[darkmatic]

Hello people,

I posted on the 23th of september 2005 a question concerning the shrinkage of polymers on curing from liquid to hard materials (some like epoxy, polyester, vinylester, methylmetacrylics, furan, ...) and I was asking myself if there where more peoply who could answer my question?

Thank you in advance...

[P]

Hi - I didn't see your origional question, but the shrinkage of polymers when curing is usually due to crosslinking along the polymer chains, As more cross links form, the polymer chains are pulled tighter into each other causing a reduction in volume. 

Also, during the initial drying phase - if the polymer is disolved in a solvent for purposes of coating like a varnish - shrinkage could be due to the loss of the said solvent. But once solid,  shrinking is probably due to further X-linking on curing.

[Dude]

You probably want to either make or buy a dilatometer.  It can be as simple as a volumetric flask that you put monomer in to the mark exactly, polymerize to a glass, let cool to the same temperature and then measure the weight of water needed to reach the same line and convert to volume.

In your Sept 2005 question, polymer shrinkage would defined as:
1.  assume constant mass  (i.e. 1 g)
monomer  1 g (1 mL / 1.074 g) = 0.931 mL
polymer   1 g ( 1 mL / 1.15 g) = 0.8695 mL

% shrinkage = (0.931 mL - 0.8695 mL) / 0.931 mL * 100 = 6.6 % volume contraction

[P]

OK - I think I get you now  -  your talking about the reduction in volume from the monomer to the polymer.  Likewise - in the monomer, being liquid, the individual molecules are spaced out in a way as they would in any standard liquid.   When the polymerisation occurs the molecules are linked together with chemical bonds - I'd say that the chemical bond length in the polymer would be less than that of the average spacing between the molecules in the liquid  -  thus, less volume for the same mass.

 

[darkmatic]

Yes Dude,

That's what I ment, but I don't know if the calculation that I gave was correct, because many books and literature mention the shrinkage into a negative number (eg -6,6%)

Also many manufacturers and suppliers measure the shrinkage in a linear measuring...This cannot be as correct as a volume measuring (3 dimensions)

Otherwise I've made since semptember 2005 different samples and I came to the conclusion that this is the most correct measuring (calculation). I measured the density of the liquid polymer, let it cure, cut into pieces and the measured the mass in air and water to calculate the density.

Thanks,
greetings