Glass Transition

[prix27]

I have got a slope for finding glass transition of polymer. Does anyone have suggestions of finding exact temperature for the glass transition. If so could you please how to find that in excel...if you can direct me to any link then also its fine.....your help is highly appreciated

 

[babouic]

Good afternoon.

If you have a DSC you can easily know Tg.
If you are looking for data, Handbook is your friend. For example PvC Handbook

Cheers

 

[prix27]

the curve I have got is from DSC....I dont know hot to get the exact temp of glass transition from the curve

 

[TomDOT]

You should have software on the DSC that lets you pick out the start and end of the melt curve on your plot, then gives you a number for Tg. Even the old DSC I used which ran DOS on an 8086 processor could do this!

 

[Compositepro]

A computer can be programmed to find a Tg transition on a flat line. That doesn't mean its correct or real. If you cannot see a transition on a curve with your eyeball, I would not believe any number called "Tg".

 

[patprimmer]

I agree with CP. There should be a very distinctive change in the curve at TG. TG may be a region rather than an absolute point.

Regards
Pat
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[TomDOT]

Guys, perhaps I was unclear - I was talking about manually picking the start and endpoints of the melt/glass transition. it's almost always a range with polymers. Software just to pick the notional "Tg".

 

[Demon3]

It's essential to realise that there is no defined glass transition temperature. It's not a fundamental property and it depends on the testing speed. If you scan by DSC at one rate then another the measured Tg will change. You can measure it by NMR and that will be vastly different to the value obtained by DSC. So, the only meaninful measurement of Tg is one that relates closely to how the plastic part will be used. So, why do you want to know the Tg? Then we can tell you how to measure it.

What is Tg? It is the temperature at which 10-15 polymer segments can move together giving some flexibility to the polymer. If you test quickly the temperature needed for 10-15 units to move is much higher than if you test slowly because during a slow test the units have more time to move. Another definition is the temperature at which the free volume of the system is around 2.5 volume % (this can be measured by positron annihilation).

Sorry for the details. Did my PhD in polymer science so I have had this drilled into me. In the real world, Tg is irrelevant except as it pertains to how the material behaves in your application.

Chris DeArmitt - PhD FRSC CChem
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[TomDOT]

Chris - thanks for the detailing!

I can tell you the pipeline coating guys (Fusion Bond Epoxy/FBE) would dispute your assertion about Tg being irrelevant in the real world. Tg by DSC is the gold standard for determining whether the FBE has achieved cure or not.

 

[Demon3]

As I mentioned, it has a meaning when used to determine a real world property. In your case you use it to determine extent of cure. That is valid, but it seems you're not interested in Tg itself which was my point.

If you're not careful Tg will change as you are heating the sample to measure it. For examples, epoxies will cure until Tg = Tcure. At that point the chains can't move and cure any further. So your end Tg will be the same as the cure temperature. When you heat it past that Tg by DSC the epoxy will start curing again(because the chains are mobile again) and Tg changes.

Another way to measure extent of cure by DSC is to ramp the temperature and measure the cure that takes place. That extra cure due to heating (exotherm) shows the reactive groups that remained when your original cure was complete. That's used as standard to give an indication of cure extent.

Chris DeArmitt - PhD FRSC CChem
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[TomDOT]

Chris - change in Tg is actually the method used. It's a dual Tg determination. First run is an initial Tg on a sample from the "cured" pipe followed by a heat soak, is cooled, then Tg is determined again. If the delta Tg is greater than a predetermined amount, the pipe is rejected.

 

[Demon3]

Nice method. Try that DSC heat ramp as you'll get the same result much faster.

Chris DeArmitt - PhD FRSC CChem
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[TomDOT]

Sounds like it would be faster. Overcoming the industry inertia wouldn't be worth the effort to me though. I'd have to get on the right subcommittee, run comparison testing to validate, et cetera. I'm not running the test currently, though I may see about using your approach for FBE coated rebar sometime in the next few years...

 

[Demon3]

Haha. Point taken.

Chris DeArmitt - PhD FRSC CChem
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