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Crystallization and Gelation of Poly(4-Methyl-1-Pentene) in N-Pentane and in N-Pentane + Carbon Dioxide at High Pressures

Jian Fang and Erdogan Kiran. Chemical Engineering, Virginia Tech, 132 Randolph Hall, Blacksburg, VA 24061

Isotactic poly(4-methyl-1-pentene) (iP4MP1) is a semicrystalline polyolefin with a number of interesting properties. It is of practical importance as a membrane making polymer. It is the only semicrystalline polymer in which the density of the amorphous fraction is slightly higher than the density of the crystalline fraction at room temperature. Moreover, iP4MP1 presents a complex polymorphic behavior. Five different crystalline forms which differ with respect to their helical conformation and unit cell dimensions have been reported for this polymer when crystallized from semidilute solutions depending on the solvent and thermal history of the crystallization. Form I which is a 7/2 helix with tetragonal unit cell is the only form obtained from melt crystallization. Other crystalline forms are recovered when crystallized from different solvents. Crystallization from most common solvents at ambient pressures lead to Form I or Form III, which is a 4/1 helix with tetragonal unit cell.

In this presentation we will report on the fluid-solid boundary and specifically the crystallization and gelation behavior of iP4MP1 in n-pentane and n-pentane + CO2 mixtures. The study was conducted to explore (a) if the crystal modifications could be altered at high pressures in solutions of different carbon dioxide content, and (b) if high pressure conditions would promote gelation and if carbon dioxide would influence the gelation behavior of the polymer. Even though gelation is a well known occurrence in stereoregular polymers at ambient conditions, gelation of iP4MP1 from high pressure solutions, in particular binary in fluid mixtures containing carbon dioxide, to our knowledge has not been previously reported.

The miscibility and phase separation experiments were conducted in n-pentane and also in n-pentane + carbon dioxide mixtures in a pressure range from 10 to 50 MPa. In n-pentane experiments were conducted at polymer concentrations in the range from 1-6.5 wt %. No liquid-liquid phase separation was observed over the temperature range 298-423 K for these solutions containing. The fluid-solid phase boundaries (crystallization and melting temperatures) were determined by changing the temperature while holding pressure constant at selected pressures up to 45.5 MPa. The experiments in n-pentane + carbon dioxide were carried out only with 4.6 wt % polymer solutions, for fluid mixtures containing 5, 10, 20, 40 and 50 wt % carbon dioxide. In the systems containing 5 - 20 wt % carbon dioxide, no liquid-liquid phase separation boundary could be identified, however, compared to the n-pentane solutions, the crystallization and melting temperatures were lowered. In solutions containing 40 and 50 % carbon dioxide in the solvent mixture, instead of crystallization, a sol-gel type phase transformation was observed. The sol-gel transition temperature was observed to show higher sensitivity to pressure.

The crystals and gel samples collected from the high pressure view cell were characterized by DSC, NMR and SEM to identify the type of crystal modification of the polymer that is promoted in the different solvents and /or pressure conditions, and to assess the resulting morphological changes. The original polymer was of Form I with a DSC melting transition at 506 K. When recrystallized from n-pentane at ambient pressure it transforms to Form III as verified by NMR results. It displays a DSC endotherm at 347 K, and a multiple melting endotherm clustered around 506 K. When recrystallized in n-pentane at high pressures, it transforms to Form II crystal modification, in which the 347 K endotherm disappears, but an endotherm at 406 K appears. Crystals formed in solutions containing up to 20 wt % carbon dioxide were of Form II. The gels also showed crystalline features corresponding to crystal modification Form I which is further promoted at the higher pressure crystallizations. The present results demonstrate that the crystal modification Form II is the preferred outcome of crystallization from high pressure solutions. This is the form that is not normally obtained from recrystallizations at ambient pressure in ordinary solvents. The SEM results revealed that the polymer inner structure transforms from a lacy appearance in crystals formed in n-pentane to more porous structures in gels formed from n-pentane + carbon dioxide solutions.