Characterization of Functionalized Polyolefins by High
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Reprinted from American Laboratory January 2011 Application Note by Anton Ginzburg, Tibor Macko, and Robert Brüll way. Another drawback is that TREF is a Characterization of Functionalized relatively time-consuming technique. The method of choice when it comes to sepa- Polyolefins by High-Temperature rating complex polymers with regard to their chemical composition is HPLC. Separation in HPLC can be achieved via different mecha- nisms, including adsorption–desorption and Two-Dimensional Liquid precipitation–redissolution.6 However, HPLC has thus far been limited to polymers that are soluble at room temperature. The authors’ Chromatography group developed the first interactive HPLC systems for the separation of functionalized unctionalized polyolefins, such as such as branching, and by the chemical com- polyolefins at high temperatures.7–9 It was copolymers of ethylene vinyl ace- position via affinity toward the used solvent. shown that EVA copolymers,7–9ethylene tate (EVA), are a commercially This means that macromolecules having the methyl acrylate copolymers,8 and ethylene important polymer commodity. identical hydrodynamic volume but different butyl acrylate8 can be separated according to FDepending on their comonomer con- chemical composition may coelute at the their chemical composition at a temperature tent, these materials find application in same elution volume in SEC. of 140 °C using silica gel as stationary phase the production of foams, films, and hot and gradients of nonpolar/polar solvents as melt adhesives. The polyolefin market is Crystallization fractionation (CRYSTAF) mobile phase. To characterize the chemi- growing by 5–6% annually due to its ver- and temperature-rising elution fraction- cal heterogeneity, i.e., the relationship of satile physical and mechanical properties, ation (TREF) are commonly used to deter- CCD and MMD (hyphenation of interac- low cost, and easily available raw materi- mine the CCD of semicrystalline olefin tive HPLC with SEC), 2D-LC separation is als.1 As a result, the characterization of copolymers. Both techniques utilize the required. The advantages and disadvantages functionalized polyolefins has become an relationship between comonomer content of using either HPLC-SEC or SEC-HPLC important area in polymer research.2 and the crystallizability from a hot dilute sequences have been discussed in detail by solution, which is derived from Flory’s the- van der Horst et al.10 Olefin copolymers are generally distributed ory.3 TREF can also be hyphenated with with regard to various parameters such as SEC to reveal the full chemical heteroge- At present, standard 2D-LC procedures chemical composition, molar mass, func- neity.4,5 This can be done either off-line4 and instruments are limited to separation tionality, degree of branching, block length, or on-line.5 However, the entire approach at ambient temperatures,11 and therefore and tacticity. Comprehensive characteriza- is limited to well-crystallized samples; thus cannot be applied to the characterization tion of the interrelationship between these samples with a higher content of comono- of semicrystalline polyolefins. This paper distributions is therefore essential to under- mer (>10 wt%) cannot be studied in this describes the coupling of the interac- standing the catalyst performance, as well as to optimizing the synthesis and structure– property relationships. In the case of func- tionalized polyolefins, the most important distributions are molar mass distribution (MMD) and chemical composition distri- bution (CCD), and their interrelationship is referred to as chemical heterogeneity. A number of fractionation techniques are commonly used in polyolefin analysis. High- temperature-size exclusion chromatogra- phy (HT-SEC) is the established method to determine the MMD. SEC separation is based on the size of the molecules in solu- tion (hydrodynamic volume) and the extent to which they are excluded from the pores of a stationary phase. The MMD and cor- responding molar mass averages can be obtained from a calibration curve that relates the molar mass to the elution volume, or by using on-line molar mass detectors such as light scattering or viscosimetry. However, the size of a macromolecule in solution is also influenced by its molecular architecture, Figure 1 HT-2D-LC schematic. Figure 4 Chromatogram obtained from the Figure 2 Photograph of the HT-2D-LC system. Figure 3 Chromatogram corresponding to the SEC separation of the mixture of five polymers: PE HPLC separation of the mixture of five polymers: PE (Mw = 1.18 kg/mol), PVAc (Mw = 37 kg/ (Mw = 1.18 kg/mol), PVAc (Mw = 37 kg/mol), and mol), and EVAs (19, 45, and 80 wt% VA, tive HT-HPLC system developed by the EVAs (19, 45, and 80 wt% VA, respectively). respectively). Experimental conditions described in authors’ group with SEC, and application Experimental conditions described in the text. the text. of HT-2D-LC for the characterization of commercial functional polyolefins. column size 250 × 4.5 mm i.d., and aver- Results and discussion age particle size diameter 5 µm (MZ Experimental Analysentechnik, Mainz, Germany). TCB For the separation of commercial EVAs and cyclohexanone were used as the com- according to chemical composition, a sorbent/ Polymers and solvents ponents of the mobile phase with a flow solvent system developed in the authors’ labo- 1,2,4-Trichlorobenzene (TCB) and cyclo- rate of 0.1 mL/min. ratory7 was applied that separates the EVA hexanone (Merck, Darmstadt, Germany) macromolecules with regard to their content were used as components of the mobile A PL Rapide H column, 150 × 7.5 mm of the polar comonomer. The polar copoly- phase. The polymer samples were dissolved (Polymer Laboratories, Church Stretton, mers were first fully adsorbed on silica gel from in TCB at a concentration of 3 mg/mL at U.K.), was used in the second dimension TCB and subsequently desorbed according to a temperature of 140 °C; 200 µL of the (SEC). TCB was used as the mobile phase their comonomer content by applying a gradi- sample solutions was injected and analyzed in the SEC column at a flow rate of 2.5 ent of cyclohexanone.7 A blend of five com- by 2D-LC. mL/min. A calibration curve for that col- ponents, including the PVAc, polyethylene umn was obtained with 11 polystyrene (PE), and three EVA copolymers varying in EVA copolymers were products of Bayer standards at a flow rate of 2.5 mL/min. The their average VA content, was prepared. The (Leverkusen, Germany). Polystyrene stan- temperature of the autosampler and both chromatogram in SEC and HPLC are shown dards (PS) with weight-average molar masses column ovens was set to 140 °C. in Figures 3 and 4. (Mw) in the range 0.687–2570 kg/mol (poly- dispersity index between 1.02 and 1.07) as The HPLC and SEC columns were con- SEC shows a multimodal distribution, but well as polyvinyl acetate (PVAc) (Mw = nected via an electronically controlled it is not possible to recognize that the num- 37 kg/mol) were obtained from Polymer eight-port valve system (VICI Valco ber of components as macromolecules with Standards Service (Mainz, Germany). Instruments, Houston, TX) with two 200- different chemical composition coelute in µL loops. From the moment of injection SEC (Figure 3). No information about the into the HPLC column, the eight-port MMD of macromolecules with different Instrumentation valve was switched every 2 min in order chemical composition can be obtained from Separations were achieved on a proto- to inject 200 µL of effluent from the HPLC the HPLC chromatogram alone (Figure 4). type system for high-temperature two- column into the SEC column. dimensional chromatography, constructed To study the chemical heterogeneity interre- by Polymer Char (Valencia, Spain). An evaporative light scattering detector lationship, HT-HPLC must be hyphenated The system consisted of an autosam- (ELSD, model PL-ELS 1000, Polymer to SEC. On-line hyphenation of HT-HPLC pler, two column ovens, valves, and two Laboratories) was used for detection after and HT-SEC was achieved in the 2D-LC pumps (Agilent, Waldbronn, Germany). the SEC. The ELSD was operated at a instrument, in which both chromatographic A binary gradient pump was used for the nebulization temperature of 140 °C, an modes were connected on-line by an elec- HPLC analysis and an isocratic pump was evaporation temperature of 260 °C, and an tronically controlled eight-port valve system used for SEC. Figure 1 is a schematic of the airflow of 1.5 L/min. equipped with two 200-µL loops. The two 2D-LC instrument; a photograph of the loops were used alternately in the symmetri- system is presented in Figure 2. 2D-LC system handling was done with cal configuration to store the effluent from software provided by Polymer Char. the first-dimension column and to subse- Separations in the first dimension were WinGPC-software (Polymer Standards quently inject it into the second-dimension carried out on a silica gel Nucleosil 500 Service) was used for data acquisition and column. While one sample fraction from the column with the following parameters: construction of 2-D contour plots. first dimension was being injected and ana- Two-dimensional liquid chromatography is experimentally more demanding than other chromatographic techniques. How- ever, the complete characterization yields qualitatively new information about the polymer material, and results can be pre- sented in an impressively simple way. References 1. Kaminsky, W. Macromol. Chem. Phys. 2008, 209, 459–66. 2. Yanjarappa, M.J.; Sivaram, S. Progress Polym. Sci. 2002, 27, 1347–98. 3. Flory, P.J. J. Chem. Phys. 1949, 17, 223– 40. 4. Wild, L. Adv. Polym. Sci. 1991, 98, 1. 5. Ortin, A.; Monrabal, B.; Sancho-Tello, J. Macromol. Symp. 2007, 257, 13–28. 6. Pasch, H.; Trathnigg, B. HPLC of Polymers; Springer: Berlin, 1997. Figure 5 HT-2D-LC separation of the mixture of five polymers: PE (Mw = 1.18 kg/mol), PVAc (Mw = 37 7.