Local chain rigidity

The dipole associated with the maleimide unit, which is located in the plane of the maleimide cycle and perpendicular to the C - C bond taking part to the chain backbone. The motion of this resultant dipole implies a rotation of the rigid maleimide unit around the local chain axis. This structure is, in some way, analogous to that encountered with iso-phthalic units in aryl-aliphatic polyamides (Sect. 6) where the zso-phthalic rings only undergo small amplitude oscillations but no flips at temperatures below the glass transition temperature. Thus, it is unlikely that the maleimide dipole could be involved in the transition motions of CMIMx copolymers. 

Summary Solid state NMR studies of molecular motions and network structure in poly(dimethylsiloxane) (PDMS) filled with hydrophilic and hydrophobic Aerosil are reviewed and compared with the results provided by other methods. It is shown that two microphases with significantly different local chain mobility are observed in filled PDMS above the glass transition, namely immobilized chain units adsorbed at the filler surface and mobile chain units outside this adsorption layer. The thickness of the adsorption layer is in the range of one to two diameters of the monomer unit ( 1 nm). Chain units in the adsorption layer are not rigidly linked to the surface of Aerosil. The chain motion in the adsorption layer depends significantly on temperature and on type of the filler surface. With increasing temperature, both the fiaction of less mobile adsorbed chain units and the lifetime of the chain units in the adsorbed state decrease. The lifetime of chain units in the adsorbed state approaches zero at approximately 200 K and 500 K for PDMS chains at the surface of hydrophobic and hydrophilic Aerosil, respectively. 

Adsorption junctions at the surface of active fillers are of importance due to the large total elastomer-filler interfacial area. The adsorption of chain units at the Aerosil surface causes a significant restriction of local chain motions in the first layer adjacent to the filler surface. The low mobile adsorbed chain units represents another type of network junction in filled elastomers. However, the adsorbed chain units are not rigidly linked to the surface of Aerosil above T. The lifetime of chain imits in the adsorbed state is already very short at room temperature chain units adhere to the filler surface for only tens of microseconds [9], It was shown in Part I that the fraction of adsorbed chain units decreases on heating due to chain desorption. Therefore, the amount of adsorption junctions decreases with the increase of temperature as shown in Fig. 13. 

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