Molding Materials Structural Aspects

As already described earlier, three-dimensional crosslinked thermosets are relatively aging-resistant, even without stabilizing additives. Any differences result in particular from the chemical raw materials of the numerous resin versions. Glass transition temperature is one important parameter, characterizing the heat distortion stability of the resin and simultaneously providing information on crosslinking density. 

Any structural and property differences between the various UP resins are determined by their raw materials components used during the esterification process in their formulation [575]. For the basic dicarbonic acids or anhydrides, the following relationships are noted  

Orthophthalic acid anhydride is an esterification component of most standard 

Isophthalic acid is used for resins with increased hydrolysis resistance and increased glass transition temperature, 

Adipic acid is used in elastic types because of the aliphatic chain length between the functional groups, 

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In the final step, the embossed substrate is cooled to a temperature below the Tg, where the plastic material transitions back to the rigid state. After reaching this lower cycle temperature, the embossing master is mechanically separated from the polymer substrate that now contains the desired microfeatures. Separation is a critical step due to the large forces that act on the polymer structure and mold. This is particularly tme for microstructures with vertical sidewalls and high aspect ratios. The overall cycle time of the embossing process is typically 1-10 min. 

The increased interest in thermotropic wholly aromatic LC polyesters is related above all to their commercial applications. Their ability to exist as anisotropic melts, combined with high molding and extrusion characteristics, allows very easy processing of these materials into various pats and articles. The fundamental aspect of this interest is associated with the fact that, with these polymers, it is possible to examine the effect of chain stiffness and of the influence of regular chain structure on evolution of macromolecular order in polymers on the whole. 

Figure 5.6 Lithography by hard X-rays using a proximity gold mask. The X-rays produce a different solubility in etching solutions between the exposed and unexposed regions. This approach is typically used for the fabrication of polymer-based resist with high aspect ratio (a). After that the resist is used as a mold or for structural application (b). The proper choice of a functional material in a weakly or non-cross-linked state such as sol-gel in the xerogel state can offer several advantages for the preparation of functional devices in a one-or two-step process. Film patterning can...

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