SINGLE-CHAIN SINGLE-CRYSTALS

Single-chain single crystals of isotactic polystyrene and poly(ethylen e oxide) were studied by using transmission electron microscopy, high resolution electron microscopy, electron diffraction. Single-chain sin gle crystals were prepared by spreading a dilute solution of polymers on a water surface and collecting the resulting single-chain particles on copper grids, followed by isothermal crystallization. A statistica l analysis of the sizes of single-chain crystals was found to match wi th the known molecular weight distribution of original sample, indicat ing the particles to be composed of single chain. Observation of the m orphology and electron diffraction gave evidence of the single crystal nature. Regular-shaped single-chain crystals were obtained after isot hermal crystallization for a longer lime. By close observation, severa l types of morphologies were found for single-chain crystals of isotac tic polystyrene and poly(ethylene oxide); in addition to the conventio nal morphologies observed for multi-chain crystals, new morphologies w ere observed in both cases. The morphologies of poly(ethylene oxide) w ere explained according to the crystal structure and twin modes. Tent- like single-chain crystals were often observed. Because of the small s ize of the crystals, they can avoid collapse on the substrate. The cry stalline c-axis of single-chain crystals were found to orient preferab ly in the direction normal to the substrate. The investigation of elec tron diffraction and high resolution electron microscopy revealed that the structure of the single-chain crystals of isotactic polystyrene i s the same as far multi-chain crystals. A reasonable explanation is gi ven for the unusual resistance to electron irradiation and the missing of lower-index reflections. Regular periodic stripes were found on th e top surface of single-chain crystal of isotactic polystyrene with an average periodic length in accordance with (220) spacing. In addition , a statistical thermodynamics theory was developed for single-chain c rystal. It is found that the equilibrium dimensions are related to mol ecular weight and annealing temperature, while the equilibrium melting temperature depends on molecular weight.