Polymers interactions between layers

A different approach to the synthesis of nanosized macromolecules through hierarchical self-assembly is based on Layer-by-Layer (LbL) chemistry. LbL allows the deposition of ultra thin films whose thickness can be controlled by the chemical structure of the molecules and number of deposited layers. The interactions between layers can be ionic, covalent, hydrogen-bonding, and charge-transfer, depending upon the nature of the polymer used in the preparation.The layer-by-layer assembly of an electroactive polymer nanocomposite thin film of cationic linear poly(ethyleneimine) and Prussian Blue nanoparticles, has been exploided... 

In contrast with the tactoid structure predominating in microcomposites (conventional composites), in which the polymer and the clay tactoids remain immiscible, resulting in agglomeration of the clay in the matrix and poor macroscopic properties of the material (Alexandre et al., 2009 Luduena, Alvarez, and Vasquez, 2007), the interaction between layered silicates and polymer chains may produce two types of ideal nanoscale composites. The properties of the resulting material are dependent on the state of the nanoclay in the nanocomposite, that is, if it is exfoliate or intercalate. Intercalation is the state in which polymer chains are present between the clay layers, resulting in a multilayered structure with alternating polymer/inorganic... 

In an effort to create model surfaces to correlate the conformations of the polymers with the measured interactions, we have been examinming the interactions between layers of 2-vinylpyridine(PVP)-styrene(PS) block copolymers adsorbed onto mica substrates (in the surface force apparatus). " In these systems, we observed repulsive forces at distances many ( 10) times larger than the dimension of those polymers in solutions. In particular, we found that the distance at which the repulsive force becomes significant is a linear function of PS-block length. 

Free volume present in nanocomposite systems plays a major role in determining the overall performance of the membranes. Positron annihilation lifetime spectroscopy (PALS) is an efficient technique used for the analysis of free volume. The diffusion of permeant through polymeric membranes can be described by two theories, namely, molecular and free-volume theories. According to the free-volume theory, the diffusion is not a thermally activated process as in the molecular model, but it is assumed to be the result of random redistributions of free-volume voids within a polymer matrix. Cohen and Turnbull developed the free-volume models that describe the diffusion process when a molecule moves into a void larger than a critical size, Vc- Voids are formed during the statistical redistribution of free volume within the polymer. It is found that the relative fractional free volume of unfilled polymer decreases on the addition of layered silicates. The decrease is attributed to the interaction between layered silicate and polymer because of the platelet structure and high aspect ratio of layered silicates. The decrease is explained to the restricted mobility of the chain segments in the presence of layered silicates. This results in reduced free-volume concentration or relative fractional free volume [49]. 

In many colloidal systems, both in practice and in model studies, soluble polymers are used to control the particle interactions and the suspension stability. Here we distinguish tliree scenarios interactions between particles bearing a grafted polymer layer, forces due to the presence of non-adsorbing polymers in solution, and finally the interactions due to adsorbing polymer chains. Although these cases are discussed separately here, in practice more than one mechanism may be in operation for a given sample. 

Israelachvili and his colleagues have used the SEA to study the interactions between surface layers of surfactant and of other molecules representing functionalised polymer chains, adhesion promoters or additives. Typically a monolayer of the molecule concerned is deposited onto cleaved mica sheets. The values of surface energies obtained from the JKR equation (Eq. 18) throw some interesting light on the nature and roughness of surface layers in contact. 

The situation becomes most complicated in multicomponent systems, for example, if we speak about filling of plasticized polymers and solutions. The viscosity of a dispersion medium may vary here due to different reasons, namely a change in the nature of the solvent, concentration of the solution, molecular weight of the polymer. Naturally, here the interaction between the liquid and the filler changes, for one, a distinct adsorption layer, which modifies the surface and hence the activity (net-formation ability) of the filler, arises. Therefore in such multicomponent systems in the general case we can hardly expect universal values of yield stress, depending only on the concentration of the filler. Experimental data also confirm this conclusion [13],... 

The importance of polydispersity is an interesting clue that it may be possible to tailor the weak interactions between polymer brushes by controlled polydispersity, that is, designed mixtures of molecular weight. A mixture of two chain lengths in a flat tethered layer can be analyzed via the Alexander model since the extra chain length in the longer chains, like free chains, will not penetrate the denser, shorter brush. This is one aspect of the vertical segregation phenomenon discussed in the next section. 

Carbon black is reinforced in polymer and mbber engineering as filler since many decades. Automotive and tmck tires are the best examples of exploitation of carbon black in mbber components. Wu and Wang [28] studied that the interaction between carbon black and mbber macromolecules is better than that of nanoclay and mbber macromolecules, the bound mbber content of SBR-clay nanocompound with 30 phr is still of high interest. This could be ascribed to the huge surface area of clay dispersed at nanometer level and the largest aspect ratio of silicate layers, which result in the increased silicate layer networking [29-32]. 

The polymer layers, however, also introduces new contributions to the overall interaction between the particles. As two particles approach one another, compression of the polymer layer may occur which is unfavourable. Associated with this compression, is an increase in the local polymer concentration - this can be favourable or unfavourable depending on the solubility of the polymer. 

A review on drag-reducing polymers is given in the literature [1359]. It has been suggested that drag reduction occurs by the interactions between elastic macromolecules and turbulent-flow macrostructures. In turbulent pipe flow, the region near the wall, composed of a viscous sublayer and a buffer layer, plays a major role in drag reduction. 

A number of these dyes were applied, mixed with a polymer for the control of the aggregation state, to CD-R and DVD-R recording systems. The aggregation state in the recording layer was controlled by choosing the set of axial substituents (Scheme 8 R1, R2, and R3). The interaction between the phthalocyanine dyes and the polymers was dependent on the length of the axial substituents or, more carbon atoms in the alkyl group were found to be necessary for the axial substituents to mix with the polymers.218... 

Galgali and his colleagues [46] have also shown that the typical rheological response in nanocomposites arises from frictional interactions between the silicate layers and not from the immobilization of confined polymer chains between the silicate layers. They have also shown a dramatic decrease in the creep compliance for the PP-based nanocomposite with 9 wt% MMT. They showed a dramatic three orders of magnitude drop in the zero shear viscosity beyond the apparent yield stress, suggesting that the solid-like behavior in the quiescent state is a result of the percolated structure of the layered silicate. 

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