(315b) Gelation, Crytallization and Polymorphic Transformations of Syndiotactic Polystyrene in Acetophenone + Carbon Dioxide Mixtures at High Pressures

Syndiotactic polystyrene (s-PS) is known to have five polymorphic states, namely the alpha, beta, gamma, delta, and epsilon forms. The alpha- and beta-forms are typically obtained from melt. They are in planar zigzag chain conformations. While the gamma, delta and epsilon forms are normally formed in the presence of solvents. They are in the 21 helical chain conformations.

How the polymorphic states can be modulated in dense fluids is of current interest. In this presentation, we will report on the gelation and crystallization processes and the polymorphic states that evolve in solutions of s-PS in acetophenone + carbon dioxide fluid mixtures at high pressures.

In pure acetophenone only a gelation phase boundary is observed. Solutions containing 25 wt % CO2 display a sol-gel and a liquid-liquid phase boundary. Solutions containing 40 wt % CO2 display a crystallization and a liquid-liquid phase boundary. It is shown that when the gelation boundary is crossed first, the resulting crystal structure is the ä form. When the L-L phase boundary is crossed first, polymer-rich phase leads to a mixture of ?delta + beta? crystal forms, while only the delta-form is found in the polymer-lean phase from solutions with 25 wt % CO2. In solutions with 40 wt % CO2, crossing the crystallization boundary first, leads to the beta form with no presence of the delta crystal form. However, when the liquid-liquid phase boundary is crossed first, the polymer-rich phase forms only the beta crystal form and the polymer-lean phase leads to a mixed crystal structure containing the beta and delta forms.

Formations of the polymorphs were verified by XRD, FTIR and SEM. These observations were then used to propose a phase diagram which provides a rational picture of the consequences of phase separation paths.