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### Topic: Polymer characterisation  (Read 2828 times)

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#### emel

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##### Polymer characterisation
« on: October 17, 2014, 10:34:45 AM »
Hey guys,
I'm not sure if this is the right board for this but I have a question about polymers. I'm currently working with a thiol terminated polystyrene that I will be coating onto a gold surface. The only information I have on the polystyrene is that its average Mn is 5000. I was wondering how I would calculate its extended and coiled length as well as its overall Mw. I'm new to polymer chemistry so even pointing me in the direction of a good book would be appreciated! Thanks in advance!

The polystyrene I'm using: http://www.sigmaaldrich.com/catalog/product/aldrich/746924?lang=en&region=IE

#### mjc123

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##### Re: Polymer characterisation
« Reply #1 on: October 17, 2014, 12:27:07 PM »
Aldrich tell you the PDI (polydispersity index) < 1.1. The PDI, equal to Mw/Mn, is a measure of the breadth of the MW distribution. A value < 1.1 is considered very narrow; essentially you can treat it as if all the chains have the same molecular weight.
For the extended length ("contour length"), consider the backbone carbon chain. What is the length of a repeat unit if the C-C distance is 154 pm and the C-C-C angle is 109°? How many repeat units are there in a molecule of MW ca. 5000?
For the coiled length (not sure what you mean by this - radius of gyration?) - depends on solvent quality. Bigger the better the solvent.
I used to do quite a bit of polymer characterisation, but since I left that company I haven't got access to the books, and don't remember all the equations.

#### emel

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##### Re: Polymer characterisation
« Reply #2 on: October 18, 2014, 06:43:34 AM »
Thank you! So for the Mw I should just multiply my Mn by 1.1? So around 5500 g/mol. That seems straightforward enough!
So for calculating the number of repeating polymer units

n=Mn/Mw

=5500g/mol/104g/mol
=52.88 so around 53 units.

To calculate the length I've found the formula L = (N)(d) sin/2
since I have 2 CC bonds I multiply 53 by 2 and sub in to get
(106)(.154nm)(sin109/2) and I get a length of 13.3nm

Does that seem right? (sorry for poor formatting, I'm still getting used to the site!)

Thanks for everything
« Last Edit: October 18, 2014, 07:33:57 AM by emel »

#### mjc123

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##### Re: Polymer characterisation
« Reply #3 on: October 18, 2014, 06:37:21 PM »
Quote
n=Mn/Mw
No, n = MW(polymer)/MW(repeat unit)   (For convenience ignoring the terminal groups)
But you used the right numbers. Result looks OK to me.

#### emel

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##### Re: Polymer characterisation
« Reply #4 on: October 18, 2014, 08:19:16 PM »