Copolymer differential refractive index

The use of multidetector GPC greatly increases the power of SEC, particularly in the case of copolymers. For copolymers of styrene-maleic anhydride (SMA), not only can the molecular weight distribution be determined, using a differential refractive index (DRI) detector, but also the compositional information of SMA (styrene content or acid number) by combining chromatograms from DRI and UV detectors [9]. 

One or more detectors is attached to the output of the columns. For routine analysis of linear homopolymers, this is most often a Differential Refractive Index (DRI) or a UV detector. For branched or copolymers, however, it is necessary to have at least two sequential detectors to determine molecular weight accurately. Branched polymers can be analyzed using a DRI detector coupled with a "molecular weight sensitive" detector such as an on-line viscometer (VIS) or a low-angle laser light scattering (LALLS) detector. 

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 AC, a differential refractive index detector (RI) to measure the refractive index difference An 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 ... 

Molecular-weight determinations are performed in good solvents for both blocks of the PEO copolymers. The most widely used analysis conditions are as follows eluents-tetrahydrofuran (THE) and chloroform (CHE) flow rate—1.0 ml/min detection—differential refractive index (dRI) detector. The temperature interval is between... 

Figure 6.12 is an example of a molar mass distribution of a cationic copolymer floccu-lant of acrylamide measured using a SEC system coupled to a differential refractive index detector and a multi-angle light scattering detector. 

As mentioned, the minimnm nnmber N of different solvents in which scattering measnrements on the copolymers must be made increases at the alarming rate of N(N + l)/2 and the nnmber of refractive increments needed grows at the even higher rate of N N + If 12. For the simplest case, that of 1V= 2, nine different differential refractive index values must be known. For a terpolymer, this becomes 24 values, and for N = 5, 90 values Clearly an alternative method is needed. 

Trathnigg and co-workers [97] have quantitated copolymers and blends using a dual concentration detector. Xu and co-workers [98] used UV absorbance and differential refractive index detectors in conjunction with SEC to study the copolymer composition of chlorinated butyl rubber/PS comb graft copolymers as a function elution volume. 

Refractometry can be used to determine the composition of a copolymer. In addition, differential refractometry has been used to study micellization in dilute block copolymer solutions (Tfizar and Kratochvfl 1972). The refractive index (n) is obtained in an Abbe refractometer via measurements of the critical angle for external reflection. The refractive index increment dn/dc, where c is the polymer concentration, can be related to the molecular weight of particles in solution. Further details of the method are provided by Pepper and Samuels (1989). 

The application of refractive index and differential viscometer detection in SEC has been discussed by a number of authors [66-68]. Lew et al. presented the quantitative analysis of polyolefins by high-temperature SEC and dual refractive index-viscosity detection [69]. They applied a systematic approach for multidetector operation, assessed the effect of branching on the SEC calibration curve, and used a signal averaging procedure to better define intrinsic viscosity as a function of retention volume. The combination of SEC with refractive index, UV, and viscosity detectors was used to determine molar mass and functionality of polytetrahydrofuran simultaneously [70]. Long chain branching in EPDM copolymers by SEC-viscometry was analyzed by Chiantore et al. [71]. 

Krause, S., Lu, Z.-H., and Iskander, M., Rroperties of low molecular weight block copolymers 4. Differential scanning calorimetry and refractive index-temperature measurements on styrene-butadiene diblock copolymers and styrene-butadiene-styrene triblock copolymers. Macromolecules, 15,1076-1082 (1982b). 

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