Liquid Chromatography At Critical Conditions Research Articles

Influence of Microstructure on the Elution Behavior of Gradient Copolymers in Different Modes of Liquid Interaction Chromatography.

We studied the influence of microstructure on the chromatographic behavior of gradient copolymers with different gradient strengths and block copolymer with completely segregated blocks by using gradient liquid adsorption chromatography (gLAC) and liquid chromatography at critical conditions (LCCC) for one of the copolymer constituents. The copolymers consist of repeating units of poly(propylene oxide) and poly(propylene phthalate) and have comparable average chemical composition and molar mass, and a narrow molar mass distribution to avoid as much as possible the influence of these parameters on the elution behavior of the copolymers. On both reversed stationary phases, the elution volume of gradient copolymers increases with the increasing strength of the gradient. The results indicate that for both modes of liquid interaction chromatography, it is important to consider the effect of microstructure on the elution behavior of the gradient copolymers in addition to the copolymer chemical composition and molar mass in the case of gLAC and the length of the chromatographically visible copolymer constituent in the case of LCCC.

Liquid chromatography at critical conditions (LCCC): Capabilities and limitations for polymer analysis

This paper investigates liquid chromatography at critical condition (LCCC) for polymer analysis. Based on controversial claims on the separation of cyclic polymers from linear analogues in the literature, the efficiency of LCCC for separation and purity analysis is questioned. Polyisobutylene (PIB) and poly(3,6-dioxa-1,8-octanedithiols) (polyDODT) were used for the study. The structure of low molecular weight cyclic and linear polyDODT was demonstrated by MALDI-ToF. NMR did not show the presence of thiol end groups in higher molecular weight PIB-disulfide and polyDODT samples, so they were considered cyclic polymers. When a low molecular weight polyDODT oligomer with only traces of cycles, as demonstrated by MALDI-ToF, was mixed with an Mn = 27 K g/mol cyclic sample, LCCC did not detect the presence of linear oligomers at 6 wt%. Based on the data presented here, it can be concluded that the LCCC method is not capable of measuring <6 wt% linear contamination so earlier claims for cyclic polystyrene (PS) samples purified by LCCC having <3% linear contaminants are questioned.

Alkylene oxide poylmerizations: identification of side reactions and by-products

In this study microwave-assisted anionic ring opening polymerization (ROP) of alkylene oxides is reported. Low molar mass polymers of propylene oxide (PO), butylene oxide (BO) and hexylene oxide (HO) are synthesized by anionic ring opening using various monohydric alcohols as initiators. The monohydric alcohols and lower molar masses allowed for monitoring of different types of possible unwanted by-products and their sources. Liquid chromatography at critical conditions (LCCC) and liquid adsorption chromatography (LAC) are used to determine the presence of oligomers other than targeted. Finally, MALDI-TOF MS of the products have clearly shown the trends and extents of different types of side reactions that are possible in anionic ring opening of alkylene oxides. Propylene oxide is most vulnerable to chain transfer reactions compared to higher alkylene oxide.

Chromatography under critical conditions: An analogy between functionalized and partially cyclic polymers

An analogy is established between functionalized polymers and partially cyclic macromolecules (PCMs) in the liquid chromatography at critical conditions (LCCC). Application of the functionalized chain analogy (FCA) for prediction of the behavior of complex multi-cyclic PCMs in the LCCC mode is demonstrated. By using FCA, we discuss possibilities of LCCC to separate multi-cyclic PCMs by the number of cycles, and with respect to molecular topology. FCA is also extended to describe PCMs with specifically adsorbing groups; this results in a simplified theory of LCCC of functionalized PCMs. By simulating chromatograms of heterogeneous functionalized PCMs at the conditions of LCCC, we show possible dramatic effects of functional groups on the topological separation of PCMs: even the retention order of components may change to opposite.

Estimation of critical conditions of polymers based on monitoring the polymer recovery

Liquid chromatography at critical conditions (LCCC) is a very attractive chromatographic technique on the border between the size exclusion and liquid adsorption mode of the liquid chromatography. The strong interest in LCCC arises from the fact that it is well suited to analyze the block lengths in segmented copolymers or the heterogeneities with regard to end groups present, for example, in functionalized polymers e.g., telechelics. In this paper a new method for identification of the critical conditions of synthetic polymers is proposed, which requires only one polymer sample with higher molar mass. The method is based on monitoring the recovery of the polymer sample from a column. The composition of the mobile phase is modified until the polymer sample is fully recovered from the column. The corresponding composition of the mobile phase is composition corresponding to LCCC. This new method was applied for the determination of critical conditions for polyethylene, syndiotactic polypropylene and isotactic polypropylene. The results of the new method will be compared to those of classical approaches and advantages will be pointed out.

Liquid Chromatography at Critical Conditions of Poly(propylene)

Liquid chromatography at critical conditions (LCCC) of poly(propylene) (PP) holds unique potential to further augment the understanding of molecular heterogeneities present in PP. The critical conditions for isotactic poly(propylene) (iPP) and syndiotactic poly(propylene) (sPP) have been identified using porous graphitic carbon as stationary phase and combinations of adsorption and desorption promoting solvents. It is found that 1,2,4‐trichlorobenzene is a stronger desorption promoting eluent compared to 1,2‐dichlorobenzene, while 2‐octanol shows a weaker adsorption promoting effect compared to 2‐ethyl‐1‐hexanol for all stereo‐isomeric forms of PP. The fraction of desorption promoting solvent needs to reach critical conditions decreased in a linear manner with the temperature. High temperature 2D liquid chromatography with infrared detection provides quantitative information about the fractions of the constituents (iPP and ethylene–propylene copolymer) of a model high impact PP sample at LCCC of iPP. image

High temperature size exclusion–liquid adsorption chromatography (HT-SEC–LAC): Full isocratic separation of parent isotactic polypropylene homopolymer from ethylene-propylene copolymers

A simple and highly selective chromatographic separation method named high temperature size exclusion–liquid adsorption chromatography (SEC–LAC) was developed for the separation of the parent isotactic polypropylene (iPP) homopolymer from either segmented or random ethylene-propylene copolymers (EPCs). The compositions of the adsorption promoting and desorption promoting solvents were carefully optimized, so that an intermediate elution mode between total liquid adsorption (LAC) and liquid chromatography at critical conditions (LCCC) of iPP was established. At the same time selectivity in the separation of PPs of different tacticity was not altered. This was encouraging and showed that this approach should be applicable across samples of different stereoregularity. The main advantage of the SEC–LAC method over LAC methods is the completely independent first step SEC analysis of the iPP homopolymers and the subsequent LAC analysis of all other olefin homopolymers and copolymers over a wide range of chemical compositions. Therefore, SEC–LAC combination can overcome the major drawbacks in LAC such as loss of separation selectivity depending on the molar masses of iPP homopolymers. In this way, the present study clearly proves that this separation protocol can discriminate the molar mass heterogeneity of iPP homopolymers present in complex EP copolymer systems without using multidimensional analytical approaches such as high temperature two-dimensional liquid chromatography (HT-2D LC). FTIR measurements were performed to obtain structural information on the separated components for the proper understanding of the separation mechanism. In conclusion, this study clearly shows that SEC–LAC coupled to FTIR is a powerful analytical method that is ideal for quantifying small amounts of iPP homopolymer contaminations in propylene/α-olefin copolymers and other multicomponent polyolefin blends.

Onflow liquid chromatography at critical conditions coupled to 1H and 2H nuclear magnetic resonance as powerful tools for the separation of poly(methylmethacrylate) according to isotopic composition

The present work addresses a major challenge in polymer chromatography by developing a method to separate and analyze polymers with identical molar masses, chemical structures and tacticities that is solely based on differences in isotope composition. For the first time, liquid chromatography at critical conditions (LCCC) was used to separate PMMA regarding the H and D isotopes. At critical conditions of H-PMMA, D-PMMA eluted in the adsorption mode and vice versa. By online onflow LCCC-NMR, both PMMA species were clearly identified. Different from other detectors, NMR can distinguish between H and D. Onflow LCCC-H/NMR and LCCC-D/NMR measurements were carried out and the H/D-blend components were detected. 1H and 13C NMR provided the tacticity of protonated PMMA. Double resonance 13C{H} and triple resonance 13C{H,D} provided the tacticity of the deuterated samples. Samples with similar tacticities were used to ensure that separation occurs solely regarding the isotope labeling.

Structural characterization of telechelic polyisobutylene diol

The chemical homogeneity of telechelic polyisobutylene diol (PIB-diol), prepared by hydroboration-oxidation of allyl telechelic PIB obtained by reacting living PIB with allyltrimethylsilane, was investigated by liquid chromatography at critical conditions (LCCC) and HPLC coupled with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). A normal phase gradient HPLC method was developed that was able to separate the as-synthesized PIB-diol into three components; PIB-diol, PIB-monool and PIB without any OH functionality. These were analyzed by MALDI-TOF MS, which suggested that the reaction of living PIB with allyltrimethylsilane was incomplete. LCCC using refractive index (RI) detector as a concentration detector allowed separation and quantification of PIB species according to their chemical heterogeneity (PIB-diol=95.3%, PIB-monool=3.3%, non-functional PIB=1.4%). The calculated number average functionality (Fn) of PIB-diol=1.94 suggests high quality of PIB-diol suitable for high molecular weight polyurethane synthesis.

HPLC‐1H‐NMR Characterization of Polystyrene‐block‐Polyisoprene Copolymers: LCCC‐1H‐NMR Using a Single Mobile Phase

SummaryThe analysis of PS‐b‐PI copolymers, synthesized by sequential living anionic polymerization was conducted by on‐line hyphenation of liquid chromatography at critical conditions (LCCC) and proton nuclear magnetic resonance spectroscopy (1H‐NMR). Critical conditions were established for polyisoprene using 1,4‐dioxane as the mobile phase and varying the column oven temperature. At these critical conditions, the polyisoprene homopolymer formed during synthesis was separated from the copolymer. The molar mass of the PS block, the chemical composition of the block copolymers as well as the microstructure of PI were determined in a single experiment.

Liquid chromatography at critical conditions of polyethylene

Olefin copolymers are of increasing scientific interest due to their important application potential. Liquid chromatography can deliver important information, especially on their molecular structure. In particular, liquid chromatography at critical conditions (LCCC) is of interest because, in this chromatographic mode, the elution is molar mass independent for a given repeat unit. LCCC is conducted at conditions where the entropic effect of size exclusion is equal to the enthalpic effect of interaction between the macromolecules and the stationary phase and can be used to separate copolymers as well as some homopolymers containing one single repeat unit but having different end groups. For the first time, critical conditions for linear PE were experimentally identified by using porous graphite (Hypercarb™) as the stationary phase, and either 1-decanol/ortho-dichlorobenzene (ODCB), 1-decanol/1,2,4-trichlorobenzene (TCB), n-decane/ODCB, or n-decane/TCB as the mobile phase at 160 °C. The identified critical conditions for PE using the above approach in the above solvents have been verified to be correct, barring slight deviations, by two different techniques which were previously used to determine critical conditions for polymers soluble at ambient temperature. The critical conditions for polyethylene were applied to separate statistical copolymers of ethylene/1-octene with similar molar mass.

Quantitative analysis of oleic acid and three types of polyethers according to the number of hydroxy end groups in Polysorbate 80 by hydrophilic interaction chromatography at critical conditions

A quantitative characterization of Polysorbate 80 is crucial for its many applications. In this paper we report a quick RP-HPLC method for the quantitative determination of Polysorbate 80. The hydrolysis of Polysorbate 80 to release oleic acid and three types of polyethers was first carried out. A chromatographic method based on liquid chromatography at critical conditions (LCCC) was then developed for an endgroup-based separation of low-molecular-mass polyether. With this method the polyether, irrespective of its molecular-mass, is separated according to endgroups (functionality) due to interactions of the polar endgroups with the hydrophilic stationary phase. The different types of polyethers and oleic acid were identified using on-line electrospray ionization mass spectrometry and quantified by evaporative light scattering detection.

Comprehensive two-dimensional liquid chromatography for the separation of protonated and deuterated polystyrene

Liquid chromatography at critical conditions (LCCC) has been shown to be a powerful method for the separation of complex polymers regarding chemical composition, functionality, or molecular topology. LCCC has never been used, however, to separate polymers according to the degree of deuteration. This is a very challenging task since polymers shall be separated that are identical regarding molar mass, endgroups and chemical composition. In the present work, critical conditions were established in such a way that one component of a complex mixture elutes at critical conditions, whereas the other component shows size exclusion chromatography (SEC) behaviour. Blends of protonated (h) and deuterated (d) polystyrene (PS) were separated by LCCC at critical conditions of both h-PS and d-PS. Depending on the molar masses of the blend components, baseline separation could be achieved. In order to improve the separation further, comprehensive two-dimensional liquid chromatography was carried out on a number of model blends. In the first dimension LCCC was used, which separated the blends according to isotopic effects whereas in the second dimension the separation took place with respect to hydrodynamic volume. In order to further improve the separation of a number of blends a separation protocol was used where one component shows SEC conditions whereas the other component shows liquid adsorption chromatography (LAC) conditions. This separation protocol was achieved by varying the column temperature.

Online Coupling of Two-Dimensional Liquid Chromatography and NMR for the Analysis of Complex Polymers

For the first time, comprehensive two-dimensional liquid chromatography (2D-LC) of complex polymers is coupled online to 1H NMR. 2D-LC is used to separate mixtures of poly(ethylene oxide)s with regard to chemical composition and molar mass. The present samples contain polymers with different end groups and chain distributions. In the first LC dimension, liquid chromatography at critical conditions (LCCC) is used for the selective separation according to the end groups. Fractions that are then uniform regarding their end groups are automatically transferred into the second LC dimension which separates the fractions regarding their chain length distributions using liquid adsorption chromatography. The eluate from 2D-LC is directly introduced into the 1H NMR for on-flow analysis. The online coupling of one- and two-dimensional chromatography with 1H NMR detection is demonstrated. The NMR is coupled to both individual separations as well as to the entire two-dimensional separation. As a result of this multidi...

Multi-Block Polyurethanes via RAFT End-Group Switching and Their Characterization by Advanced Hyphenated Techniques

The detailed characterization of poly(styrene)-b-poly(tetrahydrofuran) (pS-b-pTHF) multiblock copolymers (17800 g mol–1 ≤ Mn ≤ 46800 g mol–1) generated via urethane linkages is presented. The synthesis of the block copolymers is enabled via a mechanistic switch of the thiocarbonyl thio end group of a poly(styrene) to dihydroxyl terminated polymers that subsequently react with a diisocyanate terminated polytetrahydrofuran based prepolymer to form multiblock copolymer structures. The characterization of the multiblock copolymers and their substructures includes size exclusion chromatography (SEC), liquid chromatography at critical conditions (LCCC), nuclear magnetic resonance (NMR), and infrared (IR) spectroscopy as well as matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. To obtain even further details of the polymer size and its composition, SEC with triple detection as well as newly developed SEC coupled online to IR spectroscopy was carried out. The quantification of the average blo...

Characterization of Polystyrene‐block‐Polyethylene Oxide Diblock Copolymers and Blends of Homopolymers by Liquid Chromatography at Critical Conditions (LCCC)

AbstractIn the present study, diblock copolymers of PS‐b‐PEO synthesized by sequential living anionic polymerization as well as the blends of PS, PEO homopolymers will be investigated by using liquid chromatography at critical conditions. Critical conditions will be established for the PS block of the copolymer. At these conditions, the PS block is chromatographically invisible and does not contribute to retention so the block copolymer is separated according to the block length of the PEO block. At the critical conditions of PS, it is also possible to separate the PS homopolymer formed. The critical conditions will be established in such a way that the non‐critical PEO block elutes in size exclusion chromatography (SEC) mode.

Characterization of Polydimethylsiloxane‐block‐polystyrene (PDMS‐b‐PS) Copolymers by Liquid Chromatography at Critical Conditions

AbstractIn this study, liquid chromatography at critical conditions (LCCC) is used to determine the block lengths and molar masses of polydimethylsiloxane‐block‐polystyrene copolymers synthesized via atom transfer radical polymerization (ATRP) techniques. In addition, the analysis allows for the determination of the presence of homopolymers in the samples. LCCC at the critical point of polydimethylsiloxane (PDMS) on a reversed phase column allows for the determination of PDMS homopolymer and the length of the polystyrene (PS) block in the block copolymer which elutes in the size exclusion chromatography (SEC) regime under these conditions. The PDMS block length and PS homopolymer can be obtained under normal phase conditions. The results of LCCC are compared with data from conventional SEC. magnified image

Separation Analysis of Polyisoprenes Regarding Microstructure by Online LCCC-NMR and SEC-NMR

It is shown that online coupling of liquid chromatography at critical conditions (LCCC) and 1H NMR spectroscopy can be used to separate and analyze polyisoprenes with respect to their isomeric microstructures. It was possible to separate blends of copolymers consisting predominantly of 3,4- and 1,4-isoprene units by using critical conditions of 1,4-PI. Critical conditions of 1,4-PI were established such that polymers containing predominantly 3,4-PI elute in the size exclusion mode. Furthermore, on-flow NMR detection allows for the complete eluate analysis and the detection of all three isomeric species, such as 1,2-, 1,4-, and 3,4-isoprene, in the eluting fractions. These three individual moieties were correctly quantified in the individual blend components. It was also found that the 3,4-PI samples were random terpolymers of 1,2-, 1,4-, and 3,4-isoprene, whereas the 1,4-PI samples were copolymers of 1,4- and 3,4-isoprene. In addition, SEC-NMR allowed for a fast and precise molar mass calibration and the calculation of the relevant molar mass parameters Mw and Mn.

Analysis of polyethyleneoxide macromonomers by liquid chromatography along the critical adsorption line

Polyoxyethylene macromonomers are analyzed by one-dimensional liquid chromatography under different conditions, depending on the required information. These samples may contain polyethylene glycol (PEG) and the corresponding di(meth)acrylate besides the desired mono(meth)acrylate. The molar mass distribution (MMD) of the PEG and the monoester can be obtained by liquid adsorption chromatography (LAC) on a reversed-phase column in acetone-water with a gradient from 10% to 20% acetone. The MMD of the diesters can be obtained with isocratic elution by liquid chromatography at critical conditions (LCCC) on a reversed-phase column in 31% acetone, or using size-exclusion conditions for PEG and LAC conditions for the end groups, which is the case in 40-55% acetone. The absolute amount of the series with different functionality can be obtained by LCCC in ternary mobile phases consisting of acetone, methanol, and water along the critical adsorption line. Under such conditions, all series elute as narrow peaks (regardless their MMD), which can easily be integrated and quantified.

Coupling of NMR and Liquid Chromatography at Critical Conditions: A New Tool for the Block Length and Microstructure Analysis of Block Copolymers

The online coupling of liquid chromatography at critical conditions (LCCC) and NMR was used for the analysis of block copolymers. Polyisoprene-b-poly(methyl methacrylate) (PI-b-PMMA) copolymers synthesized by living anionic polymerization were separated regarding chemical composition byLCCCusingsinglesolventsasmobilephases.Theanalysis ofthechemically differentLCCCfractionswas conducted by on-flow 1 H NMR. To separate the copolymers from the respective homopolymers, critical conditions of polyisoprene (PI) and poly(methyl methacrylate) (PMMA) were used. Critical conditions were obtained by varying the column temperature. The comprehensive chemical composition and microstructure analysis by 1 H NMR revealed that quantitative information on the PI/PMMA content as well as the microstructure of PMMA (isotactic, atactic, and syndiotactic triads) and PI (1,2-PI, 1,4-PI, 3,4-PI) was obtained.For thefirst time thedetermination ofthetruemolarmasses and truechemicalcompositions ofthe block copolymers was achieved by determining the microstructure distribution.