Thermally reversible covalent bond polymer

Nitroxide mediated polymerization (NMP) is another type of controlled radical polymerization technique used to synthesize polymer hybrids. It relies on the reversible trapping of growing macro-radicals by nitroxide to form dormant species in which the C-ON covalent bond is thermally cleavaged (Fig. 19). At a polymerization temperature, the equilibrium between dormant and active species is strongly shifted to the dormant side, which Emits the irreversible chain termination reaction. 

In chemical gels the network connection (crosslink) is usually a covalent bond, which leads to a thermally irreversible gel. When the crosslinking is purely physical in nature, a physical gel is formed which is thermally reversible. There have been at least two excellent works published in the last few years. Reversible Polymeric Gels and Related Systems by Russo [2] and Thermoreversible Gelation of Polymers and... 

It should be noted that although no distinction is made between the endo and exo stereoisomers (Fig. 12.25) of furan-maleimide adduct (assuming that their thermal behavior is at least similar if not equal), differences in thermal reversibility (rDA, DA) can have an important in uence on the material properties, especially in the case of thermoremendable polymers and networks. For instance, by breaking only the covalent bonds of the endo DA-isomers inside the network imder mild conditions and reforming them in a less stressed state, stresses can be relaxed. Important material properties such as durability, adhesion to other materials, appearance of micro-voids and micro-cracks, etc., can be improved (Canadell et al., 2010). 

Thermoplastic Elastomers These new materials contain physical cross-links rather than chemical cross-links. A physical cross-link can be defined as a non-covalent bond that is stable under one condition but not under another. Thermal stability is the most important case. These materials behave like cross-Unked elastomers at ambient temperatures but as linear polymers at elevated temperatures, having reversible properties as the temperature is raised or lowered. 

Healable polymeric systems may, for example, contain encapsulated monomers and polymerization catalysts or latent functionalities, which are able to participate in thermally reversible (Cho et al, 2006), covalent bond-forming reactions (Adzima et al, 2008). It has also been shown that non-covalent interactions, specifically hydrogen bonds (Fouquey et al., 1990), may be used to promote healing within a supramolecular polymer blend (albeit in the presence of a plasticizing solvent). In the latter system, it is proposed that fracture propagates via the dissociation of the weak supramolecular interactions rather thanby scission of the covalent bonds, so that re-assembly of the supramolecular netwoik restores the original physical properties of the material. 

Since the ions in ionic polymers are held by chemical bonds within a low dielectric medium consisting of a covalent polymer backbone material with which they are incompatible, the polymer backbone is forced into conformations that allow the ions to associate with each other. Because these ionic associations involve ions from different chains they behave as crosslinks, but because they are thermally labile they reversibly break down on heating. lonomers therefore behave as cross-Unked, yet melt-processable, thermoplastic materials, or if the backbone is elastomeric, as thermoplastic rubbers. It should be noted that it is with the slightly ionic polymers, the ionomers, where the effect of ion aggregation is exploited to produce meltprocessable, specialist thermoplastic materials. With highly ionic polymers, the polyelectrolytes, the ionic cross-linking is so extreme that the polymers decompose on melting or are too viscous for use as thermoplastics. 

---