Blends of polyethylene (PE) and polypropylene (PP) have always been the subject of intense reasearch for encouraging polymer waste recycling while producing new materials for specific applications ...

Review on modification strategies of polyethylene/polypropylene immiscible thermoplastic polymer blends for enhancing their mechanical behavior:

Brief intervals of shear prior to a temperature quench accelerate crystallization, resulting in much smaller spherulites. Crystallization kinetics of five commercial linear isotactic polypropylenes...

Onset of Flow-Induced Crystallization Kinetics of Highly Isotactic Polypropylene

Chain topology has a profound impact on the flow behavior of single macromolecules. For circular polymers, the absence of free ends results in a unique chain architecture compared to linear or branched chains, thereby generating distinct molecular dynamics. Here, we report the direct observation of circular DNA dynamics in transient and steady flows for molecular sizes spanning the range of 25.0–114.8 kilobase pairs (kbp). Our results show that the longest relaxation times of the rings follow a power-law scaling relation with molecular weight that differs from that of linear chains. Also, relative to their linear counterparts, circular DNA molecules show a shifted coil-to-stretch transition and less diverse “molecular individualism” behavior as evidenced by their conformational stretching pathways. These results show the impact of chain topology on dynamics and reveal commonalities in the steady state behavior of circular and linear DNA that extends beyond chain architecture.

When Ends Meet: Circular DNA Stretches Differently in Elongational Flows

Brief intervals of strong flow stretch chains in a semicrystalline polymer melt to form flow-induced precursors, which accelerate crystallization kinetics and transform the morphology. Using commercial isotactic polypropylene, the persistence and lifetime of flow-induced precursors were investigated, focusing on the effects of specific work and annealing time and temperature. Precursors were formed by shearing polypropylene in a rotational rheometer and then quenching to a desired temperature. The crystallization time and crystallization temperature of the sheared samples were investigated using novel rheology and DSC experiments. For sufficiently large shear rates, the appearance of flow-induced precursors is controlled by the applied work. A qualitative change in many properties related to precursors takes place at a critical work value Wc, about 7 MPa for our iPP material. To erase the persistent “memory” of flow-induced nuclei, samples must be annealed for a very long time below 210 °C (>3000 min) or ...

Lifetime of Flow-Induced Precursors in Isotactic Polypropylene

To solve problems in polymer fluid dynamics, one needs the equations of continuity, motion, and energy. The last two equations contain the stress tensor and the heat-flux vector for the material. There are two ways to formulate the stress tensor: (a) One can write a continuum expression for the stress tensor in terms of kinematic tensors, or (b) one can select a molecular model that represents the polymer molecule and then develop an expression for the stress tensor from kinetic theory. The advantage of the kinetic theory approach is that one gets information about the relation between the molecular structure of the polymers and the rheological properties. We restrict the discussion primarily to the simplest stress tensor expressions or constitutive equations containing from two to four adjustable parameters, although we do indicate how these formulations may be extended to give more complicated expressions. We also explore how these simplest expressions are recovered as special cases of a more general framework, the Oldroyd 8-constant model. Studying the simplest models allows us to discover which types of empiricisms or molecular models seem to be worth investigating further. We also explore equivalences between continuum and molecular approaches. We restrict the discussion to several types of simple flows, such as shearing flows and extensional flows, which are of greatest importance in industrial operations. Furthermore, if these simple flows cannot be well described by continuum or molecular models, then it is not necessary to lavish time and energy to apply them to more complex flow problems.

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Polymer Fluid Dynamics: Continuum and Molecular Approaches.

In this article, normal stresses and shear stresses in oscillatory shear flow are measured at small and large deformation amplitudes. New material parameters are then introduced based on Fourier transform rheology and stress decomposition analysis of normal and shear stress measurements. Furthermore, uniaxial extensional measurements are performed and compared to simulation results using the molecular stress function model. Different behaviors were observed for the polyethylene and polypropylene type blends, which are believed to arise from the different types of long-chain branching (LCB) topology present in each of the systems. The use of the new material parameters proposed and described within this article has the potential to allow for a better understanding of structure-property relationships in industrial LCB materials.

Isabel Neuhaus

Papers

Large amplitude oscillatory shear and uniaxial extensional rheology of blends from linear and long-chain branched polyethylene and polypropylene