Fast and high-resolution analysis of low molecular weight polymers using ultra-efficient polymer chromatography (APC) systems
Mia Summers and Michael O'Leary
Waters Corporation (Milford, MA, USA)
Application advantages
â– Can quickly characterize polymers without reducing performance levels
â– Compared with conventional GPC analysis, it can improve the resolution of low molecular weight oligomers
â– Compared with conventional GPC analysis, it can improve the calibration level and thus make more accurate determination of low molecular weight oligomers
â– The polymer can be quickly monitored, so that early changes in product development can be discovered
Waters Solutions
ACQUITY® Ultra High Performance Polymer Chromatography (APC ™) System
ACQUITY APC XT column
Waters Polymer Standard
Empower® 3 chromatography data software with GPC option
Key words
Polymer, SEC, GPC, APC, polymer characterization, low molecular weight polymer, oligomer, epoxy resin
introduction
Gel permeation chromatography (GPC) is a widely recognized and effective method for polymer characterization. However, despite the large amount of information available using this technique, this type of analysis itself still has shortcomings. The column is usually packed with styrene-divinylbenzene, and it needs to be properly aged and should be operated at low back pressure to ensure its long-term stability. The filler particles are usually large (≥5 μm), and the resolution is generally affected by this. Columns packed with smaller particles (<5 μm) have been put on the market and can increase the separation speed of GPC, but the separation speed will be limited by the lower maximum working pressure of the column itself. In addition, conventional GPC instruments have a larger system volume, which requires the use of larger diameter columns to slow down the system peak broadening that may result in reduced resolution. Waters ACQUITY Ultra High Performance Polymer Chromatography (APC) system combined with a sub-3 μm hybrid particle chromatography column can enhance system stability and ensure flow rate accuracy at higher pressures. In addition, the overall diffusivity of the APC system is low, which can significantly improve the resolution, especially when analyzing low molecular weight oligomers. Improving the resolution of separating low molecular weight oligomers and shortening the running time enables rapid monitoring of polymer process development, early detection of new polymer types and overall acceleration of new polymer product launches.
This application note compares the separation based on the ACQUITY APC system with the separation based on conventional GPC. This article will show that the use of a low-diffusion system with a sub-3 μm hybrid particle technology column can speed up analysis, increase resolution, and help calibrate low molecular weight oligomers. The combined use of these techniques can more reliably and accurately determine 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 conditions
Detector: 2414 RI (Differential Refractive Index Detector)
RI flow cell: 35 ℃
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 ℃
Sample thinner: THF
Injection volume: 20 μL
ACQUITY APC system conditions
Detector: ACQUITY RI (Differential Refractive Index Detector)
RI flow cell: 35 ℃
Mobile phase: THF
Flow rate: 1 mL / min
Columns: ACQUITY APC XT 200 Ã… column and two 45 Ã… columns, 4.6 x 150 mm (3 columns in series)
Column temperature: 35 ℃
Sample thinner: 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 polymers using SEC, it is important to use appropriate standards to generate a calibration curve to determine the separation range of the column currently used. The analysis of standards and samples using conventional GPC is quite time-consuming and can run up to 1 hour (or longer). Since the data generated by the sample will be compared with the calibrated standard to determine the molecular weight, the accuracy of the standard's analysis results is crucial to obtain accurate results regarding the polymer sample. In addition to the longer running time of the GPC itself, the larger extra column volume of conventional GPC systems can also cause peak broadening, which reduces the resolution and thus the accuracy of the calibration data points. Compared with the conventional GPC system, the ACQUITY APC system has a lower diffusivity, so the resulting peak broadening is less, and the chromatographic peaks of the narrow distribution standard are also clearer, as shown in Figure 1. In addition, the low-diffusivity APC system combined with a stable sub-3 μm APC column technology that supports higher flow rates and back pressures can also improve the resolution of 1K polystyrene standards and shorten the analysis time to the original 1/5.
Figure 1. Comparison of running time and resolution of polystyrene standards (Mp: 100K, 10K, and 1K) analyzed in conventional GPC system and ACQUITY APC system
The improved resolution using the APC system adds more identifiable chromatographic peaks to determine the molecular weight of 1K polystyrene standards. As shown in Figure 2, by using the value provided by the standard supplier or the molecular weight information determined by the external standard measurement value, more data points can thus be added to the calibration curve, which is based on The result of the sample calculated by this curve increases the credibility.
Figure 2. When using the ACQUITY APC system, more data points for polystyrene standards (100K, 10K, and 1K) are drawn on the calibration curve due to the improved resolution of 1K low molecular weight standards
In general, a series of standards need to be run to derive the data points used to generate the calibration curve. When using conventional GPC, it can take several hours to several days to equilibrate, formulate, and analyze each standard. Therefore, usually no calibration is performed and the analysis result is determined according to the original calibration curve. The ACQUITY APC system significantly speeds up the equilibrium due to its low retention volume, and the operating time is significantly shortened due to the use of smaller particles at higher flow rates. The shortened running time makes balancing and calibration operations easy to complete within an hour. Finally, thanks to the improved resolution, it may only be necessary to prepare and inject fewer standards to obtain a stable curve that can be used for calibration. When analyzing samples, the increased stability of the calibration operation makes the molecular weight determination of low molecular weight oligomers more reliable.
Figure 3 shows the analysis results of an epoxy resin sample versus calibration with polystyrene standards. The results show that using three ACQUITY APC XT 4.6 x 150 mm tandem columns can distinguish different oligomers in less than 5 minutes of running time.
Figure 3. A sample of epoxy resin dissolved in tetrahydrofuran was analyzed using three ACQUITY APC XT 4.6 x 150 mm series columns equipped with ACQUITY RI detectors. Low molecular weight oligomers (shown as peak-tip molecular weights) can be resolved in less than 5 minutes.
The feature of APC that can shorten the running time helps to monitor the reaction during the 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 polymers and their expected properties, which can facilitate the development of new polymers and accelerate the time to market.
in conclusion
Because the ultra-high performance polymer chromatography system has a lower diffusivity and can withstand a higher back pressure to allow the use of smaller hybrid particles, this system is significantly better than conventional GPC systems. By combining with the latest chromatographic column technology, the APC system also improves the resolution of low molecular weight oligomers compared to conventional GPC. APC's performance advantages include more reliable calibration results, which are essential for generating accurate measurements for polymer characterization. The improvement of the detection speed and resolution of low molecular weight polymers can achieve rapid and reliable characterization of polymers during the development process, thereby promoting close market tracking of new polymers.
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