Surface rheology of block-copolymer stabilized interfaces: a combined computational & experimental study. SNF project 200021_156106 at www.complexfluids.ethz.ch/snf15
We carried out molecular dynamics simulations to study the crystallization of polymer melts subjected to confinement between two parallel walls. Two types of walls, bare and grafted walls, are studied. The bare walls are slippery for chain motions, and the interactions between polymer chains and the walls are moreover chosen either attractive or repulsive. The crystallization in the case of bare walls generally consists of surface-induced processes close to the walls followed by homogeneous nucleation in both the boundary and middle regions. In the case of grafted walls, parts of polymer chains residing close to the walls are adhesive to the surfaces and become permanent graft points. We find that the surface-induced crystallization is increasingly suppressed with increasing grafting density. At high grafting densities, only crystallization in the middle regions is observed. We calculated the spatial distribution of entanglement lengths and related it to the crystallization behavior. The entanglement length close to the walls is found to decrease with increasing grafting density, as the adhesion points act as effective entanglement knots. In light of our recent results that less entangled polymer melts lead to faster crystallization and higher crystallization order, we now show that this conclusion stands also for the case of confined polymer melts. Our results suggest entanglements to be an universal factor towards the understanding of polymer crystallization under different situations, in particular at supercooling. (C) 2016 Elsevier Ltd. All rights reserved. Kroger, Martin/C-1946-2008 Kroger, Martin/0000-0003-1402-6714 DFG [SO 277/6-2]; Swiss National Foundation [SNF 200021-156106] CL and JUS gratefully acknowledge the financial support by DFG (SO 277/6-2) and computing time from ZIH of TU Dresden. MK acknowledges financial support by the Swiss National Foundation through grant SNF 200021-156106. The authors thank Huanhuan Gao for discussions. [hide]
Principal Investigators
Leonard Sagis (PL)
Polymer Physics, ETH Zurich, Switzerland ►
Wageningen University, Netherlands ►
Patrick Ilg (PL)
Polymer Physics, ETH Zurich, Switzerland ►
University of Reading, United Kingdom ►
Peter Fischer (Co-PI)
Inst. Food, nutrition and health, ETH Zurich, Switzerland ►
Martin Kröger (PI)
Polymer Physics, ETH Zurich, Switzerland ►
Secretary
Patricia Horn
Polymer Physics, ETH Zurich, Switzerland ►
Involved Students
Ahmad Moghimikheirabadi
Polymer Physics, ETH Zurich, Switzerland ►
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Selected conferences (co-)organized by project members
IWNET 2015
05 Jul - 10 Jul 2015, 7th International workshop on nonequilibrium thermodynamics (IWNET 2015), Hilvarenbeek, The Netherlands ►
learn more ► About this project
Complex fluid-fluid interfaces are interfaces in which the adsorbed species self-assemble into complex microstructures. Such interfaces are ubiquitous in nature, industrial processes, and consumer products, and can be found in living cells, nano- and microcapsules, vesicles, food emulsions, or foam. Compared to simple liquid-like interfaces (stabilized by low molecular weight surfactants), complex interfaces display significant viscoelasticity, with high values for their surface shear and dilatational moduli. Their stress-deformation behavior dominates the macroscopic dynamics of multiphase materials that contain such interfaces, and when this occurs those materials can be referred to as Interface-Dominated Materials (IDMs).Complex interfaces can be formed by a wide range of surface active components, such as proteins, colloidal particles, polymers, lipids, or mixtures of these components. In this proposal we will focus on complex interfaces stabilized by amphiphilic multi-block copolymers. These polymers consist of alternating blocks of a hydrophilic repeating unit A, and a hydrophobic repeating unit B. Amphiphilic copolymers can form interfaces with exceptional mechanical properties. This makes them ideal candidates for application in highly stable emulsions, or encapsulation systems with high mechanical stability, for application in food and pharmaceutical products.
Amphiphilic copolymers may form 2d gels, 2d (soft) glass phases, 2d (liquid) crystalline phases, or even 2d metastable emulsions (phase-separated mixtures of immiscible copolymers) after adsorption. The type of structure formed depends on surface concentration, and length, distribution, rigidity, and hydrophobicity of the sub-blocks of the copolymer. The response of polymer stabilized fluid-fluid interfaces to deformations or gradients in temperature is often highly nonlinear. The nonlinearity in their response to perturbations is a result of changes in this interfacial microstructure, induced by the applied gradients. The effect of deformations on interfacial microstructure, and the effect of these changes on macroscopic dynamics of interface-dominated materials is still poorly understood. A more fundamental understanding of the nonlinear response of polymer interfaces, is essential for a targeted design of high-end polymer stabilized IDMs, such as encapsulation systems with environmental triggers, nanoparticles with structured interfaces, or foam and emulsions with extreme stability. In view of the widespread occurrence of IDMs, the study of dynamic mechanical properties of these interfaces is highly relevant for many disciplines, such as colloid and interface science, physical chemistry, polymer physics, pharmaceutical science, food science, coating technology, or soft matter physics.
The aim of this project is to characterize the microstructure and mechanical properties of interfaces stabilized by multi-block copolymers, using a multiscale multidisciplinary approach, which integrates state of the art computational methods with surface rheological experiments, and experimental interfacial structure evaluation. The computational modeling will be done using Monte Carlo (MC) and Molecular Dynamics (MD) simulations. We will measure both shear and dilatational surface properties, and the microstructure will be evaluated using various forms of microscopy (AFM, TEM, SEM), and neutron and X-ray reflectivity measurements. We will determine the mechanical properties and interfacial structure as a function of surface polymer concentration, chemical structure of the polymers (variation of number, size, and distribution of blocks), and degree of hydrophobicity and rigidity of the sub-blocks. A detailed insight in the dynamic behavior of copolymer interfaces will provide new insight in the macroscopic dynamic behavior of polymer stabilized interface-dominated materials (emulsions, foam, encapsulation systems, nanoparticles), and will allow a more targeted design of these systems with tailor made properties, tuned for specific industrial applications.
29 April 2024 mk