Abstract
We simulated the crystallization and melting behavior of entangled polymer melts using molecular dynamics where each chain is subject to a force dipole acting on its ends. This mimics the deformation of chains in a flow field but represents a well-defined equilibrium system in the melt state. Under weak extension within the linear response of the chains, the mechanical work done on the system is about two orders of magnitude smaller as compared with the heat of fusion. As a consequence, thermodynamic and simple arguments following the secondary nucleation model predict only small changes of the crystalline phase. By contrast, an increase of the stem length up to a factor of two is observed in our simulations. On the other hand, the lamellar thickening induced by the external force is proportional to the increase of the entanglement length in the melt prior to crystallization as measured by the primitive path method. While the mechanical work done on the system is only a small perturbation for thermodynamics of polymer crystallization, the change of the primitive path is large. This suggests that a strong increase in the lamellar thickness induced, by external deformation, a topological rather than a thermodynamic origin.
Highlights
Polymer materials undergo crystallization via the partial alignment of their molecular chains
While the mechanical work done on the system is only a small perturbation for thermodynamics of polymer crystallization, the change of the primitive path is large
The rest of this work is structured as follows: In Section 2, we present the thermodynamic analysis of polymer crystallization on weak forces and its consequences on the secondary nucleation model
Summary
Polymer materials undergo crystallization via the partial alignment of their molecular chains. While in the case of a flow field, non-equilibrium effects overlay the thermodynamics; in our case, the application of a dipole force field gives rise to a well-defined equilibrium state of the polymer melt. The ratio between the mechanical work done on the chains and the latent heat of fusion is very small (of the order of percent) which makes a simple thermodynamic explanation of the resulting effects unlikely We exemplify this by applying the classical Lauritzen–Hoffman model [33] to include the effect of external force. From the viewpoint of the secondary nucleation theory, this result is intuitive: The shift of the equilibrium melting temperature due to the entropy reduction of the melt state leads to a stronger effective under-cooling at a given crystallization temperature and to quicker growth and thinner lamellae. Under the consideration that a weak force (η 1) does not influence the kinetics of the crystallization process on the scale of monomers, the application of the force can be mapped to equivalent system with a higher under-cooling only
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