Impact of molar mass on melt memory effect of semicrystalline polymers

The crystalline properties of polymers depend on the cooling rate, heterogeneities and the melt state from which the material is cooled down. To obtain a standard crystalline state semicrystalline materials are heated to temperatures well above the melting peak and are cooled at a certain rate. Selecting appropriate temperatures the thermal history is erased obtaining an isotropic melt state which results in a standard Tc that will depend on the selected cooling rate. However, if the material is not heated to temperatures high enough to remove the thermal history, some self-nuclei can survive. Those self-nuclei increase the nucleation density increasing the crystallization temperature of the material, this effect has been termed melt memory effect.

Melt memory depends on several parameters including intermolecular interactions, topology and architecture, or chain rigidity. A few works have shown that molecular mass could have an impact on the melt memory effect, however systematic studies have not been performed. It is well known that when the molar mass is very low oligomer chains can adopt extended conformation within the crystal. Increasing molar mass chain folding occurs. Furthermore, entanglements depend on molar mass, for very short molar masses chains are unentangled. Above the critical molecular weight entanglements are formed which constitute a physical network. The conformation of the chain within the crystal and entanglements can impact melt memory but this has not been studied in literature. In this work we address this knowledge gap to gain insight in the origin of melt memory. By studying PEO standards we demonstrate that melt memory is trigger by entanglements, independently whether the chains adopt an extended or folded conformation within the crystals.

References

L. Sangroniz, D. Cavallo, and A. J. Müller, Macromolecules 53, 4581 (2020).

L. Sangroniz, A.J. Müller, and D. Cavallo, Macromol. Rapid Comm. 2400011 (2024).