Polymer Structures

You can't alter the structure of these much, with the possible exception of crosslinking. Any other changes (i.e., adding sidechains, chemically modifiying them pre/post extrusion,...) will change the of the polymer enough that it should be considered a new polymer with a new Tg and Tm. Crosslinking them will have a minimal impact on Tg and Tm, except that their flow will be greatly reduced.

As an aside, these are all crystalline polymers. Tm will be the dominate characteristic although Tg certainly exists for these polymers as well.

Polyprop and nylon are extrudable. ePTFE is a finished product.

John Aspen Research -

Opinions expressed herein are my own and may not represent those of my employer.

John at Aspen is generally correct with the exception that polypropylene already comes in three forms structurally. Isotatctic, Atactic and now Syndiotactic and its latest form cyclolefins. These structure all have different Tg and Tm . Now if you take a typical polypropyene homopolymer and alter its Mw by a post polymerization process you will change its Tg and Tm but not alter its main structural characteristics but you will alter its molecular weight distribution or Q or polydispersity index making it more narrow. There are ways to alter its Tg and Tm without changing structure too. New molecular modifiers have been discovered that can be added to PP homopolymer to alter both Tg and Tm significantly. Tg of PP is -5C to 0 depending on how you measure and the starting PP homopolymer. Molecular modifiers can drive Tg down to -25 C without altering structure or Mw.

JWebster Stabilizati> Hi i was investigating the use of polymers in medical applications and

But it's very difficult to change the structure _after_ polymerization.

What to you mean by cyclolefinic polypropylene?

If you significantly change the molecular weight distribution you change the type of polymer. That's one of the key concepts, if you are dealing with polymers: a simple formula like H-(CH2)n-H, n=1000 is not sufficient to describe your substance. By altering the molecular weight you also change the (average) structure. The effect of end-groups get's more pronounced, and this then leads to the change in Tm and Tg (although for very high molecular weight polymers the effect might be negligible).

If you accept adding plasticers then we could also talk about dissolving the polymer in a solvent...

Tg and Tm are dictated by both, intra- and inter-molecular interactions.

Regards, Oliver

I'm wondering what kind of additives can be used to change Tg and Tm. Tg : By adding some kind of plasticizer (polymeric? in order to prevent migration of the plasticizer...). Tm : If you can disturb the crystallite, you will bring the melting peak down, but also the strength. But may be you can add some kind of polymer which forms together with the polypropene another new crystallite with a higher (lower ?) Tm. Theoretically seems this possible... But what is such a additive? What is a brand name?