Telechelic Associative Polymers Research Articles

Stress Buildup under Start-Up Shear Flows in Self-Assembled Transient Networks of Telechelic Associating Polymers

The nonaffine transient network theory is used to study the time development of the shear and normal stresses under start-up shear flows in networks formed by self-assembled telechelic, hydrophobically modified water-soluble polymers. The initial slope, strain hardening, and overshoot of the shear stress are studied in detail in relation to the nonlinear tension-elongation curve of the elastically active chains in the network. The condition for the occurrence of strain hardening (upward deviation of the stress from the reference curve defined by the linear moduli) is found to be gamma > gammac(A), where gamma is the shear rate, gamma(c) is its critical value for strain hardening, and A is the amplitude of the nonlinear term in the tension of a chain. The critical shear rate gamma(c) is calculated as a function of A. It is approximately 6.3 (in the time unit of the reciprocal thermal dissociation rate) for a nonlinear chain with A = 10. The overshoot time t(max) when the stress reaches a maximum and the total deformation gamma(max) = gamma(t max) accumulated before the peak time are obtained in terms of the molecular parameters of the polymer chain. The maximum deformation gamma(max) turns out to depend weakly upon the shear rate gamma. The first and second normal stress differences are also studied on the basis of the exact numerical integration of the theoretical model by paying special attention to their overshoot, undershoot, and sign change as a function of the shear rate. These theoretical results are compared with recent rheological experiments of the solutions of telechelic hydrophobically modified poly(ethylene oxide)s carrying short branched alkyl chains (2-decyl-tetradecyl) at both ends.

Asymmetric End-Capped Poly(ethylene oxide). Synthesis and Rheological Behavior in Aqueous Solution

A method was developed to prepare asymmetric end-capped poly(ethylene oxide)s (PEOs), containing an alkyl group on one chain end and a perfluoroalkyl group on the other one (C18H37−PEO−C2H4−C8F17 and C18H37−PEO−C10H20−C8F17). These new telechelic associative polymers (APs) were synthesized by anionic polymerization of ethylene oxide initiated by an alkoxide (C18H37−O−K+) followed by esterification. The rheological behavior of asymmetric and analogous symmetric APs in aqueous solutions has been investigated as a function of polymer concentration (C), hydrophobic end-cap and surfactant used to homogenize the system: sodium dodecyl sulfate (SDS) or lithium perfluorooctyl sulfonate (LiFOS). A steep increase of the static viscosity η, attributed to the formation of a multiconnected network, is observed for C ≈ 1 wt %. Semidilute solutions present a Maxwellian rheological behavior with only one relaxation time τ associated with the least hydrophobic end group. The viscoelastic response is modified by the interactions between hydrophobic end groups and surfactant inside mixed aggregates.

Simulations and experiments of self-associating telechelic polymer solutions

A Brownian dynamics computer simulation study of a highly coarse-grained model oftelechelic associating polymers has been carried out. In a critical concentration range themodel produces the so-called ‘loops-to-bridges’ transition, thought to exist in theexperimental systems, in which the two hydrophobic groups are in different micelles,thereby forming a highly interconnected, ultimately percolating, network. The fraction ofbridged polymers produced by the model correlates well with the experimental viscosity atcorresponding concentrations. The distribution of micelle sizes compares favorably with thepredictions of the Meng–Russell free energy theory. The mean cluster size scales well withvolume occupancy according to a simple mean-field theory. The stress relaxation function isa stretched exponential at short times and not too high concentrations but develops alonger time plateau in the percolation region, both in agreement with experiment. Newexperimental data for the concentration dependence of the self-diffusion coefficient,viscosity, elastic modulus and relaxation time of telechelic associative polymers arepresented, which show broad qualitative agreement with the simulation data.

Phase behavior of flowerlike micelles in a SCF cell model

We study the interactions between flowerlike micelles, self-assembled from telechelic associative polymers, using a molecular self-consistent field (SCF) theory and discuss the corresponding phase behavior. In these calculations we do not impose properties such as aggregation number, micellar structure and number of bridging chains. Adopting a SCF cell model, we calculate the free energy of interaction between a central micelle surrounded by others. Based on these results, we predict the binodal for coexistence of dilute and dense liquid phases, as a function of the length of the hydrophobic and hydrophilic blocks. In the same cell model we compute the number of bridges between micelles, allowing us to predict the network transition. Several quantitative trends obtained from the numerical results can be rationalized in terms of transparent scaling arguments.

Shear banding and rheochaos in associative polymer networks.

We present experimental evidence of an instability in the shear flow of transient networks formed by telechelic associative polymers. Velocimetry experiments show the formation of shear bands, following a complex pattern upon increasing the overall shear rate. The chaotic nature of the stress response in transient flow is indicative of spatiotemporal fluctuations of the banded structure. This is supported by time-resolved velocimetry measurements.

Effect of Polydispersity on Fluorescence Quenching in Micelles Formed by Telechelic Associative Polymers

Time-resolved fluorescence quenching studies, using pyrene as a probe for micellar environments in solutions of a telechelic associating polymer based on PEG with n-C12 (dodecyl) hydrophobic end groups, have been carried out to determine the aggregation number(s) of the micelles. At low [pyrene]/[hydrophobe] values, η, quencher-average aggregation numbers, Nq, calculated from the fitting of the fluorescence decay curves to the Infelta equation were found to decrease rapidly as η increased. By fitting the Nq vs η plots to a cubic curve [Warr, G. G.; Grieser, F. J. Chem. Soc., Faraday Trans. 1 1986, 82, 1813], this observation was shown to be consistent with a highly skewed, polydisperse distribution of micelle aggregation numbers, described by a weight-average aggregation number of 75 and a standard deviation of 86. For a given polymer concentration in the range 1.0−2.5 g dL-1, the fluorescence decay curves did not produce parallel log(I/I0) vs time plots, as would be expected for a monodisperse array of micelles. Instead, the plots showed a characteristic intersection. Simulation of these curves using Infelta-type equations could not reproduce this behavior, indicating that a more refined model may be needed. This detailed study of the distribution of aggregation number in associative telechelic polymers is discussed in the context of recent investigations [Meng, X.-X.; Russel, W. B. Macromolecules 2005, 38, 593], from which it is suggested that polydispersity of micelle size is to be expected as a normal feature of such systems.

Micellization of Telechelic Associative Polymers: Self-Consistent Field Modeling and Comparison with Scaling Concepts

We present numerical results from self-consistent field calculations on the micellization of telechelic associative polymers and their mono-functional analogues. These results are confronted with relatively simple scaling concepts. The proportionality of the critical micelle concentration (CMC) with the hydrophilic backbone length, as found in the calculations, shows good correspondence with a scaling argument based on the entropic penalty of loop formation. It is also shown that models for the conformation of spherical brushes can be applied to predict the structure of the flowerlike micelles formed by these telechelic polymers. Furthermore, we find good agreement between the numerical dependence of the aggregation number upon both backbone and terminal hydrophobe length and an analytical expression derived from the well-known Daoud-Cotton model by introducing a correction for the finite size of the micellar core.

Rheological Properties of a Telechelic Associative Polymer in the Presence of α- and Methylated β-Cyclodextrins

The viscosity of hydrophobic ethoxylated urethane (HEUR) solution decreased in the presence of alpha-CD or m-beta-CD; however their interactions were quite different. When the alpha-CD/hydrophobe molar ratio exceeded 5.0, the viscosity was close to that of a PEO solution of similar molecular weight. Oscillatory shear indicated that the mechanically active chains in HEUR solution decreased with the addition of alpha-CD. This agreed with the hypothesis that alpha-CD formed an inclusion complex with the hydrophobic moiety of the HEUR polymer, thereby destroying the transient hydrophobic associative network. The viscosity/temperature relationship of the alpha-CD/HEUR system (for HEUR with 70% of the PEO chains capped at both ends) did not obey the Arrhenius relationship for alpha-CD/hydrophobe molar ratio in the range 0.8-5.0. The low shear viscosity increased with increasing temperature at molar ratio of 1.0, and this was attributed to the competitive complexation of the alpha-CD/hydrophobe and the alpha-CD/PEO chain. Increasing temperature favored alpha-CD/PEO complexation. Comparison between the behavior of alpha-CD/HEUR and m-beta-CD/HEUR resulting from the different binding characteristics was discussed.

Telechelic associative polymers: Interaction potential and high frequency modulus

Previous studies establish that adsorbed layers of telechelic polymers can exchange ends, increasing the entropy and allowing the loops to relax. The resulting entropic attraction amounts to approximately 1kT per bridging chain for ideal chains but decreases with increasing degree of stretching. From the interactions between planar brushes of highly stretched chains, we calculate the pair potential between spheres via the Derjaguin approximation. The softness of the repulsion and the depth of the attraction depend on the degree of stretching of the chains, the aggregation number or adsorption density, and the ratio of the overall size to the layer thickness. From the interaction potential, second virial coefficients in the osmotic pressure and the high-frequency modulus, characterizing dilute micellar solutions, follow directly. The second virial coefficient in the osmotic pressure is very negative with modest stretching but approaches the hard sphere limit as the chains become very strongly stretched. Th...

Rheology of telechelic associative polymers in aqueous solutions

Employing theory derived in previous papers [Meng and Russel, Macromolecules 36, 10112–10119 (2003); 38, 593–600 (2005)] to describe the micellar structure of aqueous solutions of telechelic polymers with a poly(ethylene oxide) (PEO) backbone and alkane end groups, we predict the high-frequency modulus and low shear viscosity for comparison with data from the literature and our own experiments. In these solutions the molecular structure determines the aggregation number and radius of the micelles. Interactions between micelles lead to exchange of end blocks and compression of loops, generating a pair potential with an attractive well and soft core. Computing the high-frequency modulus from the intermicellar potential and constructing a correlation for the relaxation time from experimental data allows us to predict the low shear viscosity. Comparison with measurements for our model associative polymers, as well as relatively monodisperse systems from the conventional synthesis, suggests an alternative to the structure-property relations constructed by Annable et al., J. Rheol. 37, 695 (1993) by modifying reversible network theory. The distinctions lie in our incorporation of excluded volume repulsions and entropic attractions between micelles, which produce strong correlations that determine the concentration dependence.

Rheology of Block Polyelectrolyte Solutions and Gels: A Review

We review recent experimental results on the rheology of block polyelectrolyte solutions, focusing mainly on diblock and triblock architectures and the effects of block polyelectrolyte concentration, block composition, solution pH, and ionic strength. Micellar solutions of diblock polyelectrolytes are typically viscoelastic liquids at a low concentration; however, gel formation can occur at higher concentrations when micelle corona chains begin to interpenetrate or interact. The rheology can be tuned by varying pH or ionic strength or through the addition of hydrophobic groups along the corona chain. Triblock architectures form elastic gels, analogous to neutral telechelic associative polymers. However, the charged nature of the backbone promotes the formation of a highly networked structure, enabling gel formation at lower concentrations than those observed with neutral telechelics. Finally, we briefly describe recent work on creating block copolypeptide gels with unique rheological and self-assembly characteristics and new architectures for creating thermoreversible block polyelectrolyte gels.

Deviations from the Stress−Optical Rule in Telechelic Associative Polymer Solutions

The stress−optical rule is known to be valid for a wide range of polymeric fluids, as long as the chains are only slightly extended. In solutions of associative polymers, strain hardening and shear thickening are known to occur, and non-Gaussian chain stretching has been proposed as an explanation for these phenomena. Hence, the stress−optical rule could be expected to break down for these materials. Here, a rheooptical investigation of a telechelic associative polymer is presented. Various nonlinear flow fields have been applied. When strain hardening or shear thickening occurs, it is shown that the stress−optical rule is indeed not valid anymore. An analysis of the rather small values of the strains at the onset of the nonlinearities suggests that the bridging chains are probably in a stretched state even in the absence of flow. On the other hand, the birefringence data indicate that the local deformation of the polymeric chains is more complex. The final result is that the mechanical behavior can be described quite well by a transient network model, assuming initially strained chains. The rheooptical data, however, prove that the actual structural changes in associative polymers are even more complex, possibly because of the presence of micelles.

The steady state and dynamic rheological properties of telechelic associative polymer solutions

In this study, steady state, dynamic shear and normal stress differences of the polymer solutions containing telechelic hydrophobically modified ethoxylated urethane (HEUR) associative polymers have been investigated. The intermediate steady state and dynamic shear thickening were observed for both ascending and descending shear rate and frequency, respectively. The dynamic amplitude stress sweep measurements showed that maxima in G′ and G″ occurred at the frequencies of 1, 5 and 10 rad/s. At lower shear rates and frequencies, the ratio of complex viscosity to steady shear viscosity was observed to be greater than unity. This phenomenon is believed to be associated with the formation of a micro-gel like structure. The modified Cox–Merz rule (Delaware–Rutgers rule) explained the dynamic and steady state viscosity behaviors well in the shear thinning zone but not at the plateau or shear thickening regions.

Characterization and rheological properties of dilute-solutions of three different families of water-soluble copolymers prepared by solution polymerization

Water-soluble polyacrylamides hydrophobically modified with small amounts ofN,N-dialkylacrylamides [N,N-dihexylacrylamide (DHAM) andN,N-dioctylacrylamide (DOAM)] have been prepared through free radical solution polymerizations using two hydrophobic initiators derived from 4,4′-azobis(4-cyanopentanoic acid) (ACVA) and long linear chains consisting of 12 and 16 carbon atoms (C12 and C16). This procedure resulted in polyacrylamides containing hydrophobic groups along the chain as well as at the chain ends.We compare the properties of this class of polymers, termed “combined associative polymers”, with those of the multisticker (with hydrophobic groups along the polymer chain) and telechelic (with hydrophobic groups at the chain ends) associative polymers. These materials were prepared using DHAM or DOAM and a hydrophobic initiator (ACVA) modified with alkyl chains of two different lengths. Polymers having molecular weights (M w ) of ca. 175,000 and hydrophobic contents [H] of ca. 0.8 mol% were prepared using 0.07 mol% of initiator relative to the total monomer feed. We investigated the effects that the type, localization, and concentration of the hydrophobic groups have on the viscosities of the associative polymer solutions.

Evaluation of a transient network model for telechelic associative polymers

A number of models are available to describe the rheological behaviour of associative polymers. Model evaluation has most often been limited to the linear properties and to a qualitative prediction of the characteristic shear thickening at intermediate shear rates. Several other nonlinear features of associative behaviour have, however, been identified. These include failure of the Cox–Merz analogy and an unusual response in nonlinear stress relaxation, large amplitude oscillatory flow and in superposition flows. Here, experimental results for such experiments are compared with the predictions of a transient network model, i.e. the Vaccaro–Marrucci model [J. Non-Newtonian Fluid Mech. 92 (2000) 261], which has only a single adjustable parameter to describe nonlinear behaviour. It is shown that the various nonlinearities are well predicted qualitatively. The only deviation is a substantial overprediction of the critical strain or strain rate for the onset of these nonlinearities. The effect of changing the value of the model parameters and of relaxing the model assumptions on the predictions is investigated. The results suggest that the presence of a small number of highly stretched chains could be responsible for the observed deviation from the experimental data.

Shear in Surface Gel of Associative Polymer

The normal and shear forces between adsorbed layers of telechelic associative polymer poly(ethylene oxide) end-terminated with octadecyl groups have been studied using a surface force balance. The normal forces profiles measured for polymer monolayers and surface gels were similar to those reported for the same polymer in earlier studies. The shear forces between the polymer-covered surfaces were very weak up to compression of ca. 0.27 of the original thickness of the polymer layers, corresponding to the effective friction coefficient μ < 0.04. Weak frictional forces were attributed to osmotic repulsion between poly(ethylene oxide) chains and shear thinning of the gelled polymer. At higher polymer compression the shear forces gradually increased due to compression-forced increase in interpenetration and entanglement of the macromolecules adhering to the two opposite surfaces and interactions between the octadecyl groups. Shear thinning was also observed for long-time shear and for shear velocity v s exceeding 600 nm/s. For strongly compressed polymer and v s < 600 nm/s the dependence of frictional forces on the shear velocity was weak, caused by a thinning of the interpenetration zone between the sheared polymer layers and/or a transition of the slip plane to the polymer-substrate interface.

General nonlinear rheological behavior of associative polymers

Shear thickening is very common in telechelic associative polymers. Yet, it is not a universal characteristic of the whole class of associative polymers. Here, more general features of associative behavior are investigated. In particular, nonlinear rheological properties have been studied for both associative hydrophobically modified ethoxylated urethanes and hydrophobically modified alkali-soluble emulsions (HASE) polymers. A known common characteristic is the failure of the Cox-Merz rule for the viscosities. The equivalent relation for the first normal stress is examined here, a parallelism with the results for the viscosities has been observed. More pronounced similarities between telechelics and the more complex HASE systems can be seen in some other nonlinear properties. The dynamic moduli display strain hardening for both moduli at intermediate strains at all but the lowest frequencies, even when no shear thickening can be detected in steady state shear flow. The linear relaxation functions differ quite strongly among associative polymers. However, they all display a specific non-linear relaxation. It includes short time strain hardening followed by strain softening starting at long relaxation times. The results indicate that the same microstructural mechanisms are responsible for the rheological behavior of both classes of associative polymer.

Study of three different families of water-soluble copolymers: synthesis, characterization and viscoelastic behavior of semidilute solutions of polymers prepared by solution polymerization

Polymer chains consisting of water-soluble polyacrylamides, hydrophobically modified with low amounts of N, N-dialkylacrylamides ( N, N-dihexylacrylamide (DHAM) and N, N-dioctylacrylamide (DOAM)) have been prepared via free radical solution polymerization, using two hydrophobic initiators derived from 4,4′-azobis(4-cyanopentanoic acid) (ACVA) containing long linear chains of 12 (C12) and 16 (C16) carbon atoms. This procedure resulted in polyacrylamides containing hydrophobic groups along the chain as well as at the chain ends. This class of polymers, termed ‘combined associative polymers’, has been studied and compared with the multisticker (with hydrophobic groups along the polymer chain) and telechelic (with hydrophobic groups at the chain ends) associative polymers, which were prepared with DHAM or DOAM and with the hydrophobic initiator (ACVA) modified with alkyl chains of two different lengths. The viscoelastic properties of these different families of associative polymers were investigated using steady-flow and oscillatory experiments. The effect of type, localization and concentration of the hydrophobic groups on the viscosity of the associative polymer solution was investigated. All viscosity curves clearly show two different regimes within the semidilute range: a first unentangled regime where the viscosity increases moderately; and a second entangled regime where the viscosity varies according to a power law, proportional to C 4. The relaxation time, T R, and the plateau modulus, G 0, showed relatively high values which increased with the number of carbon atoms in the hydrophobic groups. The combined associative polymer (PAM- co-DHAM/ACVA12) showed relaxation times that remained relatively constant along the concentrations studied, but very high values of G 0.

The effect of (2-hydroxypropyl)-β-cyclodextrin on rheology of hydrophobically end-capped poly(ethylene glycol) aqueous solutions

(2-Hydroxypropyl)-β-cyclodextrin (HPBCD), a modified cyclic oligosaccharide, changes the flow behavior of aqueous solutions of a model telechelic associative polymer, hydrophobically end-capped poly(ethylene glycol) (ODU-12), because the inclusion complexation of HPBCD and octadecyl end-capping groups interferes with aggregation of the polymer end groups. The HPBCD-induced decrease in the high-frequency modulus can be well described assuming a 1:1 binding isotherm, but cannot fully explain the decrease in the low shear rate Newtonian viscosity, because HPBCD also strongly decreases the relaxation time. Consequently, besides decreasing low shear rate Newtonian viscosity, HPBCD also extends the Newtonian behavior to higher shear rates as predicted by the free-path version of the transient network theory [Marrucci et al., Macromolecules 26, 6483–6488 (1993)].

Telechelic Associative Polymers: Interactions between Strongly Stretched Planar Adsorbed Layers

The interactions between planar brushes of adsorbed telechelic polymers are investigated via a self-consistent mean-field calculation via the strong stretching approximation. By allowing the relaxation of the loops upon the initiation of bridging, we obtain a stronger attraction than from Milner and Witten's analysis that excludes this effect. The entropy of ends is the origin of the attraction, but excluded-volume interactions play a role. The interaction potential in kT per chain is determined by the dimensionless separation and a single dimensionless parameter, the extent of stretching normalized by the size of a Gaussian coil.