Terminally anchored polymer

Factor, B. J., Theobald, W. and Toprakcioglu, C. (1996) Probing collective motions of terminally anchored polymers. Science, 274, 2041-2044. 

Several experimental parameters have been used to describe the conformation of a polymer adsorbed at the solid-solution interface these include the thickness of the adsorbed layer (photon correlation spectroscopy(J ) (p.c.s.), small angle neutron scattering (2) (s.a.n.s.), ellipsometry (3) and force-distance measurements between adsorbed layers (A), and the surface bound fraction (e.s.r. (5), n.m.r. ( 6), calorimetry (7) and i.r. (8)). However, it is very difficult to describe the adsorbed layer with a single parameter and ideally the segment density profile of the adsorbed chain is required. Recently s.a.n.s. (9) has been used to obtain segment density profiles for polyethylene oxide (PEO) and partially hydrolysed polyvinyl alcohol adsorbed on polystyrene latex. For PEO, two types of system were examined one where the chains were terminally-anchored and the other where the polymer was physically adsorbed from solution. The profiles for these two... 

Scheme 1 Scheme of grafting to approach of carboxy-terminated polymer onto anchoring layer of GPS... 

Comparison of these potentials with those for the terminally anchored chains shows the interaction to be relatively weak. For example, experiments with polystyrene in cyclohexane, which does not adsorb on mica, yielded no detectable forces between mica surfaces because of the polymer (Luckham and Klein, 1985). Indeed, estimates of the potential from Eq. (130) at the experimental conditions fall several orders of magnitude below the detection limit for the instrument. 

Fig. 32. E>elineation of regimes for interaction between free polymer at bulk concentration /3pb and terminally anchored chains of graft density l2a for n = 5000, p = 500, and v/l3 = 1 (Gast and Leibler, 1986) 1, Negligible interpenetration and layer thickness unaffected by free polymer 11, slight interpenetration, but layer significantly compressed by free polymer III, complete interpenetration and relaxed layer.

The interaction potentials described in previous sections for adsorbing homopolymer and terminally anchored layers in good solvents clearly indicate the ability of polymers to stabilize colloidal dispersions against flocculation due to van der Waals dispersion forces. Indeed, the practice preceeded the analyses by centuries in some cases and decades in others, since the use of adsorbing polymers dates to ancient times, and block copolymer stabilizers emerged from industrial laboratories in the 1960s (Napper, 1983). 

Poly(NIPAM-co-MAA) with MAA content above 5 mol% is soluble under both physiological pH (7.4) and temperature (37°C), when its carboxylic functions are ionized. However, when the pH is decreased to the pH of late endosomes and lysosomes ( pH = 5-5.5) (1), MAA protonation triggers dehydration of the polymer chains. If the polymer is anchored to a phospholipid membrane through randomly or terminally incorporated alkyl chains, transition, to a globule conformation induces reorganization of the bilayer that leads to massive content leakage (3). 

FIGURE 5.25 Polymeric chains adsorbed at an interface (a) terminally anchored polymer chain of mean end-to-end distance L (b) a brush of anchored chains (c) adsorbed (but not anchored) polymer coils (d) configuration with a loop, trains, and tails (e) bridging of two surfaces by adsorbed polymer chains. 

In steric stabilization adsorbed polymer molecules must extend outward from the particle surface yet be strongly enough attached to the surface that they remain adsorbed in the presence of applied shear. An example is a system of particles containing terminally anchored block copolymer chains having a hydrophobic portion of the molecule that is very strongly adsorbed on the particle surfaces and a hydrophilic part... 

This diagram is able to explain some puzzling observations disclosed by Cowell and Vincent (1982). They report clear differences between the stability behaviour of latex dispersions depending upon whether the particles are naked or pre-coated by terminally anchored chains. The addition of free polymer to the sterically stabilized particles resulted in the transition sequence stability- instability stability. This is precisely what would be predicted from Fig. 17.20 if bridging flocculation is absent (as it must be if the free polymer and stabilizing moieties are identical in chemical composition). 

The method of liberation of the end product from its support depends, first, on the type of C-terminal anchor bond to the polymer and, second, on the question of whether the synthetic material has to be released fully protected, partially masked but with free carboxylic terminus, or entirely deprotected. These aspects are part of the strategic concept — as mentioned in the beginning — which have to be carefully considered before the Merrifield synthesis is initiated. The conditions of deprotection of the mobile temporary N-terminal protecting group and those to cleave masking groups from side functions of the desired peptide are interrelated with the detachment conditions projected to liberate the end product from its support. 

The melt is assumed to be in thermodynamic equilibrium with the clay particles. Each clay sheet is modeled as a planar surface of area A. Due to the attraction between the end-groups and the sheets, the functionalized chains become terminally-anchored to these surfaces and effectively push the sheets apart. We assume that at any distance 2H between the surfaces, there is an equilibrium between the anchored and free functionalized chains. In other words, the degree to which the functionalized polymers bind to the surface is determined by the distance between tte particles. 

Parnas, R. S., and Cohen, Y. 1994. A terminally anchored polymer chain in shear flow Self-consistent velocity and segment density profiles. 

FIG. 2a Illustration of vinylpyrrolidone graft polymerization onto silica (A) surface sily-lation (B) graft polymerization (C) terminally anchored polymer chain. 

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