A comparison of glass transition temperatures measured by differential
scanning calorimetry (DSC) and dynamic mechanical thermal analysis (D
MTA) was made using microlayer composites with many alternating layers
of two polymers. What appeared to be a third transition at a temperat
ure intermediate between the glass transitions of the two components w
as observed in DMTA but not DSC. The mechanical origin of this interme
diate damping peak in the layered composites was established. The visc
oelastic properties of each polymer were modelled by a spring and dash
pot in series, and the composite as a parallel arrangement of two Maxw
ell elements. It was shown that an anomalous damping peak can appear b
etween the glass transition temperatures when the modulus of the compo
site can not drop below the modulus of the glassy layers, although the
viscosity of the composite continues to be defined by the viscosity o
f the rubbery layers in this temperature range. The intermediate dampi
ng peak may be a general feature in the dynamic mechanical behavior of
co-continuous multiphase systems including polymer blends and continu
ous fiber composites.