Hydrophobically modified polymers solutions

Abstract This paper reviews possible phase diagrams of associating polymer solutions in which phase separation and molecular association interfere. Paying special attention on the structure and reorganisation of the network junctions, we study competition between phase separation and gelation. The molecular structure of associating micelles, or multiple cross-link junctions, in the networks is analyzed from the sol/gel transition lines. The effect of added surfactants on the formation of reversible gels in hydrophobically modified polymer solutions is also studied under the assumption of the existenee of a minimum multiplicity required for stable cross links. To describe... 

Recently, many studies have focused on self-assembled biodegradable nanoparticles for biomedical and pharmaceutical applications. Nanoparticles fabricated by the self-assembly of amphiphilic block copolymers or hydrophobically modified polymers have been explored as drug carrier systems. In general, these amphiphilic copolymers consisting of hydrophilic and hydrophobic segments are capable of forming polymeric structures in aqueous solutions via hydrophobic interactions. These self-assembled nanoparticles are composed of an inner core of hydrophobic moieties and an outer shell of hydrophilic groups [35, 36]. 

In a dilute solution, when the polymer is in a coil state (Fig. 6a), the diffusion of hydrophobic particles into the coil is normally faster than the chemical reaction [53]. In this case, the local concentration of particles H inside the coil is practically the same as in the bulk. Therefore, we expect that at the initial stage, the reaction will lead to a random copolymer some of the P monomeric units will attach to H reagent and thereby they will acquire amphiphilic (A) properties P + H —A (Fig. 6b). As long as the number of modified A units is not too large, the chain remains in a swollen coillike conformation (Fig. 6b). However, when this number becomes sufficiently large, the hydrophobically modified polymer segments would tend to form... 

SURFACE ACTIVITY. The surface tension results for aqueous solutions of Polymer JR and Oxiatrlsof t are given In Figure 1. The hydrophobe modified polymers clearly show more surface activity than the unmodified polymer. The surface activity of the modified polymers as measured hy the surface tension criterion Is only moderate compared to conventional surfactants which exhibit ultimate surface tension values In the range of 20-40 mN/m. The effect of the molecular changes resulting In this moderate surface activity can, however, be considerable on other properties of the polymer, as will- be shown In subsequent sections. 

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). 

Similarly, Wasserman and coworkers have studied a wide selection of polymeric materials in aqueous solution that are associative of some kind, i.e., that form some sort of self-assembly through non-covalent interactions [96]. Their study mainly deals with hydrogels of hydrophobically modified polymers, aqueous solutions of polymeric micelles created by block copolymers, and hydrogels based on poly (acrylic acid) and macrodiisocyanates. The spin probes of choice were hydrophobic, such as 5- and 16-DSA (see Eig. 2) or even spin labeled polymers. It was, e.g., possible to screen for the effect of chemical stmcture on the gel formation by recording and understanding the local mobility of the hydrophobic, long chain spin probes as a function of temperature. 

Figure 20.10. Addition of sodium dodecyl sulfate to a solution of a hydrophobically modified polymer (EHEC) gives a strong increase in viscosity at first and then a decrease to a low value. For the unmodified polymer, changes in viscosity are small. (By courtesy of K. Thuresson)...

As surfactants will compete for hydrophobic sites in an aqueous solution (or interface), it can be expected that many properties of hydrophobically modified polymers will change, depending on the exact conditions of the solution. These can include increases, or decreases, in viscosity and such properties as foaming, emulsification, and wetting. For example, Danino et al. (94) report a rather complete study, using several techniques, of the polyamphiphile poly(disodium maleate-coalkylvinylether) and its mixtures with anionic or nonionic surfactants that have a disruptive effect on the association of this polymer. 

Wang Y, Han B, Yan H. Microcalorrmetry study of interaction between ionic surfactants and hydrophobically modified polymers in aqueous solutions. Langmuir 1997 13 3119-3123. 

Piculell L, Thuresson K, Lindman B (2001) Mixed solutions of surfactant and hydrophobically modified polymer. Polym Adv Technol 12 44... 

The excimer fluorescence studies of hydrophobically modified polymers, developed by Winnik, Yekta, Duhamel, and others, which have been widely used also by other groups for solution of various problems, are treated in a chapter Pyrene-Labeled Water-Soluble Macromolecules as Huorescent Mimics of Associative Thickeners. ... 

When So = 2, this model reduces to the variable multiplicity model in which junctions of arbitrary multiplicity can coexist at the probability determined by the thermodynamic balance. In the case of micro-crystalline junctions, for instance, it is natural to assume that a minimum number Sq greater than 2 of the crystalline chains is required for a junction formation. This is because, the surface energy terms will prevent small-k units from being stable, leading to the existence of the critical multiplicity for the nucleation of the crystallites. Similarly, a minimum aggregation number is required for the stability of micelles formed by hydrophobes on water-soluble polymers. As we will see later, surfactants added to the solution cause complex interaction with hydrophobically modified polymers due to the existence of this minimum multiplicity. 

Panmai, S., Prud hortune, R. K., Peiffer, D. G. et al. (1998) Rheology of solutions of hydrophobically modified polymers with spherical and lod-hke surfactant micelles. Polym. Mater. Sci. Engng, 79, 419-420. 

Fluidized aqueous suspensions of 15% by weight or more of hydroxyethyl-cellulose, hydrophobically modified cellulose ether, hydrophobically modified hydroxyethylcellulose, methylcellulose, hydroxypropylmethylcellulose, and polyethylene oxide are prepared by adding the polymer to a concentrated sodium formate solution containing xanthan gum as a stabilizer [278]. The xanthan gum is dissolved in water before sodium formate is added. Then the polymer is added to the solution to form a fluid suspension of the polymers. The polymer suspension can serve as an aqueous concentrate for further use. 

A class of systems extensively investigated by means of PFG-NMR are colloids. They are usually hydrophobically modified water-soluble polymers, that is, polymers with a water-soluble skeleton bearing one or more hydrophobic units, which allow the self-assembling of the polymer in water solution and the interaction with surfactants.77... 

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