Thermoplastic polymers are long-chain, more or less flexible molecules. In the melt, they have a random, coil-like conformation, whereby the polymer coils penetrate each other. It is known from scattering experiments (in particular neutron scattering ) that the mass distribution of a single chain in the melt can be described to a good approximation by a three-dimensional Gaussian distribution . On length scales that are larger than the persistence length and smaller than the chain's radius of gyration, the random conformation of the chain resembles the trace of a three-dimensional Brownian motion . Thus, a polymer ball in the melt is a self-similar object with the fractal dimension 2.
The complex structure of a polymer melt from interpenetrating, tangled macromolecules leads to complex dynamic properties. The mutual penetration drastically impedes the diffusion of the molecules. Polymer melts are therefore very viscous . The entanglements and entanglements of the interpenetrating balls have a similar effect on short time scales as cross-linking points in an elastomer , which is why a polymer melt reacts elastically to rapid deformation for a short time, similar to rubber. A polymer melt is a non-Newtonian fluid : With increasing shear rate, a decrease in viscosity can be observed in many cases ("shear thinning"): This can be attributed to a stretching of the polymer coils in the direction of flow and a decreasing density of entanglements with neighboring chains.