Search results for "MOLA"
showing 10 items of 1216 documents
A Monte Carlo study of diffusion in "living polymers"
1996
We report the first numeric experiments on diffusion in living polymers (polymers that can break and recombine reversibly, and are characterized by an exponential molecular weight distribution). In the simulation we use a modification of the bond fluctuation model which is known to reproduce the correct Rouse dynamics of polymer chains. The diffusion coefficient D reveals a Rouse-type behaviour D ∝ 1/L, where L is the average chain length of the polydisperse system. We also find a D ∝ exp [ − V/2kBT] dependence on the bond energy V, whereas at constant temperature the diffusion coefficient turns out to be inversely proportional, D ∝ ρ−1, to the monomer density of the system ρ in agreement w…
Polymer-Polymer Interaction: Consistent Modeling in Terms of Chain Connectivity and Conformational Response
2006
An approach developed for the modeling of polymer solutions is extended to polymer blends. It accounts explicitly for the fact that the segments of a given macromolecule cannot spread out over the entire volume of the system (chain connectivity) and that the space a polymer molecule occupies may change after contact formation between the components of a mixture (conformational response ζ). The validity of the equation obtained for the Flory-Huggins interaction parameter between polymers is tested by means of critical data published for the system PVME/PS. The measured phase diagrams can be modeled equally well by two limiting assumptions concerning the temperature dependence of the conforma…
Phase Behavior of the System Linear Polyglycerol + Methanol + Carbon Dioxide
2011
To compare the phase behavior of linear and hyperbranched polymers, the phase envelopes of the ternary system linear polyglycerol + methanol + carbon dioxide were determined for polymers of varying molar mass. Phase changes were detected by a static synthetic method using the Cailletet setup for temperatures between 331 K and 421 K and pressures up to 13.1 MPa. Besides the vapor–liquid and liquid–liquid equilibria, also the vapor–liquid to vapor–liquid–liquid and vapor–liquid–liquid to liquid–liquid phase boundaries are reported. The experimental results are similar to systems with hyperbranched polymers (rather than linear polymers). For the systems with linear polymers, however, the bubbl…
Prediction of maxima and minima in the curve of total sorption parameter in ternary polymer systems. Influence of ternary interaction parameter
1993
Flory-Huggins theory modified by Pouchly has been applied to predict maxima and minima in the curve of total sorption in ternary polymer systems formed by a polymer and two liquids. In this work, different diagrams based on experimental magnitudes easily obtained such as the difference in affinities of liquids, solvents and non solvents, and the solvent molar volume ratio. Total sorption parameter has been considered to be the decisive magnitude to define extrema conditions in both cosolvent and cononsolvent ternary polymer systems. The theoretical prediction is not altered by the inclusion of ternary interactions. Different examples of ternary systems dealing with vinyl polymers and polydi…
Influence of gamma-irradiation on thermally-induced mesoscopic gelation of degalactosylated xyloglucans
2014
Thermoresponsive degalactosylated xyloglucans have been already proposed as in situ gelling scaffolds for tissue engineering, due to their reversible macroscopic thermal gelation at body temperature and biodegradability. The highly branched, hydroxyl group-rich molecular structure renders xyloglucans interesting raw materials also in the form of micro/nanoparticles for application as nanoscalar drug delivery devices in cosmetic and pharmaceutical formulations. Owing to their natural source, xyloglucans show high average molecular weight, broad molecular weight distribution and poor water solubility, as large and compact aggregates usually form via inter-molecular hydrogen bonding. Co-60 gam…
γ-radiation-initiated polymerization of vinylidene fluoride in dense carbon dioxide
2002
The γ-ray-initiated batch polymerization of vinylidene fluoride (VF2) has been investigated in dense carbon dioxide under relatively mild operative conditions (T ≤ 40 °C and P < 25 MPa). When the initial VF2 molar concentration was increased from 3.4 to 6.4 mol/L, monomer conversion increased from 20 to 73%; a similar trend was observed for the number-average molecular weight and the molecular complexity of synthesized poly(vinylidene fluoride) (PVDF) as determined from rheological measurements. Under all adopted experimental conditions, a synthesized PVDF polymer was collected in the form of a white powder. Despite the inherent heterogeneous character of the polymerization process, a homog…
Insertion of One, Two, and Three Molecules of Alkyne into the Pd−C Bond of Ortho-palladated Primary and Secondary Arylalkylamines
2009
The ortho-metalated complex [Pd2{κ2(C,N)-C6H4CH2CH2NHMe-2}2(μ-Br)2] (1) can be prepared by refluxing a 1:1 mixture of Pd(OAc)2 and N-methylphenethylamine in acetonitrile, followed by addition of an excess of NaBr. Complex 1 reacts with PPh3 to give the mononuclear derivative [Pd{κ2(C,N)-C6H4CH2CH2NHMe-2}Br(PPh3)] (2). The cationic complex [Pd{κ2(C,N)-C6H4CH2CH2NHMe-2}(py)2]ClO4 (3) can be obtained by reacting 1 with AgClO4 and pyridine. The previously reported complex [Pd2{κ2(C,N)-C6H4CH2NHCH2Ph-2}2(μ-Br)2] (A) reacts with PhC≡CCO2R (R = Me, Et) to give the monoinserted derivatives [Pd2{κ2(C,N)-C(Ph)═C(CO2R)C6H4CH2NHCH2Ph-2}2(μ-Br)2] (R = Me (4a), Et (4b)). These dimers react with neutral l…
Reactivity Towards Acidic Protic Ligands of Cyclopalladated Di‐μ‐hydroxo Complexes
2008
The dinuclear hydroxo complexes [{Pd(μ-OH)(C∧N)}2] [C∧N = 2-(2-pyridyl)phenyl (Phpy) (I), 7,8-benzoquinolyl (Bzq) (II) and 2-(2-oxazolinyl)phenyl (Phox) (III)] react in a 1:2 molar ratio with a wide variety of protic electrophiles H(L∧L) bearing different sets of donor atoms (L∧L = O∧O or O∧N) to give the mononuclear neutral palladium(II) derivatives with the general formula [Pd(L∧L)(C∧N)] [O∧O = salicylaldehydate (sal) (1), acetylacetonate (acac) (2) and benzoylacetonate (bzac) (3); O∧N = N-phenylsalicylaldiminate (N-Phsal) (4), N-p-chlorophenylsalicylaldiminate (N-pClPhsal) (5), 2-pyrrolecarbaldeydate (2-pcal) (6), 8-hydroxyquinolinate (oxin) (7)]. Structural characterisation of complexes…
A green route to synthesize poly(lactic acid)-based macromonomers in scCO2 for biodegradable nanoparticle production
2014
Poly(lactic acid)-based macromonomers, aimed at biomedical applications and with well-defined average chain length, are produced through catalytic ring-opening polymerization of L,L-lactide co-initiated by a co-monomer bearing a double bond. Reactions have been carried out in supercritical carbon dioxide (scCO2) at different temperatures, ranging from 90 to 130 °C. The resulting oligomers have been characterized by different techniques (1H-NMR, 13CNMR, MALDI-TOF, ESI, GPC, FT-IR, TGA), which show that oligomers with narrower molecular weight distribution are produced at the lowest temperature. In addition, a significant reduction of the impact of the secondary reactions has been found at th…
On The Incompatibility of Dextran and Pullulan in Aqueous Solutions and Its Modeling
2012
Joint aqueous solutions of branched dextran and linear pullulan are investigated with respect to their phase separation. The experiments demonstrate that the polymers are – depending on the molar mass of dextran – incompatible in aqueous solutions despite their chemical similarity. This finding can be modeled on the basis of an approach accounting for chain connectivity and conformational relaxation of the components. According to these calculations, the polymers exhibit a miscibility gap in joint solutions despite the favorable interactions between them. Using information on the subsystems H2O/dextran and H2O/pullulan, the assumption of complete miscibility of the polysaccharides is requir…