Topological trapping

Scheme 2. Pictorial representation of an entanglement 4, an interpenetrated polymer network 5, and a topologically trapped macrocycle in a polymer network 6.

After the initial reports on dendritic molecules [19], proposals have been made for the construction and applications of guest-host systems made out of dendrimers [1,2,35]. The concept of topological trapping by core-shell molecules is based on... 

Semiyen et al. have studied topological trapping of siloxane polymers.249,250 They measured the radii of different-sized siloxane rings and computed the potential energies for threading polymer chains through them. 

The reticulated structures are made up of clusters. If all the clusters have a finite size, the system is soluble and the solution is called sol. On the other hand, if the structure contains a cluster of infinite size, the system is a gel which is not soluble but which may swell in a solvent. The same reaction may lead either to sols or to gels according to the final branching rate. The sol-gel transition may be considered as a percolation transition. Note that an infinite cluster can be made either by chemical binding or (partially) by topological trapping [see Fig. 1.8]. From a mechanical point of view, a sol is viscous, a gel is elastic. Thus a piece of vulcanized rubber can be considered as a gel. 

Fig. 1.8. Cluster consisting of two disconnected parts, bound by topological trapping.

Sauvage et al. have elaborated special molecules with the topologies of catenated rings and knots [29]. The syntheses of these depend on metal coordination sites which direct the assembly of the components, but in general these catenanes and knots have few bonded connections. They are special because non-bonded sections are topologically trapped to coexist, thereby creating new opportunities for investigation of supramolecular relationships. A... 

Fig. 1. TEM picture of one gold cluster topologically trapped within a micro-network.

The active micro-reactors described above cannot be recycled because the SiH moieties cannot be renewed. Recyelable micro-networks may be realized in the form of passive miero-reactors which do not actively take part in the reaction but merely provide the confined reaction space. For this purpose hollow micro-networks are synthesized first, a micro-emulsion of linear poly(dimethyl-siloxane) (PDMS) of low molar mass (M = 2000-3000 g/mol) is prepared and the endgroups are deactivated by reaction with methoxytrimethylsilane. Subsequent addition of trimethoxymethyl-silane leads to core-shell particles with the core formed by linear PDMS surrounded by a crosslinked network shell. Due to the extremely small mesh size of the outer network shell the PDMS ehains become topologically trapped and do not diffuse out of the micro-network over periods of several months (Fig. 3). However, if the mesh size of the outer shell is increased by condensation of trimethoxymethylsilane and dimethoxydimethylsilane the linear PDMS chains readily diffuse out of the network core and are removed by ultrafiltration. The remaining empty or hollow micro-network collapses upon drying (Fig. 4). So far, shape-persistent, hollow particles are prepared of approximately 20 nm radius, which may be viewed as structures similar to crosslinked vesicles. At this stage the reactants cannot be concentrated within the micro-network in respect to the continuous phase. 

Clarson, S. J. Mark, J. E. Semiyen, J. A., Studies of Cyclic and Linear Poly(dimethylsiloxanes) 24. Topological Trapping of Cyclic Polymers into Unimodal and Bimodal Model Network Structures. Polym. Comm. 1987,28, 151-153. 

Huang, W. Frisch, H. L. Hua, Y. Semiyen, J. A., A Study of the Properties and Topological Trapping of Cyclic Poly(dimethylsiloxane) in Poly(2,6-dimethyl-1,4-phenylene oxide) Networks. J. Polym. Set, Part A Polym. Chem. 1990,... 

Galiatsatos, V. Eichinger, B. E., An Interpretation of the Topological Trapping of Cyclic Poly(dimethylsiloxane) in PDMS Network Structures. Polym. 

At e = 2, the interface starts to shed defects and doubles (Figure 15.2). Figure 15.3 is a snapshot after the start of defect shedding. The memory of the position of the defect line at the interface at the time it was shed is topologically trapped thus decorating the breathing mode for Figure 15.3. 

Topological Trapping to Stabilize Incompatible Blends and Introduce Localized Mobility... 

Huang, W., Frisch, H.L., Hua, Y., and Semiyen, J.A. (1990) A study of the properties and topological trapping of cyclic poly(dimethylsiloxane) in poly(2,6-dimethyl-l,4-phenylene oxide) networks. Journal of Polymer Science Part A-Polymer Chemistry, 28,1807—1812. 

Wood, B.R., Joyce, SJ., Scrivens, G. et aL (1993) Cychc polyesters 2. Topological trapping experiments and... 

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