Constraining polymer chains

Very interesting studies of natural rubber reinforcement with ZnO nanoparticles were performed by scientists from India, under the direction of Sabu Thomas [62]. The goal of these studies was to characterize the viscoelastic behavior and reinforcement mechanism of ZnO nanoparticles introduced into the rubber matrix. They have presented a constrained polymer model based on a rubbery region and a ZnO nanoparticle. Very interestingly, the authors presented a core-shell morphology model and constrained polymer model to explain the constrained polymer chains in NR/ZnO nanocomposites [62]. Thanks to this research and the proposed models, it is possible to understand the behavior of nanofillers in the polymer matrix and maybe in the future to develop an ideal nanofiller for use in the rubber matrix. 

The deformation of polymer chains in stretched and swollen networks can be investigated by SANS, A few such studies have been carried out, and some theoretical results based on Gaussian models of networks have been presented. The possible defects in network formation may invalidate an otherwise well planned experiment, and because of this uncertainty, conclusions based on current experiments must be viewed as tentative. It is also true that theoretical calculations have been restricted thus far to only a few simple models of an elastomeric network. An appropriate method of calculation for trapped entanglements has not been constructed, nor has any calculation of the SANS pattern of a network which is constrained according to the reptation models of de Gennes (24) or Doi-Edwards (25,26) appeared. 

Recently, a very interesting example of solvatochromism was reported by Fujiki and co-workers.206 Poly(methyl-3,3,3-trifluoropropylsilylene), 87, synthesized via Wurtz coupling, showed solvatochromism as a result of weak, non-covalent intramolecular Si- -F-G interactions which rendered the conformation of the polysilane uniquely controllable by solvent choice and molecular weight. UV, shown in Figure 18, photoluminescence, NMR, and viscosity studies on the polymer indicated a 73 helical rod-like conformation at room temperature in non-coordinating solvents (e.g., toluene and decane), since the intramolecular interaction resulted in constraining the chain in a rigid helix. 

Thermodynamic Considerations. Most polymerisations are characterised by a reduction in entropy as a large number of monomer molecules with the freedom to move in three dimensions are joined together and ultimately constrained to a linear ID polymer chain, where motion is much more restricted. Hence, for a ROP to be thermodynamically favourable under such circumstances, ring strain is usually needed to provide an enthalpic driving force, A/Zrop, that overcomes the unfavourable TAArop term that contributes to AGrop [eqn (8.1)]. 

There has been much general interest in polymer chains constrained by a second phase, in a variety of structures, as illustrated by a recent symposium on this topic [33]. Some particularly interesting examples of such constraining second phases are the zeolites, for which there is a very extensive literature, covering a considerable period of time [207-213]. The goal in the present application of a zeolite was to thread a polymer chain through its cavity, in the hope that the... 

In this type of system, the polymer chains are constrained by a surface. They can lie between two hard surfaces such as in the galleries within two parallel clay platelets (as is illustrated in Figure 7), have one layer absorbed on to a hard surface as a coating, with the other free (as in Figure 8), they can be absorbed by the surfaces of exfoliated clay platelets (Figure 9), or by the surface of a solid reinforcing particle completely surrounded by an elastomeric phase (Figure 10). 

Figure 7. Sketch of a layer of polymer chains constrained between two hard surfaces.

Figure 8. Sketch of a coating of polymer chains constrained below by a hard surface, but with a free upper surface.

Figure 10. Sketch of polymer chains in an elastomer constrained in the vicinity of a reinforcing filler particle. The constrained chains are arbitrarily shown by the heavier lines, and are referred to in the elastomer literature as bound rubber [233, 236-238].

Here, the polymer chains are constrained within a volume. An example is the collection of polymer chains going through the relatively large pores of a nanotube, as is illustrated in Figure 11. 

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