2023-07-03 21:27:48
With more than 100 million tonnes produced per year worldwide, polyethylene (PE) is the chemically simplest and most widespread polymer. And yet, understanding how the molecular architecture, namely the length distribution of the polymer chains and their degree of branching, controls the properties of the final plastic material remains a real challenge. In particular, crystallization, which largely governs the appearance and properties of PE, depends on the chain architecture. Directly observing this phenomenon is nearly impossible, even with state-of-the-art high-resolution instruments.
As early as the 1960s, experiments were carried out on model systems made up of shorter chains, alkanes. The results obtained have largely contributed to the understanding of the crystallization of polymers, with in particular the discovery of the folding of the chains on themselves to form a crystal, the influence of the architecture of the chains on the thickness of the crystal and the temperature of merger. These advances have aided in the design of the industrial polymers used today. But the latter have complex hierarchical structures consisting of mixtures of branched polymers of different lengths. Their crystallization is a kinetic, non-equilibrium process, which greatly complicates the correlation between structure, properties and processing conditions.
Molecular simulations appear today as a new way to understand these correlations. Increasingly realistic polymer models, highly efficient molecular simulation codes, a sharp increase in computing power and the availability of high-performance computing clusters, such as the GENCI/IDRIS National Supercomputing Center for the CNRS, allow simulations on increasingly long and complex polymer chains.
As part of a recent collaboration between TotalEnergies and the CNRS, researchers from the Charles Sadron Institute (CNRS/University of Strasbourg) simulated the growth of PE single crystals of an unprecedented size. Their results reveal the multi-layered structure of the crystals in unprecedented detail. Furthermore, they demonstrate that it is possible to control the structure and the thickness of the layers by simply introducing a few additional branches along the chains. This is particularly important for industrial applications since the small-scale molecular structure determines the large-scale mechanical properties, such as fracture toughness or impact resistance. Optimizing this process might lead to the design and manufacture of new, even better performing polymers. These results can be found in the journal ACS Macro Letters.
1688422682
#Molecular #simulations #control #crystallization #industrial #polymers
