Molecular Weight of PLA with the BI-MwA

Nov 8, 2019
Applications: Polymers
Instruments: BI-MwA

Polylactic Acid (Polylactide, PLA) Molecular Weight Determination with the BI-MwA

Polylactic acid, (polylactide, PLA) polymers are being used in a wide range of biomedical, commercial, pharmaceutical and academic applications due to their biocompatibility and biodegradability. Applications include stents, sutures and a wide range of uses in food packaging and disposable tableware. However, the molecular weight of the polymer can affect the properties of the end product. Therefore rapid, reliable characterization is important to better understand manufacturing performance, processing, and structure property relationships.

image of BI-MWA

Molecular weight determination of PLA by solution based techniques such as Static Light Scattering (SLS) or GPC with refractive index detection is a challenge due to the low value for the refractive index increment, dn/dc. For example the dn/dc of PLA in THF is 0.042 mL/g, which is 4.5 times less than a polystyrene solution with the same molecular weight and concentration in THF (dn/dc: 0.19 mL/g).

image of PLA molecular weight

An industrial PLA sample was dissolved in THF and data were collected with a BI-MwA in batch mode. Commercial polymers contain significant amounts of impurities and dust, which are a large problem in light scattering. The BI-MwA’s closed flow system prevents entry of airborne dust after filtration. Also, the BI-MwA’s software features automated dust rejection algorithms. Both of these features reduce the effects of dust on measurement results.

Light scattering data is typically analyzed with the Zimm equation:

image of Zimm equation

Here, K is the Debye constant, a constant of the polymer/solvent system and proportional to the square of the refractive index increment, dn/dc1.

Polymer concentration, c, is determined when sample solutions are prepared, and ∆R is proportional to the excess scattered intensity and measured by the BI-MwA. The scattering vector, q is given by (4pn/∆)sin(T/2). Here, n is the refractive index of the solvent, ∆ is wavelength, and T is the scattering angle.

To determine molecular weight, the BI-MwA software generates a Zimm plot based on the Zimm equation. After extrapolation to zero angle and concentration, values for absolute molecular weight, Mw, radius of gyration, Rg and second virial coefficient, A2 are calculated and displayed.

For this sample:

Mw1.598 x 105 g/mol
Rg32.3 nm (indicating either a stiff polymer chain or a broad molecular weight distribution)
A21.18 x 10-3 cm3 mL/g2 (indicating that THF is a thermodynamically good solvent for PLA)

These data show that the BI-MwA can be used to determine the molecular weight of polylactides.

image of Zimm plot software