HDPE is very stiff, MDPE is kinda stiff and LDPE is flimsy. Same monomer just cross linked differently with a different production process. Plastic is chemistry magic.

It's just carbon and hydrogen chains. How complex can it be? Surely not so complex to require its own field.

/s

For those unaware of the joke, Organic Chemistry is quite complex. Hydrogen+Carbon can make plastic or Gasoline depending on the details of how it chains.

I’m aware, but HDPE is less stiff than other commodity plastics like ABS or PLA.