Crystallization and melting behavior of nanopolymeric particles containing single 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 isotactic polystyrene (iPS) in benzene. Thermal analysis of the particle samples revealed that the cold crystallization temperature greatly decreases and the crystallization rate remarkably increases as the solution concentration decreases. The increase in crystallization rate can be attributed to fewer interchain entanglements within and between particles, thus proving experimentally 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 interdiffusion of chains between these particles is rather sluggish in the vicinity of T-g of bulk iPS. Crystallization is assumed to be accomplished before the chain interdiffusion, and nanocrystals form in situ within compact single- and few-chain globules. The average size of crystals is larger than that of single-chain crystals because the single-chain crystals may initiate 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.