Today's KNOWLEDGE Share :How Polymer blends get unique properties

Today's KNOWLEDGE Share

Ever wonder how polymer blends get their unique properties?

It all starts with the fundamental thermodynamics dictated truth that most polymers are immiscible – they simply don't mix, with rare exceptions like PMMA/PVDF.

In these immiscible blends, the continuous phase often determines the material's thermo-mechanical, chemical, and even aesthetic properties.
The dispersed phase acts more like a filler.

But how do you predict which polymer will form that continuous phase?

It's not just about which polymer is present in a larger volume!
Viscosity is a critical factor.
A lower viscosity polymer tends to become the continuous phase.

Take a 60/40 PC/PBT blend, for instance: if the PBT is a high-flow lubricant (to the PC phase), its lower viscosity can make it the continuous phase, even if it's the minority component.

Key considerations:
- Ensure your blend ratios are expressed by volume, weight ratio at room T would be misleading.
- Volume ratios are only meaningful here at processing temperatures, since PBT, a crystalline polymer, will shrink 3X more than PC when solidifying!
- Compatibilizers are crucial for controlling the fineness of the dispersed structure.

Of course high shear in Injection Molding will modify the blend morphology, sometimes to the point of phase inversion, if one component is way more shear-thinning than the other for instance.

Hydrodynamic forces in the flow will always push the low viscosity component towards the outer layers of the flow (minimum total energy).
This can sometime lead to delamination failure of the molded part, a well known issue when molding Xenoy in automotive bumpers.

The dispersed phase will also depart from spherical inclusion shape and possibly show very elongated morphologies, especially in the “frozen skin”.

The very unique “co-continuous” morphology is very difficult to obtain reliably in Injection Molding because of processing effects mentioned above.

Note also that in all immiscible blends, a small fraction of each polymer is miscible with the other one, which can be studied in a phase diagram.
For entropic reasons, lower molecular weight grades are always more miscible than higher molecular weight grades.
So in a given blend, the short chains in the distributions are responsible for the partial miscibility observed, contributing to the structural integrity of the blends and a small shift to the two glass transition temperatures..

source : Vito leo

#polymers #polymerblends  #mixturemorphology