Effect of chain-length of n-alkane on solvent-induced crystallization and solvent exchange phenomenon in syndiotactic polystyrene

Abstract

The effect of molecular size and the vapor pressure of a series of n-alkanes as a solvent on solvent-induced crystallization of amorphous sPS and also the solvent exchange phenomenon in the delta form of syndiotactic polystyrene (sPS) were investigated by means of FT-IR spectroscopy and X-ray diffraction. The crystallization of amorphous sPS was found to be very much influenced by the molecular size and the vapor pressure of the n-alkanes used. At room temperature, n-alkanes with six and seven carbon atoms crystallize the sPS into the delta form, whereas the longer n-alkanes did not induce the crystallization of the amorphous sPS. By increasing the crystallization temperature to 50 degrees C, the vapor pressure of n-alkanes increases and as a result n-alkanes with eight to ten carbon atoms crystallize the amorphous sPS into the gamma form unlike the cases of n-hexane and n-heptane. On further increasing the chain-length of n-alkanes to n-tridecane and n-hexadecane, no crystallization of amorphous sPS was observed even at 50 degrees C. By keeping the crystallization behavior in mind, we used these n-alkanes to exchange the existing solvent in the delta form of sPS/chloroform complex. n-Alkanes with six and seven carbon atoms easily replace the chloroform enclosed in the crystal lattice at room temperature and the d(010) lattice spacing was found to increase according to the molecular size of the solvent used in the exchange process. n-Alkanes with eight to ten carbon atoms could also replace the chloroform enclosed in the crystal lattice at room temperature. But in this case the d(010) lattice spacing was found to be similar or lower than that of the delta form of sPS/chloroform complex and a new reflection was observed 2 theta = 6.6 degrees, indicating the formation of the epsilon form. On the other hand, longer n-alkanes (C13 and C16) did not intrude into the cavities of the delta form at room temperature. By increasing the solvent exchange temperature to 50 degrees C, the longer n-alkanes (C13 and C16) also replaced the existing chloroform in the delta form and structure transformed to the epsilon form. In this way, we found that the crystallization and solvent exchange process of sPS using n-alkanes is quite complicated and depends strongly on the chain-length of n-alkane molecule