Why is LDPE softer than HDPE?

望‵:

size of the polymer molecule is not a major factor, given that molecular size of the polymers are already large; in this case, further increase in size has small effect on its physical properties.


even when we consider LDPE and HDPE ith similar molecular size, we can still observe that HDPE has greater physical strength than LDPE.


HDPE is catalyzed by ethene and ziegler-natta catalyst, so it's basically straight chain;
LDPE is catalyzed by free-radical initiators like organic peroxides, so it has more branching.

difference in structure (molecular shape) leads to different physical properties:
- straight-chain polymer has higher packing efficiency, can be packed closer, so intermolecular attraction is more effective in binding the molecules. hence HDPE is more stiff and hard.

- highly-branched polymer packs poorly, so the crystalline regions are less abundant; weaker intermolecular force leads to softer nature.

you are right
intermolecular strength cause this phenomenon
LDPE contains only about 5000 ethene molecules
the force is quite small when compare to HDPE
HDPE contains around 5000-30000 ethene molecules
for this reason
LDPE is more flexible while HDPE is tough.