Loops and tails

Fig. XI-4. Schematic diagram of the structure of an adsorbed polymer chain. Segments are distributed into trains directly attached to the surface and loops and tails extending into solution.

Fig. XI-6. Polymer segment volume fraction profiles for N = 10, = 0-5, and Xi = 1, on a semilogarithinic plot against distance from the surface scaled on the polymer radius of gyration showing contributions from loops and tails. The inset shows the overall profile on a linear scale, from Ref. 65.

The consideration made above allows us to predict good chromatographic properties of the bonded phases composed of the adsorbed macromolecules. On the one hand, steric repulsion of the macromolecular solute by the loops and tails of the modifying polymer ensures the suppressed nonspecific adsorptivity of a carrier. On the other hand, the extended structure of the bonded phase may improve the adaptivity of the grafted functions and facilitate thereby the complex formation between the adsorbent and solute. The examples listed below illustrate the applicability of the composite sorbents to the different modes of liquid chromatography of biopolymers. 

Indeed, the polymeric interface seems to be highly diffuse and hydrophilic because copolymers of N-vinylpyrrolidone and N- (2-hydroxyethyl) acrylamide are readily soluble in water [53]. Besides, aminopropyl-glass adsorbs the acryloyl chloride copolymer so that only 10% of its active functions become amidated. The rest is located on the loops and tails of the attached macromolecules [51]. Thus the steric repulsion of the bonded phase is a probable reason for the high inertness of the packing towards viruses. 

Adsorption on Kaolinite. As for polyacrylamides, adsorption of XCPS on kaolinite is conducted as a function of S/L and the results extrapolated to S/L=0. However, the S/L dependence of XCPS adsorption on kaolinite is considerably less than that for HPAM. This is due to the flat conformation of the adsorbed molecules of semirigid xanthan (25) compared to the more extended conformation of flexible HPAM (27). The absence of loops and tails in the adsorbed XCPS layer thus diminishes the probability of flocculation of particles by polymer bridging. The slight dependence in adsorption on S/L may therefore be attributed to coagulation of particles induced by Ca. ... 

Since the contour lengths of loops and tails are quite short (c /p is small), as evident in the simulations, gioop and gtaii approach the limits... 

Macromolecules have also been specifically designed and synthesized for use as emulsifiers for lipophilic materials and as stabilizers in the colloidal dispersion of lipophilic, hydrocarbon polymers in C02. We have demonstrated the amphiphilicity of fluorinated acrylate homopolymers, such as PFOA, which contain a lipophilic, acrylate like backbone and C02-philic, fluorinated side chains (see Fig. 3) [103]. It has been demonstrated that a homopolymer which physically adsorbs to the surface of a polymer colloid prevents agglomeration by the presence of loops and tails (see Fig. 4) [113]. The synthesis of this type of... 

Poly(methyl methacrylate) provides a level of stabilization even though the solution in CCl is below the 0-temperature. All the copolymers, both random and block, are better stabilizers than PMM, the methacrylate units acting as anchors, with stabilizing sequences of styrene loops, of block copolymers, or mixed loops and tails, of random copolymers, at better than 0-conditions. Higher M.W. polystyrenes give silica dispersions too unstable to measure by our optical method the sediment volumes are between those of poly(methyl methacrylate) solutions and pure solvent. 

Scheutjens, J. M. H. M., and G. J. Fleer (1980), "Statistical Theory of the Adsorption of Interacting Chain Molecules. 2. Train, Loop, and Tail Size Distribution", J. Phys. Chem. 84/2, 178-90. 

Double-layer forces are commonly used to induce repulsive interactions in colloidal systems. However, the range of electrostatic forces is strongly reduced by increasing the ionic strength of the continuous phase. Also, electrostatic effects are strong only in polar solvents, which is a severe restriction. An alternative way to create long-range repulsion is to adsorb macromolecules at the interface between the dispersed and the continuous phase. Polymer chains may be densely adsorbed on surfaces where they form loops and tails with a very broad distribution of sizes... 

The portion of the protein that protrudes into the cytoplasm is the intracellular domain, which may be composed of a single folded section of polypeptide or by several loops and tails. 

Dissolved polymer molecules can be adsorbed by polymer particles via electrostatic attractive force or hydrophobic interaction. When polyelectrolyte is adsorbed on an opposite-charge particle, the polymer molecules usually have a loop-and-tail conformation and, as a result, inversion of charge occurs. For example, sulfatecarrying particles behave as cationic ones after they adsorb poly(lysine). Then poly(-styrene sulfonate) can be adsorbed on such cationic particles and reinvert the charge of particles to anionic (14). Okubo et al. pointed out that the alternate adsorption of cationic and anionic polymers formed a piled layer of polyelectrolytes on the particle, but the increment of adsorbed layer thickness was much less than expected. This was attributed to synchronized piling of two oppositely charged polyelectrolytes (15). 

Loops and tails of an isolated adsorbed polymer chain assume a number of different configurations and they substantially determine the configurational entropy of the adsorbed polymer, while the interaction energy between trains and the surface determines the enthalpy of adsorption. 

Scheutjens and Fleer further calculated50 the concentration distributions of loop and tail segments, the root-mean-square thickness of the adsorbed layer, and the average numbers and lengths of trains, loops, and tails. They also computed the distributions of trains, loops, and tails from the free segment probability Pj, which may be represented by... 

B.3.4 Root-Mean-Square Thicknesses of Loops and Tails B.3.4.1 Root-Mean-Square Thickness of Loops... 

Experimentally, the root-mean-square thicknesses of loops and tails can be measured by ellipsometry. It is thus necessary to relate them to the average numbers of segments in loops and tails for deducing the conformation of an adsorbed polymer. 

The segment-density distributions of train, loop, and tail form can be calculated by LDFT,... 

Zheligovskaya et al. [55] have simulated the adsorption of quasirandom adsorption-tuned copolymers (ATC). The critical adsorption energy as well as some characteristics of the adsorbed single chains (statistics of trains, loops, and tails) were studied. All these properties were compared with those... 

Comparable experiments were performed with DexP-coated macroporous polystyrene-divinylbenzene (PS-DVB) particles [264] and with DexP, labelled with 4-amino-TEMPO, using EPR spectroscopy to study the conformation of the polymer chains [265]. Low substituted DexP gave thicker layers with lower density than highly substituted derivatives due to the presence of more loops and tails. With increasing DS of DexP, the stiffness of the adsorbed layers and, therefore, the density increases and the non-specific interaction of BSA with the DexP-coated PS-DVB surfaces seems to be restricted to the top of the adsorbed layer. 

The spectral line shape is concentration dependent, but remains motionally narrowed until relatively high polymer concentrations are reached (Figure 1-B,C and reference 27). Loop and tail segment density near the surface may be large, but falls off rapidly with distance above the surface. We therefore expected tails and all but the smallest loops (and surface extended loops) would exhibit three line spectra similar to that observed with the free, isolated polymer molecule. 

By contrast a nitroxide attached to monomeric units rigidly held to a solid surface should show a spectral line shape similar to the bulk polymer in the glassy state (Figure IE). Thus a polymer molecule adsorbed at a solid-liquid interface should exhibit a composite spectrum, from which one can deduce the fraction of the monomeric units in loops and tails, and their mean motion. The sensitivity of the method to small amounts of units with motional freedom can be seen from Figure 2. 

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