Dumitru Pavel

Molecular simulation of thermophysical properties of aromatic polymers containing oxetane ring in the main chain

Molecular simulation techniques have been applied to newly synthesized aromatic polymers, containing oxetane rings in the main chain, to characterize the shape of rod-like macromolecules. Single chains and periodic unit cells of a series of aromatic polymers with degree of polymerization 15 were used in the simulations, in accordance with the experimentally obtained one. The total potential energy was minimized and then NVE and NPT molecular dynamics simulations were performed for 1,000 ps at 11 temperatures between 10 and 1,000 K. The coefficient of asymmetry was calculated from the computer-generated structures. The predictive capability of the NPT molecular dynamics simulation and Polymer Properties modules of Cerius2 were used to estimate the orientational properties (order parameter), glass transition temperature, cohesive energy, and decomposition temperature of the polymers simulated. In general, there is a good-to-excellent agreement between simulated results and available experimental data of the above investigated properties.

Liquid crystalline polymers: molecular simulation of some polyethers containing oxetanic rings in the main chain

Abstract This paper presents a conformational analysis of some polyethers containing the oxetanic ring in the main chain. The studied polymers were synthesized by phase transfer catalysis, using 3,3-bis(chloromethyl)-oxetane and various bisphenols. The analysis was performed using the Cerius 2 program (version 2.0) molecular simulation software for material science, designed by Molecular Simulations Incorporated (MSI). This study tries to elucidate some aspects concerning the difficulties appearing in the liquid crystalline propertys characterization. These difficulties are induced by the cross-linking processes that take place in certain situations due to the opening of the oxetanic rings at high temperature.

Molecular simulation of aromatic polyesters containing oxetane rings in the main chain

Abstract Molecular simulation is a powerful research tool for gaining new insights into polymer chemical structures and processes. This paper presents a computational conformational analysis of some aromatic polyesters containing either an oxetane ring or propylene moieties in the main chain. The studied polyesters were synthesised by phase transfer catalysis using 3,3-bis-(chloromethyl)-oxetane, 1,3-dibromopropane and various aromatic diacids. The computational analysis and calculations were performed using the Cerius2 program (version 3.5), molecular simulation software for material science, designed by Molecular Simulations Incorporated. This study elucidates some aspects and properties dependent upon supramolecular arrangement of the macromolecular chains. In order to verify the agreement between simulated and experimental results the coefficient of asymmetry, order parameter and glass transition temperature were calculated for each studied aromatic polymer.

Simulation of diffusion of O 2 and CO 2 in amorphous poly(ethylene terephthalate) and related alkylene and isomeric polyesters

The diffusion of small molecules through polymers is important in many areas of polymer science, such as gas barrier and separation membrane materials, polymeric foams, and in the processing and properties of polymers. Molecular simulation techniques have been applied to study the diffusion of oxygen and carbon dioxide as small molecule penetrants in models of bulk amorphous poly(ethylene terephthalate) (PET) and related alkylene and isomeric polyesters. A bulk amorphous configuration with periodic boundary conditions made into a unit cell whose dimensions were determined for each of the simulated polyesters in the cell having the experimental density. The diffusion coefficients for O 2 and CO 2 were determined via NVE molecular dynamics simulations using the Dreiding 2.21 molecular mechanics force field over a range of temperatures (300, 500 and 600 K) using up to 3 ns simulation time. We have focussed on the influence of the temperature, polymer dynamics, number of CH 2 groups, density and free volume distribution on the diffusion properties. Correlation of diffusion coefficients with free volume and number of CH 2 groups was found.

Thermal behavior and molecular simulation of liquid crystalline polymers containing a pentamethylenic spacer

Abstract The paper presents a study concerning the thermal stability and molecular simulation of some aromatic polyethers, containing a pentamethylenic spacer. The polymers were synthesised using a phase transfer catalysis technique, starting from 1,5-dichoropentane and different bisphenols: 4,4′-dihidroxyazobenzene, 4,4′-dihidroxydiphenyl and bisphenol A. For the investigated polymers the molecular simulation was performed prior the synthesis in order to predict the possibility of liquid crystalline behavior. Molecular simulation was also used as a complementary analysis method for a better understanding of the thermal behavior. Thermogravimetric analysis, in static air atmosphere, with a heating rate of 10 °C/min, was used. Cerius2 and Hyperchem programs were used to perform the molecular simulations. All the polymers present a good thermostability with weight loss being up to 300 °C. The kinetic characteristics suggest a complex degradation mechanism, based on successive reactions. The inter-chain interaction estimated using the polar surface and the chain conformation do not significantly influence the polymer thermostabilities. A comparison between simulated and experimental values of the isotropisation temperature and temperature corresponding to 50% weight loss was performed.

Liquid crystalline polymers. III. Characterization by thermo-optometry of some copolyethers containing an oxetane ring in the main chain

The suitabilities of thermo-optometry and differential scanning calorimetry for the characterization of certain copolyethers were compared. Under certain conditions, such polymers do not exhibit the endotherm signal corresponding to the solid/liquid crystalline transition in the DSC curves. Thermo-optometry provides evidence of these phase transitions into the liquid crystalline state, and is a very useful additional method.

Thermal behaviour and molecular modelling of some aromatic polyethers containing a hexamethylenic spacer

We report a study of the thermal stability of some aromatic copolyethers containing a hexamethylenic spacer. The polymers were synthesized by phase transfer catalysis (in a liquid/liquid system) starting from 1,6-dichlorohexane and different bisphenols: 4,4′-dihydroxyazobenzene, 4,4′-dihydroxydiphenyl, bisphenol A and 2,7-dihydroxynaphthalene. Thermal stability was investigated by thermogravimetric analysis, in a static air atmosphere the heating rate being 10°C/min. Molecular modeling was used as a complementary analysis method for the best understanding of the relationship between the chain conformation and polarity and the thermal behavior. CERIUS2 and HYPERCHEM programs were used to perform the molecular modelling. All the synthesized polymers present similar values of the starting point of the weight loss. This behavior can be explained by supra-molecular ordering, which is probably more important than the chemical structure. The presence of the hexamethylenic spacer leads to a micro-phase separation, with a favourable influence on the ordering in the solid state. All polymers showed low values of the polar surface, with interchain interactions playing a secondary role in the thermal stability.

Thermal properties and molecular simulation of some polyethers and copolyethers with semi-rigid chains

Abstract Thermal stability of some aromatic polyethers and copolyethers, containing oxetanic structures in the main chain has been studied. The polymers were synthesized using a phase transfer catalysis technique, starting from 3,3-bis(chloromethyl) oxetane and various bisphenols: 4,4′-dihydroxyazobenzene, 4,4′-dihydroxydiphenyl, 4,4′-dihydroxydiphenylether and bisphenol A. The chain geometry (in the case of homopolymers), flexibility and polarity were correlated with thermal stability, molecular modeling being used as an additional method. All polymers showed good thermal stability, the weight loss starting above 300°C. The lowest stability was shown by the polymers containing azobenzene units. The most stable product was a copolymer based on diphenyl and diphenylether units. In the case of homopolymers, the chain geometry influences thermal stability, a linear conformation giving better results compared to a helical one. Increasing of chain flexibility can be compensated, from the thermostability point of view, by increased chain polarity, but only up to a certain value. The heating rate does not influence the degradation mechanism, as confirmed by a compensation effect investigation.

Liquid Crystalline Polymers. 15. Synthesis and Thermal Behavior of Some Aromatic Polyethers Containing Different Spacers

A series of aromatic homo- and copolyethers containing 6, 7 or 8 methylenic groups as spacers are presented. The polymers were synthesized by phase-transfer catalysis and they were characterized by using 1H-NMR, polarized light microscopy, differential scanning calorimetry and thermogravimetric analyses. All copolymers have low molecular weight, situated within the oligomer domain. They have high transition temperatures except for the case when bent or semi-flexible bisphenols were used as comonomers. Only 4,4 -dihydroxyazobenzene-based polymers showed liquid crystalline properties with a mesophase range over 50° C, and with the clearing temperatures situated near their thermal decomposition limit. The insertion of kink or flexible bisphenols was not favorable to mesophase formation (it induced too high a disorder in the system), but it lowered the transition temperatures, as expected. Few exceptions occurred in the case of bisphenol A and 4,4 -dihydroxybenzophenone. The biphasic nature of these copolyethers is most probably due to both the polydispersity and the chemical heterogeneity of the samples. Some of the semi-crystalline copolymers may exhibit virtual mesophase that can be uncovered by increasing their molecular weight or by raising the free energy of the isotropic liquid state.