ALBERT

Notes: Molecular simulation of single chain in the vicinity of nanoparticle in comparison withpure system is presented. According to the Rouse theory, chains were considered as a sequence ofbeads connected together by harmonic springs. The motion of atoms was supported by thermal energyand retarded by the resistance of surrounding. New atom position, in given time, was determined bythe Smoluchowski equation, that consists of two terms: first one includes the influence of theinter-atomic collisions, the sterical obstacles and the strong intermolecular interactions in frictioncoefficient, second one express the energy field aggregated from potentials of all atoms. Sinusoidalshear stress was applied to the chain. The output of the model was energy as a function of time. Theenergy course was also sinusoidal but shifted according to the deformation. The amplitudes and phaseshifts were analyzed for the chains under different conditions .The chains were subjected to the modelfirst as the standalone objects. Then, barrier was defined and chains placed in the vicinity of it. Thebarrier acted as a volume excluded hindrance. This type of chain molecular dynamics could be used asa stand-alone model or it could be suitable component for complex models, for example networkmodel of polymer nanocomposite

Notes: This paper describes the viscoelastic properties of network model. In the firstapproximation, nanocomposite was modeled as a 3D tetrafunctional network consideringentanglements to act as the physical x-links. Nano-sized non-deformable domains of defined shapeand size were introduced into the network. The chains in the vicinity of the inclusions wereconsidered immobilized. Hence, the semicrystalline polymer was considered a three-phase systemcontaining flexible matrix bulk phase, immobilized chains near the inclusion surface and rigidcrystalline domains. The crystallites were characterized by their Young's modulus and their tractionproperties were calculated using the Hooke's law. Unlike the model, the real polymer has viscoelasticdeformation properties. The components which could cause viscoelastic properties were introducedand their impact on viscoelastic properties of whole network was investigated. The components werefor example the reptation motion of chain in entanglements or chain, whose motion was retarded byimpact of surroundings. The model enabled to investigate the influence of each component, as well asthe influence of distribution of each component. The types of nods, whose influence was investigatedin this contribution, were fast knot, free entanglement, one-way entanglement and energy barrier

Notes: Abstract In this paper we report on the morphogenesis of dental resins formed by a thermally initiated radical cross-linking copolymerization of model dental resins. These systems were considered reasonable model simplifications of the actual resins used in commercial filling composites, veneers, etc. Effect of bis-GMA content varying from 0 to 100% on the degree of conversion and morphology of the cured resin was investigated. Dynamic scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), dynamic mechanical thermal analysis (DMTA) and high temperature solvent extraction experiments were performed in order to determine the nature and location of unreacted unsaturations in relation to bis-GMA concentration. The interval of cure temperatures, ranging from 23 to 250°C, may seem irrelevant from the clinical point of view but is instrumental for an understanding of the processes involved. Single glass transition temperature (Tg) found in DMTA experiments showed that the copolymerization process between bis-GMA and TEGDMA is random in nature, however, the overall cure reaction is inhomogeneous and this inhomogeneity is increasing with increasing bis-GMA molar fraction. Heterogeneous character of the cure was reflected into the network morphogenesis characterized by microgellation preceded by cyclization and, following connection of microgels. These cure kinetics resulted in a heterogeneous morphology of the cured resin. Moreover, it was shown that the cure kinetics controls to a great extent the degree of conversion of the reactive vinylidene C=C bonds in a fully cured resin. A decrease in bis-GMA concentration in the resin mixture led to a higher degree of conversion, however, even a neat TEGDMA homopolymer did not exhibit C=C conversion greater than 65–70% under the cure conditions used. Leaching out tests, consisting of an extraction of unreacted monomer from a finely ground cured resin in boiling cyclohexanone at 160°C over a period of 5 d, revealed total weight loss of the order of 3.2–5.6 wt % regardless of the total conversion. No uncured monomer was detected in the extract solution using FTIR spectroscopy, however, the method used is not sensitive to the presence of oligomers. It was concluded that the remaining unreacted double bonds are the inevitable result of the diffusion controled heterogeneous cure reaction. The low rate of diffusion in the resin beyond its gel point is controlled by high monomer rigidity, low mobility and radius of gyration of the monomers. The unreacted C=C bonds exist in the form of pendant, side groups chemically attached to the network. In addition, a hypothesis has been proposed that for concentrations of rigid bis-GMA monomers above 50 wt %, preferably only one methacrylate group of its molecule has reacted on the time span of the experiment performed and has been incorporated in the cured network. Hence, a substantial part of unsaturation exists in the solid polymer in the form of pendant groups formed by mechanically ineffective bis-GMA monomers. The results are in good qualitative agreement with computer simulations based on the kinetic gelation model. © 2000 Kluwer Academic Publishers

Notes: Clinical implementation of fibre-reinforced composites (FRCs) for treatment of misaligned teeth requires a stability of the material under stress in a moist environment. The performance of the material is strongly dependent on its resistance to hydrolytic deterioration of the components. Of particular concern are the quality and durability of the interfaces between the fibres and the matrix. Scanning electron microscopy (SEM) was utilized to evaluate qualitatively the effect of water immersion on the interfaces between constituents in a variety of E-glass fibre-reinforced thermoplastics. A particular series of organosilane-coated fibres embedded in polycarbonate (PC) were chosen for further quantitative study of the residual shear strength of the interfaces using an embedded-single-fibre test. Annealed PC and maleated polypropylene (MPP) reinforced with bare E-glass fibres have an appropriate combination of mechanical properties and environmental stability for potential orthodontic applications. The usual sizing agents used for commercial E-glass fibres interfere with the bonding of the fibres to the matrix and the usual silane treatments used to promote bonding could lead to problems of hydrolytic stability under extreme conditions of stress and moisture.

Notes: The effect of a deterioration of matrix and matrix-fibre interface, caused by moisture, on the flexural properties (modulus, strength and ultimate strain) of unidirectional E-glass fibre-reinforced thermoplastics [polycarbonate, poly(ethyleneterephthalate glycol) and nylon 12] was investigated. Specimens of span/thickness ratio varying from 15 to 30, prepared from pultruded thermoplastic prepregs, were tested in directions parallel to and perpendicular to the fibre orientation as moulded and after immersion in water at 85°C for 100 h. No significant reduction in longitudinal properties, controlled by fibre behaviour, resulted from exposure to water. A significant reduction in transverse properties, controlled by the matrix and interface behaviour, was observed. Polycarbonate/bare E-glass fibre composite annealed at 275°C for 1 h before immersion in water exhibited a superior resistance to moisture attack. This phenomenon is attributed to the “transcrystalline” interphase near the fibre surface and good wetting of fibres by the matrix. Resistance against moisture favours this material for further clinical application as an orthodontic wire.

Notes: The mechanical properties of an orthodontic wire pultruded from S2-glass-reinforced polyethyleneterephthalate glycol (PETG) were measured using two experimental devices simulating clinical conditions. A comparison of moduli measured in the clinically relevant devices with those measured in a standard flexural test reveals that data obtained using small cross-section, short span length clinical specimens require corrections associated with clamping and shearing effects. The clamping effect dominates and is caused by the softening of the material near the clamps. The shear effect becomes important at high fibre volume fractions and small span/thickness ratios. With adoption of these corrections, good agreement between moduli calculated using rule of mixtures and those measured in clinical tests is achieved. The analytical base developed for prediction of the stiffness of the orthodontic wire for different span/thickness ratios improves the procedure for design of dental appliances.

Notes: Abstract A spin-trapping technique and direct kinetic measurements of electron paramagnetic resonance spectra were used to investigate the kinetics of radical production in the cross-linking photo-initiator system based on camphoroquinone and dimethyl-aminoethylmethacrylate in the presence of the corresponding monomer dimethacrylate copolymer mixture, as well as in the pure photo-initiator system. Consequently, in both cases, the kinetics of radical production and decay was measured. From the measured data the formal kinetic parameters of particular radical processes were calculated by the non-linear least-squares regression method. © 1998 Chapman & Hall