1. Scanning Electron Microscopy (SEM) Conclusions  Changing the solvent that PHB-HHx is dissolved in, greatly alters the mechanical properties  Water replaced gels are stronger but deform more easily than solvent filled gels  Electrospinning with DMF requires a higher weight percent of Nodax  The working distance or applied voltage charge to electrospin a gel must be greater than that of just the solution to cause the DMF to evaporate quickly enough and not cause the fibers to form a crystalline sheet Future Work  Cell Culturing - determine how cells react to Nodax and determine how bacteria will degrade Nodax; can this be applied to tissue engineering applications?  Varied Mechanical Testing – repeat tests on the same gels to gain conclusive information on whether lower concentrations of HHx in the gel affect its strength  SEM and Transmission Electron Microscopy (TEM) work – further determine the structure of the gels and how they differ at varying mol% of HHx  Electrospinning – vary the techniques of electrospinning to enable more successful spinning of gels and spin gels at different concentrations and mol% HHx References 1.Noda, I. 2010. Nodax Class PHA Copolymers : Their Properties and Applications Acknowledgements Frank Kriss for SEM training Differential Scanning Calorimetry (DSC) DSC results show what temperatures the gels should be heated to in order to fall back into solution, the onset temperature is also used to calculate the heat needed to surround the syringe when electrospinning. Dynamic Mechanical Analysis (DMA) DMA results show that the structure of gels including CHF are much stronger, both when replaced with water and while still in CHF/DMF solvent. Also, once the gel’s solvent is replaced with water the structure is less fluid and deforms easily, as shown by the plot moving to the right with each cycle. Rheometry What is PHB-HHx (Nodax)? Poly (hydroxybutyrate) (PHB) is a biodegradable, aliphatic polyester that can be produced by chemical processes or bacterial fermentation. However, bacterially produced PHB results polymer that is brittle and lacks flexibility. In order to modify these properties, 3-hydroxyhexanoate (3HHx) has been added as a co-monomer and these new materials, referred to as PHB-HHx, which have a significantly reduced crystalline content. The degradability of this thermoplastic polymer can change with varying amounts of the 3HHx comonomer from 0mol% (pure PHB) to 13mol%HHx. Some have been observed to degrade in 30-45 days 1. We have discovered that these polymers undergo thermoreversible gelation when dissolved in poor solvents, which significantly changes the polymers structural properties. The motivation for studying PHB-HHx gels is that when electrospun or processed as a gel it can be used to make scaffolds for tissue engineering and water filtration. In the latter case, the electrospun PHB-HHx used as the first filter and the more tightly packed gel matrix used to filter the smaller particles. PHB is completely biodegradable, renewable, not water soluble, and by changing the mol% of HHx, many different types of plastics from grocery bags to toys can be made, allowing it to be a desirable material for many applications. Materials The material being focused on here is the PHB 13mol% HHx (now referred to as Nodax) in gel form at 10wt% and 15wt% PHB-HHx to solvent. The PHB is dissolved in chloroform(CHF), dimethylformamide (DMF), and a solution of equal parts DMF and CHF, where it is heated in the solvents to approximately 60 o C and stirred until dissolution occurs. To form a gel, the heated Nodax solution can be either left at room temperature to gel, or have its gelation quickened at cooler temperatures as I did for these experiments at 4 o C. PHB-HHx is a thermoreversible physical gel- meaning it does not gel because of cross linking in a chemical reaction, but because of the interactions of its chain with the solvent. Upon gelation it is possible to liquefy and re-gel the Nodax by changing the temperature. Gelation of this polymer occurs only in poor solvents (such as DMF), the polymer experiencing attractive and repulsive forces toward the solvent molecules, and the balance of attraction and repulsion allows the polymer not to fall out of solution or completely dissolve but to exist a stage in between – a gel. The polymer in solution takes so long to gel even at cold temperatures because the polymeric chains are long and need time to adjust to the lowest and most stable energy state. The gel is thermoreversible because upon heating the polymer gel, the physical crosslinks “melt” and become a solution once again. Analysis of Thermoreversible Gels Comprised of Biodegradable PHB-HHx Polymers Nile Bunce, Brian Sobieski, Liang Gong, Bruce Chase, Isao Noda and John Rabolt This poster was made possible by the National Science Foundation EPSCoR Grant No. IIA-1301765 and the State of Delaware. Onset Temperature: 43.919 o C Peak Temperature: 49.926 o C Onset Temperature: 26.256 o C Peak Temperature: 33.353 o C Figure 7: Cyclic compression of 15wt% Nodax in DMF after overnight refrigeration at 4 o C Figure 8: Cyclic compression of 15wt% Nodax in DMF structure, solvent replaced with water Figure 9: Cyclic compression of 15wt% Nodax in CHF/DMF after overnight refrigeration at 4 o C Figure 10: Cyclic compression of 15wt% Nodax in CHF/DMF structure, solvent replaced with water Figure 3: 13mol%HHx 15wt% thermoreversible gel in a 1:1 CHF/DMF solution. A. Fluid PHB after heating for 10 minutes at 54 o C B. PHB after cooling at 4 o C over night AB Figure 6: 15wt% Nodax gel used for DMA before (A) and after (B) solvent replacement with water AB Figure 4: 10wt% Nodax in DMF during ramp Heat /Cool/Heat Onset Temperature: 26.256 o C Peak Temperature: 33.353 o C Figure 5: 15wt% Nodax in CHF/DMF during ramp Heat /Cool/Heat Figure 1: Structure of PHB(left) and 3HHx(right) by varying the repeats of x and y in the chain the properties of the material change Figure 2: Chain of PHB-13mol%HHx showing how the different comonomers interact Figure 11: Elastic modulus of 10wt% and 15wt% Nodax in CHF/DMF solution at 15 o C This experiment shows the differences in the strength and deformability (the elastic modulus (G’)) of the 10wt% and 15wt% gels. This test measures the changes in the resistance of the material as the Nodax solution changes from a liquid to a gel. As shown in the graph, the 15wt% gel is much stronger than the 10wt% gel. Figure 12 : Apparatus made to keep syringe contents warm during the electrospinning of gels with antifreeze running through it from a heat exchanger Figure 14: 15wt% Nodax dissolved in CHF and electrospun at 0.2mL/hour for 10 minutes at 11kV, fibers vary in width from about 4µm to 17µm Figure 15: 15wt% Nodax dissolved in CHF/DMF and electrospun at 0.5 mL/hour for 10 minutes at 8.5kV, fibers vary in width from about 1µm to 1.3µm Figure 13: 15wt% Nodax gel in CHF/DMF and critical point dried, fibers vary in width from 100nm to 600nm SEM photos show-  The fibers and structure of dried gels are much smaller than that of the elsectrospun fibers  In Figure 15 there is a crystalline layer of Nodax under the fibers where previous fibers had re-dissolved into DMF that had not yet evaporated.  Figure 14 vs. Figure 15 shows differences that can occur when electrospinning a gel vs. Nodax in CHF y x