Molecular Simulation of Thermoplastic Polyurethanes under Large Tensile Deformation

Abstract

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.

Strain hardening behaviors and mechanisms of polyurethane under various strain rate loading

“…Whereas, little attention is given to the mechanism of high strain hardening ratio for elastomers. For most elastomers, strain‐induced crystallizations are usually found during deformation . Then the crystallization zones present hard inclusions in elastomers.…”

“…Molecular simulations were also used to study the molecular‐level mechanism of mechanical response under large tensile for yielding, toughening and Mullins effect, and some mechanisms were found for yielding and plastic flow. They were categorized as (a) cavitation, (b) chain pull‐out, (c) localized melting with shear band formation, and (d) block slip …”

Molecular Simulation of Thermoplastic Polyurethanes under Large Compressive Deformation

“…For this study of TPU response in compression, we use the same systems and methods as described in our previous report of TPU response in extension, 11 where a more detailed explanation on model construction can be found. For the sake of completeness, the details of models construction are briefly recapped below, but the reader is referred to the previous report 11 for a more complete description. We use the Interphase Monte Carlo (IMC) method of Rutledge and co-workers [13][14][15][16][17][18][19] as implemented in the EMC (Enhanced Monte Carlo) 16 software (version 9.3.7) to construct the model.…”

All-atomic and coarse-grained molecular dynamics investigation of deformation in semi-crystalline lamellar polyethylene

“…To gain insight into the complex behaviour of semicrystalline polymers, atomistic modelling is a useful numerical modelling tool that has been widely used in the literature [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44]. Most of these works have been based on simplified and coarse-grained interaction models, such as the united atom (UA) interaction model.…”

“…Among the aforementioned works, Rutledge and co-workers recently published a series of papers dealing with molecular dynamics (MD) modelling of yielding of PE [25][26][27][28][29][30] and other semi-crystalline polymers, such as polyurethane [31,32] and polyether [33]. These investigations have revealed that there is a strain rate dependence that dictates the observed stressstrain behaviour and yielding mechanisms [25][26][27][28].…”