Similar Number-average Molecular Weights Research Articles

Basic research about corncob residue as Lyocell spinning material

Using the corncob residue as a new cheaper source material for Lyocell spinning technology. Chemical properties of the corncob residue after extraction of hemicellulose and lignin were investigated in this paper. It was found that the main composition of corncob residue is cellulose, accompanied by slight hemicellulose and very tiny amount of spinning insoluble components. Compared to wood pulp, corncob residue has a similar number-average molecular weight, a slightly larger weight-average molecular weight, a lower peak-relative molecular weight, and a larger polydispersity. All those properties suggest that this kind of corncob residue has big potential to be used as spinning material for regenerated cellulose fiber. A new type corncob residue made fiber was produced, using the Lyocell spinning technology. Mechanical properties of the corncob residue fiber were analyzed. The corncob residue fiber has a tensile strength value between that of viscose fiber and Lyocell fiber, indicating its good application prospects. However, the corncob residue fiber has a high crystallinity and the orientation value with large fiber linear density, suggesting that the spinning technology needs to be further improved.

Synthesis of meso-lactide by thermal configurational inversion and depolymerization of poly(l-lactide) and thermal configurational inversion of lactides

Meso-lactide (MLA) was synthesized by thermal degradation of poly(l-lactide) [i.e., poly(l-lactic acid) (PLLA)] at relatively high temperatures of 250 and 300 °C for thermal degradation periods up to 750 and 360 min, respectively. Maximum MLA yields as high as 30 and 40 mol% were successfully attained at 250 and 300 °C, respectively, when thermal degradation was continued for 750 and 180 min, respectively. This indicates that the intentional thermal configurational inversion and depolymerization of PLLA and configurational inversion of lactides (LAs) at the higher temperature are favorable for the synthesis of MLA in a short period. The molecular weight and optical purity and tacticity of poly(lactide) [i.e., poly(lactic acid) (PLA) decreased and the d-lactyl unit content of PLA increased with an increase in reaction time. The higher MLA fraction for 250 °C compared to that for 300 °C at the reaction times when PLA samples had similar number-average molecular weights can be ascribed to the lower tacticity or higher racemization at chiral carbons in PLA chains.

A Comparison of Three Different Biodegradable Aliphatic Oligoesters (PGA, PLLA, and PCL) with Similar Linear Alkyl End Groups by DSC and SAXS

The biodegradable aliphatic oligoesters polyglycolide (PGA), poly(L-lactide) (PLLA), and poly(ϵ-caprolactone) (PCL) with similar number-average molecular weight Mn values but different linear alkyl end groups [CH3−[CH2−CH2]m−CH2−] were compared in terms of their physical properties, parameters such as melting temperature (Tm), crystallinity (xi), long period (L), and lamella thickness (D). They were analyzed by DSC and SAXS. The effect of a longer and nonpolar alkyl end group such as docosyl [CH3−[CH2−CH2]10−CH2−] on the long period (L) was more evident for PCL because it was the most nonpolar species in the family of oligoesters analyzed.

Smectic-phase-induced interdigitated orientation of polythiophenes bearing phenylnaphthalene mesogen

Polythiophenes bearing mesogenic phenylnaphthalene side chains at 3-position, poly(3-{10-[6-(4-octylphenyl)naphthalene-2-yloxy]decyl}thiophene-2,5-diyl) and poly(3-{10-[4-(6-octyl-2-naphthyl)phenyloxy]decyl}thiophene-2,5-diyl), in which each mesogenic core linked reversely, and their model liquid crystals corresponding to each phenylnaphthalene side chains were synthesised and compared each other to investigate liquid crystalline property and molecular arrangement. Yamamoto polymerisation of corresponding 2,5-dibromothiophene monomers afforded soluble polythiophenes. They had similar number average molecular weights about 10 kDa and exhibited similar photophysical properties, i.e, UV–vis absorption λmax due to polythiophene backbones around 390 nm and fluorescence λmax around 540 nm in solution (560 nm in film state) with energy transfer from an excited phenylnaphthalene side group to the backbone. The respective model liquid crystals showed different behaviours of phase transition temperatures, enthalpies, and reversibility, whilst both polymers had similar phase transition behaviours. A well-developed fan-shaped texture in the temperature range between isotropic liquid and glass phases was observed for the polymers by polarising optical microscopy. It is suggested that both polymers had an almost identical smectic A1 phase, which led to glass state by retaining the interdigitated molecular arrangement through a crystal E phase on cooling.

Single crystals morphology of biodegradable double crystalline PLLA-b-PCL diblock copolymers

The morphology of solution grown single crystals of a series of double crystalline diblock copolymers derived from l-lactide and ɛ-caprolactone has been investigated by means of transmission electron microscopy. The copolymers had a variable composition with a poly(l-lactide) weight percentage that ranged between 81 and 10%. All samples had a low polydispersity index (1.4–1.1) and a similar number average molecular weight (20,000–35,000g/mol).Bulk crystallization and melting behaviour of diblock copolymers were evaluated by DSC and the results demonstrated the double crystalline nature of the samples. Fractionated crystallization clearly occurred in copolymers having an intermediate composition.Isothermal crystallizations were performed in dilute n-hexanol solutions at temperatures that ranged between 80 and 50°C. Crystal morphologies were dependent on the crystallization temperature and even on the composition. Thus, the inability of poly(ɛ-caprolactone) (PCL) blocks to crystallize between 80 and 70°C rendered lozenge, truncated and spindle-shaped crystals associated to the poly(l-lactide) (PLLA) block. These usually had thicker edges due to PLLA overgrowths that mainly took place in their periphery. However, an overgrowth of irregular PCL crystals during subsequent cooling and crystallization at room temperature was also detected. Complex morphologies constituted by lamellar crystals of both PCL and PLLA blocks were developed at intermediate temperatures (70–65°C), whereas elongated hexagonal morphologies mainly associated to the PCL block were detected at the lowest crystallization temperature. In general, electron diffraction patterns showed for all samples’ reflections associated to both poly(ɛ-caprolactone) and poly(l-lactide) (α-form) crystals. The relative intensity between the two types of reflections varied according to the copolymer composition.

Thermoplastic polyester amides derived from oleic acid

Three lipid-based Polyester Amides (PEAs) with varying ratios of ester and amide linkages were synthesized. Oleic acid was used as the starting material to produce the intermediates, characterized by MS and NMR, used for polymerization. PEAs were characterized by FTIR and GPC. The PEAs were constrained to have similar number average molecular weights, in the 2 × 104 range, thereby enabling comparison of their physical properties from a structural perspective. The thermal behavior of the polymers was assessed by DSC, DMA and TGA. Thermal degradation was not affected by ester/amide ratios, but Tg increased non-linearly with decreasing ester/amide ratios and correlated with hydrogen-bond density and repeating unit chain length. Crystallinity was studied by XRD and DSC. Degree of crystallization and multiple melting behavior as a function of cooling kinetics were explained well by hydrogen-bond density, repeating unit chain length and density of ester moieties. Mechanical properties were investigated by DMA and Tensile Analysis, with a non-linear increase of storage and tensile moduli recorded as a function of decreasing ester/amide ratios. The findings suggest how approaches to the synthesis of lipid-based PEAs can be targeted to the delivery of specific physical properties.

Origin of High Segmental Mobility at Chain Ends of Polystyrene

Effects of solvents on segmental dynamics of polystyrene (PS) at the chain end have been studied by the site-specific spin-label method of electron spin resonance (ESR). PS’s having similar number-average molecular weight (ca. 25 kDa) but labeled with a nitroxde at the chain end (E-PS) or midchain segments (M-PS) individually were used for the measurements. Toluene and dioctyl phthalate (DOP) which are good and theta solvents, respectively, for PS were selected as solvents. The temperature variation of ESR spectra was simulated based on the “macroscopic order with microscopic disorder” model to determine the segmental mobility of the labeled segment. The E-PS showed obviously and slightly higher mobility compared to the M-PS in the bulk PS and in the concentrated PS/DOP (50/50) solution, respectively. On the other hand, the E-PS showed the same mobility with the M-PS within experimental uncertainties in the dilute PS/toluene (1/99), concentrated PS/toluene (60/40), and dilute PS/DOP (3/97) solutions. Name...

Influence of polydispersity of poly(lactic acid) on particle formation by rapid expansion of supercritical CO 2 solutions

Poly(lactic acid) (PLA) particles were generated by rapid expansion of supercritical PLA/CO 2 solutions (RESS). Two different PLA samples, one with high (PDI = 2.4) and the other one with low (PDI = 1.4) polydispersity but similar number average molecular weight, were compared. After micronization, the polymers were analysed by rapid-scan infrared spectroscopy, scanning electron microscopy, size-exclusion chromatography, differential scanning calorimetry, and NMR spectroscopy. Our investigation reveals that the polydispersity of the polymers strongly affects the size but not the shape of the particles. We found larger particles (∼730 nm) for the PLA with high polydispersity than for the PLA with low polydispersity (∼270 nm). In both cases, spherical particles were formed. Moreover, our results clearly show that PLA with high polydispersity is less suitable for RESS processing because the low-molecular weight chains are depleted over time and process conditions are thus not constant.

Crosslinking of Polyolefin Foams

This study investigates peroxide crosslinking of two low-density polyethylene grades of similar number average molecular weight but differing molecular weight distribution. At gel contents typically associated with dicumyl peroxide (DCP) crosslinked foams, crosslinking and melt tensile properties of the solid matrix, foaming behaviour and foam properties were not significantly different. The main work compared the effects of crosslinking systems based on DCP alone and DCP with the polyfunctional monomer, triallylcyanurate (TAC). The presence of TAC allowed specific gel contents to be achieved at significantly reduced DCP concentrations compared with DCP alone. It was also determined that cost-effective use of TAC was limited to approximately 0.5 parts per hundred resin. When comparing results of solid and foamed samples of identical gel content, the presence of TAC significantly increases the melt modulus and reduces extensibility of the solid matrix compared with DCP alone and promotes a higher nucleation density in foam. This is considered to be due to a higher crosslink density in the network fraction which introduces complications in prediction of foaming behaviour if gel content alone is considered.

Size analysis of heterogeneous nuclear RNA in an aqueous gel system

We have analyzed the size distribution of heterogeneous nuclear RNA (hnRNA) from cultured murine cells in an aqueous gel system consisting of 1.8% polyacrylamide and 0.5% agarose. hnRNA, labeled in vivo with 32P at a low specific activity, was labeled in vitro at its 3′-end with NaB 3H 4, and fractionated by gel electrophoresis. The ratio of log 32 P 3 H cpm was determined for each gel fraction and shown to be linearly related to distance migrated. The ratio of 32 P 3 H cpm was used to predict the molecular weight of 28-S and 18-S RNAs and shown to be within approx. 10% of the known molecular weight. Denaturation of hnRNA with heat or dimethyl sulfoxide gave similar number and weight average molecular weight when analyzed by gel electrophoresis. The same hnRNA analyzed by sucrose gradient velocity sedimentation had a similar number average molecular weight but much higher weight average molecular weight.

The activity of mammalian polynucleotide ligase on X-irradiated DNAs

Selected samples of heterogeneous DNA from calf thymus with similar number-average molecular weight, M n, and a low incidence of single-strand breaks were exposed in aqueous solutions to a mild X-ray dose of 1500 rads. The irradiation produced on the average about 0.2 bihelical and 2.2 monohelical scissions per DNA molecule of 1708 000 M n. The percent distribution of the chemical termini released at the radiation nicks of DNA was as follows: 64.0 OH, 9.0 PO 4 and 27.0 unknowns at the 3′ ends; 3.8 OH, 68.2 PO 4 and 28.0 unknowns at the 5′ ends. A nuclease-free polynucleotide ligase I purified about 3000-fold over the crude homogenate from calf thymus succeeded in rejoining 50% of the breaks in the X-irradiated DNA. The ability of the enzyme to close radiation nicks in DNA directly was confirmed also by experiments on synthetic poly(dA) · poly-([ 3H]dT),poly(dT)-cellulose substrates with an irradiated dT chain at either the 3′ or the 5′ side of the functional break. The poor discrimination of mammalian ligase versus nicked DNA containing radiation damage is of practical relevance. While rejoining altered nucleotide chains in the helices of DNA, the enzyme might contribute to the fixation of premutational lesions in the genetic material.