SEARCH · PubMed Health

Results for “Polymers”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 163 records · Page 9Linked to original sources

PubMed1992

Evaluation of mucoadhesive polymers in ocular drug delivery. II. Polymer-coated vesicles.

Association of Carbopol 934P and Carbopol 1342 (a hydrophobic modified Carbopol resin) with phospholipid vesicles was assessed by photon correlation spectroscopy and microelectrophoresis at pH 7.4 and 5. The precorneal clearance of the polymer-coated vesicles was compared to that of uncoated vesicles by lacrimal dacryoscintigraphy in the rabbit. The mucoadhesive polymer-coated vesicles demonstrated significantly enhanced precorneal retention compared to noncoated vesicles only at pH 5 (P less than 0.005). The entrapment and subsequent release of tropicamide from Carbopol 1342-coated and uncoated liposomes were determined in vitro together with an in vivo evaluation of the vesicles formulated at the lower pH. Mucoadhesive polymer-coated vesicles failed to increase significantly the bioavailability of the entrapped tropicamide compared to uncoated vesicles and aqueous solution.

Acrylic Resins↗

PubMed1996

Effects of sugars and polymers on crystallization of poly(ethylene glycol) in frozen solutions: phase separation between incompatible polymers.

PURPOSE: This study examined the effect of third components (low-molecular-weight saccharides and polymers) on the crystallization of poly(ethylene) glycol (PEG) in frozen solutions, focusing on the relationship between their crystallization-inhibiting ability and molecular compatibility. METHODS: Effects of sugars and polymers on the crystallization of PEG 3000 in frozen solution were monitored by differential scanning calorimetry (DSC). Pulsed-NMR was employed to monitor the molecular mobility of water and solutes in the frozen solutions. Miscibility between PEG and third components in aqueous solution was estimated from the lowering of cloud point of PEG 20,000. Thermal analysis of frozen solutions containing some non-crystallizing solutes was used to examine the possibility of phase separation in frozen solutions. RESULTS: Some sugars and polymers inhibited the crystallization of PEG and formed practically stable amorphous phases among ice crystals. The mobility of solute molecules in the amorphous phase increased above the softening temperature of maximally concentrated solutions (Ts), whereas that of water molecules appeared at a lower temperature. Mono- and disaccharides that are relatively less miscible with PEG in solution inhibit PEG crystallization to a lesser degree. Two Ts regions were observed in frozen solutions containing both polyvinylpyrrolidone (PVP) and dextran, at much lower concentrations than those causing aqueous two-phase separation at ambient temperatures. CONCLUSIONS: Ice crystallization raises the concentration of solutes in the remaining solution, which can lead to phase separation in the amorphous phase. Molecular compatibility between components is an important factor determining their propensity to phase separate and crystallize.

Calorimetry, Differential Scanning↗

PubMed2005

Heterogeneity in polymer melts from melting of polymer crystals.

Semi-crystalline polymers containing amorphous and crystalline regions usually have intimately mixed chains. The resulting topological constraints (entanglements) in the amorphous regions limit the drawability in the solid state. By controlled synthesis the number of entanglements can be reduced. Ultimately, crystals composed of single chains are feasible, where the chains are fully separated from each other. If such separation can be maintained in the melt a new melt state can be formed. Here we show that through slow and carefully controlled melting such polymer crystals form a heterogeneous melt with more entangled regions, where the chains are mixed, and less entangled ones, composed of individually separated chains. Chain reptation, required for the homogenization of the entanglement distribution, is found to be considerably hindered. The long-lived heterogeneous melt shows decreased melt viscosity and provides enhanced drawability on crystallization. This novel route to create heterogeneous melt should be applicable to polymers in general.

Journal Article↗

PubMed2004

Simulation of polymer--polymer interdiffusion using the dynamic lattice liquid model.

In this paper, we present computer simulation results concerning interdiffusion of fully compatible components in symmetric binary (AB) polymer mixtures in solutions. The simulation is performed in two dimensions using the algorithm based on the dynamic lattice liquid model. The solvent molecules are taken into account explicitly. The evolution of the concentration profiles in time at an interface is studied for chain lengths N=2,4,8,16 for three polymer concentrations phi=0.1,0.5,0.9. The tracer diffusion coefficients for polymer chains and for the solvent are obtained by monitoring the mean square displacements of their center of mass. The relationships between coefficients of interdiffusion and self-diffusion are tested.

Journal Article↗

PubMed2005

Transient interfacial tension and dilatational rheology of diffuse polymer-polymer interfaces.

We demonstrate the influence of molecular weight and molecular weight asymmetry across an interface on the transient behavior of the interfacial tension. The interfacial tension was measured as a function of time for a range of polymer combinations with a broad range of interfacial properties using a pendant/sessile drop apparatus. The results show that neglecting mutual solubility, assumed to be a reasonable approximation in many cases, very often does not sustain. Instead, a diffuse interface layer develops in time with a corresponding transient interfacial tension. Depending on the specific combination of polymers, the transient interfacial tension is found to increase or decrease with time. The results are interpreted in terms of a recently proposed model [Shi et al., Macromolecules 37, 1591 (2004)], giving relative characteristic diffusion time scales in terms of molecular weight, molecular weight distribution, and viscosities. However, the time scales obtained from this theoretical approach do not give a conclusive trend. Using oscillatory dilatational interfacial experiments the viscoelastic behavior of these diffusive interfaces is demonstrated. The time evolution of the interfacial tension and the dilatational elasticity show the same trend as predicted by the theory of diffuse interfaces, supporting the idea that the polymer combinations under consideration indeed form diffuse interfaces. The dilatational elasticity and the dilatational viscosity show a frequency dependency that is described qualitatively by a simple Fickian diffusion model and quantitatively by a Maxwell model. The characteristic diffusion times provided by the latter show that the systems with thick interfaces (tens of microseconds and more) can be considered as slower diffusive systems compared to the systems with thinner interfaces (a few micrometers in thickness and less) can be considered as fast diffusive systems.

Journal Article↗

PubMed2005

Approaching criticality in polymer-polymer systems.

The interfacial width of polyolefins blends has been probed as a function of distance away from the critical point by using neutron reflectivity. For strongly immiscible polymer pairs, the width of the interface increases slowly when the degree of immiscibility is decreased and the interfacial width varies with the interaction parameter chi of the polymers. Closer to the critical point the dependence on the degree of miscibility becomes stronger and the way in which the interfacial width diverges, as criticality is approached, is related to both the chain length and chi. The self-consistent field theory numerical calculations, with the additional contribution due to capillary waves, provides a good description of the width of the interface between two polymer bulk phases in particular at intermediate values of the degree of immiscibility.

Journal Article↗

PubMed2004

Polymer elasticity-driven wrinkling and coarsening in high temperature buckling of metal-capped polymer thin films.

We report the critical effects the deformational stress from the elastic nature of a confined polymer layer has during the relaxation process on the buckling of thin metal-polymer bilayer systems (less than 100 nm) even above the temperature at which the polymer is in the liquid flow region. In contrast with what is generally believed, the dispersion force does not play a significant role in the buckling. We also find that the final wrinkled waves take on the shape of wormlike islands. The coarsening leading to the island structure is driven by the growth in amplitude of the dominant wave at the expense of less dominant ones.

Journal Article↗

PubMed2006

Single polymer molecules in a protein nanopore in the limit of a strong polymer-pore attraction.

The capture and release of single poly(ethylene glycol) molecules by the alpha-Hemolysin pore are observed as time-resolved reversible steps in ion conductance. The capture on rate, inferred from the step frequency, decreases monotonically with polymer size. However, the polymer residence time shows a crossover behavior, first increasing and then decreasing with molecular weight. Our interpretation is that, in the case of polymers which are too large to be accommodated within the pore, the out-of-the-pore part of the molecule pulls on the trapped part, thus acting as an entropic spring.

Bacterial Toxins↗

PubMed2002

Thermally and electrically switchable gratings based on polymer-ball-type polymer-dispersed liquid-crystal films.

We focus on the fabrication and study of controllable holographic gratings based on azo-dye-doped and undoped polymer-ball-type polymer-dispersed liquid-crystal films. Experimental results indicate that the next step of photopolymerization of the sample with the illumination of Ar+ laser beams after UV curing causes a latent density grating to be recorded. This grating is formed by a selective secondary photopolymerization. Heating and applying a voltage change the structure of the liquid crystal and induce the appearance of the latent grating. Diffraction efficiencies versus temperature, voltage, and state of polarization are studied for both dye-doped and undoped cells and are found to be quite different. This discrepancy is attributable to the reorientation effect of liquid crystals through their interaction with the photo-induced adsorption of the doped dyes on the surface of polymer balls in the dye-doped cell.

Journal Article↗

PubMed2004

Characterization of an optoelectronic polymer, poly(2-phenoxy p-phenylene vinylene), and its precursor polymer by dynamic infrared spectroscopy.

Numerous applications of dynamic infrared spectroscopy to study a variety of polymer systems have been described in the literature. Typically, dynamic spectral changes are used to determine the molecular and submolecular reorientations that give rise to a material's observable mechanical properties. In the present study, the normal modes are characterized by their time-dependent response to an applied perturbation as an aid to assignment of the observed vibrational bands. Characterization of a newly synthesized optoelectronic polymer, poly(2-phenoxy p-phenylene vinylene), and its precursor polymer, is described. Vibrational modes along the backbone and side chain are expected to exhibit significantly different responses to mechanical perturbation due to delayed phase response of the phenoxy substituent. In-phase spectra, quadrature spectra, and two-dimensional infrared correlation maps are included in this characterization. This study has demonstrated that dynamic infrared spectroscopy can be used to distinguish backbone phenylene ring stretches from ring stretches associated with the phenoxy substituent. Density functional theory calculations are applied to confirm infrared spectral assignments. The mechanical properties are briefly discussed in light of the dynamic response.

Journal Article↗

PubMedDate not supplied

Growing cholesterol-dependent NS0 myeloma cell line in the wave bioreactor system: overcoming cholesterol-polymer interaction by using pretreated polymer or inert fluorinated ethylene propylene.

Difficulty in growing cholesterol-dependent NS0 cells in the Wave bioreactor using the original low-density polypropylene (LDPE) bags has been encountered. It has been shown that in these bags chemically defined cholesterol is depleted from solution and therefore unavailable for the cells. Our data suggest that the cause of the depletion is not chemical but is due to the physical structure of the polymer. It is proposed that polymer structures with inkbottle pores retain cholesterol, whereas structures with V-shaped pores adsorb cholesterol reversibly. Ultra-low-density polyethylene (ULDPE) bags can support cell growth but need to be pretreated with excess cholesterol. Another material, fluorinated ethylene propylene (FEP) does not need to be pretreated and is found to be superior (negligible cholesterol adsorption) as a result of its inert characteristics.

Animals↗

PubMed2006

About a formamide-based origin of informational polymers: syntheses of nucleobases and favourable thermodynamic niches for early polymers.

Formamide NH(2)CHO chemistry provides a unitary frame into which several pieces of the origin-of-life puzzle may be adjusted. Synthetic processes were uncovered which, starting from formamide and prebiotically easily available common catalysts, yield all the necessary nucleic bases precursors, including acyclonucleosides. Formamide allows phosphorylations and trans-phosphorylations, favours the micellar aggregation of surfactants and, most importantly, determines conditions in which the formation of nucleic polymers is thermodynamically favoured. In the detected conditions, the phosphoester bonds are more stable in the polymeric than in the monomeric form, thus allowing formation and survival of informational nucleic polymers.

Evolution, Chemical↗

PubMed2006

Screening of lipid carriers and characterization of drug-polymer-lipid interactions for the rational design of polymer-lipid hybrid nanoparticles (PLN).

PURPOSE: The thermodynamics and solid state properties of components and their interactions in a formulation for polymer-lipid hybrid nanoparticles (PLN) were characterized for screening lead lipid carriers and rational design of PLN. METHODS: Verapamil HCI (VRP) was chosen as a model drug and dextran sulfate sodium (DS) as a counter-ionic polymer. Solubility parameters of VRP, VRP-DS complex, and various lipids were calculated and partition of VRP and VRP-DS in lipids was determined. Thermodynamics of VRP binding to DS was determined by isothermal titration calorimetry (ITC). The solid state properties of individual components and their interactions were characterized using differential scanning calorimetry (DSC) and powder X-ray diffraction (PXRD). RESULTS: Dodecanoic acid (DA) was identified as the best lipid carrier among all lipids tested based on the solubility parameters and partition coefficients. VRP-DS complexation was a thermodynamically favorable process. Maximum binding capacity of DS and the highest drug loading capacity of DA were obtained at an equal ionic molar ratio of DS to VRP. In the PLN formulation, DA remained its crystal structure but had a slightly lower melting point, while VRP-DS complex was in an amorphous form. CONCLUSIONS: Drug loading efficiency and capacity of a lipid matrix depend on the VRP-DS binding and the interactions of the complex with the lipid. A combined analysis of solubility parameters and partition coefficients is useful for screening lipid candidates for PLN preparation.

Calcium Channel Blockers↗

PubMed1991

Microcapsules through polymer complexation. I: Complex coacervation of polymers containing a high charge density.

Acidic and basic methacrylate co- and ter-polymers based on (respectively) methacrylic acid (MAA) and dimethylaminoethyl methacrylate (DMAEMA) were prepared by solution free radical polymerization and structurally characterized by NMR spectroscopy and dilute solution viscometry. Relationships between pKa/pKb/pH, solubility and extent of ionization were determined by acid/base titration. Yields and equilibrium water contents of complex coacervates from these polymers were measured, and microcapsule forming systems based on this effect were developed as potential prostheses for organ transplantation. Short term cell viability in these capsules was demonstrated using erythrocytes as a model cell line.

Animals↗

PubMed1994

Hydrophilic-hydrophobic biodegradable polymers: release characteristics of hydrogen-bonded, ring-containing polymer matrices.

Biodegradable hydrogen-bonded, ring-containing polymers were prepared. These included poly(enolketones) by the controlled oxidation of poly(vinyl alcohol), and poly(amide-amines) and poly(amide-enamine-esters) by the reaction of diketene with diamines. These polymers had both hydrophilic and hydrophobic properties and are potentially matrix materials for the controlled release of drugs.

Biodegradation, Environmental↗

PubMed2004

Flexural properties of denture base polymers reinforced with a glass cloth-urethane polymer composite.

OBJECTIVES: A newly designed light-cured reinforcement made from urethanemethacrylate oligomer and woven glass cloth has orthotropic anisotropy. This is produced for incorporation into the outermost position under the greatest tension in denture base resins. In this study, the flexural properties of self-, heat-, and light-curing reinforced resins were determined. METHODS: The silanized glass cloth was soaked in urethanemethacrylate oligomer containing camphorquinone and 2-(dimethylamino)ethylmethacrylate. It was sandwiched between two pieces of polyethylene film and pressed to form a reinforcement sheet 0.3 mm in thickness, which was light-cured and prepared using four different surface conditions: with or without the polyethylene film and with or without a bonding agent. The reinforcement sheet was fixed in a fluorocarbon resin mold 3 mm in thickness, which was filled with self-, heat-, or light-curing resin and cured. The cured laminated plate was cut for flexural testing (40 x 7 x 3 mm3). A three-point flexural test was carried out at a crosshead speed of 2 mm/min and a span length of 30 mm. In this study, the glass fiber content was measured at percentages by weight because it was not possible to determine accurately the volume of the various polymers. RESULTS: The baseline flexural strengths of the self-, heat-, and light-curing resins were 76.2, 68.6, and 55.6 MPa, respectively, and these values were increased to 271.7, 216.4, and 266.5 MPa by the reinforcement sheet. The baseline flexural moduli of self-, heat-, and light-curing resins were 2.0, 2.4, and 2.1 GPa, respectively. These values were increased to 7.2, 5.1, and 6.6 GPa by the reinforcement sheet. SEM photographs revealed good impregnation of the glass fiber within the polymer matrix. SIGNIFICANCE: The differences in the flexural strengths and flexural moduli of the control and reinforced specimens were significant (p < 0.01).

Composite Resins↗

PubMed2004

Lignin dehydrogenative polymerization mechanism: a poplar cell wall peroxidase directly oxidizes polymer lignin and produces in vitro dehydrogenative polymer rich in beta-O-4 linkage.

An investigation was performed to determine whether lignin dehydrogenative polymerization proceeds via radical mediation or direct oxidation by peroxidases. It was found that coniferyl alcohol radical transferred quickly to sinapyl alcohol. The transfer to syringaresinol was slower, however, the transfer to polymeric lignols occurred very slightly. This result suggests that the radical mediator theory does not sufficiently explain the mechanism for dehydrogenative polymerization of lignin. A cationic cell wall peroxidase (CWPO-C) from poplar (Populus alba L.) callus showed a strong substrate preference for sinapyl alcohol and the sinapyl alcohol dimer, syringaresinol. Moreover, CWPO-C was capable of oxidizing high-molecular-weight sinapyl alcohol polymers and ferrocytochrome c. Therefore, the CWPO-C characteristics are important to produce polymer lignin. The results suggest that CWPO-C may be a peroxidase isoenzyme responsible for the lignification of plant cell walls.

Alcohols↗

PubMed2002

Physical properties and blood compatibility of surface-modified segmented polyurethane by semi-interpenetrating polymer networks with a phospholipid polymer.

Segmented polyurethanes, (SPU)s, are widely used in the biomedical fields because of their excellent mechanical property. However, when blood is in contact with the SPU, non-specific biofouling on the SPU occurs which reduces its mechanical property. To obtain novel blood compatible elastomers, the surface of the SPU was modified with 2-methacryloyloxyethyl phosphorylcholine (MPC) by forming a semi-interpenetrating polymer network (semi-IPN). The SPU film modified by MPC polymer with the semi-IPN (MS-IPN film) was prepared by visible light irradiation of the SPU film in which the monomers were diffused. X-ray photoelectron spectroscopy confirmed that the MPC units were exposed on the MS-IPN film surface. The mechanical properties of the MS-IPN film characterized by tensile testing were similar to those of the SPU film. Platelet adhesion on MS-IPN films was also investigated before and after stress loading to determine the effects of the surface modification on the blood compatibility. Many platelets did adhere on the SPU film before and after stress loading. On the other hand, the MS-IPN film prevented platelet adhesion even after repeated stress loading.

Blood Platelets↗