Characterisation of poly(N-isopropylacrylamide) by asymmetrical flow field-flow fractionation, dynamic light scattering, and size exclusion chromatography

Asymmetrical flow field-flow fractionation (AsFIFFF) was used to determine the hydrodynamic particle sizes, molar masses, and phase transition behaviour of various poly(N-isopropylacrylamide) (PNIPAM) samples synthesised by reversible addition--fragmentation chain transfer (RAFT) and conventional free radical polymerisation processes. The results were compared with corresponding data obtained by dynamic light scattering (DLS) and size exclusion chromatography (SEC). Agreement between the three methods was good except at higher molar masses, where the molar mass averages obtained by SEC were much lower than those obtained by AsFIFFF and light scattering. The aggregation of the polymers, which are thermally sensitive, was studied by DLS and AsFIFFF at various temperatures. In deionised water there was an abrupt change in the particle size due to phase separation at approximately equal to 32-35 degrees C. The critical temperatures determined by AsFIFFF were 3-5 degrees C higher than those obtained by DLS.     

Analysis of polyamides by size exclusion chromatography and laser light scattering

The molar mass analysis of polyamides is complicated due to the fact that only a limited range of solvents can be used and association and aggregation phenomena have to be screened by adding electrolytes to the mobile phase. Optimum SEC behaviour is obtained when hexafluoroisopropanol + 0.05 mol/L potassium trifluoroacetate is used as the mobile phase. The calibration of the SEC system can be conducted in different ways. While a calibration with narrow disperse polymethyl methacrylate standards does not yield accurate molar mass information, the quantification can be done using an “artificial” calibration curve. This calibration curve is obtained by correcting the PMMA calibration curve with polyamide molar mass data from light scattering. The resulting molar mass distributions for different types of polyamides are compared with molar masses that are determined by size exclusion chromatography with a light scattering detector and an excellent correlation is obtained.     

Determination of lignin by size exclusion chromatography using multi angle laser light scattering

A method was developed using high-performance size exclusion liquid chromatography (HPSEC) with multi-angle laser light scattering (MALLS), quasi-elastic light scattering (QELS), interferometric refractometry (RI) and UV detection to characterize and monitor lignin. The combination proved very effective at tracking changes in molecular conformation of lignin molecules over time; i.e. changes in molecular weight distribution, radius of gyration, and hydrodynamic radius. Until this study, UV detection (280 nm) had been the primary lignin determination method for chromatography. Three different HPLC columns were used to study the effects of pH, flow conditions, and mobile phase compositions (dimethyl sulphoxide, water, 0.1M NaOH, and lithium bromide) on the chromatography of lignin. Since light scattering accuracy is highly dependent on solute concentration, both the UV and RI detectors were calibrated for use as concentration detectors. Shodex Asahipak GS-320 HQ column with 0.1M NaOH (pH 12.0) run at 0.5 ml/min was found to give the highest separation and most consistent recovery. The study also revealed that the lignin aggregated at pH below 8.5. This aggregation was detected only by MALLS and was not observed on UV or RI detectors. It is very important to take this loss in apparent concentration due to aggregation into consideration before collecting reliable depolymerization data.     

Characterization of suspension poly(vinyl chloride) resins and narrow polystyrene standards by size exclusion chromatography with multiple detectors: Online right angle laser-light scattering and differential viscometric detectors

This work reports the utilization of a multi-detector size chromatography for the characterization of poly(vinyl chloride) (PVC) resins prepared by suspension polymerization in the range of temperatures between 21 and 75 °C. The chromatography equipment offers the possibility of analyzing the samples in terms of their absolute molecular mass using a combination of three detectors (TriSEC): right angle light scattering (RALLS), a differential viscometer (DV) and refractive index (RI). The PVC resins were fully characterized concerning the molecular weight distribution (MWD), its dependence with intrinsic viscosity (η) and molecular sizes (radius of gyration, R_g and hydrodynamic radius, R_h). Additionally, it is also presented the characterization of polystyrene narrow standards serving as reference polymers.It is possible to find in the literature several methodologies concerning the breaking of typical aggregates presented in PVC solutions. The most suitable for the experiments were chosen, adapted and analysed by light scattering. It was observed that the application of the TriSEC to study PVC solutions was effective and it was concluded that this is an important tool for the polymer characterization, opening the possibility of running experiments avoiding the need of fractionation of the polydisperse PVC in order to obtain the Mark-Houwink-Sakurada (MHS) constants, or the utilization of MHS, that are quite diverse in literature.     

Characterization of a poly(ethersulphone) by on-line coupling of size exclusion chromatography with NMR and FTIR spectroscopy

An on-line detection „on the flow”︁ during the size exclusion chromatography run using FTIR and NMR spectrometers was applied to investigate the liquid chromatographic separation process of oligomers and to identify the separated chromatographic compounds. The FTIR and NMR detectors give real-time information about the chemical structure and functional groups of the eluated species. The preferences and limits of the on-line coupling were discussed using an oligomeric hydroxyl terminated poly(ethersulphone) as an example. Molecular weights could be calculated from the ¹H-NMR spectra of the separated species using the intensities of end-group and main-chain signals. Residues of N-methylpyrrolidone which was used as solvent in the oligomer synthesis were identified by the FTIR detector.     

Random cross-linked poly(styrene-co-maleic anhydride): Characterization of cross-linking intermediates by size exclusion chromatography

The cross-linking reaction of poly(styrene-co-maleic anhydride) (SMA) with ethylene glycol (EG) was monitored using size exclusion chromatography. On-line viscosity and static light scattering detection provided [η] and R_g for structural analysis of the cross-linking intermediates. With increasing reaction time a decrease of the expansion exponent α of the Kuhn-Mark-Houwink-Sakurada relationship was observed. A change from coiled structures to spherical molecules during the cross-linking reaction was confirmed by analyzing the fractal dimensions of the molecules after different reaction times. The calculation of the branching parameters g and g′ revealed the formation of contracted branched molecules. The connection of SMA chains by EG leads to a contraction of the polymers with increasing molecular weight.     

Composition determination of binary polymer mixtures by size exclusion chromatography with light scattering detection

<正>Base on the principle of absolute quantification of size exclusion chromatography(SEC),a light scattering(LS) detector coupled with a concentration detector(refractive index detector) is utilized to determine the compositions of complicated binary mixtures.A theoretical analysis predicts that the response factors for both LS and RI detectors are linear functions with the composition of any specified polymer mixtures in the binary polymer mixtures.Two pairs of complicated binary mixtures were used to test the theory mentioned in the present paper,and the experimental results show an excellent accordance with the theory.     

Processing of poly(lactic acid): Characterization of chemical structure, thermal stability and mechanical properties

The processing of poly(lactic acid) (injection and extrusion/injection) as well as annealing of processed materials were studied in order to analyze the variation of its chemical structure, thermal degradation and mechanical properties. Processing of PLA was responsible for a decrease in molecular weight, as determined by GPC, due to chain scission. The degree of crystallinity was evaluated by means of differential scanning calorimetry and X-ray diffraction. It was found that mechanical processing led to the quasi disappearance of crystal structure whereas it was recovered after annealing. These findings were qualitatively corroborated by means of FTIR. By analyzing ¹H NMR and ¹³C NMR chemical shifts and peak areas, it was possible to affirm that the chemical composition of PLA did not change after processing, but the proportion of methyl groups increased, thus indicating the presence of a different molecular environment. The thermal stability of the various materials was established by calculating various characteristic temperatures from thermograms as well as conversion and conversion derivative curves. Finally, the mechanical behaviour was determined by means of tensile testing (Young modulus, yield strength and elongation at break).     

Determining the molar mass of a plasma substitute succinylated gelatin by size exclusion chromatography-multi-angle laser light scattering,sedimentation equilibrium and conventional size exclusion chromatography

The clinical effectiveness of succinylated gelatin as a plasma substitute depends strongly on its molar mass, determined conventionally by size exclusion chromatography (SEC). This study evaluates different SEC calibration standards in comparison with two independent "absolute" methods for determining the weight average molar mass (M(w)) of a succinylated gelatin sample. SEC calibrated using succinylated gelatin fractions correlated well with size exclusion chromatography-multi-angle laser light scattering (SEC-MALLS) and sedimentation equilibrium whereas SEC calibrated with unmodified gelatin, sodium polystyrene sulfonates or pullulans overestimated M(w) by over 20%. Universal calibration was equivocal. The problems associated with the preparation of succinylated gelatin fractions suggest that an absolute method such as SEC-MALLS may be a more suitable choice for determining the M(w) in succinylated gelatins.     

Characterization of poly(D,L-lactic acid) by gel permeation chromatography

A number of samples of poly(D ,L -lactic acid) (PLA) with weight-average molecular weights M?_w in the range 15,000–350,000 were prepared by a ring-opening polymerization. The molecular weight distributions (MWDs) of these samples were determined by gel permeation chromatography (GPC). The method involves a universal calibration of the columns on the basis of polystyrene standards and a rapid iteration algorithm leading to the establishment of the Mark–Houwink relationship. In addition, osmometry and viscometry data are presented. The effect of hydrolytic degradation on the MWD of two PLA samples was studied by GPC.     

Surface characterization of poly(lactic acid) and polycaprolactone by inverse gas chromatography

Inverse gas chromatography (IGC) has been used to characterise the surface properties of polycaprolactone (PCL) and poly(lactic acid) (PLA). The dispersive component of the surface free energy (gamma(S)(D)) was found to be very small for both of them--values close to 30 mJ/m(2) in the case of the PLA and ca. 40 mJ/m(2) for the PCL. The retention times of the n-alkanes, necessary to calculate the dispersive component of the surface energy, were obtained from the maximum, the centre at half height and the centre of mass of the chromatographic peak. While the values obtained using the first two parameters appear not to be affected by the peak asymmetry, in spite of having been obtained above the glass transition temperature of the polymer, the values obtained using the latter have been found to be not reliable. The drawbacks of using n-alkanes with a very small retention time have also been discussed, estimating the error it can introduce in the final results. Finally, the acid-base properties of the two biopolymers were determined using the approaches suggested by Schultz et al. and by St. Flour and Papirer. Although both methods describe the surfaces of PLA and PCL as neutral ones, differences between the values of the parameters K(A), K(D) and S(C) were obtained.     

Influences of poly(lactic acid)‐grafted carbon nanotube on thermal,mechanical, and electrical properties of poly(lactic acid)

Poly(lactic acid)‐grafted multiwalled carbon nanotubes (MWNT‐g‐PLA) were prepared by the direct melt‐polycondensation of L ‐lactic acid with carboxylic acid‐functionalized MWNT (MWNT‐COOH) and then mixed with a commercially available neat PLA to prepare PLA/MWNT‐g‐PLA nanocomposites. Morphological, thermal, mechanical, and electrical characteristics of PLA/MWNT‐g‐PLA nanocomposites were investigated as a function of the MWNT content and compared with those of the neat PLA, PLA/MWNT, and PLA/MWNT‐COOH nanocomposites. It was identified from FE‐SEM images that PLA/MWNT‐g‐PLA nanocomposites exhibit good dispersion of MWNT‐g‐PLA in the PLA matrix, while PLA/MWNT and PLA/MWNT‐COOH nanocomposites display MWNT aggregates. As a result, initial moduli and tensile strengths of PLA/MWNT‐g‐PLA composites are much higher than those of neat PLA, PLA/MWNT, and PLA/MWNT‐COOH, which stems from the efficient reinforcing effect of MWNT‐g‐PLA in the PLA matrix. In addition, the crystallization rate of PLA/MWNT‐g‐PLA nanocomposites is faster than those of neat PLA, PLA/MWNT, and PLA/MWNT‐COOH, since MWNT‐g‐PLA dispersed in the PLA matrix serves efficiently as a nucleating agent. It is interesting that, unlike PLA/MWNT nanocomposites, surface resistivities of PLA/MWNT‐g‐PLA nanocomposites did not change noticeably depending on the MWNT content, demonstrating that MWNTs in PLA/MWNT‐g‐PLA are wrapped with the PLA chains of MWNT‐g‐PLA. Copyright © 2008 John Wiley & Sons, Ltd.     

Characterization of large water-soluble macromolecules by size exclusion chromatography

A method for characterizing very large, water-soluble polymers by size exclusion chromotography (SEC) has been developed. Sephacryl S1000 packing material, a precision syringe pump, and an eluent pressure detector have been utilized to produce highly accurate chromatograms of polymers having molecular hydrodynamic diameters up to 250 nm. Previous SEC analysis has been limited to polymers having hydrodynamic diameters of less than 120 nm.     

Interactions of poly(2-methacryloyloxyethyl phosphorylcholine) with various salts studied by size exclusion chromatography

The objective of this research is to develop a relationship between salt type and concentration to poly(2-methacyloyloxyethyl phosphorylcholine) (PMPC) zwitterionic polymer solution behaviors. In particular, polyelectrolyte hydrodynamic volumes were analyzed through size exclusion chromatography in relation to the addition of various salts at various concentrations. The salt properties examined were salt concentration, ionic strength, solution pH, cation type/size, anion type/size, valency, and configuration. It was found that the effect of ion properties is related to mechanisms associated with the geometry of the polyelectrolyte. The negative charge group of the polyelectrolyte situated closer to the backbone (inside) is less important to the change in hydrodynamic volume than the positive charge group situated at the end of the side chain (outside). The extensive amount of data generated in this study provides a strong background for possible accurate formulation of a theory based on the salt effect on PMPC polyelectrolyte solution behavior.     

Molecular weight determination of hypromellose phthalate (HPMCP) using size exclusion chromatography with a multi-angle laser light scattering detector

The molecular weight of hypromellose phthalate (HPMCP), a polymer used for enteric coating, was determined using size exclusion chromatography with a multi-angle laser light scattering detector. The values of weight-average molecular weight (Mw) of commercially available grades (HP-55, HP-55S, and HP-50) were 45600, 60200, and 37900, respectively. Their inter-day precisions expressed in terms of the coefficient of variation were less than 3%. A correlation curve between Mw and solution viscosity was prepared so that Mw could be easily estimated from the solution viscosity measured by the compendial method.     

Study of polymer complexes by size exclusion chromatography coupled with light scattering in combination with fluorescence spectroscopy

Poly(methyl methacrylate) stereocomplexes prepared at different concentration in dilute tetrahydrofuran solutions were studied by size exclusion chromatography coupled with refractive and light scattering detectors in combination with fluorescence spectroscopy. A considerable increase in segment density due to complexation compared with free poly(methyl methacrylate) chain was only slightly affected by the polymer concentration in solution where stereocomplexes were formed. At polymer concentrations up to 3×10⁻³ g cm⁻³, an increase in non‐uniformity of polymer complex molecular weight and size and a shift to higher values of both were observed. In semidilute solutions (at c > 3×10⁻³ g cm⁻³) stereocomplexes virtually did not become heavier and larger.     

Kinetics of the thermal decomposition of processed poly(lactic acid)

The kinetics of the thermal decomposition of processed poly(lactic acid) has been studied and compared to that of raw material. Processing consisted of two different industrial processes: 1) Injection (with or without further annealing); 2) Extrusion followed by injection (with or without further annealing). For this study, an integral method (based on the general analytical solution), differential methods (based on the first conversion derivative and on the 2nd derivative) and special methods have been used. On the other hand, a method based on the maximum decomposition rate has been considered. By doing that, the kinetic parameters (reaction order, frequency factor and activation energy) have been determined. It has been demonstrated that there was only one first-order reaction for the entire conversion range. A new equation (based on the second conversion derivative plot as a function of temperature) was developed allowing the calculation of the reaction order. This method quantifies peak areas (and not peak heights, as reported by Kissinger). It is very useful because it considers both peak shape and width. Activation energy, as determined by using the general analytical solution, was 318 kJ/mol for unprocessed poly(lactic acid) whereas it was 280 ± 5 kJ/mol for processed materials. All the processed materials had approximately the same thermal stability (T₅ = 333.0-335.8 °C, at 95% confidence level), which was slightly lower than that of unprocessed materials (T₅ = 337.5 °C). PLA melting (during extrusion and injection) was responsible for depolymerization reactions (the small molecules formed during melting processes can volatilize readily).