Session V59: Structure/Property Relationships in Polyolefins

Linking synthesis with entanglement state in Ultra High Molecular Weight Polyethylene; a route for solvent free processing

In this presentation we will demonstrate differences in the density of the entangled state that could be realised on synthesising Ultra High Molecular Weight Polyethylene (UHMWPE); having number average molar mass greater than a million g/mol. The differences in the entangled state, having more than 10⁵ repeat units, have implications on processing in the solid state (below the equilibrium melting point) and chain diffusion of chain segments from non-crystalline to crystalline region. The polymers having reduced number of entanglements could be processed, free of solvent, into uniaxial and biaxial products leading to unprecedented mechanical properties required for the manufacturing of light weight recyclable composites. On melting the low entangled crystals of UHMWPE, a non-equilibrium melt state could be realised. The time required for the melt to transform from non-equilibrium to equilibrium state is shown to be dependent on the molar mass.
References: 1. Rastogi, S.; Lippits, DR.; Peters, GWM.; Graf, R.; Yefeng, Y.; Spiess, HW.; Nature Materials 2005, 4, 635-641 2. Rastogi, S.; Yao, Y.; Ronca, S.; Bos, J.; van der Eeem, J. Macromolecules 2011, 44, 5558-556; 3. Yao, Y.F.; Rastogi, S.; Xue, H.J.; Chen, Q.; Graf, R.; Verhoef, R. Polymer 2013, 54 (1), 411-422; 4. Andablo-Reyes EA; Auhl D; De Boer EL; Romano D; Rastogi S; Journal of Rheology 2017, 61 (3), 503-513; 5. Romano D; Tops N; Bos J; Rastogi S; Macromolecules 2017, 50 (5), 2033-2042; 6. Ronca S; Igarashi T.; Forte G.; Rastogi S.; Polymer 2017, 123, 203-210   

Deuterium-labeled polyolefins: Exchange, characterization, and applications

The polyolefins manufacturing industry has taken advantage of the broad array of physical properties available from simple combinations of hydrocarbon monomers, leading to the ubiquity of these materials in our daily lives. Properties can be tuned by the type, amount, and arrangement of monomers and comonomers, as well as through long-chain branching; each of these structural elements additionally has some distribution within and across molecular weight, making resin design a complex multi-dimensional problem. While material performance continues to improve, understanding of structure-property relationships often remains elusive due to the combination of structural complexity and chemical simplicity, which masks the role of particular components in resin performance.

Isotopic labeling is a powerful tool that is well-suited to the challenges posed by polyolefin systems. Recent work has identified facile, inexpensive hydrogen-deuterium exchange reactions for accomplishing such labeling on polyolefins. These labeling reactions enable new studies isolating particular labeled components in polyolefin systems, but also create a need for new characterization and analysis tools to identify the arrangement and distribution of deuterium on the polymer chains. Additionally, the exchange itself is surprisingly sensitive to the solvent used during reaction as well as the stereochemistry of the polymer, pointing toward mechanisms of adsorption that have analogues in other polymer systems. Here, studies relating the exchange reactions, characterization methods, and use of labeled polymers in applied systems will be reviewed.
  

Can the failure of tube models in blends of branched polymers be salvaged by slip-link models?

A recent comparison between experiments and theories for the dynamic modulus of linear/star-branched entangled polymer blends revealed that tube models were unable to describe simultaneously two different sets of data. Both the branch-on-branch (BoB) computer algorithm, and the hierarchical model were used with various sets of parameter values. While one set of values could describe one data set, it failed to describe the other.
On the other hand, the discrete slip-link model was able to describe both sets simultaneously, using parameters determined for monodisperse linear chains only. This observation naturally raises the question in our title. Hence, we attempt further coarse graining of the slip-link model to a level of description more like a tube. We consider the simplest case: the primitive path length of a monodisperse star arm without constraint dynamics. Specification of the coarse-grained level of description requires determination of just two 1D objects: free energy and friction, as functions of primitive path length. Statistical mechanics determines the former, and we devise an efficient and accurate method to determine the latter. We can then compare predictions of the coarse-grained and detailed models.   

Molecular simulation of polyolefin crystallization: Nucleation Phenomena

The properties and performance of polyolefins depend strongly upon both the molecular scale structure of the chains and the manner in which they are processed to form engineering materials. Much of their utility derives from the spectrum of semicrystalline morphologies that can be realized. The development of semicrystalline morphology is controlled through processes of crystal nucleation and growth as the polymer is cooled from the melt, in either the presence or absence of process flows. Due to the small spatiotemporal scales involved, however, it remains a challenge to examine these processes directly and determine their underlying physical mechanisms.

In this talk, we describe the use of molecular dynamics simulations to characterize nucleation and growth of a new crystal phase from the polymer melt. Homogeneous nucleation is analyzed in terms of classical nucleation theory using a method based on mean first passage times (MFPT) of crystalline clusters of varying size. This MFPT analysis is then applied in two dimensions within layers near a crystal surface, to examine the so-called secondary (or surface) nucleation theory for chain molecules; within this analysis, crystal growth can be decomposed into sequential and simultaneous processes of surface cluster nucleation and spreading of the crystal cluster across the growth front. Finally, we examine surface nucleation and spreading upon surfaces of foreign materials, in an effort to illuminate the design criteria for effective nucleating agents (NA). We envision a “materials genome” of NAs, and use high throughput simulation to screen entire classes of materials for NA efficiency. Lessons learned from the families of diamond-like and 2D, graphene-like materials will be discussed.   

Polymer Modeling At ExxonMobil: Reaction Kinetics And Computational Rheology

Industrially produced polymers generally contain molecules with a wide range of architectures and sizes, which are responsible for their unique flow properties. For example, polyethylene (PE), despite its apparent chemical simplicity, is a complex mixture characterized by three distributions: (i) Molecular weight, (ii) Comonomer composition (e.g. butene or hexene), and (iii)Branching (long chain branching, where the branch length exceeds about 1,000 g/mol for PE). \\ \\ When developing a new plastic, it is essential to understand how to relate these various distributions to its melt flow performance. Recent theoretical advances in reaction kinetics and computational rheology have enabled the prediction of polymer melt flow performance from molecular structure. \\ \\ In this presentation, we describe a modeling platform which combines polymerization and computational rheology models in order to predict the flow of various branched polyolefins. In particular, we show how the polymer branching distribution controls the ultimate resin flow performance (e.g. extrudability and bubble stability for film blowing operations). This approach combines multiple characterization techniques such as gel permeation chromatography, nuclear magnetic resonance, or shear and extensional rheology, in order to establish a detailed inventory of branched species. Decoding the branching distribution is critical as rheology is highly sensitive to the molecular architecture and amount. In addition, there exists today no stand-alone experimental characterization tool capable of providing quantitative polymer branching information. In this context, this framework opens up unique opportunities for the design of resins with targeted flow performance.