Cyclic trapping network

Poly(dlmethyl siloxane) 92 Cyclics trapped to form catenanes with poly(2,6-dlmethyl-1,4-phenylene oxide). 26% w/w cyclics permanently captured In network. Ip, phase domains seen after extraction mechanical and thermal properties discussed. [104]... 

Effect of Trapped Cyclics in Networks, if cyclic molecules are present during the end linking of chains, some of them will be trapped because of having been threaded by the linear chains prior to the latter being chemically bonded into the network structure (5,102). The fraction trapped is readily estimated from solvent extraction studies. Some typical results, in terms of the fraction trapped as a function of degree of polymerization of the cyclic (103), showed that, as expected, very small cyclics do not get trapped at all, but almost all of the largest cyclics do. 

If cyclic molecules of PDMS are present during the end linking, they are trapped within the network if they are large enough to be penetrated by any of the precursor chains [5]. This "incarceration" process has also been successfully simulated [51]. 

The cyclic diluents can also be sorbed into the networks after the end-linking process [191] or they can be present during the process [193-195]. In the latter case, some are permanently trapped as will be described below, making difficult the calculation of diffusion coefficients from the extraction data. In the former case, however, D is readily calculable. For both the cyclic and linear chains, D was found to decrease with increase in Afd, and with decrease in Mc, as expected. The cyclics were found to have values of D larger than those for the linear chains, presumably because their greater compactness facilitates their transport through the network structure. 

Figure 4. Sketch of the effects of having cyclics present during the end-linking of functionally-terminated chains to form a network structure. Cyclics such as a, which are not threaded by such a chain before its end linking will be extractable from the subsequently prepared network. Cyclics which have been threaded, such as b, would be permanently trapped, and thus unextractable [196],...

These cyclics can change the properties of the network in which they are trapped. Because they restrict to some extent the motions of the network chains, they should increase the modulus of an elastomer. Some small but possibly significant increases in low-deformation moduli have, in fact, been observed [192], Also, when PDMS cyclics are trapped in a thermoplastic material, they can act as a plasticizer that is in a sense intermediate to the usual external (dissolved) and internal (copolymerized) varieties. Interesting changes in mechanical properties have been observed in materials of this type [197]. 

This trapping technique can also be used to form networks with no cross-links. Mixing the same types of linear chain with large amounts of the cyclics and then functionally end-linking them could give sufficient cyclic interlinking to yield an Olympic or chain-mail network [3, 193, 200, 203], as is illustrated in Figure 5 [193], Attempts have been made to prepare and characterize such materials, because they could well have unusual elastomeric properties [204],... 

Figure 4.12 Sketch of the trapping of cyclics during the end-linking preparation of a network.292 Reproduced by permission of the American Chemical Society.

Figure 9. Trapping of cyclic molecules during end-linking preparation of a network. (Reproduced from reference 47. Copyright 1987 American Chemical...

Mark and Semiyen, in a series of papers, have studied the mechanism and the effect of trapping cyclics in end-linked elatomeric networks [100-103], Sharp fractions of cyclics of polyfdimethylsiloxane) (PDMS), varying in size from 31 to 517 skeletal atoms, were mixed with linear chains for different periods of time and the linear chains were then end-linked using a tetrafunctional silane. The untrapped cyclics were extracted to determine the amount trapped. It was found that while cyclics with less than 38 skeletal atoms were not at all trapped, for n>38, the percentage of cycUcs trapped increased with size, with 94% trapped in the case of the cychc with 517 skeletal atoms. In effect, the system of trapped cycUcs in the end linked PDMS network is a polymeric catenane. It is thus possible to control the elastomeric properties of the network by incorporating the appropriate sized cyclics. This study has been extended to cyclic PDMS in poly(2,6-dimethyl-l,4-phenylene oxide) [104,105] and cyclic polyesters in PDMS [106]. 

TRAPPING OF CYCLIC OLIGOMERS WITHIN NETWORK STRUCTURES... 

If cyclic molecules are present during the end linking of chains, some will be trapped because of threading by the linear chains prior to the latter being chemically bonded into the network structure (figure 7.28). - The... 

A tetrafunctional network containing cyclics (heavy lines). Cyclics a and b were trapped by linear chains that passed through them prior to end linking into a network structure. 

It may also be possible to use the trapping technique to prepare networks having no cross links whatsoever. Mixed linear chains, with large amounts of cyclics, are difunctionally end linked to yield an Olympic or chain-mail network (figure 7.30). Such materials are similar in some respects to the catenanes and rotaxanes that have long been of interest to a variety of scientists and mathematicians. " Computer simulations could establish the conditions most likely to produce these novel structures. 

Mark, J. E., Sorption, Extraction, and Trapping of Linear and Cyclic Molecules in Model Elastomeric Networks. J.Appl. Polym. Sci. Symp. 1989,44, 209-216. 

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. 

DeBolt, L. C. Mark, J. E., Models for the Trapping of Cyclic Poly(dimethylsiloxane) (PDMS) Chains in PDMS Networks. Macromolecules 1987,20,2369-2374. 

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. 

---