Hello

Nowsheen,

      It would help to understand what you are specifically looking for.   The term morphology can be related to shape of the fiber, the architecture of the non-woven textile, the porosity of the fiber, the texture on the fiber surface, or the phase domain distribution within individual fibers. 

To start at the beginning, the stability of the electrospinning jet, and ultimately the shape of nanofiber, is the result of a balancing act between solution viscoelastic response, solution surface tension, and electrostatic forces.   At the extreme cases, if surface tension dominates, then you get drops.  If electrostatic forces dominate, you get filaments.  The viscoelastic response of the solution acts counter to which ever of the other two is dominate.  The other main ingredient is the rate of solvent evaporation, which is integrally coupled with the three previously mentioned variables. 

As an example,  the "string of pearls, or beaded" morphology is a case where surface tension is clearly dominate, but the viscoelastic response of the solution(as the solvent evaporates) stabilizes the Rayleigh instability. This prevents viscous waves from separating completely into drops, freezing in what is essentially a metastable morphology as the solvent evaporates.  Conversely, if the temperature is kept constant and one increases the  polymer concentration (increasing the viscosity), the average fiber diameter increases and eventually, spinning becomes impossible, regardless of the voltage applied (see Baumgarton reference in the previous poster's articles).   One good reference discussing this sort of thing would be

H. Fong and D. H. Reneker, J. Polyym Sci Part B, 37,3488 (1999).

There are, of course a number of others.  

Generally speaking, there are a myriad morphologies, textile architectures, porosities, and blends of fibers that have been reported in the literature over the last 30 years, and I do not think there is a single review article that captures them all.   Author's of articles that address different aspects of your question include Greg Rutledge,JJ Feng,  Darrell Reneker, Alex Yarin, John Rabolt and David Martin,  among many, many others.   To my knowledge, there is no Universal model that will predict all the many morphologies reported, but the authors I have mentioned have made contributions in many areas that could be of interest to you.

I hope that this helps.

Joe Deitzel

SEM

Thank you all for your replies. I apologise for not being clear enough. I understand that processing parameters influence fiber morphology. What I wanted to know is whether there is a way of predicting how polymers will be distributed when electrospinning a solution consisting of polymer blends i.e. core-shell, shish-kebab etc.

According to my experience, the morphology depends on the used solvent(s). However,  the compatibility of the polymers in the blend is very important. There is not way of prediction, well maybe the Flory Huggins theory of polymer solutions might help to understand the compatibility of them before electrospinning. Good luck

Kind regards

Nowsheen,

     The short answer to your question is no, I am not aware of any model that will predict the phase separation and distribution of polymer blends in an electrospinning jet.  As Gloria has suggested, phase separation under equilibrium conditions has been studied quite a lot and is driven by fairly straight forward thermodynamics.  However, most polymer processing is no where NEAR equilibrium conditions, which makes things challenging.  This is especially true in the case of electrospinning.   As I indicated in my previous comments, we really do not have a comprehensive understanding needed to model a "Homogeneous" electrospinning jet, let alone one containing multiple components.   Most of the work of which I am aware has been experimental in nature.   You might look up work by Reneker, and Lynn Samuelson, as some of the first who have done work in this area.  There are of course others.

Generally speaking we have seen in our own lab

1.Relative concentrations of the blend components will determine which phase is continuous and which is discontinuous.

2. Polymers with minimal surface free energy (fluorene containing polymers) will bloom to the surface

3. Electrically conductive materials will tend to move to the surface during spinning as well. 

Most of the work looking at core/shell morphologies has involved the use of concentric spinnerettes. 

Hope this helps.

JMD

Dear Joel,

Thanks a lot for this comprehensive reply. Sure helps!

Cheers

Nowsheen

the most important parameter defining morphology of electrospun polymeric blend nanofibers is the ratio of polymeric blend, for instance in spinning of chitosan/PCL with increasing ratio of PCL to chitosan we obtain fibers with homogeneous and less beaded structure and in chitosan/PCL solutions of 75:25 ratio of beads and size distribution of nanofibers increases 

With respect to the previous answer, it is important to understand what the governing variable IS that determines your morphology of the fiber.  The blend ratio is a SECONDARY variable that encompasses multiple independent variables, so when you change it, you are changing several things at once.

It is likely that there is a significant difference in the molecular wt between the two components.  By changing the ratio of Chitosan to PCL, you are likely primarily changing the viscosity of your solution.   Beading occurs when the viscoelastic response of the solution is overwhelmed by  the surface tension.  This allows the Rayleigh(varicose) instability  to form, resulting in the beads.   The fundamental way to shift that balance is to increase either the concentration or the molecular wt. to increase the viscosity.  To a smaller extent, you can modify the morphology by increasing the spinning voltage or adding an ionic species(salt) which increases the electrostatic forces that act to counter the varicose instability of the solution, but there are limits. I refer you to Taylor's original publications and later works by Melcher, Rutledge and Feng.    

In the case cited above, it is likely that changing the blend ratio is changing the viscosity of the solution, which determines the final shape of the electrospun fiber.  In reference to the original posted question, the blend ratio will have a pronounced effect on the distribution of the polymers through out the electrospun fiber.  

Hope this helps.

JMD