Short-time interdiffusion at polymer interfaces: A probe to study polymer motion
Agrawal, Gaurav
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Permalink
https://hdl.handle.net/2142/20784
Description
Title
Short-time interdiffusion at polymer interfaces: A probe to study polymer motion
Author(s)
Agrawal, Gaurav
Issue Date
1994
Department of Study
Materials Science and Engineering
Discipline
Materials Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Materials Science
Language
eng
Abstract
The aim of this work was to design experiments that could probe the motion of the polymer chains at the molecular level, and thereby suggest the mechanism of polymer-polymer interdiffusion. In the first experiment, a thin layer ($\sim$400-1500 A) of triblock polystyrene (HDH), wherein the central half of the chain was deuterated, was interdiffused into a thin layer of similar polymer (DHD), wherein the labeling was reversed, i.e., the two end fractions were deuterated. Since, the total deuteration was equal in both the polymers, initially, the concentration of H and D segments was 0.5 at all depths in the HDH/DHD bilayer. Upon interdiffusion, if the polymer chains reptate, i.e., move ends first, a local enrichment of D in the HDH, and H in the DHD, at the interface would develop. Upon further diffusion, this enrichment would decrease, and eventually vanish as the bilayers homogenize. On the contrary, if the chain ends and the chain centers diffused at the same rate, the initial concentration profile would be retained at all times. In the second experiment, interdiffusion between HDH/HPS (normal polystyrene) and HDH/DPS (deuterated polystyrene) was investigated. In the first, the change in the concentration profiles of D yielded the penetration depth of the chain centers, and in the latter, the concentration profiles of H reflected the diffusion of the chain ends. If reptation occurs, there would be a finite time, to, before which the chain centers do not cross the interface. Concentration profiles were measured with neutron reflectivity and secondary ions mass spectroscopy, up to the times of the order of the reptation time ($\tau\sb{\rm d}$) of the polymers. The following results were obtained.
Concentration ripples were observed in 230,000 and 110,000 molecular weight (MW) HDH/DHD. The resulting concentration ripple heights and its time dependence agreed well with that found from the reptation simulations. The agreement with the Rouse simulations was not satisfactory. Furthermore, these ripples were completely absent in 30,000 and 50,000 MW where the diffusion is governed by Rouse dynamics. From the HDH/HPS and HDH/DPS experiments, the existence of t$\sb{\rm o}$ ($\sim$0.03 $\tau\sb{\rm d}$) was confirmed. This observation is unique to reptation. The results, therefore, support reptation as the predominant mechanism of diffusion in polymers.
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