Polymer water plasticizing effect

The oxygen diffusion coefficient as a function of water activity for 98L-PLA is presented in Fig. 3a. An exponential increase of the oxygen diffusion coefficients from 2 X 10 " to 9 X 10 " m /s at 23°C can be observed as water activity increased from 0 to 0.9. The observed increase of the oxygen diffusion coefficient in the range of = 0-0.9 can be attributed to the plasticization effect on the amorphous phase by the water molecules. Plasticization effects tend to increase the mobility of the oxygen molecules in the polymer matrix. Plasticization effects are evidenced by a reduction of the polymer s 7 g [43]. 

Lai, M.C., Hageman, M.J., Schowen, R.L., Borchardt, R.T., Laiard, B.B., and Topp, E.M. 1999b. Chemical stability of peptides in polymers. 2. Discriminating between solvent and plasticizing effects of water on peptide demidation in poly(vinyl pyrrolidone). J. Pharm. Sci. 88, 1081-1089. 

Thermo-mechanical analysis (TMA)26 and DMTA292 have been used to study the viscoelastic properties of bread, a composite two-phase system. The moisture dependence of Tg was similar to that of pure amorphous starch and gluten.242,282 293-295 At the moisture content of bread, water exerts its full plasticization effect on the composite polymer matrix, reducing the effective Tg to about -10° to - 12°C,26,292 well below normal shelf life storage temperatures of this product. 

For example, poly(vinyl alcohol) (PVOH) finds application as an effective adhesive for paper products such as the old-fashioned postage stamps or envelopes. Licking the stamp or envelope flap causes the adhesive to become sticky because water plasticizes the PVOH and lowers the Tg below room temperature. On pressing the stamp to a piece of paper, the polymer chains can entangle and hydrogen bond with the cellulose in the paper. As the water... 

Hydrophobic polymers and films, such as paraffin wax and poly (vinyl chloride), show no change in stiffness modulus when flooded with water. On the other hand, butyl alcohol on a paraffin wax film causes a slow, slight increase of stiffness modulus over a period of several minutes. The effect of benzene on paraffin films is similar to that of methanol on PVA. A poly (vinyl chloride) film is unaffected by plasticizers such as dimethyl and diethyl phthalate over short time periods. However, PVC gives rise to the ultrasonic modulus anomaly when contacted with tetrahydrofuran, a strong solvent for PVC. The plasticizing effect of dibutyl phthalate on polystyrene has been demonstrated by ultrasonic impedometry (16). No anomalies were found. 

As can be seen from Table 9.12, photochromism is also strongly affected by humidity. It is well known that thiazine and its derivatives undergo photoreduction smoothly in the presence of such activated surfaces as silica gel and alumina with water molecules,37 and that methylene blue is also photochemically reduced in acid solution, even with only available water as the reducing agent38 Therefore the water present in polymer films must produce a thionine-water hydration system, which accelerates the rate of the photoreduction of thionine, as well as promoting the contact of reductants by a plasticizer effect. PVA, used as the matrix, can also play the role of reductant, but its extent may be minor as compared with the added reductant. The effect of water is supported by the fact that a less hydrophilic polymer matrix such as poly(methyl methacrylate) does not exhibit photochromism even though the system contains an appreciable amount of reductants. 

The water present in foods may act as a plasticizer. Plasticizers increase plasticity and flexibility of food polymers as a result of weakening of the intermolecular forces existing between molecules. Increasing water content decreases Tg. Roos and Karel (1991a) studied the plasticizing effect of water on thermal behavior and crystallization of amorphous food models. They found that dried foods containing sugars behave like amorphous materials, and that small amounts of water decrease Tg to room temperature with... 

Nevertheless, the migration of water into the fiber is sufficient to produce the above-mentioned plasticization effect. The chain mobility is inaeased, as indicated by a decrease of the glass transition temperature by 35—50 C This is a very fortunate fact, because dyeing of the fibers is possible only above the Tg, where the increased polymer segment mobility permits dye diffusion within the fiber. For the commercial polyacrylonitrile copolymer fibers, the Tg in water hes in the r on of= 80 so that dyeing at, or slightly below, the boiling point of water becomes... 

Lai MC, Hageman MJ, Schowen RL, Borchardt RT, Laird BB, and Topp EM. Chemical Stability of Peptides in Polymers. 2. Discriminating between Solvent and Plasticizing Effects of Water on Peptide Deamidation in Poly(vinylpyrrolidone).yP/i(zm Sci 1999b 88 1081-1089. 

---