Characterization of branched ultrahigh molar mass polymers by asymmetrical flow field-flow fractionation and size exclusion chromatography

The molar mass distribution (MMD) of synthetic polymers is frequently analyzed by size exclusion chromatography (SEC) coupled to multi angle light scattering (MALS) detection. For ultrahigh molar mass (UHM) or branched polymers this method is not sufficient, because shear degradation and abnormal elution effects falsify the calculated molar mass distribution and information on branching. High temperatures above 130 °C have to be applied for dissolution and separation of semi-crystalline materials like polyolefins which requires special hardware setups. Asymmetrical flow field-flow fractionation (AF4) offers the possibility to overcome some of the main problems of SEC due to the absence of an obstructing porous stationary phase. The SEC-separation mainly depends on the pore size distribution of the used column set. The analyte molecules can enter the pores of the stationary phase in dependence on their hydrodynamic volume. The archived separation is a result of the retention time of the analyte species inside SEC-column which depends on the accessibility of the pores, the residence time inside the pores and the diffusion ability of the analyte molecules. The elution order in SEC is typically from low to high hydrodynamic volume. On the contrary AF4 separates according to the diffusion coefficient of the analyte molecules as long as the chosen conditions support the normal FFF-separation mechanism. The separation takes place in an empty channel and is caused by a cross-flow field perpendicular to the solvent flow. The analyte molecules will arrange in different channel heights depending on the diffusion coefficients. The parabolic-shaped flow profile inside the channel leads to different elution velocities. The species with low hydrodynamic volume will elute first while the species with high hydrodynamic volume elute later. The AF4 can be performed at ambient or high temperature (AT-/HT-AF4). We have analyzed one low molar mass polyethylene sample and a number of narrow distributed polystyrene standards as reference materials with known structure by AT/HT-SEC and AT/HT-AF4. Low density polyethylenes as well as polypropylene and polybutadiene, containing high degrees of branching and high molar masses, have been analyzed with both methods. As in SEC the relationship between the radius of gyration (R(g)) or the molar mass and the elution volume is curved up towards high elution volumes, a correct calculation of the MMD and the molar mass average or branching ratio is not possible using the data from the SEC measurements. In contrast to SEC, AF4 allows the precise determination of the MMD, the molar mass averages as well as the degree of branching because the molar mass vs. elution volume curve and the conformation plot is not falsified in this technique. In addition, higher molar masses can be detected using HT-AF4 due to the absence of significant shear degradation in the channel. As a result the average molar masses obtained from AF4 are higher compared to SEC. The analysis time in AF4 is comparable to that of SEC but the adjustable cross-flow program allows the user to influence the separation efficiency which is not possible in SEC without a costly change of the whole column combination.     

Natural sample fractionation by FlFFF-MALLS-TEM: sample stabilization, preparation, pre-concentration and fractionation

Two flow field flow fractionation (FlFFF) systems: symmetrical (SFlFFF) and asymmetrical (ASFlFFF) were evaluated to fractionate river colloids. Samples stability during storage and colloids concentration are the main challenges limiting their fractionation and characterization by FlFFF. A pre-fractionation (<0.45 microm) and addition of a bactericide such as NaN3 into river colloidal samples allowed obtaining stable samples without inducing any modification to their size. Stirred cell ultra-filtration allowed colloidal concentration enrichment of 25-folds. Scanning electron microscope (SEM) micrographs confirmed the gentle pre-concentration of river samples using the ultra-filtration stirred cell. Additionally, larger sample injection volume in the case of SFlFFF and on channel concentration in the case of ASFlFFF were applied to minimize the required pre-concentration. Multi angle laser light scattering (MALLS), and transmission electron microscope (TEM) techniques are used to evaluate FlFFF fractionation behavior and the possible artifacts during fractionation process. This study demonstrates that, FlFFF-MALLS-TEM coupling is a valuable method to fractionate and characterize colloids. Results prove an ideal fractionation behavior in case of Brugeilles sample and steric effect influencing the elution mode in case of Cézerat and Chatillon. Furthermore, comparison of SFlFFF and ASFlFFF fractograms for the same sample shows small differences in particle size distributions.     

Precipitation chromatography fractionation: The effect of the ratio support weight/sample size

The effect of the gravimetric ratio, packing beads/polymer (r), in the chromatographic fractionation of polystyrene has been studied. Anionic polystyrene of bimodal molecular weight distribution was fractionated in three experiments with r values of 25, 50 and 130; a value of about 50 provided the most efficient separation. A GPC self-consistency analysis of this fractionation has been done with very satisfactory results. On the other hand, GPC analysis could not show any significant broadening of the molecular weight distributions of the fractions in going from r values of 130 down to 25.     

Direct measurement of free-volume hole distributions in polymers by using a positronium probe

We have directly measured the free-volume hole distributions in semicrystalline polypropylene by positron lifetime annihilation spectroscopy. A Laplace inversion technique was engaged to analyze the positron lifetime spectra measured under quasi-isotropic external pressures of 0, 4.2, and 14.7 kbar into continuous lifetime distributions. The hole radii distributions as determined from the ortho-positronium lifetime distributions are found to be between 4.0 and 0.5 Å and to have maxima at 3.0, 1.9, and 1.1 Å under the external pressures of 0, 4.2, and 14.7 kbar, respectively. © 1992 John Wiley & Sons, Inc.     

Effect of sample size and environment on the thermal degradation of polymers

A model that takes into account mass transfer resistances at the polymer-melt and at the melt-carrier gas interfaces during the course of thermal degradation of relatively ‘large’ samples of polymer, is proposed. It is tested against experimental weight loss data for polypropylene, in the temperature range 390–430°C, obtained from the pyrolysis of 0·1–5 g samples. The weight loss data do not follow first-order kinetics and depend on the sample size. It is found that the model is capable of predicting the experimental observations.     

Determination of thickness, aspect ratio and size distributions for platey particles using sedimentation field-flow fractionation and electron microscopy

Sedimentation field-flow fractionation (SdFFF) can be used to prepare fractions of very narrow mass range for electron microscopic (EM) analysis. Assuming the particle density is the same for all particles within that fraction the equivalent spherical diameter for the particles can be calculated from SdFFF theory. Integration of the micrograph image of each particle yields an area measurement which, when used in conjunction with the equivalent spherical particle diameter (from SdFFF), provides information about the particle thickness and aspect ratio. Thus SdFFF-SEM can be used to provide detailed information about clay morphology across the particle size distribution of the sample. Three clay minerals have been studied using the methodologies outlined in this paper. The aspect ratio for the Purvis School Mine kaolinite ranged from 2.8–5.9, for RM30 illite from 11.3–24.3, and for Muloorina illite from 3.1–4.3.     

Influence of ionic strength, sample size, and flow conditions on the retention behavior of pullulan in flow field-flow fractionation

Polymer molecular parameters such as hydrodynamic size are expected to be invariant regardless of the technique used to measure them, and to vary only, to some extent, with the solvent power and the polymer structure and properties as predicted from polymer chemistry. The hydrodynamic size of five pullulan standards derived from FlFFF in solutions of different ionic strength appears to correlate well to molecular mass as expected for neutral polymers for all fractions except that of lower mass. The correlation also holds for large amounts of injected sample even though with a slope which increases with rising polymer load. The evidence that the same result is obtained also for low sample amounts but with a higher cross-flow rate is interpreted as the manifestation of the presence of hydrodynamic interactions in concentrated polymer systems.     

Accurate mass measurement in nano-electrospray ionization mass spectrometry by alternate switching of high voltage between sample and reference sprayers

An electrospray dual sprayer, which generates separate sample and reference sprays by alternately switching the high voltage between the two sprayers, is described. The technique permits accurate mass measurements in nano-electrospray ionization mass spectrometry (ESI-MS) to be obtained using a quadrupole/orthogonal acceleration time-of-flight mass spectrometer (Q-TOF). Similar to the method employed with a dual ESI source (Wolff JC et al., Anal. Chem. 2001; 73: 2605), the two sprays are orthogonal with respect to each other, but can be independently sampled without any baffle between these sprays. The reference sprayer is used in the original configuration of the ESI source and was optimized for a 1-2 muL/min flow, whereas the sample sprayer can be either a conventional glass capillary or a borosilicate tip of the type used for nano-ESI. Both sprayers can be positioned close to the cone so as to give maximum ion currents. The sample and reference sprays are independently generated by raising the potentials on the sample and reference sprayers to 1.4 and 3.0 kV, respectively; the high voltages can be rapidly turned on and off in ca. 1 ms. A nano-ESI-MS or nano-flow LC/ESI-MS experiment using a Q-TOF coupled with the above system gave mass accuracies within 3 ppm for measurements of ions up to m/z 1000 using subpicomole samples.     

Field-flow fractionation of proteins, polysaccharides, synthetic polymers, and supramolecular assemblies

This review summarizes developments and applications of flow and thermal field-flow fractionation (FFF) in the areas of macromolecules and supramolecular assemblies. In the past 10 years, the use of these FFF techniques has extended beyond determining diffusion coefficients, hydrodynamic diameters, and molecular weights of standards. Complex samples as diverse as polysaccharides, prion particles, and block copolymers have been characterized and processes such as aggregation, stability, and infectivity have been monitored. The open channel design used in FFF makes it a gentle separation technique for high- and ultrahigh-molecular weight macromolecules, aggregates, and self-assembled complexes. Coupling FFF with other techniques such as multiangle light scattering and MS provides additional invaluable information about conformation, branching, and identity.     

Analysis of branched polymers by high resolution multidetector size exclusion chromatography: Separation of the effects of branching and molecular weight distribution

This article presents the SEC analysis of branched polyisobutylene PIB and polystyrene PS with high molecular weight and broad multimodal molecular weight distribution. Both polymers were synthesized using an inimer technique, which results in long‐chain branched polymers with statistical branching and broad multimodal distributions. Using high resolution multidetector Size Exclusion Chromatography SEC the polymers were analyzed based on three branching factors: g = (R_z,br/R_z,lin)_Mw; h = (〈R_h〉_z,br/〈R_h〉_z,lin)_{Mw ;} and ρ = (R ^1/2/〈R_h〉_z). It is generally accepted that for monodisperse branched polymers g and h < 1. In the case of our polydisperse PIB and PS, it was seen that g and h > 1, and ρ increases with molar mass and the number of chain ends as predicted earlier. The multidetector SEC system allowed for the separation of branching and polydispersity, reported here for the first time experimentally. The g parameter as a function of DP_i was compared to the theory developed by Zimm and Stockmayer. The plots followed a similar trend, but were shifted by a factor related to the average chain length between branching points. The ρ parameter decreased with increasing DP_i, as predicted theoretically by Kajiwara. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

用于食品安全分析样品前处理的共价有机聚合物的制备及应用进展

食品安全关系身体健康和生命安全,是全球关注的热点之一。食品基质复杂,痕量有毒有害物质分析之前必须经过有效的前处理。目前发展的前处理技术如固相萃取、磁固相萃取、固相微萃取等,其关键是吸附介质。共价有机聚合物是一类通过共价键连接而成的有机多孔材料,具有质轻、稳定性好、比表面积大、结构可控、易于修饰等特性,是一类优异的新型吸附材料。该文综述了近年来共价有机聚合物(COPs)在食品安全分析前处理中的应用进展。共价有机聚合物及其功能化复合材料通过简单的装填、聚合反应或化学键合固定到小柱或毛细管柱中用作固相萃取的吸附介质;通过一锅法、原位还原法、原位生长法或共沉淀法生成具有磁性的固相萃取吸附介质;或者通过物理涂覆、化学键合、溶胶凝胶法及原位生长法制备固相微萃取纤维。基于以上高吸附容量共价有机聚合物的样品前处理技术,食品中农残兽残、添加剂、环境污染物及生物毒素等得到了有效富集。最后,展望了COPs在食品分析样品前处理应用中的发展方向:简单高效绿色制备方法的开发,功能化COPs的设计合成;萃取机理的研究;高通量、高灵敏度分析方法研究。这些研究将促进COPs在样品前处理领域获得更广泛的应用。     

Investigating the effect of mixing ratio on molar mass distributions of synthetic polymers determined by MALDI-TOF mass spectrometry using design of experiments

It is well known that the mixing ratio affects the molar mass distribution of synthetic polymers determined by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). Surely, the molar mixing ratio determines whether a mass spectrum will be obtained or not. However, depending on the mass range, several effects such as multimer formation occur, which might be a source of errors in molar mass distribution calculations. In this study, the effect of mixing ratio was investigated for several synthetic polymers, including polystyrene (PS), poly(dimethylsiloxane) (PDMS), poly(ethylene glycol) (PEG), and poly (methyl methacrylate) (PMMA) using statistical designs of experiments. The 2³ full factorial design was found to be suitable in the study of more than 1000 samples. The obtained MALDI mass spectra as well as the ANOVA statistics show that the mixing ratio affects the molar mass distribution. The optimal mixing ratio for a defined synthetic polymer depends on the studied combination (matrix, cationization reagent, solvent).     

Effect of post-excitation radius on ion abundance, mass measurement accuracy, and isotopic distributions in Fourier transform ion cyclotron resonance mass spectrometry

We report an evaluation of a modern Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) instrument to determine the general trend of post-excitation radius on total ion abundance, mass measurement accuracy, and isotopic distributions for internally calibrated mass spectra. The optimum post-excitation radius was determined using total ion abundance, mass measurement accuracy (MMA), and isotope ratios. However, despite the utility of internal calibration for achieving ultimate MMA, the internal calibrant ions were insufficient for compensating for sub-optimum ICR cell conditions. The findings presented herein underscore the importance of determining the optimal post-excitation radius in FT-ICR-MS to achieve high ion abundance (low limits of detection), high MMA, and valid isotopic distributions.     

Influence of reactor operational policies on distributions of chemical composition,molar mass,sequence length and sequence frequency of multicomponent polymers obtained by free-radical polymerization

Remarkable shifts in chemical composition, molar mass, sequence length and sequence frequency will occur in the course of free-radical multicomponent polymerizations, if the reactivities of the comonomers are different and a simple batch reactor is used. It is shown for the terpolymerization system of methyl methacrylate/styrene/maleic anhydride that a semi-batch reactor with appropriate regimes is suitable to obtain products with improved chemical, molecular and sequential homogeneity. However, if polymers with defined inhomogeneities like bimodal molar mass or chemical composition distributions are desired, these are also obtainable by use of appropriate operational policies, which will be illustrated for the homopolymerization of methyl methacrylate and the binary copolymerization of methyl methacrylate/maleic anhydride. Concerning the instantaneous mean chemical composition of polymer molecules, which cannot be measured directly, a new procedure is presented to determine this quantity experimentally.     

CE and asymmetrical flow‐field flow fractionation studies of polymer interactions with surfaces and solutes reveal conformation changes of polymers

Amphiphilic diblock copolymers consisting of a hydrophobic core containing a polymerized ionic liquid and an outer shell composed of poly(N‐isoprolylacrylamide) were investigated by capillary electrophoresis and asymmetrical flow‐field flow fractionation. The polymerized ionic liquid comprised poly(2‐(1‐butylimidazolium‐3‐yl)ethyl methacrylate tetrafluoroborate) with a constant block length (n = 24), while the length of the poly(N‐isoprolylacrylamide) block varied (n = 14; 26; 59; 88). Possible adsorption of the block copolymer on the fused silica capillary, due to alterations in the polymeric conformation upon a change in the temperature (25 and 45 °C), was initially studied. For comparison, the effect of temperature on the copolymer conformation/hydrodynamic size was determined with the aid of asymmetrical flow‐field flow fractionation and light scattering. To get more information about the hydrophilic/hydrophobic properties of the synthesized block copolymers, they were used as a pseudostationary phase in electrokinetic chromatography for the separation of some model compounds, that is, benzoates and steroids. Of particular interest was to find out whether a change in the length or concentration of the poly(N‐isoprolylacrylamide) block would affect the separation of the model compounds. Overall, our results show that capillary electrophoresis and asymmetrical flow‐field flow fractionation are suitable methods for characterizing conformational changes of such diblock copolymers.