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

J. Bodycomb,

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 dis-posable tableware. However, the molecular weight of the polymer can affect the properties of the end product. Therefore rapid, reliable charac-terization is important to better understand manufacturing performance, processing, and structure-property relationships.

Molecular weight determination of PLA by solution based techniques such as Static Light Scatter-ing (SLS) or GPC with refractive index detection is a challenge due to the low value for the refrac-tive 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).

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.

Polylactic Acid BI-MwA

Light scattering data is typically analyzed with the Zimm equation:

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 ΔR is proportional to the excess scattered intensity and measured by the BI-MwA. The scattering vector, q is given by (4πn/λ)sin(Θ/2). Here, n is the refractive index of the solvent, λ is wavelength, and Θ 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:
Mw: 1.598 x 105 g/mol
Rg: 32.3 nm
(indicating either a stiff polymer chain or a broad molecular weight distribution)
A2: 1.18 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.

Zimm Plot of PLA

1: For vertically polarized light, as in the case of the BI-MwA and other modern light scattering instruments, K = 4 π² n² (dn/dc)²/(Nλ4) where n is the solvent refractive index, N is Avogadro’s number, and λ is the wavelength of the laser in vacuum.

With kind permission of Brookhaven Instruments Corp.