Narrow molecular weight distribution fraction analysis

In principle, SEC can be calibrated with standards of narrow molecular weight distribution of the polymer that one wishes to analyze. In this way, a calibration curve relating molecular weight to elution volume is obtained, in a fashion similar to what is done for the calibration of TREF or crystallization analysis fractionation. This approach, however, suffers from the limitation that the calibration curve is only applicable to one specific polymer. Additionally, narrow standards are not available for many different types of polymers. In principle, the batch fractionation techniques discussed above could be used to obtain such narrow standards, but these processes are time-consuming and, as explained, do not permit the isolation of samples with very narrow molecular weight distributions. 

SEC-ESIMS is a valuable tool for polymer characterization. Compounds are separated based on their hydrodynamic size in solution, but upon detection, an absolute molecular weight is also furnished. Only 1% of the SEC effluent is required for ESIMS analysis, thereby accommodating the popular SEC detectors. SEC-ESIMS provides an attractive solution to the calibration of SEC without the use of external calibrants. Chemical composition distribution information on copolymers is easily afforded provided the individual monomers differ in molecular weight. The successively acquired mass spectra contain narrow fractions of the overall distribution that simplifies the analysis of complex formulations. Unfortunately, we have not been able to provide structured details on materials beyond 5000 Da due to the low resolution of the quadrupole mass spectrometer. Nevertheless, SEC-ESIMS is an exciting hyphenated techniques for polymer characterization. 

The initial use of analytical TREF was to characterize polyethylene fractions [120]. Mirabella and Ford used TREF to fractionate LDPE and LLDPE for further characterization using SEC, X-ray C-NMR, DSC and viscosity measurements [121]. These investigators determined that the melting behavior of LLDPE correlated well with a multimodal SCB distribution and the distribution became narrower with increasing molecular weight. Mingozzi and collaborators [122] have applied both analytical and preparative TREF to the analysis of tacticity distribution in polypropylene. The results showed that both methods could be used successfully analytical TREF gave faster qualitative results on the polymer microstructure, while preparative TREF, with subsequent analysis of the fractions, could yield detailed quantitative tacticity information. Hazlitt has described an automatic TREF instrument to measure SCB distributions for ethylene/ a-olefin copolymers [123]. 

The chromatogram was calibrated using the result of the analysis of MALDI-ToF spectra of selected fractions (see Table 5.1). The average molar masses (number average molecular weight (M ) and MJ of the copolymer were measured using the Caliber software distributed by Polymer Lab. The type of calibration selected by Montaudo and co-workers [10] was a narrow standards the calibration function was polynomial of order 1 and the calculation method was area based . 

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