Absolute Molar Masses for Phenol Formaldehyde Resins with GPC/SEC-ESI

Introduction

Phenol formaldehyde resins are synthesized by a step-growth polymerization. GPC/SEC with RI and UV detection is often used to characterize resins and to quantify the amount of different oligomeric species. Additional information can be obtained if an ESI-MS-spectrometer is added in-inline to the GPC/SEC system. This technique combines the separation ability of GPC/SEC with the sensitivity and specificity of detection from MS and allows the identification of oligomeric species and gives information about the degree of CH₂-OH substitution.

Experimental

GPC/SEC analysis was performed on a PSS SECcurity 1200 System consisting of:

• an isocratic pump

• an autosampler with variable injection volume

• a differential refractometer (RI)

• a Thermo Fisher LXQ ESI spectrometer

Figure 1: RI trace oligomeric phenol formaldehyed resin, degree of polymerization identified from corresponding mass spectrum.

Conditions:

Columns: PSS SDV, 5 μm, 8 × 300 mm; 50 Å, 100 Å, 1000 Å, precolumn

Solvent: THF

Flow-rate: 0.3 mL/min

Inject volume: 20 μL

Software: PSS WinGPC Unity 7.4, Thermo Fisher Excalibur 2.07

Figure 2: Mass spectrum for n = 1 showing the 3 different species (CH2 –OH substitution).

Results

Figure 1 shows the elugram of an oligomeric phenol formaldehyde resin, separated into three different peaks with 1, 2 and 3 repeat units. The mass spectra are then measured for each species. Figure 2 shows as an example the mass spectrum for peak n = 1 while Table 1 summarizes the masses possible in theory and identified (green) or not detected (grey). This combined approach can be used for molar masses up to approx. 2000 Da, higher degrees of polymerization have also been investigated.

Table 1: Expected molar masses for different degrees of polymerization (n) and for CH2OH units at the aromatic ring. Colour code: grey: possible in theory but not found, green: identified in MS spectrum.

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