Since the block copolymer order-disorder transition (ODT) represents a balance between enthalpic and entropic interactions, measuring the ODT temperature (TODT) provides a means to quantify the strength of those enthalpic interactions (Flory-Huggins interaction parameter χ). With a database of such χ values, polymer-polymer miscibility—a key factor in any application—can be predicted. But since χ is a joint property of the two polymers, such a database is intrinsically quite large (of order ~n2, where n is the number of polymers)—far too large to measure in practice. If χ could be predicted from pure-component properties, this would reduce the required database to order ~n, making materials design much more tractable. The simplest theory is regular mixing, wherein each polymer is assigned a value of the Hildebrand solubility parameter. Recently, we have found that regular mixing is closely obeyed in all hydrogenated derivatives of styrene-isoprene copolymers, including “block-random” copolymers wherein one or more blocks is itself a random copolymer. This allows a decoupling of TODT from the molecular weight, and hence the block copolymer domain spacing; by reducing the compositional difference between the two blocks, which may be done continuously in the block-random architecture, the molecular weight and domain spacing at a constant TODT can be made arbitrarily large, as shown below for a range of near-symmetric block-random copolymers. Currently, we are exploiting this same block-random architecture to create polymers which have a very low χ against polyethylene, and will thus show miscibility in the melt.
Another way to increase the domain spacing (d) is to introduce polydispersity into the blocks. In joint work with researchers from Dow Chemical, we have investigated the structure of AB diblock and (AB)n multiblock copolymers prepared by a novel “chain-shuttling” polymerization, in continuous-flow stirred-tank reactors. A and B are both ethylene-octene random copolymers, but with very different octene contents; these polymers are thus also block-random copolymers. When such polydisperse block copolymers are in the weakly-segregated regime, chains with a short A block tend to dissolve in the B microdomains, and vice versa, greatly swelling the domain structure. Synchrotron SAXS data taken at the Advanced Photon Source on a flow-aligned specimen of one such diblock copolymer (below) shows an enormous d-spacing of 123 nm, even though the diblock molecular weight is only Mn = 69 kg/mol. This huge d-spacing imbues these materials with structural color: they appear blue in reflection and orange in transmission, and have been dubbed “photonic polyethylene”.
Supported by the National Science Foundation, Polymers Program
Current/Recent Group Members, and Their Project Titles:
Will Mulhearn PhD *18 – “Melt-Miscibility in Block Copolymers Containing Polyethylene”
Adam Burns PhD *17 – “Thermoplastic Elastomers with Composite Crystalline-Glassy Hard Domains via Crystallization from a Single-Phase Melt”
Bryan Beckingham PhD *13 – “Mixing Thermodynamics of Block-Random Copolymers”
Sheng Li PhD *13 – “Structure and Properties of Novel Homopolymers and Block Copolymers Synthesized by Ring-Opening Metathesis Polymerization or Chain Shuttling Polymerization”
Selected Recent Publications
B.S. Beckingham and R.A. Register, "Regular Mixing Thermodynamics of Hydrogenated Styrene-Isoprene Block-Random Copolymers", Macromolecules, 46, 3084-3091 (2013).
B.S. Beckingham, A.B. Burns, and R.A. Register, "Mixing Thermodynamics of Ternary Block-Random Copolymers Containing a Polyethylene Block", Macromolecules, 46, 2760-2766 (2013).
S. Li, R.A. Register, J.D. Weinhold, and B.G. Landes, "Melt and Solid-State Structure of Polydisperse Polyolefin Multiblock Copolymers", Macromolecules, 45, 5773-5781 (2012).
S. Li, R.A. Register, B.G. Landes, P.D. Hustad, and J.D. Weinhold, "Crystallization in Ordered Polydisperse Polyolefin Diblock Copolymers", Macromolecules, 43, 4761-4770 (2010).