Grafting, homopolymer conformation

The efficiency of the copolymers, either block or graft, acting as the compatibilizer depends on the structure of the copolymers. One of the primary requirements to get maximum efficiency is that the copolymer should be located, preferentially at the blend interface (Figs. 2a, b, and c). There are three possible conformations, as shown in the figure. Many researchers [10-12] found that the actual conformation is neither fully extended nor flat (Fig. 2c). A portion of the copolymers penetrates into the corresponding homopolymer and the rest re-... 

A block copolymer is expected to be superior to a graft copolymer in stabilizing dispersions of one polymer in another because there will be fewer conformational restraints to the penetration of each segment type into the homopolymer with which it is compatible. Similarly, diblock copolymers might be more effective than triblock copolymers, for the same reason, although tri- and multiblock copolymers may confer other advantages on the blend because of the different mechanical properties of these copolymers. 

The possibility of grafting block copolymer chains via a two-step SI-ATRP was studied. The concomitant increase in molar mass and thickness of the hydrophilic layer proved both the living character of the polymer grafted in the first step and the efficient reinitiation in the second step [152]. An example of block copolymer used was polyDMAAm-h-polyNIPAAm, which exhibited a change in the chain conformation leading to a reduction of the hydrodynamic diameter of the particles upon an increase in temperature above the LCST, in a similar way as the polyNIPAAm homopolymer. 

Apart from knowing the above interactions, one of the most fundamental considerations is the conformation of the polymer molecule at the interface. This is determined by the structure of the polymer molecule (whether this is a homopolymer, a block or a graft copolymer), its flexibility, molecular weight and the various environmental conditions such as solvency, temperature, addition of electrolyte, etc. 

Figure 16.1. Various conformations of polymeric surfactants adsorbed on a plane surface (a) random conformations of loops-trains-tails (homopolymer) (b) preferential adsorption of short blocks (c) chain lying flat on the surface (d) AB block copolymer with loop-train conformation of B and long tail of A (e) ABA block copolymer, as in (d) (f) BA graft with backbone B forming small loops and several tails of A ( teeth )...

Due to the relative ease of control, temperature is one of the most widely used external stimuli for the synthesis of stimulus-responsive bmshes. In this case, thermoresponsive polymer bmshes from poly(N-isopropylacrylamide) (PNIPAM) are the most intensively studied responsive bmshes that display a lower critical solution temperature (LOST) in a suitable solvent. Below the critical point, the polymer chains interact preferentially with the solvent and adopt a swollen, extended conformation. Above the critical point, the polymer chains collapse as they become more solvophobic. Jayachandran et reported the synthesis of PNIPAM homopolymer and block copolymer brushes on the surface of latex particles by aqueous ATRP. Urey demonstrated that PNIPAM brushes were sensitive to temperature and salt concentration. Zhu et synthesized Au-NPs stabilized with thiol-terminated PNIPAM via the grafting to approach. These thermosensitive Au-NPs exhibit a sharp, reversible, dear opaque transition in solution between 25 and 30 °C. Shan et al. prepared PNIPAM-coated Au-NPs using both grafting to and graft from approaches. Lv et al. prepared dual-sensitive polymer by reversible addition-fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide from trithiocarbonate groups linked to dextran and sucdnoylation of dextran after polymerization. Such dextran-based dual-sensitive polymer is employed to endow Au-NPs with stability and pH and temperature sensitivity. 

When a block copolymer is dissolved in a solvent that is a good one for one set of units and a poor one for the other a micellar structure forms.( 183,284) The ability to form micelles is a distinguishing feature of block and graft copolymers. Homopolymers and random type copolymers do not form micellar structures in solution. A micelle usually consists of a swollen core of the insoluble block connected to and surrounded by the soluble blocks. As the copolymer concentration is increased the micelles aggregate and organize into structures that have been termed mesomorphic gels. It is from this organized structure, where the chains themselves are in nonordered conformation, that crystallization takes place. 

It is important to compare the structural features of proteins with those of synthetic polymers or of polysaccharides in order to determine the specific properties of the derivative materials. Contrary to homopolymers or copolymers in which one or two monomers are repeated, proteins are heteropolymers consisting of amino acids. Proteins have a specific amino acid sequence and spatial conformation which determine their chemical reactivity and thus their potential for the formation of linkages that differ with respect to their position, nature and/or energy. This heterogenic structure provides many opportunities for potential crosslinking or chemical grafting—it even facilitates modification of the film-forming properties and end-product properties. 

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