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Chromatographic Investigations of Macromolecules in the Critical Range of Liquid Chromatography, 14. Analysis of Miktoarm Star (μ‐Star) Polymers
Abstract
Miktoarm (μ-star) polymers are star-shaped block copolymers where chemically different homopolymers constitute the different arms. The quantitative determination of different segments of the star polymers by liquid chromatography under critical conditions (LC-CC) is described. In the present study tri-arm and tetra-arm model polymers with polystyrene, polybutadiene, polyisoprene and poly--methyl styrene arms are analyzed. Operating at the respective critical conditions, homopolymer arms are made chromatographically invisible. Under these conditions the residual (chromatographically visible) arms can be analyzed with regard to their molar masses. The accuracy of the quantification depends on the fit of the calibration procedure and the detector response to the actual chemical compositions. The results clearly indicate that LC-CC yields valuable structural information on block copolymers with complex architectures.
... Nevertheless, the critical condition has been successfully employed for the chromatographic separation of the components in polymer blends [27][28][29][30][31][32][33], and for the separation of polymers with respect to the functional groups [33][34][35][36][37][38][39][40][41] as well as for the characterization of individual block of block copolymers [22,35,[42][43][44][45][46][47][48][49]. This technique is variously termed as liquid chromatography at the critical condition (LCCC), liquid chromatography at the point of exclusion-adsorption transition (LC-PEAT), or liquid chromatography at the critical adsorption point (LC-CAP). ...
... Most of the LCCC applications to the characterization of block copolymers have been made in this mode and the determination of the molecular weight distribution of the "visible" block has been made using the [131,132]. Subsequently Pasch and coworkers have done extensive works on the LCCC characterization of various diblock copolymers [10,11,49,[133][134][135]. ...
Synthetic polymers are
rarely homogeneous chemical species but have multivariate distributions
in molecular weight, chemical composition, chain architecture, functionality,
and so on. For a precise characterization of synthetic polymers,
all the distributions need to be determined, which is a difficult
task, if not impossible. Fortunately in most of the cases it is
sufficient to analyze a limited number of molecular characteristics
in order to obtain the information required for a given purpose.
Nonetheless, it is still nontrivial if there exists distributions
for more than one molecular characteristic. There have been continuing efforts
to solve the problem. One approach is to find chromatographic methods
sensitive to one molecular characteristic only. In favorable cases,
the effect of all but one molecular characteristic can be suppressed
to a negligible level. Various interaction chromatographic techniques
as well as size exclusion chromatography are employed for the purpose.
Also the multiple detection methods each sensitive to a specific
molecular characteristic can provide additional information. Various
detection methods developed recently such as FT-IR, FT-NMR, and
mass spectrometry brought about significant progress in the characterization
of complex polymers. This review presents the recent developments
in the analysis of various heterogeneities in synthetic polymers
by a variety of liquid chromatographic separation as well as detection
methods.
... Individual block size analysis at the critical condition of the other block is usually carried out in the SEC mode because the IC retention would be too large under the isocratic LCCC elution condition. After the development of LCCC theory for the separation of diblock copolymer by Gorbunov and Skvortsov [166], MW determination of the "visible" block in a block copolymer eluting in the SEC mode at the CAP of the "invisible" block has been used widely [33,[167][168][169][170][171][172][173][174]. The MWD of the "visible" block could be determined using a SEC-type calibration curve made with homo-polymers of the "visible" block. ...
While mother nature can produce uniform macromolecules as found in many biological systems, man‐made polymers are hardly homogeneous. Synthetic polymers are usually complex mixtures of molecules differing in various molecular characteristics such as molecular weight (MW), chemical composition, functionality, microstructure, and chain architecture. For precise characterization of polymers, the multivariate distributions in all these molecular characteristics need to be measured, which is an extremely difficult task, if not impossible. In practice, however, it is often sufficient to analyze a limited number of molecular characteristics. For the purpose, liquid chromatography is the most widely used method that has made a remarkable progress during the last several decades in both instrumentation and understanding of the separation mechanism. Size exclusion chromatography (SEC) has been the most widely employed chromatographic separation technique for the characterization of synthetic polymers since its inception about a half‐century ago. Compared to traditional fractionation methods of polymers, SEC is an excellent method in precision, analysis speed, required amount of a sample, etc. Nonetheless, SEC separates the polymer molecules according to the chain size that depends on not only MW but also other molecular characteristics such as composition, chain architecture, and microstructure. Therefore, for instance, SEC cannot separate nonlinear homo‐polymers according to MW or copolymers according to the composition. In contrast to SEC in which the different pore‐accessibility of macromolecules is the main separation mechanism, interaction chromatography (IC) fractionates polymers according to the interaction strength between the polymer molecules and the sorbent. In results, IC is more sensitive to the chemical nature of the polymers than SEC. By judiciously selecting the IC separation condition, it is possible to separate polymers according to specific molecular properties with higher resolution than SEC. Furthermore, one can access to the chromatographic critical condition for a given polymer system by adjusting the solute interaction strength with the sorbent until the exclusion and interaction contributions to the solute retention are exactly compensated. At the critical condition, polymers of different MW coelute. Therefore, LC at the critical condition (LCCC) can be used as a unique tool for the characterization of polymers since one can suppress the peak dispersion by MW distribution and focus on other molecular characteristics. Recently, polymer separations using two‐dimensional LC by a rational combination of SEC, IC, or LCCC have become more and more popular for the characterization of polymers with multivariate distributions. In this article, chromatographic separation methods of synthetic polymers are reviewed with an emphasis on non‐SEC methods.
... Despite all the abovementioned caveats and peculiarities, LCCC-SEC is still an excellent and the only available technique for fair estimation of individual block length of the block copolymers [3-5, 8, 9]. Further applications of LCCC-SEC include analysis of star-shaped copolymers [33][34][35][36]. ...
Liquid chromatography at critical conditions is one of the major chromatographic techniques for characterization of complex polymers beyond mere total molar mass. In this perspective, different approaches of analyses using critical conditions are elaborated. Strengths, opportunities, and caveats associated with different LCCC approaches are discussed. Finally, the future challenges and opportunities are highlighted. © 2021, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.
... Individual block size analysis at the critical condition of the other block is usually carried out in the LCCC-SEC mode because the IC retention time could become too long under the isocratic LCCC elution condition. After the development of LCCC theory for the separation of diblock copolymer by Gorbunov and Skvortsov [139], MW determination of the "visible" block in a block copolymer eluting in the SEC mode at the CAP of the "invisible" block has been used widely [140][141][142][143][144][145][146][147][148]. The MW distribution of the "visible" block could be determined using a SEC calibration curve made with the homopolymers of the "visible" block. ...
Size exclusion chromatography (SEC) is the most widely employed chromatographic separation technique for characterization of synthetic polymers. SEC separates polymer molecules according to their chain size utilizing size-dependent pore accessibility of polymer molecules. Since the polymer chain size depends on not only molecular weight (MW) but also other molecular characteristics such as composition, chain architecture, and microstructure, SEC cannot separate, for instance, nonlinear homopolymers according to MW or copolymers according to their composition.
In contrast to SEC, interaction chromatography (IC) fractionates polymers according to the interaction strength between the polymer molecules and the sorbent. As a result, IC separation is sensitive to chemical nature and MW of polymers unlike the size-sensitive SEC. Furthermore, one can access to the chromatographic critical condition by adjusting the solute interaction strength with the sorbent until the exclusion and interaction contributions to the solute retention are exactly compensated. At the critical condition, polymers of different MW coelute. Therefore liquid chromatography (LC) at the critical condition (LCCC) is a unique tool for LC separation of polymers since the peak dispersion of polymers due to their MW distribution can be suppressed for effective analyses of other molecular characteristics. Recently, polymer separations using two-dimensional LC by various combinations of SEC, IC, or LCCC are increasingly used for the characterization of polymers with multivariate distributions.
For effective IC separations, solute interaction strength needs to be controlled judiciously during the elution. For this purpose, either solvent gradient or temperature gradient elution is often used. In this chapter, principles and applications of IC and LCCC separation methods of synthetic polymers are reviewed with an emphasis on the temperature gradient elution method.
... For example, LCCC has been applied for separations of block copolymers [3][4][5][6][7][8][9][10][11], to deter- mine the average molar mass of constituent blocks in di-and tri-block copolymer (type A-B and A-B-A) [12,13] as well as for selective separations of end-functionalized polymers according to their functional groups [2,3,14,15]. Several groups have described successful separations of macromolecules according to their archi- tecture (for example, linear from star shaped or linear from rings) [16][17][18][19] or their tacticity [20][21][22][23][24][25] with LCCC. ...
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.
... Irrespective of these challenges, LCCC is an excellent technique for the analysis of individual blocks in block copolymers [22,[27][28][29][30]32,34,41,[43][44][45][46][47][48][49][50]66,105,205,256,[260][261][262][263][264][265][266][267][268][269][270][271][272]. LCCC has also been used for analysis of miktoarm star polymers [273]. The second option in LCCC is to obtain separation with respect to the non-critical segment of the segmented copolymer in an interaction regime being a viable option, however, only for low molar mass samples. ...
Controlled radical polymerization (CRP) provides the polymer chemist with the ability to produce tailor-made polymers with controlled molar masses, molar mass distributions, chemical compositions and macromolecular architectures. Segmented copolymers can be synthesized having polymer segments arranged in a linear fashion (linear block copolymers), however, polymer segments can also be attached to pre-synthesized macromolecules or to multifunctional core molecules to produce branched (graft) copolymers, polymer stars or dendrimers. Although there are many ways to control the chain growth and the architecture of the target macromolecules, side reactions cannot be completely avoided. Accordingly, even with CRP, obtained products exhibit chemical composition and topology distributions along with the molar mass distribution.
In this review article, recent developments in multidimensional characterization of segmented copolymers are presented. Liquid chromatography (LC) is the most widely used and versatile polymer fractionation technique to address different aspects of copolymer heterogeneity. The potential and limitations of different modes of liquid chromatography of polymers (size exclusion chromatography, different modes of interaction chromatography) and spectroscopic techniques (FTIR, NMR, and MALDI-TOF-MS) are elaborated. Possible method combinations including comprehensive two-dimensional LC and the hyphenation of various modes of LC with spectroscopic techniques are discussed. Advantages and limitations of various off-line and on-line approaches to method hyphenation are highlighted. Examples from recent literature with special focus on segmented copolymers (e.g. block, graft, miktoarm, multibranched or star copolymers) will be reviewed.
... Diblock copolymers can be separated under conditions where one block elutes irrespective of molar mass at LCCC conditions (the "invisible" block) while the other block elutes at SEC conditions (the "visible" block). If suitable detection and calibration procedures are used the molar mass of the "visible" block can be quantified in the diblock copolymer [17,[65][66][67][68]. Other applications of LCCC include the separation of homo-and copolymers according to functional endgroups. ...
The present review is the latest and most comprehensive summary on advanced developments in the field of LC-NMR of polymers. Different from LC-NMR applications in biochemistry, pharmaceutics and other fields that target at the identification and quantitative determination of specific molecules, LC-NMR of polymers is aimed at the analysis of the molecular heterogeneity of complex macromolecular mixtures. Using LC-NMR the distributions in molar mass, chemical composition and molecular microstructure can be addressed. Although LC-NMR is the most expensive version of LC-spectroscopy couplings, it provides a wealth of qualitative and quantitative information that cannot be matched by any other coupling technique. The review presents the latest results on the coupling of NMR with SEC, HPLC, 2D-LC and field-flow fractionation (FFF) for the analysis of oligomers, copolymers and polymer blends. The given examples clearly prove that LC-NMR of polymers has matured and is now a powerful tool in advanced analytical polymer science.
... LCCC has been shown to separate cyclic and linear homopolymers more efficiently than SEC [20e22]. Furthermore, critical conditions were applied by Pasch et al. to characterize miktoarm (m) stars [23]. ...
Abstract In the last decade, the use of interaction chromatography and hyphenated techniques has become increasingly important for the characterization of polymeric materials. Interaction chromatography allows separation by other structural features than molar mass, while hyphenation with mass spectroscopy or spectroscopic techniques provides detailed characterization of the separated chromatographic fractions. This chapter gives an overview of the principles and applications of interaction chromatography and the information that can be determined by hyphenation of polymer chromatography with mass spectrometry and spectroscopic techniques.
The chapter provides a comprehensive description of possibilities of coupling of different modes of polymer LC. Approaches of two-dimensional analysis are followed by description of the experimental setup for such hyphenations. The application section is broadly divided into two subsections based on the technique used for 2nd dimension analysis such as SEC and IC. The two subsections are further divided into several segments based on the technique used for the first dimensional analysis. Finally, limitations and opportunities of using different approaches of 2d-LC in polymer analysis are critically discussed.
The most important method of polymer LC based on entropy/enthalpy compensation namely LCCC is comprehensively discussed in this chapter. The detailed discussion focuses on the establishment of the critical conditions, their sensitivity, the working principle, and their recent applications for analysis of complex polymers. The application section is broadly divided into two subsections namely LCCC-SEC and LCCC-IC which are further divided into numerous segments based on the classifications of the analyzed polymer samples. Finally, limitations and opportunities of using LCCC in polymer analysis are critically discussed.
This chapter describes the practical applications of vibrational spectroscopy, both infrared absorption and Raman scattering, to characterize the molecular structure of polymers. The features of crystalline and amorphous chains have been summarized. There are many techniques to measure the molecular structure of the ordered or crystalline phase. However, vibrational spectroscopy serves a specialized role to describe the structure of disordered chains. The use of spectroscopy to measure crystallite size is also shown. Often polymers constrained to surfaces are difficult to characterize because of the limited signals that can be obtained. Specialized techniques are described here. Aging of polymers as a function of time, temperature, and especially in the presence of humidity can influence the physical properties severely and often in an unexpected fashion. In this case, vibrational spectroscopy can provide possible molecular mechanisms responsible for the macroscopic physical property changes.
The chapter covers theoretical background and basic concept of chromatography of polymers encompassing size exclusion and liquid interaction chromatography. Multidimensional approaches by hyphenation of different modes of HPLC of polymers and chromatographic separations with spectroscopic techniques are discussed with emphasis on instrumentation, technical details, and possible information that can be acquired. The chapter is more of a tutorial nature intended to guide new workers in the field. Recent advances and applications are highlighted, with focus on merits and limitations of various chromatographic techniques and their hyphenations, to update more experienced practitioners.
Recent progress in modern polymer synthesis techniques have led to the design of complex functional materials, which can be difficult to analyse accurately. While size-exclusion chromatography (SEC) or mass spectrometry (MS) are typically used to gain information about molecular weight distribution, chemical structure and molecular architecture, there is a lack of available method for characterising compositional heterogeneity (i.e. monomer distribution). In contrast with SEC in which separation occurs by hydrodynamic volume, interaction-based chromatography (IC) separates compounds according to their affinity for a stationary phase, which has proven useful on gaining information about the general chemical structure of copolymers in the past. Here, we explore the potential of reverse-phase high pressure liquid chromatography (RP-HPLC) as a tool for the characterisation of monomer segmentation in charged water-soluble copolymers. A library of acrylamide copolymeric systems, prepared via reversible addition-fragmentation chain transfer (RAFT) polymerisation, is used to demonstrate the influence of monomer distribution (diblock, multiblock and statistical) on the elution time. The robustness of the method is tested by studying a range of copolymers with varying charge, charge content and hydrophobicity, as well as by using various solvent systems or column lengths. Results highlight the efficiency of RP-HPLC to separate copolymers with varying segmentation, with a limitation observed for branched architecture.
Interaction chromatography is based on the retention of solute molecules by interaction with the surface of the stationary phase including the pore surface. This interaction can be due to adsorption, hydrophobic, polar or ionic interactions or dispersive forces. Intermittent capture and release of solute molecules by the stationary phase are controlled by two basically different mechanisms or some combinations thereof. In regard to adsorption-desorption phenomena, an abrupt process is the critical step leading to sorption or desorption. This process is typified by molecular desorption from surfaces where molecules can detach, and then do so suddenly, if they possess sufficient activation energy to cause the necessary rearrangement or rupture of chemical or physical bonding. Quite different in effect are the diffusion-controlled sorption-desorption kinetics where a change occurs only gradually as molecules diffuse in and out of localized regions [1].
Complex polymers are distributed in more than one direction of molecular heterogeneity. In addition to the molar mass distribution (MMD), they are frequently distributed with respect to chemical composition, functionality, and molecular architecture. Liquid chromatography (LC) is one of the workhorses for the analysis of complex polymer distributions.This chapter presents the principal ideas of LC of polymers. In addition to classical size-exclusion chromatography (SEC), the most important methods of interaction polymer chromatography, comprehensive two-dimensional liquid chromatography (2D-LC), and the hyphenation of LC with spectroscopic techniques are addressed and the high selectivity of these methods regarding chemical composition and functionality is highlighted.
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.
http://onlinelibrary.wiley.com/doi/10.1002/macp.201500303/abstract
Two series of 3-12 multiarm star polymers of butadiene and styrene (S-PB and S-PS) with high Mn of arm (≥ 20 kg/mol) and narrow PDI (≤1.04) were synthesized via click chemistry between azide-terminated polymers and multialkynyl organic molecules. Comparing to the previous report, higher yield of coupling reaction (≥ 85%) was obtained along with the increasing of arm number and molecular weight of single-arm polymer (over 4-5 times than before). Specially, 96.1%, as the highest efficiency, occurred in the synthesis of 12-arm star PB. 4-Miktoarm star copolymer of butadiene and styrene was also synthesized with high yield (95.1%), high arm Mn and narrow PDI (1.04) by one pot synthesis of click reaction between two different linear polymers (PS-N3 and PB-N3) and 4-arm core. All the star polymers were characterized by GPC-MALLS-Viscosity-DRI. These multiarm star polymers exhibit morphologies from random coil to hard sphere depending on the arm-number of star polymers. Intrinsic viscosity appeared the maximum with increasing arm numbers in star PB and PS, in which PB-4arm and PS-6arm had the highest values, respectively.
Complex polymers are distributed in more than one direction of molecular heterogeneity. In addition to the molar mass distribution, they are frequently distributed with respect to chemical composition, functionality, and molecular architecture. For the characterization of the different types of molecular heterogeneity it is necessary to use a wide range of analytical techniques. Preferably, these techniques should be selective towards a specific type of heterogeneity. The combination of two selective analytical techniques is assumed to yield a two-dimensional information on the molecular heterogeneity. For the analysis of complex polymers different liquid chromatographic techniques have been developed, including size exclusion chromatography (SEC) separating with respect to hydrodynamic volume, and liquid adsorption chromatography (LAC) which is used to separate according to chemical composition. Liquid chromatography at the critical point of adsorption (LC-CC) has been shown to be a versatile method for the determination of the functionality type distribution of macromonomers, the molecular architecture of homopolymers and the chemical heterogeneity of block and graft copolymers.The present paper presents the principle ideas of combining different analytical techniques in multidimensional analysis schemes for the analysis of polymers with complex architectures. Branched block and graft copolymers can efficiently be analyzed with respect to chemical composition and molar mass by LC-CC and two-dimensional chromatography. The chemical heterogeneity as a function of molar mass can be determined by combining interaction chromatography and FTIR spectroscopy. For the analysis of star-like polymers LC-CC is shown to be a powerful technique when the molar mass of different segments (blocks, grafts) must be determined.
We analyzed various miktoarm star copolymers of the PS(PI)x type (x = 2, 3, 5, 7), which consist of one long polystyrene (PS) arm (82 or 105 kDa) and various numbers of short polyisoprene (PI) arms (from 11.3 to 39.7 kDa), prepared by anionic polymerization and selective chlorosilane chemistry. The length of the PI arm in stars decreases with the number of arms, so that the chemical compositions of all PS(PI)x samples were comparable. Our aim was to determine the purity of samples and to identify exactly the constituents of individual samples. For this purpose we used a variety of separation techniques (size-exclusion chromatography (SEC), reversed-phase liquid-adsorption chromatography (RP-LAC), and two-dimensional liquid chromatography (2D-LC)) and characterization techniques (UV-MALS-RI multidetection SEC system, NMR, and MALDI-TOF MS). The best separation and identification of the samples’ constituents were achieved by RP-LAC, which separates macromolecules according to their chemical composition, and a subsequent analysis of the off-line collected fractions from the RP-C18 column by SEC/UV-MALS-RI multidetection system. The results showed that all PS(PI)x samples contained the homo-PS and homo-PI in minor amounts and the high-molar-mass (PS)y(PI)z (y > 1) species, the content of which is higher in the samples PS(PI)5 and PS(PI)7 than in the samples PS(PI)2 and PS(PI)3. The major constituent of the PS(PI)2 sample was the one with the predicted structure. On the other hand, the major components of the PS(PI)x (x = 3, 5, and 7) samples were the stars consisting of a smaller number of PI arms than predicted from the functionalities of chlorosilane coupling agents. These results are in agreement with the average chemical composition of samples determined by proton NMR spectroscopy and characterization of the constituents by MALDI-TOF MS.
Over the last two decades the evaporative light scattering detector (ELSD) has found more and more use in liquid chromatography (LC) of synthetic polymers. The reason behind this is that it can be used for a significantly wider variety of analyte/solvent combinations. Although in many of the applications the ELSD has been used in a qualitative manner, it can also be used quantitatively. For quantitative interpretation of analyses it is, in the case of synthetic polymers, essential to know how parameters, which characterize a polymer sample (i.e., molar mass and chemical composition), as well as parameters, which are a consequence of the LC separation (i.e., composition and flow rate of the mobile phase, its composition), influence the response of the ELSD. This review gives a tabulated overview over applications of ELS detectors in polymer analysis. The influence of parameters arising from either the polymer side or the chromatographic separation is discussed in detail and, in addition, the influence of the ELS detector's running conditions, i.e. type and flow rate of gas and temperature of nebulizer and evaporator), will be reviewed. This information will prove valuable whenever the calibration of an ELSD for the quantitative analysis of synthetic polymers is attempted.
This review presents the precise syntheses of a variety of new chain-end- and in-chain-multi-functionalized polymers, regular and asymmetric star-branched polymers, star-linear block polymers, densely branched and graft copolymers carrying two branches in each repeating unit, and dendritic hyperbranched and star-branched polymers. These specialty polymers in terms of function and branched architecture are synthesized by recently developed methods using functionalized 1,1-diphenylethylene derivatives in conjunction with living anionic polymers based on the ‘iterative’ concept. In iterative methods, the reaction sites are regenerated after each iterative reaction sequence, and the same reactions may be repeated to afford successively and, in principle unlimitedly, the objective polymers until a steric limitation is encountered. Indeed, the above-mentioned polymers have been successively synthesized by the iterative methods. The scope, limitations, and possibilities for iterative methods will be discussed.
An efficient and reliable size exclusion chromatographic (SEC) method has been developed allowing satisfactory insight into distribution of both molecular weight of polybutylacrylate-b-polyvinylpyridine (PBA-b-PVP) block copolymers as well as the relative amount of polyvinylpyridine (PVP) versus polybutylacrylate (PBA) along the molecular weight axis. This aim was accomplished by SEC using a rather new polyester-based GRAM stationary phase support of intermediate polarity and a polar eluent system of N,N-dimethylacetamide containing small amounts of lithium bromide and acetic acid for extensive suppression of adsorptive solute-stationary phase interactions.
Star-shaped poly(lactide)s (PLAs) with various numbers of secondary OH-groups at the PLA's arm ends and primary OH-groups attached directly to the core of the star were analyzed under critical conditions determined for linear PLAs. The same set of PLAs was then analyzed after blocking the core OH-groups with benzyl moieties. Depending on the number and the placement of the OH-groups in the star polymer different elution behavior was observed. As expected, on a ‘normal’ phase column, increasing the number of functional OH-groups results in a stronger interaction with the stationary phase and therefore in a larger retention volume. Keeping the number of functional groups constant, but varying the number of arms leads to a minimum in retention volume for the four arm star-shaped PLAs. The elution behavior could be discussed quantitatively on the basis of theories for polymer chromatography for large and small pores. It was possible to derive the parameters necessary to quantitatively fit the experimental results. The model for large pores and the model for small pores result in equally good descriptions of the experimental data.
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.
While nature is capable of producing uniform macromolecules as found in complex biosystems, man-made polymers are hardly homogeneous. They exhibit inherent distributions at least in molecular weight, and often come with multivariate distributions in chemical composition, functionality microstructure, chain architecture, and so on. Size-exclusion chromatography (SEC) has been the most widely employed chromatographic technique for the characterization of molecular weight and its distribution of synthetic polymers since its inception about half a century ago. Compared to conventional molecular weight characterization methods, SEC is an excellent method with regard to precision, analysis speed, required amount of samples, etc. Nonetheless, SEC separates the polymer molecules in terms of their chain size, which is not only a function of molecular weight but also depends on other molecular characteristics such as composition, branching, and microstructure. Therefore, SEC cannot characterize nonlinear polymers or copolymers effectively. In contrast to SEC, in which the different pore accessibility of the macromolecules is the major separation mechanism, interaction chromatography (IC) fractionates the polymer sample by enthalpic interaction between the polymer solutes and the stationary phase. As a result, IC is sensitive to the chemical nature as well as the molecular weight of the polymers, whereas it is less sensitive to long-chain branching than SEC. By judiciously selecting the IC separation condition, one can characterize specific molecular properties of polymers with much higher resolution than SEC. Furthermore, one can access the chromatographic critical condition (CC) for a given polymer system by adjusting the solute interaction strength with the stationary phase until the exclusion and interaction contributions to the solute retention are exactly compensated. Liquid chromatography at the critical condition (LCCC) can be used as a unique tool for the characterization of polymers. Of late, 2D liquid chromatography (LC) separations by a rational combination of SEC, IC, and LCCC has become more and more popular for the characterization of multivariate polymers. In this article, recent advances in non-SEC analysis methods of a variety of synthetic polymers are reviewed.
Keywords:
interaction chromatography;
chromatographic critical condition;
two-dimensional chromatography;
polymer characterization
The structure of polyacetal copolymers with respect to their end groups is investigated. Several techniques, such as liquid chromatography, MALDI‐TOF MS, IR and NMR spectroscopy, as well as combined techniques like 2D chromatography have been applied. Cyclic oligomers and polymers with formyl, hydroxy, and aliphatic end groups are identified. 2D chromatography shows that all the separated species are homogenously distributed with respect to their molar mass.
2D chromatograms of sample F97B3.
magnified image 2D chromatograms of sample F97B3.
Most synthetic polymers are distributed in more than one parameter of molecular heterogeneity. For hydrophobic copolymers there are different chromatographic techniques available to analyse these distributions. As a result of the increasing interest in hydrophilic polymers and copolymers new chromatographic techniques are developed for the characterization of these polymers as well. However, very frequently these polymers contain highly polar or charged functional groups making them soluble only in aqueous mobile phases. There are several problems related to the use of aqueous mobile phases in polymer chromatography. Even the SEC analysis of such copolymers is not straightforward. As for HPLC in aqueous mobile phases, there are only a few applications in the literature so far. In addition to the fact that only a very limited number of stationary phases is available for aqueous HPLC of polymers, the interactions of polyelectrolytes in such chromatographic systems are not well understood.
The present paper addresses the problems related to the application of SEC and HPLC in aqueous mobile phases. For graft copolymers with a polyethylene oxide backbone, e.g. PEG-g-polymethacrylic acid and PEG-g-polyvinyl alcohol, it will be shown that methods can be developed that give accurate molar mass and chemical composition information. Two-dimensional chromatography where aqueous HPLC and SEC are coupled on-line will be shown to be the most powerful analysis tool for the analysis of such copolymers. The hyphenation of the chromatographic separation techniques with spectroscopic detection techniques provides further insight into the molecular complexity of these copolymers.
Complex polymers are distributed in more than one direction of molecular heterogeneity. In addition to the molar mass distribution, they are frequently distributed with respect to chemical composition, functionality, and molecular heterogeneity. One approach for the analysis of the heterogeneity of complex polymers is their chromatographic separation by combining different separation mechanisms. A typical experimental protocol includes the separation of the sample according to composition to yield fractions that are chemically homogeneous. These fractions are transferred to a size‐selective separation method and analyzed with respect to molar mass. As a result of this two‐dimensional (2D) separation, information on both types of molecular heterogeneity is obtained. So far, 2D chromatography has been applied mostly to polymers that are soluble in organic solvents. There are several problems related to the use of aqueous mobile phases in polymer chromatography. These problems relate to the very polar or ionic character of the polymers and the experimental conditions, including the use of salt‐containing eluents. The present paper addresses the different parameters that influence the chromatographic experiments. For a model polymer system resulting from the grafting of methacrylic acid (MAA) onto poly(ethylene glycol) (PEG), i.e., PEG‐ g ‐PMAA, it will be shown that different chromatographic techniques including SEC, LC‐CC, and 2D chromatography, as well as coupled LC‐CC/FT‐IR can be used to analyze the molecular complexity of the copolymers.
LC‐CC/FT‐IR spectra of a PEG‐ g ‐PMAA sample as function of the elution volume.
image LC‐CC/FT‐IR spectra of a PEG‐ g ‐PMAA sample as function of the elution volume.
The chromatographic analysis of hydrophilic copolymers is complicated due to the fact that in most cases aqueous eluents must be used. In aqueous eluents different polar and ionic effects may disturb the selective interactions between the macromolecules and the stationary phase making it impossible to separate such copolymers with regard to chemical composition. Therefore, 2D chromatography combining a separation according to composition with a separation according to molar mass has been applied mostly to polymers that are soluble in organic solvents. The present contribution describes experimental approaches to analyze such hydrophilic copolymers by 2D‐chromatography. For a model polymer system resulting from the copolymerization of methacrylic acid and a poly(ethylene glycol) macromonomer, it is shown that different analytical techniques including SEC, LC‐CC, MALDI‐TOF MS and 2D chromatography can be used to analyze the different parameters of molecular heterogeneity of such copolymers.
2D separation of poly(MPEG‐MM 2), 1 st dimension: LC‐CC, 2 nd dimension: SEC.
magnified image 2D separation of poly(MPEG‐MM 2), 1 st dimension: LC‐CC, 2 nd dimension: SEC.
Liquid chromatography (LC) is a powerful tool for the characterization of synthetic polymers, that are inherently heterogeneous in molecular weight, chain architecture, chemical composition, and microstructure. Of different versions of the LC methods, size exclusion chromatography (SEC) is most commonly used for the molecular weight distribution analysis. SEC separates the polymer molecules according to the size of a polymer chain, a well-defined function of molecular weight for linear homopolymers. The same, however, cannot be said of nonlinear polymers or copolymers. Hence, SEC is ill suited for and inefficient in separating the molecules in terms of chemical heterogeneity, such as differences in chemical composition of copolymers, tacticity, and functionality. For these purposes, another chromatographic method called interaction chromatography (IC) is found as a better tool because its separation mechanism is sensitive to the chemical nature of the molecules. The IC separation utilizes the enthalpic interactions to vary adsorption or partition of solute molecules to the stationary phase. Thus, it is used to separate polymers in terms of their chemical composition distribution or functionality. Further, the IC method has been shown to give rise to much higher resolution over SEC in separating polymers by molecular weight. We present here our recent progress in polymer characterization with this method. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1591-1607, 2005
Blends of polystyrene (PS) and polyisoprene (PI) were analysed by on-line hyphenation of LC at critical conditions and (1) H-NMR. Chromatography at critical conditions was established for both PS and PI. At both critical conditions, a perfect separation into the blend components was achieved. By operating at critical conditions of one blend component and size exclusion mode for the other it is possible to determine the molar mass using a suitable calibration. By using NMR as a detector, the microstructure of PI can be identified, quantified and the chemical composition of the blends can be calculated by monitoring the signal intensities of the olefinic protons of isoprene and the aromatic protons of PS.
Structural information on humic acids is difficult to obtain because of the heterogeneity of the acids. Herein liquid chromatography at the critical condition, LCCC, is used to provide a sorting mechanism for the diverse types of molecules contained in humic acids. The critical condition of polymers that are believed to model some subunit of the humic acid is determined. Humic acids from three different terrestrial sources (soil, compost, and peat) are then separated under these chromatographic conditions. The portion of the humic acid that has structure similar to that of the model polymer elutes at the retention volume of the critical condition of the model. Next, fractions are collected and further characterized. This detailed characterization includes high-efficiency size-exclusion chromatography and electrospray mass spectrometry. The size-exclusion chromatograms of the fractions were found to be markedly different from that of the original humic acid sample. This is strong evidence that the LCCC separation mechanism is different from size fractionation. The mass spectra of the humic acid fractions were also markedly different from those of the bulk humic acids previously reported. The mass spectra of specific fractions collected had repeating clusters of m/z values, which is more evidence that the critical condition separation is a powerful sort function.
A characteristic feature of synthetic polymers is their dispersity in molar mass and, in many cases, chemical composition. Since dispersity is highly relevant in relation to polymer properties, ongoing efforts are being put in the development of appropriate analysis methods. In this respect, size-exclusion chromatography (SEC) is well known for the determination of molar mass distributions. Methods for chemical composition distributions are less mature than SEC and mainly include liquid chromatography and mass spectrometry and the combination of these techniques. The term chemical composition distribution is considered broad in this paper, i.e. for the chemical composition distribution of a (co)polymer backbone, for the functionality type distribution of a polymers' functional end groups, for the block length distribution of a block copolymer, for the branching distribution and for the tacticity distribution. In this paper, analysis methods for all types of chemical composition distributions are reviewed. Special attention is paid to practical requirements and common misconceptions that sometimes arise. Applications within the last 5 years are summarized.
Equations for the distribution coefficient of heteroarm stars are derived by using a model of an ideal chain in a slit-like pore; these equations together with those previously reported for linear block-copolymers are applied to describe chromatography of such copolymers. According to the theory, the retention generally depends on molar mass, composition, and architecture (microstructure and topology) of copolymers, on pore size and on adsorption interaction of chain units A and B. Three special modes of chromatography are studied in detail. (i) If interactions for A and B are close to the critical point of adsorption (CPA), the retention practically does not depend on architecture, and high molar mass copolymers can be separated by composition. (ii) At SEC condition for B and strong adsorption for A, copolymers in principle can be separated by architecture; better separation is expected in wide pores. Retention of linear block-copolymers decreases with increasing of the number of blocks; for heteroarm stars the theory predicts retention decreasing as: AB > StarAAB > StarABB; StarAAAB > StarABBB > StarAABB; StarAAAAB > StarABBBB > StarAAABB > StarAABBB. (iii) At the CPA for B copolymers AB, BAB and heteroarm stars regardless molar mass of B, M(B), can be separated by M(A). The same is true for ABA and ABAB...A in narrow pores. While the retention of AB, BAB, Star AB...B and StarAAB...B is the same, copolymers AB, ABA and linear multiblock-copolymers can be separated, as well as symmetric and very asymmetric triblock-copolymers ABA.
Two-dimensional polymer characterization is used for a simultaneous analysis of molar masses and chemical heterogeneities (e.g., end groups, copolymer composition, etc.). This principle is based on coupling of two different chromatographic modes. Liquid adsorption chromatography at critical conditions (LACCC) is applied for a separation according to the chemical heterogeneity, whereas in the second-dimension fractions are analyzed with regard to their molar mass distribution by means of size exclusion chromatography (SEC). Because appropriate standards for a calibration of the SEC are seldom available, matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI-TOF MS) was used to substitute the SEC. The LACCC-MALDI MS coupling enables acquiring additional structural information on copolymer composition, which can considerably enhance the performance of this coupled method.
Three-arm polybutadienes with one, two, and three functional end groups were prepared by anionic polymerization using [3-(dimethylamino)propyl]lithium and sec-butyllithium as initiators and methyltrichlorosilane as linking agent. Characterization carried out on the dimethylamine-capped polymers by size exclusion chromatography (SEC), in THF, low-angle laser light scattering (LALLS) in THF, and membrane osmometry (MO) in toluene indicates a high degree of molecular and structural homogeneity. The 1,2 content, determined by NMR spectroscopy, of the arms having the functional groups was higher (12-32%) than that of the arms without the functional groups (7-10%), owing to the tertiary amine group of the initiator. The dimethylamine end groups were transformed to the highly polar sulfozwitterionic ones by reaction with cyclopropanesultone. Association of the different omega-functionalized star polybutadienes was studied in dilute solutions of cyclohexane by MO and LALLS. It was found that although the dimethylamine-capped polybutadienes do not associate, the corresponding zwitterionic species associate strongly in this solvent. At fixed molecular weight of the arms, the degree of association increases with decreasing number of functional groups and, for the same number of functional groups, with decreasing molecular weight of the arms. The trifunctional species form gels at concentrations needed for MO experiments. Comparison with linear omega-zwitterionic polybutadienes in cyclohexane shows that the stars associate less strongly, maybe due to the sterically hindered star structure (monofunctional species) and to intramolecular interactions (di- and trifunctional species).
A method for the analysis and separation of macromolecules is proposed, based on the effect of the chromatographic “invisibility” of some chain fragments, namely blocks of a certain type in block copolymers, grafted chains in graft copolymers and linear elements of macromolecules with rings. When certain critical conditions for such fragments of polymer molecules are created neither the length nor the molecules weight distribution of these molecules influences the retention volume, and this effect is treated as a chromatographic “invisibility” of the elements involved.Results of the modern theory of chromatography and the direct simulation of the equilibrium behaviour of macromolecules in a chromatographic column have beent aken into account and conclusion is draw that this method of “invisibles” provides an efficient separation of macrocycles and linear polymers and the separation of macromolecules according to the number of functional groups. The method of “invisibles” also makes it possible to separate two-block copolymers according to the size of the “visible” block only, and to separate grafted polymers according to the backbone length. The scope of the application of the method to chromatographic analyses of copolymers is discussed.
The separation of di- and triblock copolymers of styrene and butadiene was accomplished by liquid chromatography at the critical point of adsorption. The size of the polybutadiene blocks was determined under chromatographic conditions corresponding to the critical point of adsorption of polystyrene using silica gel as the adsorbent and tetrahydrofuran-hexane as the eluent. The polystyrene blocks were analyzed on a nonpolar stationary phase and methyl ethyl ketone-cyclohexane as the eluent. using chromatographic conditions, corresponding to the critical point of adsorption of polybutadiene. The deviations of the experimentally determined molar masses from the nominal values were discussed considering the complex structure of the block copolymers. For triblock copolymers with sharp transitions between the blocks, it was demonstrated that the block lengths of the outer, as well as the center blocks, can be determined with high accuracy by liquid chromatography at the critical point of adsorption. For di- and triblock copolymers with tapered transitions, the observed deviations are caused by the random sequences in the macromolecules.
The elution chromatography of flexible polymer molecules flowing through a microporous particle media is described by a combination of the Casassa model of flow segregation and the Di Marzio−Rubin lattice method for calculating the partition function of confined polymers. This combination of models allows for the treatment of polymer-surface interactions so that polymer chromatography in the exclusion and adsorption regimes can be described within a unified framework. The compensation point where repulsive polymer-surface excluded volume forces and short-range polymer-surface attractive forces counterbalance each other offers opportunities for separating complex molecules. For example, calculations for a diblock copolymer where one of the components is at the compensation point (“adsorption ϑ point”) indicate that only the remaining block influences the elution of the block copolymer as a whole. This theoretical result accords with experiments on block copolymers. This singular observation provides support for the Casassa viewpoint of molecular partitioning dominated polymer elution. The chromatography of triblock copolymers, stars, and combs is also examined to determine the selectivity of elution chromatography for separating these molecular architectures. The theoretical development in the present paper should lead to improved methods for the characterization of polymers with different molecular architectures. These developments also suggest new tools for studying polymer adsorption from dilute solution.
The synthesis of well-defined, nearly monodisperse, 4-miktoarm (from the Greek word mulkappatauos meaning mixed) star co- and quaterpolymers of the A2B2 and ABCD types, is described. A is polystyrene, B polybutadiene, C polyisoprene, and D poly(4-methylstyrene). The synthetic approach involves the reaction of tetrachlorosilane with the active chain end centers of the first arm under conditions unfavorable to chain coupling or linking, followed by a stoichiometric addition (titration) of the living polymer of the second or the third arm, and finally by an excess of the living polymer of the last arm. The sequence of addition of the different living polymers and the monitoring of the progress of the linking reaction by size-exclusion chromatography (SEC) are the key for successful synthesis. Characterization was carried out by SEC, low-angle laser light scattering, laser differential refractometry, membrane and vapor pressure osmometry, and NMR spectroscopy.
The synthesis of a well-defined, near monodisperse, star terpolymer with three different arms, i.e., polyisoprene, polystyrene, and polybutadiene, is described. The synthetic approach involves the reaction of polyisoprenyllithium with an excess of methyltrichlorosilane followed, after removal of the excess of methyltrichlorosilane, by a stoichiometric addition (titration) of polystyrillithium and finally by a small excess of polybutadienyllithium. The sequence of the addition of the different living polymers is very critical. After linking, the excess of the polybutadienyllithium was terminated with methanol and removed by fractionation. Characterization was carried out by size-exclusion chromatography, low-angle laser light scattering, laser differential refractometry, osmometry, and NMR and UV spectroscopy.
Star-branched poly(methyl methacrylate)s (PMMA) were synthesized by linking ‘living’ arms (produced by anionic polymerzation) with ethylene glycol dimethacrylate. Stars having arm molecular weights of 10000 and 40000 and between 4.9 and 18.7 branches were produced. The polymers were characterized using light scattering, size exclusion chromatography, and viscometry. It was found that well-defined PMMA stars were obtained only at the higher (40000) arm molecular weight. The stars prepared using the lower molecular weight (c. 10000) arms contained very high molecular weight gel components.
Poly(styrene-b-methyl methacrylate)s were analysed by liquid chromatography at the critical point of adsorption. By operating at chromatographic conditions corresponding to the critical mode for poly(methyl methacrylate) and the size exclusion mode for polystyrene, the molar mass and the polydispersity of the polystyrene block in the block copolymers may be determined. The data were found to be in excellent agreement with the corresponding parameters of the polystyrene precursor homopolymers.
The complete chromatographic characterization of block copolymers (molecular weight versus chemical composition) is an important problem in polymer chemistry. The experimental validity of the concept of so-called “chromatographic invisibility”, predicted theoretically by Gurbunov and Skvortsov on the basis of the phenomenon of critical conditions known in liquid chromatography, was examined. The theoretical approach predicts the possibility of one component of an A-B block copolyner being eluted under gel permeation chromatographic conditions, whereas the size of the alternate “invisible” component exerts no effect on the overall elution profile of the block copolymer. This applies only when special thermodynamic conditions i.e., eluent composition and temperature, are fulfilled, where the distribution coefficient K is unity, regardless of molecular weight. Block copolymers of poly(styrene-methyl methacrylate) and poly(styrene-tert.-butyl methacrylate) wre used as examples with binary and ternary mixtures of acetonitrile-dichloromethane, methanol-chloroform, tetrahydrofuran-dichloromethane and tetrahydrofuran-dicloromethane-hexane as eluents for chromatography under critical conditions on wide-pore silica gel in narrow-bore columns.