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

(1)        Loo, Y. L.; Register, R. A.; Ryan, A. J. Macromolecules, 2002, 35, 2365-2374.

(2)        Smith, S. D.; Ashraf, A.; Satkowski, M. M.; Spontak, R. J. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1994, 35(1), 651-652.

(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.