Molecular weight and its distribution of polymers determined by using phase-modulated flow birefringence measurement

Abstract

Recently the phase-modulated flow birefringence (PMFB) method has been developed by Frattini and Fuller, which is widely accepted as one of the most sensible and accurate technique suitable for experimental studies on the molecular theory of the polymer solutions. The objective of this study is to develop a systematic method to determine molecular weight and its distribution of not only rigid polymers but also flexible ones by using the PMFB technique. Firstly, calf skin collagen was considered as an example of rigid polymers with bimodal distribution. Following the cessation of simple shear flow, the relaxation of birefringence was recorded to show two distinct relaxation times due to the molecular weight distribution of the calf skin collagen, which turns out to be bimodal. Initial rapid drop corresponds to the relaxation of a short chain(monomer) as well as a long chain(dimer), but slow decay follows due to relaxation of long chains only. This approach was extended for the system of multimodal molecular weight distribution. Using two molecular models on rigid rod and flexible polymer theories for $^\triangle{n}$ and $\tan2\chi$ have been expressed as functions of molecular weight and molecular weight distribution. For the rigid rod model, $^\triangle{n}$ has been shown to be proportional to $\Sigma{c}_iM_i^4$, and $\cot2\chi$ turns out to has a linear relationship with $\Sigma{c}_iM_i^7/\Sigma{c}_iM_i^4$. For the flexible polymer, $^\triangle{n}$ has been shown to be proportional to $\Sigma{c}_iM_i^{1.4-1.8}$ and $\cot2\chi$ is linearly dependent on $\Sigma{c}_iM_i^2$. In order to confirm these relationships, poly($\gamma$-benzyl-L-glutamate) dissolved in m-cresol was tested, which can form the liquid crystalline phase at high concentrations due to its rigid rod characteristics. The experimental results revealed that $^\triangle$n was proportional to $\Sigma{c}_iM_i^4$ and $\cot2\chi$ had linear dependence on $\Sigma{c}_iM_i^7/\Sigma{c}_iM_i^4$. The orientat...