Adhesion_Simulation_TetheredChains_Stevens - Macromolecules...

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Large-Scale Simulation of Adhesion Dynamics for End-Grafted Polymers Scott W. Sides,* Gary S. Grest, and Mark J. Stevens Sandia National Laboratories, ² Albuquerque, New Mexico 87185-1411 Received August 16, 2001; Revised Manuscript Received October 19, 2001 ABSTRACT: The adhesion between a polymer melt and substrate is studied in the presence of chemically attached chains on the substrate surface. Extensive molecular dynamics simulations have been carried out to study the effect of temperature, tethered chain areal density ( Σ ), tethered chain length ( N t ), chain bending energy ( k θ ), and tensile pull velocity ( v ) on the adhesive failure mechanisms of pullout and/or scission of the tethered chains. We observe a crossover from pure chain pullout to chain scission as N t is increased. Below the glass transition, the value of N t for which this crossover begins approaches the bulk entanglement length N e . For the values of N t and Σ used here, no crossover to crazing is observed. I. Introduction Adhesion at polymer interfaces is important in many diverse applications such as colloidal stabilization, 1,2 filler modification of polymeric materials and lubrica- tion, 3 and enhancing mechanical properties of polymer blends. 4,5 The interface of interest can be either (i) between two homopolymer melts (A + B) or (ii) between a homopolymer melt and a hard surface (A + substrate). Most polymer blends do not mix, because even a weak repulsive interaction causes the system to phase sepa- rate as the chain length increases. This phase separa- tion reduces entanglements at the interface, and the adhesive strength is then due to relatively weak van der Waals forces alone. For case ii, the interfacial strength depends on the number of chemical bonds between monomers of the polymer and sites on the substrate. In the absence of strong chemical bonding, the adhesive strength is again dominated by weak van der Waals attractions. For both cases certain additives or compatibilizers can increase the adhesive strength of the interfaces. For case i, the additive can be an AB diblock copolymer. The A(B) block of the copolymer can form entanglements with the A(B) melt side. In case ii, the additive is a chain of the A species with a function- alized end group able to react with the substrate, thereby forming a strong chemical bond. The tethered chain of the A species can then become entangled with the melt. In both cases, larger numbers of polymer entanglements result in increased interfacial strength. The systems simulated in this paper consist of substrate- tethered chains in contact with a polymer melt (i.e., case ii), but the results can easily be generalized to polymer/ polymer interfaces. Two key parameters govern the amount of interfacial
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This note was uploaded on 07/20/2011 for the course EMA 6165 taught by Professor Brennan during the Spring '08 term at University of Florida.

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Adhesion_Simulation_TetheredChains_Stevens - Macromolecules...

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