Influence of the molecular structure of azelaic and sebacic acid esters on rheology and thermodynamics of viscous flow of PVC melts

Abstract

Polyvinyl chloride (PVC) remains one of the most sought-after polymers. Plasticizers play a key role in making PVC flexible, elastic, and processable during processing. In this regard, the development and implementation of safe alternative plasticizers is an urgent task. Research and optimization of the effect of the structure of new plasticizers on the rheological and structural-viscous properties of PVC melts are critically important for creating environmentally friendly, high-tech compositions with specified performance characteristics and improved processing parameters. The paper presents the synthesis and rheological evaluation of symmetric esters of azelaic and sebacic acids and ethoxylated butanol and phenol as PVC plasticizers. Esters dibutoxyethylazelate, dibutoxyethylsebacate, diphenoxyethylazelate, and diphenoxyethylsebacate have been found to provide a higher melt flow rate (MFI) compared with industrial dioctyl phthalate (DOP). DBES demonstrated the greatest efficiency. The structure of the plasticizer has a key influence on rheology: the lengthening of the dicarboxylic acid chain and the use of a linear alkyl fragment (butyl) instead of a volumetric aromatic one (phenoxyl) significantly increase the MFI due to better flexibility of the molecules. Thermodynamic analysis of the viscous flow revealed the dominance of the enthalpy barrier (ΔH) in the activation energy (Eₐ ≈ ΔH) associated with overcoming the dipole-dipole interactions of PVC. The observed maximum ΔH at ~165 °C is explained by a change in the molecular mechanism of the flow: from the local movement of segments to a cooperative restructuring with the destruction of temporary structures (entanglements, plasticizer clusters) in the intermediate range, requiring high energy consumption. The results confirm the prospects of DBES and DBEAz as highly effective PVC plasticizers combining improved workability and environmental safety