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Notes: Abstract A theoretical model was introduced for the evaluation of the boundary layer developed between the main phases during the preparation of unidirectional fiber composites. It has been shown that this thin layer influences considerably the physical properties of the composite. It was assumed that the physical properties of themesophase unfold from those of the hard-core fibers to those of the softer matrix. Thus, a multicylinder model was assumed improving the classical two-cylinder model introduced by Hashin and Rosen for the representative volume element of the composite. Based on thermodynamic phenomena appearing at the glass transition temperatures of the composite and concerning the positions and the sizes of the heat-capacity jumps there, as well as on the experimental values of the longitudinal elastic modulus of the composite, the extent of the mesophase and the mechanical properties of the composite may be accurately evaluated. This version of the model is based on a previous one concerning a multilayer model, but it is considerably improved in order to take into consideration, in a realistic manner, the physical phenomena developed in fiber reinforced composites.

Notes: Abstract The size of the mesophase, which constitutes a boundary layer between fillers and matrix in composites, has been efficiently evaluated by the modified two-term unfolding model, which was based on delicate DSC measurements of the heat capacity jumps at the glass transitions of the composite and its constituent phases [1,2]. This model is now used to evaluate the mesophase along the whole viscoelastic spectrum of the composite, by making measurements of the storage and loss compliances or moduli of the composite and matrix and without making recourse to any other type of special measurement at the glass transition temperature of the substances. By applying this model the following important results were derived: i) Lipatov's empirical formula for defining the mesophase atT g was shown to yield reasonable results and ii) the evaluation of the size of mesophase over the entire viscoelastic spectrum was shown to remain almost constant and in conformity with the values defined by the other versions of the model. Extensive application of the experimental results of the literature indicated the mutual proof of the validity of these affine models.

Notes: Abstract The degree of adhesion developed between matrix and inclusions in composites is among the main factors characterizing their mechanical and physical behavior. The quality of adhesion depends mainly on the boundary layer created between inclusions and matrix because of chemisorption, physisorption and mechanical constraint phenomena developed between the main phases in the RVE of a composite. The extent of this boundary layer, which is called mesophase or interphase, may be a potential means for defining the quality of adhesion. While almost all previous models describing the mechanical and physical properties of composites are based on the concept of mathematical and smooth interfaces constituting the boundaries of the phases, a series of recent models developed by the author and his collaborators consider a more pragmatic situation at the interfaces between phases assuming the existence of boundary layers between phases ensuring a continuous transition of the properties of adjacent phases, which should be accepted as being in conformity with the physical and chemical procedures happening at these boundaries. The unfolding type of models introduced by the author aims to fill the gap by trying to accommodate the properties of neighbouring phases by transition boundary layers with varying properties between the bounds of the limiting phases. Thus, the unfolding models constitute a powerful means, where the notion of mesophase was introduced for defininig the RVE of a composite. The RVE was considered as consisting of the two main phases (the reinforcement and the matrix), coupled together by the intermediate phase, whose variable mechanical properties unfold from those of the reinforcement to those of the matrix. The extent of mesophase was evaluated by the three different and alternate methods, that is: i) by considering the variations in the heat capacity jumps,ΔC p , of the matrix material and the respective composite, appearing at the respective glass-transition temperatures of both substances. Based on thermodynamic measurements with differential scanning calorimetry, the extents of these jumps were accurately measured and these defined the thickness of the mesophase. It was further assumed that the steep variations of the mechanical properties in the mesophase follows negative-power laws, whose exponents were derived by measuring the moduli of the matrix, inclusions and the composite and assuming the validity of an improved law of mixtures. § ii) by evaluating the extent of mesophase along the whole range of temperature by using exclusively the mechanical properties of the storage and loss compliances and moduli of the composite and the matrix, without making recourse to thermal or other types of measurements and without limitations at the glass transition temperatures, and § iii) by defining the extent of the mesophase by the same method, but evaluating the properties of the mesophase or mesophases by methods based on diffusion laws of mutually soluble phases or impregnations. This method is convenient for studying polymer-polymer composites and composites with encapsulated or sized phases. By applying all three variations of the unfolding model it was shown that all three possibilities of defining the extent and the variable properties of mesophases are equivalent and, furthermore, they yield reasonable results. Moreover, experimental evidence with either particulates, or fiber composites indicated clearly that the introduction of the mesophase yields a better and more flexible model for interpreting in a realistic manner the complicated phenomena appearing in all composites used in engineering applications.

Notes: The thermomechanical behavior of particle composites was investigated in their transition region. In particular, the value of the glass-transition temperature Tg, which constitutes an upper limit for the structurally important glassy region, was examined. According to experimental evidence existing in the literature the introduction of a reinforcing filler in a polymeric matrix causes Tg of the latter to increase, unless mechanical imperfections counterbalance the reinforcing effect or even produce a Tg for the composite which is lower than that of the matrix. Based on mechanical theories, valid for the mechanical moduli of viscoelastic particle composites, a model was introduced that explains why the glass transition of composite materials may be reduced in some cases, whereas it may be increased in others. The concept of interphase between inclusions and matrix was used for the development of the model. Interphase is assumed to be a region, which is created between the matrix material and the filler particles, both considered as homogeneous and isotropic, whose thermomechanical properties and volume fraction may be determined from the overall thermomechanical behavior of the composite.

Notes: Retardation spectra, derived from dynamic measurements of extension compliances along three decades on the logarithmic scale of frequencies in standard specimens prepared from a fiber-reinforced composite with their fibers parallel to the longitudinal axis of the specimens, have revealed the structure of the matrix material of the composite. The experimental results were used to prove that the physicochemical rearrangements in the vicinity of the inclusions, consisting of restrained development of the macromolecules and especially their side chains due to the presence of the other phase, concentration of voids and dirt, shrinkage stresses developed during curing and creating microcracks (radial as well as along the interface), are activated by the existence of high stress gradients and eventually stress singularities due to the strong adhesion developed between phases.

Notes: The adhesion between matrix and inclusions (fibers or particulates) in a composite material is one of principal factors characterizing the mechanical and physical behavior of the modern composite materials. All theoretical models describing these substances neglect to consider the influence of the boundary layer developed between phases during the preparation of the composite. In this paper, two versions of a theoretical model were introduced for the evaluation of this mesophase layer. It had been shown that this thin layer influences considerably the physical properties of the composite. It was assumed that the physical properties of the mesophase unfold from those of the hard-core fibers to those of the softer matrix. Thus, a multicylinder model was assumed, improving the classical two-cylinder model introduced by Hashin and Rosen for the representative volume element of the composite. Based on thermodynamic phenomena appearing at the glass transition temperatures of the composite and concerning the positions and the sizes of the heat-capacity jumps there, as well as on the experimental values of the longitudinal elastic modulus of the composite, the extent of the mesophase and the mechanical properties of the composite may be accurately evaluated. These versions of model are based on a previous one concerning a multilayer model, but they are considerably improved, in order to take into consideration, in a realistic manner, the physical phenomena developed in fiber-reinforced composites.

Notes: Structural changes which take place in many amorphous polymers, when they are annealed at temperatures near the glass transition temperature, have important theoretical, physical, and mechanical consequences. In this paper the possible existence of some local ordering in highlycrosslinked epoxy resins is studied. Three kinds of tests - TMA, DSC, and dynamic experiments - are used for a type of epoxy resin, cured with six different amounts of curing agent. In order to study the effect of the thermal history on the behavior of the polymer at its transition region, as well as the morphology of the materials tested, three different thermal treatments have been followed. Interesting results were derived concerning the influence of these parameters to these parameters to the mechanical characterization of the polymer.

Notes: Four polystyrene-polyurethane mechanical blends were prepared with 5, 10, 20, and 40% thermoplastic polyurethane, respectively. Their impact properties were compared with pure polystyrene and commerical types of impact polystyrene. The rheological properties of the blends were studied with DSC and dynamic mechanical spectroscopy. It was found that addition of softer polyurethane conglomerates embedded inside the polystyrene matrix, although increasing the toughness of the blend as expected from addition of the softer particulate, also increased the glassy region of the blends by shifting their Tgs to higher temperatures. A theory based on the interaction of phases was propounded explaining this phenomenon.

Notes: A class of amine-cured epoxy resins containing various amounts of polysulfide plasticizer were subjected to dynamic testing during curing at high temperature. Both E modulus and loss factor were determined simultaneously. It was proved that the method allowed for rapid determination of the general pattern of the crosslinking procedure and the necessary curing time and, in general, that dynamic methods are most suitable for the mechanical characterization of polymers under curing at each stage of the curing cycle.

Notes: The influence of moisture absorption on the extent of the boundary interphase in particulate composites is thoroughly studied. It was found that, during the process of moisture absorption there is a variation of the extent of the boundary interphase, closely related to the degradation of the mechanical behavior of the composite, as well as to the percentage amount of moisture absorbed. An explanation of the observed relationship was advanced, based on a theoretical mechanism of absorption. This study complements a previous one, where the observed degradation of the thermomechanical properties of particulate composites due to moisture absorption was shown to be intimately interrelated with the state and extent of the interphase between fillers and matrix.