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GPC Analysis of Polymers with an On-Line Viscometer Detector

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GPC Analysis of Polymers with an On-Line Viscometer Detector GPCAnalysisof Polymers with an On-LineViscometerDetector JurisL. Ekmanis- Waters Chromatography Division, Millipore Corporation I I I / I I I I Waters DMsionof MILLIPORE FirstPresentedat the 1989 InternationalGPC Symposium,October 1-4, 1989 wf " : GPC Analysis of Polymers with an On-Line Viscometer Detector Juris L. Ekmanis Waters Chromatography Division Millipore Corporation 34 Maple Street Milford, Massachusetts 01757 ABSTRACT An on-line capillary viscometer detector for GPC, when used in conjunc- tion with a concentration detector (e.g. refractive index) and appropriate software, can be used to generate calculations of absolute molecular weight averages and intrinsic viscosity and provide data on the extent of any long chain branching in a polymer sample• This paper describes the chromatographic performance of a single-capillary viscometer detector that has been built into the Waters 150C high temperature GPC system• The complete viscometer detector system consists of hardware (capillary, transducer, electronics, etc.) incorporated within the new 150CV instrument along with data reduction software that is in the final stages of development (for Waters 845 and 860 data systems). This paper describes the performance characteristics of the viscometer detector and demonstrates the use of this 150CV integrated GPC/visco- meter system for the analysis of polymers in tetrahydrofuran (THF) at 35oC (polystyrene, polyvinyl chloride, polycarbonate) as well as in 1,2,4-tri- chlorobenzene (TCB) at 135oC (polyethylene and polypropylene). High temperature operation has been optimized by locating the viscometer components totally within the 150CV system. This eliminates the need for additional temperature-controlled zones that would be required if the visco- meter were located in an external module• Introduction The technique of Gel Permeation Chromatography (GPC) was developed by John Moore at the Dow Chemical Co. (Freeport, TX) in the early 1960's 1 and the proprie- tary column technology developed by Moore was licensed to Waters Associates by Dow. Waters subsequently commercialized the GPC technique by introducing the first GPC instrument (GPC-100) to the market in 1964. The GPC-100 was a large, floor-standing system (6 feet tall) that could be used to determine molecular weight distributions of polymers in 3 to 4 hours, depending on column selection. This was a -1- i. m'ajor, advance over the classical fractionation procedures that typically required many days, even weeks, to fully characterize the molecular weight distribution of a polymer sample before the advent of GPC. In 1967, the GPC-100 was replaced by the GPC-200 which was externally similar to its predecessor but included an updated pumping system and an improved RI detector (better temperature control and optics). In 1979, the GPC-200 was replaced by the Waters 150C high temperature GPC system. The 150C is a fully integrated benchtop instrument which is used with high efficiency, microparticulate GPC columns, e.g. Waters i_StyragelTM and Ultrastyra- gel® columns, to generate molecular weight distributions of polymers in approxi- mately 30 to 60 min. with much higher resolution than was possible with the larger particle size columns (Waters Styragel®, 37 - 75 p.m) that were used with the GPC-200. The 150C system includes a refractive index (RI) detector that monitors the differ- ence in RI between the sample side of the RI cell through which the column effluent is flowing and the reference side of the RI cell which is filled with pure solvent. The refractometer is a concentration detector that can be used to calculate true mole- cular weight averages when an absolute calibration curve is available. An absolute calibration curve for a GPC system can be generated by using narrow distribution polymer standards, if available. For organic solvents, only well characterized, narrow distribution polystyrene standards are readily available for calibration purposes. For polymers other than polystyrene, molecular weight averages based on polystyrene standards are often used for comparison among samples. Alternately, computerized techniques have been developed to generate an absolute GPC calibration curve from a single well characterized, broad distribution sample of the polymer of interest. The broad standard technique has its limits and must be used with care to generate good data. In 1989, Waters added an on-line, single-capillary viscometer detector to the 150C. The new, dual-detector (viscometer, RI) 150CV system affords the following benefits. 1) Since the viscometer detector responds relatively more to high molecular weight materials and relatively less to low molecular weight fractions than does a concentration detector (e.g. RI), the viscometer will magnify small dif- ferences in the high molecular weight regions of the molecular weight distribu- tions of polymers. This is important since small differences in this region often have a significant influence on the physical properties of a polymer. 2) A single-capillary viscometer is also a very sensitive flow meter and, as such, the viscometer is an excellent system diagnostic tool. Flow control in any GPC system is a critical element in generating accurate molecular weight averages and molecular weight distributions. With this viscometer, the opera- tor can be aware of any flow discrepancy as it occurs so that appropriate corrective action can immediately be taken in order to restore the system to proper operation. Conversely, examination of the viscometer baseline for a series of overnight runs can be used to confirm that the GPC system was operating properly and that the chromatography is worthy of subsequent data reduction to calculate molecular parameters. 3) When used with appropriate data reduction software, which is in the final stages of development for the Waters 845 and 860 DEC-based data systems, the 150CV viscometer and RI detector signals can be used to calculate: • absolute molecular weight averages (via Universal Calibration 2) • intrinsic viscosity (T1)of the whole polymer • branching information across the molecular weight distribution via the branching parameter (g') g' = 1] branched/ 1] linear This paper describes the design and performance characteristics of the single- capillary viscometer detector that has been built into the existing Waters 150C GPC system and demonstrates the use of the new 150CV system for the analysis of poly- mers in tetrahydrofuran (THF) at 35oC (polystyrene, polyvinyl chloride, polycarbon- ate) as well as in 1,2,4-trichlorobenzene (TCB) at 135oC (polyethylene and polypropylene). -3- Description of the 150CV and the Viscometer Detector The 150CV GPC/Viscometer system is shown in Figure 1 along with the Waters 845 data station that is used for viscometrycalculations. The viscometer detector has been integrated withinthe chromatographicsystem and, except for the 150CV nameplate, the only external evidence that a viscometer detector is included in this unit is the viscometer electronics control panel in the lower left corner on the front of the instrument. The block diagram (Figure 2) of the 150CV system indicates the key elements that constitute the viscometer detector. The internal solvent reservoir (rarely used) has been eliminated and solvent is supplied from an external source to the main pumping system (pump compartment) and the total system pressure transducer (PT) in standard fashion. In a regular 150C, the outlet of this pressure transducer is routed to the injector compartment. However, the viscometer detector is sensitive enough to detect the minor flow fluctuations due to the piston cross-over in a properly operating dual-piston pump. In order to successfully use the detector as a visco- meter, we have included a pulse dampening system, or baseline optimization box (BOB), to reduce the fluctuations by a factor of more than 100. Since there is not enough space to locate this dampening system in the injector compartment, the dampeners are located in the pump compartment on the drawer just in front of the main pumping system. The baseline optimization box consists of a series of eight dampening and eight restrictive elements in alternating order and enclosed in a dual-wall container so as to minimize the effects of any temperature variations. The flow from the system pressure transducer is routed back into the pump compartment, through the BOB, and then to the injector. From the injector, the solvent flows into the columns that are located in the column compartment. In the 150CV, the outlet of the last column, which is connected to the inlet of the RI detector in a regular 150C system, is instead connected first to the viscometer capillary and then to the inlet of the RI detector. The viscometer capil- lary is a 6 inch length of stainless steel tubing (1/16" O.D., 0.014" I.D.). The columns and viscometer capillary are both enclosed in a dual-wall container to isolate them from any temperature fluctuations in the column compartment. From tees at either end of the capillary, connecting tubing (stainless steel, 1/16" O.D., 0.020" I.D.) is routed into the pump compartment and connected to each side of a variable -4- ,. r_luctance differential pressure transducer (Visc PT). Under normal operation, the solvent in these connectors is stationary. By opening outlet fittings at each side of the transducer, these lines can be flushed as needed to remove air or to change solvents. This viscometer transducer is also enclosed in a dual-wall container to insulate it from the effects of any temperature variations and the complete transducer assembly is located on the top of the dampening system (BOB). The outlet of the RI detector is directed into a modified internal waste container in the pump compartment, just below the column compartment, and is continually drained to external waste• The output from the viscometer and RI detectors is displayed on the electronics control panel for each detector. The viscometer electronics panel is located in the lower left corner of the front of the 150CV in place of the blank panel (150C) that had been reserved for a future detector.
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