Flexible macromolecules easily become entangled with neighboring macromolecules. The resulting network determines many polymer properties, including rheological and mechanical properties. Therefore, a number of experimental and modeling studies were performed to describe the relationship between the degree of entanglement of macromolecules and polymer properties. The introduction presents general information about the entanglements of macromolecule chains, collected on the basis of studies of equilibrium entangled polymers. It is also shown how the density of entanglements can be reduced. The second chapter presents experiments and models leading to the description of the movement of a single macromolecule. The next part of the text discusses how the rheological properties change after partial disentangling of the polymer. The results on the influence of the degree of chain entanglement on mechanical properties are presented.

Macromolecule entanglements are common in polymers. The first part of this review describes their influence on the properties of entangled polymers. Then, methods for reducing the entanglement density of macromolecule chains are discussed. It has been shown that research on partially disentangled polymers has provided a lot of new information about the relationship between the entangled state and properties of polymers. This research concerns, among others, mechanical and thermal properties and the crystallization process. A special disentangled polymer case, ultra-high-molecular-weight polyethylene, is also discussed. The results of research on polymer composites in which macromolecules were disentangled via processing and composites were produced using already disentangled polymers are presented in particular detail. It has been indicated that such composites and blends of disentangled polymers are promising and will probably be intensively researched in the near future.

The influence of decreasing the entanglement density of macromolecules on the crystallization of the β-form of polypropylene was investigated. Polypropylene with seven times less entanglement was obtained from a solution in xylene, and its properties were compared with those of fully entangled polypropylene. To obtain a high β-phase content, the polymer was nucleated using calcium pimelate. In non-isothermal crystallization studies, accelerated growth of β-crystals was found, increasing the crystallization temperature. Also, the isothermal crystallization was fastest in the nucleated, partially disentangled polypropylene. Increased growth rate of spherulites and enhanced nucleation activity in the presence of more mobile macromolecules were responsible for the high rate of melt conversion to crystals in the disentangled polypropylene. It was also observed that the equilibrium melting temperature of β-crystals is lower after disentangling macromolecules. Better conditions for crystal building after reduction of entanglements resulted in enhanced crystallization according to regime II.

The properties of compatibilized blends of polyethylene (PE) and polypropylene (PP), having reduced macromolecular entanglements, were studied. The density of PP macromolecular entanglements was controlled by prior disentangling in solution. The polymer ratio in the blend was 4:1 or 1:4. An ethylene–octene copolymer was used as a compatibilizer. The melt blending process resulted in good dispersion of the minority component, with slightly larger inclusions when more disentangled PP was used. Rheological studies confirmed the achievement of different entanglement densities of PP macromolecules in the blends. The partial disentanglement did not affect the thermal stability of the blends. During the isothermal crystallization studies, faster growth of PP spherulites was observed in the blend with reduced entanglements, which also influenced the entire crystallization process. The recovery time of equilibrium entanglement was investigated and it turned out to be 45 min if the blend was annealed at 190 °C, which was shorter than in the analogous homopolymer. Studies of tensile properties showed that in blends with a majority share of polyethylene, the elongation at break increased with the disentanglement of the minority component, due to better bonding of the blend components and thus the reduction in microcavitation.

Polylactide (PLA) having partially disentangled macromolecules was blended by extrusion with an equilibrium-entangled poly(methyl methacrylate) (PMMA). Partial disentanglement of PLA macromolecules was obtained by dissolving the polymer in methylene chloride, freezing the diluted solution, and removing the solvent. Blends with PLA macromolecules with three levels of entanglements, containing 10, 20 or 80% of PMMA, were characterized using rheological studies and by conducting non-isothermal and isothermal crystallization. Scanning electron microscopy showed that only a small fraction of PMMA did not disperse at the molecular level. The blends prepared using less entangled PLA had lower values of rheological moduli and viscosity, which confirmed the limited density of macromolecular entanglements in them. It was found that the crystallization of PLA in blends was controlled by two main factors: the disentanglement of PLA macromolecules, which promoted faster crystal formation, and the presence of PMMA macromolecules, which hindered the movement of PLA macromolecules to crystal growth sites, making crystallization more difficult. Spherulites grew in blends containing 0–20% PMMA. PMMA macromolecules were incorporated into their structure.