POLYMER COMPOSITE SCIENCE AND HAIR GELS 499 perforated, Teflon-coated plates and clamped using spacers to maintain a flat, rectan­ gular geometry while the samples were dried for 24 hours at 23 ° C, 50% RH. For high-humidity testing, the dried samples were conditioned for an additional 24 hours at 25°C, 90% RH. Testing was done using a support span of 2.54 cm and a flexure rate of 40 mm/s. The data were plotted as force versus time. Peak force was used as a measure of stiffness instead of Young's modulus (5 ), which is in the linear deformation region of the curve, because it is a better probe of adhesion (6). Five samples were prepared per gel, and the averages were reported. High-humidity spiral curl retention (HHSCR) testing was performed on fixative­ composite samples. Gel (0.1 g) was applied to a European brown hair tress weighing 0.5 g and measuring 16.5 cm in length. Application and distribution of the gel was done with the tress on a balance to ensure accurate loading. The treated tresses were wrapped onto curlers with a machined spiral groove, secured, and dried for 24 hours at 23 ° C, 50% RH. The dried tresses were carefully unwrapped and hung on a ruled peg board. The initial length of the curls was recorded, and the peg board assembly was placed into a humidity cabinet at 25°C, 90% RH. Readings were taken every 15 minutes for the first hour, then every half hour for hours two through four, and every hour for hours five through eight. The curl retention is calculated as: % Curl retention= (L - L r )/(L - L 0) X 100 (1) where L is the length of the uncurled tress, L r is the length of the curl at the time of the reading, and L0 is the length of the curl at the start of the experiment. The length of the tress is denoted as the lowest line number (on the peg board) that is below the entire tress. Ten tresses were prepared per gel, and the averages were reported. RESULTS AND DISCUSSION Traditionally, rigid polymers have been used in fixative applications to create stiff-hold hairstyles. It is generally believed that the fixative polymer stiffens the hair since the stiffness of treated hair is more rigid than that of the untreated hair fibers. However, it will be demonstrated that fixative-treated hair is a polymer-fiber composite. Thus, the hair fibers are the primary source of strength of the composite, and adhesion is of prime importance in achieving polymer composite properties. Nonetheless, this does not mean that the cohesive properties are unimportant. With adequate adhesion, the cohesive strength of the polymer affects the stiffness of the composite. Therefore, the cohesive properties of potential fixative polymers must also be considered. Cohesion of fixative polymers is probed by mechanical tests on thin films. FILM PROPERTIES Dynamic mechanical analysis (DMA) and tensile testing are conducted to give compli­ mentary information on polymer cohesion. DMA is used to measure the glass transition temperature (T g ), melting temperature(s), and crosslinking, all of which affect the stiffness of the polymer. Since most fixative polymers are amorphous, T g is the most important parameter for describing rigidity. T g is a measure of the cohesive energy of the polymer. It represents the thermal energy needed to overcome attractive forces and

500 JOURNAL OF COSMETIC SCIENCE morphological factors to allow the polymer chains to move past each other with long­ range coordination. In the example shown in Figure 1, the effect of the neutralizing base on an amorphous (meth)acrylic acid-ester copolymer (polyacrylate-2 crosspolymer) is shown relative to the unneutralized (acid form) of the polymer. The elastic modulus, E', is plotted versus temperature, and the onset of T g is indicated by the temperature at which E' begins to decrease. The neutralizing bases have very different effects on the polymer. Relative to the acid form of the polymer, AMP causes very little change, while TEA causes a significant decrease and NaOH causes a major increase in T g· In fact, T g for the sample neutralized with NaOH is indiscernible. The effects of the neutralizers on polymer cohesion were confirmed with tensile testing (Figure 2). The sample neutralized with TEA was softened, as evidenced by a significantly lower Young's modulus and a significantly higher elongation to break. TEA plasticizes the polymer, lowering Tg and allowing the polymer to relax instead of breaking under the applied stress (increased elongation to break) (7). On the other hand, tensile testing of the polymer film neu­ tralized with NaOH revealed that the polymer was hardened, as demonstrated by a higher Young's modulus and a lower elongation to break. This phenomenon is explained by ionomeric crosslinking (8). In polymers with ionizable groups, such as the carboxylic acid groups in polyacrylate-2 crosspolymer, electrostatic attraction between ionic species can create physical crosslinks in the polymer. The potential effects on polymer properties 1010 NaOH 109 TEA 108 AMP •□ - •□ □ -None ca � □ D.. 107 -\_□ - � #,fn LL.I � 106 � 105 104 -100.0 -50.0 0.0 50.0 100.0 150.0 200.0 Temperature (°C) Figure 1. DMA temperature sweep data for thin films of polyacrylate-2 crosspolymer, neutralized to pH 7 with NaOH, AMP, or TEA. The unneutralized polymer (none) is also shown for comparison.