Gel Permeation ChromatographyâFourier Transform IR Spectroscopy

Chapter 18

Gel Permeation Chromatography—Fourier Transform IR Spectroscopy To Characterize Ethylene-Based Polyolefin Copolymers 1

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Downloaded by NORTH CAROLINA STATE UNIV on October 29, 2012 | http://pubs.acs.org Publication Date: February 26, 1993 | doi: 10.1021/bk-1993-0521.ch018

R. P. Markovich , L. G. Hazlitt , and Linley Smith-Courtney 1

Polyolefins Research, Dow Chemical USA, Freeport, TX 77541 Division of Mathematical and Information Sciences, Sam Houston State University, Huntsville, TX 77340

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A Fourier transform infrared (FT-IR) spectrometer has been coupled with a high temperature gel permeation chromatography (GPC) instrument to provide a powerful tool for the characterization of ethylene based polyolefin copolymers. The combination of these devices provided the ability to simultaneously characterize the molecular weight (MW), the molecular weight distribution (MWD), and chemical composition (such as, the comonomer or branching concentration) as a function of molecular weight. Unique problems and solutions associated with development of GPC/FT-IR system for ethylene based copolymers will be presented. GPC is a powerful analytical tool frequently used for characterization of polymers. GPC separates the polymer molecules according to size. The size of any polymer molecule is dependent on its molecular weight, composition, branching and microstructure. With appropriate calibration between molecular weight and molecular size, GPC can provide information about the molecular weight and the molecular weight distribution of the whole polymer.(i) The molecular weight and molecular weight distribution data are used to correlate with polymer properties, especially polymer physical properties. IR spectroscopy is also a powerful tool frequently used for characterization of polymers. The IR spectra can provide quantitative and qualitative information about IR active functional groups in polymers and therefore can indirectly provide information about the microstructure of polymers (including information about branching and comonomer concentration). This information can be provided almost instantaneously by the use of an FT-IR because it is capable of obtaining a IR spectrum from 4000-700 cmin less than 1 second. Use of an FT-IR as a detector for GPC provides the ability to simultaneously characterize the molecular weight, the molecular weight distribution and to identify and quantify IR active functional groups. One thus has the ability to characterize the concentration of IR active functional groups as a function of molecular weight. GPC/FT-IR has been used to provide such information for different applications and different materials(2) including high density polyethylene (HDPE) and linear low density polyethylene (LLDPE) (3,4). In this paper, unique problems and solutions associated with development of GPC/FT-IR for ethylene based copolymers will be presented. Further, examples of the characterization information obtained via this method are presented. 1

0097-6156/93/0521-0270$06.00/0 © 1993 American Chemical Society

In Chromatography of Polymers; Provder, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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Experimental Details


Materials. Experiments were conducted with the following ethylene based copolymer types: ultra low density polyethylene (ULDPE), linear low density polyethylene (LLDPE), ethylene acrylic acid (EAA) and ethylene carbon monoxide (ECO) copolymers. Conditions for GPC/FT-IR Analysis. Figure 1 illustrates the key features of the GPC/FT-IR setup. A Waters 150C ALC/GPC was connected via a heated transfer line to a BioRad FTS-60 FT-IR. The Gel Permeation Chromatography operating conditions were: column - Polymer Laboratories Gel Mixed Bed 20 Micron; operating temperature = 140°C; mobile phase - 1,2,4-Trichlorobenzene; Flow rate -1.0 ml/min; sample concentration 0.25 g/50ml; injection size - 400 jil; Run time - 60 minutes; mass detectors - FT-IR and differential refractometer; molecular weight standards - a combination of the polystyrene standards ranging in M from 600 to 8,400,000. Column calibration for molecular weight estimation was achieved separately (column effluent directed to the differential refractive index mass detector) using the method described by Williams(5) for use with linear polyethylene systems. During the actual analysis the column effluent was directed exclusively to the spectrometer with appropriate adjustments for elution elution volume to compensate for detector offset. Heated transfer lines were maintained at 140°C. The FT-IR optical bench operating conditions were: sample cell - heated, stainless steel flow thru cell with zinc selenide windows; sample cell operating temperature - 140°C; cell pathlength - 1 mm; spectrum type - single beam; scan number - 50 (coadded); scan frequency - 20,000 Hz; Resolution - 8 cm ; aperture - open; detector - wide band, mercury cadmium telluride (MCT); data station - Model 3260; 40 spectra were saved per GPC analysis; up to eight samples could be analyzed unattended in a mn. The Bio-Rad FTS-60 data station was connected via RS232 cable to an IBM AT clone running GPC data acquisition software. Single beam spectra were taken in real time at 30 second intervals by commands sent from the PC to the FT-IR data station. The single beam spectra were stored on the FT-IR data station hard disk. Forty single beam spectrum were taken per sample. Up to eight samples were analyzed per mn. w

-1

TCB

COLUMNS DETECTOR

HEATED TRANSFER LINE

TEMP PC RHEOSTA1

PUMP

ill.

FT-IR

HEATED

BIO-RAD COMPUTER

^ E L L

I

K WASTE

Fig. 1. GPC/FT-IR instrumental set-up.

PLOTTERl & PRINTER

HHHEOSTAI

H

TEMPI


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CHROMATOGRAPHY OF POLYMERS


Data Processing and Analysis. After a GPC/FT-IR run was completed, data from the single beam spectrum stored on the FT-IR data station hard disk were transferred to the PC for further processing and analysis. For each single beam spectra, only the single beam spectral intensities for a selected set of wavelengths were transferred to the PC. This was done to provide sufficient information for the features of interest and at the same time minimize both memory requirements and data processing time. The wavelengths that were used are described below in Table 1 along with the features associated with them. Software programs on the PC processed the data and generated normalized molecular weight and chemical composition distribution curves. Some of the software was developed in-house specifically for this application. Table 1 Description of Absorbance Wavenumber (cm" ) Carbonyl 1706, 1714 Methylene Symmetrical 2846, 2854, 2862 Stretching Methylene Asymmetrical 2918, 2826, 2934 Stretching Methyls 2950, 2958, 2966 1

Background Intensity Baseline Method for Calculation of Solute Absorbance. It was necessary to develop a non-classical method to calculate solute absorbance. Determination of absorbance values via the classic method of using the ratio of a background or reference single beam spectra (radiant intensity transmitted through solvent and sample cell) from a sample single beam spectra (radiant intensity transmitted through solute, solvent and sample cell) was not possible because of the following factors: low concentrations of sample, baseline drift and significant loss of energy due to the combined effect of the solvent, the cell pathlength, and the zinc selenide IR cell windows. The unique feature of the method was the use of background intensity baseline functions, in place of a discrete background single beam spectra, to determine background (or reference) intensities. The background intensity baseline is a line equation relating background intensity to elution volume. A unique background intensity baseline was determined for each of the wavenumbeis used during an analysis. The line equation was determined by calculating the least-squares best line fit between the first three and last three single beam intensity values obtained at that wavenumber during a GPC/FT-IR analysis. An example of background intensity baseline is illustrated in Figure 2 for 2926 cm . The absorbance of the solute at any one of the forty sample points for a given wavenumber was calculated using the following derivation of Beer's law: -1

Aij=logf where i is the wavenumber, j is the elution volume (or sample point), B is the background intensity at the given wavenumber and elution volume which was calculated from the background intensity baseline for that wavenumber and S is the single beam intensity value at the corresponding wavenumber and elution volume. In Figure 2, the corresponding background intensity (B) and sample single beam intensity (S) values which would be used to calculate the solute absorbance at wavenumber 2926 cm-1 and elution volume 21.25 cc are highlighted for illustration.


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2.6-r

ELUTION

VOLUME

(cc)

Fig. 2. Background intensity baseline method. Mass Detection and Determination of Weight Percent Versus Elution Volume. The FT-IR was used as the mass detector. All eleven of the wavenumbers listed in Table 1 were used for mass detection. Since spectra were taken at 40 equally spaced times during the GPC analysis; we were able to determine the weight percent of polymer at each these times. The weight % of polymer was determined using the following equation: Weight % -100x(Ai/ATotal) where Ai is the sum of the absorbance values for the eleven wavenumbers for a specific spectra and Ajotal is sum of the absorbance values for the eleven wavenumbers for all 40 of the spectra obtained during a GPC analysis. Since the spectra were taken at known times during the GPC analysis, the weight % data could be reported as a function of time, elution volume or molecular weight. Quantitative Determination of Methyls and Olefin Copolymer Concentration. Standards ranging in methyl concentrations from 1 to 100 methyls per 1000C carbons (M/1000C) were used to define a calibration curve. The standards were NBS 1483 (0.97 M/1000C), NBS 1482 (2.46 M/1000C), Dotriacontane (62.5 M/1000C), and Eicosane (100.0 M/1000Q. The calibration curve related M/1000C to the ratio of methyl absorbance divided by total absorbance. The methyls absorbance was defined to be the sum of the absorbances at 2950, 2958 and 2966 cm" (6,7). The total absorbance was the sum of the absorbances for all of the wavenumbers in Table 1. The resulting calibration plot is shown in Figure 3. Copolymer concentrations for LLDPE and ULDPE resins were assumed to be proportional to the concentration of methyl groups. This assumption was made because in most cases molecular weight corrections for chain ends were small (see discussion). 1


274

CHROMATOGRAPHY O F POLYMERS

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T

100.00 80.00


o §

60.00 o ^ 40.00 ag 20.00 0.00 -20.00 0.07

0.09

0.11

0.13

0.15

0.17

RATIO

Fig. 3. GPC/FT-IR calibration plot of methyls per thousand carbons versus absorbance ratio (the absorbance ratio is the ratio of the methyl absorbance divided by the total absorbance). The plot contains actual and calculated values for the standards used in the calibration. Quantitative Determination of Polar Comonomer Concentration. The polar comonomers in ethylene acrylic acid and ethylene carbon monoxide copolymers each possess a carbonyl functionality. Therefore, the carbonyl absorbance was used to determine the comonomer concentrations. The comonomer concentration was reported on a relative basis as the ratio of carbonyl absorbance divided by the total absorbance. The carbonyl absorbance was the sum of the absorbances at 1706 and 1714 cm~l. The total absorbance was the sum of the absorbances for all of the wavenumbers in Table 1. Since spectra were taken at 40 equally spaced times during the GPC analysis; we were able to determine the polar comonomer concentration ratio at each of these times. Results and Discussion Figures 4, 5 and 6 illustrate the type of data which was obtained via GPC/FT-IR. Figures 4 and 5 are graphs of the molecular weight distribution (weight% versus Log molecular weight) and the methyls/1000C carbons distribution (methyls/lOOOC carbons versus log molecular weight) for two ULDPE resins. All of the molecular weight determinations were based on a single elution volume to molecular weight relationship (intended for linear polyethylene systems) as described previously. No attempt was made to correct for structural deviations from linear polyethylene. Therefore, the molecular weight data reported herein are meant for relative comparison between similar copolymers only. The reported branching data is uncorrected for molecular weight. Attempts to correct for chain ends using the molecular weight resulted in over correction (negative methyls/lOOOC) at low molecular weights. Alternative corrections using only one methyl termination per chain (assuming vinyls resulted from elimination reactions) gave more reasonable results. However, since the correction was only important for a very small percentage of the molecular weight distribution (< 10%) and since other effects, such as column resolution could not be ruled out, no correction was included. The methyls/1000C was plotted over a molecular weight range which corresponds to 97% of the polymer. The methyls/lOOOC data for points corresponding to molecular weights outside the range shown was subject to significant error because of the


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extremely low sample concentrations. Thefigures4 and 5 indicate that the two resins have similar molecular weight distributions but very different methyls/lOOOC distributions.

LOG M

Fig. 4. GPC/FT-IR chromatogram for an ULDPE resin. Plot of weight % and methyls /1000C versus log molecular weight.

LOG M

Fig. 5. GPC/FT-IR chromatogram for an ULDPE resin. Plots of weight % and methyls /1000C carbons versus log molecular weight. Figure 6 is a graph of the molecular weight distribution and the acrylic acid comonomer distribution for an EAA resin. The acrylic acid distribution is represented as the ratio of the carbonyl absorbance divided by the total absorbance. The CO absorbance/total absorbance was plotted over a molecular weight range which corresponds to 97% of the polymer. Just as for the methyls/1000C data infigures4 and 5, the carbonyl ratio data for points outside this molecular weights range were not shown because they were subject to significant error due to the extremely low sample concentrations.



276

CHROMATOGRAPHY OF POLYMERS

LOG M

Fig. 6. GPC/FT-IR chromatogram for an EAA resin. Plot of weight % versus log molecular weight and of the carbonyl absorbance ratio (the ratio of the carbonyl absorbance divided by the total absorbance) versus log molecular weight. Conclusions The results demonstrate that GPC/FT-IR can be used to characterize ethylene based copolymers. GPC/FT-IR provides three key advantages: 1. One is able to simultaneously characterize molecular weight, molecular weight distribution, and comonomer concentrations (as a function of molecular weight). 2. The technique is much faster and simpler than alternative techniques; for example, preparative GPC fractionation and sample collection followed by infrared analysis and solid state IR versus solution IR. 3. The method is adaptable to many copolymer systems (or any other IR active functional groups). The data obtained from the analyses are solid evidence for claims concerning polymer structure, especially where molecular weight is a determining factor. Literature Cited 1. Moore, J. C. J. Polym. Sci., Part A2, 1964, 835. 2. Brehon, F. Spectra 2000 (Deux Mille), 1990, 145, 48. 3. Housaki, T.; Satoh, K.; Nishikida, K.; Morimoto, M. Makromol. Chem. Rapid Commun., 1988, 9, 525. 4. Nishikida, K.; Housaki, T.; Morimoto, M.; Kinoshita, T. J. Chromatogr., 1990, 517, 209. 5. Williams, T.; Ward, I. J. Polym. Sci., Polym. Letters, 1968,Vol.6, 621. 6. Bellamy, L. J. The Infrared Spectra of Complex Molecules, Wiley, New York 1975. 7. Bellamy, L. J. Advances in Infrared Group Frequencies, Barnes and Nobel, New York 1968. RECEIVED August 27, 1992


Gel Permeation ChromatographyâFourier Transform IR Spectroscopy

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