Hairy Rodlike Polymers

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Hairy Rod Polymers in Supercritical Fluids

Update (January 2, 2004).  This project has never been populated.  Interested students are encouraged to enquire with any of the investigators who may still have an interest in this or related research.    Professor Radosz has moved to the University of Wyoming, but can participate as an outside member. 

This project will: enable two IGERT students to synthesize, characterize and process materials based on rod-shaped polymers using environmentally safe supercritical fluids.

Primary Faculty co-Advisors:

• William H. Daly, Chemistry Department (Synthetic Polymer Chemistry)
• Maciej Radosz, Chemical Engineering Department (Thermodynamics)
• Paul S. Russo, Chemistry Department (Polymer Physical Chemistry)

Off-campus Participant: Gerhard Wegner, Director, Max Planck Institut für Polymerforschung, Mainz, Germany

Technical Proposal: Nature often selects rodlike macromolecules for structural applications, an important example being collagen fibers in skin and tendon. Human attempts to design structures with rodlike polymers are primitive by comparison. One of the main problems is that most rodlike polymers do not melt; unlike polyethylene and many other common polymers, it is usually impossible to mold rodlike polymers at high temperature. Rodlike polymers such as Kevlar are usually processed from solvents, often rather harsh or environmentally damaging solvents to overcome their poor solubility. The solubility of rodlike polymers improves if they contain spacer groups or flexible "side chains". Here is depicted a "hairy" rodlike macromolecule with flexible side chains. 

 At sufficiently high concentrations, rodlike macromolecules spontaneously form liquid crystals, which can be processed into very strong fibers or films. Films can be produced by successively dipping a plate into a liquid on whose surface floats a single layer of carefully spread rods (Langmuir-Blodgett method).

Production of three-dimensional, solid materials is not so highly developed, but such materials would offer many desirable features: light weight, directed strength, stability to high temperature and harsh chemicals and, possibly, directed response to stimulus. The objective of our IGERT team will be to understand and process hairy rodlike macromolecules into solid materials using supercritical fluids, including environmentally benign carbon dioxide. A supercritical fluid is a solvent that is neither gas nor liquid; like a gas, it expands to fill the available volume, but its density lies closer to that of a liquid. The most common example is carbon dioxide at pressures exceeding about 60 times atmospheric and temperature exceeding 30^oC (Although CO₂ is a greenhouse gas, the CO₂ is easily recovered and recycled, as will be the other supercritical fluids used in our work). Supercritical fluids do not have any surface tension, so evaporating and recovering the solvent does not generate the capillary forces that would destroy materials during processing. The research requires synthesis, molecular and thermodynamic analysis that underlies processing from supercritical fluids, and characterization of solid materials and precursor solutions.

Synthesis. Some of the most appealing macromolecules for fundamental study are based on a synthetic polypeptide structure, shown below.  Proteins share the same general structure, but the substituent groups (indicated by R, R', R" etc.) appear in a particular sequence in proteins.  Here only a few special R groups will be used. The resulting polypeptides will generally be soluble in organic solvents such as supercritical

propane; unlike proteins, they are not usually soluble in water.  These properties, and excellent size uniformity, make polypeptides great model systems for studying the fundamental characteristics of other rodlike polymers that may ultimately prove more practical in some applications, such as the poly(phenylenes) and functionalized cellulosics being developed at the Max Planck Institute.

To enhance the solubility of the peptides in supercritical CO₂, some of the substituent groups might be fluorinated alkyl chains. A typical polypeptide for this study poly(e-carbobenzoxy-L-lysine-co-decenyl-glutamate), is shown below (without fluorination).

The poly(e-carbobenzoxy-L-lysine-co-decenyl-glutamate) shown at left would be expected to form liquid crystals. The backbone amino acid used easily undergoes thermal transitions from helix to coil, making the polymer and structures made from it sensitive to temperature change. The decene double bond allows this polymer to be crosslinked efficiently and rapidly with catalysts developed by Grubbs' group (Schwab, 1995) as shown below.

Covalent networks of rods can be prepared from solutions as shown above (at right) for randomly oriented (isotropic) and aligned (liquid crystalline) cases. The March ACS National Meeting in San Francisco will feature an entire symposium devoted to production of aligned networks, reflecting the excellent control over materials properties such systems provide. After trying slower and harsher methods of crosslinking developed elsewhere,(Kishi,1990) the above chemistry was developed by former LSU student Drew Poche’ (Daly advisor, with an assist from Russo). It works quickly and reliably, but needs to be tested further and optimized. Mild discoloration must be reversed, and we must confirm that alignment is preserved when crosslinking liquid crystalline solutions. The student performing this research will learn in an integral fashion organometallic catalysis, spectroscopic techniques, light microscopy and small angle X-ray scattering for confirmation of alignment. Even with a somewhat smaller skill set, the student would be highly marketable: Poche' flirted with academia before taking a job in polyolefin characterization.

Molecular Analysis in Supercritical Fluids. The rodlike helical structure of the polypeptides is not guaranteed to exist in all solvents; it must be confirmed in each, including the supercritical fluids. Also, the state of aggregation of the rods needs to be assessed. These molecular scale analyses will begin in liquid solvents (as a control) and progress with student and instrumental capabilities towards supercritical fluids. The primary tool will be static and dynamic light scattering, which can measure the size of molecules as a function of their mass. Such size vs. mass relationships provide shape information (size increases linearly with mass for rods, whereas for spherical solids it increases as the cube root of mass). In simple solvents, this information is rapidly available from gel permeation chromatography/light scattering/viscosity (triple detector) measurements increasingly used in industrial labs. The student must master thermodynamics, optics, and data analysis to perform such research, reinforcing and motivating classroom instruction. We do not contemplate triple detector methods for supercritical fluids. Instead, dynamic light scattering will be correlated with the chromatographic results in simple solvents and then developed for supercritical fluids. We are now upgrading our NSF-funded dynamic light scattering facility; the IGERT student will make further alterations to enable measurements in supercritical fluids following recent progress here (Chan, in press) and elsewhere.(Szydlowski, 1998). This student will acquire the different skills of both Radosz and Russo.

Thermodynamic Analysis of Processing from Supercritical Fluids. The crucial challenge in processing polymeric materials from solution is how to separate the solvent. Sub-critical liquid solvents are separated from polymer by evaporation or by dilution with an antisolvent. The supercritical solvents, on the other hand, can be separated by rapid expansion, which is much faster and allows far greater flexibility in controlling the shape and size of particles and pores. Rapid-expansion separation is applicable to supercritical-fluid and polymer pairs that mix completely. The pairs that do not exhibit complete miscibility can be processed using a hybrid approach: we dissolve the polymer in a sub-critical liquid, but instead of evaporating the solvent, we pressurize the solution with a supercritical antisolvent to form the solid material.

In all these approaches, the final material morphology sensitively depends on the phase-diagram path and the rates of changing the pressure, temperature, and composition from the initial solution to the solvent-free material. Optimization of this path is the key to making desirable materials reproducibly. This, in turn, calls for understanding the solution phase behavior at high-pressures. While the phase behavior of polypeptide solutions in sub-critical liquid solvents is relatively well established, the behavior of rigid polypeptide solutions in supercritical and near-critical solutions is completely unknown.

In separate but related projects, however, we have studied the phase behavior of flexible polymers and copolymers, such as polyolefins, in supercritical fluids. We found that supercritical fluids can significantly shift the crystallizable solid-liquid and fluid-liquid transitions, relative to liquid solvents. We learned how to relate these shifts to the solvent and polymer structure. Appearing at right is a generic example of the pressure-temperature phase diagram at constant composition that captures the behavior of many amorphous-polymer solutions in supercritical and sub-critical solvents: Here, L stands for liquid, V for vapor, UCST for the upper-critical solution temperature, and LCST for the lower-critical solution temperature. The different U-LCST pairs of phase boundaries correspond to different polymer-solvent mixtures. These mixtures become completely miscible at pressures above the phase boundary. We need such a complete map of phase boundaries as a function of polymer characteristics (molecular weight, side-chain density and distribution, and rigidity) in order to design a solvent-separation paths and rates for material processing from supercritical fluids.

An example of relevant research is taken from our phase behavior and particle-formation study (Yeo, 1995) on rigid-rod aromatic-polyamide materials, similar to Kevlar. The hybrid approach (supercritical antisolvent precipitation) was used, where the primary solvent was a sub-critical liquid (for example dimethylsulfoxide = DMSO) and the antisolvent was a supercritical fluid (for example CO₂). The main result was that aromatic-polyamide type rigid-rods can be made miscible in DMSO-like aprotic solvents, and processed with supercritical CO₂, by attaching spacer groups along the aromatic-polyamide rods to reduce hydrogen bonding, and hence improve miscibility.   Our proposal to attach the hairy side chains to the polypeptide rods rests on a slightly different premise–we want to improve their chemical affinity to the solvent and sidechain mixing entropy– but it should produce a similar result: the rigid polypeptide rods should become miscible in supercritical fluids.

A second example confirms this hypothesis and proves the feasibility of this project. The polymeric solute in this preliminary experiment is a poly(stearyl-glutamate) (PSLG) having a mass of about 9000/mol. The solvent is propane. The pressure-temperature phase diagram for a 5 % solution suggests that PSLG is immiscible with propane below about 50^oC. The good news is that propane seems to be a good solvent for PSLG at higher temperatures as long as pressure is high enough, at least 300 bar in this case.  Such pressures are easily achieved in Radosz' lab. This phase diagram also suggests creative processing approaches: for example, dissolve PSLG in sub- or supercritical propane at a high pressure (say above 50^oC and 300 bar) and then precipitate by expansion, cooling or both. In addition to probing fluid-liquid and solid-liquid transitions, the team will look into isotropic-liquid crystal transitions and other solution properties. The chemical-engineering student working on this project will learn the high-pressure techniques and thermodynamics underlying high-pressure phase transitions in macromolecular systems, including the elements of modeling using equations of state. Industrial advisors confirm that these skills are highly marketable.

Characterization of Materials. Isotropic covalently crosslinked gels can be characterized by a correlation length, which may be thought of as the average distance between crosslinks. This will be determined from small angle X-ray scattering (the variation of intensity with scattering provides the necessary information). By the same method, liquid crystalline gels can be assessed for the degree of order that remains after crosslinking. To assess the damage that may occur during supercritical fluid extraction, the same parameters must be measured in the precursor solutions.

Number of IGERT apprentices to be recruited and probable home departments: Three--one from Chemistry & two from Engineering (theory & experiment).

Consistency with the Macromolecular Education, Research & Training theme: Macromolecules will be made from small monomers distantly related to biopolymers, characterized under challenging conditions no biopolymer ever saw, and processed into novel materials.

How does the project form a vector cross-product of existing research themes by the participants?

Existing research directions. Russo’s research group has a longstanding program concerned with the dynamics of rodlike polymers in normal liquid solvents, such pyridine or 98% sulfuric acid. Daly’s research group has a well-established program in the synthesis and modification of biosimilar macromolecules, including semi-rigid polymers. Radosz’ group has expertise in behavior of macromolecules in supercritical fluids. The Wegner group at the Max-Planck Institute in Germany is world-renowned for the development of hairy rod polymers as novel materials.

New research direction. This is a first attempt to characterize and process hairy-rod polymers in supercritical fluids. The probability of success, however, is high because this team has a unique set of resources and expertise to synthesize, characterize and process hairy rod polymers in supercritical fluids.

How do students benefit from the team-oriented research, beyond what would be available to them from either advisor separately? The immediate benefit from the student’s perspective is a larger pool of multidisciplinary expertise on which to draw. The long-term benefit is that such research, which cannot now be performed efficiently in any single group, places the student at the technical forefront. Our goal is to train a student who can "do it all" in this new field. Such a student would occupy a unique niche in the academic world, should he or she choose that path. With a broad background in synthesis, scattering, microscopy and high-pressure methods, the student would do equally well in an industrial environment.

Briefly describe the support level available to each individual faculty or off-campus participant (i.e., without IGERT) The LSU faculty involved are all independently supported for research in related fields, with a "net worth" of 8 grants totaling $700,000/year. Truly excellent resources await the student(s) who visit the MPIP in Mainz. This ensures a stable environment for the students, including healthy exchange of skills and ideas with postdocs, graduate students and undergrads.

Interdisciplinary strengths of the team project: Daly and Russo are at opposite ends of the wide spectrum of scientists that populate modern chemistry departments, while Radosz is an engineer. Daly is trained as an organic polymer chemist; typical activities in his group include synthesis of monomers, polymerization into macromolecules, purification, and various spectral identification techniques and simple analytical methods such as viscosity. Russo is trained in physical chemistry; typical activities in his group include preparation of solutions, light scattering and fluorescence photobleaching recovery measurements for physical properties like mass or diffusion, optical microscopy, optical design, and writing custom programs for instrument interface or data analysis. Radosz is an engineer. Typical activities in his group include high-pressure fluidics, polymer fractionation, developing thermodynamic theory and computer simulations of phase separation.

Commitment of faculty & off-campus participants to work side-by-side with apprentices: Russo enthusiastically commits one month full time to the side-by-side participation during winter holiday, between fall and spring semesters. We will dive right in to light scattering of polymers, but under conditions of high temperature not high pressure. The purpose will be an instructive miniproject involving molecular weight and diffusion measurements of one or two carefully fractionated polyethylene supplied by Dow Chemical. The polymers are not those ultimately to be studied, but this project provides a good opportunity to learn some basic experimental light scattering, face some difficult challenges (high temperature requires resourcefulness), strengthen industrial contacts in the key polyolefin area, and write a report—all in a reasonable time frame. Daly enjoys working with undergraduate students during the summer. This summer four undergraduate students worked with him on various aspects of polymer grafting and had the opportunity to learn organic synthesis and polymer modification techniques along with the typical approaches to structure identification. One of the polymers studied is used in a commercial pull-trusion process; future work will involve interaction with industrial scientists to ascertain if a concurrent pull-trusion/grafting process would yield superior composites. The project will give the students an opportunity to interact with an industrial processing laboratory and introduce them to some of the applied aspects of polymer science. Radosz also enthusiastically commits a total of at least one full month to the side-by-side lab and computational work with the student. He will start from the well-established high-pressure experimental approaches on polypeptide solutions in propane. Next, he will extend these experiments to other solvents, such as CO₂, and attempt to model the effect of variable rigidity on the phase behavior with equations of state. These faculty have worked together for several years on other projects [Chan, Poche' 1]. They often serve on student and university policy committees together, helped design the new curriculum, and have an excellent working relationship. If one must be gone for a short while, another could easily "look after" the student, offering general advice and clarifying the project vision but with less specific technical expertise.

References:

Poche', D. S., Daly, W.H., Russo, P. S., "Synthesis and Some Solution Properties of Poly(g -stearyl-a,L-glutamate)" Macromolecules, 1995, 28, 6745-6753.

Yeo, S. D.; Debenedetti, P. G.; Radosz, M.; Schmidt, H-W. "Supercritical Anti-Solvent (SAS) Process for Substituted Para-Linked Aromatic Polyamides: Phase Equilibrium and Morphology Study" Macromolecules 1993, 26, 6207 and 1995, 28, 1316.