Light Scattering Studies of Polymer Solutions and Melts

Abstract

Static and dynamic properties of polymer solutions and melts can be investigated by means of modern scattering techniques. While small angle X-ray scattering (SAXS) and small angle neutron scattering (SANS) have made advances particularly in studies related to the static structure factor S(K), laser light scattering including the use of Fabry–Perot interferometry and photon correlation spectroscopy has become a standard tool in studying polymer molecular motions. In polymer solutions, the main technique is to use measurements of angular distribution of integrated scattered intensity by means of visible light, SAXS or SANS for S(K) and measurements of angular distribution of the spectrum of scattered light by means of photon correlation spectroscopy for the dynamic structure factor, S(K,ω). Recent advances have been made in the method of data analysis related to the ill-posed Laplace inversion problem. The new approaches include the singular value decomposition technique and methods of regularization with different criteria for the smoothing operator. By combining static and dynamic light scattering measurements with appropriate algorithms for the Laplace inversion of the time correlation function, a new analytical technique has been developed for polymer molecular weight characterizations. The non-intrusive method has been applied to determine the molecular weight distributions of linear and branched polyethylene in 1,2,4-trichlorobenzene at 135°C and of poly(1,4-phenylenetere-phthalamide) in concentrated sulfuric acid. In addition, a new prism light scattering cell is being developed to integrate the above capabilities with chromatographic and other separation techniques. Aside from translational motions of the center of mass of polymers in dilute solution, photon correlation spectroscopy also permits us to investigate rotational, flexual and internal segmental motions. Polymer molecules entangle in semidilute solution. Light-scattering spectroscopy measures a cooperative diffusion coefficient and a slow mode which has been shown to be different in magnitude from the self-diffusion coefficient. The entanglement behavior varies from coils to rod-like polymers. Static and dynamic properties of polymer solutions in semidilute and semicoricentrated regimes can be related to those of bulk polymer melts where measurements of polarized Rayleigh–Brillouin spectra and depolarized Rayleigh spectra yield information on localized structural relaxation and collective segmental/molecular orientational motions. Relaxation times covering a very broad frequency range will be discussed.