material. Acrylic acid (AA) and acrylamide (AM) were grafted onto the pretreated flax yarn waste (PFYW) by free-radical graft copolymerization in homogeneous aqueous solution. As a result, the prepared FYW/PAA attained the best water absorbency of 875 g/g in distilled water, 490 g/g in rainwater and 90 g/g in 0.9 wt% NaCl solution. 

Furthermore, glycerol is not added to the mixture. This reduces the water absorbency of the final product (13). An example of a grafting compound is maleic anhydride (MA). An example of a good compatibilizer is poly(ethylene) (PE) containing approximately 5% MA which is commercially available and only slightly more expensive than pure poly (ethylene). 

Poly(vlnyl acetate) PVAc 349 8.86 101.6 305 [67,68] aj s of the entire softening dispersion from dynamic mechanical J f) with 10equilibrium with ambient moisture and the water absorbed lower the sample s Tg. Its T-dependence Is considerably weaker than either of the shift factors obtained from creep compliance In dried samples given Immediately below. 

Table 13.1 % Water absorbance by neat UPE and its composites reinforced with particle and short form of raw, mercerized, silanated, benzoylated, Grewia optiva-g-poly (AN) and Grewia optivfl-g-poly-(AAc) fibers.

Methyl methacrylate (mMA) was graft pol3nnerized onto dextran in the presence of Ce " " salts. The resulting polymer is neither soluble in water, an excellent solvent for dextran, nor in acetone, an excellent solvent for poly-(methyl methacrylate) (PMMA). A hot-pressed film of the copol3mier not only showed better wettability and water absorbing power than PMMA but also thromboresistance. There was also correlation like matrix mechanism between the molecular weight of the reactant dextran and that of the branch PMMA in the product. 

The absorption of water or moisture by solid polymers may be one of important phenomena in pure and applied polymer science. Therefore, the behaviors of water absorbed by a variety of polymers have been investigated by several physicochemical methods. In most cases, however, water is absorbed only in amorphous part, and thus the structure of water at the molecular level is not always definite. As for linear poly(ethylenimine) [poly(El)], we found two distinct crystalline hydrates, in which water exists stoi-chiometrically as "water of crystallization", and it was confirmed by the X-ray structure analyses that the formation of the hydrates is responsible for the remarkably hygroscopic nature of this crystalline polymer. 

Construction Materials. Superabsorbent polsrmers are used to control liquid water in a variety of construction-related products. Joint-sealing composites are made by blending superabsorbents into chloroprene rubber (54) or into poly(ethylene-co-vinyl acetate) (55). These composites are used like mortar in the concrete block walls of the structure. Gaps left during construction are subsequently filled as the superabsorbent swells in any water, and subsequent leaks are prevented. A water-blocking construction backfill has also been developed from cement, water absorbing polymer, and an asphalt emulsion (56). 

Polyester-based PUs are more sensitive to hydrolytic cleavage than polyether-based PUs. However, the introduction of bulky alkyl side groups into a hydroxyl-terminated polyester (e.g., poly(2,4-diethyl-l,5-pentamethyleneadipate) yields PUs that are more hydrolytically stable with regard to hydrolysis [16]. Polyethylene oxide-based PUs exhibit water sensitivity due to their hydrophihcity and the water-absorbing capacity... 

---