Tuesday, November 11, 2014

ε-Polypropylene - "And now I've lost it."

Crystalline polypropylene (PP) has always been one of the more interesting polymers because it is polymorphic - there are multiple crystalline phases that can form. Without any special processing, the α- phase dominates. This is the one that melts at about 170 oC or so. In the past few decades, nucleating agents for the β-phase have become commercialized. I've not worked with any β-PP, but it according to literature reports, it melts at about 150 oC or so. If the crystals are drawn during processing, they will revert to the α-phase, so you need to be careful about that. I'm not aware of any commercial applications, but there is probably something out there. There is also the the γ-phase, of which even less is known.

Those three phases have been known for about 20 years and are at least mentioned in passing on most books on PP. So imagine my surprise last week to find that they have now discovered the ε-phase! Wait - I didn't even know about the δ-phase Did I miss something?

Apparently I did. Or maybe not. I'll let you decide. A frantic Google search for the delta phase turned up nothing, but one of the introductory paragraphs of the ε-phase article mentions that the δ-phase occurs for copolymers of propylene and hexene or pentene when the comonomer content is great than about 10%. So is that really a polypropylene crystalline phase? Before you decide, consider what we know about the ε-phase.

Just like with the δ-phase, the ε-phase doesn't occur with stereotactic, defect-free polypropylene, but requires specially prepared material, where "...the polymers produced have progressively lower melting transition temperature, lower homosteric pentad sequence population, and higher solubility in low-boiling solvents, indicating frequent stereochemical inversion in monomer enchainment." In other words, this polypropylene has some significant "issues". ("Normal" polyproplyene has a high melting temperature as noted above, high pentad regularity, and is insoluble in low-boiling solvents.) Somehow the defects are not enough to make this completely amorphous. So again I ask, is this really a polypropylene crystalline phase?

My opinion (which comes with a full-money-back guarantee if not completely satisfied): No, the δ- and ε-phases are not part of the polypropylene polymoprhs. For "normal" polypropylene, you can make the α-, β-, and γ-phases from the same starting material, but you can't make the δ- and ε-phases and so they are not part of the set.

Before I finish, consider this example of life imitating art:
"Unfortunately, both the details of the material and the material itself were lost when our laboratory moved to its new location."
Que up the theme from "The Medicine Man". I can only imagine the scene within the new lab: "...now I've lost it. Haven't you ever lost anything doctor Bronx? Your purse? Your car keys? Well, it's rather like that: Now you have it and now you don't." That would be pretty painful to discover and hats of to the researchers for being upfront about it.

Previous Years

November 11, 2011 - Birefringence in Polycarbonate

November 11, 2011 - Flashing Labels

November 11, 2010 - Olefin Metathesis


Mitch said...

It was featured in C&EN this past week.


John said...

Good! The more people we have looking for the lost material, the better ;)

Eric F. Brown said...

So, how many people catch that popular reference to a 20+ year old movie?

John said...

Well, there's you, and there's me and I think Sean Connery probably still remembers it. So 3. Is that good?