J U N E 2 0 1 7 PRACTICAL COPOLYMER ANALYSIS USING GPC/SEC AND RELATED TECHNIQUES Sponsored by Presented in partnership with INTRODUCTION el permeation chromatography/size-exclusion chromatography (GPC/SEC) is a powerful analytical technique that provides critical information about polymer performance. Unlike other methods, GPC/ SEC does not simply identify average molar mass; rather, this method Gdetermines the complete molar mass distribution of macromolecules. Advances in instrumentation design are creating new opportunities to use GPC/ SEC for more challenging analyses such as copolymers. In this eBook devoted to Practical Copolymer Analysis using GPC/SEC and Related Techniques, sponsored by PSS Polymer Standards Service GmbH, experts discuss some of the challenges inherent in analyzing copolymers with different compositions and molar masses. Readers will learn how several GPC/SEC methods may improve copolymer sample characterization. First, Peter Kilz, director of PSS, explains the benefits of using multiple concentration detectors in GPC/SEC mode for chemical composition analysis. He highlights an investigation of block copolymers using a dual detector approach as an example. Kilz then suggests another way to measure the chemical composition of copolymers using liquid adsorption chromatography, also known as polymer- HPLC. Readers will also learn why a combination of various separation techniques, such as 2D chromatography, can overcome several major challenges inherent in copolymer analysis by allowing samples to be simultaneously separated by composition and molar mass. Last, Daniela Held of PSS Polymer Standards Service GmbH, explains how fractionation or hyphenation (e.g., with chemical detectors) solves the problem of identifying unknown macromolecules, which is often problematic with GPC/SEC systems. While numerous analytical methods are available to laboratories, GPC/SEC remains an important and effective option. With the tips presented in this eBook, there are even more applications of the technique available to analysts. TABLE OF PRACTICAL COPOLYMER AnALYSIS CONTenTS USIng GPC/SEC AND ReLATED TeCHNIQueS Dual Detection Compositional Analysis by GPC/ SEC With Multiple Detection Peter Kilz 4 Interaction Chromatography of Polymers Polymer Liquid Adsorption Chromatography 10 Peter Kilz 2D Analysis Two-Dimensional Chromatography of Copolymers Peter Kilz 15 GPC/SEC–MS for Comonomers Comonomer Identification: Strategies and Detection Options 23 Daniela Held COMPOSITIONAL ANALYSIS BY GPC/SEC WITH MULTIPLE DETECTION Peter Kilz, PSS Polymer Standards Service GmbH, Mainz, Germany SPONSOred PSS Column & Application GPC/SEC analysis reveals a multitude of molar Finder mass information about unknown samples that Click here are chemically simple or homogenous. to launch app However, results for copolymers may be finder compromised by the fact that fractions with different composition and different molar mass may co-elute. There are different methods on how to characterize such samples: SPONSOred • chemical composition analysis studied in GPC/ SEC mode by concentration detectors Copolymer Analysis • chemical heterogeneity analysis investigated by HPLC based on retention differences of Click to launch samples with different composition webinar • comprehensive 2D analysis that allows samples to be separated by composition and molar mass simultaneously. This article will discuss the chemical composition analysis with multiple detection, while the latter two will be discussed in the next two articles in this eBook. What Is the Advantage of Using Multiple Concentration Detectors? This approach provides information on the chemical composition of copolymers and its variation with molar mass (chain length). GETTY IMAGES/XXXXXXXX SHUTTERSTOCK.COM/VALEX 4 | JUNE 2017 | LCGC INTERACTION GPC/SEC–MS FOR DuAL DETECTION CHROMATOGRAPHY OF 2D ANALYSIS COMONOMERS POLYMERS These results are obtained from the from a detector calibration for each differences in detector response when component. To determine fi different concentration detectors are calibrated concentrations of a homopolymer i are for the components of a copolymer. This injected. The detector area is then plotted method is also applicable to determine versus the injected mass. Figure 1 shows compositions of polymer blends. such a plot for a two detector set-up (UV For each comonomer in the and RI) measured for one component macromolecule an independent detector using its homopolymer. signal is required: The detector response calibration should • When two components with unknown not be confused with the molar mass composition are present, two detector calibration of a GPC/SEC system where signals are required. the elution volume of a reference material • n components will require n with a known molar mass is measured.1 independent detection signals. Obviously, there is a practical limit (four How Can Copolymer signals have been reported) because of Molar Masses be Measured? detector applicability and selection, and An independent task of copolymer band broadening becomes a significant analysis is the determination of meaningful problem. Many results have been copolymer molar masses. Obviously, results published on chemically heterogeneous based on a single molar mass calibration samples using two detectors. Possible with standards will not give accurate results detector combinations and their benefits because the calibration will depend on are shown in Table 1. the local composition of the species. Its To measure the chemical composition, a hydrodynamic radius will depend on the detector response calibration is required. chemistry of the comonomers and their In each detector cell the sample fraction location (sequence distribution) in the will generate a detector signal that is macromolecule. the superposition of the responses of all The best way to overcome the calibration components present. In a general form dilemma is the use of on-line viscometers this can be described as overall detector that measure the intrinsic viscosity even for signal = c1*f1 + c2*f2 + ... = ∑(ci*fi) with ci complex molecular architectures. Molar concentration component i in copolymer masses are available with the universal and fi response factor component i in calibration concept described by Benoit and copolymer. coworkers.2 Please note that light-scattering The individual concentrations of the detection is not generally applicable components are unknowns, while the because the local scattering contrast (dn/dc) response factors can easily be determined will change with composition.3 5 | JUNE 2017 | LCGC INTERACTION GPC/SEC–MS FOR DuAL DETECTION CHROMATOGRAPHY OF 2D ANALYSIS COMONOMERS POLYMERS Figure 1: Detector calibration using a homopolymer and injecting different concentrations. Since not every lab has an on- block copolymer will hydrodynamically line viscometer there is an empirical behave like a pure homopolymer of method for samples with few hetero- the same chain length. In the case of contacts (e.g., block copolymers, long A and B segments in the AB block graft copolymers) using the multiple copolymer, only the A–B link acts as concentration detector approach a defect position and will not change described above.4 the overall hydrodynamic behavior From a GPC/SEC point-of-view the of the AB block copolymer chain. most simple copolymer is an alternating Consequently, the molar mass of the copolymer (AB)n that can be treated copolymer chain can be approximated exactly like a homopolymer with a by the molar masses of the respective repeating unit (AB). The next simple segments. Similar considerations are copolymer architecture is an AB block true for ABA, ABC and other types copolymer, where a sequence of of block structures and for comb- comonomer A is followed by a block of shaped copolymers with low side-chain B units. The only hetero-contact in this densities. chain is the A–B link that can influence In such cases the copolymer molar the size of the macromolecule. The A mass Mc can be determined from segment and the B segment of the AB the interpolation of homopolymer 6 | JUNE 2017 | LCGC INTERACTION GPC/SEC–MS FOR DuAL DETECTION CHROMATOGRAPHY OF 2D ANALYSIS COMONOMERS POLYMERS Figure 2: Molar mass distribution with overlaid chemical composition distribution of a styrene/MMA block copolymer with poor block formation. calibration curves Mi(V) and the weight more powerful and universal methods 5 fractions wi of the comonomers i. have to be employed, such as 2D separations. lg Mc(V) = ∑ wi(V) lg Mi(V) [1] Application Example: The calculation of copolymer molar Investigation of a Block Copolymer mass averages (Mn,c, Mw,c etc.) and Block copolymers are an important class copolymer polydispersity (Dc) is of polymers used in many applications performed as in conventional GPC/SEC from thermoplastic elastomers to polymer calculations. blend stabilizers. The synthesis is most In situations where the number of often done by ionic polymerization, which hetero-contacts can no longer be is both costly and sometimes difficult neglected, this simplified reasoning to control. However, block copolymer breaks down and copolymer molar properties strongly depend on the exact masses cannot be measured accurately chemical composition, block molar mass by GPC/SEC alone. This applies to and block yield, for example. These statistical copolymers, polymers with parameters can be evaluated in a single only short comonomer sequences and experiment using copolymer GPC/SEC high side chain densities.6 In such cases with