(415a) Phase Behavior of "Block-Random" Copolymers

            ?Block-random? copolymers?represented generally
by the structure (A_xB_1-x)-(A_yB_1-y),
where each of the two blocks is a random copolymer of monomers A and B, simply
with different fractions of A (x and y)?present a convenient and useful
variation on the typical block copolymer architecture, as the interblock
interactions and physical properties can be tuned continuously, via x and y,
through the random block's composition. The ability to tune the effective
interaction parameter between the blocks continuously, allows for the
order-disorder transition temperature (T_ODT) to be tuned independently
of molecular weight using only two monomers, via the difference¹
between x and y. This flexibility makes block-random copolymers a versatile
platform for the exploration of polymer phase behavior and structure-property
relationships.

Typical living or controlled
polymerizations produce compositional gradients along the random block, since
the reactivity ratios are generally different from unity, which can in turn
influence the phase behavior. Living polymerizations which proceed with random
monomer addition (no gradient) are consequently sought because they would
permit the synthesis of well-defined polymers, and block copolymers, of tunable
composition (hence tunable properties) which are effectively homogeneous on
length scales larger than the monomer size.² It is desirable to
achieve such random copolymerizations of styrene and isoprene (SI), to expand
the accessible range of polymer properties. In particular, hydrogenated
high-vinyl polyisoprene has an exceptionally low cohesive energy density,³
lower than that for hydrogenated high-vinyl polybutadiene, meaning that a broad
range of solubility parameters (>2 MPa^1/2) could be accessed in
random copolymers containing styrene and hydrogenated vinyl isoprene (hI)
units. This increased range of solubility parameters translates directly into
an increased range of accessible values of the interblock Flory-Huggins
interaction parameter χ in block-random copolymers.

The reactivity ratios for SI in
hydrocarbon and ether solvents are qualitatively similar to those for SB,⁴
but despite the seemingly minor difference between butadiene and isoprene, N,N,N¢,N¢-tetramethylethylenediamine,
TMEDA is not an effective randomizer for SI copolymerizations.⁵ Over
half a century ago, Kelley and Tobolsky⁶ showed that triethylamine
(TEA) is an excellent randomizer for equimolar SI copolymerizations (~60 wt%
S). Later, Annighöfer and Gronski⁷ used a TEA/benzene mixture (20/80
v/v) to synthesize S-SrI-I triblock copolymers with styrene-ran-isoprene
(SrI) midblocks (~50 wt% S); they reported reactivity ratios r_I
= 1.0 and r_S = 0.8, implying that effectively random
copolymers could be synthesized at essentially any S:I ratio with this
approach.

Here, organolithium initiation in a
cyclohexane/triethylamine mixture yields narrow-distribution copolymers of
styrene and isoprene of any desired composition, with no measurable down-chain
gradient. These random copolymers (SrI) have been successfully
incorporated into well-defined symmetric block copolymers (I-SrI diblocks)
and subsequent isoprene-selective hydrogenation yields thermally stable hI-SrhI
diblocks, which self-assemble into well-defined lamellar morphologies with
sharply-defined order-disorder transitions, whose temperatures scale
predictably with diblock molecular weight. The use of SrhI in lieu of a
styrene homopolymer block effectively dilutes the unfavorable contacts between
the two blocks in the homogeneous phase reducing the effective X, where X
if the interaction energy density (Χ = χ(Nρ/M)RT = (χN)_ODTρRT_ODT/M)
by a factor of ~8, and allowing for greatly elevated molecular weights and d
spacings at a given value of T_ODT. The measured interaction energy
density between hI and SrhI is consistent with the mean-field ?copolymer
equation?⁸, predicts factor of ~5 reduction in X, providing a
first step towards the design of styrene-isoprene block-random copolymers of
desired molecular weight and T_ODT.

Within the context of mean-field theory,
as in the Flory-Huggins model, X is related to the difference in
solubility parameters (d) of the two blocks as X=(Δd)².
Having measured values of X we can rank SrhI in solubility
parameter relative to other polymers, particularly the saturated hydrocarbon
polymers studied extensively by Graessley and coworkers.^3,9 We find
that our determined solubility parameter for SrhI matches well with what would
be calculated from homopolymer solubility parameters, for hI and S, using a
volume fraction weighted average. Interestingly, the value of d_SrhI
determined here is close to that for polyethylene,⁹ suggesting
potential miscibility between polyethylene and SrI, an idea we are in
the process of testing.

This work was generously
supported by the National Science Foundation, Polymers Program (DMR-1003942).

References

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