Physical crosslinking point

Physical hydrogels were formed induced by self-assembled water-insoluble and crystalline polypseudorotaxane domains which act as physical crosslinking points. Such physical hydrogels include the polypseudorotaxane systems of a-CD threading on PEO or its copolymers, a-CD threaded on PEI, PL, or their copolymers, and even 3-CD or y-CD threaded PPO or PEI copolymers. The thermo-reversible and thixotropic properties of these supramolecular hydrogels have inspired their applications as injectable drug delivery systems. Physical hydrogels induced by physical interaction of threaded CD molecules in polyrotaxanes were also developed. 

TTie presence of crystallites is very important for product performance because crystallites serve as physical crosslinking points which reinforce material and improve its viscoelastic properties. It is therefore very important to notice that the mechanism peculiar to PVC allows for melting of crystallites and their formation on cooling. Melting of crystallites improves PVC processability (lowers melt viscosity and improves its flow), and recovery of crystallites on cooling improves performance characteristics of PVC. The gelation depends on the melting enthalpies of primary and secondary crystallites. 

The elongation at the break of TPNR with MWCNTs is shown in Table 17.1. It can be deduced that the reinforcing effect of MWCNTs is very marked. As the MWCNTs content in the TPNR increases, the stress level gradually increases but at the same time the strain of the nanocomposites decreased. This is because the MWCNTs included in the TPNR matrix behave like physical crosslinking points and restrict the movement of polymer chains. This shows that the inclusion of MWCNTs makes the TPNR stronger but more brittle. 

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