Microgel viscosity

The intrinsic viscosity of microgels described in [9] decreased with increasing fractions of the crosslinking monomer to about 8 ml/g which was still above the theoretical value for hard spheres of about 2.36 ml/g according to the Einstein equation and assuming a density of 1.1 g/ml. Obviously, due to the relatively low fraction of the crosslinking monomer, these microgels did not behave like hard spheres and were still swellable to some extent. 

Fig. 11. Variation of Qv/Q°° ratio ( ) and the reduced intrinsic viscosity of microgels [ird/frilo (O) with the DVB content in the monomer mixture. Experimental data points were taken from Hoffmann [70]. The dotted horizontal line represents the critical Qv/Q°° value for the onset of a phase separation.

Because the presence of an electrolyte increases the dimensions of micelles and microemulsion droplets [115], it may be expected that in presence of ions the size of microgels is also increased. This expectation could be confirmed external electrolyte increases Mw (Fig. 21) as well as dz and [r ] (Fig. 22) up to the limit of the emulsion stability. Therefore, the addition of an external electrolyte to the reaction mixture for the ECP of EUP and comonomers is a means to vary the molar mass, the diameter and the intrinsic viscosity of microgels from EUP and comonomers deliberately. 

Fig. 33. Dependence of viscosity on the shear rate of microgel solutions in C2H5OC2 H4OCOCCH3. EUP(MA+HD), c/t 70/30, EUP/S and EUP/EDMA(D), AIBN, P.-S. polystyrene [136].

Compared with rigid microgels, the intrinsic viscosity of microgels prepared from the comonomer mixture mentioned before is higher, but the slope of the curve in Fig. 56 is still low because the composition of these microgels was close to the limit of stability. 

Fig. 56. Dependence of Mwof the microgels on the polymer yield in the anionic polymerization of EDMA in toluene by n-BuLi [254] (see Figure 53 caption for the reaction conditions). Reduced viscosity vs concentration of microgels a) Composition (mol %) N,N -methyl-enebisacrylamide (55%), methacrylamide (33%), methacrylic acid (2%), methacrylamido acetaldehyd-dimethylacetal (10%),measured at 20 °C in water, b) Composition (mol %) 1,4-DVB (35%), propenic acid amide-2-methyl-N-(4-methyl-2-butyl-l,3-dioxolane prepared by emulsion copolymerization and measured in dimethylformamide.

TFFF has been employed for the investigation of linear polymers in various solvents [19, 20, 21], copolymers [22, 23], branched polymers [23], and cross-linked microgels and block-copolymer micelles [24]. Kirkland coupled TFFF with an online viscosity detector [25]. 

The addition of OMM increases the steepness of the curve owing to the higher viscosity of exfoliated microgel type network structure [13] at low shear rates and its orientation that enhances the decrease of viscosity with increasing shear rate. 

When the slurry ages, the pH rises, and, consequently, the viscosity rapidly elevates, there appears to be a breakdown in the mechanism that binds the solid particles in the silica matrix. With a well-developed sol, this behavior is more controllable. We believe that, at longer aging times, the microgel development proceeds by mechanisms more favorable to a finely textured, fibrillar structure (19). Hence, catalysts bound with this gel are more attrition resistant. We suggest that the dependence of attrition on slurry age/viscosity may be a consequence of an inhibition to forming a strong matrix. The most important conclusion that can be drawn from... 

The fact that a viscosity increase after phase segregation (for t > tp) is connected with such mechanism is evidenced by the results of gel chromatographic (GPC) analysis of solfi action in the network formation process of low-molecular siloxane rubbers (Fig. 15). As the reaction proceeds the molecular mass of the sol fraction decreases and so does its viscosity. However, network formation of a number of epoxy resins cured with amines or other curing agents conform the homogeneous model without microgel formation [88 a]. 

Rheokinetic analysis shows that rheokinetic equations su ested to describe changes in viscosity and elastic modulus reflect the characteristics of this process. Therefore, the nature of the viscosity increase up to the gel point is defined not only by a variation of the molecular structure of the material but sometimes also by the effect of formation of a new phase (microgel) from particles of a cured product. 

---