574 JOURNAL OF COSMETIC SCIENCE 5.0 4.0 3.0 2.0 1.0 0.0 0 50 100 150 200 250 Strain (%) Figure 3. Stress-strain curves of biphasic polymer latex films: (a) PBMA. (b) NAS. (c) NAS-M5. M5), the biphasic latex coated the entire hair (Figure 4c), which is attributed to the ion-ion interaction between hairs and positively charged latexes. The topology of latex films on the hair surface was examined by AFM. Figure 5 shows the AFM images of the hair surface. A naked hair exhibits a rough surface because of the cuticles located in the exterior layer. However, after the hair was coated with NAS300- M5, it could be observed that the latexes deposited evenly on the hair surface and formed a film topology corresponding to a spherical cap structure. In addition, even after the film was annealed thermally at a high temperature, the spherical cap structure could be observed in the AFM images (Figure 5c). This film morphology was attributed to the limited interdiffusion between polymer chains, which was induced by the distinctly phase-separated latex structure. In general, the latex film is formed in three steps: water evaporation, particle deformation, and interdiffusion between particles (12,13). This process is largely influenced by several factors, such as the transition temperature of polymers, water resistance, and the surfactants (14-18). In addition to those factors, the compatibility between polymers also plays an important role in determining the final morphology of films (19). In our study, the biphasic polymer latex is composed of hydrophobic PBMA phase in the core and hydrophilic PEG and the cationic group in the shell. With reference to the topology shown in Figure 5, it can be said that PEG-rich shells were mixed selectively by the chain migration between adjacent particles and that the phase mixing between the hydrophobic PBMA core and the PEG-rich shell did not occur even at the high annealing temperature. CONCLUSIONS In this study, the biphasic polymer latexes composed of PBMA in the core and the

POLYMER LATEXES ON THE HAIR SURFACE 575 Figure 4. SEM images of human hairs treated with biphasic polymer latexes: (a) a naked hair. (b) a hair treated with NAS300. (c) a hair treated with NAS300-M5. PEG/cationic group in the shell were synthesized by surfactant-free emulsion polymer- ization, and its applicability to a film former on the hair surface was evaluated. It was found that the PEG chains on the latexes help the formation of a stable latex dispersion. In the microscopic observations, the biphasic latexes show favorable deposition on the hair surface, forming a nano-sized spherical cap structure thereon. The biphasic polymer latexes considered in this study can give a viscoelastic property to human hairs, which is an important factor that determines the styling and feel of hair. ACKNOWLEDGMENTS This work was supported in part by the National Research Laboratory (NRL) program