Measuring Compositional Heterogeneity in Polyolefins Using Size

Chapter 13

Measuring Compositional Heterogeneity in Polyolefins Using Size-Exclusion Chromatography/ Fourier Transform Infrared Spectroscopy Downloaded by UNIV OF GUELPH LIBRARY on September 7, 2012 | http://pubs.acs.org Publication Date: November 4, 2004 | doi: 10.1021/bk-2005-0893.ch013

Paul J. DesLauriers Chevron Phillips Chemical Company LP, Phillips R&D Center, Highways 60 and 123, Bartlesville, O K 74004

This chapter provides an introductory overview for using size exclusion chromatography (SEC) and on-line, Fourier transform infrared spectroscopy (FTIR) to characterize comonomer content across the molecular weight distribution in polyolefins. The basic spectral aspects of the method, considerations for FTIR as an on-line detector, and the use and limits of on-line SEC-FTIR to detect polymer compositional heterogeneity are addressed. Although on-line SECFTIR is a rapid and powerful tool to detect trends resulting from catalyst and process changes, the method's ability to discern a sample's compositional heterogeneity is dependent upon the extent to which the molecular weight distributions of its components overlap. In some cases, SEC-FTIR cannot fully elucidate a sample's compositional heterogeneity. Other techniques such as analytical temperature rising elutionfractionation(ATREF) must then be used.

210

© 2005 American Chemical Society

In Multiple Detection in Size-Exclusion Chromatography; Striegel, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

Downloaded by UNIV OF GUELPH LIBRARY on September 7, 2012 | http://pubs.acs.org Publication Date: November 4, 2004 | doi: 10.1021/bk-2005-0893.ch013

211 To say that most commercial LLDPE and HOPE polyethylene resins produced today are compositionally heterogeneous is a gross understatement. Primary structural variations in these polymers include chemical structure (end group chemistry and comonomer content), heterogeneity in the molecular weight distributions arising from multi-site catalysts or dual catalyst systems, and topological variations (short and long chain branching architectures). These variations in turn lead to secondary and tertiary structural (morphological) variations that arise during the extrusion and/or manufacturing process of the final product. Of course, the multitude of additive formulations and filler types blended with these polymers further augment the heterogeneities of the final products. As polyolefins become more complex in their composition, characterization of the polymer's molecular weight and polydispersity by size exclusion chromatography (SEC) using one detector system is no longer sufficient to define the architecture of the resin or the subsequent structure property relationships that shed light on product performance, it is not surprising that considerable effort has been made in recent years to develop analytical techniques that characterize the heterogeneity of polyolefin resins. These techniques include a number of hyphenated methods, many of which are described by other contributors to this text. In this chapter, the focus is on how comonomer content is characterized across the molecular weight distribution in polyolefins using SEC and on-line Fourier transform infrared spectroscopy (FTIR). Although all of the variations mentioned above play important roles in determining the resin performance properties, molecular weight, comonomer content and its distribution over the polymer's molecular weight are critical to understanding the physical properties of the resin. Molecular weight and its distribution give the polymer its mechanical properties and will influence resin processability. Comonomer content and distribution, at both the inter-molecular and intra-molecular levels, will result in pronounced differences in density as well as in resin performance properties such as thermoxidative stability, stress crack resistance, impact strength, hot tack, heat seal and hexane extractables, to name just a few (1-5). On-line SEC-FTIR provides a convenient way to gain both MW data as well as comonomer distribution across the MWD. In the following sections, the basic spectral aspects, considerations for FTIR as an on-line detector, and use and limits of this method to detect polymer structural heterogeneity are briefly reviewed. Experimental details for much of the work presented in this chapter can be found in Reference 11.

IR Absorption Bands and Polyolefin Topology Three absorption regions in polyolefin FTIR spectra are used to glean information about the type and level of short chain branching (SCB) in these resins: the C-H deformation or bending region (1376-1384 cm" ), the methyl and 1


212 1

1

methylene rocking regions (1200 to 800 cm" and 770 - 720 cm' , respectively) and the C-H stretching bands found between 2980 and 2950 cm" . All threeabsorption regions and their relative molar absorbtivities are illustrated in Figure 1. 1


C-H stretch

C-H bend

3000

-(CH )-rocking 2

2000

1000

cm

1

Figure 1. ATR spectrum for an ethylene 1-olefin copolymerfilmsample showing the three sets of IR absorption bands used to detect methyl (SCB) content. Historically, peaks arising from the C-H bending modes have been used to determine SCB (6,7) in solid-state characterization of polyolefin samples. These bands have medium absorbencies and can be easily detected using either double beam or Fourier transform infrared spectrophotometers and deuterated triglycine sulfate (DTGS) detectors (7). However, both peak position and the measured absorbance for C-H deformations are affected by going to methyl, ethyl, and longer branch lengths (6). The absorption contributions from SCB and chain end methyls, as well as, the change in molar absorptivety, make it difficult to acquire an accurate measurement of the SCB level using this spectral region since the measured value is a weighted average of the various methyl types. The rocking modes have also been used to determine the type and amount of SCB in polyolefin film samples (6). As in the C-H bending modes, both peak position and molar absorptivity are affected by chain length. However, since the absorbencies of various chain lengths occur in different spectral regions, the use of these vibrational modes has proved useful for identifying specific chain types. For example, Blitz et. al. report (6) that various branch types were qualitatively and quantitatively characterized in LLDPE copolymers, LLDPE terpolymers and LDPE resins using these absorption bands. Methyl branches were characterized by an absorbance at 935 cm" , ethyl branches at 770 cm" , butyl branches at 893 cm" , isobutyl branches at 920 cm" , and hexyl branches at 888 cm" . Although useful, the molar absorptivity of these bands are very low as illustrated 1

1

1

1


1

213 in Figure 1 and typically requires a good spectrometer coupled with a mercury cadmium telluride (MCT) detector in order to properly quantify these peaks. The C-H stretching bands, (3000 to 2800 cm" ) for simple n-alkanes and polyolefins are the strongest of the three absorptions. This spectral region is composed of a complex over-lapping band system due to vibrations from both methyl and methylene moieties. Four distinct absorption bands are typically observed in simple n-alkanes (8-11). Absorption bands at 2855 and 2928 cm" are due to symmetrical (CH v ) and asymmetrical (CH v ) stretching of the carbon hydrogen bond in methylene groups, respectively. The other two bands arise from the symmetrical (CH v ) at 2874 cm" and asymmetrical (CH v ) vibrations at 2957 cm" . In addition to these four fundamental bands, a possible C H combination band (11) is detected as a broad shoulder on the side of the 2928 cm" methylene peak (-2900 cm' ). The majority of ethylene 1-olefin copolymers contain isolated short chain branches. That is, the branches off the main chain backbone are not adjacent to each other. When methyl moieties appear as chain ends to isolated short chain branches of four carbons or greater, spectral profiles similar to those found for straight-chain hydrocarbons are observed (i.e., at least five main peaks) (11). However for the branched resins, slight differences in peak positions for the methyl C-H stretch for methyl groups attached to side chains with one methylene unit or less (i.e., methyl and ethyl branches) and a broadening of the 2930 to 2980 cm" region occur as well. The carbon hydrogen stretching vibration in the methyne moiety is very weak and is reported to occur at -2890 cm" (10). Although, this absorption is seldom identifiable due to its low absorptivity and the presence of the C H combination bands, it's contribution to the noted broadening effect for short chain branched samples cannot be ruled out. Lastly, in the case of 1-olefin homopolymer the short chain branches can be considered adjacent to each other. The presence of adjacent SCB branches in particular 1-olefin homopolymers further complicates the FTIR spectra of these samples compared to samples containing only isolated SCB. For example, although the spectrum of poly(l-hexene) is very similar to that observed for nalkanes and ethylene 1-olefin copolymers, the spectra for polypropylene and , poly(l-butene) both show the presence of more than one absorption peak in the asymmetric methyl stretching region of the spectrum (ca. 2960 cm' ), as well as shifts in other peak absorptions (Figure 2). Although peak position of the methyl group can vary in some ethylene 1olefin copolymers and poly (1-olefin) homopolymers, the absoptivities for these moieties are unaffected by chain length (11). This observation and the fact that the C-H stretch in these polymers is the strongest absorption, makes this absorption region ideal for monitoring concentration in on-line SEC-FTIR analysis. 1

1

2

s

2

a

1


3

s

3

1

2

1

1

1

1

2

1


a

214


0.018

3000

2950

2900

2850

2800

1

Wavenumbers (cm )

Figure 2. FTIR spectra ofpoly(l-hexene), polypropylene and poly (I-butène). Spectra acquired by SEC-FTIR at the peak max of the eluting sample. t

FTIR as an on-line SEC concentration detector The extent to which the above structural information can be acquired by SEC-FTIR is dependent on the chosen methodology. Two types of SEC-FTIR methods are typically used. In one method, sample eluent from the SEC column is deposited on a rotating germanium disk (12). The solid deposit is subsequently analyzed off-line by FTIR for branching content using absorption bands associated with the bending and/or rocking modes (13). However, deposition heterogeneity, loss of volatile components and other factors can prove problematic (14). In on-line FTIR methods (11,15-18), branching levels in the SEC eluent are measure in a heated flow cell (Figure 3). In this method the absorption characteristics of the solvent dictates which spectral region is accessible for analysis of the polymer itself. Although this latter consideration has been cited as a limiting factor in the use of FTIR detectors with flow cells (14), this methodology remains far more sensitive and flexible than those methods which employ single-beam photometers and detectors fitted with fixedwavelength interference filters.


215 Temperature control unit

Heated transfer line flow

FTIR

Column

spectrometer

bank


Solvent waste

flow

Pump

Solvent

HT-SEC Unit

Figure 3. Schematic for a typical SEC-FTIR unit and cell usedfor on-line SECFTIR characterization (11). This is a standard arrangement for high temperature SEC with the obvious addition of the external transfer line, heated flow cell, and narrow band, mercury cadmium telluride (MCT) FTIR detector.

Developments in spectrographic software (19) and the availability of a dependable high-temperature flow cell (20), has helped furthered the application of FTÏR as an on-line detector for polyolefins. The schematic shown in Figure 4 illustrates a high temperature flow cell well suited for use in this method. Generally speaking, the cell consists of IR transparent windows separated by a spacer. The IR beam passes perpendicular to the effluent flow as the sample enters and exits the heated flow cell. For on-line SEC-FTIR characterization of semi crystalline polyolefins, 1,2,4-trichlorobenzene (TCB) is commonly used as the solvent. However, other high boiling point solvents with the appropriate spectral window and thermal stabilities can be used. Varieties of optical materials are available for use as the IR cell windows under these conditions and include quartz, KBr, ZnSe, and CaF. The cell volumes typically vary between 25 to 70 μ ι and the optimum path length for high temperature work was found to be 1.6 mm (21). However, a 1 mm path length seems to be adequate for most work (11,15,16,18).



216

Figure 4. Exploded view of heatedflowcell (Polymer Laboratories, Ltd.).

Chromatograms are generated from ratio-recorded transmittance spectra (22). As shown in Figure 5, base line spectra is first acquired over the 3000-2700 cm" spectral region. The appropriate absorption band measurement is then acquired and compared against the acquired back-ground. Although 260 background scans (at 8 cm' resolution) and up to 50 scans have been used for each MW slice (18), 8 to 16 scans at this resolution for both background and data collection are adequate (11,16). The final chromatogram profile is generated by using the Gram Smitt algorithm or root mean square (RMS) absorption methods. Figure 6 shows a chromatographic profile generated for a polyethylene sample using the RMS method. The level of baseline noise is also shown (insert). The detection limit for any detector is typically described as when a transducer (detector) signal is twice the noise level of the detector (23). In the case of the FTIR as an on-line SEC concentration detector using the 3000-2700 cm-1 spectral region, the detection limit occurs at 2 χ 10" RMS absorption units and corresponds to ~^g/mL polymer sample concentration at the detector. 1

1

4


217 2.9

0.2

Neat solvent (background)

0.009 3000-2820 cm-1

-0.006 3300

3200

3100

3000

2900

2800

2700

2600

Wavenumbers (cm" ) 1

1

Figure 5. Absorption for the TCB mobile phase over the 3000-2700 cm' spectral region (a) and the same spectral region after subtracting out the solvent background (b).

1.2 1

Sample peak S/N ~

1.0

1000 /1

εο

Ο

ο ι Ο Ο Ο

0.8

0.6

$2-Ο

0.4

RM


Solvent blank

0.2

Measuring Compositional Heterogeneity in Polyolefins Using Size

Recommend Documents