Controlled Reinforcement Orientation

If an op-director and an m-director are not reinforcing, the op-director controls the orientation. (The incoming group goes mainly ortho to the m-director). 

This brings us to double stereoselection and reinforcement of the mechanisms. If the site (a)symmetry were to control the orientation of the chain, and if, then, the orientation of the incoming propene is controlled by both the chain and the site, the highest stereoselection is obtained when the two influences reinforce one another. For 1,2-insertion this can be done most effectively for isotactic polymerization, since chain-end control naturally leads to isotactic polymer and this we can reinforce by site control with ligands of the bis(indenyl)ethane type. The chain-end influence of short chains is smaller than that of longer polymer chain and therefore short chain ends lead to lower selectivities. It may also be irrferred that making syndiotactic polymer via a 1,2-insertion mechanism on Ti or Zr complexes is indeed more difficult than making an isotactic polymer, because the two mechanisms now play a counterproductive role. 

It was also noticed that the use of prepreg enables reinforcement to be achieved without prior melt compounding, and it is easier to control the orientation needed in obtaining a particular laminate system, for example a quasi-istropic form. 

See Chapter 8 for details on the coining process. With reinforced plastic this process (also called injection-compression molding) provides a means of controlling fiber orientation, and so on. 

Quantitative predictions of the effects of fillers on the properties of the final product are difficult to make, considering that they also depend on the method of manufacture, which controls the dispersion and orientation of the filler and its distribution in the final part. Short-fiber- and flake-filled thermoplastics are usually anisotropic products with variable aspect ratio distribution and orientation varying across the thickness of a molded part. The situation becomes more complex if one considers anisotropy, not only in the macroscopic composite but also in the matrix (as a result of molecular orientation) and in the filler itself (e.g., graphite and aramid fibers and mica fiakes have directional properties). Thus, thermoplastic composites are not always amenable to rigorous analytical treatments, in contrast to continuous thermoset composites, which usually have controlled macrostructures and reinforcement orientation [8, 17]. 

An interesting application of surface-induced alignment of LCTs is orientation at the surface of fibers in fiber-reinforced composites. Adams and Mallon have shown that a low molar mass liquid crystal is oriented at the surface of a carbon fiber parallel to the fiber direction (111), while Sue and co-workers foimd that 1 can be oriented along the fiber direction, depending on the cure schedule and the epoxy/hardener formulation (112). These findings raise the possibility of creating fiber-reinforced composites with controlled matrix orientation and/or tailored interfaces, which may improve the fiber/matrix interface and result in improved properties (see Reinforcement). 

Uniaxial tensile deformations give prolate (needle-shaped) ellipsoids, and compressive or biaxial deformations give oblate (disc-shaped) ellipsoids. Prolate particles can be thought of as a conceptual bridge between the roughly spherical particles used to reinforce elastomers and the long fibers frequently used for this purpose in thermoplastics and thermosets. Similarly, oblate particles can be considered as analogs of clay platelets used to reinforce a variety of materials with dimensions that are controllable. " The orientation of nonspherical particles is also of considerable importance because of the anisotropic reinforcements such particles provide. 

Expansion or contraction can be controlled in the plastic by orientation, cross-linking, adding fillers and/or reinforcements, etc. Any cross-linking has a substantial beneficial ef-... 

A designer can produce RP products whose mechanical properties in any direction will be both predictable and controllable. This is done by carefully selecting the plastic and the reinforcement in terms of both their composition and their orientation, and... 

Expansion and contraction can be controlled in plastic by its orientation, cross-linking, adding fillers or reinforcements, and so on. With certain additives the CLTE value could be zero or near zero. For example, plastic with a graphite filler contracts rather than expands during a temperature rise. RPs with only glass fiber reinforcement can be used to match those of metal and other materials. In fact, TSs can be specifically compounded to have little or no change. 

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