Calibration procedures in gel permeation chromatography

Procedures for the determination of the log molecular weight vs. elution volume calibration relation are reviewed for linear homopolymers. The calibration curve is readily established with narrow molecular weight distribution fractions. For broad molecular weight distribution fractions the peak molecular weight at the peak elution volume of a fraction's gel permeation chromatogram has to be calculated from average molecular weights. When well-characterised fractions of the polymer requiring analysis are unavailable, a calibration curve can be established by procedures which assume that the hydrodynamic volume of a polymer molecule in solution controls fractionation in gel permeation chromatography. These universal calibration procedures require information on Mark-Houwink viscosity constants or polymer unperturbed dimensions. The validity of hydrodynamic volume as the universal calibration parameter is discussed with special reference to the goodness of the solvent for a polymer and polymer polarity. Examples are given of the various calibration procedures which are employed in the determination of molecular weight distribution and average molecular weights for poly(methylmethacrylate), polyethylene and polyisoprene.     

超临界丙烷分级聚苯乙烯

利用超临界流体的溶剂强度随温度、压力的变化而变化和超临界流体泄压至常压时溶质完全析出的特点,采用超临界丙烷取代常规溶剂对聚苯乙烯进行分级研究,以期柔性地调节操作温度和压力,获得分子量分布较窄的聚合物级分.结果表明,对多分散系数为4.225的聚苯乙烯进行等温超临界分级和等压超临界分级实验能够得到多分散系数分别为1.0～2.0和1.3～2.0的级分.并且发现,压力和温度越高,溶剂的溶解能力越大,分级得到的级分分子量越大.同时,从高聚物溶液理论出发,结合超临界溶液的溶解特性,建立了超临界流体分级高聚物的级分分子量的预测模型.利用实验数据对模型参数优化结果表明,当压力大于25 MPa时,超临界等温分级模型的平均相对误差为5.32％;当温度大于413.15 K时,超临界等压分级模型的平均相对误差为18.03%.     

Field-flow fractionation: New and exciting perspectives in polymer analysis

The development of advanced polymeric materials requires state-of-the-art synthesis and molecular characterization protocols. Only the precise knowledge of molecular structure–property correlations allows achieving optimum performance properties of novel materials. The analysis of the molecular composition of a complex polymeric material requires the determination of its molar mass, chemical composition, functionality and molecular topology among other (less important) parameters.A number of column-based fractionation methods, including size exclusion chromatography (SEC) and high performance interaction chromatography (HPLC) are the standard techniques for the analysis of complex polymers. These methods work well as long as the molar mass is not too high and/or the macromolecules do not exhibit undesired interactions with the stationary phase (column). Certain polymers form large aggregates or other entities (micelles, liposomes) in solution that typically cannot be analyzed by column-based fractionation methods.One alternative for the fractionation of such complex materials is field-flow fractionation (FFF), an open-channel technique which does not use a stationary phase. In FFF, all problems related to the stationary phase such as undesired adsorption, shear degradation of large macromolecules, co-elution of linear and branched macromolecules, can be avoided. Different sub-techniques of FFF render the fractionation of complex polymer systems according to molecular size, chemical composition or molecular topology.In this review article, most recent developments of FFF in polymer analysis are addressed. Natural and synthetic polymers, polyolefins and polymeric nanocomposites are embraced. The most important FFF sub-techniques in polymer analysis include asymmetric flow field-flow fractionation (AF4) and thermal field-flow fractionation (ThFFF). Major developments in these very topics since 2008 are critically discussed following a previous review article that summarized earlier work (see Prog. Polym. Sci. 2009; 34: 351–68). The potentials and limitations of the different FFF sub-techniques for polymer analysis are elaborated and most recent methods of hyphenating FFF with other techniques are highlighted.     

A new instrument for preparative gel permeation chromatography

A preparative liquid-phase chromatograph was built for the purpose of obtaining sufficiently large quantities of very narrow fractions of different polymeric species, such as polystyrene, PVC, polybutadiene, and polyethylene. This apparatus allows the fractionation of approximately 20 g of polymer per day; the fractions so obtained have polydispersities of about 1.1 over a very wide range of molecular weights. Polydispersities of less than 1.01 were obtained after recycling of the sample.     

Preparative gel permeation chromatography of Athabasca asphaltene and the relative polymer-forming propensity of the fractions

Athabasca asphaltene has been separated according to molecular weight on Bio-Beads SX-1 gel. The number-average molecular weights of the five arbitrary fractions obtained by this fractionation range from 1200 to 17000. The chemical, spectral and thermal properties of the fractions are all similar but their polymer-forming propensities are markedly different. The significance of this latter property, which is defined in terms of the amount of CH₂Cl₂-insoluble material produced upon thermolysis at 300 °C, increases rapidly with increasing molecular weight of the fraction. In contrast the whole asphaltene does not form polymer at 300 °C under the same conditions and it is concluded that the chain propagating steps are terminated by a variety of inhibitors that are contained in the asphaltene agglomerate. During gel permeation chromatography separation the clay present in the asphaltene concentrates in the higher-molecular-weight fractions. This affinity to attract the clay is thought to be related to the physical, and not the chemical, properties of these higher-molecular-weight materials. The clay also exerts a catalytic effect on the polymerization of the asphaltene fractions which is most pronounced in the highest-molecular-weight fraction and gradually decreases with decreasing molecular weight.     

Preparation of monodisperse polymer samples by solubility difference method

An attempt was made to prepare the polymer fractions having extremely sharp molecular weight distribution (MWD), by using a successive solutional fractionation (SSF) method, in which a polymer-lean phase was separated as a fraction from a polymer-rich phase. For this purpose a large-scale preparative SSF apparatus was constructed. Atactic polystyrene (PS) high-density polyethylene (PE), and cellulose di- and tri-acetates (CDA and CTA) were fractionated by SSF. The fractions isolated from a quasi-binary mixture (polymer/solvent system) have the same MWD as that predicted by the computer simulation technique. Even under the conventional fractionation conditions (initial polymer volume fraction v^o_p = ～ 0.01, total number of fractions n_t = 10 ～ 20) the fractions with the ratio of the weight to number-average molecular weight M_w/M_n less than 1.1 for PS, 1.2 for PE, 1.3 for CDA and 1.4 for CTA were obtained, with exception of a few initial fractions. The advantage of the SSF method was clarified over the conventional preparative methods such as gel permeation chromatography and the column fractionation method.     

Fractionation of low molecular weight polyethylene by high-pressure soxhlet extraction

The use of quantitative, high-pressure Soxhlet extraction for the fractionation of low molecular weight polyethylene samples is described. Liquid carbon dioxide was found to be a suitable solvent for the lowest molecular weight hydrocarbons but failed to solubilize hydrocarbons with molecular weights greater than C-40—C-50. Liquid pentane was found to be an effective solvent for hydrocarbons that were insoluble in liquid CO₂. Careful, stepwise adjustment of the extraction solvent temperature produced polymer fractions with molecular weight distributions substantially narrower than those of the parent materials. Polymer fractions with molecular weights up to C-90 were analyzed by high-temperature gas chromatography. These analyses demonstrated the effectiveness of the technique in fractionating polymers according to molecular weight. Further evidence was provided by thermal analysis of the fractions that indicated melting-point transitions that were much sharper than those of the parent materials. High-pressure Soxhlet extraction offers considerable potential as a general method for purification and fractionation of synthetic and natural polymers. © 1993 John Wiley & Sons, Inc.     

THE EFFECT OF COLUMN GEOMETRY ON SEPARATION EFFECTIVENESS OF AGAROSE FOR POLY(VINYL PYRROLIDONE)

Glass columns packed with agarose gets were evaluated for the analytical fractionation of poly(vinyl pyrrolidone). Columns were studied with inside diameters ranging from 0.56 to 1.58 cm. and lengths ranging from 28 to 120 cm. Column efficiencies were found to vary from 250 plates per foot for the least effective column to 1600 plates per foot for a 1.10 by 44 cm. column. The efficiency was judged by the usual criterion of band spreading for a low molecular weight material, in this case 2-pyrrolidone. Another measure of effectiveness used was the separation obtained between two fractions of poly(vinyl pyrrolidone) with molecular weights of 12,700 and 58,500. The column geometry giving the highest plate count also separated the polymer fractions most effectively.     

Studies of the fractionation of polyoxymethylene

The precipitation of polyoxymethylene in p-chlorophenol solution and the molecular weight fractionation of the polymer by mechanical agitation were investigated. The agitation of the solution was carried out in a glass vessel with a stirrer, usually at 60°C. After agitation for several minutes a fibrous polymer precipitated. High molecular weight polymer precipitated around the stirrer in an early stage, and therefore the method might be applied to the fractionation of polyoxymethylene. The method was applied to the fractionation of polyoxymethylene prepared in a solid-state and in a solution polymerization of trioxane, catalyzed by BF₃.OEt₂. It was found that the polymer from the solid state contained a small amount of extremely high molecular weight fraction, and that obtained from the solution had a relatively narrow distribution of molecular weight.     

THE EFFECT OF COLUMN GEOMETRY ON SEPARATION EFFECTIVENESS OF AGAROSE FOR POLY(VINYL PYRROLIDONE)

Glass columns packed with agarose gets were evaluated for the analytical fractionation of poly(vinyl pyrrolidone). Columns were studied with inside diameters ranging from 0.56 to 1.58 cm. and lengths ranging from 28 to 120 cm. Column efficiencies were found to vary from 250 plates per foot for the least effective column to 1600 plates per foot for a 1.10 by 44 cm. column. The efficiency was judged by the usual criterion of band spreading for a low molecular weight material, in this case 2-pyrrolidone. Another measure of effectiveness used was the separation obtained between two fractions of poly(vinyl pyrrolidone) with molecular weights of 12,700 and 58,500. The column geometry giving the highest plate count also separated the polymer fractions most effectively.     

Controlled antibody release from a matrix of poly(ethylene-co-vinyl acetate) fractionated with a supercritical fluid

A new method is presented for controlling the rate of antibody (Ab) release from an inert matrix composed of poly(ethylene-co-vinyl acetate) (EVAc), a biocompatible polymer that is frequently used to achieve controlled release. Using supercritical propane, a parent EVAc sample (M_n = 70 kDa, M_w/M_n = 2.4) was separated into narrow fractions with a range of molecular weights (8.7 < M_n < 165 kDa, 1.4 < M_w/M_n < 1.7). Solid particles of Ab were dispersed in matrices composed of different polymer fractions and the rate of Ab release into buffered saline was measured. The rate of Ab release from the EVAc matrix depended on molecular weight: > 90% of the incorporated Ab was released from low molecular weight fractions (M_n < 40 kDa) during the first 5 days of release, while < 10% was released from the high molecular weight fraction (M_n > 160 kDa) during 14 days of release. No significant differences in polymer composition, glass-transition temperature, or crystallinity were identified in the different molecular weight fractions of EVAc. Mechanical properties of the polymer did depend on the molecular weight distribution, and correlated directly with Ab release rates. Because it permits rapid and reproducible fractionation of polymers, supercritical fluid extraction can be used to modify the performance of polymeric biomaterials. © 1993 John Wiley & Sons, Inc.     

Gas antisolvent fractionation of semicrystalline and amorphous poly(lactic acid) using compressed CO2

Semicrystalline and amorphous poly(lactic acid) (l-PLA and d,l-PLA, respectively) were fractionated from chloroform solutions using compressed CO₂ as an antisolvent. The following process variables were used to precipitate normalized molecular weight fractions (NMW) of l-PLA ranging from 0.81 to 1.54 relative to the starting material: polymer concentration, initial organic solution volume, and the rate of antisolvent addition. An analysis of variance (ANOVA) used to quantify the importance of these variables determined that polymer concentration had the most significant impact on the NMW of l-PLA precipitated in this gas antisolvent (GAS) precipitation process. The results of the ANOVA also suggest a predictive approach to polymer fractionation in this complex system. The analysis also highlights the differences and similarities between the fractionation of semicrystalline and amorphous polymers using compressed antisolvents.     

Comparative study of complex mechanisms of various polystyrene degradations and oxidations

Comparative investigations of the polymeric residues remained after degrading polystyrene by heat or by UV irradiation reveal numerous profound changes of the structural and molecular parameters of the polymer when subjecting it to either of these main types of degradation. The polymeric residues of the commercial and of the prepared polystyrene samples were investigated in view of the following characteristics: discolouration, IR, UV and ¹³C NMR spectra, DSC-analysis, GPC-elution curves and X-ray patterns. The results show pronounced heterogeneity of the degradative reactions occuring in the framework of the thermal and photochemical degradation of polystyrene, including the existence of crosslinking processes and producing thus very inhomogeneous polymeric material. The presumption of the existence of different mechanism paths occuring in various polystyrene structural fractions was confirmed and further promoted, especially from the standpoint that the lower-molecular fractions of the polymer should behave more sensitively against the degradative attacks. Many-sided meanings of the informations obtained by analysing degraded polystyrene samples allow to postulate the present degradative mechanism by comparing among them various types of de gradative processes. The gained informations about the locations and ranges of absorption bands in UV, IR and ¹³C NMR spectra, as well as the complementary evidence from other determinations enabled the claim that polystyrene thermo-oxidative degradation would preferentially take place in the way to produce acetophenone structures near the chain ends. In the initial degradation periods this would be related with substantial lowering of the molecular mass. The photo-oxidative degradation mechanisms cause the build-up ketone and aldehyde structures and ring-opening reactions concurrent with chain crosslinking reactions. Comparing long-wave and short-wave UV irradiations of polystyrene does not only show the existence of inequal induction times, but additionally the presence of two sets of similar reactions leading to the production of many oxidized structures. The investigations would show that oxidative polystyrene degradation is a very complex phenomenon in which the prefered courses of chemical reactions are in the relations to the initial reaction steps, namely they do depend on structures formed in the early stages of the thermal and photo-degradative processes.     

Performance of crystallization analysis fractionation and preparative fractionation on the characterization of γ‐irradiated low‐density polyethylene

The effects of γ‐radiation on a low‐density polyethylene (LDPE) were investigated by novel techniques, such as crystallization analysis fractionation and preparative fractionation, to analyze and compare their performance with other analytical procedures such as DSC, FTIR, and GPC. The LDPE was thus irradiated with four different doses of γ‐radiation. Different fractions were obtained from these irradiated materials by preparative fractionation, which were characterized by the above‐mentioned analysis techniques. The changes in the morphology and chemical structure of LDPE after the irradiation were analyzed and it was found that both oxidative scission and crosslinking are phenomena related to the exposure of LDPE at high‐energy radiation. Crystallization analysis fractionation and preparative fractionation proved to be suitable techniques to characterize the effects of γ‐radiation on a low‐density polyethylene material. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1803–1814, 2004     

Polymer‐Derived Ceramic Microspheres with Controlled Morphology via Novel Phase Separation‐Assisted Pyrolysis

In this contribution, a novel and versatile strategy named phase separation assisted pyrolysis for the preparation of polymer‐derived ceramic (PDC) microspheres with controlled morphology is reported. Various Si–C–N–(Ni) magnetic microspheres with a dimension from micro‐ to nanoscale can be conveniently prepared by using polymeric precursor with an assistance of polymer template of linear polystyrene. The morphology of PDC microspheres could be controlled through the feed ratio of precursor and template, pyrolysis temperature, and centrifugal fractionation, which were proved with dense structure, tunable crystals, and ferromagnetic property with an ultralow hysteresis loss. During phase separation and pyrolysis, the polymeric precursor or molecular precursor as well as various polymer templates can be employed. So, this strategy to synthesize ceramic microspheres with a dimension from micro‐ to nanoscale is indeed general and convenient for functional PDC microspheres with well‐controlled morphology.     

Preparative fractionation and characterization of polycarbonate/eugenol-siloxane copolymers

Bisphenol-A polycarbonate/eugenol-siloxane copolymers were fractionated at the preparative scale by the continuous polymer fractionation (CPF) technique. It is the first example of copolymer fractionation by CPF. The distribution of siloxane species across the fractions was assessed for copolymers differing in initial siloxane concentration and block length. On- and off-line combinations of size exclusion chromatography and infrared spectroscopy were used to analyze chemical composition (CC) of the unfractionated samples across the molecular weight distribution enabling comparison with the fractions. A polycarbonate-siloxane copolymer containing 10 wt% of very short siloxane blocks (dp=2) was fractionated solely according to molecular weight (MW). By contrast, a copolymer containing 5 wt% siloxane blocks with a larger degree of polymerization (dp=23) was fractionated according to MW, as well as to CC. This ‘chemical drift’ effect for the larger siloxane block length can be ascribed to large solubility differences of low MW chains, which drastically vary in composition according to the (small) number of siloxane blocks they contain.     

Efficiency of Gold Nano Particles on the Autoxidized Soybean Oil Polymer: Fractionation and Structural Analysis

Polyunsaturated plant oils have gained great interest as monomers to produce biodegradable polymers obtained from renewable resources due to the limited existing sources of petroleum oil and environmental issues. Soybean oil was autoxidized by exposure to atomospheric oxygen at room temperature with or without the presence of gold nanoparticles (Au NPs) 5–41 days. When the autoxidation process was catalyzed with Au NPs, the molecular weight of the oxidized oil was increased in 5 days. In contrast to this, without Au NPs, the oxidized oil was still a fluidized liquid. Autoxidized soybean oil polymer in toluene solution with gold NP showed a surface plasmon resonance at λ_max = 540 nm in a UV–VIS spectrometer and a fluorescence emission spectrum at λ_max = 450 nm, when it was irradiated at λ_max = 390 nm. The higher molecular weight of the polymeric oils was successively fractionated by the extraction from the solvent‐non‐solvent mixture CHCl₃/petroleum ether with the volume ratio of 5:15. Three polymeric oils fractions with different molecular weight (ca 1000, 4000, and 40,000 g/mol) were obtained. GC–MS analysis, ¹H‐NMR and GPC techniques were used in the structural analysis of the fractionated polymeric oils.     

Oligomer-Rich Fraction from Polydisperse Sample of Poly(bis(β-phenoxyethoxy)phosphazene)

Poly[bis(β-phenoxyethoxy)phosphazene] [PBPEP] had been shown in our previous paper to be a very useful polymer for investigating the crystallization mechanism of polymers, as the crystallization rate of PBPEP is extraordinarily small when isothermally crystallized from the melt. The crystallization of the low molecular weight oligomers of PBPEP was first studied in comparison to the high molecular weight polymers. The oligomer-rich fraction was obtained by fractionation of the as-polymerized sample, which had a broad molecular weight distribution. The fractions thus obtained were characterized by solution viscometry and size exclusion chromatography. The melting temperature and the growth rate of the spherulite from the melt were investigated by differential scanning calorimetry and optical microscopy. The growth rate was one or two orders of magnitude smaller in the oligomer-rich fraction than in the other high molecular weight fractions. A collapsed spherulite appeared in the oligomer-rich fraction at high crystallization temperatures. It is speculated that in the oligomer-rich fraction there is an excess free energy due to defects in the crystal phase. This defect is considerably larger in the oligomer-rich fraction than in the other fractions because a large quantity of short length chains is present.     

Use of solution crystallization analysis by laser light scattering for studying the solution crystallization of various Ziegler–Natta‐catalysed polypropylenes

Solution crystallization analysis by laser light scattering (SCALLS) involves the observation of the scattering of diode mercury laser lamp light after it passes through a polymer solution. An increase in turbidity occurs when the hot polymer solution is cooled and the polymer starts to crystallize out of solution. This causes a decrease in the amount of laser light that can pass through the solution and an increase in the amount of scattered light. The reverse of this process leads to the turbidity decreasing with an increase in temperature. According to this concept, it is possible to follow the solution crystallization of various polypropylenes under controlled cooling. In this study, SCALLS was able to differentiate between different isotactic and syndiotactic polypropylenes with similar chemical structures, but different tacticity and molecular weights. Furthermore, SCALLS provided good crystallization information that is similar to that from crystallization analysis fractionation and temperature rising elution fractionation. In addition, SCALLS can be used as a quantitative tool for the measurement of weight fractions during dissolution. © 2014 Society of Chemical Industry     

A high hydroxyl group content found in high molecular weight fractions of poly(oxyethylene glycol)

The results of a fractionation of high molecular weight poly(oxyethylene glycol) with the mixture benzene—isooctane are presented. The fractions are characterized by gel permeation chromatography (GPC), infrared spectroscopy, viscometry, and dialysis. A high hydroxyl content was found for the higher fractions, which is not compatible with a linear polyoxyethylene glycol molecule with hydroxyl endgroups. The presence of hydroxyl groups on the chain is improbable. The dialysis of the higher fractions in CCl₄ and toluene shows that a surprising amount passes through the dialysis bag. The possibility of degradation of the polymer is considered. However, GPC analysis of the products of the dialysis suggest that the high molecular weight is made up of aggregates of middle-sized molecules and low molecular weight ones, held together by hydrogen bonding between hydroxyl and ether groups. Some results of a fractionation in water with the lower critical solubility temperature at 99°C. are given.