Conformation, polymer, near interfaces

The randomization stage refers to the equilibration of the nonequilibrium conformations of the chains near the surfaces and in the case of crack healing and processing, the restoration of the molecular weight distribution and random orientation of chain segments near the interface. The conformational relaxation is of particular importance in the strength development at incompatible interfaces and affects molecular connectivity at polymer-solid interfaces. 

Starting from previous results [186, 187] the authors have ascribed this effect to the formation of a polymer layer adsorbed on the support surface where naacromolecules are less mobile and to a lower density of packing due to conformational restrictions near the interface. These conformational restrictions result in a modification of the crystallization conditions and therefore the crystalline polymer fraction is smaller, the less the film thickness. 

While thin polymer films may be very smooth and homogeneous, the chain conformation may be largely distorted due to the influence of the interfaces. Since the size of the polymer molecules is comparable to the film thickness those effects may play a significant role with ultra-thin polymer films. Several recent theoretical treatments are available [136-144,127,128] based on Monte Carlo [137-141,127, 128], molecular dynamics [142], variable density [143], cooperative motion [144], and bond fluctuation [136] model calculations. The distortion of the chain conformation near the interface, the segment orientation distribution, end distribution etc. are calculated as a function of film thickness and distance from the surface. In the limit of two-dimensional systems chains segregate and specific power laws are predicted [136, 137]. In 2D-blends of polymers a particular microdomain morphology may be expected [139]. Experiments on polymers in this area are presently, however, not available on a molecular level. Indications of order on an... 

The occupied areas Sl per adsorbed polymer at the air-water interface estimated from adsorbances for IEI and POE were almost equal and independent of the KBr concentration. The thickness of the adsorbed POE did not exceed 15 A while that of the adsorbed IEI increased with decreasing KBr concentration and was about 1.5 times as large as the root mean-square end-to-end distance of the ionene homopolymer with the same M in bulk solution. It was concluded that POE, which is non-ionic and hydrophilic, is adsorbed in a nearly flattened conformation with short loops and trains. On the other hand, IEI is adsorbed a in tail-train-tail conformation in which the oxyethylene block lies flat on the air-water interface and the ionene tails are elongated to the bulk solution. 

The result of the interactions of some copolymer mimics of AMP with model bacterial membranes has been studied via atomistic molecular dynamics simulation (Figure 3.2). The model bacterial membrane expands homogeneously in a lateral manner in the membrane thickness profile compared with the polymer-free system. The individual polymers taken together are released into the bacterial membrane in a phased manner and the simulations propose that the most possible location of the partitioned polymers is near the l-palmitoyl-2-oleoyl-phosphatidylglycerol clusters. The partitioned polymers preferentially adopt facially amphiphilic conformations at the lipid-water interface, although lack intrinsic secondary structures, such as an a-helix or P-sheet, found in naturally occurring AMP [23]. 

Considering, for instance, a system containing 1 nm thick plates, Ipm in diameter, the distance between plates would approach 10 nm at only 7 vol% of plates [217]. The behavior of PNCs can be rationalized as follows. The proliferation of internal inorganic-polymer interfaces means the majority of polymer chains reside near an inorganic surface. Since an interface restricts the conformations that polymer molecules can adopt, and since in PNCs with only a few volume percent of dispersed nanoparticles the entire matrix polymer may be considered as nanoscopically confined interfacial polymer, the restrictions in chain conformations will alter molecular mobility, relaxation behavior, and the consequent thermal transitions such as glass transition temperature of the composites [217]. 

Here, the second virial coefficient (excluded-volume parameter), vb = [1 -2x T)]<0, is negative because water is a poor solvent for the hydrophobic block B. The third virial coefficient, Wb, is positive, and x= T- e /d is the relative deviation from the theta temperature. At small deviations from the theta point, r < 1, the surface tension y and the polymer volume fraction (p are related as y/k T = (p. However, at larger deviations from the theta point, (p becomes comparable to unity and the latter relationship breaks down. Because in a typical experimental situation (p = 1, we treat (p and y as two independent parameters. Note that in a general case, surface tension y and width A of the core-corona interface depend on both the polymer-solvent interaction parameter Xbs T) for the core-forming block and the incompatibility Xab between monomers of blocks A and B. That is, y could depend on the concentration of monomers of the coronal block A near the core surface. We, however, neglect this (weak) dependence and assume that the surface tension y is not affected by conformations of the coronal blocks in a micelle. 

The polymers are designed such that the backbones could fold and position the relatively hydrophilic bridging groups near the water interface, and the relatively hydrophobic bridging groups near the air interface. As multiple layers are deposited by the Y-type method, in alternating (AB) fashion, the hydrophilic surfaces of adjacent layers are in contact, as are the hydrophobic surfaces of adjacent layers. This would likely be the most thermodynamically stable conformation. 

The coupling reaction proceeds very quickly, caused by concentrating the reactive moieties (MAH, epoxide, NH2, COOH, etc.) at the interface. The polymer chain end generally prefers to locate at the interface, because such a chain conformation is more probable, compared with the case where a mid-segment locates near the interface. Then, the amino chain ends of PA may be cmicentrated at the interface. The MAH unit is highly polar and is unstable in the nmi-polar PP-MAH phase, and it tends to segregate at the interface to contact with the polar chain of PA. Thus, both reactive sites may be concentrated near the interface to provide a favorable situation for the coupling reaction. 

Polymer Adsorption. The driving force for adsorption typically is the en-thalpic interaction between the interface and the polymer segments. Exceptions include aqueous solutions near hydrophobic substrates. In that case, the driving force may include the hydrophobic interaction between water and the surface. The enthalpic interactions may be of various forms such as hydrogen bonding, inter-facial tension, van der Waals attraction, polar interactions, and electrostatic attractions. This enthalpic interaction is offset by the loss in conformational entropy... 

Nearly Gaussian Chains—For weak adsorption, The conformational properties of adsorbing chains are assumed to be only weakly perturbed upon adsorption. This might not be true well above the adsorption threshold, when quite dense polymer layers are formed next to the interface. For critical adsorption conditions, however, this approximation is expected to be valid. 

Since P.G. de Gennes introduced the scaling method for the study of polymer conformations near an interface, many predictions were given to polymer concentration profiles. These are different from mean field results and depend in a subtle manner on surface interaction and bulk properties. ... 

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