Advances in Hair Styling 436 very smooth force vs. distance curve indicating the polymer and polymer-fiber bonds have not broken in not only the first pass of the probe but also, and most importantly, subsequent cycles.30 The mechanical properties of the film and the resultant performance on hair are dependent on the cohesive properties. These are dictated by how the actual molecules of the polymeric chains interact with each other. One measure of cohesion is the glass transition temperature or T g . When a polymer film is heated, there is a phase transition as the polymer goes from a crystalline to a highly viscous amorphous liquid. This transition temperature is the T g. . T g is a factor in the way the polymer breaks through stress. With a high T g the polymer will be brittle and can easily fracture, whereas a low T g polymer will be more elastic and breakage will ultimately occur at higher elongation. There are several factors that can affect the T g of a polymer. The first is the chemical structure and chain flexibility. In general, polymers with a flexible backbone chain have a low T g and those with stiff chains have a high T g . This depends on the chemistry of the polymer, such as the nature of the functional groups attached to its backbone. Copolymerizing two or more monomers with different Figure 6. Examples of stress-strain curves produced on a texture analyzer illustrating performance attributes