Layered fillers glass transition temperature

In adhesive joints, for example with epoxy resin adhesives, internal stress in the adhesive layer may result from different coefficients of expansion in the glued materials, whereby their moduli of elasticity are important factors. The glass transition temperature, and thus the curing temperature, also play a role. The reaction shrinkage of the resin is another source of internal stress. Suitable formulations with added fillers, oligomers or copolymers are among the measures taken to reduce these influences. 

The hardening of the boundary layer of epoxy and polyester compositions was studied by IR spectroscopy (of repeatedly disturbed total internal reflection—RDTIR) on a UR-20 spectrometer. A KRS-5 prism N =18, C = 45°) and a germanium prism N =14, C = 45°) were used for simulation of the substrate surface. The thickness of a polymer layer was 0.5-1 nm for the element made of germanium and 2-3 nm for KRS-5. Inhibition of polyester resin polymerization by atmospheric oxygen was eliminated by applying a polyethylene film to the surface of a system to be hardened. The thickness of the applied epoxy and polyester composition was up to 20-30 nm. To evaluate the glass-transition temperature and the polymer-solvent interaction parameter, we used reversed-phase gas chromatography. Basalt-1 and basalt-2 (see below) served as fillers and as a model substrate. The... 

The basis for a thermodynamic probe to describe interphase microstructure is founded on fundamental concepts of the abrupt change in molecular mobility at the glass transition temperature. If filler-polymer matrix interactions were limited to a thin interfacial boundary layer, it would not be possible to observe any difference in glass transition temperature behavior for the composite materials having different levels of filler loading or glass fiber reinforcement. 

Let the volume fraction of filler be (pf, transition layer - cpi and matrix - (pm which have the corresponding expansion coefficients Uf, a , anda . The transition region has its own glass transition temperature, Tgi. In accordance with Figure 4.6, the expansion coefficient of the filled polymer at T < T is given by ... 

For systems in which the polymer and filler have an affinity for each other [strong adsorption], increases in the surface area/ volume ratio of the filler can result in large changes in the volume fraction of polymer that is henceforth considered to be "bound" to the filler interface. Many changes in physical phenomena related to the polymer chain dynamics, e.g., the glass transition temperature [Tg] and degrees and rates of polymer crystallization, could be drastically altered due to this bound layer. This has been referred to as the "nano-effect." ° In cases where Tg shifts are observed, the effect is somewhat similar to that reported for thin polymer films. The most important result of an increased bound-polymer layer is the consequent changes in mechanical properties of the final composite.i ... 

The surface modification of fillers with polymeric substances is discussed with reference to improvement of the properties of filled polymers. When the fillers are modified with polymers, the boundary layer contains not only the matrix filler, but also the modifying polymer, and when the modifying polymer interacts with the surface filler, the properties of both the mineral component and the modifier are altered. One characteristic which reflects the result of polymer-filler interaction is the glass transition temperature of the modifier. To determine this parameter for a very low content of polymer, the method of inverse gas chromatography was used using the modified filler as the stationery phase. The study investigated the use of travertine and tuff, the surfaces of which were modified with latices of a copolymer of styrene and methacrylic acid. 3 refs. (Article translated from Plasticheskie Massy, No.8, 2000, p.36)... 

Temperature dependences tg 5 in the region of glass-transition temperature of the polymer matrix were obtained for the same specimens filled with The increase in the concentration of the polymeric filler shifts the maximum of losses toward lower temperatures. It indicates that in specimens with a greater degree of filling the molecular interaction weakens as a consequence of a looser packing of segments in the boundary layer. 

Moreover, in many cases, a shift of Tg to lower values of temperature has been detected, but in these cases the quality of adhesion between phases may be the main reason for the reversing of this attitude 11,14). If calorimetric measurements are executed in the neighbourhood of the glass transition zone, it is easy to show that jumps of energies appear in this neighbourhood. These jumps are very sensitive to the amount of filler added to the matrix polymer and they were used for the evaluation of the boundary layers developed around fillers. 

Summary Solid state NMR studies of molecular motions and network structure in poly(dimethylsiloxane) (PDMS) filled with hydrophilic and hydrophobic Aerosil are reviewed and compared with the results provided by other methods. It is shown that two microphases with significantly different local chain mobility are observed in filled PDMS above the glass transition, namely immobilized chain units adsorbed at the filler surface and mobile chain units outside this adsorption layer. The thickness of the adsorption layer is in the range of one to two diameters of the monomer unit ( 1 nm). Chain units in the adsorption layer are not rigidly linked to the surface of Aerosil. The chain motion in the adsorption layer depends significantly on temperature and on type of the filler surface. With increasing temperature, both the fiaction of less mobile adsorbed chain units and the lifetime of the chain units in the adsorbed state decrease. The lifetime of chain units in the adsorbed state approaches zero at approximately 200 K and 500 K for PDMS chains at the surface of hydrophobic and hydrophilic Aerosil, respectively. 

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