2008 TRI/PRINCETON CONFERENCE 253 Dynamic mechanical analysis (DMA) was performed on a TA RSA3 Dynamic Mechanical Analyzer (TA Instruments) using rectangular samples of polymer fi lm. Strain sweeps were fi rst done in tension to determine the linear range of the polymers. Temperature sweeps were then done in tension from -50 to 250°C at a frequency of 1 Hz and a strain of 0.001% in a nitrogen-purged atmosphere. The glass transition temperature (Tg) was chosen as the onset of the decrease in the elastic modulus (E ). Frequency sweeps were done in extension at 23°C and 50% or 90% RH. The stiffness of the fi xative-hair composite samples was performed using a TA.XT.Plus® Texture Analyser in a three-point bend confi guration. Composite samples for this test were prepared by applying 0.8 g of polymer dispersion to virgin Chinese hair tresses, which were 2.5 g in weight and 16.5 cm in length. The prepared tresses were sandwiched between perforated, Tefl on-coated plates and clamped using spacers to maintain a fl at, rectangular 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 Figure 1. Average repeat unit for cassia. Figure 2. Average repeat unit for cassia hydroxypropyltrimonium chloride at a substitution level of 3.0 meq/g.

JOURNAL OF COSMETIC SCIENCE 254 25°C, 90% RH. Testing was done using a support span of 2.54 cm and a fl exure 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, (6) which is in the linear deformation region of the curve, because it is a better probe of adhesion. (7) Five samples were prepared per poly- mer, and the averages are reported. RESULTS Traditionally, rigid polymers have been used in fi xative applications to create stiff-hold hairstyles. It is generally believed that the fi xative polymer stiffens the hair since the stiff- ness of treated hair is more rigid than the untreated hair fi bers. However, it will be dem- onstrated that fi xative-treated hair is a polymer-fi ber composite. Thus, the hair fi bers are the primary source of strength of the composite, and adhesion is of prime importance in achieving polymer composite properties. This does not mean that the cohesive properties are unimportant, as the results for the chosen example will demonstrate. With adequate adhesion, the cohesive strength of the polymer affects the stiffness of the composite. Therefore, the cohesive properties of potential fi xative polymers must also be considered. Cohesion of fi xative polymers is probed by mechanical tests on thin fi lms. FILM PROPERTIES Dynamic mechanical analysis (DMA) and tensile testing were conducted to give compli- mentary information on polymer cohesion. Tensile data for cassia and two cationic de- rivatives are shown in Figure 3, plotted as percent elongation to break and tensile strength versus charge density. These results show that as the cationic charge substitu- tion increases, the tensile strength decreases, and the elongation increases. In other words, the quaternary substitution in this example appears to plasticize and thus in- crease the stress relaxation properties of the polymer. With respect to the chemical structure of cassia, intermolecular hydrogen bonding is expected to drive close packing of the polymer chains and act as a primary contributor to the cohesive strength. The cationic substitution in this example adds fl exible short chain branches to the polymer backbone, which cause steric hindrance to close packing, creating additional free volume that gives a plasticization effect to the polymer. Thus, a decrease in the glass transition temperature (Tg) is expected. DMA was used to measure the Tg, an important parameter for describing amorphous polymer rigidity. The Tg is related to the activation energy needed to enable long-range, coordinated molecular motion hence, the Tg is related to the cohesive energy density (8) of the polymer and is affected by inter- and intramolecular forces and molecular architec- ture (9,10). The effect of the level of cationic substitution on the Tg of cassia is shown for the example polymers in Figure 4. The elastic modulus, E , is plotted versus temperature, and the onset of Tg is indicated by the temperature at which E begins to decrease. The DMA data reinforce the concept that cationic charge substitution causes steric hindrance to intermolecular attractive forces and consequently lowers the cohesive en- ergy density. The glass transition temperatures for these cassia polymers decrease with increased charge density.