Controlling the Polymer Microstructure of Biodegradable

Chapter 11

Controlling the Polymer Microstructure of Biodegradable Polyhydroxyalkanoates 2

Downloaded by UNIV OF ARIZONA on November 23, 2012 | http://pubs.acs.org Publication Date: December 10, 2002 | doi: 10.1021/bk-2003-0840.ch011

Aaron S. Kelley1 and Friedrich Srienc 1

Department of Chemical Engineering and Materials Science, Bio-Process Technology Institute, University of Minnesota, Minneapolis, MN 55455 Department of Chemical Engineering and Materials Science, Bio-Process Technology Institute, University of Minnesota, St. Paul, MN 55108

2

Polyhydroxyalkanoates (PHAs) are biodegradable polyesters produced by many bacterial species. They are a natural part of the renewable carbon life cycle and are a sustainable source of plastics into the future. PHA production, however, cost about ten times as much as petroleum-derived thermoplastics. The value of the polymers can be increased by controlling the polymer's microstructure. By changing what substrates are available to the bacteria during polymer synthesis, different compositions of PHA can be synthesized. This technique can be used to synthesize core and shell latexes, giving specific properties to film and coatings. Additionally, alternating substrates at a faster rate can be used to synthesize different polymers within an individual polymer chain, thereby producing block copolymers. Block copolymers have uses as blend compatibilizers and thermoplastic elastomers. Producing PHAs with controlled microstructures gives control of the physical properties and allows for the production of high value polymers.

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© 2003 American Chemical Society

In Biocatalysis in Polymer Science; Gross, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

125 Polyhydroxyalkanoates (PHAs) are a class of biodegradable polyesters produced by many bacterial species. The polymer is synthesized when growth is limited by lack of a nutrient but carbon is available in excess. Polyhydroxybutyrate (PHB), the most prevalent PHA, has high crystallinity which leads to brittle failure. Copolymers of PHB incorporating polyhydroxyvalerate (PHV) have proven to increase the toughness of the polymer. Blends of PHB with other biodegradable materials such as starch or polycaprolactone (PCL) have also lead to some improvements while maintaining biodegradability. Phase separated blends present yet another way to improve the physical properties of brittle PHB. It has been shown that blends of PHB and polyhydroxybutyrate-co-valerate (PHBV) will phase separate with as little as 8% HV differences between the two polymers. 1


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Synthesizing polymer granules within bacteria in a "living" polymerization gives a unique opportunity to control the microstructure of the polymer. By controlling the substrate available to the bacteria, different polymer types can be synthesized within the same granule, or even within the same polymer chain. Curley et al. (1996) demonstrated the synthesis of "layered" granules in Pseudomonas oleovorans with diauxic use of substrates. Kelley and Srienc (1999) showed that 'layered' granules could be synthesized by Ralstonia eutropha as well, consisting of a core of PHBV and a shell of PHB. 4

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Results and Discussion In order to benefit from controlled PHA microstructures, a system had to be developed in R. eutropha to produce two different polymers that would phase separate. PHB and PHBV have been shown to phase separate. Furthermore, hydroxyvaleric acid is only incorporated into the polymer chain if an odd chained fatty acid is present in the media. This presents a unique opportunity to control when PHBV is synthesized. 3

Figure 1 shows the approach used to synthesize controlled microstructures in R. eutropha. With an excess of fructose present in the media, control of PHBV synthesis is dependant upon valeric acid additions. By controlling when and how much valerate is added, the microstructures can be controlled. Valerate additions that remain in the media for 8-12 hours before exhaustion produce layered granules. Multiple additions lasting approximately 4 hours each will produce multiple layered granules (work not shown). 5

If switches in synthesis conditions can be made to occur on a timescale equal to the synthesis time of individual polymer chains, block copolymers will be produced. Literature estimates of chain synthesis times vary from 10 minutes to 5 hours. 67



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Figure 1. Thisfigureoutlines the approach to synthesizing controlled PHA microstructures. Valeric acid is added in limited amounts to media containing an excess of fructose. The number of switches and in what timeframe determine the microstructure. Switches on the order of hours produce layered granules. Switches within the synthesis time of a polymer chain produce block copolymers.

Conclusions Polymer synthesis in microorganisms presents a unique opportunity to control microstructures to produce novel architectures. Core and shell granules should have uses as specialized latexes. Block copolymers could be used to compatabilize blends, as thermoplastic elastomers and possibly as biocompatible adhesives. Controlled deposition of materials can aid in the production of scaffolding for further nanoconstruction, presenting opportunities in the unfolding nanotechnology arena.

Acknowledgements This work has been supported in part by the Consortium for Plant Biotechnology Research and by the National Science Foundation (BES9708146).


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3. 4. 5. 6. 7.

Luzier, W.D. Proc. Natl Acad. Sci. USA 1992, 89, 839-842. Ramsay, BA, Langlade V, Carreau PJ, Ramsay JA. Appl. Environ Microbiol. 1993, 59, 1242-1246. Barham PJ, Organ SJ, J. Mater.Sci.,1994, 29, 1676-1679. Curley JM, Lenz RW, Fuller RC. Int. J.Biol.Macromol., 1996, 19, 29-34. Kelley AS, Srienc F. Int. J.Biol.Macromol.,1999, 25, 61-67. Kelley AS, Jackson DE, Macosko C, Srienc F. Poly. Degrad. Stab., 1998, 59, 187-190. Steinbûchel A, Aerts K, Babel W, et al., Can. J. Microbiol., 1995, 41, 94105.


Controlling the Polymer Microstructure of Biodegradable

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