In other words, the bonds are only broken if new ones are already formed, as a single thermally activated process. Vitrimers 14, 15, 16 are a unique sub-class of the dynamic covalent networks, distinguished by the associative covalent bond exchange chemistry, where the total number of the covalent cross-links remains constant maintaining the material integrity. However, in the last decade, there has been a rapid rise of examples of dynamic covalent networks, also called ‘covalent adaptable networks’, where the cross-links are exchangeable covalent bonds 11, 12, 13. Traditionally, this concept referred to physically cross-linked networks, held by hydrogen bonds 4, 5, hydrophobic interactions 6, 7, or other self-assembled constraints 8, 9, 10. Transient polymer networks can exhibit excellent mechanical and thermal properties at low temperatures, and can also be reprocessable and recyclable at high temperatures as thermoplastics 1, 2, 3. In creep, such materials can exhibit either strain thinning or strain thickening, depending on applied load, and present the phase diagram of this response.
We also investigate partial vitrimers including a permanent sub-network and an exchangeable sub-network where the bond exchange occurs. At large plastic creep, we describe the strain thinning when the material is subjected to a constant stress or force, and suggest another method to characterize the material parameters from the creep curves. We identify the elastic-plastic transition at a time scale comparable to the life-time of the exchangeable bonds in the vitrimer network, and propose a new method to deduce material parameters using the Master Curves. We describe the full rheology profile of vitrimers, from small deformation (linear) to large deformation (non-linear) viscoelastic behaviour, providing concise analytical expressions to assist the experimental data analysis, and also clarify the emerging insights and rheological concepts in the subject.
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