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Polydisperse polymer liquid crystals near the anisotropic-isotropic transition

Polydisperse systems of polymer liquid crystal (PLC) macromolecules consisting of LC and flexible sequences are considered, particularly at the phase transition LC-i from an anisotropic to the isotropic phase. The Flory and Matheson lattice model is used for formulation of the partition function; the orientation distribution function for LC orienting potential interactions introduced by Flory and Ronca is included. Using a Landau approach to calculating the Helmholtz function of the system as a function of the orientation parameter s, we obtain the formulae for parameters of the model in the vicinity of the LC-i transition. The critical values at the transition of s, of the strength of orienting interactions, the system temperature, and the anisotropy of LC sequences are calculated and discussed as functions of the polydispersity parameter d. Regions of d are found where polydispersity has a weak influence on the LC-i equilibrium parameters. There is also a region of d values in which the influence of polydispersity is quite pronounced.     

Epoxy networks modified by unsaturated oligoesters and acrylates or methacrylates

We have studied the formation of networks on the basis of an epoxy resin called ED-20, the same resin modified with tert -butylhydroperoxide (called ED-20P), unsaturated oligoesters and polyethylenepolyamine in the presence of acrylates and methacrylates at 20°C, 100°C and 130°C. The gel-fraction contents in the blends and the pendulum damping hardness of films made from the blends have been determined. The progress of gelation has been related to the composition, time and crosslinking temperature. The presence of peroxide groups enhances the extent of gelation. When those groups are present, acrylates and methacrylates provide still higher values of the ultimate gel fractions, acting as diluents (lowering viscosity of the initial blends) and simultaneously as crosslinking agents. The pedulum damping hardness tests for organic films performed as a function of time provide results parallel to those obtained by the gel fraction determination via Soxhlett extraction with acetone. Thus, both methods can be used to pursue the progress of gelation.     

Chain overlap and entanglements in dilute polymer solutions: Brownian dynamics simulation

We have analyzed chain conformations and the existence — or otherwise — of chain overlaps and entanglements in dilute polymer solutions (at concentrations c < C^*, c^* = critical concentration). The fundamental problem of existence of chain overlaps in dilute solutions is also related to the drag reduction phenomenon (DR). Some experimental results pertinent to DR are explained in terms of entanglements even for solutions at concentrations defined in ppm. We report results of Brownian dynamics simulations of polymer solutions in which the equations of motion of the chains are solved by using the Langevin equation. Chains move according to actions of a systematic frictional force and a randomly fluctuating force w(t), where t is time. In addition, a shear flow field can be introduced into the model. To evaluate the structure of polymer chains in solution we have devised a measure of interchain contacts and two different measures of entanglements. The results for c = 0.3 c^* demonstrate that both chain entanglements and overlaps take place even in dilute solution. They also confirm predictions from an earlier combinatorial model.     

Synthesis of polysiloxanes with electron-donating groups by anionic ring-opening polymerization

Anionic kinetically controlled ring opening polymerization of cyclotrisiloxane is explored as a method of synthesis of copolymers having the chain composed of dimethylsiloxane units and siloxane units bearing an uncharged nucleophilic (electron-donating, ED) group. Model monomers for these studies are derived from hexamethylcyclotrisiloxane (D₃) in which one methyl group is replaced by the organophosphorus group of general formula - (CH₂) _nP(X)Ph₂ where X is lone pair, O or S. This method is shown to give significant advantages over the thermodynamically controlled homopolymerization and copolymerization of nucleophile-substituted cyclosiloxanes which have so far been used for synthesis of ED substituted siloxane copolymers. The yield of the copolymers is much higher and the molecular weight distribution is narrow. The arrangement of the ED substituents along the chain, although not purely uniform, is much more regular as compared with the statistical distribution. The content of cyclic oligomers was higher than in the polymerization of unsubstituted D₃, which is rationalized on the basis of intramolecular interaction of the counter-ion with the ED groups.     

In situ vibrational spectroscopy of conducting polymer electrodes

The application of in situ Fourier transform infrared spectroscopy for the study of electrochemical redox reactions of conducting polymers is described. The experimental setup of two different methods, external reflection absorption spectroscopy and attenuated total reflection spectroscopy, is shown. The results of spectroelectrochemical investigations of the reactions of polyaniline in different electrolyte solutions are discussed.