Macromolecular brush

The observation that a macromolecular brush gets stretched as the side chains get adsorbed on a flat surface provides a means to stimulate molecular motility by desorption of the brush molecule or a segment of it. If the molecule is in a subsequent period allowed to relax to the adsorbed stretched state it will eventually do a step forward. This is depicted schematically in Figure 28 as a sort of a creep motion. Here, the desorbed state might be characterized as an excited state whose formation requires input of energy. In the case that the structure of the surface and of the molecule favor relaxation into a distinct direction, i.e., in the case of an asymmetric potential, the motion of the molecule can be become directed. 

Scheme 30.8 Preparation of macromolecular brushes via the grafting-onto method, using a combination of ATRP and CuAAC click chemistry. Reproduced with permission from Ref [85] 2007, American Chemical Society.

Dense grafting of side chains onto linear backbones, and homopolymerization of macromonomers, are both used to synthesize macromolecular brushes. Steric repulsion of the side chains forces the main chain into an extended wormlike conformation, resulting in liquid-crystalline phases, and lower dynamic shear moduli than linear flexible coils in concentrated solutions [93, 94]. Densely grafted polymeric brushes on sliding surfaces have been found to reduce friction, and therefore have potential for providing biolubrication for artificial implants [95]. 

A macromolecular, photochemical surface labeling reagent was developed by Louvard et al. (1976). It comprised a Fab fragment of a human myeloma protein with which 4-fluoro-3-nitrophenylazide (Table 5.1.1c) had been reacted. Used at a concentration of 0.1 mM, this conjugate could be used to label the surfaces of sealed vesicles or it could be trapped inside vesicles and used to label from within. Louvard and colleagues focused their attention on the modification of aminopeptidase in intact brush-border membranes and demonstrated that the enzyme spans the bilayer. From the... 

Abstract The major enzymatic barrier to the absorption of macromolecules, particularly therapeutic peptides, is the pancreatic enzymes the peptidases, nucleases, lipases and esterases that are secreted in considerable quantities into the intestinal lumen and rapidly hydrolyse macromolecules and lipids. In the case of the peptidases, they work in a co-ordinated fashion, whereby the action of the pancreatic enzymes is augmented by those in the brush borders of the intestinal cells. The sloughing-off of mucosal cells into the lumen also furnishes a mixture of enzymes that are a threat to macromolecules. As the specificity and activity of the enzymes are not always predictable, during pharmaceutical development it is important to test the stability of therapeutic macromolecules, and novel macromolecular-containing or lipid-containing formulations, in the presence of mixtures of pancreatic enzymes and bile salts, or in animal intestinal washouts or ideally, aspirates of human intestinal contents. 

The lysosomal enzymes The lysosomes are membrane vesicles ubiquitous to mammalian cells and contain a panoply of hydrolytic enzymes, estimated to be over 60 in number, that function to digest practically any biological macromolecule. They are important to the discussion of oral macromolecular drug delivery for two reasons. First, any macromolecules that escape digestion by the pancreatic and brush border enzymes are likely to be taken up into the epithelial cells by the process of endocytosis. In this process, the apical membrane invaginates and the target molecules enter endocytic vesicles that then fuse with the lysosomes and are subjected to intracellular hydrolysis by the lysosomal enzymes. Second, the sloughing-off of the epithelial cells means that the lysosomal enzymes will be released into the lumen of the intestine. They may be... 

Polyelectrolyte brushes are macromolecular monolayers where the chains are attached by one end on the surface and, at the same time, the chains carry a considerable amount of charged groups. Such poly electrolyte structures have received thorough theoretical treatment, and experimental interest has been vast due to the potential of brushes for stabilising colloidal particle dispersions or for... 

Figure 1 Macromolecular architectures linear macromolecular chains (homopolymer, block-copolymer and statistical copolymer [14]), brushed-polymer (= linear chains attached to a polymer-chain brush-polymer, brush-copolymers [14]), star polymer [4], mikto-star-polymer [16], arborescent graft polymer (=repeated grafting of linear chains on a macromolecule [17,18]), dendrimer (= maximally branched, regular polymer [15])...

Figure 35 Sketch of a brush-polymer , illustrating the sterical overcrowding of the side-chains to force the macromolecular backbone into a stretched conformation [404]...

How adsorption of the side chains to a flat substrate effects the backbone conformation has been observed in further microscopic detail for brush molecules with a methacrylate backbone and poly-(n-butyl acrylate) side chains. These poly(/v-butyl acrylate) brushes were prepared by living radical grafting from a multifunctional macromolecular initiator.38 The synthetic approach allowed observation of the same batch of molecules without (macroinitiator) and with poly(n-butyl acrylate) side chains (brush). 

Because the adsorption of the side chains is such a strong factor in controlling the macromolecular conformation, not only the length of side chains but also their composition is of great importance. This can be demonstrated by comparing the structures for the brush molecules with poly(n-butylacrylate) side... 

This review deals with recently obtained experimental results on IPECs based on branched PE species, specifically including PE stars, star-like micelles generated in aqueous solutions of ionic amphiphilic block co- and terpolymers, and cylindrical PE brushes. In addition, we will also present the results of molecular dynamics (MD) simulations performed for some of these systems, which enable the possible structural organization of the formed macromolecular co-assemblies to be revealed. 

Using atomic force microscopy (AFM), Xu et al. [85] showed that similar macromolecular co-assemblies derived from the cylindrical PE brush based on quat-ernized poly[2-(dimethylamino) ethyl methacrylate] (PDMAEMAQ) complexed with short poly(sodium styrene sulfonate) (PSSNa) have distinct longitudinal undulations (Fig. 8), thereby apparently providing experimental proof of a peculiar pearl-necklace structure of IPECs based on such branched PEs. 

The results of experimental and theoretical research on water-soluble (nonstoichio-metric) IPECs based on nonlinear (branched) polyionic species (HPE) complexed with oppositely charged linear PEs (GPE) demonstrated that the main feature of such macromolecular co-assemblies is their pronounced compartmentalized structure, which results from a distinctly nonuniform distribution of the linear GPE chains within the intramolecular volume of the branched HPE. In the case of star-shaped PEs or star-like micelles of ionic amphiphilic block copolymers, this com-partmentalization leads to the formation of water-soluble IPECs with core-corona (complex coacervate core) or core-shell-corona (complex coacervate shell) structures, respectively. Water-soluble IPECs based on cylindrical PE brushes appear to exhibit longitudinally undulating structures (necklace) of complex coacervate pearls decorated by the cylindrical PE corona. 

Recent advances in polymer chemistry, in particular, in controlled radical polymerization, have enabled the synthesis of complex macromolecular architectures with controlled topology, which comprise chemically different (functional) blocks of controlled length in well-defined positions. Block co- and terpolymers, molecular and colloidal polymer brushes, and star-like polymers present just a few typical examples. Furthermore, miktoarm stars, core-shell stars and molecular brushes, etc. exemplify structures where chemical and topological complexity are combined in one macromolecule. 

Huck et al. prepared Au NPs inside IL-based polyelectrolytes [70], The nanocomposite synthesis relies on loading the macromolecular film with AuCl precursor ions followed by their in situ reduction to Au nanoparticles. It was observed that the nanoparticles are uniform in size and are fully stabilized by the surrounding polyelectrolyte chains. Moreover, XRR analysis revealed that the Au NPs are formed within the polymer-brush layer. Interestingly, AFM experiments confirmed that the swelling behavior of the brush layer is not perturbed by the presence of the loaded NPs (Fig. 4.22]. The Au NP-poly-METAC nanocomposite is remarkably stable to aqueous environments, suggesting the feasibility of using this kind of nanocomposite systems as robust and reliable stimuli-responsive platforms. 

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