Amphipathic copolymers

CMC of the amphipathic copolymers is low interaction between polymer units is so strong that blood components cannot break them up. 

As mentioned above, dilute solutions of copolymers in solvents that are good for both components exhibit similar behaviour to homopolymer chains. However, in a selective solvent, whereby the medium is a good solvent for one component, say A, and a poor solvent for the second component B, one part of the amphipathic block or graft separates as a distinct phase, whereas the other stays in solution. The insoluble portion of the amphipathic copolymer will aggregate reversibly to form micelles. It is believed that the polymeric micelles are spherical [4]. The critical micelle concentration of these block and graft copolymers is usually very low. 

Nonaqueous Dispersion Polymerization. Nonaqueous dispersion polymers are prepared by polymerizing a methacryhc monomer dissolved in an organic solvent to form an insoluble polymer in the presence of an amphipathic graft or block copolymer. This graft or block copolymer, commonly called a stabilizer, lends coUoidal stabiUty to the insoluble polymer. Particle sizes in the range of 0.1—1.0 pm were typical in earlier studies (70), however particles up to 15 pm have been reported (71). 

Micellar systems based on amphipathic block-copolymers have gained most attention as intravenously administered drug carrier systems over the years. These block-copolymers are composed of a hydrophilic PEG block and a hydrophobic block based on doxorubicin conjugated poly(aspartic acid) or poly(/i-bcnzyl L-aspartate). 

Non-Aqueous Processes. Dispersions of composite particles in non-aqueous media (12) have been prepared. The particles were sterically stabilised to prevent flocculation and aggregation. This was achieved by physical absorption of amphipathic graft or block copolymer (13,14) or by covalent attachment of diluent-soluble oligomer or polymer chains (15) at the particle surface so that by definition different polymers were situated at the surface and in the bulk of the particles, even for single-polymer particles. Composite particles were prepared by slow addition of the second monomer which was fully miscible with the diluent phase, obviating a monomer droplet phase further monomer-soluble initiation and amphipathic graft stabiliser was included as appropriate so that the process comprised continued dispersion... 

Piirma I. Synthesis of amphipathic polymeric material block copolymers. In Polymeric Surfactants, Surfactant Science Series. Vol. 42. New York Marcel Dekker, 1992 17-34. 

The best steric stabilizers are found empirically to be amphipathic block or graft copolymers. One of the comonomers should generate a homopolymer that is nominally insoluble in the dispersion medium whereas the other should polymerize to form a polymer that is soluble in the dispersion medium. 

Fig. 2.2. Schematic lepreseotation of the steric stabilization of a colloidal particle by a amphipathic block copolymer (only some of the stabilizing chains are shown).

Experience shows that the most effective stabilizers are amphipathic block or graft copolymers, which contain anchor groups that are strongly held to the surface and are insoluble in the medium, and stabilizing chains that are soluble in the medium and extended from the surface. High surface coverage is essential. 

The best steric stabilizers are amphipathic block or graft copolymers such as poly(oxyethylene lauryl ether) (Mr 1200). Commercial nonionic surfactants are classified according to the hydrophilic-lipophilic balance (HLB), which scales the relative solubilities of the two components in aqueous and nonaqueous media. The need for the anchor part of the stabilizing molecule can be eliminated if the stabilizing moieties can be covalently bonded to the latex particles. 

An alternative (and perhaps more efficient) polymeric snrfactant is the amphipathic graft copolymer consisting of a polymeric backbone B (polystyrene or polymethylmethacrylate) and several A chains (teeth) snch as polyethylene oxide [4], This graft copolymer is sometimes referred to as a comb stabilizer. This copolymer is nsnally prepared by grafting a macromonomer such methoxy polyethylene oxide methacrylate with polymethylmethacrylate. The grafting onto technique has also been nsed to synthesize polystyrene-polyethylene oxide graft copolymers. 

PUrma, I. (ed.) (1992) Synthesis of amphipathic polymeric materials graft copolymers. In Polymeric Surfactants. Marcel Dekker, New York, pp. 35-48. 

It can be seen that this polymer is amphipathic (p. 167), being made up of both hydrophobic and hydrophilic elements— the branched hydrocarbon chains and the relatively polar amide groups. It sits, in fact, on a knife edge between the forms of behaviour that characterize the two extremes, and depending on conditions, one or the other will prevail. At room temperature or below it is soluble in water, but when the temperature rises its hydrophobicity comes to the fore, the bound water is released, and the polymer falls out of solution as a compact white solid. In copolymers with other monomer units this process can be finely tuned by selecting the transition temperature or the polymer can be made pH-sensitive by incorporating some acrylic acid units into the chain ... 

We will start our discussion with the grafting to approach, which may be characterized as the simplest. Depending on the nature of the surface of the porous membrane that we want to decorate with a brush, we must choose an end-functionalized polymer chain (or a highly asymmetric block copolymer of an amphipathic nature) that may strongly adsorb from its functionalized part on the inner surface. Then in principle we may use the same method as for the case of a flat surface, i.e., dip the membrane in a solution that contains these polymers and then wait for the full formation of the brush. However, it has been shown [21] that self-assembly inside the pores is prohibited when the ratio of pore diameter to chain radius of gyration... 

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