Modern Polymer Spect..

1.5.5 Block Copolymers
Block copolymers made of dissimilar polymer segments joined by a covalent bond
are interesting systems to study by 2D IR spectroscopy. Because of the repulsive
interaction between the dissimilar segments, block copolymers often phase separate
at the submolecular scale to form microdomains. It has been speculated for some
time [56, 571 that the composition of block segments comprising such microdomains
does not change sharply at the domain interface. Instead, a diffuse interlayer is
thought to exist where substantial mixing of dissimilar block segments actually takes
place. Analysis based on 2D 1R was used to probe the existence of such an iiitrrphirse
region between microdomains [58, 591.
A thin solution-cast film of a diblock copolymer consisting of a polystyrene block
(a,, 50,000) and a deuterium-substituted polyisoprene block (a,, 100,000 J was
stimulated at room temperature with a 0.1%)-amplitude, 23-Hz dynamic tensile
strain. In this composition range, diblock copolymers of styrene and isoprene produce
a hexagonally packed regular array of rod-like polystyrene microdomains
embedded in a continuous matrix of the polyisoprene block. The diameter of each
polystyrene microdomain is well below 0.1 pm.
Figure 1-25 shows the synchronous 2D IR spectrum of this block copolymer
in the CH-stretching region. Since the polyisoprene block of this polymer is fully
deuterium substituted, the responses shown in Figure 1-25 represent exclusively the
contribution from the polystyrene segment. One may expect the effect of the applied
mechanical perturbation on the molecular reorientation of the block copolymer will
be felt predominantly by the rubbery polyisoprene segments, since the dispersed
polystyrene microdomains are in the glassy state at room temperature. It is, therefore,
rather surprising to find autopeaks indicating substantial molecular mobility
of polystyrene segments under dynamic strain at this temperature.
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Figure 1-25. A synchronous 2D IR
spectrum in the CH-stretching vibration
region of a microphase-separated diblock
copolymer consisting of polystyrene and
perdeuterated polyisoprene segments.