Why do big thermoplastics (like Polytetrafluoroethylene) behave like thermosets?
Thermosets are liquids or soft solids which irreversibly 'set' or 'cure' when heated. This 'setting' produces covalently  bonded cross-linkages between adjacent molecular chains, resulting in in their mechanical rigidity.

Thermoplastics too are formed from linear polymer chains, but their molecules typically have very long chains with high molecular weight, with very few cross-linkages. The weaker van der Waals bonds which link adjacent molecules in thermoplastics are easily broken when heated, allowing the chains to 'slide' past each other, thereby giving them the property of 'softening' when heated.


  Why does a big molecule break down before the van der Waals forces
  break?


In the case of polytetrafluoroethylene (PTFE), we have a very long, linear polymer chain of carbons covalently bonded with fluorine. PTFE is quite unique because it doesn't have any hydrogen atoms. Instead, it has these large highly electronegative fluorine atoms.



The bonds within the PTFE molecule are very strong (C-C bonds are 607kJ/mole and C-F bonds 552kJ/mole, much stronger than the typical C-H bonds found in other plastics such as polyethylene).  In effect, the PTFE molecule is like a 'rod' of carbons bonded to a surrounding 'blanket' of fluorine atoms (arranged in a slight helical twist).

Although the van der Waals bonds which form between adjacent the adjacent linear 'rod' molecules of PTFE are much weaker than the C-C and C-F bonds within the molecules, these van der Waals forces are able to combine in an additive manner, so the long straight PTFE 'rods' are able to be 'packed' together in a hexagonal arrangement, a bit like pencils in a box. This results in PTFE having a relatively high density and melting point of $327^o C$.

But when PTFE melts, its extremely long molecular chain have a high probability of entanglement, resulting in a extremely high melt viscosity. Although it's melt viscosity varies with the shear stress applied and temperature, in the temperature range between $360^o C$ and $380^o C$ and shear stresses of $0.1MPa$, commercial samples of the granular polymer display viscosities of the order of $10^{10}Pa.s$. This is exceptionally high compared with most other engineering thermoplastic melts. In fact, at atmospheric pressure, it hardly seems to 'melt' in the conventional sense of 'becoming a flowing liquid', although it becomes slightly softer, it forms a near-clear gel with very high elasticity.

Even below it's melting point, PTFE will start to decompose at $260^o C$, releasing toxic compounds through sublimation. When heated above $360^o C$ thermal decomposition is more rapid and the PTFE degrades noticeably. As a consequence, PTFE can not be processed by many 'conventional' thermoplastic techniques, such as injection molding. Instead, it is processed in a finely powdered or granular form, using modified powder metalurgical techniques such as cold-molding, sintering, compression and extrusion, to fabricate rods, tapes, wire, tubing, sheeting and other profiles which are typically suitable for further machining.

Above $480^o C$ PTFE thermally decomposes in air to release a mixture of vapours and particulates, including carbonyl fluoride, hydrogen fluoride, tetrafluoroethylene and other fluorinated hydrocarbon products, including a small amount of some highly toxic species such as perfluoroisobutylene.

The precise nature of the reactions responsible for this decomposition is still subject to research, but it appears likely that it is initiated by random-chain incisions in the PTFE chain, followed by depolymerization.