APC Analysis of Polyamides in HFIP as an Alternative to High Temperature GPC

This application note demonstrates the Waters ACQUITY Advanced Polymer Chromatography (APC)™ System for polyamide GPC analysis using HFIP as a more sustainable alternative when compared to traditional separation techniques.

Benefits

• Alternative analytical approach to high temperature Gel Permeation Chromatography (GPC) applications for polymers soluble in HFIP
• A 15-minute per sample analysis time compared to traditional GPC of 45-minute run time
• Lower hazardous waste and organic solvent consumption Polyamide GPC option
• APC Isocratic Solvent Manager (ISM) system with years of consistently successful use with HFIP

Polyamides are an integral part of our daily lives, even if we do not readily see them performing their functions. Nylon 6,6 is a commodity polyamide named for its six-carbon monomer starting materials, adipic acid and hexamethyl diamine. Two examples of nylon products are car parts and electrical wiring, and these require heat and chemical resistance.¹ Due to the chemical resistance of polyamides, dissolving the polymer in an organic solvent for GPC analysis is challenging. Many manufacturers use high temperature GPC (>100 °C) to determine the molecular weight distribution of these nylon polymers, and the analysis often uses m-cresol or N-Methyl pyrrolidone as the mobile phase.^2,3

An alternative GPC analysis of difficult to dissolve polyamide Nylon 6,6 is using hexafluoro isopropanol (HFIP) as a mobile phase. HFIP is a viscous solvent at room temperature: 1.65 cP at 20 °C. Compared to the viscosity of tetrahydrofuran, 0.48 cP at 25 °C, HFIP as a mobile phase requires consideration for LC system operating parameters with a 45-minute analysis time with liters of very expensive solvent and hazardous waste.^4–7

HFIP as a mobile phase in a high-pressure system, such as an Advanced Polymer Chromatography (APC) System with a limit of 15,000 psi, adds new advantages to this routine analysis. This low dispersion system has a unique capability of being designed to maintain the high resolution of the Waters BEH small particle columns. One can imagine the challenges of putting a viscous mobile phase through a small particle column at high pressure and needing some extra consideration to overcome the physical limitations of HFIP. Many scientists find the UPLC advantages of efficient method development and faster analysis time as a strong motivator and have succeeded is using HFIP in their experiments with published results.⁸

In this APC analysis, an HFIP mobile phase with salt additive is used to run a size-based separation of various polyamides, and the instrument parameters are listed in the LC Conditions table. Samples are dissolved in HFIP at 1 mg/mL overnight in scintillation vials and transferred to instrument vials. 

Column Bank Assembly

Note the column bank selection of five columns. This is a creative solution to managing five columns in a space typically used for three 150 mm length columns. The column bank example shown in Figure 1 does not have a 150 mm 200 A column that covers a molecular weight range of 3K–70K Daltons. The column compartment has four eCord™ connections which enable traceability. The fifth column will not have a connection for the e-cord magnet, and therefore will not be captured in the Empower™ Software. This fifth column is helpful during screening for the molecular weight range of an unknown sample. Once the range is determined, the column bank can be optimized to four columns.

The picture displayed here is an example of a four-column bank. To fit two 75 mm length columns in the space of the 900 Å column shown (150 mm length), the Pre-column filter will have to be removed, and the connectors are shaped for compacting into the column oven. Once the column bank is optimized to four columns, the pre-column filter can be re-installed.

Data Management

Instrument control as well as data acquisition and processing were done by Empower 3 Chromatography Data System, FR5

Once the column bank and method are optimized to 15 minutes per injection, the polymethyl methacrylate (PMMA) calibration standards are run to establish a range of molecular weight range in Figure 2 and relative calibration curve (Figure 3).⁹

The highest molecular weight points are on a bend in the calibration curve, and this is due to the lower resolution of a shorter separation path and shorter column, yet this does not affect the quality of the unknown polyamide sample analysis due to their eluting in the linear range of the curve. A 5^th order curve was used to obtain the best R² value of 0.998453 (Figure 3).

The unknown polyamide samples were integrated and calculated using the relative calibration PMMA standard curve (Figure 4).

The results from the experiments described demonstrate that the APC has the capability to analyze polyamides dissolved in HFIP with a calibration curve having and R² value of 0.9985.

This analytical method delivers the results three times faster than traditional GPC with a 15-minute analysis per injection. The shorter run time uses less than 7 mL of HFIP solvent per injection, as compared to traditional GPC using greater than 22 mL per injection.¹⁰

Taken together, this method described here has the potential to increase laboratories sample throughput offering a more sustainable, cost-effective separation option and a viable alternative to high temperature GPC. 

1. Palmer, Robert J. (2001). "Polyamides, Plastics". Polyamides, Plastics. Encyclopedia Of Polymer Science and Technology (4th ed.). John Wiley & Sons, Inc. doi:10.1002/0471440264.pst251
2. Beginners Guide to Size Exclusion Chromatography.
3. Wudy, Katrin & Drummer, Dietmar. (2018). Aging Effects of Polyamide 12 in Selective Laser Sintering: Molecular Weight Distribution and Thermal Properties. Additive Manufacturing. 25. 10.1016/j.addma.2018.11.007.
4. Chen, An-Liu & Wei, Kuan-liang & Jeng, Ru-Jong & Lin, Jiang-Jen & Dai, Shenghong. (2010). Well-Defined Polyamide Synthesis from Diisocyanates and Diacids Involving Hindered Carbodiimide Intermediates. Macromolecules. 44. 10.1021/ma1022378.
5. APC System Chemical Compatibility Guide.
6. Wikipedia, https://en.wikipedia.org/wiki/Hexafluoro-2-propanol
7. Sigma Aldrich. 
8. Christian Wold, Elena Uliyanchenko (LCGC Europe), Ultra-High Size-Exclusion Separations of Engineering Plastics: Challenges and Opportunities,
9. Don Trinite, Waters Corp.,1360 N. Wood Dale Rd., Suite C, Wood Dale IL 60191.
10. Richard Mendelsohn, Jennifer Gough, Fast, High-Resolution Analysis of Polydimethylsiloxanes in Toluene with Advanced Polymer Chromatography Coupled to Refractive Index Detection. Waters Application Note 2022, 720007658.