The above drawing represents
the crosslinking of polymer chains by covalent bonds (chains-black lines
and crosslinks are green stars). When polymers with pendant reactive
groups such as unsaturated polyesters are cured (reacted through the double
bonds) crosslinking occurs. The crosslinking sites act as anchors
and significantly affect the properties of the final material. In
the case of fiberglass, unsaturated polyester matrix resin is reinforced
with glass fibers and then cured. The crosslinked polymer is much
more brittle and harder than an uncrosslinked polymer.
An important property
from a characterization standpoint of crosslinked composites is the number
of crosslinked sites (green stars) and the molecular weight or length of
chain between sites. The molecular weight between crosslinks can
be obtained by swelling the polymer in a solvent. The following equations
relate the molecular weight per crosslink unit to the amount of solvent
uptake which occurs when you swell a polymer in a solvent.
Qualitative analysis of
crosslinking can be achieved by DMA and DSC
by noticing the degree in which crosslinking alters the Tg in the thermal
analysis. Mechanical tests (tension, compression, and creep) can
also be performed to investigate the affect of crosslinking on mechanical
properties.
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