Today's KNOWLEDGE Share : Breaking Polymer Chains

Today's KNOWLEDGE Share

Have you ever asked yourself if breaking a plastic part (or tensile bar) always means you are breaking polymer chains ?

It is not a trivial question and it is actually quite an important aspect to address if we want to better understand a polymer performance.

As it turns out it has a lot to do with the polymer chain entanglement density of the polymer of interest (and the temperature).

In a loosely entangled polymer, like Polystyrene, the lower ability to delocalize stress inside the network will allow an easier reach of the carbon-carbon bond strength limit, allowing thus significant chain scission when breaking a PS sample.

On the other side, highly entangled polymers like PC or PSU/PES/PPSU will spread the stress around the much denser entangled network, making carbon-carbon bonds way more unlikely to fail. The result is that failure will be dominated by disentanglement.

This has been proven by observing the significant appearance of free radicals (testifying chain scissions) on the PS fracture surface, contrary to the lack of free radicals for a PC fractured sample.

Of course, at very low temperatures, plasticity is almost totally suppressed, leaving chain scission as the only failure mechanism for all polymers, regardless of their entanglement density.

The Physics at play is not so different from what we observe in GF filled grades. Classical short GF (150-250 micron long) are too short to develop a stress higher than the glass stress at break, so fibers will be pulled out when breaking a sample.

LGF (long glass fibers, say longer than 1 mm) will typically break because the fibers are beyond the “critical length”, allowing the maximum stress in the fiber to reach the strength of glass.

source:Vito leo