A new method for determining molecular weight distributions of copolymers

Abstract

Measurement of the molecular weight distributions of copolymers by size exclusion chromatography (SEC) presents problems because the elution volume of any species may depend on its composition as well as its molecular weight. Also, the response of the usual concentration detectors may also be influenced by the copolymer composition as well as its concentration. These problems arise when the copolymer composition may vary with molecular size. Conventional SEC techniques are suitable for copolymers with invariant compositions. This article describes and illustrates a method for measuring molecular weight distributions of copolymers. In many cases, the variation of copolymer composition with molecular weight can also be determined. The technique uses three detectors: (a) an evaporative detector (ED) to measure the concentration, Δc, of the eluting species; (b) a differential refractive index detector (DRI) to measure the refractive index difference, Δn, between the solution and solvent at any given elution volume; and (c) a low‐angle laser light scattering (LALLS) detector that measures the corresponding molecular weight of the eluting solutes. This latter measurement is possible because the appropriate values of Δn/Δc are available from the outputs of the other two detectors. For LALLS measurements of molecular weight all the species in the detector cell at any instant must have the same composition or, at least not have Δn/Δc that varies with composition. The method is illustrated with data from ethylene‐propylene and ethylene‐propylene‐diene copolymers.

Block copolymer molecular weight determination via gel permeation chromatography: Choosing a combining rule

“…Several methods have been proposed for calculating the molecular weight of a block copolymer from information on the properties of its component homopolymers and the block copolymer's composition. When significant compositional heterogeneity exists between chains, the copolymer's chemical composition distribution (CCD)1–3, 6, 19, 23, 30–35 must be considered; but for near‐monodisperse block copolymers, the CCD is not broad enough to be significant 5, 36. This simpler case describes typical styrenic block copolymers and is the case that we consider here.…”

A Comprehensive Review on Controlled Synthesis of Long‐Chain Branched Polyolefins: Part 3, Characterization of Long‐Chain Branched Polymers

“…A single dn/dc value can be found for homopolymers [42] or copolymers that contain an invariant composition, independent of MW. [69] For copolymers with varied compositions over the MW range, an average dn/dc of the whole sample is applied instead of the true dn/dc for each slice. Therefore, only some apparent MW can be obtained.…”

Copolymer analysis

“…Ethylene content was rich at the higher-and lower-molecular-mass ranges in the copolymer. As already explained in section 5.2.2, the use of three detectors, an evaporative detector (ED), RI, and LALLS, is very attractive for measuring variations of both copolymer composition and molecular weight [37]. This technique was applied to the analysis of EPM and EPDM.…”

“…This discrepancy might be in part due to neglecting the CCD of the copolymer. An SEC system with three different detectors has been applied for the characterization of copolymers EPM and EPDM: an evaporation detector (ED) to measure the concentration dC, a differential refractive index detector (RI) to measure the refractive index difference dn between the solution and the solvent (the mobile phase), and a LALLS detector to measure the corresponding molecular weight of the eluting solutes, in the effluent from a column [37]. All three detectors were interfaced with a microcomputer, and computations were based on the following equations:…”