Search results for "Polymers"
showing 10 items of 3567 documents
A thermodynamic analysis on the coincidence of extrema conditions in the sorption equilibrium for ternary polymer systems
1993
Flory-Huggins theory of polymer solutions has been used to express the condition of extrema values in the total sorption, as well as the inversion point in the preferential adsorption parameters for termary polymer systems. Two approaches have been followed, the first considers the binary and ternary interaction parameters independent of polymer concentration and solvent composition. In the second one, this dependence has been introduced. Our attention is focused on the volume fraction of solvent mixture dependence of the above parameters, in order to confirm or not the coincidence between the extrema values and the inversion point. Several cosolvent and cononsolvent ternary polymer systems…
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…
A comparative study between cosolvent and cononsolvent ternary polymer systems through the preferential adsorption coefficient
1993
Sorption equilibrium of polystyrene and poly (dimethyl siloxane) in mixed solvents has been monitored by means of the preferential adsorption coefficient experimentally determined from intensity light scattering measurements. The pairs of solvents used to dissolve each polymer sample have been selected for the purpose to perform cosolvent and cononsolvent ternary polymer systems. Flory-Huggins formalism including ternary interactions has been used to predict the sorption equilibrium for the cosolvent system and, for the first time, for cononsolvent ones. Moreover, the proportionality between binary and ternary interactions, recognized by Pouchly, is also corroborated for both ternary polyme…
Hydrodynamic mechanisms of spinodal decomposition in confined colloid-polymer mixtures: A multiparticle collision dynamics study
2013
A multiscale model for a colloid-polymer mixture is developed. The colloids are described as point particles interacting with each other and with the polymers with strongly repulsive potentials, while polymers interact with each other with a softer potential. The fluid in the suspension is taken into account by the multiparticle collision dynamics method (MPC). Considering a slit geometry where the suspension is confined between parallel repulsive walls, different possibilities for the hydrodynamic boundary conditions (b.c.) at the walls (slip versus stick) are treated. Quenching experiments are considered, where the system volume is suddenly reduced (keeping the density of the solvent flui…
Spinodal demixing, percolation and gelation of biostructural polymers
1990
We present a variety of new experiments which concern the self-assembly of a polymeric network from homogeneous solutions of Agarose, a representative biostructural polysaccharide used for previous studies at our laboratories. They allow deriving a semi-quantitative phase diagram in the T, C plane. The diagram includes both the spinodal and gelation lines. Below a value of about 2% w/v, concentration is not sufficient for direct gelation; however, quenching of the sol from high temperatures to below the spinodal line initiates the spinodal demixing. The latter generates two sets of regions having respectively, higher- and lower-than-average polymer concentrations. In the higher-concentratio…
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…
Interfacial properties of glassy polymer melts: A Monte Carlo study
1996
The properties of the interface between a polymer melt and a solid wall are studied over a wide range of temperatures by dynamic Monte Carlo simulations. It is shown that in the supercooled state near the glass transition of the melt an “interphase” forms, the structure of which is influenced by the wall. The thickness of this interphase is determined from the monomer density profile near the surface and is strongly temperature dependent. At low glass-like temperatures it is larger than the bulk radius of gyration of the chains.
Demixing and polymerization in systems of anisotropic globular particles: A molecular dynamics simulation study
1991
We report a first set of results of Molecular Dynamics simulations of phase separation in a two-dimensional system of identical particles bearing attachment sites at their surface. Morphology of regions of aggregation so obtained appear similar to images of biologically self-assembled structures. Also, they evidence the relevance of the region of thermodynamic instability and of the presence and range of interaction of specific attachment sites. Particles of our system are apt to represent indifferently, though grossly, globular proteins or rigid cells, bearing specific and fixed adhesion sites at their surface. Results illustrate some basic aspects of the origin of the morphology of extend…
Mechanism of anionic polymerization of (meth)acrylates in the presence of aluminium alkyls
1995
Summary Methyl methacrylate was polymerized with tert-butyl lithium in the presence of triethylaluminum or triisobutylaluminium at -78~ in toluene. As indicated by GPC and MALDI-TOF mass spectrometry, the polymerization is accompanied by side reactions. The lower oligomers were fractionated by distillation and characterised by NMR, FT-IR, UV and electron impact (EI) mass spectrometry. All these data show that part of the polymer chains are carrying exactly one tert-butyl isoprenyl ketone unit. In order to avoid the formation of tert-butyl isoprenyl ketone a molar ratio of A1/Li > 2 is necessary.
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…