Hydrophobes: Adsorption and Rheology - American Chemical Society

Chapter 13

Dispersions Containing PEO with C Hydrophobes: Adsorption and Rheology 16

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Downloaded by NORTH CAROLINA STATE UNIV on September 22, 2012 | http://pubs.acs.org Publication Date: August 10, 2000 | doi: 10.1021/bk-2000-0765.ch013

Q. T. Pham , J. C. Thibeault , W. Lau , and W. B. Russel 1

Department of Chemical Engineering, Princeton University, Princeton, NJ 08544 Research Laboratory, Rohm and Haas Company, Spring House, PA 19477-0904

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The rheology of polymer latices containing 35 kg/mole polyethylene oxide chains with C terminal hydrophobes is related quantitatively to the composition. Correcting the solution concentration and the particle volume fraction to account for adsorption of polymer permits a clear distinction between the contributions the solution and those from the particles. The relaxation spectrum then clearly direct interactions between the particles that increase the low shear viscosity and produce a power law spectrum that controls the modulus at low frequencies. 16

Introduction The rheology of latex dispersions containing associative polymers depends not only on the dynamics of the associated solution but also on polymer-particle interactions that control the structure and stability of the dispersion (1,2,3). Associative polymers adsorb onto polymer latices via hydrophobic interactions, forming dense layers that increase the hydrodynamic size of the particles while reducing the polymer concentration in solution. These processes have opposite effects on the magnitude of the stresses, but generally preserve the relaxation times as those of the associated solution. Attractions between dense layers on interacting particles, e.g. due to bridging chains, also increase the stress, while introducing slower relaxations that require diffusion of the particles. The overall dispersion rheology depends qualitatively and quantitatively on the relative importance of these processes. Our previous work (4) demonstrates that narrow distribution polyethylene oxide (PEO) chains with C terminal hydrophobes (ODU=octadecyl unimers) adsorb on poly(methylmethacrylate) (PMMA) latices from water as dense layers of moderately stretched chains in very dilute solutions and appear to reorganize as whole or hemimicelles at higher concentrations. In either case chains in an adsorbed layer associate hydrophobically with micelles in solution and with adsorbed layers on other particles. At finite particle volumefractionsφ, adsorption increases both the intrinsic viscosity 1 8

© 2000 American Chemical Society

In Associative Polymers in Aqueous Media; Glass, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

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[η] by an amount consistent with the measured layer thickness, and the Huggins coefficient k . The former indicates strong coupling with the associated solution, equivalent to a no-slip boundary condition. The value of k exceeds that of hard spheres, implying direct coupling between adsorbed layers on interacting particles. Thus, the observed enhancement of the dispersion viscosity arises from the high viscosity of the polymer solution, the increased hydrodynamic volume of the particles, and direct interactions between adsorbed layers. Likewise, the viscoelasticity of the dispersion resembles that of the associated solution with elasticity at highfrequencygoverned by a single relaxation time of 0(5-15 ms), while the particles impart a power law distribution of longer relaxation times that dominate the elasticity at low frequencies. The latter is not sufficiently strong to affect the shear rate dependence of the viscosity. Here we examine the effect of hydrophobe size on the behavior with the same dialyzed PMMA latices (diameter 2a=224 nm) and PEO backbones (35 kg/mole) but with C | (HDU=hexadecyl unimers) instead of C hydophobes. To facilitate direct comparison between the two hydrophobes, we perform the same experiments on adsorption and rheology and use the data as a second test for the validity of the correlations presented previously. The materials and methods are described in detail elsewhere (4,5). 9

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Adsorption Adsorbed amounts for HDU, ODU, and PEO generally increase monotonically with the polymer concentration c in the aqueous phase and approach a plateau F at high concentration (Table I) that increases with hydrophobe size, translating into a reduced area per chain for the bigger hydrophobe (6). Unlike ODU, for which a small initial plateau was observed at low c , HDU and PEO increase more smoothly and steeply to the apparent plateau (Figure 1). s

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Table I: Adsorbed amounts and layer thicknesses

r

Polymer

m 2

ODU HDU PEO

(mg/m ) 2.69 1.79 0.90

1/σ (nm /chain) 21.6 32.4 64.9 2

δ (nm) 26±5 18±6 10±4

m

0.35N(ovP) (nm) 10-15 9-14 —

Unmodified PEO adsorbs on latex particles at many points along its backbone, producing loops, trains, and tails with the tails governing the hydrodynamic thickness (7). On polystyrene particles with a=\20 nm, monodisperse PEO (40 kg/mol) has a plateau absorbed amount r = 0.68±0.50 mg/m and a layer thickness δ= 12+2 nm 2

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(8). Both r and δ increase monotonically with molecular weight (9). In good solvents, δ is about 1.5 to 2 times the radius of gyration of PEO in bulk solution, indicating a moderately stretched configuration for the adsorbed chain. Although polystyrene has a slightly different surface polarity than PMMA, the r are comparable, implying a loop-train-tail arrangement for the adsorbed PEO on PMMA. Comparison with the unmodified PEO suggests that the associative polymers adsorb in crowded layers of stretched chains. From the adsorbed amounts and the molecular weights, the layer thicknesses can be estimated by simple theories for neutral polymer brushes by treating each triblock as two diblock copolymers adsorbed by terminal hydrophobes. Adapting the mean field theories of Alexander (10) and de Gennes (//) for dense brushes of terminally anchored chains in a good solvent to triblocks adsorbed with both hydrophobes on the surface yields the thickness δ on a spherical particle of radius α»δ, as m


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(1) with L (=285 nm) the contour length of the PEO block, / (=0.456 nm) the Kuhn length, and υ the excluded volume. The number of chains per unit area ais related to the measured surface coverage r by m

σ

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~ M where Ν a is the Avogadro's number and M is molecular weight of the PEO. The uncertainty in the dimensionless excluded volume, v/^ =0.12-0.33, reflects variations in the reported radius of gyration of PEO in solution from which it is estimated (4). The latter is accomplished via a mean field theory comparable to (1), on the presumption that uncertainties in the theories themselves should cancel at least partially. The predicted layer thicknesses for ODU and HDU in Table I increase with hydrophobe size and exceed by more than 20% the end-to-end distance of the homopolymer (11-16 nm). Hydrodynamic diameters of particles with adsorbed layers measured by dynamic light scattering increase rapidly with total polymer concentration c and reach full thickness at concentrations below the plateau adsorbed amount r in Figure 1. On the plateau the layers are Significantly thicker for the associative polymers (Table I). For unmodified PEO, £=10±4 nm is similar to those adsorbed onto polystyrene particles (£=6-12 nm for 20-40 kg/mol PEO and a=120 nm) (8,9), but smaller than predicted for chains that adsorb terminally. Thus, PEO must contact the particle surface at many points along its backbone. The increase in δ with hydrophobe size is also consistent with predictions from Eqn (2), corroborating the notion that associative polymers adsorb via their hydrophobe endcap, essentially forming dense terminally-anchored brushes. If hydrophobic interactions dictate adsorption, then the bigger hydrophobes adsorb more strongly, form denser layers, and hence have a more stretched configuration. However, the prediction for ODU is considerably smaller than the measured value, whereas that for HDU just falls within the error bars, supporting the argument that ODU adsorbs as micelles or hemi-micelles instead of 3

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individual chains. Furthermore, both adsorbed amount (Figure 1) and layer thickness increase smoothly for HDU, while ODU exhibits discontinuities near the cmc (c ~ 100-200 ppm). Of course, definitive resolution of the actual configuration would require more direct visualization of the adsorbed layer. s


Viscosity The steady shear profiles of PMMA dispersions thickened with HDU (Figure 2) generally exhibit a Newtonian viscosity at sufficiently low shear rates, shear thickening at intermediate shear rates, and thinning at shear rates of 100 s*, as reported previously for dispersions containing ODU. The dispersion viscosity increases monotonically with polymer concentration and is comparable to that of the neat solution, but can decrease with the addition of particles. Compared to ODU, HDU enhances the viscosity less and causes shear thinning that moves to lower shear rates (0.04-0.2 s") at higher volume fractions and is more gradual. Several observations are unique to PMMA+HDU. For example, at high polymer concentration (cp>3 wt%) and high particle volume fraction (^=0.17), the dispersion viscosity gradually decreases over all accessible shear rates (Figure 2). At the other extreme for Cp\ reveals non-hydrodynamic interactions between particles, presumably due to association between adsorbed layers. The comparable [η] and k for ODU and HDU suggest that the hydrophobe size does not change the nature of the interactions between particles and polymers. The dispersion viscosity is still enhanced primarily by the more viscous associated solution and augmented by particles coupled through their adsorbed layers. Figure 5 compares the correlation (lines from Eqn (4)) with the full set of data. For cp>2 wt%, the high solution viscosity % and increased particle interaction kn maintain a high dispersion viscosity η that increases monotonically with φ β·. This regime persists to lower c than with ODU, since the weaker adsorption depletes the solution polymer less drastically. For cp=l.5 wt%, addition of particles reduces ησ more than an order of magnitude below the original solution viscosity, while for c G' and « > l / 2 ; after gelation a crossover emerges at high ω, with G" ) 0

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°°Γ

dX

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Numerical solution yieldsA/û> =0.285, allowing λι to be deduced from the crossover. For example, in Figure 9b û? =21.9 rad/s determines A/=0.013 s. Then combining the power law and discrete models and adjusting H X and G' produces quite a good fit of both the residuals and the full spectrum. However, with more strongly associated samples, such as ^>0.17 and Cp=2.5-3.5 wt%, setting «=0.5 and following the same procedure errs at 0(1) for the lowest frequencies. Introduction of a static limit G'->G as ω-+0 might help but has not been attempted. c

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Figures 10-13 represent the results as a function of effective volume fraction φφ normalized where appropriate by the properties of the polymer solution at concentration c . The highfrequencymodulus of the dispersion G ' (ο ,φ) generally increases with c but falls below the modulus for the solution G ' (c 0) except at the highestfaff.Normalized as the ratio of G ' (ρ ,φ) to G ' (c ,0), with the latter calculated from curve fitting the measured values (5), shows that G' (c ,0G' (c ,Q)

Hydrophobes: Adsorption and Rheology - American Chemical Society

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