In this poster I will highlight two projects that I have been involved with during my postdoctoral work as a member of the Bates group at the University of Minnesota. Following the conclusion of my postdoctoral work, I would like to begin a tenure track faculty position in a chemical engineering department. As a faculty member, the overarching theme of my research program will be focused on nanostructured polymeric materials including block copolymers, nanocomposites, thin films and membranes. One of the cornerstones of my research program will be polymer synthesis. The current palette of synthetic tools available to polymer scientists has facilitated nearly unbridled control of molecular scale architecture. I view synthetic capability to be paramount in designing and studying novel polymeric materials. The projects that I am currently involved with can be summarized as follows:

Polydispersity Effects in the Phase Behavior of Poly(isoprene-b-styrene-b-ethylene oxide) (ISO) Triblock Copolymers

The phase behavior of block copolymers has been studied intensely for several decades, both theoretically and experimentally. Despite this long history, relatively little attention has been focused on understanding the impact of polydispersity. Presumably, this has been a consequence of the living synthetic methods commonly employed for block copolymer synthesis that naturally produce polydispersities < 1.1. Nevertheless, the importance of polydispersity was noted as early as 1980 (1) when Leibler stated: “Much more important than fluctuation effects are those effects resulting from the polydispersity of the copolymer blocks…it seems that even a small polydispersity of the sample may be crucial.” Since 1980, there have been only a few experimental studies which have aimed to address this issue. We have recently studied (2) functional organolithiums that can be used to prepare α-hydroxy polystyrene with polydispersity indices ranging from ~1.1-1.6. Here we used 3-triisopropylsilyloxy-1-propyllithium (TIPSOPrLi) to prepare monomodal α-hydroxy poly(styrene-b-isoprene) diblocks containing a polystyrene block with M_w/M_n = 1.31 or 1.44. More than 15 ISO triblocks were synthesized from these two parent diblocks along the f_I = f_S isopleth. These studies (3) of polydispersity effects are unique in that they address the impact of polydispersity in the S block which is forced to bridge between the I and O domains. Surprisingly, we did not observe the orthorhombic network morphology which has been observed in monodisperse analogs. These results clearly illustrate the important role that polydispersity can play in the phase behavior of block copolymers and are a demonstration of the delicate nature of the free energy balance governing morphological behavior.

(1) Leibler, L. Macromolecules 1980, 13, 1602-1617. (2) Meuler, A.J.; Mahanthappa, M.K.; Hillmyer, M.A.; Bates, F.S. Macromolecules 2007, 40, 760-762. (3) Meuler, A.J.; Ellison, C.J.; Evans, C.M.; Hillmyer, M.A.; Bates, F.S. Macromolecules, submitted.

Nanofibers Produced by Melt Blowing

Melt blowing is a more than 50 year old polymer processing method for generating polymer fibers often as nonwovens. It employs a jet of hot air which creates an extensional force on an extruded polymer filament resulting in attenuation of its diameter by more than 1000 times in some cases. Although historically this method has been limited to the production of fibers exceeding 1-2 microns, we have recently demonstrated the ability to produce long, defect free fibers several hundred nanometers in average diameter. Traditionally, electrospinning has been the only processing method available for producing fibers on this size scale. However, electrospinning is inherently less desirable than melt blowing due to its low production rate, solvent requirement, and high operating voltage. In this study (1), nanofibers were produced from commercial melt blowing grades of polypropylene and poly(butylene terephthalate) using a laboratory melt blowing device (both single- and multi-orifice) which was designed after commercial equipment. This demonstration of melt blowing capabilities is a step towards closing the gap between electrospinning and melt blowing in terms of the ability to produce nanoscale fibers. In addition, it highlights the potential for increasing the number of applications served by melt blowing. Finally, we have attempted to understand the ultimate limitations of this process by exploring the processability of a series of low and high molecular weight polystyrenes and their blends. Surprisingly, conditions can be identified where exceedingly low molecular weight polystyrene (M_n = 2.6 kg/mol and PDI = 1.09) can be readily melt blown into defect free fibers with an average diameter less than 1 micron. This result illustrates how much is yet to be learned about this process.

(1) Ellison, C.J.; Phatak, A.; Giles, D.W.; Macosko, C.W.; Bates, F.S. Polymer 2007, 48, 3306-3316.