Full copolymer characterization by SEC–NMR combined with SEC–MALDI

Semiquantitative Approach for Polyester Characterization Using Matrix-Assisted Laser Desorption Ionization/Time-of-Flight Mass Spectrometry Approved by 1H NMR

Matrix-assisted laser desorption ionization/time-of-flight (MALDI/ToF) mass spectrometry and ¹H NMR were used for the structural investigation of isophthalic and maleic acid copolyesters with neopentyl glycol. Since both methods provided information on the ratio of incorporated acid components and terminating groups, results were compared and linear correlations (R² = 0.96-0.98) could be found. This suggests that MALDI/ToF MS is a suitable tool for the semiquantitative characterization of polyester systems. For the isophthalic/maleic acid ratio, MALDI results yielded constantly lower values than ¹H NMR, which was attributed to varying ionization efficiencies of homo- and copolyesters. Ratios of carboxylic and hydroxylic terminating groups, which are conventionally still measured by time consuming complex titrations, were measured with MALDI and ¹H NMR and were in good agreement. Both methods either excluded or distinguished unreacted monomers in the polyester bulk in contrast to acid-base titrations where those monomers severely distort the results. Additional structural information could be gained including the observation of cyclic structures (MALDI), E/Z isomerism from maleic to fumaric acid, and the statistical distribution of the acid components within the polyester chain (¹H NMR). While ¹H NMR peak assignments have to be verified by ¹³C NMR and multidimensional techniques, MALDI/ToF MS provides a straightforward technique that can be applied to other polyester systems without major alterations.

On-line SEC-MR-NMR hyphenation: optimization of sensitivity and selectivity on a 62 MHz benchtop NMR spectrometer

The development of sophisticated synthetic routes for polymeric materials and more complex formulation used in current polymers require more advanced analytical techniques. A direct correlation between molar mass distribution and chemical composition is provided.

On-line size exclusion chromatography–pyrolysis-gas chromatography–mass spectrometry for copolymer characterization and additive analysis

On-line coupled size exclusion chromatography-pyrolysis gas chromatography mass spectrometry (SEC-Py-GC-MS) is studied as a novel tool for the characterization of complex polymer samples. An automated system for on-line SEC-Py-GC-MS allowing transfer of multiple fractions was developed based on stop-flow operation of the SEC dimension, syringe-based transfer of the SEC fraction to the GC instrument and solvent elimination with subsequent pyrolysis in a programmed temperature vaporization (PTV) injector. After optimization the system was applied to the characterization of a complex terpolymer composed of very similar monomers. The use of the system for combined pyrolysis and additive analyses in polycarbonate was also demonstrated. Results obtained with the new method indicate the interesting potentials of the method for detailed characterization of polymeric materials.

Structural analysis of poly[(R,S)-3-hydroxybutyrate-co-L-lactide] copolyesters by electrospray ionization ion trap mass spectrometry

Two random copolyesters of poly[(R,S)-3-hydroxybutyrate-co-L-lactide] (P[(R,S)-3HB-co-LA]), prepared by equimolar reaction of (R,S)-beta-butyrolactone with L-lactic acid and (R,S)-3-hydroxybutyric acid with L-lactide, respectively, were characterized by electrospray ionization ion trap mass spectrometry (ESI-ITMS). Detailed studies of these copolyesters were performed by means of collision-induced dissociation (CID). The molecular architecture of individual copolyester macromolecules, including chemical structures of their end groups (hydroxyl and carboxylate), were established on the basis of their ESI mass spectra. The influence of an intermolecular transesterification reaction on the microstructure of the copolyester synthesized by equimolar reaction of (R,S)-3-hydroxybutyric acid with L-lactide was observed. The mass spectra provided information on sequence distribution and indicated that, despite the synthetic pathway applied, random P[(R,S)-3HB-co-LA] copolyesters were formed predominantly. The arrangements of comonomer structural units along the copolyester chains were evaluated by the respective ESI-MS/MS fragmentation pathways.

Diffusion-induced growth of compositional heterogeneity in polymer blends containing random copolymers

The compositional relaxation in random copolymer systems on a macroscopic scale is considered in theory. A set of diffusion equations is derived that describes the motion of chains of different composition and then converted into coupled equations for statistical moments of the compositional distribution. Several ways to solve the closure problem for these equations are discussed. The simplest is the situation when the shape of the transient compositional distribution can be predicted a priori, for example, a bimodal distribution is kept during interdiffusion of two copolymers that are not very close in composition. For a general case, it is shown that the cumulant-neglect closure based on the truncation of high-order cumulants is an effective method to get an approximate solution in terms of the time-dependent local mean composition and its dispersion. This method is applied to non-homogeneous compatible polymer systems, such as a random copolymer AB of a composition varying in space, a bilayer of Bernoullian copolymers AB of different composition, and a bilayer of homopolymers A and B, in which an autocatalytic polymer-analogous reaction A --> B takes place, with possibility of the neighbor group effect. It is found that the interdiffusion can lead to a substantial broadening of the local compositional distribution, which, in turn, accelerates the system dynamics and promotes chemical reactions.

Interface for Direct and Continuous Sample−Matrix Deposition onto a MALDI Probe for Polymer Analysis by Thermal Field Flow Fractionation and Off-Line MALDI-MS

A simple interface based on an oscillating capillary nebulizer (OCN) is described for direct deposition of eluate from a thermal field-flow fractionation (ThFFF) system onto a matrix-assisted laser desorption/ionization (MALDI) probe. In this study, the polymer-containing eluent from the ThFFF system was mixed on-line with MALDI matrix solution and deposited directly onto a moving MALDI probe. The result was a continuous sample track representative of the fractionation process. Subsequent off-line MALDI-mass spectrometry analysis was performed in automated and manual modes. Polystyrene samples of broad polydispersity were used to characterize the overall system performance. The OCN interface is easy to build and operate without the use of heaters or high voltages and is compatible with any MALDI probe format.

Size-exclusion chromatography-a review of calibration methodologies

Recent developments are reviewed in size exclusion chromatographic calibration methodologies, including direct calibration by using narrow and broad polymer standards and various instrumental methods (nuclear magnetic resonance, mass spectrometry, light scattering) as well as universal calibration with and without viscometry detectors, for simple and complex polymers.

On-line gel permeation chromatography/nuclear magnetic resonance of complex polymer formulations

Separation of synthetic polymer mixtures by gel permeation chromatography (GPC), followed by on-line detection using a 500 MHz nuclear magnetic resonance (NMR) spectrometer, has been demonstrated using three different polymer formulations as examples. The mobile phase used in all cases was deutero-chloroform, and an inexpensive commercially available flow cell was used as a link between the separation and detection stages of the experiment. Using this technique it is possible to derive chemical information relating to specific molecular sizes of polymer mixtures, and not just the size-averaged information that would be obtained from standard NMR experiments. This provides an invaluable tool for the deformulation of complex mixtures such as those found in the surfactants and adhesives industries.

Comparison of on-line flow-cell and off-line solvent-elimination interfaces for size-exclusion chromatography and Fourier-transform infrared spectroscopy in polymer analysis

Two commercial liquid chromatography-Fourier-transform infrared spectroscopy interfaces (LC-FTIR), viz. a flow cell and a solvent-elimination interface have been assessed for use in size-exclusion chromatography (SEC) with respect to their chromatographic integrity (i.e. peak asymmetry, chromatographic resolution), quantitative and qualitative aspects. A polycarbonate/aliphatic polyester (PC/APE) blend and a polycarbonate-co-polydimethylsiloxane (PC-co-PDMS) copolymer were selected for the assessment. Both samples were successfully and selectively analyzed. The relatively large volume of the flow cell and the inherent deposition characteristics of the solvent-elimination interface led to a comparable decrease in the chromatographic resolution. The separation of oligomers was diminished in comparison with SEC-ultra-violet (UV). However, the peak asymmetry was not significantly affected by either interface. For both interfaces, a linear relationship was obtained for the FTIR response versus the injected concentration. The sensitivity was found to be higher for the solvent-elimination interface. For the current model compounds, the flow-cell interface detection limits are worse. However, the repeatability of flow-cell SEC-FTIR, evaluated by means of four SEC-FTIR analyses of polycarbonate, was considerably better than for solvent-elimination SEC-FTIR. This is probably due to the well-defined optical path length of the sample in the flow cell. By spectral subtraction, it was very well possible to obtain qualitative (functional group) information for compound identification also with flow-cell SEC-FTIR.

Coupling thermal field-flow fractionation with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry for the analysis of synthetic polymers

Thermal field-flow fractionation (ThFFF) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) have been coupled to yield a powerful combination of techniques for polymer analysis. Thermal FFF's high molecular weight (MW) selectivity and sensitivity to chemical composition are used to separate polydisperse polymers and polymer mixtures into the narrow polydispersity and homogeneous chemical composition fractions essential for MALDI-TOFMS analyses. On the other hand, MALDI-TOFMS's ability to directly measure molecular weight alleviates the need for polymer standards for ThFFF. In this first-time coupling of ThFFF and MALDI-TOFMS, compatibility issues were addressed and optimum conditions and procedures were identified and developed to maximize the capabilities of the combined technique. Depending on the polymer MW and the method of MALDI sample deposition, fractions from 1-10 ThFFF runs were combined for MALDI-TOFMS analysis. Binary solvents were used to enhance ThFFF retention and resolution of low-MW (<15-kDa) polymers, and methods were developed to allow routine MALDI-TOFMS analyses of polystyrene polymers up to 575 kDa. Overall, the MW compatibility of the two techniques was extended from several kilodaltons to several hundred kilodaltons. Polymer fractions were collected after separation by ThFFF and analyzed either by MALDI-TOFMS or reinjection into the ThFFF system. Good agreement was observed between the MW distribution data obtained by MALDI-TOFMS and ThFFF. The application of ThFFF/MALDI-TOFMS to polydisperse polymers and polymer mixtures was demonstrated. This combined technique was also shown to be a viable means for preparing standards from the original polymer sample.

Characterization of polyesters by matrix-assisted laser desorption/ionization and Fourier transform mass spectrometry

Two homopolyesters, poly(neopentyl glycol-alt-isophthalic acid) and poly(hexanediol-alt-azelaic acid), and two copolyesters, poly(dipropoxylated bisphenol-A-alt-(isophthalic acid-co-adipic acid)) and poly(neopentyl glycol-alt-(adipic acid-co-isophthalic acid)) were analyzed by internal source matrix assisted laser desorption/ionization Fourier transform mass spectrometry (MALDI-FTMS). The high resolution and high mass accuracy provided by FTMS greatly facilitate the characterization of the polyester and copolyester samples. Isobaric resolution allows the ion abundances of overlapping isotopic envelopes to be assessed. Repeat units were confirmed and end functionality assigned. Single shot mass spectra of the entire polymeric distribution demonstrate that the dynamic range of this internal MALDI source instrument and the analyzer cell exceeds performance of those previously reported for higher field instruments. Corrections of space charge mass shift effects are demonstrated for the analytes using an external calibrant and (subsequent to confirmation of structure) via internal calibration which removes ambiguity due to space charge differences in calibrant and analyte spectra. Capillary gel permeation chromatography was used to prepare low polydispersity samples from a high polydispersity polyester, improving the measurement of molecular weight distribution two-fold while retaining the benefits of high resolution mass spectrometry for elucidation of oligomer identity.

Mass spectra of copolymers

Recent and older literature (covering the last 12-13 years) in the field of mass spectra of random and block copolymers is reviewed. A detailed description is given of the information on copolymer properties that can be recovered from the analysis of the low-mass region of the spectrum (the region below 500 Da) and the high-mass region. The features of mass spectra of copolymers obtained by different synthetic routes are discussed, such as free radical, condensation, ring-chain equilibration, microbial synthesis, ring-opening, simple anionic, cationic, Ziegler-Natta, and/or metallocene catalysis, along with some random and block copolymers that occur in Nature. The emphasis is on copolymer composition and average molar mass determination, and on the benefits of coupling mass spectrometry (MS) with separation techniques such as size-exclusion chromatography (SEC) and high performance liquid chromatography (HPLC).