(648a) Deformation and Yield in Semicrystalline Polymers

Many polymers are heterogeneous in the solid state, comprising both ordered and disordered domains with nanometric dimensions that are intimately related through long chain molecules that traverse from one domain to another. Semicrystalline polymers as well as many copolymers such as thermoplastic polyurethanes fall into this category. Modeling the thermomechanical properties of these materials is complicated by the need to capture not only the behavior of the ordered and disordered domains, respectively, but also their coupled response through the interface. To study such materials with atomistic resolution, we use the Lamellar Stack Model [1], which serves as the elementary building block for more complicated morphologies. By studying the more tractable problem of molecular deformation mechanisms within the lamellar stack using molecular dynamics simulations, one can begin to understand the processes operative throughout the complicated, heterogeneous morphology.

Results are reported for atomistic models of semicrystalline linear polyethylene (PE) and phase-segregated thermoplastic polyurethanes (TPU) containing amorphous polytetramethylene oxide (PTMO) soft segments and 4,4’-diphenylmethane diisocyanate (MDI) hard segments with n-butanediol chain extenders. The Lamellar Stack Model was constructed in each case using the Interphase Monte Carlo (IMC) procedure [2,3] to equilibrate both the packing and topology. Following equilibration, representative ensembles of configurations were then transferred to LAMMPS [4] for re-equilibration in the NPT ensemble, followed by deformation at constant strain rate to large strain, typically under uniaxial strain (constant lateral dimensions) or uniaxial stress (constant lateral stress) conditions.

The response of the lamellar stack was found to depend on both the mode and rate of deformation. In semicrystalline PE, mechanisms of response were categorized as fine crystallographic slip, occurring at low strain, followed by either cavitation in the amorphous domain or melting/recrystallization of the crystal domain [1,5]. The signature of melting/recrystallization behavior in particular was a rise in stress up to the yield point, followed by a sudden drop in stress as material was transferred from one domain to the other; under appropriate simulation conditions, this mechanism would occur repeatedly, giving rise to a cyclic rise and fall in stress with increasing strain. These events were found to be uncorrelated between configurations, resulting in the appearance of plastic flow post-yield after ensemble-averaging [6]. In the TPU case [7], under uniaxial strain deformation, cavitation of the soft domain was universally observed, in close analogy to semicrystalline PE. Chain pull-out associated with bridges and bridging entanglements was also observed. In contrast to semicrystalline PE, shear band formation and localized block slip mechanisms were found. In this case, the different mechanisms could be traced to the soft domain topology, but also the sliding of hydrogen-bonded sheets within the hard domain. The occurrence of these several mechanisms depends uniquely on the mechanical coupling between hard and soft domains in heterogeneous polymer solids, as represented by the disordered domain topology.

[1] Kim, J.; Locker, C.R.; Rutledge, G.C. Macromolecules 2014,47 (7), 2515-2528.

[2] Balijepalli, S.; Rutledge, G.C. J. Chem. Phys. 1998,109 (16), 6523-6526.

[3] in 't Veld, P.J.; Hutter, M.; Rutledge, G.C. Macromolecules2006,39 (1), 439-447 DOI: 10.1021/ma0518961.

[4] Plimpton, S. J.Comput. Phys. 1995, 117, 1−19.

[5] Lee, S.; Rutledge, G.C. Macromolecules 2011,44 (8), 3096-3108.

[6] Yeh, I.-C.; Andzelm, J. W.; Rutledge, G. C. Macromolecules2015, 48(12), 4228–4239.

[7] Zhu, S., Lempesis, N., in ‘t Veld, P.J., Rutledge, G.C., Macromolecules 2018, 51(5), 1850-1864.