Polymers terminally anchored chains

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.

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). 

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... 

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... 

The nature of the radical within the latex particle determines its fate ie, its propensity to desorp, propagate, chain transfer, or terminate. It seems reasonable that an ionic radical will not penetrate deeply into a latex particle but rather anchor its ionic head on the surface or palisade of the latex particle, much the way a surfactant molecule does. Once anchored, the nonpolar tail containing the radical will penetrate into the particle, and reactively diffuse throughout the poljmier and monomer solution imtil either the ionic radical desorbs back into the aqueous phase, the ionic radical terminates with another radical within the particle, or the ionic radical undergoes a chain transfer event with either the monomer, polymer, or a chain-transfer agent within the latex particle. Once a chain transfer event occurs, the new radical becomes nonionic and has a markedly different solublity in the particle and aqueous phases. As the nonionic radical grows in chain length within the particle, it becomes even less soluble in the aqueous phase and becomes less likely to desorb. Such a qualitative description of radical fate was quantified... 

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. 

The first examples employed a lithium-terminated polymer chain, utilizing the primary product of the -butyl-lithium-catalyzed polymerization of a masked disilene (see Section 3.11.4.1.2) and a reactive siloxy alkylbromide anchor-derivatized quartz surface, affording the end-grafted polysilane 79, as shown in Scheme 27.191... 

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