High Molecular Weight Samples Research Articles

Study on the drug release property of cholesteryl end‐functionalized poly(ε‐caprolactone) microspheres

End-functionalized poly/oligo(epsilon-caprolactone)s were synthesized through the ring-opening polymerization of epsilon-caprolactone initiated by cholesterol with a hydroxyl group. Using the end-functionalized poly/oligo(epsilon-caprolactone)s with different molecular weights, the microsphere drug delivery systems were fabricated using a convenient melting-emulsion method. The drug release properties of microspheres were investigated with the presence of an enzyme, Pseudomonas cepacia lipase, as well as in the absence of the enzyme. The release profiles can be fitted nicely by the classical empirical exponential expression. Under the hydrolytic condition, the drug release is mainly controlled by Fickian diffusion, and the high molecular weight of the matrix results in a slower drug release rate. Under the enzymatic condition, the drug release is dominated by combined degradation and diffusion mechanism, and the high molecular weight sample exhibits a faster release rate that is mainly caused by the higher degradation rate of the sample with lower cholesteryl moiety content.

Conductive Fibre Prepared From Ultra-High Molecular Weight Polyaniline for Smart Fabric and Interactive Textile Applications

Panion™ fibre is prepared from a high molecular weight version of chlorine and defect free polyaniline (PANI). It is processed into textile fibre by continuous wet spinning techniques. The room temperature DC conductivity for unstretched and stretched (5x at 373 K) high molecular weight samples was 72 and 725 Ω −1 cm −1, respectively. Resistivity measurements of Panion™, from 4 to 350 K, indicate that the fibre manufacturing process and the polymer synthetic route both serve to minimize structural disorder in the solid-state. The transport resides on the metallic side of the metal-insulator (M-I) phase boundary for unstretched and stretched fibre samples prepared from high M w PANI emeraldine base. The monofilament fibre can be formed into yarns for weaving fabric, braided into 2- and 3- dimensional structures, or used to form knitted fabrics, all with tunable electronic functionality.

Tuning the Polymer Release from Hydrophilic Matrix Tablets by Mixing Short and Long Matrix Polymers

In this work a rotating disc method was developed for studying the dissolution process of "bimodal" polymer tablets, whose dissolution rates have been tuned by mixing low-molecular weight and high-molecular weight samples of poly(ethylene oxide) in various proportions. The tablets were prepared along different routes, by mixing the polymer fractions as powders or by mixing on a molecular level so that the effect of tablet heterogeneity could be assessed, but also by purifying the original powders so the effect of additives could be determined. When the mixed tablet was dominated by the low-molecular weight fraction, a faster dissolution was observed for the tablet mixed at the powder level. In those cases small gel pieces were released from the tablet during the whole dissolution process. As long as no gel piece erosion was observed, it did not matter if the two polymer fractions were blended on the molecular level or on the powder level, the steady-state dissolution rate was the same. The presence of small amounts of additives in the nonpurified commercial samples had no significant effect on the tablet dissolution within the uncertainty of the experiment.

Isotactic Poly(propylene) Crystallization: Role of Small Fractions of High or Low Molecular Weight Polymer

AbstractSummary: Two well‐characterized metallocene isotactic poly(propylene) (iPP) samples, one of high and one of low molecular weight, were blended together for studying the effects of a second component on quiescent and shear‐induced crystallization. A small amount of added high molecular weight (HMW) (up to 10 wt.‐%) polymer increases the quiescent crystallization rate. This was observed as a decrease in characteristic halftime of transmitted light intensity. The crystallization halftime increases again when adding more than 10 wt.‐% of HMW polymer. The crystallization halftime of pre‐sheared samples decreases with increasing HMW fraction and is lowest for the HMW sample by itself. For the specific shearing conditions (γ = 600, Tx = 145 °C), wide‐angle X‐ray diffraction (WAXD) images show the presence of the gamma (γ) crystalline phase of iPP for samples containing 25 wt.‐% of HMW and higher. DSC thermograms demonstrated higher crystalline fractions with increasing HMW fraction in pre‐sheared samples.Optical micrograph of an iPP sample after quiescent crystallization at 145 °C.magnified imageOptical micrograph of an iPP sample after quiescent crystallization at 145 °C.

The linear viscoelastic behavior of a series of molecular weights of the thermotropic main-chain liquid crystal polymers HBA/HNA 73/27

In this work the rheological properties of the thermotropic liquid crystalline polymer 1,4 hydroxybenzoic acid (HBA)/2,6 hydroxynaphatoic acid (HNA) 73/27 produced by Ticona have been investigated over the molecular weight range 30 000– > 51 000. The thermotropic main-chain liquid crystal polymers (TLCPs) show linear viscoelastic behavior at small strain amplitudes, as detected in strain sweep experiments performed at a constant frequency of 1, 5, 10, 30, 50, and 100 rad/s. The onset for nonlinearity (γc) significantly decreases (from ∼35% to ∼5%) as the applied frequency (ω) increases, independently of the molecular weight. When the results are plotted in terms of G*/G0* (the complex modulus normalized to the corresponding linear viscoelastic value) as a function of the stress amplitude, the different curves do not seem to collapse upon each other in the large frequency range investigated in this paper. Indeed, the stress onset for nonlinearity increases as the applied frequency (ω) increases (σc∼700 Pa at ω=1 rad/s to σc∼2000 Pa at ω=100 rad/s for Mw=51 000), opposite to the trend shown by the strain onset. The behavior of these TLCPs seems, therefore, intermediate between having a characteristic strain and stress level for the occurrence of nonlinearity. The nonlinear onset dependence on the frequency has been, then, analyzed by horizontally shifting the normalized G*(γ) profiles curves at different ω values and a master curve has been obtained. The frequency behavior of these TLCPs has also been studied showing that the lower molecular weight samples are characterized by a prevalent viscous behavior at high frequencies (G″ > G′), while decreasing ω the storage modulus tends to level off and dominates the loss modulus. On the other hand, the higher molecular weight samples (Mw=51 000 and Mw > 51 000) show a linear viscoelastic behavior that resembles that of conventional flexible polymers with a terminal regime at the lower frequencies and a transition to the rubber-like region at higher ω. Our results support, then, the hypothesis suggested in the literature [Romo-Uribe (2001)] of the existence of entanglements in the HBA/HNA 73/27 thermotropic copolyester, whose contribution increase with increasing the molecular weight.

Processing and formulation effects on rheological behavior of barley β-glucan aqueous dispersions

A freeze–thaw cycling process applied to barley β-glucan aqueous dispersions resulted in network structure development, which remained intact after thawing in both acidic and neutral conditions. The effects of freeze–thaw cycling were milder in the presence of sucrose where β-glucans were less prone to cryogel formation, giving weaker cryostructurates. The effect of three operating factors (β-glucan content, sucrose content and salt content) and their interactions on viscosity at shear rates of 10, 50 and 125 s −1 was studied using response surface methodology. Significant positive linear effects were identified for all the factors on the first two models, while salt was not significant for the response at 125 s −1. The β-glucan also exhibited significant negative quadratic effects for all three responses. Significant negative interactions were observed between β-glucan and sucrose, and β-glucan and salt for the viscosity at 10 and 50 s −1, while for the viscosity at 125 s −1, only the interaction between β-glucan and sucrose was significant. Response surface methodology and a second order regression model were used for the study of three factors (pH, temperature and time) influencing the viscosity of β-glucan dispersions of a low and a high molecular weight sample, following acid hydrolysis. Significant linear effects on viscosity of the low and high molecular weight samples were obtained for all the factors. A significant positive interaction effect was observed between pH and time for the viscosity of the lower molecular weight sample, while time exhibited a significant negative quadratic effect for the response of the high molecular weight sample. Acid hydrolysis had a stronger impact on viscosity reduction of the high molecular weight sample.

The flocculation efficiency of polydisperse polymer flocculants

The flocculation performance of three poly(acrylic acid) (PAA) samples ( M w=9×10 4, 2.5×10 5 and 1×10 6 g/mol) has been investigated. Colloidal alumina particles were used as a model system and tests were performed at pH 5. Using a single-component polyacid, it was found that the optimum dosage required to achieve supernatant clarity was similar between the 9×10 4 g/mol PAA (23 ppm) and 2.5×10 5 g/mol PAA (26 ppm), but increased dramatically with the 1×10 6 g/mol PAA (83 ppm). For the two lower molecular weight samples, flocculation occurs through a charge neutralisation mechanism. In contrast, polymer bridging is inferred to be the dominant flocculation mechanism for the high molecular weight sample. The flocculation performance of a polymer mixture, produced by blending the high and low molecular weight polyacids to give an average molecular weight of 2.5×10 5 g/mol, was also studied. Supernatant clarity from this system was found to be comparable to that from the single-component polyacid of the same (average) molecular weight. However, the optimum dosage required for the polymer mixture was about twice as much as that for the single-component reference polymer. The results suggest that for the polymer mixture no synergistic effects occur. Instead, analyses of aggregate sizes indicate an independent behaviour for the two polymers in the blend. We also examined re-suspension (under shear) and re-flocculation of the sediment formed in the initial flocculation experiments. For the three single-polymer systems, rapid re-flocculation after shear was seen for the two lower molecular weight samples suggesting a reversible aggregate breakage. For the high molecular weight sample, re-suspension resulted in the formation of a stable dispersion. This result was attributed to breakage of the high molecular weight polymer sample during re-suspension. In the case of the polymer mixture, rapid re-flocculation was again observed despite the presence of a large amount of the high molecular weight sample. This result may have important practical implications.

Gelation characteristics of acid-hydrolyzed oat beta-glucan solutions solubilized at a range of temperatures

It has been observed that under certain conditions, acid-hydrolyzed oat (1–3)(1–4)-β- d-glucan (β-glucan) solutions will form gels. However, the optimum conditions for gelation have not been fully explored. In the current work, one of the factors involved in determining gel integrity, initial solubilization temperature, was investigated. Six percent dispersions of two samples of acid-hydrolyzed oat β-glucan of medium and low viscosity ( M w =70,000 and 40,000 g/mol, respectively), were solubilized at 60, 70, 80 and 90 °C for 3 h with constant stirring. After solubilization, the samples were cooled to 25 °C and rheological measurements in flow mode showed that the viscosity decreased with increasing solubilization temperature. Size exclusion chromatography indicated that the decrease in viscosity resulted from better dispersion of aggregates rather than depolymerization. Solutions dissolved at higher temperatures gelled more slowly at 4 °C and had a lower storage modulus after 4 and 7 days. The lower molecular weight sample gelled more quickly and had a higher storage modulus after 4 and 7 days than the higher molecular weight sample. Thus, solubilizing acid-hydrolyzed oat β-glucan solutions at higher temperatures tend to more fully disperse the gum and disrupt small aggregates which may act as nucleation sites for gel formation.

New Methods for Determining the Molecular Weight of Polyaniline by Size Exclusion Chromatography

Aggregation of polyaniline emeraldine base dissolved in polar aprotic solvents containing lithium chloride salt results in distorted non-Gaussian elution peaks in the size exclusion chromatogram. This results in significant error for the calculation of the weight average and number average molecular weights measured by size exclusion chromatography. We compare the effectiveness of LiCl, LiBr, and LiBF 4 , for limiting polymer aggregation in a high molecular weight sample of polyaniline, and thereby improving chromatographic peak shape. Weight average and number average molecular weight values were most accurately determined for dilute polyaniline in NMP solutions containing LiBF 4 .

Dynamic Dilational Properties of Oil–Water Interfacial Films Containing Surface Active Fractions from Crude Oil

The dilational viscoelasticity behaviors of oil–water interfaces formed by surface active fractions of different average molecular weights and H/C atom ratios, which were distilled by supercritical fluid extraction and fractionation (SFEF) methods from Iranian heavy oil, and influences of pre‐equilibrium time on them were investigated. The interfacial relaxation processes were investigated by interfacial tension relaxation methods. The dilational moduli, elasticities and viscosities all increased and phase angles also regularly changed along with increasing number average molecular weights and decreasing H/C atom ratios. Along with increasing pre‐equilibrium time, the dilational moduli of samples increased and the phase angles decreased to the equilibrium values, as a result of the enrichment of surface active fractions into the interface. It took longer to reach the equilibrium for relatively high molecular weight samples than for low molecular weights samples. Along with increasing average molecular weights and decreasing H/C atom ratios of the samples, slow relaxation processes gradually appeared and their contribution increased. The characteristic relaxation times of similar processes of different samples also increased. The results of interfacial tension relaxation experiments and dilational viscoelasticity parameters determined by sinusoidal oscillation of interfacial area perfectly coincided with each other. The results for interfacial dilational viscoelasticity measurements, number average molecular weight and H/C atom ratios, and interfacial tension relaxation experiments illustrated that the fractions containing large condensed ring aromatic compounds that have large conjugated structures played a more important role in film‐forming and film rigidity than fractions of smaller molecules.

On the influence of molecular weight and crystallisation condition on the development of defect in highly drawn polyethylene

Longitudinal morphology of highly drawn high density polyethylenes has been investigated. Three grades of HDPEs with different molecular weights were used. Two different initial morphologies were obtained by quenching and slow cooling from the melt. The samples were drawn at 75 °C close to their breaking points and their longitudinal morphologies examined under SEM after etching. Two types of defects were observed. These are the structure containing a large number of parallel longitudinal etched pockets laid in the regions between almost parallel continuous transverse bands or ‘Pisa’ structure recently reported and transverse cracks. The appearance of these defects was found to correlate with molecular weight and thermal history of the samples. For all quenched samples, both Pisa structure and transverse cracks were observed. The number of transverse cracks was found to increase with increasing molecular weight. Apparently, the transverse cracks, which thought to limit the drawing to high draw ratios, can be retarded by drawing at higher temperature (100 °C). This suggests that the formation of transverse cracks relates to chain mobility and drawability of the materials. The Pisa structure was found to disappear from slowly cooled low molecular weight samples. For slowly cooled high molecular weight sample, the Pisa structure became less salient. It is suggested that the formation of Pisa structure is determined by molecular entanglement. Analysis of the band separation of the Pisa structure shows that there seems to be a characteristic value which depends on drawing temperature.

Production and characterization of pullulan from beet molasses using a nonpigmented strain of Aureobasidium pullulans in batch culture.

The production of pullulan from beet molasses by a pigment-free strain of Aztreobasidium pullulans on shake-flask culture was investigated. Combined pretreatment of molasses with sulfuric acid and activated carbon to remove potential fermentation inhibitors present in molasses resulted in a maximum pullulan concentration of 24 g/L, a biomass dry wt of 14 g/L, a pullulan yield of 52.5%, and a sugar utilization of 92% with optimum fermentation conditions (initial sugar concentration of 50 g/L and initial pH of 7.0). The addition of other nutrients as carbon and nitrogen supplements (olive oil, ammonium sulfate, yeast extract) did not further improve the production of the exopolysaccharides. Structural characterization of the isolated polysaccharides from the fermentation broths by 13C-nuclear magnetic resonance spectroscopy and pullulanase digestion combined with size-exclusion chromatography confirmed the identity of pullulan and the homogeneity (>93% dry basis) of the elaborated polysaccharides by the microorganism. Using multiangle laser light scattering and refractive index detectors in conjunction with high-performance size-exclusion chromatography molecular size distributions and estimates of the molecular weight (Mw = 2.1-4.1 x 10(5)), root mean square of the radius of gyration (R = 30-38 nm), and polydispersity index (Mw/Mn = 1.4-2.4) were obtained. The fermentation products of molasses pretreated with sulfuric acid and/or activated carbon were more homogeneous and free of contaminating proteins. In the concentration range of 2.8-10.0 (w/v), the solution's rheologic behavior of the isolated pullulans was almost Newtonian (within 1 and 1200 s(-1) at 20 degrees C); a slight shear thinning was observed at 10.0 (w/v) for the high molecular weight samples. Overall, beet molasses pretreated with sulfuric acid and activated carbon appears as an attractive fermentation medium for the production of pullulan by A. pullulans.

Melt Rheology of Highl-Content Poly(lactic acid)

The melt rheology of linear poly(lactic acid)s (PLA) characterized by a high content of the l-form of the monomer is comprehensively investigated. Measurements of dynamic, steady, and transient shear viscosities are presented. Extensional data on PLA are presented for the first time and show a strong strain hardening behavior. The Cox−Merz relationship is obeyed over a particularly wide range (roughly 3 decades of shear rate). Results for high molecular weight samples suggest that the plateau modulus is approximately 5 × 105 Pa. In addition, the zero shear viscosity, η0, for these materials is found to roughly scale with the expected 3.4 power vs molecular weight. The transient shear results are satisfactorily predicted using a truncated form of the K−BKZ constitutive equation and a set of Maxwell modes (Gk, λk) derived from the dynamic spectra. However, to capture the observed extensional hardening, an additional long time relaxation mode must be added to the spectrum. Time sweep measurements demonstrate that the melt stability of the polymer precludes long time measurements. Chemical changes manifest themselves in a lack of adherence to the principle of time−temperature superposition. It is shown that stabilization of poly(lactic acid) using tris(nonylphenyl)phosphite is possible and leads to a material that is thermorheologically simple within the experimentally assessable rates of deformation.

Ultrasonic depolymerization of hyaluronic acid

Hyaluronic acid (HA) was depolymerized by ultrasonication. Changes in molecular weight and molecular weight distribution were observed by size exclusion chromatography with a low-angle laser light scattering photometer. We investigated the influence of sonication intensity, temperature, HA concentration, coexisting cations and ionic strength. Results demonstrated that, with an increase of intensity, initial depolymerization rate ( k) increased and ultimatedepolymerized molecular weight ( M lim) converged to smaller size. The factors that change high-order structure of HA molecules had great influence upon the k, but not so much upon the M lim. For example, continuous sonication with 55 W depolymerized the HA to almost the same M lim (approximately 0.1×10 6), with a few exceptions. Where exceptions occurred, they were in concentrated monovalent cation solutions; the M lim increased up to about 0.3×10 6. Consequently, by regulating the sonication conditions, HA with the desired lower molecular weight and a narrow distribution could be prepared from high molecular weight samples.

Anomalous Fluorescence of Linear Poly(methylphenylsiloxane) in Dilute Solution at Temperatures below −50 °C

Fluorescence spectroscopy and 1H NMR were used to study linear poly(methylphenylsiloxane) (PMPS) in dilute solution (2 × 10-4 to 4 × 10-3 M) at temperatures below −50 °C. Several PMPS samples covering a range of number-average molecular weights ((1.9−93) × 103) and of low polydispersity were employed. The temperature dependence of steady-state fluorescence results show two different trends, depending on chain length: (i) for the high molecular weight samples, the Stevens−Ban plot of excimer−monomer intensities levels off when reaching temperatures below −50 °C and yields an apparent rate constant for excimer formation (ka) which is independent of temperature; (ii) for the low molecular weight samples, the expected bell-shaped plot is observed, and ka increases with temperature in the whole range here studied (153−293 K), including also the temperatures below −50 °C. Likewise, light scattering at the incident radiation wavelength increases abruptly upon decreasing temperature below −50 °C for the high molecular weight samples. An anomalous behavior of high molecular weight samples is also observed in 1H NMR spectra; the signals disappear in the baseline at temperatures below −50 °C, indicating a drastic loss of mobility in the millisecond time range. A conformational change coupled with interchain aggregation is argued to explain these results.

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.

A New Strictly Alternating Comblike Amphiphilic Polymer Based on PEG. 2. Associative Behavior of a High Molecular Weight Sample and Interaction with SDS

We describe the associative behavior of a strictly alternating comblike amphiphilic polymer in aqueous surfactant (sodium dodecyl sulfate (SDS)) solutions. The polymer contains normal stearyl pendent hydrophobic groups separated by poly(ethylene glycol) (PEG) of molecular weight 8000. The average number of hydrophobes per chain is 9. In the dilute regime in the presence of very low amount of SDS, the polymer associates mainly in flowerlike micelles very similar to those formed by a sample of similar structure but a lower degree of polymerization (on average, two to three hydrophobes per chain). Further addition of SDS leads to the formation of mixed micelles of decreasing NR (number of hydrophobes from the polymer per micelle). Flowerlike conformation is preserved until a SDS concentration which coincides with the critical aggregation concentration (cac) of SDS in the presence of poly(ethylene oxide) (PEO), after which the structure is disrupted. In the absence of SDS, below the polymer overlapping concentration c*, bridging between micelles leads to phase separation between a highly viscous phase rich in polymer (network of interconnected flowerlike micelles) and a phase of very low viscosity. In the semidilute regime, the viscosity against SDS concentration exhibits a maximum, a classical result for hydrophobically modified polymers interacting with surfactant micelles. The covalent linkage between the PEG−C18 sequences belonging to the same polymer chain that bridges multiple micelles is responsible for the enhanced viscosity as compared to the telechelic PEO. Rheological data show evidence of the transition from a network of interconnected flowerlike micelles to an extended uniformly connected network upon addition of SDS.

Synthesis and properties of poly(arylene vinylene)s with controlled structures

Poly(arylene vinylene)s (PAVs) have attracted interest as the light emitting layer in LEDs. Such polymers may be made by several routes. We have used McMurry, Yamamoto and Suzuki methodologies to make different PAVs. We synthesised polymers with para and also with meta linkages between phenylene and vinylene groups in an attempt to increase the energy gap and to shift the light emitting wavelength towards the blue region. The number and geometrical connectivities of the phenylenes constituting the arylene portion of the repeat unit and the cis/trans distribution have been varied in a controlled way. Relatively high molecular weight samples were produced by equilibrium fractionation. Results correlating the details of polymer structure with the observed photoluminescence are discussed.

Small-angle X-ray scattering from bisphenol A polycarbonate in tetrahydrofuran. Molecular characteristics and excluded-volume effects

Small-angle X-ray scattering measurements have been made on six fractions of bisphenol A polycarbonate with weight-average molecular weights Mw of 2.3 × 103 − 1.3 × 104 in tetrahydrofuran at 25°C using a new type of apparatus equipped with an imaging plate detector. The z-average radii of gyration < S2 >z12 and the particle scattering functions obtained are analysed on the basis of current theories for the worm-like chain, along with previous data for < S2 >z (from light scattering on high molecular weight samples) and the intrinsic viscosity [η]. Excluded-volume effects on < S2 >z and [η], which are significant for Mw above 104, are taken into account by the quasi-two-parameter (QTP) theory for the worm-like or helical worm-like chain. It is found from the analysis that all the properties examined are explained consistently by the worm-like chain whose contour is characterized by a Kuhn segment length of 3.0 nm and a linear mass density of 240 nm−1. In particular, the theory of Yoshizaki et al. for [η] of an unperturbed worm-like chain combined with the QTP theory is found to describe the molecular weight dependence of Flory's viscosity factor over the entire range of Mw examined, i.e. from 2.3 × 103 to 4.7 × 105.

Molecular Weight Effects on the Reorganization of Poly(Vinylidene Fluoride), Polyamide 12, and Poly(p-Phenylene Sulfide)

Although reorganization in metastable polymeric crystals has been recognized for over 30 years, the effect of molecular weight on the reorganization process has not been widely investigated. In this work three polymeric systems were studied: poly(vinylidene fluoride), polyamide 12, and poly(p-phenylene sulfide). For the polymers investigated, the lowest molecular weight sample was found to reorganize the most and, conversely, the highest molecular weight sample was found to reorganize the least. Comparisons of each system show that independent of molecular weight the reorganization rates among the three systems could be differentiated with the polyamide showing the greatest reorganization and the poly(phenylene sulfide) the least. Both conventional differential scanning calorimetry (DSC) and temperature-modulated DSC (TMDSC) were used in this study to investigate the reorganization process.