Polyelectrolyte (PE) Collapse

Interest on the PE collapse was aroused by new predictions of theoreticians, proposing anisotropic intermediate states like cigars or pearl necklaces along this shrinking process. Structural changes of PE can be induced by e.g. alkali and alkaline earth halides.
The interaction between PE and metal cations plays a central role in many industrial applications as well as in biological systems (e.g. the dispersion of inorganic pigments, scale inhibition in steam pipes or protein supported biomineralisation).
The specific binding of Ca² to the dissociated carboxylate groups of sodium polyacrylate (NaPA) induces a shrinking process (coil to globule transition) prior to the precipitation of CaPA. The phase diagram of NaPA in the presence of Ca², dissolved in 0.01 M NaCl solution, is presented in figure 1. The subtle interplay of NaPA, NaCl with Ca² gives access to well defined and extremely small increments and allowed us to address any intermediate state along the shrinking process of NaPA.
SANS measurements of different NaPA intermediates, obtained by varying the ratio of Ca² per carboxylate group, were carried out at D11. The scattering curves show a q^-4 dependency at high q (indicating densely packed objects with sharp boundaries) and a maximum with adjacent minimum at lower q. Experimental scattering curves are compared with theoretical models. Neither a sphere nor a cylinder form factor can render the measured curve and are thus clearly ruled out as possible PE shapes. Furthermore, the minimum/maximum does not stem from an interparticular interference, as proven by a reference measurement of pure, fully ionized NaPA.
Eventually, the minimum could be attributed to the distance of neighbouring pearls in a PE chain. We succeeded in recovering all features of the scattering curves and consider the results as indication for the appearance of pearl-necklace shapes during PE collapses.

Figure Caption - Phase diagram of NaPA in the presence of Ca in an acqueous 0.01M NaCl solution. The grey region marks the 2 phase region, i.e. precipitation of CaPA occurs. The horizontal arrow marks the approach of the phase boundary. Below, observed NaPA shapes (with Ca as red points) are shown.

Shearing of Wormlike Micelles

Shear thinning and orientation of cylindrical surfactant and block copolymer micelles was investigated at D11 over a broad range of shear rates and concentrations [1] with rheo-SANS, a powerful technique that can be used to obtain a detailed molecular picture of the structural changes of complex fluids under shear, For these experiments our recently developed Searle-type shear cell was used on a commercial Bohlin CVO120HR rheometer [2].

In sheared solution, elongated objects such as wormlike micelles align in the flow direction, with an orientational distribution that can be well described by an Onsager-type distribution function. The shear viscosity measured as a function of shear rate for 3 different micellar solutions is shown in figure 2 and shows pronounced shear thinning. Also shown in figure 2 are 4 examples of in situ measured anisotropic small angle neutron scattering patterns, together with the corresponding calculation. From the calculated scattering patterns the orientational order parameter S can be determined. It lies in a range 0.05 = S = 0.85 for the different wormlike micellar solutions at different concentrations and shear rates studied here.

The relation between shear viscosity and order parameter reveals important features of the molecular orientation process. In summary, this study shows that shear thinning and orientation of wormlike micelles is observed for t dis >> 1, where t dis is the disentanglement time of the micelles and is the shear rate. In this regime there is a simple functional relationship of the type h z e-as between the shear viscosity and the order parameter S, with a common slope a for all samples over a wide range of concentrations and shear rates. The study provides direct relations between bulk properties like shear rate and shear viscosity, and molecular properties such as micellar thickness and orientation distribution of long wormlike micelles or fibrous structures.

Figure caption - Shear viscosity h as a function of shear rate for cylindrical micelles of PB-PEO-16: c =10 g/l (n,r), CPySal: c =20 g/l (l,m) and CTAB: c =20 g/l (r).
Filled symbols are upward, open symbols are downward cycles. Measured and calculated SANS-patterns are shown for PB-PEO-16 at four different shear rates ( =1, 5, 50, and 500 s ^-1) together with the calculated orientational order parameter S.