Transient crosslink

The procedure used for testing the ideal Donnan theory is applicable to any model that decouples ionic effects from network elasticity and polymer/solvent interactions. Thus we require that nnet depend only on EWF and not C. While this assumption may seem natural, several models which include ionic effects do not make this assumption. For example, the state of ionization of a polymer chain in the gel and the ionic environment may affect the chain s persistence length, which in turn alters the network elasticity [26]. Similarly, a multivalent counterion can alter network elasticity by creating transient crosslinks. 

The Tg of the silver-POZ complex initially increases with the silver concentration, reaches a broad maximum, then slightly decreases again. The increase in Tg with the complexation of the silver ions is primarily attributable to both the reduced chain mobility in the transient crosslinks of the polymer segments due to the silver cations and the dangling of silver cations on the polymer chain [14]. 

Abstract This chapter presents selected literature examples to review the development of the use of donor-acceptor jr-jr stacking interactions as transient crosslinks in supramolecular polymer networks. The chapter examines notable examples of these highly specific and directional interactions and illustrates how they can be... 

Blending also addressed two aspects of PP processability high viscosity and poor melt strength. Several external lubricants were incorporated into the resin, viz. s 5 wt.% of either polydimethylsiloxane (PDMS), fluorinated polymers or elastomers. Usually, these blends also showed improved impact strength and elasticity. Another modification involved either copolymerization or grafting with acidic monomers, thus forming a transient crosslinking by the ion clusters. The presence of... 

The glass transition temperatures of these polymers also increased with salt content, because of a stiffening of the structure, which can be ascribed to the formation of transient crosslinks between solvating moieties through the metal ions. The increase in viscosity of liquid poly(propylene glycol) on salt addition was, in fact, one of the first experimental observations symptomatic of a polymer-salt interaction. ... 

Chemical crosslinks and entanglements differ, however. The former is permanent, the latter transient. Given sufficient time, even the effects of entanglements can be overcome and stationary-state flow is achieved. An... 

It is known that aromatic azides are photodecomposed to give active nitrenes as the transient species, which react with the environmental binder polymers to crosslink them. However, the mechanism of these photocrosslinking polymers has not been studied in detail. L.S.Efros et al. have proposed that the rubber polymer is crosslinkes in such a way that the aromatic nitrene inserts into an unsaturated bond of the polymer to give an aziridine ring. The experimental evidence for this, however, has not been given (8). 

Note The network strueture of a transient polymer network is based on transient junction points or crosslinks arising from interactions between polymer chains. 

Lele AK, Mashelkar R A. Energetically crosslinked transient network (ECTN) model implications in transient shear and elongation flows. J Non-Newtonian Eluid Mech 1998 75 99-115. 

A pseudo solid-like behavior of the T2 relaxation is also observed in i) high Mn fractionated linear polydimethylsiloxanes (PDMS), ii) crosslinked PDMS networks, with a single FID and the line shape follows the Weibull function (p = 1.5)88> and iii) in uncrosslinked c/.s-polyisoprenes with Mn > 30000, when the presence of entanglements produces a transient network structure. Irradiation crosslinking of polyisoprenes having smaller Mn leads to a similar effect91 . The non-Lorentzian free-induction decay can be a consequence of a) anisotropic molecular motion or b) residual dipolar interactions in the viscoelastic state. 

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