Chemically-cross-linked elastomers, synthesis

As pointed out by Hajji et al. [32], polymer blend nanocomposite systems can be prepared by various synthesis routes because of their ability to combine in different ways to introduce each phase. The organic component can be introduced as (1) a precursor, which can be a monomer or an oligomer (2) a preformed linear polymer (in molten, solution, or emulsion states) or (3) a polymer network, physically (eg, semicrystalline linear polymer) or chemically (eg, thermosets, elastomers) cross-linked. The mineral part can be introduced as (1) a precursor (eg, tetraethyl orthosilicate) or (2) preformed nanoparticles. Organic or inorganic polymerization generally becomes necessary if at least one of the starting moieties is a precursor. 

As already mentioned, four types of chemical structures are known for ferroelectric LC polymers side-chain polymers, main-chain polymers, combined ones, and cross-linked polymers, i.e., ferroelectric LC elastomers. All of them contain rodlike mesogenic moieties (Fig. la) but not disklike ones (Fig. lb). From the first publications to date, most research work has been carried out on side-chain polymers about 240 side-chain FLCPs versus 5 main-chain FLCPs [10]. Synthesis of the two other types of polymers under discussion is often related to the procedures used for side-chain FLCPs e.g., preparation of cross-linkable side-chain polymers is usually the first step in the synthesis of ferroelectric LC elastomers. That is the reason for the prior discussion of the synthesis of side-chain FLCPs. 

Figure 2-43. Chemical structure, transition temperatures and schematic presentation of the cross-linking processes involved in the synthesis of the investigated LSCE and reference elastomer [111].

Polysiloxanes gain more and more interest in recent years due to their excellent chemical and physical properties, such as thermal stability, hydrophobicity or electrical resistance (1). One synthesis route towards linear polymers is the ROP of cyclic monomers such as hexamethylcyclotrisiloxane (D3) (2). The functionalization of the substituents on the silicon atom in the polymer is performed via the modification of the cyclic monomers. Especially vinylic side groups are often introduced into the polymer backbone since they are active groups for cross-linking of polysiloxanes to three dimensional networks such as silicone elastomers (3). 

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