Evolution of Nonlinear Rheology and Network Formation during Thermoplastic Polyurethane Polymerization and Its Relationship to Reaction Kinetics, Phase Separation, and Mixing

Abstract

Understanding the evolution of nonlinear rheological properties during polymerization has been of significant industrial and academic interest [1]. The industrial interest originated from the design, operation, and control requirements of polymerization processes [2,3]. The industrial polymerization processes cover a broad spectrum from bulk production of thermoplastics, such as polyolefins, to production of thermosets, such as tires, comprising many simultaneous steps, such as chemical reactions and mixing, followed by compounding, molding, and cross-linking reactions [4,5]. The most relevant and widely documented rheological property for success of industrial processes is the viscosity of the polymerizing medium, which directly influences mass, heat, and momentum transfer processes. The academic interest, on the other hand, has its roots in utilizing rheology as a valuable tool to understand the properties of polymer chains whether in bulk, in solution, or in a polymerizing medium [6]. Monitoring the changes in rheological properties, such as normal stresses, relaxation times, storage, and loss moduli, which are usually nonlinear functions of time, temperature, and morphology, has undeniable significance in providing insights into structure and properties of polymers as they develop during polymerization or during shape-forming operations such as of extrusion, injection molding, casting, and film blowing, to name a few.The nonlinear rheological properties of growing polymer chains, irrespective of the growth mechanisms -such as free-radical or step-growth -are governed by similar parameters. Chemical structure, flexibility, and mobility of chains, the available free volume during polymerization, the degree of conversion, the extent of chain entanglements, the degree of phase separation, and the presence of chemical or physical cross-links are a few of the morphological features and physical phenomena [7] with strong influence on polymerization.