Narrow Molecular Weight Fractions Research Articles

SEC-DAD - Effective Method for the Characterization of π-Conjugated Polymers

In the article we present a unique analytical tool for the characterization of conjugated polymers – SEC-DAD (Size Exclusion Chromatography – Diode Array Detector). The chromatographic separation is performed in a conventional SEC mode which provides narrow molecular-weight fractions of the analyzed polymer. The uniqueness of the SEC-DAD combination comes out with the utilization of DAD for the monitoring the absorption characteristics of particular fractions along the molecular weight distribution. If applied in the characterization of the conjugated polymers, SEC-DAD helps to reveal the dependencies of (i) the extent of conjugation and (ii) covalent and configuration structure of the polymer on the molecular weight.

Effect of Temperature on Polyelectrolyte Expansion of Lignosulfonate

The temperature effect on the polyelectrolyte expansion of sodium lignosulfonate (SL) was studied in the range of 20 to 38 °C. A narrow molecular-weight distribution fraction of sodium lignosulfonate was first obtained by gel column chromatography, which was suitable for the hydrodynamic radius (Rh) measurement by dynamic light scattering (DLS). Dynamic light scattering experiments showed that the hydrodynamic radius of sodium lignosulfonate decreased with increasing temperature. Using a quartz crystal microbalance (QCM) and atomic force microscopy (AFM), it was found that the adsorbed sodium lignosulfonate film lost water with increasing temperature and reabsorbed water with decreasing temperature. Surface tension and contact angle experiments showed that there were more hydrophobic groups on the surface of the sodium lignosulfonate molecule as the temperature increased. It can be concluded that the sodium lignosulfonate molecule shrank and became more hydrophobic with increasing temperature. Analysis suggests that the decreasing of the hydrogen-bond interactions between the sodium lignosulfonate molecule and water molecules with increasing temperature is the primary reason for the molecular conformation change of sodium lignosulfonate.

Impact of Polymer Molecular Weight on the Dynamics of Poly(dimethylsiloxane)−Polysilicate Nanocomposites

Introducing hard spherical polysilicate nanoparticles up to 0.30 volume fraction into narrow molecular weight (M) fractions of poly(dimethylsiloxane) (PDMS) that spanned its unentangled and entangl...

A Critical Size Ratio for Viscosity Reduction in Poly(dimethylsiloxane)−Polysilicate Nanocomposites

The zero-shear-rate viscosity η0 of polymer−nanocomposites (PNCs) derived from entangled poly(dimethylsiloxane)s and rigid polysilicate nanoparticles were investigated as a function of molecular size and concentration. Narrow molecular weight fractions of polymer and nanoparticle were obtained by supercritical fluid extraction. Molecular weight properties were analyzed by size exclusion chromatography and the nanoparticle radius of gyration Rg was characterized by small-angle neutron scattering. All seven polysilicate fractions were smaller (0.75 ≤ Rg, nm ≤ 2.1) than the five polymers (3.0 ≤ Rg, nm ≤ 12). Relative to the polymer η0 the PNC η0 exhibited either plasticization (viscosity reduction) or reinforcement (viscosity increase). Only reinforcement was observed in PNCs based on the polymer below Mc—the critical molecular weight for chain entanglement effects to start influencing η0—which included an increase in the PNC η0 by over 3 orders of magnitude using 0.30 volume fraction of the largest nanoparticle. For polymers above Mc, the crossover from plasticization to reinforcement behavior could be described by a critical molecular-size ratio based on the unperturbed Rg of the nanoparticle and of the polymer. Viscosity reductions of up to 52% were achieved, and were more significant at the higher of the two nanoparticle concentrations studied. The critical nanoparticle-to-polymer Rg ratio at 298 K was 0.18 ± 0.006 and 0.13 ± 0.003 for a nanoparticle volume fraction of 0.17 and 0.30, respectively. A generalized form for the concentration dependent crossover ratio is proposed to account for perturbations in the molecular size of the PNC components that can be the basis for future studies. The effects of particle size polydispersity and temperature are discussed.

Structural Identification of the Monomeric Constituents of Petroleum Pitch

The monomer fraction of a representative petroleum pitch, M-50, was characterized in terms of both the predominant polycyclic aromatic hydrocarbon (PAH) backbones and their extent of alkylation. Such an identification of individual molecular species and their relative abundance was greatly facilitated by the use of a series of separation processes prior to analysis. In particular, low (up to 270), medium (270−335), and high (335−388) molecular-weight cuts of the monomeric fraction of the M-50 pitch were isolated by dense-gas extraction (DGE). The resultant three cuts were then further fractionated by preparative-scale gel permeation chromatography (prep-scale GPC) to obtain narrow molecular-weight fractions suitable for individual species identification by both matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry and reversed-phase high-performance liquid chromatography with photodiode array detection (HPLC/PDA). Results indicate that the monomer fraction consists of five dominant series, with each series consisting of 1−3 PAH backbones typically substituted with 0−4 methyl groups. The dominant PAH backbones include pyrene (molecular weight = 202); chrysene, benz[a]anthracene, and triphenylene (molecular weight = 228); benzo[a]pyrene and benzo[e]pyrene (molecular weight = 252); and benzo[ghi]perylene (molecular weight = 276). The highest molecular-weight series beginning at 326 could not be unambiguously identified but may in fact be a small dimer formed by the condensation reaction of pyrene and naphthalene in the starting decant oil, because the UV−vis spectra indicate the presence of a five-membered ring, something not observed in any of the other identified series.

Cylindrical Polymer Nanostructures by Solution Template Wetting

Wetting of porous templates, e.g., porous aluminum oxide with pore diameters in the range from 25 up to several 100 nm, by polymer melts has been demonstrated previously to be a highly effective way toward the preparation of polymer nanotubes. The concept evaluated in this contribution is to perform wetting from polymer solutions rather than from melts with particular emphasis on the influence of the molecular weight of the polymer used for wetting. Narrow molecular weight fractions were used for this purpose. The aim is to extend the nature of nanostructures which become available by wetting and to allow functionalization of the nanostructures by additives such as drugs or biological objects which decompose at elevated temperatures. Polystyrene was used as model polymer. Results are first of all that the quality of the solvent—good, Θ, or poor—and the polymer concentration are important wetting parameters as far as the preparation of stable cylindrical structures is concerned and second that nanowires, nanotubes, or cylindrical structures with a regular arrangement of voids can be prepared reproducibly by a proper variation of the molecular weight of the polymer used.

Field Effect Transport and Trapping in Regioregular Polythiophene Nanofibers

We report the electrical characterization of field effect transistors based on regioregular poly(3-hexylthiophene) (RRP3HT) nanofibers fabricated using nanostencil shadow masks. Mobility values were 0.02 cm2/Vs with on/off current ratios of 106. Current densities of ∼700 A/cm2 were achieved in single nanofibers. A series of Soxhlet extractions was employed to separate RRP3HT into narrow molecular weight fractions. Nanofibers made from the THF fraction exhibited superior electrical properties in terms of increased current levels and decreased activation energy. The lowest activation energies in the nanofibers were achieved by using top contacts (i.e., vapor-deposited metal on top of the nanofibers) and material purified by Soxhlet extraction. Contact effects were eliminated from bottom contact devices (i.e., nanofibers on top of metal electrodes on a dielectric substrate) with a four-probe geometry. Temperature-dependent measurements reveal two distinct regimes of transport. The high-temperature regime (355−245 K) is characterized by activation energies of 62−145 meV depending on contact geometry and RRP3HT purity with a Meyer−Neldel energy of 33 ± 3 meV. The low-temperature regime (235−85 K) has lower activation energies of 31−112 meV. Shifts in the turn-on gate voltage with temperature indicate ∼4.8 × 1012 acceptor-like states/cm2 (or ∼1 per nm of fiber length) and 3.2 × 1012 donor-like states/cm2 exist in the nanofibers. We propose that transport can be explained in terms of the multiple trap and release (MTR) or variable range hopping (VRH) formalisms of transport in a bimodal, exponential distribution of shallow and deep donor-like states.

Lamellar and Crystalline Layer Thickness of Single Crystals of Narrow Molecular Weight Fractions of Linear Polyethylene

The thermal dependence of the lamellar thickness (Lp) and the thickness of crystalline layers (Lcryst) of linear polyethylene single crystals has been revisited. LPE fractions with different average molecular weights were crystallized at the same temperatures to avoid the delicate problem of an arbitrary choice of their equilibrium dissolution temperature to calculate their degree of supercooling. This procedure allows direct check of the thicknesses of the crystalline layers dependence on the degree of supercooling. Indeed, the equilibrium melting and dissolution temperatures depend on the average molecular weight. Mats of linear polyethylene single crystals of various narrow molecular weight fractions were obtained from dilute p-xylene solutions. Their lamellar thickness was determined from the SAXS intensity curve. The crystalline layers thickness was calculated from the linear correlation function of the SAXS intensity curves and from the LAM mode frequency of the Raman spectrum. It depends as expected on the crystallization temperature. However, at a given crystallization temperature, both the thicknesses of the lamellae and of the crystalline layers are nearly independent of molecular weight and therefore also of the degree of supercooling estimated from a well-known semiempirical relationship.

Fractionation of moderate molecular weight polysiloxanes by centrifugal TLC.

Fractionation of polydispersed polysiloxanes (1-2 g) into narrow molecular weight fractions has been achieved by a rapid (30-60 min) convenient process, using the Cyclograph Centrifugal Chromatography System. These narrow molecular weight poly(dimethylsiloxane) fractions can be used as secondary standards for GPC.

Cold Crystallization of Narrow Molecular Weight Fractions of PEEK

Several amorphous narrow molecular weight fractions of poly(aryl ether ether ketone) or PEEK have been heated from below their glass transition temperature to above their final melting point. The semicrystalline morphology induced by cold crystallization and its thermal evolution are studied by time-resolved simultaneous small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction, and differential scanning calorimetry. The long period and the crystalline and the amorphous thicknesses are computed from the correlation function of the SAXS curves. The evolution of the structural parameters of the various semicrystalline samples can be reconciled with the existence of a single distribution of lamellae which undergoes reorganization during the continuous heating. The reorganization process as well as the semicrystalline morphology depends on the molecular weight of the fractions. The amorphous thickness strongly increases with the molecular weight while the thermal evolution of the thickness of the crystalline layers is similar for all fractions. These experimental results support the view that the larger degree of entanglements of the high molecular weight samples impedes the reorganization mechanism with the consequence that the apparent melting temperature decreases with increasing average molecular weight.

Morphological Study and Melting Behavior of Narrow Molecular Weight Fractions of Poly(aryl ether ether ketone) (PEEK) Annealed from the Glassy State

The morphology of several semicrystalline poly(aryl ether ether ketone) (PEEK) samples with narrow molecular weight distributions and of a commercial sample of PEEK has been investigated by means of differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS). The use of different molecular weights allows us to enlarge the macroscopic crystallinity range usually attainable for the commercial PEEK grades. WAXD and DSC data are in good agreement to estimate the weight degree of crystallinity values ranging from ∼15% for the highest molecular weight sample to ∼55% for the lowest molecular weight sample. Most of the investigated samples exhibit the well-known double melting behavior with a low and a high melting endotherm. This behavior is interpreted in the light of a melting−recrystallization mechanism which is strongly affected by the presence of entanglements in the amorphous zones. The analysis of the SAXS data shows that, under constant annealing ...

Melt memory effects in polymer crystallization

AbstractPolymer melts submitted to local shear flow fields, as well as isotropic solid samples, were treated in the melt at different temperatures and for various duration of time. The effects of thermal history on kinetics and morphological characteristics of subsequent isothermal crystallization was investigated. Results suggest that subcritical pseudocrystalline aggregates, behaving as predetermined athermal nuclei when temperature is brought below melting point, can survive in the melt for long time. Their concentration decays exponentially with a single characteristic relaxation time. For all investigated polymers, temperature dependence of relaxation times can be fitted by an Arrhenius law with high apparent activation energy. Experiments performed with narrow molecular weight fractions of poly(ethylene oxide) suggest that relaxation times relevant to destruction of oriented nucleation precursors are proportional to M1,5.

Analysis of human T cell responses to group A streptococci using fractionated Streptococcus pyogenes proteins.

Cell extract and spent culture supernatant proteins from Streptococcus pyogenes Manfredo strain (type M5) were each separated to give 22 narrow range molecular weight fractions by blot-elution from SDS-polyacrylamide gels. Eluted samples and unfractionated proteins were screened for T cell stimulatory activity using human peripheral blood mononuclear cells (PBMC) from healthy adults in proliferation assays. Responses were measured in 4- and 7d cultures. Responses to a wide range of cell extract proteins were revealed by fractionation, the degree of response to each fraction varying between donors. Unfractionated culture supernatant proteins elicited proliferative responses by PBMC from all individuals examined. Responses to culture supernatant fractions containing 25-33 kDa proteins could be attributed to known superantigens. Furthermore, samples from culture supernatants containing higher molecular weight fractions (> 45 kDa) elicited responses in 50% of donors in 7d cultures, suggesting that these fractions contained common recall antigens. The efficacy of using electroeluted samples to identify T lymphocyte stimulatory proteins was confirmed by demonstrating that a known superantigen of S. pyogenes Manfredo strain, streptococcal pyrogenic exotoxin C (SPEC), could be fractionated successfully using this method and its activity recovered. Our results show that human T cell responses to group A streptococci involve a remarkably wide range of both cell-associated and released streptococcal proteins.

Analysis of human T cell responses to group A streptococci using fractionated Streptococcus pyogenes proteins

Cell extract and spent culture supernatant proteins from Streptococcus pyogenes Manfredo strain (type M5) were each separated to give 22 narrow range molecular weight fractions by blot-elution from SDS-polyacrylamide gels. Eluted samples and unfractionated proteins were screened for T cell stimulatory activity using human peripheral blood mononuclear cells (PBMC) from healthy adults in proliferation assays. Responses were measured in 4- and 7d cultures. Responses to a wide range of cell extract proteins were revealed by fractionation, the degree of response to each fraction varying between donors. Unfractionated culture supernatant proteins elicited proliferative responses by PBMC from all individuals examined. Responses to culture supernatant fractions containing 25–33 kDa proteins could be attributed to known superantigens. Furthermore, samples from culture supernatants containing higher molecular weight fractions (>45 kDa) elicited responses in 50% of donors in 7d cultures, suggesting that these fractions contained common recall antigens. The efficacy of using electroeluted samples to identify T lymphocyte stimulatory proteins was confirmed by demonstrating that a known superantigen of S. pyogenes Manfredo strain, streptococcal pyrogenic exotoxin C (SPEC), could be fractionated successfully using this method and its activity recovered. Our results show that human T cell responses to group A streptococci involve a remarkably wide range of both cell-associated and released streptococcal proteins.

Ostwald ripening in immiscible polyolefin blends

AbstractThe coarsening in the quiescent melt of the phase‐segregated particles of a polymer blend, composed of a narrow molecular weight fraction of an unbranched high‐density polyethylene (HDPE) and a highly branched (100 ethyl branches/1000 C atoms) hydrogenated polybutadiene (HPB) was studied. The system was effectively binary, due to the narrow molecular weight and composition distributions of each component. The system was composed of 90 wt % of the HDPE and 10 wt % of the HPB and it formed a two‐phase system in the melt at 177°C. The blend was precipitated from xylene solution in order to obtain an initially intimately mixed system. This was the third study in a series of studies of the coarsening of phase‐segregated particles in polymer blends. This study was unique in that the system studied was binary in this case while the previous systems were multicomponent. Since the present system was binary, exact thermodynamic calculations of the phase state of this system could be applied with a high level of confidence. The droplet phase particles, which were mainly composed of the HPB, were observed to coarsen on storage in the melt for times of from 5 s to 1 h. At the shortest storage time of 5 s the particles had an average radius of about 0.05 μm and coarsened to about 0.2 μm after 1 h storage in the melt state. Particle dimensions were measured by scanning electron microscopy of n‐heptane‐etched and gold‐coated sections. It was found that the volume of the particles increased linearly with time and that the rate constant of coarsening was Kexp = 1.23 × 10−18 cm3/s and this agreed fairly well with the rate constant calculated from Ostwald ripening theory of Kce = 0.86 × 10−18 cm3/s. In contrast the rate constant for direct particle diffusion and coalescence was Kc = 3.6 × 10−20 cm3/s. Since this was two orders of magnitude smaller than the rate constant for Ostwald ripening, it was concluded that, although the linear increase of volume with time was also consistent with the particle diffusion and coalescence mechanism, this was not a significant contributor to the coarsening mechanism. The major cause for the insignificance of the particle diffusion and coalescence mechanism was the high melt viscosity of the matrix polymers. The application of the Ostwald ripening theory to this system could be made with a high level of confidence because it was binary. It was found that the phase concentration of the droplet phase apparently underwent a rapid increase during the first 1‐2 min of storage in the melt, indicating that the system did not reach phase equilibrium (i.e., did not completely phase‐segregate) for about 1‐2 min. This further indicated that the long‐time coarsening regime was not entered until after this length of time. The particle size distributions remained approximately self‐similar over the period of coarsening, as predicted by Ostwald ripening theory. © 1995 John Wiley & Sons, Inc.

Crystallization, melting and morphology of syndiotactic polypropylene fractions: 2. Linear crystal growth rate and crystal morphology

Linear crystal growth rates of two narrow molecular weight fractions of syndiotactic polypropylene having the same syndiotacticity have been measured using polarized light microscopy over a temperature range of > 20°C. It has been found that a regime III to regime II transition at a supercooling of ∼ 50°C exists. Structure analysis via electron diffraction (ED) experiments indicates that no change of growth planes has been found during this regime transition. Nevertheless, a gradual change of the crystal perfection due to a chain packing change from a crystal incorporated with isochiral packing defects to a majority of cell III structure in this supercooling range has been observed. The validity of the nucleation theory applied to s-PP is discussed. For the crystal morphological study, single crystals with rectangularly faceted lamellae can be grown at high crystallization temperatures (low supercooling) in thin s-PP film samples as observed via transmission electron microscopy. Similar to the results reported by Lovinger and Lotz, the ED patterns show that the long axis of the single lamellar crystal is the b-axis. On decreasing the crystallization temperature, spherulites are developed. Cracks on the lamellar crystals have been observed, and they are always perpendicular to the b-axis. This phenomenon has been explained by invoking the observation that the coefficient of thermal expansion along the b-axis is about one order of magnitude larger than that along the a-axis, as measured via wide-angle X-ray diffraction experiments. However, at high crystallization temperatures, the cracks are found less frequently. This is due to the pure cell III crystal packing that forms at these tempratures leading to the incorporation of fewer isochiral packing defects which promote crack initiation.

Measurement of Viscoelastic Properties of Polymer Solutions by Torsional Crystals

The network analyzer was used for measurement of the viscoelastic properties of dilute polymer solutions. It is a sensitive instrument to measure resonance resistance and resonant frequency of crystals by the admittance circle diagrams shown on the digital oscilloscope. Measurements can be conducted more correctly and rapidly compared to the previous measuring system. Its validity was verified by measuring the complex shear modulus of dilute polymer solutions and comparing the results with the Rouse theory. The complex shear modulus was determined by measuring changes in the properties of a torsionally vibrating crystal. Frequency dependence of complex shear modulus is investigated in the frequency range from 3 to 252 kHz and temperature range from −60 to 23°C. The samples are several kinds of dilute solutions of polystyrene in toluene. The use of polystyrene is limited in narrow molecular weight fractions ranging in number-average molecular weight from 2.1×103 to 1.60×105. The results agreed with the theory by Rouse.

Fractionation of linear saturated (co)polyesters by differential precipitation

Samples of poly(ethylene terephthalate) (PET), poly(ethylene terephthalate-co-naphthalene-2,6-dicarboxylate) (PENT) and poly(ethylene naphthalene-2,6-dicarboxylate) (PEN), materials which find application in textile and packaging industry, have been investigated, with the aim to obtain narrow molecular weight fractions suitable for further characterization. Preliminary results concerning with the fractionation of gram-quantities of the polyesters are reported, using a method based on the fractional precipitation from a phenol/1,2,4-thrichlorobenzene solution of the polymer, using n-heptane as non-solvent. Obtained fractions have been characterized by intrinsic viscosity measurements and Gel Permeation Chromatography analysis.

Synthesis and characterization of narrow molecular-weight distribution fractions of poly(aryl ether ether ketone)

Synthesis and characterization of narrow molecular-weight distribution fractions of poly(aryl ether ether ketone)

Soluble polymeric carriers for drug delivery. Part 2. Preparation and in vivo behaviour of N-acylethylenimine copolymers

The preparation, characterization and in vivo behaviour of novel N-acylethylenimine (NAEI) copolymers are described and the potential use of such materials for drug-delivery applications is discussed. 2-Methyl-2-oxazoline (MeOZO) and 2-(4-hydroxyphenyl)-2-oxazoline (HOPheOZO) were copolymerised in bulk, in vacuo to give water-soluble copolymers of weight average molecular weights ( M w ) of below 30,000 Da. M w was determined by GPC using poly(oxyethylene) (POE) as calibrant. Poly[ N-(2-hydroxypropyl)methacrylamide] [poly(HPMA)] was also examined by GPC and the results were compared with those from the NAEI copolymers. Narrow molecular weight fractions of the copolymers were radioiodinated to give two 125I-labelled copolymers of M w = 15,300 Da , and M w = 29,300 Da , with polydispersity M w M n ⩽ 1.3 , in both cases. These were administered intravenously to mice and an assessment was made of their biodistribution. 24 h after administration of the 15,300 Da ( M w ) 125I-labelled copolymer, 7% of the injected dose was found in whole blood. This compares with 28% of the injected dose remaining after 24 h, following administration of the 29,300 Da ( M w ) 125I-labelled copolymer. The overall biodistribution pattern of both iodinated copolymers was found to be very similar. A significant amount of the copolymers appeared to remain in the body after leaving the vascular compartment, with a tendency to accumulate in the skin and muscle. No accumulation of radioactivity was found associated with organs of the mononuclear phagocyte system.