Kinetic structures for glass transition in a modified 2d Fredrickson model

Abstract

Butler Harrowell pictures (BHP) are presented for a modified Fredrickson 2d-50×50 lattice model in a large temperature interval above and below the onset temperature. The fraction of “mobile” (spin down) sites in equilibrium is also numerically calculated as a function of temperature. The results are discussed with respect to a characteristic length of glass transition and to exhaustion of fluctuating free volume at about half of the onset temperature. The contributions from α cooperativity and accidental combinations of local β modes to the characteristic length cannot clearly be separated from the BHPs alone. Below the α onset temperature a cellular pattern is obtained with less mobile cell walls and islands of mobility within. Comparison with an analytical onset model and with cooperativities from heat capacity spectroscopy experiments on polymers results in the conclusion that one monomeric unit of a real glass former corresponds to an array of several or even many sites.