Dear Panarin,

I believe you have answered your own question, if I understand you well enough.

If you have polymers of an equivalent chemical composition to your PLA  with respective  known molecular weight moment values (let's say weight average molecular weight, for instance) then you can construct a viscosity versus molecular weight calibration curve.  While doing that you will need to keep your polymers solutions at a low relative viscosity (1.1 or so, I am guessing) relative to the thermodynamically good solvent you choose to do the viscosity measurement in and convert to a specific viscosity divided by polymer concentration (to obtain an intrinsic viscosity plot).  Of course, temperature control is also called for.

If your standards are not similar to your PLA in chemical composition (or perhaps even molecular weight polydispersity )then you can't really use your proposed approach to a quick molecular weight measurement with sufficient accuracy.  Even if correcting your results with Mark-Houwink-Sakarada constants reported in the literature for PLA and your standard.

Best regards,

Mike

If you have the kinematic viscosity of multiple samples of PLA and know the molecular weight of those samples, then I don’t see a problem interpolating molecular weight from a curve based on that known data.   If you’re interpreting within a known set (and the known set doesn’t contain different structures, molecules, branching etc) then you could almost relate any property to MW as long as there was a consistent and significant change (not miniscule, step-wise, or ill-conditioned) and then interpolation would be trivial.  I don’t see a problem with the interpolation but definitely not extrapolation!  Comparative data would result rather than absolute data but that may suffice. 

However the relationship between kinematic viscosity and molecular weight is not clear.   The intrinsic viscosity value Malvern's GPC uses is a solution viscosity term, which is different than a pure viscosity term, such as kinematic viscosity, which describes the neat sample.   

Dear Mr. Bender and Rawle,

Thank you for your responses. I was quite unsure as to how valid using standard curve by measuring the standard PLA's molecular weight and viscosity separately and seeing their relationship will be.

However, I have a few more questions: Mr. Bender, is the value 1.1 for relative viscosity an acceptable value based on some paper or is it based on estimation? To both, how do you decide which method to use (assuming GPC isn't allowed)? Mostly, I see Ubbelhode being used but what makes Standard Curve method less used? Is it because it is less accurate?

Lastly, is there any paper/results that you two know of that supports the use of Standard Curve? I apologize if the last question sounds demanding but I don't many that uses it and none yet with PLA.

Dear Panarin,

we have used it vice versa to describe the logarithmic corve of the viscosity from Polyether Polyols with different molecular weights. As Long as you stay with the same structure you can use the molecular weight to interpolate the  log of dynamic or kinematic viscosity or use the log of viscosity to interpolate the molecular weight.

E. g. in "The Properties of Gases and Liquids" you find some approaches to calculate the dynamic viscosity from the structure of a molecule.

Dear Panarin,

It is not clear why to use kinematic viscosity. If you know solution density you can easily calculate dynamic viscosity of the solution. You have to know PLA concentration in order to calculate reduced viscosity from specific viscosity. Once, knowing reduced viscosity and solution concentration by extrapolation you can get intrinsic viscosity. It is customary to perfom double extapolation: reduced viscosity versus polymer concentration and log of relative viscosity devided by concentration (inherent viscosity) versus concentratiuon. From this plots you can get not only intrinsic viscosity but data on solvent polymer interaction.

From intrinsic viscosuty you can calculate molecular mass (weight) if you have calibration curve (constans) or if you take the data from the literature. However, you have to be carefull  how the calibration has been done. In order to get viscosity average of molecular mass the calibration (or determination of constants K and a) should be done with monodisperse PLA fractions. If the fractions were polydisperse you do not know which average you get. If you want to constract  you own  calibration curve you need an absolute method of determinig of molecula mass (e.g. light scattering, or someone else) and monodisperse PLA fractions.

If you measure efflux time in Ubbelohde capilar viscometer it should be over 120 seconds in order to avoid necessity of dissipation of kinetic energy and capilary end corrections.

GPC is relative method of molecular mass determination, as well as viscometric one.

If you give me more details about you system I hope I can give you right advice.

Best regards

Prof Petar Dokić

Hi Panarin,

The above answers are broadly correct.  The MW is proportional to the zero shear viscosity by the Kuhn-Mark-Houwink-Sakurada relationship for a melt or concentrated solution where the exponent ~3.4 for concentrated samples.  The same relationship can be used for dilute polymer solutions (below a critical concentration) where the intrinsic viscosity is proportional to the MW raised to the power alpha (where alpha is ~0.5-0.8). In the semi concentrated range, other factors such as shear rate and MWD also play a role, and the analysis becomes more difficult.   

Normally one would measure 3 or more samples by GPC to obtain their MW and then a rheometer or viscometer to get their intrinsic or zero shear viscosity.  The data (log viscosity vs log MW) is then plotted in a spreadsheet and a correlation created using the power law relationship which relates to the Mark Houwink relationship.

Hope this helps,

Best

Phil

Dear Philip, Petar and Klaus,

Thank you for your kind responses. My initial curiosity stemmed from my doubt with the "standard curve" method with Kinematic Viscosity (e.g. from a Brookefield machine) vs. Intrinsic Viscosity + Ubbel + Mark-Houwink (what I usually see). However, from what I see, both are applicable as long as there's a basis. 

Maybe you can refer to this paper: THE RELATION BETWEEN THE MOLECULAR WEIGHT AND INTRINSIC VISCOSITY OF POLYACRYLONITRILE