Hydrophobically modified polymers

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]. 

Topical spermicides such as nonoxynol-9 (N9) and benzalkonium chloride act on sperm membranes through a detergent effect, namely, hydrophobe-hydrophobe interaction between the active and substrate (spermatozoa). The idea was to optimize the cationic/hydrophobic polymer in the drug delivery system so epithelial cells were protected without sacrificing the drug s spermicidal activity. One of the questions that needed to be answered in designing an optimum cationic/hydrophobe modified polymer was the effect of the hydrophobe on the drug activity (N9 initially, and other actives subsequently). 

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... 

In general, the microsegregated structures observed for hydrophobically modified polymers (Fig. 7) are similar to core-shell globules obtained via... 

The surface isotherms obtained with these hydrophobic modified polymers were interpreted in terms of hydrophobic and hydrophilic balance of the polymers. 

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. 

Figure 6. A schematic of the draining of a foam lamellae formed In the presence of a hydrophobe modified polymer. (Reprinted with permission from ref. 21. Copyright 1985 Allured Publishing.)...

Low charge density, hydrophobically modified polybetaines were shown to interact and comicellize with nonionic, anionic, cationic, and amphoteric surfactants [181-183] and many ionic organic dyes [264,265]. The association mechanism of hydrophobically modified polymers and surfactants in dependence on the concentration of interacting components can be modeled by two pathways (Scheme 21) [183]. 

Hydrophobically modified polymers can associate in aqueous media to form micelle-like structures above their critical association concentrations (CACs). The nanosized self-aggregates were prepared using modified natural polysaccharides such as pullulan, curdlan, and glycol chitosan. The modified polysaccharides provide excellent biocompatibility, biodegradability, low immunogenicity, and biological activities. 

Stabilization of Aqueous and Nonaqueous Suspensions with a Hydrophobically Modified Polymer... 

A number of vesicle-forming biohybrid amphiphiles based on either hydrophilically or hydrophobically modified polymers have very recently been described - see the chemical structures in Fig. 7. 

FIG. 20 Sketch of the gelation-fluidification process in protein/hydrophobically-modified polymer systems. 

The subject of surfactant-modified, water-soluble polymers, briefly discussed in Water-Soluble Polymers, is addressed in the last three sections (Chapters 16-28) of this book. These associative thickeners are covered in detail, ranging from the maleic acid copolymers of variable compositions introduced in various commercial markets in the early 1960s to the most recent entries (that is, in the open literature), hydrophobe-modified poly (acrylamide). Chapter 23 is complementary to the spectroscopic studies in Chapters 13-15 it explores new approaches to understanding associations in aqueous media. The three hydrophobe-modified polymers that have gained commercial acceptance in the 1980s, (hydroxyethyl)cellulose, eth-oxylate urethanes, and alkali-swellable emulsions, are discussed in detail. In particular, hydrophobe-modified (hydroxyethyl)cellulose, which is... 

Hydrophobically modified poly(acrylic acid) (HMPAA) thickeners, however, contain a certain proportion of long-chain alkyl groups (usually within Cs to C22 chain lengths). These alkyl groups set the hydrophobically modified polymers apart from their unmodified counterparts because they provide sites that allow anchoring of the polymer at an oil-water interface by hydrophobic interaction. The modified polymers, therefore, should behave as... 

In all cases, stabilization with unmodified polymer types A, B, C, and D yielded emulsions that broke within hours of preparation to give complete coalescence of the oil phase. Emulsions prepared with the hydrophobically modified polymer, however, were, in many cases, quite stable. The results for this hydrophobically modified polymer are summarized in Figures 21 and 22. Stable emulsions were achieved at all pH values studied at a concentration of 0.4% of the hydrophobically modified polymer. These emulsions exhibit stability for years at ambient temperature. For 0.2 and 0.6% polymer, however, emulsions were stable only below pH 8. 

The structure of the hydrophobically modified polymer is set by the number of micelles and the number of hydrophobic monomers present during polymerization. Determination of the structure of the polymer is a problem because of the extremely small concentration of the hydrophobic monomers in the polymer (< 1 mol %). Normal analytical techniques such as NMR are not suflBciently sensitive to determine the number or the sequence length of the hydrophobic regions in such polymers. 

HEUR polymers offer the greatest variation in architectural design of any hydrophobically modified polymer discussed in this text. This latitude in... 

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. 

Chem. Desa tp. Aq. sol n. of nonionic hydrophobe-modified polymer Uses Thickener for water-based paints, esp. acrylic, styrene-actylic, and vinyl-actylic systems... 

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