Hydrophobically associating polymers intermolecular association

Because of their chemical similarity, the polymer-bound hydrophobes have a tendency to interact with the hydrophobic part of the surfactant molecule. If the surfactant concentration in the system is high enough, micelles are formed. If there are enough micelles in the system, then all the hydrophobes will get bound to micelles (Figure 14). As a result, there will be no intermolecular hydrophobic association (Figure 2) and no viscosity... 

The nature of hydrophobic interactions and their effects on the structure and properties of water have been extensively studied, particularly for small molecules (i 3). In contrast, the introduction of hydrophobic associations into synthetic water-soluble polymers to control solution rheology has received rather limited and recent study (4-7). To better understand the relationships between polymer structure and solution properties, we have synthesized and characterized a series of copolymers of acrylamide and N-substituted alkylacrylamides and terpolymers containing anionically charged carboxyl groups. Solution properties of these systems have been obtained in both the dilute and semidilute concentration regime, to probe the influence of intra- and intermolecular interactions. In addition, the influence of the shear field and solvent quality on the associations was studied. 

Terpolymer solution behavior is dependent on compositional microstructure, polymer concentration, ionic strength, pH, temperature, and shear history. The viscosities of these low charge density terpolymers would be expected to be insensitive to changes in ionic strength below the critical overlap concentration, C above C intermolecular attractive forces might be expected to cause enhanced viscosities similar to those of hydrophobically associating copolymers. 

The associative mechanism of thickening has been variously described, but is generally thought to result from nonspecific hydrophobic association of water-insoluble groups in water-soluble polymers 34, 35). For associative ASTs, the terminal hydrophobes of the ethoxylated side chains are considered to be the primary interactive components. These hydrophobes can interact with each other via intermolecular association, and can also interact with hydrophobic particle surfaces when present. The specific interaction with dispersed-phase components such as latex particles has been shown to be one of surface adsorption (36). In essence, the associative component of thickening in dispersed-phase systems also has dual character resulting from the building of structure within the aqueous phase and interaction with particle surfaces. 

These can be made by copolymerizing two monomers, one of which contains a side-chain such as Cio or C12, e.g. the copolymerization of norbomene dicarboxylic esters with 5-decylnorbom-2-ene. Alternatively, the homopolymer of the diester can first be hydrolyzed to the acid form and then partially reacted with 1-dodecylamine. The viscosity of aqueous solutions of such hydrophobically modified polymers increases sharply with increase in concentration as a result of intermolecular association (McArdle 1995). 

By T,-type, we mean polymers that exhibit inverse temperature transitions in which the protein-based polymers hydrophobically associate on raising the temperature. T, represents the onset temperature for the transition. For the elastic-contractile model proteins of interest here, the inverse temperature transition is seen as a phase separation resulting from both intermolecular and intramolecular hydrophobic association. On raising the temperature... 

VID Vi l, R.R.L., Balaban, R., and Borsali, R., Amphiphihc derivatives of carboxy-methylcellulose Evidence for intra- and intermolecular hydrophobic associations in aqueous solutions, Polym. Eng. Sci., 48, 2011, 2008. 

The effect of hydrophobic association on viscosity in the semi-dilute regime is different from that observed at low polymer concentrations. Figure 40, from Bock et al. [89], shows the variation of the reduced viscosity with polymer concentration for polyacrylamide and N-octylacrylamide copolymers having hydrophobe contents of 0.75 and 1 mol%. At a hydrophobe content of 0.75 mol%, the viscosity significantly increased because of intermolecular association. Increasing hydrophobe content further to 1 mol% resulted in higher viscosities. 

Hydrophobic association is also enhanced in polymer systems. Although polymer surfactants are considered to form micelles via intrapolymer hydrophobic interaction, our recent study (2r,s) revealed that a polyionene bearing anthryl groups as the hydrophobic domain showed a clear cmc UHtical micelle concentration) at the segment concentration around 3 x 10 5m. Reference experiments with a polyionene without anthryl groups and the monomer and dimer model compounds have indicated that the cmc is particularly low for the polymer. Taking the excimer intensity of anthracene fluorescence as an index of interchromophore interaction, we confirmed the existence of interpolymer association by the concentration dependent excimer intensity. Under the same condition to the polymer, any model systems either monomeric or dimeric do not associate intermolecularly. 

Compared with the massive increase in tjo observed with increasing polymer concentration, the ratio /e//m was fotmd to increase to a much lesser extent. While t]o increased by seven orders of magnitude when the Py(35)-HASE concentration was increased from 1 to 45 g the /e//m ratio increased by less than threefold. Nevertheless, the increase in 7e//m was noticeable and indicated that intermolecular interactions occurred. Furthermore, it took place in the same concentration range where ijo increased suggesting that the increase in intermolecular hydrophobic association between the pyrene labels coincided with the increase in r/o, an expected result if it was due to the formation of an extended polymeric network between Py-HASE molecules. 

However, the rheological behaviors of associative polymers can be divided into a dilute region and a semi-dilute region by a critical concentration c. At concentrations below c, it is in a dilute region where intramolecular hydrophobic associations within the polymer dominate the behavior of the polymer, and the viscosity decreases with the solution concentration increase. At a concentration above c, it is in a semi-dilute region where intermolecular associations between polymers control the polymer rheology and the viscosity increases with the solution concentration increases (Taylor and Nasr-El-Din 2007). 

Rheological studies of solutions of these polymers in distilled water clearly identified viscosity enhancement due to the presence of the hydrophobic tert butylstyrene endblocks. Comparison with homopolymer and random copolymers of similar composition and molecular weight demonstrated the increased viscosification with the block configuration. The block polymer solutions were also observed to solubilize toluene with a marked increase in the solution viscosity. These results are interpreted as the result of polymer intermolecular association through the hydrophobic blocks similar to micellization of surfactants. 

Additional evidence for intermolecular hydrophobic association is provided by the effects on the Brookfield viscosity by addition of NaCLThus, the viscosity of the copolymer solution increases with increasing NaCl concentration, especially at lower shear rate (0.4 s ) (Fig. 7.6). At polymer concentrations above 2000 ppm, the viscosity increases are especially pronounced between 1.0 and 5.0... 

Figure 7.15 shows that the apparent intrinsic viscosity of modified polymer (14-4) is higher (10.9 dl/g) than that of unmodified HEC (6.6 dl/g). This fact is consistent with the existence of hydrophobic association formed by the bound fluorocarbon chains in this system. Apparently, intermolecular association of sample 14-4 still prevails at the lowest concentration ( 0.06 gr/dl). 

All the described properties of such a s-fraction of poly(NVCl-co-NVIAz) synthesized at the temperature above the PST of the reacting system allowed us to draw the conclusion that the chains of this type had the comonomer sequence, which at the temperatures above the conformation transition facilitated the formation of polymer particles, where H-blocks are in the interior shielded by the P-blocks against additional intermolecular association. Such a behaviour of this copolymer in aqueous media is close to that of oligomeric proteins similar to casein [46] possessing a rather hydrophobic core surrounded by the polar segments. 

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