Hydrophobically modified associating polyacrylamide (HAPAM) synthesized by micellar copolymerization at high monomer concentration
Introduction
Hydrophobically modified associating polyacrylamide (HAPAM) and its partially hydrolyzed analogues are now becoming a class of prospective candidates as thickeners or rheology modifiers for use in the formulations of tertiary oil recovery, drilling fluids, hydraulic fracturing and drag reduction. Due to association of hydrophobe moieties in nanodomains, their aqueous solutions exhibit very interesting rheological properties and better stability with respect to salts than the unmodified precursor, polyacrylamide (PAM) and partially hydrolyzed polyacrylamide (HPAM) [1], [2], [3], [4], [5], [6], [7], [8], [9], [10].
Practically, most of acrylamide-based polymers used in petroleum exploitation activities are actually partially hydrolyzed to introduce some carboxylic groups into the chain. The presence of these charged units improves water solubility and increases hydrodynamic volume of the chain due to the mutual repulsion of the negative charges. The effect of charged groups introduced into the acrylamide backbone of HAPAM remains a matter of debate in literature. To the best of our knowledge, only a few studies so far dealt with HAPAMs containing both hydrophobic and carboxylic groups from hydrolysis process [11], [12], [13].
However, almost all these HAPAMs were prepared via micellar copolymerization, and it is well known that obtained polymers are characterized by blocky distribution of the hydrophobes, compositional inhomogeneity and strong dependence of solution properties on block length [10].
The size and bulkiness of the hydrophobe can be varied by considering linear or branched alkyl chains of various lengths, typically C8–C12. Aryl or alkylaryl groups can be advantageously used for UV or fluorescence analysis. Fluorocarbon groups provide stronger hydrophobic interactions. Additionally, the hydrophobic monomer can also bear a zwitterionic group [10].
A well-known limitation of solution polymerization processes is the rather low monomer concentration used in order to prevent a too high viscosity of the reaction mixture and is favourable to heat diffusion, especially in the case of PAM where high molecular weights are usually obtained (1–5 × 106 g mol−1). For this reason, in all the studies reported on micellar copolymerization, the total monomer concentration was generally in the range 2–6 wt% [10]. However the generation of this polymer lowers production efficient and industrial drying process is difficult under this low concentration. An initiation system under lower temperature and using higher monomer concentration has been studied. This system releases free-radicals slowly, so the reaction in the initial stage conducts smoothly and steadily. The amount of initiator per molar monomer in this system is smaller than those used in low monomer concentration system. Those above mentioned factors are favourable to obtain high molecular weight polymers.
The main goal of the present work is to synthesize and characterize HAPAM containing both long hydrophobic alkyl chains (C18) and carboxylic groups from hydrolysis process. We have used micellar polymerization at unusual high monomer concentration (∼19 wt%) and the samples were carefully characterized by means of several techniques including light scattering, viscometry, 1H and 13C NMR spectroscopy. In the second part, we have discussed the rheological behaviour in brine according to micellar copolymerization parameters.
Section snippets
Materials
All solvents and reagents of the best grade available were used without further purification. The water was distilled three times with an all-glass apparatus. N-octadecylacrylamide was prepared according to literature [14].
Micellar copolymerization
A 500 ml three-necked flask equipped with a thermometer and mechanical stirrer is charged with 3.5 g of N-octadecylacrylamide, 310 g of deionized water and appropriate amount of sodium dodecylsulfate (SDS). The mixture is stirred until a homogeneous phase appears in the
Micellar copolymerization and hydrolysis
The synthesis of HAPAM by micellar copolymerization was largely described by Candau et al. [10] and derived from a method described by Valint et al. [16], [17] and Turner et al. [18], [19]. In the micellar copolymerization process, the hydrophobic monomer is solubilized within surfactant micelles, whereas the hydrophilic monomer is dissolved in the aqueous continuous medium. The reaction mixture is optically transparent, but one should be aware that it is actually a microheterogeneous system
Conclusion
In this work, a series of hydrophobically modified polyacrylamides and their hydrolyzed derivatives have been prepared via micellar polymerization at high monomer concentration (19 wt%) and post-hydrolysis. 1H and 13C NMR elucidation of polymer microstructures displayed a good agreement with feed ratio. Post-hydrolysis process seems to be a more effective route to control the hydrolysis degree. Physico-chemical parameters determination by ACM techniques in dilute solution shows that samples have
Acknowledgments
The authors acknowledge the assistance of G. Clisson in performing ACM experiments, F. Erhenfeld for the software and A. Khoukh in performing NMR experiments. We thank RIPED for its financial support and Dr. Jeanne Francois for helpful discussions.
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