Although commercial block copolymers used as melt-processible rubbers and adhesives are based on amorphous blocks, designing crystallinity into one of the blocks can confer significant property advantages, such as vastly improved solvent resistance. Combining two or more self-organizing mechanisms, such as crystallization and interblock repulsion, into a single polymer yields both morphological richness and kinetic complexity. We have shown that crystallization can radically change the mesophase diagram from the usual “spheres-cylinders-gyroid-lamellar” progression: when crystallization occurs from a single-phase melt, a lamellar morphology typically results for a wide range of compositions.
However, for thermoplastic elastomers (TPEs), a lamellar structure—containing plate-like crystals of large lateral extent—is not a desirable morphology, as it leads to a relatively stiff material, which exhibits yielding at higher strains, and relatively poor recovery (large “permanent set”). Better mechanical properties are obtained when the hard domains are discrete, as with the A cylinders or spheres formed in typical ABA triblock TPEs; however, it is difficult if not impossible to formulate a TPE which has a thermally accessible order-disorder transition (ODT), and still has sufficiently strong segregation between the blocks at use temperatures to result in good mechanical properties. Consequently, all-amorphous TPEs with the best mechanical properties are processed in the ordered state, where the material shows exceedingly high viscosities and elasticities, making it difficult and energy-intensive to process. Crystallizable block copolymers potentially offer a way out of this restrictive box, if they can be designed to show single-phase melts while simultaneously limiting the lateral growth of crystals below the freezing point.
We employed ring-opening metathesis polymerization (ROMP), followed by catalytic hydrogenation, to synthesize ABCBA pentablock copolymers, where the A block is crystallizable (hydrogenated polynorbornene, hPN); the B block is glassy at room temperature (hydrogenated poly(methyltetracyclododecene), hPMTD); and the majority C block is rubbery at room temperature (hydrogenated poly(n-hexylnorbornene), hPHN). With judicious control over the block lengths, pentablocks could be designed with easily-processed melts; upon cooling, the hPN blocks (blue in the figure below) crystallized, dragging with them the glassy hPMTD blocks, which limit growth of the hPN crystals. Glassy hPMTD enhances the tensile strength of the composite hard domains, since yielding limits the tensile strength in crystalline polymers such as hPN. We are currently exploring the potential of this idea with common commercial monomers (amenable to anionic polymerization), as well as whether the properties can be further enhanced though a starblock (vs. linear) macromolecular architecture.
While the above example has only one kind of crystallizable block (hPN), incorporating a second kind of crystallizable block into the copolymer further expands the range of structural possibilities, as well as allowing the structure to be readily manipulated through judicious choice of the processing route. Via ROMP, block copolymers of hPN with perfectly linear polyethylene, LPE (obtained via hydrogenation of polycyclopentene) were synthesized at different molecular weights (MW. At high MW, hPN crystallizes first; these high-MW polymers can be drawn into fibers, and crystallization during fiber drawing orients the crystal stems of both hPN and LPE along the fiber axis. But melting and quiescently recrystallizing the LPE blocks (without melting the hPN) leads to a completely different structure for the LPE, where the crystal stems are perpendicular to the fiber axis. At lower MW, the situation is reversed: LPE crystallizes first, due to a shallower dependence of crystallization rate on MW for LPE than for hPN. This example shows that we have just scratched the surface of the very complex processing-structure relationships present in double-crystalline block copolymers.
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”
John Bishop PhD *11 – “Structure-Property Relationships in Novel Polymers and Block Copolymers from Ring-Opening Metathesis Polymerization”
Selected Recent Publications:
B.S. Beckingham and R.A. Register, "Architecture-Induced Microphase Separation in Nonfrustrated A-B-C Triblock Copolymers", Macromolecules, 46, 3486-3496 (2013).
S. Li, S.B. Myers, and R.A. Register, "Solid-State Structure and Crystallization in Double-Crystalline Diblock Copolymers of Linear Polyethylene and Hydrogenated Polynorbornene", Macromolecules, 44, 8835-8844 (2011) [cover article].
J.P. Bishop and R.A. Register, "Thermoplastic Elastomers with Composite Crystalline-Glassy Hard Domains and Single-Phase Melts", Macromolecules, 43, 4954-4960 (2010).
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).