Infrared spectroscopy is widely used in the analysis and characterization of polymers. Polymer products are not a singular species, but rather, they are a population of polymer molecules varying in composition and configuration plus other added components. This paper describes instrumentation that provides the benefit or resolving polymer populations into discrete identifiable entities, by combining chromatographic separation with continuous spectra acquisition. The technology also provides a way to determine the mass distribution of discrete components across the chromatographic distribution of a sample. Various examples of application of this technology to polymer products are described. Examples include additives analysis, resolution of polymer blends, composition characterization of copolymers, analysis of degradation byproducts, and techniques of analysis of reactive polymer systems.
“The primary motivation for determining the structure of a polymer chain is to relate the structure to the performance properties of the polymer in end use. If a polymer chain is completely characterized and the structural basis of its properties is known, the polymerization can be optimized and controlled to produce the best possible properties from the chemical system” [
This paper addresses hyphenated chromatography-IR spectrometry instrumentation and the data processing and presentation techniques that can reveal the compositional and molecular structural properties of polymer materials.
In analysis of polymers, no single technique can provide as much information as can Fourier transform infrared spectrometry (FTIR). Many commercial polymer products are not simply a single homopolymer, but rather are multicomponent systems. To obtain maximum information regarding the product, one must utilize some fractionation process prior to spectroscopy.
Polymer products are quite complex. They may consist of mixtures of discrete components. The polymerization process typically yields variations in structure and composition as polymerization proceeds. These variations dictate the physical properties (strength, flexibility, melting point, and glass transition temperatures, to name a few) in the resulting product. In some cases, it is desirable to have invariant structure and composition throughout the population of polymer chains, while in other cases, such variations are specifically generated by the manipulation of the polymerization process. Characterization of distributed composition and structural properties, therefore, is essential to physical properties optimization and control. This paper offers a number of examples demonstrating the utility of this technology.
A limitation of classical infrared spectroscopy is the inability to characterize or identify components in a mixture. Chromatography is a powerful tool for resolving/separating solutes, but provides no molecular identification of solute components. Liquid Chromatography-FTIR (LC-FTIR) addresses this limitation by chromatographic separation of mixed solutes coupled to a spectrometer which processes the eluting resolved constituents and acquires their discrete spectra. This publication illustrates the use of hyphenated LC-FTIR to provide graphical and numeric information regarding multicomponent samples.
A hot-melt adhesive was analyzed by gel permeation chromatography-FTIR (GPC-FTIR). A sample was injected onto a suitable chromatographic column, and the column eluant passed to a continuous sample collecting module containing an FTIR spectrophotometer. The sample chromatogram is effectively “painted” onto an IR transparent medium as a continuous stripe of solutes eluate. Sequential spectra collected along this stripe are the basis of a time-arrayed data set of sample composition as a function of elution time.
Figure
Hot-melt adhesive components identification.
There have been numerous developments of instrumentation that could provide LC-FTIR capability. Two fundamental approaches have emerged: (1) flow cells and (f2) solvent elimination systems. The flow-cell approach, analogous to ubiquitous LC-UV (liquid flow cell detectors operating in the ultraviolet range) systems, has limited utility when applied to LC-IR. The latter has poor detection limits, due to limited optical path length and to interfering IR bands generated by the LC mobile phase. There is several thousand times as much mobile phase, as there is solute in the flow cell during the elution, and any absorbance by the mobile phase will swamp the sample in spectral regions, where the mobile phase has any IR absorbance. As a result, flow cells can be utilized only for the spectral regions in which the mobile phase exhibits no absorbance. LC-IR flow cell methods have been used for polymers that possess structure that manifests itself in such solvent spectral window regions, but they typically require complex chemometric statistical methods to determine composition based on the severely limited spectral regions available. Short-chain branching in polyolefins is measured via LC-FTIR using flow cell methods. Recently, the structural characterization of polycarbonates was reported, utilizing 2-dimensional chromatography, flow cells, and multivariate statistical analysis of the raw data mass [
Most of the instrumentation development has centered on solvent elimination processes. Some solvent elimination devices made use of microbore chromatography columns which would deposit eluant droplets into wells on a moving plate, which could, after evaporation, yield reflectance or transmission spectra. It was apparent that the use of nozzle systems could be used for evaporative removal of eluant, depositing the solutes as dry solids. Bieman and Gagel developed an instrument that directed column eluant through a pneumatic nebulizer, depositing the nonvolatile solutes onto the surface of a slowly revolving germanium disk as regular track [
Figure
DiscovIR HPLC-IR analyzer.
In polymer work gel permeation chromatography (GPC) is most commonly employed, but other chromatography modes, such as reverse-phase, work quite satisfactorily. Unlike HPLC-IR flow cells, this type of interface eliminates all chromatography mobile phase and has none of the spectral interference limitations encountered in use of chromatography-IR flow cells.
With the exception of the IR chromatograms shown in Figure
An overall schematic is shown in Figure
Instrument schematic representation.
Instrument automated operation and data analysis is provided by software resident on a dedicated computer. The system has all the functionality associated with FT-IR bench instruments, and including spectral library capabilities.
Equipment utilized for LC-FTIR polymer analysis is shown in Figure
This presentation focuses on the techniques of utility in extracting compositional information from LC-IR of various polymeric materials. For experimental parameters used in these analyses, please refer to the Section
Polymer industry GPC-FTIR application types.
(i) Components identification | (i) Copolymers: comonomer distribution |
(ii) Deformulation | (ii) Aging/environmental changes |
(iii) Reactive systems analysis | (iii) Lot-to-lot characterization “good versus bad” |
The “hyphenation” of liquid chromatography with Infrared spectroscopy provides the opportunity to resolve multicomponent samples and contain spectral information about the individual components. In the case of polymers, one can reveal changing composition/configuration of polymer molecules across the molecular weight distribution. Applications described include polymer characterizations based on composition, configuration, and conformation of polymer molecules. “The The The
The following example is a characterization of poylymeric additives present in a heavy duty diesel engine lubricant. Petroleum-based motor oils are manufactured with a complex “additives package” that provide for optimal performance in the harsh operating environment of internal combustion engines. A significant fraction of motor oils consists of synthetic polymers. Functions of these polymers include viscosity-temperature modulation and the dispersion of sludge generated in the environment of an operating engine.
A sample of (unused) motor oil was injected onto a gel permeation chromatography (GPC) column which was coupled to a DiscovIR FTIR system. Figure
Time-ordered spectra from the GPCFTIR analysis of diesel motor oil.
Casual inspection of the deposit spectra indicate that the polymer material is made up of at least two components. The first elution material has an intensity maximum at 9.2 minutes, corresponding to a MW of 600 k Daltons (Figure
First elution peak spectrum.
Inspection of the spectrum yielded the following observations: typical aromatic C–H stretches (3082 cm−1, 3061 cm−1, 3027 cm−1), ring breathing modes (1601 cm−1, 1493 cm−1), aromatic ring out of plane bends (698 cm−1, 756 cm−1), 1735 cm−1 carbonyl, C–O stretches in the 1200–1000 cm−1 region, bands associated with conjugated dienes are absent.
The spectra of this elution region are those of a styrene-acrylate copolymer. This is consistent with the viscosity index improvers used in engine lubricants.
The broad eluant profile in the 10–12 minute elution timeframe provided the spectrum in Figure
Second elution peak spectrum.
Note in the above spectra that a 700 cm−1 band is present only in the first solute peak. Similarly, a 1220 cm−1 band is present only in the second solute peak. The data analysis software that is incorporated with the instrument provides for the generation of infrared chromatograms of selected spectral bands. This powerful feature enables the analyst to generate a chromatogram that shows the distribution of a particular polymeric component even when the polymer components are not completely resolved chromatographically.
Figure
Infrared band chromatograms of the deposited sample.
Polymer composition or configuration chromatograms can aid in the deformulation of complex systems. Figure
Tire polymer sample.
Initial inspection of the data suggested a polymer or polymers containing (trans) butadiene and styrene. These styrene (700 cm−1) and diene (967 cm−1) chromatograms virtually overlap in shape and elution times. This indicates a styrene butadiene rubber (SBR) copolymer in which the ratio of the comonomers is invariant across the molecular weight distribution of the copolymer.
The third (maximum band intensity) chromatogram is generated from the CH stretch bands and closely corresponds to the total mass of solutes. Notice that there is a sharp rise in this IR chromatogram at 31 minutes, and that this elution profile extends to lower molecular weights than do the comonomers. Examination of a spectrum at 31 minutes elution time indicates a different composition than the copolymer.
The chromatogram of the 1707 cm−1 band has a completely different distribution from the SBR polymer. It elutes at the tail of the SBR profile and has a different spectrum. The 1700 cm−1 band is generated by a carbonyl of lower molecular weight additive, included with the polymer to promote cross-linking with other components that will be added to the final formulation. This illustrates the ability to deformulate or identify the discrete components that comprise this sample. The technique can be applied to more complex samples by selecting spectral bands unique to each component.
In many, if not most, polymer blend samples, there is significant molecular weight overlap of the different polymer components. This example illustrates the data processing techniques used to characterize such solute overlaps.
This blend sample shows two polymers that have at least one pair of mutually exclusive bands. The uppermost in Figure
Two component polymer blend: overlapping MW distributions.
Band chromatograms showing distribution of two components.
The discrete mw weight distributions of each blend component are readily apparent. If one selects two spectra from the tails of the overall distribution, as in Figure
Copolymers are complex. During polymer synthesis the molecular population varies simultaneously in molecules size, concentration, and composition.
The reaction rates of the comonomers are seldom exactly equal. As a consequence the comonomer proportions will often change as polymerization proceeds. This is called “composition drift”. Generally, composition drift will alter the finish physical properties of a copolymer. In some instance, the polymer chemist desires a minimum of composition drift, whilst in others a particular drift profile is deliberately sought [
Typically, the total mass chromatogram of the polymer chains resembles a probability distribution plot—at a maximum in the region of the average molecular weight, and tapering off to nothing at the high and low molecular weight extremes of the distribution. Both the total mass and comonomer concentration are varying across the GPC elution profile. At any point in the chromatographic elution, a comonomer spectral band intensity is a product of the instantaneous elution mass and the per cent concentration of the comonomer.
Figure
GPC mass distribution and varying concentration of comonomers.
This is the bulk spectrum of the copolymer. Useful IR bands for determination of composition are color highlighted. The (trans)butadiene is the green band, and the three styrene bands are highlighted in red (Figure
Bulk spectrum of a styrene/butadiene copolymer.
Figure
GPC-IR data map of SBR copolymer.
Assuming that the deposited chromatogram conforms to the Lambert-Beer law of band absorbances, the absorbance of a spectral band at any place along the elution curve is equal to the product of the band extinction coefficient the solute deposit thickness, and the concentration of the comonomer in the copolymer
Band chromatograms of the comonomers butadiene and styrene.
Compositional drift variations in a pharmaceutical excipient.
This example shows composition drift measurements of a styrene-butadiene copolymer. A peak ratio chromatogram of the styrene/diene absorbance ratio was generated. The ratio chromatogram was algebraically transformed to a plot of polymer styrene content as a function of molecular weight. The 968 cm−1 (diene) and 1495 cm−1 (styrene) absorbance intensities were used for the absorbance ratio. A bulk spectrum of a known overall composition copolymer will provide the value (
Polymer excipients are increasingly used to enhance and control the delivery of active pharmaceutical ingredients (APIs). They play an especially crucial role in delivery of poorly soluble drugs [ withstand the rigors of the compounding and fabrication processes, retain efficacy during shelf life, make the passage through the stomach without undue decomposition, be released in the GI tract with adequate bioavailability and with a desired release kinetic profile.
Performance is achieved by control of both the chemical and physical properties of the excipient package. Both polymer blends and copolymers are used in this field.
Figure
Infrared spectrometry not only reveals the
The following analysis of polyolefins shows how conformational spectral bands can aid in the deformulation of olefin polymers.
There are many commercially available polyolefin polymers, and they have a broad range of physical performance attributes. Polyolefins can be synthesized as high modulus structural materials, soft rubbers, and most everything in between. They are made chiefly from the monomers ethylene and propylene and possess only carbon and hydrogen atoms.
Polyolefins are, by definition, constituted of only carbon and hydrogen. Principal bands are limited. At first glance, vibrational spectroscopy might seem to be of scant utility, but the configuration/conformation properties of olefin polymers make infrared spectroscopy possibly the most useful tool available for characterization. It is also the configuration/conformation properties that determine many of the physical properties of a polyolefin.
Three samples of olefin polymers were analyzed by GPC-FTIR. Referring to Figure
IR spectra of three polyolefin products.
Sample 1 is polypropylene homopolymer.
Sample 2 is an ethylene propylene copolymer.
Sample 3 is determined to be a blend of polypropylene homopolymer, ethylene/propylene copolymer, and ethylene/butene copolymer.
The sample was analyzed for molecular and composition drift as in the previous examples. For brevity, the molecular weight distributions of composition and composition drift are not repeated here. This example points out the composition and configuration aspects used to characterize Sample 3. The 720 cm−1 evidences some splitting, suggesting crystallinity in long sequences of ethylene comonomer. The 1155 band is a methyl branch band of polypropylene and is common to all samples. The presence of the 772 cm−1 indicates the butene comonomer in Sample 3. This is supported by similar elution chromatograms (not shown) of the 720 cm−1 and 772 cm−1 bands. The ethylene/propylene, by contrast, shows no such peak splitting. This infers that the peak splitting arises from long ethylene sequences of ethylene comonomer in the ethylene/butene copolymer. Sample 1 is an isotactic polypropylene, as evidenced by the 1168 cm−1 band. Sample 2 is atactic. The presence of the isotactic PP band in sample 3 arises from the blend of sample 2 and sample 1.
Polyethylene glycol (PEG) is a polyether terminated with hydroxyl groups. It is a low molecular weight polymer with the depicted structure
Oxidative changes of Polyethylene glycol.
In such applications, degradation of the PEG can have an adverse effect on the shelf life or potency of the medication.
After the oxidation process, the bulk material showed weak IR bands at 1720 cm−1 and 1640 cm−1, which were absent from the original PEG 1000. Examination by LC-FTIR revealed two series of oxidative cleavage products: one series containing aldehyde functionality and the other a series of carboxylic acid salts, which account for the extra IR bands. This information gives insight towards preservation strategies for extending the shelf life of formulations containing PEG. RP-LC/FTIR results are shown.
Functional group chromatograms revealed a series of aldehyde and carboxylated salt oligomers produced by the oxidation process. Air oxidation of PEG generates unstable peroxides, typical of the autooxidation of ethers. The peroxides then react further, leading to cleavage of the PEG chain between oxygen and carbon atoms.
The resulting aldehydes and carboxylate salts are PEG oligomers with oxidized end groups. Both of these more polar series elute early in the reverse-phase chromatogram. The bulk material shows the carboxylate IR band at 1640, consistent with a tightly bound cation such as a transition metal. This suggests a cleanup technique for preservation of the bulk material to improve the shelf life of products containing PEG.
In a similar vein, Figure
Eudragit L100-55 excipient is a copolymer with the polymer backbone structure as shown in Figure
Eudragit backbone molecular structure.
Undesired cross-linking of a pharmaceutical polymeric excipient.
Extrusion is performed at an elevated temperature. A GPC-FTIR analysis of product under various extrusion conditions revealed that a side group was cleaved from the polymer backbone, with resultant formation of anhydride cross-links. This has a deleterious effect on release characteristics of the active ingredient.
Many coatings are based on chemical reaction for their “drying” or “curing”. Epoxy and urethane systems are examples of liquid polymer materials that treat to form hard films or objects.
Oil-based paints are the most ubiquitous of reactive coatings. Paints consist of polymers dissolved in oils such as linseed oil, soya oil, and various vegetable oils. These oils are unsaturated fatty acids, and they have the ability to form adducts with the polymer component under appropriate reaction conditions.
During the coating synthesis, some portions of the vegetable oil will become adducted to the polymer mass. Upon exposure to oxygen, these unsaturates will cross-link to form a solid film (drying).
Figure
Paint: partitioning of oil into the polymer fraction.
To make a quantitative assessment of monomer-polymer partitioning of the oil, the area under the carbonyl IR chromatogram was integrated, and a judgment was made as to the polymer/monomer partition (ca. 31 minutes). Scalar values of the integral function were taken at this time and at the maximum of the integral curve, resulting in a determination of 56% inclusion of the oil into the polymer mass.
This is a postreaction end state analysis of the sample. The technique can be extended to monitoring real-time reaction kinetics of processes whose total reaction times are greater than one hour. The limiting factor is the cycle time of the chromatography column.
Shell rotella T SAE 15 W-40 diesel engine oil Column GPC: Jordi mixed bed: 25 × 1 cm Mobile phase Tetrahydrofuran (THF) Flow rate 1 mL/min Injection volume 50 Sample concentration 90 mg/mL Cyclone temperature 260°C Pressure, Cyclone 400 torr Pressure, sample deposit chamber 6.4 torr Carrier gas flow 380 cc/min Condenser temp 5°C Zn Se disk translation rate 3 mm/min Disk temp 20°C Nebulizer power 6 Watts
Polymer blend Column GPC: waters styragel HR 4 Mobile phase Tetrahydrofuran (THF) Flow rate 1 mL/min Injection volume 100 Sample concentration 3 mg/mL Cyclone temperature 150°C Pressure, cyclone 369 torr Pressure, sample deposit chamber 4.7 torr Carrier gas flow 380 cc/min Condenser temp 15°C Zn Se disk translation rate 3 mm/min Disk temp 0°C Nebulizer power 5 Watts
Firestone 721AC styrene/butadiene copolymer Column GPC: Jordi 50 × 1 cm mixed bed: linear DVB Mobile phase Tetrahydrofuran (THF) Flow rate 1 mL/min Injection volume 5 Sample concentration 12 mg/mL Cyclone temperature 248°C Pressure, cyclone 400 torr Pressure, sample deposit chamber 5.8 torr Carrier gas flow 321 cc/min Condenser temp −10°C Zn Se disk translation rate 3 mm/min Disk temp 90°C Nebulizer power 1 Watt
The chromatograph used in this application was a Waters 150°C chromatograph, operating at 145°C.
Copovidone (BASF Kollidon VA64) Column Shodex OHpak SB-806 M HQ Mobile phase MeOH/H2O, 0.05 M acetic acid Flow rate 1 mL/min Injection volume 150 Sample concentration 0.35% Cyclone temperature 21°C Pressure, cyclone 750 torr Pressure, sample deposit chamber 2.6 torr Carrier gas flow 0 cc/min Condenser temp 20°C Zn Se disk translation rate 3 mm/min Disk temp 25°C Nebulizer power 0 Watts
A sample of PEG 1000 (polyethylene glycol of average molecular weight 1000) was subjected to vigorous air oxidation and then analyzed it by reverse-phase LC-IR to learn about the identity and distribution of the oxidation products within the bulk polymer.
Blend of polyolefins Column GPC: Jordi DVB mix bed 25 cm × 1 cm, 5 Mobile phase 1,2,4 trichlorobenzene (TCB) Flow rate 1 mL/min Injection volume 100 Sample concentration 25 mg/mL Cyclone temperature 375°C Pressure, cyclone 109 torr Pressure, sample deposit chamber 1 torr Carrier gas flow 57 cc/min Condenser temp 20°C Zn Se disk translation rate 3 mm/min Disk temp 90°C Nebulizer power 10 Watts
Polyethylene glycol (PEG 1000), partially air oxidized in 15% acetonitrile/water Column Reverse-phase eclipse C-18, 4.6 × 50 mm Mobile phase 10–90% acetonitrile gradient, 30 min Flow rate 1 mL/min Injection volume 150 Sample concentration 4.5 mg/mL Cyclone temperature 180°C Pressure, cyclone 460 torr Pressure, sample deposit chamber 5.2 torr Carrier gas flow 370 cc/min Condenser temp 5°C Zn Se disk translation rate 3 mm/min Disk temp −20°C Nebulizer Power 13.5 Watts
Eudragit L100–55 Column Jordi gel DVB mixed bed-250 × 10 mm Mobile phase THF Flow rate 1 mL/min Injection volume 100 Sample concentration 5 mg/mL Cyclone temperature 150°C Pressure, cyclone 460 torr Pressure, sample deposit chamber 5 .2 torr Zn Se disk translation rate 3 mm/min Disk temp −15 to
The multidistributed attributes of polymer systems present a huge challenge to the analyst. Infrared spectrometry has proved to be a superior tool in polymer analysis, but is severely limited by the multicomponent nature of polymer samples. Traditional sample fractionation followed by FTIR analysis can yield results but is extremely costly on terms of time and effort.
The advent on practical LC-FTIR technology addresses this need. It is suited to multiple applications for the analysis and characterization of polymer materials. Several examples of utility are presented in this paper. The combination of LC-FTIR instrumentation coupled with the interpretative capabilities of infrared software greatly assists in the interpretative aspects of infrared spectra and renders hyphenate LC-FTIR a practical working technique for polymer scientists and synthesis chemists.