Analysis of hyperbranched polyesters

In this note we analyze two hyperbranched polyesters samples by advanced size exclusion chromatography to assess the branching. We show that even for low dn/dc samples of hyperbranched polyesters the results are reproducible

Introduction

Polyesters are long chain polymers containing an ester linkage. They are used as films or fibre but also as composites, elastomers and for coating. The hyperbranched polyester has a core shell structure with tree like branches. The number of branches will determine the ratio of the molecular weight to size while the solubility and physical properties of the hyperbranched polyesters will be dictated mainly by the end groups exposed.
In this application communication we analyze two hyperbranched polyesters samples by advanced size exclusion chromatography to assess the branching.

Experimental

The samples were found to be soluble in tetrahydrofuran (THF) containing 1% acetic acid. The analysis was carried out on a triple detector gel permeation chromatography (TD-GPC) system. The Viscotek GPCmax solvent delivery and autosampler was used for the isocratic delivery of THF at 1 mL/minute. The injection volume was 100 µL. The separation was made using 3 columns (I-MBLMW-3078) in series. The columns are 30 cm long and the internal diameter is 7.8 mm. They were kept at a constant temperature of 45 °C inside the Viscotek triple detector array (TDA) system. After the columns, the sample passes through the detectors in series: light scattering, differential refractometer (RI), with wavelength match to the light scattering detector, and the viscometer. The RI is used to calculate the concentration while the light scattering detector is used to measure the molecular weight of the sample eluting.
The viscometer measures the intrinsic viscosity at each elution slice.

Results

Figure 1: Triple chromatogram of the two samples of hyperbranched polyesters.
MRK1331_fig01

Figure 1 shows the triple detector chromatogram of the sample A and sample B. The red curve is the RI signal. The blue curve is the digital differential pressure transducer signal that, when combined with the inlet pressure, is used to measure the intrinsic viscosity. The green curve is the light scattering signal. The LS signal is proportional to the molecular weight, the concentration and the square of the value of dn/dc for the polymer-solvent combination. The light scattering signal is quite weak in this case as the value of dn/dc is small. However, good signal to noise and good resolution are obtained.

Table 1: Molecular weight and size data derived from the triple detection chromatograms using OmniSEC software.
Sample IDMZ (Da)Mw (Da)Mn(Da)Mw/MnIV (dL/g)RH (nm)dn/dc (cm3/g)
Sample A12,8907,9414,2951.850.0521.770.074
Sample A13,0767,8924,3551.810.0511.760.074
Average12,9837,9174,3251.830.0521.770.074
%RSD1.00.440.981.41.40.400
Sample B9,2085,8763,9281.500.0441.540.068
Sample B9,3766,0964,0601.500.0441.560.069
Average9,2925,9863,9941.500.0441.550.069
%RSD1.32.62.3000.911.0

Table 1 shows the results for the molecular weights of the hyperbranched polyester sample A and B. Both samples have low molecular weight and a fairly low polydispersity, defined as Mw/Mn. The intrinsic viscosity is low for the hyperbranched sample. The IV and the molecular weight is used to calculate the hydrodynamic radius (Rh) for the samples. Both have hydrodynamic radius less than 2 nm. For comparison, a linear random coil with an average molecular weight of 7,000 g/mol would have a hydrodynamic radius of >2 nm. The smaller size calculated for these samples indicates the branching. Measurement of linear polyester would be needed to obtain the branching ratio of the sample A and B.

Figure 2: Repeat injection of the hyperbranched polyesters. Sample B has a higher density, hence a higher branching ratio than sample A.
MRK1331_fig02

Figure 2 shows the Mark-Houwink plots of the samples. The Mark-Houwink plot is the double logarithmic plot of the intrinsic viscosity measured by the viscometer versus the molecular weight measured by the LS detector. The samples were injected twice and the results are very repeatable especially if you consider the low molecular weight and low dn/dc values. The two samples are clearly different in branching as the sample B is lower on the MH plot than sample A.

Conclusion

We have shown that even for low dn/dc samples of hyperbranched polyesters the results are reproducible and the triple detection technique can measure accurately the physical properties of the samples.

The Mark Houwink plot also shows that TD-GPC is perfectly able to distinguish between slightly differently branched samples.

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