Do PLA and PA6 have a common solvent? If yes, then why are their blends always (at least to my best knowledge ) prepared by melt blending?
HFIP is a good solvent for hard-to-dissolve polyesters, including PA6 and PLA
Concerning the latter question:
There is only a very minute number in polymers that readily mix. The situation in polymers with H-bond capacity is more complex though.
You may want to look up Flory-Huggings theroy for polymer blends.
Dispersion force bound polymers don't mix well due to their long chain length.
Roughly speaking, the likely-hood of mixture is antiproportional to the degree of polymerization (due to decrease of the entropy of mixing for long chain molecules). Thus even if monomers and mers are pretty similar in chemical nature, polymerized monomers, i.e., the polymer, may precipitate from the monomer at some point during chain growth.
Another suggestion: a practical route to polymer solutions is to look-up Hansen solubility parameters, which is often an OK starting point, when you do not have a clue on which solvent to buy(try).
Thank you Matthias for your answer. You mean blend of these two immiscible polymers can not be prepared by solution casting (HFIP as solvent)?
It depends on what you want to get. You can get a smooth film, but I would not consider this a blend then, because it is will be trapped far way from equilibrium, which is, admittedly, often the case for polymers in application. So it depends what you want to do with the film. If you use it only below Tg of both polymers it might fit your needs.
I'm totally confused Matthias. I know without using a compatibilizer, i won't get good properties, but their blend has been prepared by melt blending with no compatibilizer. so if I find some conditions (composition, temperature, pressure) that the mixture (PA6, PLA, HFIP) is miscible, why do you think it's not ok to use solution casting method for this blend? as you know blend of many immiscible polymers such as PLA/PCL has been prepared by solution casting. could you please explain that again?
Dear Catherine,
The reason why melt blending is used is many applications, is because it can easily process large quantities and the shear forces in a twin screw extruder used for melt blending is very high, which results in good dispersion of phase A in phase B.
And don't be confused, my remarks where rather generic in nature:
I just said it depends on what you assume you want to get out of it. With blends you can have miscible blends, immiscible blends and compatible blends. The difference is in the number of phases. With miscible blends you have only one phase. With immiscible blends you have two phases. With compatible blends you have two phases but not a complete phase separation and macroscopically it behaves as one phase.
So if your PA6 is incompatible with PLA, you will not get a miscible blend, only because polymer A and polymer B have the share a common solvent.
I know solution casting is neither green nor economic and i understand the difference between miscible and immiscible blends. but PLA and the nanofiller i am working with degrade to some extent at the high processing temperature of PA6. anyway, i am going to melt blend them and decrease the degradation by chemical modification . i was just curious to know whether it is possible to prepare the blend by solution casting. i think if two immiscible polymers have a good common solvent, you will probably find a range of composition and temperature in their phase diagram (three component system) that they form a miscible mixture due the interactions with the solvent and of course after evaporation of the solvent, phase segregation will occur and you get an immiscible blend again. but nanofiller can stabilize the morphology via thermodynamically localization at the interface, and of course rate of evaporation of the solvent plays an important role. thank you for taking time to answer my question. that helped me a lot.
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