Specifically binding ions induce the transition of anionic polyacrylate coils from
extended conformation to collapsed globules passing through a cascade of intermediate
states when solution conditions approach the L-type precipitation threshold. It is the
conformation of these intermediate states on surfaces and in solution which is at the focus
of this thesis. In comparing the surface and solution conformations of intermediate states,
we were able to qualitatively and quantitatively underline the effects of sample history.
Two types of quantitative comparisons have been emphasized. In real space, the radius of
gyration values of adsorbed molecules have been evaluated incorporating fully the x, y
and z axes. These values have been compared with radius of gyration values of the very
same sample solution obtained using SLS. In reciprocal space, a novel image processing
protocol has been used to generate the 2D form factor curve wherein the correlation
maxima have been compared with corresponding maxima obtained for the very same
sample solution using small angle scattering techniques like the SANS.
The influence of bivalent ions, respectively, Strontium, Lead and Calcium, on the shape
of polyacrylate coils is studied. In the last case, temperature has been introduced as a
secondary parameter to shed further light on the mechanism by which polyelectrolytebivalent
ion complexation takes place. Both scattering and AFM experiments reveal
formation of necklace-like structures as intermediates for NaPA-Sr2+ system. Since the
mol. wt. of the NaPA coils used was relatively large in this case, adsorption on mica
surfaces was strong. Under such conditions, the molecules undergo a z collapse upon flux
drying but do not get altered in x and y directions. The ratio Rg(AFM)/Rg(SLS) was
found to be in the range 0.7-0.9. The remaining (insignificant) differences in the Rg
Abstract
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values arise due to the fact that AFM gives the square root of number averaged mean
squared radius of gyration while SLS gives the square root of z-averaged mean squared
radius of gyration. The differences in radius of gyration values observed in solution and
on surfaces were more prominent for NaPA-Pb2+ system. Again, although both scattering
and AFM reveal necklace-like structures as intermediates, the ratio Rg(AFM)/Rg(SLS)
was now found to be nearly 0.6. The fact that Rg(AFM) is the square root of number
averaged and Rg(SLS) is the square root of z-averaged mean squared radius of gyration,
alone cannot explain this low value. Since the mol. wt. of NaPA coils used in this case
was quite low, adsorption on mica surfaces was weak. Under such conditions, the
molecule does not only undergo a z collapse upon flux drying, but also shrinks in the x
and y directions due to capillary forces. Finally, with NaPA-Ca2+ system, the picture did
not show a one-to-one correspondence between solution and surface conformations at all.
In fact, it showed a one-step-ahead correspondence. As already stated, the coil to globule
transition was induced by increasing the equilibration temperature from 15°C to 30°C in
this case. SANS could not identify any necklace-like intermediates in solution at the
equilibration temperature of 15°C while AFM scans at this temperature showed the
beginning of formation of pearls. Likewise, at the equilibration temperature of 30°C,
SANS could identify necklace-like intermediates in solution with a large majority of
dumbbells while AFM scans at this temperature showed a mix of dumbbells, sausage-like
structures and globules. Indeed, we were witnessing an accelerated coil to globule
transition on surfaces as compared to the situation in solution resulting in a pre-emption
in the formation of intermediate states on surfaces. Since the ratio Rg(AFM)/Rg(SLS)
(given the square root of number averaged and the square root of z-averaged mean
Abstract
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squared values respectively) at the equilibration temperature of 30°C showed a range of
0.7-0.9 indicating strong adsorption of the relatively high mol. wt. NaPA coil on mica
surfaces, our suspect were the substrate-sample interaction forces. The AFM scans were
therefore analysed with 2D form factor curves, a better protocol when no assumptions
about the shape of adsorbed molecules are made a priori, to trace the effects of sample
history.
The thesis establishes the general utility of AFM to capture the essential features of a
collapsing coil which the very coil exhibits in solution. The shape of the coil on surface
and in solution may not be exactly the same, yet reveal the same characteristics. The
comparative advantages and disadvantages of salt pre-treated mica surfaces and
chemically modified mica surfaces have been brought out. Finally, a definitive new
insight is gained as regards the mechanism of coil collapse induced by specifically
binding ions. The entropic nature of the process as well as the visualized shape of the
collapsing intermediates does not support a mechanism along an electrostatically driven
shrinking with linear, rod-like arrays of pearls as intermediates. On a molecular level, it is
the liberation of water molecules and Na+ ions which promotes binding of bivalent ions
to COO- residues. This binding in turn increases the hydrophobicity of the polyacrylate
chains. As a consequence, the chains shrink due to an increased propensity for polymerpolymer
contacts (and finally precipitate).
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa.de:bsz:14-qucosa-24839 |
| Date | 15 October 2009 |
| Creators | Sinha, Prashant |
| Contributors | Technische Universität Dresden, Fakultät Mathematik und Naturwissenschaften, Prof. Dr. M. Stamm, Prof. Dr. M. Stamm, Prof. Dr. K.-F. Arndt |
| Publisher | Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis |
| Format | application/pdf |
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