Mechanical hole burning spectroscopy to probe polymer heterogeneity

Mechanical Hole Burning Spectroscopy (MSHB) was recently proposed as a method to probe the dynamic heterogeneity of polymeric materials. Here we show how the MSHB technique can be applied to a triblock copolymer and to polymer solutions. The heterogeneity of a styrene-isoprene-styrene block copolymer was investigated in the vicinity of its order-disorder transition temperature (ODT). It was found that MSHB is not only able to qualitatively distinguish the heterogeneous region from the homogenous one, but also is sensitive to the "depth" in the heterogeneous regime. We also examined the heterogeneity length scale represented by entanglement density by performing MSHB experiments on polystyrene solutions of various concentrations and molecular weights. We find that the MSHB response is not sensitive to the entanglement density while the behavior in the Rouse regime provides evidence of dynamic heterogeneity. The response appears homogeneous in the terminal regime.