Small monomer units (spheres, colour-coded by polymer type) enter
from the left and link end-to-end into a single long polymer chain —
the degree of polymerization (DP) is simply how many monomers are
joined. Ordered, tightly-packed stretches of chain form crystalline
regions (drawn straighter and brighter); disordered stretches stay
amorphous. Hitting "Start degrading" simulates hydrolysis: ester or
other hydrolysable bonds in the backbone break, chain length falls,
and monomer fragments detach.
Mn = DP × M0 (number-avg molecular weight)
d(DP)/dt = −k_hydrolysis(T) · [hydrolysable bonds] (chain scission)
- Polymer type — PLA and PHA carry backbone ester bonds that microbes/water can hydrolyse (compostable); conventional PE has an all-carbon backbone with no such weak point, so it barely degrades on human timescales.
- Chain length (DP) — sets molecular weight and, generally, mechanical strength; very short chains behave like a viscous liquid, not a solid plastic.
- Compost temperature — industrial composting (~55-60 °C) speeds hydrolysis and microbial attack far beyond ambient conditions.
This is the core trade-off in sustainable plastics: durability in
use versus breakdown at end-of-life — a backbone chemistry that
resists water while in service but hydrolyses once composted.