Effect of temperature on the viscoelastic behaviour of entangled solutions of multisticker associating polyacrylamides
Introduction
The incorporation of a few hydrophobic groups in a hydrophilic macromolecular chain results in systems with unique rheological characteristics in aqueous solution [1], [2], [3], [4], [5], [6], [7]. Above a certain polymer concentration, the hydrophobic moieties associate and build a transitory three-dimensional network. Recently, we have reported measurements of linear and non-linear viscoelasticity for aqueous solutions of multisticker polymer chains consisting of hydrophobically modified polyacrylamides (HMPAM), in which the number and length of the hydrophobic blocks could be adjusted in a controlled manner [8], [9]. This was achieved by free radical copolymerization of acrylamide (AM) with a hydrophobic comonomer solubilised into micelles dispersed in an aqueous continuous medium [10]. An appropriate choice of the hydrophobic comonomer, the N,N-dihexylacrylamide (DiHexAM), allowed us to obtain samples homogeneous in composition with an average copolymer composition independent of the degree of conversion [11].
The main conclusion drawn from the rheological studies performed on these samples was the existence of three distinct concentration regimes: (i) a dilute regime where the viscosity is essentially controlled by intramolecular interactions, (ii) a semidilute unentangled regime, dominated by intermolecular hydrophobic associations, and (iii) a semidilute entangled regime for which the viscoelastic behaviour can be described by a sticky reptation mechanism [12], [13]. The results obtained in the latter case suggested that the dynamic properties are strongly dependent on the hydrophobic characteristics of the copolymer whereas the elasticity is mainly controlled by the density of entanglements and only depends on the polymer concentration.
A priori, the above conclusion could be comforted from the comparison between the elastic behaviour of entangled solutions of hydrophobically modified and unmodified polymers, respectively. If entirely controlled by the entanglements, the plateau modulus, G0, which is proportional to kBTνe (where kB is the Boltzmann constant, T is the absolute temperature, and νe is the entanglement density), should then be the same for both materials. Furthermore, the quantity G0/T should be independent on temperature for both modified and unmodified polymers.
Some additional information can also be given by studying the samples in the presence of a surfactant susceptible to bind to the hydrophobic blocks contained in the associating polymer. It is indeed well known that the interactions of hydrophobically modified water-soluble polymers with small surfactant molecules affect drastically their rheological behaviour [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. In particular, experiments performed on semidilute solutions of HMPAM showed a strong increase of the terminal time of the stress relaxation at surfactant concentrations around its critical micelle concentration, CMC, whereas the plateau modulus was found to be constant over an extended range of surfactant concentrations [27].
In this paper, we report measurements of linear and non-linear viscoelasticity of HMPAM solutions in the entangled regime. The temperature dependences of the rheological parameters (terminal time, zero shear viscosity and plateau modulus) have been compared to those obtained for the unmodified parent polyacrylamide. The effect of the addition of surfactant (sodium dodecyl sulfate) on the rheological behaviour of these systems was also studied as a function of temperature.
Section snippets
Experimental section
The synthesis of the samples has been described in detail in previous papers [9], [11], [28]. The associating copolymers are polyacrylamides hydrophobically modified with a small amount of N,N-dihexylacrylamide (DiHexAM). They were synthesized in aqueous solution by using the micellar technique of Valint et al. [29] with sodium dodecyl sulfate (SDS) as the surfactant and 4,4′-azobis(4-cyanovaleric acid) (ACVA) as the initiator. In this process, the high density of hydrophobic molecules in the
Data analysis. Oscillatory experiments
Fig. 1 shows typical frequency dependences of the storage modulus G′(ω) and of the loss modulus G″(ω) for the homopolyacrylamide PAM and the corresponding hydrophobically modified HMPAM. At low frequency, the behaviour of the complex shear modulus is Maxwellian, as ascertained by the variations of G′(ω) and G″(ω) that scale, respectively, like ω2 and ω. The curves G′(ω) and G″(ω) cross each other at a circular frequency ωc. The inverse of ωc is often taken as the characteristic time of the
Conclusion
This study shows that in entangled solutions of multisticker associating polyacrylamides, a change in temperature or addition of surfactant affects essentially the lifetime of the crosslinks formed by the associating units, whereas the plateau modulus remains roughly unchanged. However, the comparison between the moduli of a hydrophobically modified polymer and the unmodified analogue reveals that the elasticity of the solution is strongly enhanced by the presence of associating units. This
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