DETECTOR RESPONSE AS A FUNCTION OF MOLECULAR-WEIGHT AND ITS EFFECT ONSIZE-EXCLUSION CHROMATOGRAPHY MOLECULAR-WEIGHT DETERMINATION OF POLYMER DISTRIBUTIONS

In size exclusion chromatography (SEC), if the response of the detecto r is not constant with molecular weight, the calculated molecular weig hts are not a true representation for the sample. Without response fac tors, or some other method of correcting the response for the specific polymer calibration standards used, the resulting distribution is bia sed. From literature references, it is known that polystyrene has a no n-constant, refractive index (RI) detector response with molecular wei ght. However, this is generally ignored since the effect is relatively small and does not preclude their use as calibration standards due to their narrow dispersities. Analysis within Dow Coming has confirmed t he literature result. Polydimethylsiloxane (PDMS) has a relatively con stant refractive index response (confirmed by Analysis of Variance sta tistical method). Therefore, analysis of PDMS polymers using (RI) dete ction gives a true molecular weight distribution with little bias in t he distribution. For siloxane resins, the refractive index response is not constant with molecular weight. So far, only infrared (IR) and vi scometry detection have the necessary consistancy for a true molecular weight distribution determination. The IR detection is readily explai ned from IR principles, but the viscosity can Only be explained by hyp othesising the siloxane resin to adopt essentially a spherical configu ration in solvent. Polyethylene glycol, used for analysis of silicone polyethers, shows a marked non-constant response with molecular weight using both RI and IR detection. Unless the molecular weight data are transformed using the equation of the fit line or response factors are used to correct the detector responses, the molecular weight distribu tions calculated will have a bias associated with them. The variabilit y in response seems to correlate with molecular weight, suggesting eit her that polymer endgroups may be having a significant effect upon det ection, or the polymers may be preferentially adsorbing on to the colu mns. Polyisobutylene seems to have two areas of constant RI detector r esponse with a step change between them. The standards used in this st udy were purchased from two separate vendors and may have been manufac tured by different processes, which could account for this difference in response. Otherwise, the calibration is virtually constant, and wou ld give effectively unbiased distributions. (C) 1997 Elsevier Science Ltd. All rights reserved.