Atomistic simulation of the structure and mechanics of semicrystalline and heterogeneous polymer systems
N. Lempesis (1), P. J. in ‘t Veld (2), G. C. Rutledge (3)
(1) Massachusetts Institute of Technology, Department of Chemical Engineering, USA, Email: firstname.lastname@example.org
(2) BASF, Polymer Physics, Germany. Email: email@example.com
(3) Massachusetts Institute of Technology, Department of Chemical Engineering, USA, Email: firstname.lastname@example.org
Abstract: It is generally known that polymer melts rarely crystallize completely. Instead, one obtains a heterogeneous, semicrystalline phase composed of crystalline lamellae interspersed with layers of an amorphous matrix, usually at various orientations within the semicrystalline material. It is believed that the interaction of the crystalline and amorphous domains plays a dominant role in determining thermodynamical, dynamical and mechanical properties of the semicrystalline material. We study amorphous poly(tetramethylene oxide) (PTMO) melt, semicrystalline PTMO and crystalline diphenylmethane-4,4'-diisocyanate (MDI) with 1,4-butanediol as chain extender, as auxiliary systems that are closely related to the components of a common thermoplastic polyurethane (TPU) . This work addresses the challenging question of how the existence of two phases (crystalline and amorphous) exhibiting different chemistry, geometry and composition influences the thermodynamical and mechanical behavior of the composite TPU.
Our approach to understand the structure-property relationships of TPUs entails the development and application of theoretical and computational modeling at the atomistic length scale, where the chemistry, structure and geometrical constraints of individual domain types and the interfaces between them are paramount. In particular, it is widely known that in TPUs, phase separation prevails, where hard, organized and soft, amorphous domains coexist, rendering TPUs in a heterogeneous state. The hard, organized regions comprise predominantly crystalline MDI segments, whereas the soft, amorphous regions consist of amorphous PTMO. In order to simulate the behavior of the composite TPU, a set of auxiliary systems (PTMO melt, MDI and PTMO perfect crystals and semicrystalline PTMO) was initially studied. At a later step, these systems were appropriately combined so that the TPU is obtained. By comparing simulations of the PTMO melt and the PTMO/MDI crystals with experimental data, we have validated an appropriate force field that captures well both intra- and intermolecular interactions for both chemical systems (PTMO and MDI). To model the interface between the crystalline and amorphous domains, we have employed a modified version of the Interphase Monte Carlo algorithm . In general, this algorithm allows the creation of a semicrystalline system by using topology-altering Monte Carlo moves at a targeted area of the perfect crystal. By appropriately utilizing this algorithm we have constructed the semicrystalline PTMO and the heterogeneous TPU. Topological features (tails, loops and bridges) observed in these systems in nature are examined from a statistical point of view. In the process of quantifying the effect of the interface between domains, a criterion has been developed for identifying the relevant interface between domains and a methodology was established that identifies the energetically most favorable interface between any two neighboring components in a heterogeneous material. Results on deformation of crystal and semicrystalline PTMO, as well as crystalline MDI to large strain levels are reported, along with some preliminary results on the deformation of the composite TPU.
Acknowledgement: We gratefully acknowledge financial support from BASF for the research project entitled “Multi-scale Theoretical Modelling of Thermoplastic Polyurethanes”.
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 In ‘t Veld P. J, Rutledge G. C., 2003, Macromolecules, 36, 7358–7365
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