posted on 2018-02-22, 20:08authored byShuze Zhu, Nikolaos Lempesis, Pieter J. in ‘t Veld, Gregory C. Rutledge
Thermoplastic
polyurethanes (TPUs) are useful materials for numerous
applications due in part to their outstanding resilience and ability
to dissipate energy under large mechanical deformation. However, the
mechanistic understanding of the origins of these mechanical properties
at the molecular level remains elusive, largely due to the complex,
heterogeneous structure of these materials, which arises from the
segregation of chemically distinct segments into hard and soft domains.
In this work, molecular simulations are used to identify the mechanism
of mechanical response under large tensile deformation of a common
thermoplastic polyurethane comprising 4,4′-diphenylmethane
diisocyanate and n-butanediol (hard segment) and
poly(tetramethylene oxide) (soft segment), with atomic resolution.
The simulation employs a lamellar stack model constructed using the
Interphase Monte Carlo method established previously for semicrystalline
polymers, which models the interfacial zone between hard and soft
domains with thermodynamically rigorous distributions of bridges,
loops, and tails. Molecular-level mechanisms responsible for yield,
toughening, and the Mullins effect are reported. We have found several
distinct mechanisms for yield and plastic flow, which we categorize
as (i) cavitation, (ii) chain pull-out, (iii) localized melting with
shear band formation, and (iv) block slip. The activity of these mechanisms
depends on the topology of chains in the soft domain and the direction
of loading (e.g., parallel or perpendicular to the interface). Further
insights regarding toughening mechanisms and the Mullins effect are
obtained from cyclic loading, where mechanisms ii to iv were found
to be irreversible and account for the superior resilience and dissipation
at large tensile strains in thermoplastic polyurethanes.