P849: Physical chemistry pedagogy via macromolecules: Polymer blend thermodynamics and ionic polymerization kinetics

Author: Sianna Bates, Northeastern State University, USA

Co-Author: Brittney Rogers and Carl Aronson, Northeastern State University, USA

Date: 8/6/14

Time: 2:05 PM2:25 PM

Room: MAN 123

Related Symposium: S56

Within an undergraduate physical chemistry course sequence, macromolecules are routinely isolated and treated as a group of materials with specialized structure, size, dynamics, and self-assembly properties. Nevertheless, polymeric materials can be incorporated into the natural flow of the course and utilized to illustrate both thermodynamic properties and chemical reaction kinetics in the laboratory and lecture. Polymer blends were synthesized within the laboratory course portion wherein the composition dependence of the resulting blend’s glass transition temperature was measured using differential scanning calorimetry. Varying the macromolecular architecture allowed students to gauge the overall accessibility of functional groups involving a thermodynamic competition between hydrogen bonding and steric hindrance. In a separate application, anionic polymerization has been used to extend the lecture course portion on the kinetics of complex reaction mechanisms. Living anionic polymerization provides a mechanistic pathway to vary macromolecular architecture. Although commonplace in research, anionic polymerization lab techniques remain challenging due to the inherent air and moisture sensitivity of the reaction. Student generated computational pedagogical kinetic modeling of living anionic homopolymerization has been used to investigate the effects of temperature and impurity concentration on molecular weight and polydispersity. Arrhenius energetics were included for each mechanistic step. Thermodynamics governing the dissociation equilibrium constant between propagating free ions and ion pair chain ends was modeled using the van’t Hoff equation. The kinetic model allowed for identity changes in monomer, solvent, initiator, and counter cation as well as impurity concentration. The initial model was extended to compute the kinetics of sequential addition anionic block copolymerization.

P553: Physical chemistry pedagogy via macromolecules: Solvent diffusion from polymer solutions with lyotropic liquid crystalline capability

Author: Sianna Bates, Northeastern State University, USA

Co-Author: Brittney Rogers and Carl Aronson, Northeastern State University, USA

Date: 8/5/14

Time: 3:40 PM4:00 PM

Room: ASH 2302

Related Symposium: S23

Within an undergraduate physical chemistry course sequence, polymers are routinely isolated and treated as a group of materials with specialized structure, size, dynamics, and self-assembly properties. Nevertheless, macromolecular chemistry can be incorporated into the natural flow of the course and utilized to illustrate thermodynamic principles in the laboratory such as chemical potential gradients in non-equilibrium dynamics involving diffusion. In focusing on pedagogy specific to phenomenological transport fluxes and forces, thin films of either functionalized cellulose or poly(n-alkyl isocyanate) solutions were juxtaposed against air in a diffusion couple geometry at room temperature. The solvent was allowed to diffuse away and evaporate from the solution in a controlled manner. The diffusion couple geometry produced a uniform film for optical assessment of liquid crystalline potential between crossed polarizers. After an induction period, a stable microstructure developed in which the interior of the sample remained isotropic followed by a liquid crystalline band, with characteristic disclination defects and texture, followed by a crystalline band nearest to the external surface. The width of the total characteristic birefringent band was measured over time and provided information concerning the dynamics and trajectory of solvent transport and evaporation from the cover slip edge. The apparent solvent diffusion coefficient for each system was measured at room temperature as a function of initial polymer concentration. Diffusion couple optical microscopy data from the physical chemistry teaching laboratory were compared to engineering literature data concerning rheological characteristics as well as finite difference calculations in order to validate the observed concentration dependence of diffusion.