Topological crosslinking

In conclusion, simple symmetry considerations allow for a successful orientation of poly domain elastomers using mechanical fields. In principle knowledge is only needed of the local chain conformation of the LC polymer on which the elastomer is based, and the consistent mechanical deformation must be applied. Nevertheless, the chemical constitution of the whole polymer network has to be considered. Often, the orientational behavior is strongly influenced by the crosslinking topology. As a mle of thumb, prolate chain conformations are increasingly preferred when the crosslinker concentration is increased and when the crosslinker molecules are more rod-shaped [90, 91]. 

Thus, the increase of the crosslinking density in the LCEs results in a shift of their phase-transition behaviour towards and beyond supercritical. As this trend is now confirmed for both the side- and main-chain nematic LCEs, i.e. two LCE families with drastically different crosslinking topologies, it can be anticipated that the link between the crosslinking concentration and the phase-transition criticality holds universally for most types of LCEs. 

Greve A, Finkelmami H (2001) Nematic elastomers the dependence of phase transformation and orientation processes mi crosslinking topology. Macnnnol Chem Phys 202 2926-2946... 

Ziabicki, A, Topological Structure and Macroscopic Behavior of Permanently Crosslinked Polymer Systems, Polymer 20, 1373, 1979. 

The topological structure of condensation polymers is predetermined by the functionality of the initial monomers. If all of them are bifunctional then linear polymers are known to form. Branched and crosslinked molecules are prepared only when at least one of the monomers involves three or more functional groups. 

We have investigated the static and dynamic mechanical properties of networks of different chemical and topological structures ( 19,20). In a previous paper, we reported results obtained on networks with crosslink functionality four (21). In the present study, we investigated the effect of the structure of junctions on the mechanical behaviour of PDMS. Rather uncommon networks with comb-like crosslinks were employed, intending that these would be most challenging to theoretical predictions. 

The division by two denotes the topological requirement (6) of two inter-crosslink chains per DVB, in the present model of a closed X-type network structure. (Consider two X units with their chain ends joined together. There are four chain lines and two vertices.)... 

A greater molecular understanding will be required to interpret the difference between fixed (covalent) and mobile (polar) crosslinks and topological entanglement. 

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