In-situ SAXS-Deformation Studies on Highly Ordered Lamellar Block Copolymers
Yachin Cohen
Department of Chemical Engineering, Technion , Haifa, Israel.
Microphase-separated block copolymers exhibit interesting mechanical behavior due to
their uniquely ordered morphologies. They find many useful applications, from the more
conventional thermoplastic elastomers and toughened thermoplastics to more novel
optical active devices. The mechanical response of a lamellar block copolymer structure,
composed of alternating glassy/rubbery layers, is particularly unique: it exhibits a rigid
elastic modulus at low strain followed by yielding and drawing to extremely high strain,
with very high energy absorption. A reversible rubbery behavior is observed upon
unloading and immediate re-stressing, Moreover, full recovery of the initial rigidity is
achieved by annealing.
The objective of this study is to elucidate the microstructural basis for the
mechanical response of lamellar block copolymers by combining mechanical
deformation with small-angle x-ray scattering (SAXS) measurements using the NSLS
synchrotron facility. Electron and scanning probe microscopies were used to obtain
complementary images of the morphologies. In this study polystyrene-polybutadienepolystyrene triblock copolymers were roll-cast to form globally oriented films with a
lamellar morphology. In-situ deformation-SAXS experiments were conducted in three
directions: perpendicular, parallel and diagonal (at 45 degrees) to the lamellar planes.
In parallel deformation, macroscopic necking is due to break-up of the lamellar
structure. The main deformation mechanism observed when stretching in the parallel and
diagonal directions is formation of kink-grain boundaries oriented along the stretching
direction. Symmetric kink boundaries, giving rise to the “chevron” morphology, are
observed in parallel deformation, whereas diagonal deformation results in assymetric
kink boundaries. With increasing strain the layers tilt in a manner which is affine with the
macroscopic elongation, leading to rupture of the glassy layers at tilt boundaries.
When the block copolymer structure is deformed above the glass transition
temperature of the poly(styrene) layers, dilation of the layer spacing is observed, in an
affine manner with the macroscopic elongation. At an intermediate temperature (80oC),
layer dilation is observed at low strain, which transforms at the mechanical yield point to
a combination of dilation with tilting of the layers at higher strains. An affine
deformation model, accounting for these phenomena, is presented.
(work done at: Dept. of Materials Sci. and Eng. MIT, with E.L. Thomas and R.J. Albalak)