Rapid, high-resolution analysis of low molecular weight polymers using ultra-efficient polymer chromatography (APC) systems

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Key words

Polymer, SEC, GPC, APC, polymer characterization, low molecular weight polymers, oligomers, epoxies

introduction

Gel Permeation Chromatography (GPC) is a widely recognized and effective method for polymer characterization. However, although a large amount of information can be obtained using this technique, such analysis itself still has drawbacks. The column is typically filled with styrene-divinylbenzene and requires proper aging and should be operated at low back pressure to ensure long-term stability. Filler particles are usually large (≥5 μm) and resolution is generally affected as a result. Columns packed with smaller particles (<5 μm) are on the market and can increase the rate of GPC separation, but the separation speed is limited by the low maximum operating pressure of the column itself. In addition, conventional GPC instruments have a large system size, which requires the use of larger diameter columns to slow down the system peak broadening that may result in reduced resolution. The Waters ACQUITY Ultra High Performance Polymer Chromatography (APC) system combined with a sub-3 μm hybrid particle column enhances system stability and ensures flow rate accuracy at higher pressures. In addition, the overall diffusivity of the APC system is low, which can significantly increase the resolution, especially when analyzing low molecular weight oligomers. Increasing the resolution of the separation of low molecular weight oligomers and reducing run time enables rapid monitoring of polymer process development, early detection of new polymer types and overall acceleration of the market for new polymer products.

This application note compares separation based on the ACQUITY APC system with conventional GPC-based separation. This article will show that using a low-diffusion system with a sub-3 μm hybrid particle technology column speeds up the analysis, increases resolution, and aids in the correction of low molecular weight oligomers. The combined use of these techniques enables a more stable and accurate determination of the molecular weight parameters of low molecular weight polymer samples. Early identification of even minor changes in a polymer can significantly accelerate the development of polymers in chemical and biomaterial applications.

experiment

Alliance ^® GPC System Condition <br>Detector: 2414RI (Differential Refractive Detector)
RI flow cell: 35 °C
Mobile phase: THF
Flow rate: 1mL/min
Column: Styragel 4e, 2 and 0.5, 7.8 x 300 mm (3 in series)
Column temperature: 35 °C
Sample diluent: THF
Injection volume: 20 μL

ACQUITY APC System Conditioner <br>Detector: ACQUITY RI (Differential Refractive Detector)
RI flow cell: 35 °C
Mobile phase: THF
Flow rate: 1 mL/min
Column: ACQUITY APC XT 200 Ã… column and two 45 Ã… columns, 4.6 x 150 mm (3 columns in series)
Column temperature: 35 °C
Sample diluent: THF
Injection volume: 20 μL

Data management

Empower 3 Chromatography Data Software

sample

1 mg/mL Waters polystyrene standard (100K, 10K and 1K) epoxy resin (2 mg/mL)

Results and discussion

In order to properly characterize a polymer using SEC, it is important to generate a calibration curve using the appropriate standards to determine the separation range of the currently used column. Using standard GPC to analyze standards and samples is quite time consuming and can run for up to 1 hour (or longer). Since the data produced by the sample will be compared to the calibrated standard to determine the molecular weight, the accuracy of the standard analysis results is critical to obtaining accurate results for the polymer sample. In addition to the long run time of the GPC itself, the extra column size of a conventional GPC system can also cause peak broadening, which reduces resolution and thus reduces the accuracy of the calibration data points. Compared to conventional GPC systems, the ACQUITY APC system has a lower degree of diffusion, resulting in fewer peak broadening and clearer peaks for narrowly distributed standards, as shown in Figure 1. In addition, the low-diffusion APC system combined with the stable sub-3 μm APC column technology supporting higher flow rates and back pressures also improves the resolution of 1K polystyrene standards and reduces analysis time to the original 1/5.

Figure 1. Comparison of run time and resolution of polystyrene standards (Mp: 100K, 10K, and 1K) in a conventional GPC system and an ACQUITY APC system

The increased resolution using the APC system adds more identifiable peaks to determine the molecular weight of the 1K polystyrene standard. As shown in Fig. 2, by using the value provided by the standard supplier or the molecular weight information determined from the standard measurement value obtained by the external method, more data points can be added to the calibration curve, thereby The sample results calculated from this curve increase the confidence.

Figure 2. Using the ACQUITY APC System, more data points on polystyrene standards (100K, 10K, and 1K) are obtained on the calibration curve due to increased resolution of 1K low molecular weight standards.

In general, you need to run a series of standards to derive the data points used to generate the calibration curve. When using conventional GPC, it can take hours to days to equilibrate, formulate, and analyze each standard. Therefore, calibration is usually not performed and the analysis results are determined based on the original calibration curve. The ACQUITY APC system significantly accelerates the equilibrium speed due to its low system retention volume and significantly reduces run time due to the use of smaller particles at higher flow rates. The reduced run time allows balance and calibration operations to be easily completed in less than an hour. Finally, thanks to the increased resolution, it may only be necessary to prepare and inject fewer standards to obtain a stable curve that can be used for calibration. When the sample is analyzed, the stability of the calibration operation is increased to make the molecular weight determination of the low molecular weight oligomer more reliable.

Figure 3 shows the results of an analysis of an epoxy resin sample compared to calibration with polystyrene standards. This result indicates that three ACQUITY APC XT 4.6 x 150 mm tandem columns can be used to resolve different oligomers in less than 5 minutes of run time.

Figure 3. A sample of epoxy resin dissolved in tetrahydrofuran was analyzed using three ACQUITY APC XT 4.6 x 150 mm tandem columns equipped with an ACQUITY RI detector. Low molecular weight oligomers (shown as peak tip molecular weight) can be resolved in less than 5 minutes.

APC's ability to reduce run time helps to monitor reactions during process development. Increased resolution can facilitate faster identification of polymer changes that may occur in synthetic applications or degradation studies. Early detection of process changes by monitoring various molecular weights helps to better understand the polymer and its intended properties, thereby facilitating the development of new polymers and speeding time-to-market.

in conclusion

This system is significantly superior to conventional GPC systems due to its lower diffusivity and higher back pressure to allow for the use of smaller hybrid particles. Combined with the latest column technology, the APC system also improves the resolution of low molecular weight oligomers compared to conventional GPC. The performance advantages of APC include more reliable calibration results, which are essential for generating accurate measurements for polymer characterization. Simultaneous improvement in detection speed and resolution of low molecular weight polymers enables rapid and reliable characterization of polymers during development, facilitating close market tracking of new polymers.

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