(321a) Melt and Solid-State Structures of Semicrystalline Linear ABC "Block-Random" Copolymers

The solid-state structure of
semicrystalline block copolymers is set either by block incompatibility or by
crystallization of one or more blocks. A variety of solid-state morphologies
may be observed depending on the block interaction strength, ranging from
spherulitic to confined crystallization within preexisting microphase-separated
domains¹. We aim to explore crystallization from both homogeneous
and microphase-separated melts and to characterize the resulting solid-state
structure for linear ABC ?block-random? copolymers that incorporate a
semicrystalline polyethylene endblock.

?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 difference²
between x and y. This flexibility makes block-random copolymers a versatile
platform for the exploration of polymer phase behavior and structure-property
relationships. 

In previous
work we showed that in a cyclohexane/triethylamine mixture, narrow-distribution
copolymers of styrene (S) and isoprene (I) of any desired composition, with no
measurable down-chain gradient could be synthesized. 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 χ; this reduction
was generally consistent with the mean-field ?copolymer equation?³.
By studying these block-random diblocks we were able to extract solubility
parameters for both hI and SrhI.  By comparison with the saturated
hydrocarbon polymers studied extensively by Graessley and coworkers,^4,5
we find that our determined solubility parameter for SrhI matched well
with that calculated from homopolymer solubility parameters, for hI and S,
using a volume-fraction-weighted average and, interestingly, that the
determined value of d_SrhI is close to that for
polyethylene,⁵ suggesting potential miscibility between polyethylene
and SrI which would facilitate the formation of homogeneous melts. Here,
we extend these hI-SrhI studies by adding a semicrystalline endblock,
polyethylene (E), via hydrogenation of high-1,4-polybutadiene (B), to form
linear ABC (E-hI-SrhI) and ACB (E-SrhI-hI) ?block-random?
copolymers. Such ABC triblock copolymers are more complex than more typical AB
diblock copolymers, with two additional independent interaction parameters and
two independent volume fractions. 

We synthesize linear triblock
copolymers, B-I-SrI and B-SrI-I, via organolithium-initiated
anionic polymerization in cyclohexane. After polymerization of the butadiene
block, triethylamine is added facilitating the random copolymerization of
styrene and isoprene while also increasing the vinyl content of the isoprene
block.  Selective hydrogenation of the diene units with a Ni-Al catalyst yields
a semicrystalline polyethylene endblock. For E-hI-SrhI triblock-random
copolymer with block molecular weights of 30-14-14 kg/mol, small-angle x-ray
scattering has revealed the formation of a well-ordered three-domain lamellar
melt from which crystallization of the hydrogenated polybutadiene
(polyethylene) block proceeds. Currently this polymer is being studied after
orientation, compression in a channel die⁶ above T_m(E),
to further examine the solid state structure, to confirm confinement of
crystallization into the melt morphology, and to reveal the orientation of the
crystal stems relative to the pre-existing lamellar interfaces formed by
self-assembly in the melt. We are currently synthesizing, hydrogenating and
characterizing additional E-hI-SrhI and E-SrhI-hI triblock-random
copolymers to explore the resulting morphologies.

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

References

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(2)        Smith,
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(3)        Graessley,
W. W. Polymeric Liquids & Networks:  Structure and Properties;
Garland Science:  New York, 2004, pp. 341-407.

(4)        Graessley,
W.W.; Krishnamoorti, R.; Reichart, G. C.; Balsara, N. P.; Fetters, L. J.;
Lohse, D. J. Macromolecules 1995, 28, 1260-1270.

(5)        Krishnamoorti,
R.; Graessley, W. W.; Dee, G. T.; Walsh, D. J.; Fetters, L. J.; Lohse, D. J. Macromolecules
1996, 29, 367-376.

(6)        Lee,
H. H.; Register, R. A.; Hajduk, D. A.; Gruner, S. M; Polymer Engineering and
Science, 1996, 36, 1414-1424.