Use google and google scholar to obtain more info on wax hybrids. Unfortuately many of the references from google scholar are from professional technical journals that require a fee or membership for downloading.
You shouldn't be surprised that the specific impulse of wax is high. After all, the only difference between methane, propane, gasoline, kerosine (RP-1), fuel oils, wax and lubricating oils, bunker oil, and tar and polyethylene is the the number of carbons in the molecular chain.
The generic chemical formula for a saturated hydrocarbon is CnH(2n+2).
Methane is CH4. Propane is C3H8, gasoline, kerosine, fuel oils, wax and lubricating oils, bunker oil, and tar are mixtures of hydrocarbons have average molecular of C5-C10, C10-C16, C14-C20, C20-C50, C20-C70, C70 and larger respectively.
https://www.schoolscience.co.uk/content/4/chemistry/fossils/p8.html
Polyethylene is a simply a polymer of ethylene with the formula CH3(CH2)nCH3 where n is a big number.
The biggest physical difference between the different classes of hydrocarbons is that some are gases, some are liquids and some are solids, and that the liquids and solids must first vaporize before they react completely.
They all react the same way in a rocket motor, and when they burn make CO, CO2, H2O and H2 in varying amounts depending on the oxidizer to fuel ratio.
The great advantage of a wax hybrid is that has the inherent simplicity of a solid and the Isp of a liquid. The primary problem with wax is that is that you have to add an Infrared opacificer (1-2% carbon black, not graphite) to it so that the thermal radiation from the flame does not melt the wax in depth. Other than that, it's a great fuel.
You can buy waxes that melt from 100 F to 600 F, and you can even buy black wax that is a high melting wax containing carbon black and used in industry for the temporary sealing of leaks in vacuum systems. Not all waxes have high shrinkage near the melting point either. It pays to shop around for the properties you want.
https://www.mcmaster.com has a fairly wide variety of waxes if you want to experiment.
Some black polyethylene rods and tubes contain carbon black and are the machinable equivalents of black wax suitable fo hybrid experiments.
Bob Krech