last update: March 28, 2001
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STC-ERSP Program Details Principal Investigator: Michael Rubinstein Project Title: Association and Formation of Reversible Networks (#21) Phone/Fax: (919)-962-3544/(919)-962-9312 E-mail: mr@unc.edu Research Plan Connectivity Requested Budget Allocation - Year 1 Research Plan Overall objectives Rubinstein proposes to develop a theoretical approach to elucidate the static and dynamic properties of associating polymers. A systematic theoretical study of the effect of specific segment-segment interactions will allow us to understand the key processes governing the phase separation and reversible network formation in highly compressible solvents like CO2. We will investigate the thermodynamic and dynamic properties of the reversible networks as a function of the material variables and CO2 pressure and temperature. Relation to overall objectives of the Center The rheological properties of polymer solutions and reversible networks in liquid and supercritical CO2 systems are extremely important in future applications of this environmentally friendly technological platform. Methods of controlling the viscosity of these systems can be utilized to improve processability and to reduce energy costs. Approach and Year 1-Year 5 timelines Years 1-2: We will develop theoretical and computer models to describe the thermodynamic properties of associating polymers in dilute and semidilute polymer solution. The result of this study will be the theoretical phase diagram of associating polymers as a function of the polymer concentration, polymer molecular weight, strength and number of the reversible bonds per chain (length and strength of CO2-phobic segments), CO2 pressure and temperature. Years 2-3: We will investigate the dynamic properties of the reversible networks. By the analogy with polymer solutions the dynamics of reversible networks can be separated into two regimes - unentangled and entangled networks. In the unentangled networks the motion of the chains is not topologically constrained by the surrounding polymers. But reversible bridges between neighboring chains act as high friction centers and significantly slow down the motion of the chains. The dynamics of the unentangled reversible gels will be described by the 'sticky-Rouse' model. In the framework of this bead and spring theory the mobility of beads (the friction coefficient of beads) is controlled by the lifetime of the reversible interchain bridges. Above the entanglement threshold the sticky motion of the chains has to be modified to allow polymers to reptate along the confining tubes formed by their neighbors (the sticky-reptation model). Using these models we will describe the linear dynamics of the system and will calculate the dependence of the visco-elastic properties of the solution of associative polymers on the polymer concentrations, polymer molecular weight, the number and molecular weight of CO2-phobic and CO2-philic blocks. Years 3-5: We propose to investigate the non-linear dynamics of the reversible networks. The viscosity of the solution can be independent, decrease (shear thinning), or even increase (shear thickening) with shear rate depending on the content of CO2-phobic block, polymer concentration, solvent quality, and shear rate. The shear thickening phenomenon results from increased overlap between stretched elastic strands (bridges between micelles). This higher overlap leads to the increase in the number of bridges that provides an additional resistance to the flow and leads to the increase of viscosity. Shear thinning begins at the shear rates at which shear forces disrupt the reversible network by reducing the number of bridges lowering the resistance to the flow as well as the solution viscosity. We are planning to extend models to calculate non-linear rheological properties of associative polymers in supercritical CO2. Thrust area of this proposal Thrust Area C: Rate Processes Connectivity Collaborators, multi-institutional, multi-disciplinary components Dynamic properties (rheology) of associating polymers will be investigated in collaboration with Khan (NCSU). Related work in other thrust areas The parameters essential for this study such as solvent quality for CO2-phobic and CO2-philic blocks will be measured by DeSimone (UNC-CH) and M. Adam (Paris, France) and will be calculated in computer simulations by Berkowitz (UNC-CH). The researchers in Thrust Area A (Interfacial and Colloidal Science in Compressible Media) will be studying thermodynamic properties (phase diagram) of associating polymers. Sharing of resources (students, supplies, equipment, etc.) The computer system used for numerical simulations will be shared with Berkowitz (UNC-CH). Requested Budget Allocation - Year 1 Personnel salaries
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