LDPE
Low density polyethylene contains significant chain branching so the polymer chains cannot pack close together. This means that solvents can enter between the chains causing the plastic to swell. The loose chain packing has a number of results including a low maximum stable temperature of 80 degrees C as well as making the plastic translucent, flexible and tough.
The branching results in weak intermolecular forces and low tensile strength.
HDPE
High density polyethylene contains little chain branching which results in the polymer chains packing closer together. Therefore there are stronger intermolecular forces and tensile strength.
The close chain packing means that the resulting plastic is hard, opaque and stable at up to 110 degrees C.