Series of copolymers ethylene oxide and alkylglycidyl ether (EO-co-AGE), ethylene oxide-3,3-di(alkoxymethyl)propyl glycidyl ether (EO-co-DAGE) and poly(alkyl glycidyl ether) ((POM/POE)–PAGE) have been synthesised, as associating water-soluble polymers, where the hydrophobic substituents (less than 1% mole fraction) are, respectively, randomly distributed along the chain or present as dimers or as long sequences. The synthesis of initial hydrophobic monomers and the copolymer preparation are shortly described. The associative properties of these new associating samples were studied as functions of length, distribution, mole fraction of hydrophobes and polymer concentration, using fluorescence, light scattering and viscosimetry. Generally, these comb-like associating polymers do not exhibit thickening properties as good as those of the equivalent telechelic polymers of the same chemical composition. This difference is attributed to the formation of intra-molecular hydrophobic association favoured in cases of comb structure with respect to the telechelic ones. However, a great influence of the sequentiality of the hydrophobe distribution was observed.
Introduction
The term “water-soluble associating polymer” is given to a polymer constituted by a hydrophilic skeleton that bears some hydrophobic groups either randomly distributed along the chain (grafted or comb-like) or fixed at one or two extremities (telechelic) [1]. The skeleton of branched associating polymers is most frequently polyacrylamide (PAM) [2], [3], [4], [5], [6], [7], polyacrylic acid [8], [9] or acrylamide–acrylic acid copolymers [10], [11]. Telechelic polymers are more usually based on poly(ethylene oxide) (PEO) [12], [13], [14], [15], [16], [17], [18]. Hydrophobic groups can be either aliphatic (with a number of carbons ranging between 8 and 20), aromatic or fluorinated. Many works have been devoted to such polymers because of their attractive properties. In particular, it is well known that aqueous solutions exhibit shear thickening and shear thinning behaviours owing to the fact that hydrophobic groups gather in nano-domains, which act as temporary cross-links. Different techniques have been used to elucidate the association mechanism and relate the solution structure and rheological properties (fluorescence [17], [18], [19], light [20], [21], [22], [23], X-rays and neutron scattering [15], [16], NMR [24], [25]).
Very often, rigorous interpretation is made difficult by the sample polydispersity and by a bad control of its chemistry. For example, it is known that the distribution of the hydrophobic groups along the chain plays an important role, a sequential distribution leading to a higher viscosity of the aqueous solutions than a random one [26]. However, owing to the very low amount of hydrophobes, sequentiality cannot be directly evaluated by NMR, but only deduced from kinetics arguments.
On the contrary, telechelic polymers, prepared from PEO, constitute the best models for fundamental studies, since they have a very low polydispersity and the degree of functionalisation of the extremities can be perfectly controlled [27], [28], [29]. Besides, methods of characterisation have been developed for such polymers [29]. Thus, a good picture emerges from many physico-chemical and rheological investigations, as schematised in Fig. 1. At a given critical concentration, CAC (critical association concentration), ω or α,ω-functionalised PEO associate in micelles or “flower-like” aggregates, respectively. Recent light scattering studies show that this first association step can be considered as a “closed association”, which means that it is rather co-operative. In the second step, above a second critical concentration, which corresponds to the overlap of micelles or “flowers”, viscosity diverges from that of unmodified PEO [22], [23]. Very soon, for a liquid order appears in the solutions as revealed by a small angle peak, in X-rays or neutron scattering curves [15], [16]. For the more-associated systems, molecular weight of PEO below 6000, several narrow peaks (up to seven) indicate that the structure is cubic and for the less-associated ones, the broad peak corresponds to a degeneration of such structure. It has been shown that the elastic modulus at the plateau and the viscosity enhancing is related to a reinforcement of the organisation [29].
Whatever the systems are, branched or telechelic, the dependencies of rheological properties on number, length and chemical nature of hydrophobic groups is empirically well known. However, there are very few works dealing with the advantages and disadvantages of the one type of architecture (branched or telechelic) with respect to the other. This is due to the chemical problems encountered in preparing the two types of architecture with exactly the same chemical composition. An attempt was made to prepare telechelic associating polymers from PAM [28], but the samples were polydisperse and it was not possible to obtain directly difunctionalised samples alone; then, a separation step became necessary.
PEO seems to be a better candidate to make such a systematic comparison between branched and telechelic architecture. In this work, we present the synthesis of hydrophobic PEO with pendant aliphatic chains. We have focused our attention on the problem of the hydrophobe distribution. For this purpose, copolymerisations of ethylene oxide with comonomers or macromonomers having one or more aliphatic groups were performed (as schematised in Fig. 2). The association was studied by fluorescence, light scattering, neutron scattering and viscosimetry. The phase diagrams of these copolymers were also established. The results were systematically compared with those previously obtained with telechelic PEO.
Section snippets
Copolymer synthesis methods
(i) Ethylene oxide–alkyl glycidyl ether copolymers (EO-co-AGE). In a first step, alkyl glycidyl ethers (AGE) were obtained by reaction of epichlorydrine (EC) with the corresponding alcohol, in the presence of sodium hydroxide and trimethylammonium hydrogenosulphate ((But)4NHSO4), as described in Ref. [30]. Reaction 1:Copolymerisations of EO and AGE were performed with several catalysts of the type described by Vandenberg [31], [32], in various solvents, as reported in Table 1.
(ii) Ethylene
Results
Table 1, Table 2give information about sample synthesis and characteristics: molar fraction of pendant chains τ and weight average molecular weight. Sample nomenclature is defined by one letter M, D or B for (EO-co-AGE), (EO-co-DAGE) and (MO/PEO-co-PAGE), respectively. The first number is the total number of carbons in pendant aliphatic chains n′: if the precursor alcohol has six carbons, n′=8. The second number is τ×100. The third one is the weight average molecular weight divided by 1000.
Discussion
These experimental results show that grafted or comb-like samples behave very differently from α–ω telechelic ones, when compared at the same average grafting ratio.
The main differences are:
•
a higher solubility characterised by a lower discrepancy of LCST with respect to that of the parent PEO, at least for τ<0.7%;
•
lower values of CAC, measured by fluorescence but a much broader range where I1/I3 values change from the value characteristic of an aqueous environment to that corresponding to a
Conclusions
The synthesis of a series of comb-like hydrophobically modified PEO and the physico-chemical studies of their association in aqueous solution have allowed us to demonstrate the influence of polymer architecture on the hydrophobic interactions. We conclude that a comb-like architecture favours intra-molecular association with respect to the inter-molecular ones, which leads to aqueous solutions of much lower viscosity than these of telechelic polymers. On the other hand, we confirm that for
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Hydrophobe polymers, performance with environmental acceptability
Water-soluble polymers have been used as drag reducer in slick-water hydrofracking for unconventional hydrocarbons, however, the fatal “thermothinning” behavior and weak salt tolerance significantly impedes their practical use in more hostile environment. To alleviate these issues, here we present the first attempt to use a polyether-based thermoviscosifying polymer (TVP) as the drag reducer. Comparative studies on rheological behaviors and drag reduction performance were performed using TVP and currently-used partially hydrolyzed polyacrylamide (HPAM) with similar molecular weight. It is found that TVP solutions show thermothickening ability even at concentration as low as 200 mg⋅L−1, whereas HPAM solution only exhibits thermothinning behavior. The drag reduction of TVP is 73.47% while that of HPAM is 64.29% in 5.0 × 104 mg⋅L−1 NaCl saline water. More significantly, TVP provides improving drag reduction performance at elevated temperature. A higher drag reduction of 79.77% can be attained by TVP at 65 °C as compared to 70.18% for HPAM. The TVP displays a high drag reduction retention rate (94.57–97.77%) after shearing for 1 h at temperature range of 35–65 °C. These findings offer new possibilities for developing drag reducers for slick-water hydrofracking under harsh conditions.
To further demonstrate the existence of aggregates, fluorescence spectroscopy characterization and TEM imaging were carried out. The ratio I1/I3 of intensities of the first and the third vibronic peaks for the emission spectrum provides an estimate of the relative hydrophobicity of the local environment [52,53]. As shown in Fig. 4A, the I1/I3 values of HPAM-1 and HPAM-2 in the entire range of concentration remain unchanged and are close to that of pure water (1.7) [54], which can be interpreted as the immunity of pyrene to amide functions in aqueous solution [52].
Considerable success has been achieved in enhanced oil recovery by polymer flooding from high- to medium-permeability reservoirs; however, the poor injectivity and ease of mechanical degradation of the commonly used high-molecular-weight (high-MW) partially hydrolyzed polyacrylamides (HPAM) impede their use in low-permeability reservoirs. To alleviate these issues, we proposed the use of the self-adaptive polymer (SAP). SAP with a relatively low MW (8.7 × 106 g mol−1), a rigid unit, and hydrophobic pendant group was prepared and compared with its counterparts HPAM-1 (8.2 × 106 g mol−1) and HPAM-2 (24.9 × 106 g mol−1). It was found that while the molecular size of SAP is similar to that of HPAM-1, its thickening capability is stronger under identical conditions, and both of them can be smoothly injected into the 60-mDarcy cores. By contrast, HPAM-2 causes fatal plugging in this scenario. The favorable injectivity of SAP solution stems from its reversible viscosity change in response to shear forces without any deterioration, which is achieved by the association-disassociation transition of molecular chains. In the elongational field, both SAP and HPAM-1 do not degrade mechanically, as opposed to the case of HPAM-2. More significantly, SAP can improve oil recovery up to 18.7%, which is around three times higher than that of HPAM-1. This self-adaptive concept may open a new pathway for the molecular design of polymers used for enhancing oil recovery in low-permeability reservoirs.
Controlled polymerization of long-chain alkyl glycidyl ethers (AlkGE) under anionic ring opening polymerization conditions is enabled by the addition of a crown ether (18-crown-6, K+). Homopolymers with molecular weights in the range of 4000 to 9000 g mol−1 and side chain melting temperatures of 14 °C (C12-AlkGE) and 43 °C (C16-AlkGE), respectively were synthesized. Furthermore, a series of amphiphilic ABA polyether triblock copolymers with polyethylene glycol (PEG) as the hydrophilic central block (Mn = 6k, 10k, and 20k g mol−1) with total molecular weights in the range of 7000 to 28 000 g mol−1 and narrow dispersity (1.12 to 1.34) were prepared. Separate melting endotherms of the triblock copolymers were observed for these double-crystalline materials. Due to self-assembly of the hydrophobic alkyl chains, the addition of water or water/ethanol mixtures to the triblock copolymers leads to the formation of micellar, one-component hydrogels. By varying the repeating units of the hydrophobic PAlkGE blocks, a tunable hydrophobic effect can be achieved. Variation of the alkyl chain length enables tailoring of the melting temperatures of the PAlkGE-blocks both in bulk and hydrogels. Thermal and rheological investigation of ethanol/water gels revealed temperature-dependent mechanical behavior, indicating a transition from crystalline to flexible micellar hydrophobic domains in the range of 12 °C to 31 °C. The amphiphilic polyether triblock copolymers enable the preparation of guest-loaded thermo-responsive gels and hydrogels and bear promise for pharmaceutical applications, e.g. as drug release systems.
That is why an intense micellization of TBCs in the range of high ethanol content could be attributed to a selective insolubility of PAAm blocks. In this case, the appearance of “flower-like” micelles, which are well known for A-b-B-b-A triblock copolymers with insoluble side blocks or for hydrophilic polymers with some hydrophobic grafts [36,37], could be waiting (the right part of the scheme in Fig. 4d). Insoluble “tails” of PAAm would form a large micellar “core” but soluble “loops” of PEO would be concentrated in a “corona”, thus providing stabilization for the whole micellar structures.
Micellization of asymmetric triblock copolymers (TBCs) contained chemically complementary poly(ethylene oxide) (Mn = 6, 14 and 35 kDa) and polyacrylamide of different chain length (PAAm-b-PEO-b-PAAm) was studied using visible spectroscopy, static light scattering, photography and transmission electron microscopy. The formation of “hairy-type” and “flower-like” micelles consequently in aqueous and aqueous/ethanol (30/70 v/v) solutions was established. An unusual morphology of the “hairy-type” micelles was found in aqueous solutions of TBC, which comprised the lengthiest PEO and PAAm blocks. Significant encapsulation of a model crystalline drug prednisolon (PS) by the “hairy” micelles due to hydrogen bonds and hydrophobic interactions was determined by ultraviolet–visible spectroscopy and Fourier transform infrared spectroscopy. This resulted in the appearance of “snow-flakes-like” micellar structures. The possibility of regulating crystalline properties of the drug in micellar nanocontainers was shown with differential scanning calorimetry and wide-angle X-ray scattering.
When each of the hydrophobic moieties consist of two hydrocarbon groups the thickening capability is significantly enhanced. This is similar to the behavior observed for twin-tailed HMPAM polymer 15 [321]. The hydrophobicity of the groups increases and therefore a stronger interaction gives rise to the observed enhanced thickening capability.
Recent developments in the field of water-soluble polymers aimed at enhancing the aqueous solution viscosity are reviewed. Classic and novel associating water-soluble polymers for enhanced oil recovery (EOR) applications are discussed along with their limitations. Particular emphasis is placed on the structure–property correlations and the synthetic methods. The observed rheological properties are conceptually linked to the polymer chemical structure (1) and topology (2). In addition, the influence of external parameters, e.g. temperature, pH, salt, and surfactant, on the rheological behavior is reviewed. Progress booked in deeper understanding of the structure–property relationship is thoroughly discussed. Furthermore, a critical overview of the synthetic methods as well as of the solution properties of these polymers is provided. In this respect the influence of “internal” (i.e. chemical structure) and “external” (vide supra) factors on these properties provide a conceptual toolbox for the rationalization of the response of water-soluble polymers to external stimuli. In turn, such rationalization constitutes the basis for the design of new polymeric structures for EOR applications.
Just like star copolymers, the multimolecular micelles may be a kind of multimolecular aggregates with the basic building units of unimolecular micelles, as reported by Hong et al. [15]. The large micelles may be connected together by lyophobic interaction of the PS side chains and necklace-like morphologies of the micelles are formed [46]. The self-assembly of EC-g-PS dense graft copolymers with different side-chain lengths in acetone was studied.
The self-assembly of the dense graft copolymers ethyl cellulose-graft-polystyrene (EC-g-PS) in acetone was investigated by using dynamic light scattering (DLS), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). It was found that EC-g-PS copolymers can self-assemble into spherical micelles in acetone. The micelles' size increases with increasing the copolymer concentration and the PS side-chain length. Moreover, the copolymers can self-assemble into multimolecular micelles at relatively high polymer concentration while unimolecular micelles can be formed at low concentration. The micelles have a core–shell structure with the ethyl cellulose main chains in the shell and the side polystyrene chains in the core of the micelles.
