Polymer surfaces, crystal nucleation affected

In semi-crystalline polymers the interaction of the matrix and the tiller changes both the structure and the crystallinity of the interphase. The changes induced by the interaction in bulk properties are reflected by increased nucleation or by the formation of a transcrystalline layer on the surface of anisotropic particles [48]. The structure of the interphase, however, differs drastically from that of the matrix polymer [49,50]. Because of the preferred adsorption of large molecules, the dimensions of crystalline units can change, and usually decrease. Preferential adsorption of large molecules has also been proved by GPC measurements after separation of adsorbed and non-attached molecules of the matrix [49,50]. Decreased mobility of the chains affects also the kinetics of crystallization. Kinetic hindrance leads to the development of small, imperfect crystallites, forming a crystalline phase of low heat of fusion [51]. 

Finally, shear viscosity is strongly affected by the clay in the blends, especially at high PEN contents. A lubricating effect rather than a filler effect reveals the possibility that the clay is not well dispersed in the polymer blend, and migration of particles in the flow to the wall region can explain the observed reduction in shear viscosity. When MMT clay is mixed with crystallizable polymers such as polyesters, some processing problems arise because the crystallization process is modifled by nucleation effects induced by the nanoparticles. Moreover, these particles also influence the kinetics of transesteriflcation between PET and PEN, besides other factors such as the reaction time and extruder processing temperature. In Reference 72, a quaternary alkyl ammonium compound (Cl8) and MAH were used to modify the surface properties of the clay... 

The rate and degree of crystallization may also be affected. Any interface, any surface, any defect in any system may act as a nucleation point, and the addition of more of these nucleation points wiU necessarily produce systems with finer-grained crystalline micro-structures. In nanocomposites, the material is full of nucleation sites as long as silicate dispersion is favorable, materials containing finer-grained polymer crystallites (versus the... 

In immiscible polymer blends with a high degree of immiscibility such as PP/PS, it has been shown that nucleation at the interface affects the crystallization behavior. Wenig et al. (1990) showed that with increasing the amount of PS in a blend with PP, the nucleation shifted from preferentially thermal (related to the degree of undercooling) to more athermal. This was explained by the effect of heterogeneous surface nucleation at PS interfaces (Fig. 3.53). 

For semicrystalline polymers, fillers may affect crystallinity, size of crystallites, and direction of crystal growth. Filler surface may provide a large number of nucleation sites, although this also depends on surface functional groups and surface treatments. In certain polymers, fillers may promote transcrystallinity, which can improve adhesion and other properties [10]. 

To understand and predict the properties of composites, it is necessary to realise that adding filler may affect the polymer phase, both chemically and physically. Chemical changes may occur if the filler, or impurities on the filler surface, catalyse degradation of the polymer. Alternatively, various physical changes may result from the incorporation of filler. Some fillers nucleate crystal growth in certain polymers, which in turn influences manufacturing and mechanical properties. 

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