Long Chain Branching Analysis

Two analytical methods, size exclusion chromatography and rheology, provide a long chain branching index. The application of size exclusion chromatography to long chain branching analysis was described in Section 5.2.3.1. 

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FIGURE 16.16 Nonbranched/long chain branched glucans of potato starch dissolved in hot water-steam and 0.1 M NaOH 1.2 ml of the 18-mg/ml solution was separated on Sepharose CL 2B (88 X 1.6 cm) 3-ml fractions were collected for further analysis normalized (area = 1.0) eluogram profiles (ev) constructed from an off-line determined mass of carbohydrates of each of the fractions flow rate 0.15 ml/min V. , = 70 ml, = 180 ml eluent 0.01 tA NaOH. 

The proposal that PVAc also has non-hydrolyzable long chain branches stems from the finding that PVA also possesses long chain branches. No/akura et a/.171 "07 suggested, on the basis of kinetic measurements coupled with chemical analysis, that chain transfer to PVAc involves preferential abstraction of backbone (methine) hydrogens (ca 5 1 v,v the acetate methyl hydrogens at 60 °C). 

Gel Permeation Chromatography (GPC), also known as Size Exclusion Chromatography (SEC), is a technique used to determine the average molecular weight distribution of a polymer sample. Using the appropriate detectors and analysis procedure it is also possible to obtain qualitative information on long chain branching or determine the composition distribution of copolymers. 

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]. 

Table II contains the results of the analysis of star-branched copolymers (Mw, Mn, and polydispersity d) by both the viscometric coupling (V) and the light-scattering coupling (L). The numerical values are in agreement, especially for the values. These results confirm that the universal calibration is perfectly valid for branched molecules, even for a high degree of long-chain branching.

Molar mass distribution is a dominant microstracture parameter that, in copolymers, needs to be measured with additional information to account for long chain branching, comonomer incorporation, or ethylene propylene combinations (in the case of EP copolymers). The combination of GPC and IR spectroscopy has been shown to be of great value in the characterization of copolymers. The importance of automation and sample care, especially in the case of polypropylene, has been discussed as well as the significant improvement in sensitivity by the use of IR MCT detectors. There are big expectations for the analysis of ultrahigh molar mass polyolefins by the new AF4 technology. 

It is interesting to note that the development of TREF as a routine polymer analysis tool has until recently taken place almost exclusively within industrial research laboratories. Further, the more sophisticated versions of the TREF have emerged from laboratories associated with companies engaged in the production of linear low density polyethylenes (LLDPE). Clearly this has been driven by the need to understand the nature of LLDPE which exhibits behavior indicative of considerable structural heterogeneity. This is in spite of the fact that, compared to conventional low density polyethylene (LDPE), it is narrow in MWD and contains little or no long-chain branching. 

In this chapter, the analysis of LCB in macromolecules using SEC with multiple detection is described. While the qualitative information obtained from this type of analysis is mentioned, particular attention is paid to the requirements necessary for accurate, quantitative determination of LCB, of the long-chain branching distribution (LCBD), and of the fractal dimension (dt) of macromolecules. 

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