Chain elastomers

The characteristic property of elastomers is their rubber-elastic behavior. Their softening temperature lies below room temperature. In the unvulcanized state, i.e. without crosslinking of the molecular chains, elastomers are plastic and thermo-formable, but in the vulcanized state—within a certain temperature range — they deform elastically. Vulcanization converts natural rubber into the elastic state. A large number of synthetic rubber types and elastomers are known and available on the market. They have a number of specially improved properties over crude rubber, some of them having substantially improved elasticity, heat, low-temperature, weathering and oxidation resistance, wear resistance, resistance to different chemicals, oils etc. 

This was the first type of polymer used in pharmaceutical applications, and was found to have desirable characteristics in that its resilience provided sealing properties and this resilience could be developed to allow the rubber to be pierced by a hypodermic needle, resealing after removal. This high level of resilience is partially due to its chemical structure, it being a straight chain elastomer (Figure 12.1). 

Polyolefins A broad class of hydrocarbon-chain elastomers or thermoplastics usually prepared by addition (co)polymerization of alkenes such as ethylene. There are branehed and linear polyolefins and some are chemically or physically modified. Unmodified polyolefins have relatively low thermal stability and a nonporous, nonpolar surface with poor adhesive properties. Proeessed injection, blow, and rotational molding and extrusion. Polyolefins are used more and have more applications than any other polymers. Also called Olefinie Resins, Olefin Resins, and Polyolefin Resins. 

A second series of LARC-13 adhesives was synthesized containing aromatic amine-terminated silicone elastomers of varying chain lengths. Changing the elastomer repeat unit from n = 10 to n = 105 had very little effect on polymer properties. However when a 50 50 combination of long and short-chained elastomers were reacted into the LARC-13 amic acid, adhesive strengths were enhanced. A concentration of 15% by weight of elastomers was found to maximize adhesive properties for this system. 

Bergmann, G. H. E. Einkelmann, H. Percec, V. Zhao, M., Liquid-Crystalline Main-Chain Elastomers. Makromol. Rapid Commun. 1997,18, 353-360. 

The X-ray pattern taken at room temperature from a stretched sample is reported in Figure 11. A broadened wide angle reflection at the equator is evident. The smectic layer reflections can be observed at the meridian, indicating a perpendicular orientation of the smectic layers to the stress direction. The orientation is therefore dominated by the polymer chains, as it was reported also in the case of highly stretched fibers of a smectic main chain elastomer (35). 

Long, Highiy Flexible Chains. Elastomers consist of polymeric chains which are able to alter their arrangements and extensions in space in response to an imposed stress. Only long polymeric molecules have the required exceedingly large number of available configurations. 

The application of the previously discussed techniques to induce monodomain structures in side-chain liquid-crystalline polymers by the application of electric or electromagnetic fields, by shearing or on anisotropic surfaces, frequently leads to comparatively low, macroscopically uniform orientation. Additionally, the methods are limited to a sample thickness of about 100 pm. Liquid-crystalline side-chain elastomers do not have this restriction, because a high macroscopic orientation can be induced in polymeric networks by mechanical deformation up to a sample thickness of about a centimeter [103, 109]. The synthesis of such systems can be performed by crosslinking linear, side-chain liquid-crystalline polymers to networks [llOj. The inherent combination of rubber elasticity and liquid-crystalline phase behavior, may then be exploited for the induction of a macroscopic mesogen orientation by mechanical deformation. 

To obtain the liquid crystalline state in a polymer network, several strategies are conceivable. They are all based on well known principles evaluated during the last few decades for linear liquid crystalline polymers. The monomer units of the network have to consist of mesogenic moieties, which are either rigid rods or discs in the case of thermotropic polymorphism or amphiphiles in the case of lyotropic polymorphism. The mesogenic units can be attached either as side chains to the monomer units yielding side chain elastomers (Fig. la, b) or directly linked... 

Fig. 1 Different attachmcait geometries for the synthesis of LCEs side chain elastomers with end-on (a) or side-on (b) attached mesogenic side chains and main chain elastomers with mesogenic units incorporated end-on (c) or side-on (d) into the polymer main chain...

In the following sections some selected examples of the chemistry of LC networks will be summarized. While the synthesis of LC side chain elastomers mainly follows the radical polymerization technique and the polymer analogous addition reaction, LC main chain elastomers are exclusively synthesized by polycondensation or polyaddition reactions. 

In the following we will outUne two basic methods to synthesize LC side chain elastomers. As a starting point for the synthesis of LC elastomers a mixture of mesogenic monomers and bi- or multi-functional crossUnker molecules may serve. This will be discussed in the first part of the section. Alternatively, polymer analogous reactions, where the mesogenic moieties are attached to a polymer backbone, can be employed, which will be discussed in the second part of the section. 

An example for the synthesis of a side chain elastomer using radical polymerization of acrylates is presented in Scheme 1 and was demonstrated by Thomsen and co-workers [14]. Here, the mesogenic groups are attached side-on to the polymer-backbone. This attachment geometry is useful for a number of applications, because... 

The chemistry of main chain elastomers is limited to step-growth reactions, i.e., polycondensation and polyaddition reactions, which demand the highest purity of the starting materials and experimental conditions which exclude side reactions. 

Compared to side chain elastomers the preparation of main chain elastomers with suitable transition temperatures is rather challenging. What is more, due to the rigid rod-like mesogenic moieties in the polymer main chain, most linear main chain liquid crystalline polymers exhibit high clearing temperatures and tend to crystallize [23], making them unfavorable for many LC elastomer applications. 

Despite their more complex chemistry, main chain LC elastomers have gained much interest in recent years due to the direct coupling of the liquid crystalline order and the polymer backbone conformation. The ground breaking predications of de Gennes were also based on main chain elastomers [3]. 

Scheme 5 Combined main chain side chain elastomer synthesized by Wennter et al. [29]...

It is also possible to use the vinyl-terminated pre-polymer to crosslink a side chain LC polymer. This was demonstrated by Wermter et al. [29] who crosslinked a side chain end-on polymer similar to that shown in Scheme 3 with a main chain polymer carrying vinyl end-groups (Scheme 5). The side chain component can be understood as a multifunctiOTial crosslinker for the main chain elastomer and also acts as a plasticizer, further decreasing the transition temperature to about 90 C. Such combined main chain/side chain elastomers show, if oriented to a permanent monodomain (Sect. 4) extremely high length changes at the phase transformation to... 

Scheme 8 Components for the synthesis of main chain elastomers (MCLCEs) in a solvent-free one-pot reaction employing a photo-initialized thio-ene polycondensation [39]...

Ishige and co-workers [36-38] produced Sca main chain elastomers with transition temperatures of about 180 °C also utilizing a one-pot method, by a melt trans-... 

Fig. 5 Local polymer chain conformation of nematic side chain elastomers (with respect to the nematic director n) and resulting global chain conformation in the cholesteric phase structure (with respect to the cholesteric helix axis h)...

In some cases it is also possible to induce a global chain conformation of the LSCE opposite to the local chain conformation of the polymeric starting material which will be discussed for cholesteric and smectic-A side chain elastomers. 

This robust synthetic approach has frequently been used to produce LSCEs in the last few years as it works well for side chain as well as main chain elastomers. It is also applicable for polymer networks of different LC phase structures (smectic, cholesteric, lyotropic hexagonal) as long as they exhibit prolate chain conformations. We will therefore give a more detailed description in the following paragraphs. 

For main chain elastomers, a proper preparatirai of the reaction mixture is cmcial as volatile reactants are often involved and the exact stoichiometry has to... 

Once the reaction is successful, the elastomer strip is loosened from the centrifuge wall with the non-cutting side of a thin lancet. The elastomer film is carefully removed from the cell together with the Teflon support and cut into smaller (usually two or three) pieces. For cutting the swollen polymer gel the best method is to use a carpet knife and a hammer. That way, a clean cut of both the gel and the Teflon band is achieved. It is useful, especially if working with sticky side chain elastomers, to keep some toluene at the preparation table to clean the tools before reuse. If possible, both long rims of the film should also be cut with a carpet knife, because small defects at the rim can... 

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