Nanocluster nucleation and growth in polymeric media below the glass transition

The synthesis of metal nanoclusters in a polymeric environment has been shown to yield nearly monodisperse particles, whose size is controlled by the strength of the polymer/metal interactions. Almough the phenomenon is quite general, little is known regarding the mechanism by which die polymer controls nanocluster size. Previous studies of the kinetics of nanocluster growth in polymeric melts above die glass transition temperature (T_g) suggest that me nanoparticle size is set by the critical cluster size (nucleation stage) rather than the rate of metal precursor transport, namely, growth. In this paper, we examine me kinetics of iron oxide (Fe₂O₃) nanocluster formation below the glass temperature (T_g) in two polymer melts: polystyrene (PS) and poly(methyl memacrylate) (PMMA). We find that the morphology of the nanoclusters formed below T_g is highly sensitive to the system temperature and differs significantly from the morphology above T_g. However, the kinetics of cluster formation is exponential with time in both PS and PMMA, both above and below T_g. The glass transition does not significantly affect the rate constant in PS, thereby suggesting that the cluster formation mechanism is largely insensitive to the polymer state (glassy or melt). However, we find a significant difference in in die kinetics of PMMA above (where the rate constant increases exponentially with T) and below (where it is nearly constant) the T_g.