Adsorption diblock copolymers

Diblock copolymers consist of one sequence of anchor segments and a second sequence of backbone segments. The relative lengths of the two sequences can be controlled to provide a wide variety of adsorption and barrier characteristics. Typical commercial dispersants may use alkane... 

AB diblock copolymers in the presence of a selective surface can form an adsorbed layer, which is a planar form of aggregation or self-assembly. This is very useful in the manipulation of the surface properties of solid surfaces, especially those that are employed in liquid media. Several situations have been studied both theoretically and experimentally, among them the case of a selective surface but a nonselective solvent [75] which results in swelling of both the anchor and the buoy layers. However, we concentrate on the situation most closely related to the micelle conditions just discussed, namely, adsorption from a selective solvent. Our theoretical discussion is adapted and abbreviated from that of Marques et al. [76], who considered many features not discussed here. They began their analysis from the grand canonical free energy of a block copolymer layer in equilibrium with a reservoir containing soluble block copolymer at chemical potential peK. They also considered the possible effects of micellization in solution on the adsorption process [61]. We assume in this presentation that the anchor layer is in a solvent-free, melt state above Tg. The anchor layer is assumed to be thin and smooth, with a sharp interface between it and the solvent swollen buoy layer. 

In the case of heterogeneous polymers the experimental methods need to be refined. In order to analyze those polymers it is necessary to determine a set of functions / (M), which describe the distribution for each kind of heterogeneity i This could be the mass distributions of the blocks in a diblock copolymer. The standard SEC methods fail here and one needs to refine the method, e.g., by performing liquid chromatography at the critical point of adsorption [59] or combine SEC with methods, which are, for instance, sensitive to the chemical structure, e.g., high-pressure liquid chromatography (HPLC), infrared (IR), or nuclear magnetic resonance spectroscopy (NMR) [57],... 

In this review, we introduce another approach to study the multiscale structures of polymer materials based on a lattice model. We first show the development of a Helmholtz energy model of mixing for polymers based on close-packed lattice model by combining molecular simulation with statistical mechanics. Then, holes are introduced to account for the effect of pressure. Combined with WDA, this model of Helmholtz energy is further applied to develop a new lattice DFT to calculate the adsorption of polymers at solid-liquid interface. Finally, we develop a framework based on the strong segregation limit (SSL) theory to predict the morphologies of micro-phase separation of diblock copolymers confined in curved surfaces. 

To establish the molecular thermodynamic model for uniform systems based on concepts from statistical mechanics, an effective method by combining statistical mechanics and molecular simulation has been recommended (Hu and Liu, 2006). Here, the role of molecular simulation is not limited to be a standard to test the reliability of models. More directly, a few simulation results are used to determine the analytical form and the corresponding coefficients of the models. It retains the rigor of statistical mechanics, while mathematical difficulties are avoided by using simulation results. The method is characterized by two steps (1) based on a statistical-mechanical derivation, an analytical expression is obtained first. The expression may contain unknown functions or coefficients because of mathematical difficulty or sometimes because of the introduced simplifications. (2) The form of the unknown functions or unknown coefficients is then determined by simulation results. For the adsorption of polymers at interfaces, simulation was used to test the validity of the weighting function of the WDA in DFT. For the meso-structure of a diblock copolymer melt confined in curved surfaces, we found from MC simulation that some more complex structures exist. From the information provided by simulation, these complex structures were approximated as a combination of simple structures. Then, the Helmholtz energy of these complex structures can be calculated by summing those of the different simple structures. 

Marques et al. [116] have studied the adsorption of an A - B diblock copolymer from a dilute solution onto a solid surface that attracts the A block and repels the B block in a nonselective solvent, good for both blocks. 

The adsorption of diblock copolymers and the reduction of the interface tension are presented in Fig. 24 and compared to the prediction of the mean... 

There continues to be extensive interest in latexes and micellar systems. The structure of acrylic latex particles has been investigated by non-radiative energy transfer by labelling the co-monomers with fluorescent acceptor-donor systems. Phase separations could also be measured in this way. Excimer fluorescence has been used to measure the critical micelle temperature in diblock copolymers of polystyrene with ethylene-propylene and the results agree well with dynamic light scattering measurements. Fluorescence anisotropy has been used to measure adsorption isotherms of labelled polymers to silica as well as segmental relaxation processes in solutions of acrylic polymers. In the latter case unusual interactions were indicated between the polymers and chlorinated hydrocarbon solvents. Fluorescence analysis of hydrophobically modifled cellulose have shown the operation of slow dynamic processes while fluorescence... 

Adsorption from a Binary Mixture of Short and Long Diblock Copolymers... 

The influence of the molecular weight distribution of diblock copolymers on their segregating properties is considered in Sect. 4.2.4. It describes first experimental study [255] on a bimodal mixture of short symmetric and long asymmetric copolymers added to a polymeric matrix. Shorter copolymers were found to adsorb preferentially at the homopolymer interfaces in accord with brush formation observed from a solvent host matrix [274-276]. The mean field model is able [255] to predict the segregation isotherm of the bimodal mixture of copolymers, basing on single component adsorption data [254]. 

Bijsterbosch, H.D. Stuart, M.A.C. Fleer, G.J. Adsorption kinetics of diblock copolymers from a micellar solution on silica and titania. Macromolecules 1998, 31, 9281-9294. lijima, M. Nagasaki, Y. Okada, T. Kato, M. Kataoka, K. Core-polymerized reactive micellesm from heterotelechelic amphiphilic block copolymers. Macromolecules 1999, 32, 1140-1146. 

Sakai, K. et al.. Adsorption characteristics of zwitterionic diblock copolymers at the silica/aqueous solution interface. J. Colloid Interf. Sci., 317, 383, 2008. 

The adsorption of block and graft copolymers is more complex, as the intimate structure of the chain will determine the extent of adsorption [37]. Random copolymers adsorb in an intermediate fashion compared to that of the corresponding homopolymers. Block copolymers retain the adsorption preference of the individual blocks. The hydrophilic block (e.g., PEO the buoy) extends away from the particle surface into the bulk solution, while the hydrophobic anchor block (e.g., PS or PPO) provides a firm attachment to the surface. Figure 6.14 shows the theoretical prediction of diblock copolymer adsorption according to SF theory. In this case, the surface density cr was plotted versus the fraction of anchor segments v, and adsorption was shown to depend on the anchor/buoy composition. 

The amount of adsorption was greater than for homopolymers, and the adsorbed layer thickness was more extended and dense. For a triblock copolymer A-B-A, with two buoy chains and one anchor chain, the behaviour was similar to that of diblock copolymers this is shown in Figure 6.15 for the PEO-PPO-PEO block (Pluronic). 

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