Intermediate states in bivalent ion induced shrinking of polyacrylate coils on surfaces and in solution

• Main Author: Sinha, Prashant
• Other Authors: Technische Universität Dresden, Fakultät Mathematik und Naturwissenschaften
• Format: Doctoral Thesis
• Language: English
• Published: Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden 2009
• Subjects: AFM; Scattering; Polyacrylate; Coil to Globule transition; Coil to Globule Transition; ddc:540; rvk:VE 6307
• Online Access: http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-24839; http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-24839; http://www.qucosa.de/fileadmin/data/qucosa/documents/2483/PhD%20thesis.pdf

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 ii 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 iii 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).