1 октября, 2015 
Malyar The molecular weight distribution (MWD) can be determined by means of GPC [or size exclusion chromatography, (SEC)]. This method divides the molecules according to their hydrodynamic volume, which is proportional to their molar mass. The most important consideration in the chromatography of formaldehyde condensation resins is the poor solubility of the resins in most solvents usually used in GPC and hence the proper choice of the solvent and the mobile phase. This choice influences the solubility of the resin, the behavior of the chromatographic columns, and the effectiveness of detection. For lower molar mass PF resins, tetrahydrofuran (THF) is a suitable solvent [128], while for higher molar mass phenolics and for MF resins, dimethylformamide (DMF) can be recommended, sometimes modified e. g., by addition of small amounts of ammonium formate or other salts such as LiCl [128,340]. UF resins are only soluble in DMF (with some undissolved higher molar mass portions) and dimethylsulfoxide (DMSO). DMSO shortens the lifetime of the chromatographic columns and causes problems with high pressures because of its higher viscosity in comparison to other organic solvents and low refractive index increments [341]. The high reactivity of the functional groups of the resins
additionally requires the use of the correct solvent and mobile phase, especially concerning sample preparation, in order to obtain a satisfactory reproducibility of the results.
Another problem with GPC of condensation resins is the calibration of the columns. Because in the oligomeric and polymeric regions of the resins no compounds with a special and singular molar mass and a clear molecular structure are available, similar or chemically related substances have to be used as calibration standards. However, differences in the hydrodynamic volumes even at the same molar mass cannot be excluded totally. This uncertain calibration of the columns also induces a great uncertainty in the calculation of molar mass averages on the basis of the chromatograms obtained.
Molar mass distributions of UF resins have been reported by several authors [22,125,340-345], as have mass distributions of MUF resins [71,346-348]. The molecular characterization of PF resins can also be performed without any major problems by GPC [128,134,139,349,350]. Due to newer GPC methods, modification of the PF resin before the analysis is no longer necessary.
Figure 3 shows chromatograms of two PF resins, one with a distinct high molecular weight portion, and the other with rather lower molar masses [128]. The averages of the molar mass can be (i) calculated from the gel chromatograms, taking into consideration the above-mentioned problems with the calibration of the columns, and (ii) measured by
(a) vapor pressure osmometry for the number average molar mass (UF resins [19,22,341,344], MF resins [351], PF resins [121,352,353]) and
(b) light scattering for the weight average molar mass (UF resins [19,22,341], PF resins [354]).
The weight average molar mass at each elution volume can also be monitored directly during each GPC run using GPC-LALLS. If the weight average molar mass in this case is determined directly in the eluent by light scattering, no standard calibration of
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Figure 3 GPC plot of two PF resins: (———————————————— ) PF resin with a distinct high molecular weight portion; (———— ) PF resin with rather low molar masses. Column set: Merck HIBAR LiChrogel PS1 + PS4 + PS20 + PS400. Solvent and mobile phase: THF. Detection: UV-VIS, 254 and 280 nm, respectively. Concentration of samples: 1 mg/ml. Flow rate: 0.5ml/min. (After Ref. 128.) |
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Figure 4 GPC coupled with low angle laser light scattering (GPC-LALLS) of a UF resin: e(V) = concentration signal; E(V) = normalized response of the LALLS detector; log Mw(V) = E(V)/e(V) = measured weight average molar mass as a function of the elution volume V. Column set Varian Toyo Soda TSK G4000 H8+ G3000 H8+ G2000 H8+ G1000 H8. Solvent and mobile phase: 0.01 m solution of LiBr in DMF. Temperature: 40°C, flow rate: 1.1 ml/min, concentration of samples: 10-15mg/ml. (After ref. 18.) |
the column is necessary (GPC-LALLS). The eluent with the dissolved molecules passes a light scattering cell and the weight average molar mass is measured directly during each chromatographic run. However, this method is laborious and, therefore, described only in a few cases in the literature (UF [18] Fig. 4; PF [109,142,355,356]). During each run two curves are obtained: one is the concentration peak, and the other the light scattering peak, which is directly related to the actual molar mass average in the detection cell at each moment. Using these two curves, an individual calibration curve can be derived for each run. However, it must be taken into consideration that the light scattering signal can only be evaluated in the higher molar mass region and, therefore, the calibration curve is valid with sufficient accuracy only in this part of the chromatogram.
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