N. V. Lukasheva

Effect of the SO2 group in the diamine fragment of polyimides on their structural, thermophysical, and mechanical properties

Experimental and theoretical investigations, including an all-atom computer simulation, are performed for block samples of thermoplastic polyimides, amorphous R-BAPS (based on R dianhydride 1,3-bis(3′,4-dicarboxyphenoxy)benzene and diamine BAPS 4,4′-bis(4″-aminophenoxy)biphenyl sulfone), and crystallizable R-BAPB (based on R dianhydride and diamine BAPB 4,4’-bis(4″-aminophenoxy)biphenyl), which differ in either the presence or absence of the sulfone group in the repeating unit of the polyimide macromolecule. The features of thermophysical, structural, and mechanical properties of R-BAPS and R-BAPB are related to the formation of associates from sulfur and oxygen atoms of the sulfone group that are stabilized by electrostatic interactions.

Evaluation of the characteristic equilibration times of bulk polyimides via full-atomic computer simulation

The full-atomic computer simulation of bulk plastic polyimides based on dianhydride 1,3-bis(3′,4-dicarboxyphenoxy)benzene and two types of diamines, 4,4′-bis(4″-aminophenoxy)diphenyl sulfone and 4,4′-bis(aminophenoxy)diphenyl oxide, is performed on the microsecond scale via the moleculardynamics method. For the investigated molecules, which consist of eight repeating units, the limiting values of the characteristic sizes of individual polymer chains are established. The limiting sizes obtained via computer simulation are in good agreement with theoretical values calculated in terms of virtual-bond formalism. It is found that the time of sample equilibration for the full-atomic computer simulation of bulk plastic polyimides is ∼1 μs, which agrees in order of magnitude with the displacement time of the center of mass of an individual molecule by a distance equal to its own size.

Interaction between nanosized crystalline components of a composite based on Acetobacter xylinum cellulose and calcium phosphates

A composite consisting of two nanosized biocompatible components, Acetobacter xylinum cellulose and calcium phosphate, is prepared through aggregation in an aqueous suspension. The structures of initial components and composite are investigated by the methods of X-ray and electron diffraction and electron microscopy. The mineral component consists of two crystalline phases, hydroxyapatite and whitlockite (magnesium-containing tricalcium phosphate), which are nanosized platelike crystals. The composite preserves the crystalline structures of initial calcium phosphates and cellulose. In the course of composite formation, hydroxyapatite and whitlockite crystallites are adsorbed on the surfaces of nanofibrillar cellulose ribbons. Whitlockite nanocrystals are predominantly deposited on the surface of cellulose ribbons. The mutual orientation of the surfaces of crystalline structures of cellulose and two types of calcium phosphates, hydroxyapatite and whitlockite, is analyzed by means of computer simulation, and the variants of mutual arrangement of their surfaces during formation of the interfacial boundary are suggested.

Computational Modeling of Polylactide and Its Cellulose-Reinforced Nanocomposites

Computer simulations based on all-atom models can provide significant assistance to researchers in understanding molecular mechanisms that lead to the appearance of new properties in composites different than those of the individual components. Recently, this simulation of polymer nanocomposites has become more popular, since it provides answers that cannot be obtained experimentally. To date, there have been only a few publications devoted to the computer simulations of polymer nanocomposites based on cellulose, even though these materials are very promising and attract great attention because of their renewable character. One of the main biodegradable polymers, obtained from natural sources and used for developing such biocomposites, is polylactide (PLA). This chapter focuses on computer simulations of PLA and nanocellulose, as well as on composite materials based on these materials.

Influence of protonation on the conformations of rigid-chain polymers

The conformations, internal rotation barriers, and the energies of bending deformation of molecules that model chain segments of the rigid-chain polymer poly(p-phenylenebenzobisoxazole) were calculated with the use of the AM1 semiempirical quantum-chemical method. The model molecules included several heterocycles and had a size larger than the repeat unit of the polymer, which includes one heterocyclic ring and one phenylene ring. For molecules in which all heterocyclic rings are diprotonated (at nitrogen atoms), the planar conformation is optimal (as in the case of uncharged molecules). At the same time, the internal rotation barriers in such molecules are reduced relative to the neutral molecules. However, when not all heterocycles are protonated in the molecule, the barriers turn out to be substantially higher than in the neutral molecule. For molecules in which all heterocycles are tetraprotonated, ab initio calculations of the optimal conformation were also performed. For these molecules, the conformation in which the phenyl rings and the heterocyclic rings are turned by almost 50° with respect to one another appeared to be optimal. In this case, the height of the rotation barriers is even lower than in molecules with diprotonated heterocycles. The protonation was found to have a weak effect on the bending rigidity of the poly(p-phenylenebezobisoxazole) chain.

Conformational variability of helix sense reversals in poly(methyl isocyanate)

The conformations and energies of several helix sense reversal geometries in poly(methyl isocyanate) (PMIC) have been determined using the PCFF forcefield. In an extension of previous studies, a larger conformational variability for a helix sense reversal has been investigated. In addition to the reversal geometry previously detailed by several authors that results in a relatively small angle deviation from the rod-like polyisocyanate structure, we report the discovery of reversals of similar energy with much larger angle deviations from linearity. The effect of electrostatic interactions as controlled by the value of the dielectric constant, e, on the conformation and energy of a reversal is also shown to be important. At e=1.0 (vacuum) the conformations of the reversals with large and small angle ‘kinks’ have similar energies. However, at e=2.0 (non-polar organic solvent) and e=3.5 (bulk state) the reversals corresponding to the large angle kinks have lower energies.

Investigation of the role of the pyrimidine ring in the main chain of polyamido acids and polyimides. 1. Supermolecular structure of polypyromellitimides based on 2,5-bis(p-aminophenyl)pyrimidine and its carbocyclic analog 4,4′-diaminoterphenyl

A detailed comparative study of polypyromellitamido acids and polyimides based on 2,5-bis(paminophenyl)pyrimidine and 4,4′-diaminoterphenyl was conducted to determine the role of the nitrogencontaining pyrimidine ring in the main chain of the polymers. A model of the layer packing of polyimide chains in crystalline regions was proposed based on the data from x-ray structural analysis and quantum chemical calculations. The existence of tilted packing caused by partial displacement of the solvent molecules bound to the pyrimidine ring of the neighboring macromolecule with higher basicity in pyrimidine-containing polymers was hypothesized.

A model of supermolecular structure of stiff-chain polyimide copolymers and composites☆

Abstract Microstructure and molecular packing in stiff-chain polyimide copolymers and composites of various composition was studied by PMR, X-ray diffraction, and also by calculating the diffraction patterns for model systems. Copolymers of different microheterogeneity were synthesized; it is shown how to characterize the microheterogeneity in the stage of poly(amic acid) by means of PMR and in the polyimide stage by means of X-ray diffraction measurements. A generalized model has been proposed for the domain structure of polyimide copolymers and composites, which shows the latter to be molecular composites and elucidates the reasons underlying the improvement of physico-mechanical over those of homopolymers.

Calculation of the mutual packing of poly(4,4′-oxydiphenylene) pyromellitimide chains☆

Abstract The mutual packing of poly(4,4′-oxydiphenylene) pyromellitimide chains based on pyromellitic acid dianhydride and 4,4′-diaminodiphenyl ether in ordered regions is calculated by an atom-atom approximation and by a quantum chemical semiempirical method. The calculation indicated the most probable packing, i.e. face-centered layered packing. The presence of other packings, close in energy, can explain the experimentally observed mesomorphism of structure, which retains the layered character, but without the ideal order along the macromolecule axis.

Molecular-Level Insight into the Interaction of Phospholipid Bilayers with Cellulose

Molecular-level insight into the interactions of phospholipid molecules with cellulose is crucial for the development of novel cellulose-based materials for wound dressing. Here we employ the state-of-the-art computer simulations to unlock for the first time the molecular mechanisms behind such interactions. To this end, we performed a series of atomic-scale molecular dynamics simulations of phospholipid bilayers on a crystalline cellulose support at various hydration levels of the bilayer leaflets next to the cellulose surface. Our findings clearly demonstrate the existence of strong interactions between polar lipid head groups and the hydrophilic surface of a cellulose crystal. We identified two major types of interactions between phospholipid molecules and cellulose chains: (i) direct attractive interactions between lipid choline groups and oxygens of hydroxyl (hydroxymethyl) groups of cellulose and (ii) hydrogen bonding between phosphate groups of lipids and celluloses hydroxymethyl/hydroxyl groups. When the hydration level of the interfacial bilayer/support region is low, these interactions lead to a pronounced asymmetry in the properties of the opposite bilayer leaflets. In particular, the mass density profiles of the proximal leaflets are split into two peaks and lipid head groups become more horizontally oriented with respect to the bilayer surface. Furthermore, the lateral mobility of lipids in the leaflets next to the cellulose surface is found to slow down considerably. Most of these cellulose-induced effects are likely due to hydrogen bonding between lipid phosphate groups and hydroxymethyl/hydroxyl groups of cellulose: the lipid phosphate groups are pulled toward the water/lipid interface due to the formation of hydrogen bonds. Overall, our findings shed light on the molecular details of the interactions between phospholipid bilayers and cellulose nanocrystals and can be used for identifying possible strategies for improving the properties of cellulose-based dressing materials via, e.g., chemical modification of their surface.