Mixing Thermodynamics of Block-Random Copolymers

Abstract

Random copolymerization of A and B monomers represents a versatile method to tune interaction strengths between polymers, as ArB random copolymers will exhibit a smaller effective Flory interaction parameter &chi;; (or interaction energy density <italic>X<i/talic>) upon mixing with A or B homopolymers than upon mixing A and B homopolymers with each other, and the A<italic>r</italic>B composition can be tuned continuously. Thus, the incorporation of a random copolymer block into the classical block copolymer architecture to yield "block-random" copolymers introduces an additional tuning mechanism for the control of structure-property relationships, as the interblock interactions and physical properties can be tuned continuously through the random block's composition.

However, typical living or controlled polymerizations produce compositional gradients along the "random" block, which can in turn influence the phase behavior. This dissertation demonstrates a method by which narrow-distribution copolymers of styrene and isoprene of any desired composition, with no measurable down-chain gradient, are synthesized. This synthetic method is then utilized to incorporate random copolymers of styrene and isoprene as blocks into block-random copolymers in order to examine the resulting interblock mixing thermodynamics.

A series of well-defined near-symmetric block and block-random copolymers (S-I, Bd-S, I-S<italic>r</italic>I, S-S<italic>r</italic>I and Bd-S<italic>r</italic>I diblocks, where S is polystyrene, I is polyisoprene and Bd is polybutadiene), with varying molecular weight and random-block composition are synthesized and the mixing thermodynamics&mdash;via comparison of their interaction energy densities, <italic>X</italic>&mdash;of their hydrogenated derivatives is examined through measurement of the order-disorder transition (ODT) temperature. Hydrogenated derivatives of I-S<italic>r</italic>I and S-SrI block-random copolymers, both wherein the styrene aromaticity is retained and derivatives wherein the styrene units are saturated to vinylcyclohexane (VCH), are found to hew closely to the regular mixing prediction, <italic>X</italic>A-ArB = <italic>f</italic>B<super>2</super><italic>X</italic>A-B, thereby confirming the utility of this simple relationship in designing block copolymers with targeted interaction strengths using only these two common monomers. Thus, this <italic>f</italic>B<super>2</super> scaling is a useful "design rule" for tuning the interblock segregation strength in A-A<italic>r</italic>B (and B-A<italic>r</italic>B) block-random copolymers using styrene and isoprene as constituents.

The reduction in <italic>X</italic>A-A<italic>r</italic>B over <italic>X</italic>A-B permits the synthesis of polymers having much larger M and domain spacing <italic>d</italic> while maintaining a thermally-accessible ODT; measured domain spacings are found to closely follow the expected scaling, <italic>d</italic> ~ <italic>X</italic><super>1/6</super>M<super>2/3</super>. The decoupling of the order-disorder transition temperature from polymer molecular weight&mdash;and thereby interdomain spacing&mdash;provides an additional means to alter the polymer structure-property dynamic through synthesis, in addition to more common molecular variations, such as changes in block sequence, length of the blocks, and number of blocks.

A similar examination of the interaction energy densities between E (hydrogenated Bd) and both hydrogenated derivatives of random copolymers of styrene and isoprene (S<italic>r</italic>hI and VCH<italic>r</italic>hI) found large positive deviations from regular mixing in the E-S<italic>r</italic>hI system and smaller but significant negative deviations in the E-VCH<italic>r</italic>hI system. Nevertheless, a ternary mixing model ("copolymer equation"), using independently-determined values of the three component interaction energy densities, is found to provide a good representation of the experimental interaction energies.

Random copolymer blocks are also incorporated into linear A-B-C triblock copolymers, and the extent of block microphase separation in nonfrustrated E-hI-A<italic>r</italic>hI, where A is either S or VCH, triblock copolymers forming a "three-domain, four-layer" lamellar morphology is examined. Specifically, the extent of separation between the B and C blocks is probed, for the case where the B and C blocks are sufficiently compatible that they would not be microphase-separated if they were connected as a diblock. However, attachment of the A block, and consequent localization of the A-B block junction to the A-B lamellar interface, induces extensive separation between the B and C blocks. The organization of the crystallized E block was also examined and it was found that in a weakly-segregated triblock, crystallization of the E block destroys the melt lamellar structure, but for more segregated triblocks, E crystallization is effectively confined by the melt structure, leading to an orientation wherein the E crystallites stack orthogonally to the lamellar microdomains, as observed previously in E-containing diblock copolymers exhibiting confined crystallization.

Addditionally, the microphase separation of the two hydrogenated derivatives of an S-I-S<italic>r</italic>hI triblock, generating A-hI-A<italic>r</italic>hI triblocks, is also probed. While the two derivatives possess nearly identical block volume fractions, differences in the magnitudes of the block interaction energies results in the formation of two disparate ordered morphologies.