Interactions Between Cyclic and Linear Polyoxyethylenes in the Amorphous Phase

摘要

Spontaneous threading and the dynamics of the threaded species have been studied by simulation of two-component melts composed of monodisperse linear and cyclic polyoxyethylenes. The threaded species are homopolyrotaxanes, where the same monomer unit appears in the cyclic and linear components.. The intramolecular conformations of the coarsegrained polyoxyethylenes are controlled by the rotational isomeric model of Abe et al. for the unperturbed chain, with the added requirement of the maintenance of ring closure in the case of the cyclic species. The intermolecular interactions are controlled by a Lennard-Jones potential that has been discretized for use on the high coordination lattice employed in the simulation. The temperature of the simulation is 373 K, where the system is amorphous. The density of the system is 1.06 g/cm3. Analysis of the intramolecular pair correlation functions detects no tendency for demixing of the cyclic and linear species in the melt. The amount of spontaneous threading decreases rapidly when the number of monomer units in the cyclic species falls below 10. Spontaneous penetration of a single cyclic species by two distinct linear chains can be observed when the number of monomer units in the cyclic rises above 14. Therefore the size of the cyclic should be 14 monomer units for the optimal spontaneous formation of a classic homopolyrotaxane. The internal dynamics of the homopolyrotaxane was studied in order to distinguish between two hypothetical descriptions. Should the internal dynamics be described as the shuttling of a mobile cyclic species along a comparatively static linear species? Alternatively, is the internal dynamics better represented as the slithering of a mobile linear species through the constraint imposed by a comparatively static cyclic species? The second description more closely describes the systems studied in the simulation, where the linear species is treated in a manner appropriate for a very long, methyl terminated chain. Conceivably this description could be changed by using shorter linear chains with bulky end-caps. Acknowledgements. This work was supported by funds from NASA and NSF. Dr. Xu is now at the Department of Chemistry and Chemical Biology, Cornell University.