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Size-Exclusion ChromatographyFourier Transform IR Spectrometry Using a Solvent-Evaporative Interface P. C. C h e u n g , S. T. Balke, a n d T. C. Schunk 1
1
2
Department of Chemical Engineering and Applied Chemistry, University o f T o r o n t o , T o r o n t o , O n t a r i o M5S 1A4, C a n a d a A n a l y t i c a l T e c h n o l o g y D i v i s i o n , Research L a b o r a t o r i e s , E a s t m a n K o d a k C o m p a n y , Rochester, N Y 1 4 6 5 0 - 2 1 3 6 1
2
A solvent-evaporative interface is used to deposit each fraction obtained from size-exclusion chromatography (SEC) as a dry polymer film on an IR transparent disc for subsequent IR analysis. Spectra without solvent interference bands result. However, in addition to removing the solvent, the interface must provide polymer films that yield undistorted spectra. Christiansen distortion is particularly troublesome because it interferes with quantitative interpretation in the affected spectrum. Undistorted spectra were invariably obtained from continuous films and sometimes obtained from discontinuousfilms(separate particles). Carbon-coated KCl discs and either bare or carbon-coated germanium discs provided morphologies with good spectra. Polymer deposition on bare KCl discs often provided unacceptable morphologies. Surface-wetting properties of the substrate appear to dominate deposit morphology.
A
F O U R I E R TRANSFORM INFRARED (FTIR) SPECTROMETER is potentially
a v e r y p o w e r f u l detector for size-exclusion chromatography (SEC). F o r polyolefins i n particular, S E C - F T I R
may p r o v i d e for each molecular
size present i n the sample, measurements of such properties as degree of unsaturation, b r a n c h i n g frequency, and c h e m i c a l composition of copolymers. A l t h o u g h F T I R spectrometers have already b e e n used as S E C detectors b y e m p l o y i n g m i c r o - s i z e d flow cells (J), this m e t h o d is c o n straining: V e r y few spectral windows are available i n c o m m o n l y used m o b i l e phases, and decreasing path l e n g t h opens w i n d o w s but s i m u l 0065-2393/95/0247-0265$12.00/0 © 1995 American Chemical Society
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CHROMATOGRAPHIC CHARACTERIZATION OF POLYMERS
taneously reduces polymer signal. A n alternative to the use of a flow cell is removal of the solvent from S E C fractions followed by IR analysis of each of the d r i e d films. Preparative S E C followed b y collection of fractions and mobile-phase evaporation is impractically slow and costly for routine use. A n on-line evaporative interface can accomplish solvent removal during the S E C run. In this work, we employ an interface design based upon the one developed b y D e k m e z i a n et al. (2), for h i g h - t e m perature S E C . In recent assessments of this interface and a comparison of it w i t h a commercially available room-temperature evaporation i n terface using the Gagel and Biemann design (3-5), it was found, for b o t h devices, that the morphology of the deposited film critically affected the resulting I R spectra. T h e t e r m " f i l m m o r p h o l o g y " here refers to the structure of the polymer film on the scale of mid-IR wavelengths. F i l m morphology, the focus of this chapter, is currently the primary obstacle to the use of either interface for quantitative IR detection.
Theory Considerations for H i g h - T e m p e r a t u r e S E C Analysis o f P o l y -
olefins. In the use of a solvent-evaporative interface w i t h h i g h - t e m perature S E C , three primary complications are involved: the l o w volatility of the 1,2,4-trichlorobenzene (TCB) mobile phase, differing p o l y mer solubilities i n T C B , and the semicrystalline nature of polyolefins. T h e b o i l i n g point of T C B is 2 1 3 °C at 101.3 k P a (1 atm), and the latent heat of vaporization is 48 k j / m o l . Thus to reduce operating temperature, reduced pressure is utilized to assist evaporation. Solubilities (and hence tendency to phase-separate i n a sprayed droplet) of polyolefins depend u p o n molecular weight and temperature. Whereas polypropylene is generally less soluble than polyethylene at elevated temperatures, other polymers (e.g., polystyrene) are soluble even at room temperature. P o l y m e r blends and copolymers w o u l d be expected to display film morphologies reflecting different rates of phase separation d u r i n g d r y i n g . Crystallinity w i l l also affect film morphology and varies w i t h p o l y m e r type as w e l l as w i t h degree of branching. F i l m Properties Affecting I R Spectra.
In addition to film t h i c k -
ness, influential properties include uniformity, wedging, dispersion characteristics, molecular interactions, and degree of crystal orientation (6). F i l m nonuniformity can cause offsets i n the absorption-band intensities of the spectrum. A wedge or sloping thickness sample profile can lead to photometric error (7). T h e dispersion effect, also k n o w n as the Christiansen effect, is caused b y the scattering of radiation. T h e extent of scattering depends on the particle size and refractive index differences (8-10). Christiansen scattering may not be significant w h e n there are
20.
CHEUNG ET AL.
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SEC-FTIR Spectrometry
particulates with diameters significantly smaller than the IR wavelength. Scattering can cause both baseline curvature and derivative-like, absorption-band shape changes. M o l e c u l a r interactions can cause shifts of absorption bands. Polarization effects may lead to changes in relative intensities i n a spectrum due to crystal orientation. Film
Morphology
Effects
Observed
in Previous
Interface
Investigations. As mentioned earlier i n both previous studies (3, 4) film morphology effects sometimes strongly affected the IR spectra. T h e presence of strong derivative-like bands (Christiansen effect) as w e l l as highly sloping baselines frequently occurred. T h e latter problem was considered m u c h less serious than the former because the absorption bands involved were often quite narrow (baselines c o u l d be assumed linear under the peak). In contrast when the Christiansen effect occurred, no useful quantitative information could be obtained from the part of the absorption spectrum affected. E l e c t r o n and optical microscopy r e vealed that morphologies displaying multiple isolated particles of 1-15 /urn provided particularly distorted spectra. In one demonstration of the importance of film morphology, exposing a film of polystyrene to a solvent vapor completely removed the sloping baseline b y forming a continuous film (3, 4). Issues. Interpretation of the Christiansen distorted portion of spectra is not generally practical. Generation of film morphologies that do not adversely affect the IR spectra is the requirement. W i t h the l i m ited goal of accomplishing reliable quantitative analysis using a solventevaporative interface, it is necessary to further elucidate several interrelationships: the effect of film morphology on the IR spectra (sufficient to disclose those morphological properties causing spectral distortions), the effect of polymer type on film morphology (sufficient to reveal the material's contribution to the issue), and the effect of operating c o n d i tions on film morphology (sufficient to permit acceptable morphologies to be reliably obtained). Complexities include interactions among polymer type and operating conditions. T h e work progressed i n two phases: In Phase I, the objective was to determine the causes for film morphologies that resulted i n the C h r i s tiansen distortion; i n Phase II, the objective was to use the knowledge gained from Phase I to devise methods of experimentally eliminating these causes.
Experimental
Details
The Dekmezian solvent-evaporative interface design consists of an ultrasonic nozzle installed in a heated vacuum chamber. The mobile phase is spray-dried over discs that are sequentially placed below the nozzle in a
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CHROMATOGRAPHIC CHARACTERIZATION OF POLYMERS
programmed manner. At the end of the run, a wheel containing the discs is removed for IR analyses. Our modifications to this design included a higher frequency ultrasonic nozzle (120 versus 60 k H z ) , a cooling jacket on the nozzle, and placement of a heater between the nozzle and disc (Phase I) or below the wheel containing discs (Phase II). Figure 1 shows a schematic of the interface i n Phase II. Further details may be found elsewhere (3). The interface was at the outlet of a model 150C high-temperature size-exclusion chromatograph (SEC) (Waters Associates, Milford, M A ) . In both Phases I and II of the work, the S E C was equipped with three PLgel (Polymer Laboratories, Amherst, ΜΑ) 10-μπι mixed-bed analytical columns. T C B was the mobile phase. Injection volume was 100 #L of each 0.2 wt% polymer sample dissolved in T C B with 0.2 wt% butylated hydroxytoluene as a stabilizer. Flow rates were generally 0.5 m L / m i n . A Mattson Galaxy 6020 FTIR spectrometer and a Mattson Quantum infrared micro scope (Madison, WI) equipped with mercury cadmium telluride (MCT) de tectors as well as a Nikon S M Z - 2 T optical microscope was used to analyze the films. The IR spectra shown in this study were obtained by averaging 128 scans at 4-em~* resolution under transmission mode and were not base line corrected. Polypropylene (PP 180K, American Polymer Standards, Mentor, O H ) , polystyrene (NBS 706), and linear (high-density) polyethylene (NBS 1475, NIST, Washington, D C ) were analyzed individually and com bined pairwise in equal weights. In Phase I, the columns were bypassed for some of the runs. The vacuum oven was at 153-160 °C and 1 0 - 3 0 kPa. K B r discs (13 mm in diameter and
HEATED
TRANSFER
LINE ASSEMBLY
SEC
ELUENT
ULTRASONIC NOZZLE
•
Κ
SAMPLING W H E E L WITH DISCS
RING HEATER
TO VACUUM PUMP
