Crystallization and melting behavior of nanopolymeric particles containingsingle or a few chains

Samples of nanopolymeric particles, each containing a single chain or a few chains, were prepared by a freeze-drying method from dilute solutions of i sotactic polystyrene (iPS) in benzene. Thermal analysis of the particle sam ples revealed that the cold crystallization temperature greatly decreases a nd the crystallization rate remarkably increases as the solution concentrat ion decreases. The increase in crystallization rate can be attributed to fe wer interchain entanglements within and between particles, thus proving exp erimentally for the first time that entanglements can be a large barrier to the crystallization of polymers. Annealing of the particle samples at 373. 2 K slightly changed the crystallization behavior, indicating that the inte rdiffusion of chains between these particles is rather sluggish in the vici nity of T-g of bulk iPS. Crystallization is assumed to be accomplished befo re the chain interdiffusion, and nanocrystals form in situ within compact s ingle- and few-chain globules. The average size of crystals is larger than that of single-chain crystals because the single-chain crystals may initiat e other chains to crystallize on the growing crystal surface. Meanwhile, we also found that the nanocrystals have a lower melting temperature than the bulk polymer as a result of their small size.