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MOLECULAR STUDY OF THE SURFACE FREEZING PHENOMENON IN MATERIALS CONTAINING LONG ALKYL CHAINS

Prasad, Shishir
http://rave.ohiolink.edu/etdc/view?acc_num=akron1191522340

Abstract Details

2007, Doctor of Philosophy, University of Akron, Polymer Science.
Surfaces of most liquids in contact with air have higher entropy and lower melting temperature than the corresponding bulk liquids. Exceptions include materials consisting of linear chains such as n-alkanes, alkane analogues, and poly(n-alkyl acry-late)s, for which the surfaces remain frozen above the bulk melting temperature. We have studied the profound effect of this surface freezing on the static and the dynamic properties of these materials. Along with being a unique 2D effect, the surface frozen monolayer is a perfect example of the quasi 2D-crystal. Hence, it provides an opportunity to study the mechanical properties of 2D solids. Surface sensitive tools such as infrared-visible sum frequency generation spectroscopy, Wilhelmy balance, oscillating bubble surface rheometer, and X-ray synchrotron scattering were used in our investigation. The difference between the surface melting and the bulk melting temperatures is much higher for the poly(n-alkyl acrylate)s compared to their small molecule counterpart, n-alkanes. We have attributed this difference to the partial crystallinity of the alkyl side chains at the surface and the additional length of the surface frozen layer due to the ester linkage in the case of polymers. Surface freezing leads to a large surface rearrangement in binary blends of poly(n-alkyl acrylate)s differing only in a couple of methylene side chain units. Only 2 wt.% of the longer side chain component is enough to cover the surfaces below the surface freezing temperature. In addition, the surface transition temperature for the longer side chain component depends weakly on the bulk composition. A comparison of our experimental data with a newly developed thermodynamic model suggests that almost all the side chains of a surface molecule are present in the surface layer upon ordering. The surface transition has significant effects on the dynamic properties of these materials. The wetting dynamics of the poly(n-alkyl acrylate)s are dramatically modified by the surface frozen monolayer. The relaxation process of the polymer meniscus in contact with a thin platinum wire slows down by orders of magnitude upon formation of the 3 nm thick surface ordered layer. Based on our X-ray photon correlation spectroscopy measurements, we have attributed this slowing down to a precursor film. The mechanical properties of the surface frozen monolayer were studied by expanding and contracting an air bubble in an n-alkane melt. The surface frozen monolayer crystal ruptures at the grain boundaries. The strength of this 2D crystal depends on the rate of crystal rupture. The interfacial area formed after rupture decreases with a decrease in the rate of bubble oscillation. Interestingly, the broken crystal recovers on a subsecond time scale.
Ali Dhinojwala (Advisor)
232 p.
Surface freezing; Poly(n-alkyl acrylate); Alkane; Monolayer; 2D Crystal; 2D Crystal Rupture; Surface Segregation; Surface Freezing Temperature Range; Surface Relaxation; Meniscus Relaxation

Recommended Citations

Citations

  • Prasad, S. (2007). MOLECULAR STUDY OF THE SURFACE FREEZING PHENOMENON IN MATERIALS CONTAINING LONG ALKYL CHAINS [Doctoral dissertation, University of Akron]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=akron1191522340

    APA Style (7th edition)

  • Prasad, Shishir. MOLECULAR STUDY OF THE SURFACE FREEZING PHENOMENON IN MATERIALS CONTAINING LONG ALKYL CHAINS. 2007. University of Akron, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=akron1191522340.

    MLA Style (8th edition)

  • Prasad, Shishir. "MOLECULAR STUDY OF THE SURFACE FREEZING PHENOMENON IN MATERIALS CONTAINING LONG ALKYL CHAINS." Doctoral dissertation, University of Akron, 2007. http://rave.ohiolink.edu/etdc/view?acc_num=akron1191522340

    Chicago Manual of Style (17th edition)

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© 2007, all rights reserved.
This open access ETD is published by University of Akron and OhioLINK.