PHYSICAL BEHAVIOR OF WATER-SOLUBLE POLYMERS 119 Newtonian than the high molecular weight polymer solutions. These differences in viscosity-stress properties are shown in Fig. 3. The foregoing discussion briefly shows how water-soluble cellulose polymers can impart divergent flow characteristics to water. Many of them also impart varied thickening properties to solvent systems other than plain water. Behavior of well-dispersed derivatives in these mixed solvent systems is similar to their behavior in water. However, the vis- cosity of the solvent must be taken into account. For example, if sucrose- water is the solvent and if this solvent is 50 times thicker than water, the 100,000 ioo 1 ,001 HIGH VISCOSITY TYPE•, 1.0% w PARTICLE SUSPENSION TUMBLING ISIqIAYING AT REST, FILM SAG BROOKFIELD VISCOMETER OR ROLL • •- UNDER GRAVITY POURING APP•ICAT•N I I .01 0.1 1.0 10 100 1000 10•,000 100,000 SHEAR RATE (SEC -1) Figure &--Typical flow behavior of hydroxyethylcellulose. resulting cellulose ether solution in the mixed solvent will be 50 times thicker than it will be in water. This behavior is predictable for many solvent-water mixtures, and is shown in Fig. 4. In order to render cellulose soluble in solvent, chemical groups must be added. These substituents impart solvent attracting characteristics to the cellulose and act as wedges to force the cellulose molecule apart and break its inherent physical bonds. Some of the groups which make cel- lulose hydrophilic (warerr attracting) are shown in Fig. 5. Also shown are representations of how water is believed to interact with these groups. The ionic groups, which separate into positive and negative components, provide a strong dissolving force. The components with positive charges migrate away from the cellulose chain, leaving only negative charges. Since like charges repel, the cellulose chain expands and exposes hydrophilic sites to the solvent. The nonionic groups provide dissolving power

120 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS • 10 A. 1.0 0.1 1.0 10 100 1000 10,000 100,000 SHEAR STRESS, (DYNES / CM.2) Figure 4.--Flow behavior of well-dispersed carboxyalkyl- and hydroxyalkylcellulose ethers in (A) sucrose-water and (B) water solid curves are solutions broken curves are solvents. mainly by hydrogen bonding with the solvent. The oxygen and hydrogen of the solvent interact with like groups in the polymer. Figure 6 summa- rizes the hydrophobic (or water-repelling) and the hydrophilic groups of typical ionic and nonionic water-soluble polymers. Many of the physical properties imparted to aqueous solutions by hydroxyalkylcellulose are governed by the chemical nature of its solu- billzing substituent. When ethylene oxide is added to cellulose, no hy- droxyl groups are lost. They are merely transferred to the end of the substituent. Such a derivative is more hydrophilic than alkyl cel- lulose derivatives, which have far fewer hydroxyl groups. This fact can be used to explain why alkyl celluloses precipitate at elevated temperature, whereas hydroxyethylcellulose does not. Figure 7 depicts the solvent water interacting with hydrophilic ether and hydroxyl groups. These hydrophilic groups are separated by hydrophobic ethylene groups (--CH2- CH2--). At normal room temperature, it is suggested that these chains are relatively motionless, as depicted by the uppermost structure in the figure. However, as the temperature increases, the hydrophobic group becomes excited and begins to vibrate between the ether linkages in some-