We have analyzed the influence of different amounts
w
bc of two diblock copolymers,
poly(styrene-b-methyl methacrylate) (sm blend series) and
poly(cyclohexyl methacrylate-b-methyl
methacrylate) (cm blend series), on the morphological and rheological
characteristics of a blend containing w =
7.5 wt % polystyrene in poly(methyl methacrylate) matrix.
The morphological analysis is based on the
sphere size distribution function, which was determined from the image
analysis of the transmission
electron micrographs. Using this function and assuming that all
block copolymers are located at the
interface, the interfacial area per copolymer joint, Σ, was
calculated. From its hyperbolic dependence on
w
bc the value at the critical micelle
concentration, Σcmc, was found to be about 10
nm2 for both systems.
The rheological analysis reveals that in addition to the form
relaxation process, well-known for polymer
blends, a new relaxation process is observed for these systems.
Its relaxation time, τβ, has been
studied
in dependence on the amount of added block copolymers. The
observed phenomena for each blend series,
i.e. constant blend viscosity, slight shift of the form relaxation
times τ1, and systematic shift of the
interface
governed relaxation time τβ (τβ >
τ1), have been interpreted quantitatively. In
contrast to τ1, τβ is less
influenced by the interfacial tension but is mainly governed by an
additional contribution, the interfacial
shear modulus. Formulas were derived from an expanded version of
the Palierne emulsion model which
allows the determination of the proposed interfacial properties from
rheological measurements. In general,
the interfacial tension decreases with increasing amount of block
copolymer, and the decrease is more
pronounced for the cm blend series. The interfacial shear modulus
increases during compatibilization
from 0 to amounts which are in the range of 20−30% of the
interfacial tension. The decrease of interfacial
tension is in good agreement with predictions from Leibler's brush
model extended by Dai et al. In
conclusion, it was found that the Palierne model with an nonisotropical
interfacial stress state is
quantitatively correct to describe the observed phenomena for those
blends.
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