Scanning Electron Microscopy (SEM)

ConclusionsChanging 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 WorkCell 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

References1. Noda, I. 2010. Nodax Class PHA Copolymers : Their Properties and Applications

AcknowledgementsFrank 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 days1. 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 60o 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 4oC.

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: 43.919o CPeak Temperature: 49.926o C

Onset Temperature: 26.256o CPeak Temperature: 33.353o C

Figure 7: Cyclic compression of 15wt% Nodax in DMF after overnight refrigeration

at 4o 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 4o 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 54oC B. PHB after cooling at 4oC over night

A B

Figure 6: 15wt% Nodax gel used for DMA before (A) and after (B) solvent replacement with waterA B

Figure 4: 10wt% Nodax in DMF during ramp Heat /Cool/Heat

Onset Temperature: 26.256o CPeak Temperature: 33.353o 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 15o 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 CHFyx