Special Aspects of Nitrocellulose Molar Mass Determination by Dynamic Light Scattering

The dynamic light scattering method was successfully applied to determine the molar mass of nitrocellulose. The methodology of nitrocellulose fractionation in acetonic solutions is described in detail; six polymer fractions with monomodal distribution were obtained. It was shown that the unfractionated colloxylin with polymodal molar mass distribution had mass average molecular mass values of 87.3 ± 14.1, 28.3 ± 7.3, and 0.54 ± 0.17 kDa when investigated by the dynamic light scattering method. The viscometric method only provided integral viscosity average molar mass equal to 56.7 ± 5.8 kDa.

Liquid-Crystalline Order in the Phosphorus-Containing DenDrimers

The structure of phosphorus-containing dendrimers has been studied by IR spectroscopy and optical polarization microscopy. The repeating units of dendrimer molecules are mesogens. This property arises from the conjugation of the aromatic ring and the hydrazone group. An analysis of the IR spectra showed that, with an increase in the generation number, the width of the stretching vibration bands ν(PN) and ν(PO) increases. Difficulties in packing molecules of higher generations cause conformational diversity. The shape of the dendrimer molecules was determined by analyzing the increments of dipole moments. Additionally, the modeling of the stacking of repeating links was performed. The spherical model of molecules does not satisfy the experimental dipole moments of the dendrimers. The flat disk model is more suitable for explaining step changes in dipole moments. The liquid-crystalline ordering of dendrimers under the action of applied pressure was found. With simultaneous heating and uniaxial compression, optical anisotropy appears in dendrimers. It is associated with the formation of liquid-crystalline order. However, a thermodynamically stable liquid-crystalline phase is not formed in this case. Dendrimers most likely have disk-shaped molecules.

The fibrils untwisting limits the rate of cellulose nitration process

The rate of cellulose nitration is lower compared to the low molecular weight substances. Theoretical estimates assess that the rate of the nitrating agents' diffusion cannot provide for the characteristic time of cellulose nitration, as the densely packed regions are too small. However, the electrostatic barrier between the nitrating mixture and the microcrystallites makes the latter inaccessible for the nitronium ion. The cellulose nitration rate decreases and the transformation of elementary fibril structure occurs at the same degree of substitution corresponding to a complete nitration of the fibrils' surface. The supercoiled macromolecules in the elementary fibrils cannot dissociate without untwisting. The fibrils' untwisting as well as their swelling are very slow. Thus, we propose that the nanofibrils' untwisting limits the rate of the nitronium ion transport into the cellulose nanofibrils and, thus, the rate of the nitration reaction as a whole.