Thermal-mechanical history

Chemical composition (including moisture content) Physical composition (i.e., geometry, form, particle size) Thermal mechanical history... 

Stresses in solvent based coatings arise from the differential shrinkage between the thin film coatings and the corresponding substrates. These stresses are due to volume changes associated with solvent evaporation, chemical reaction (i.e. cyclization in polyimide formation) and differences in thermal expansion coefficients of the coating and substrate (4>5). The level of residual stress depends on the material properties such as modulus, residual solvent content and crosslinking (5) and its thermal-mechanical history. 

The properties of PVDF homopolymers and copolymers are sensitive to the chemical composition, molecular mass characteristics, and molecular architecture. These attributes are dictated by the polymerization method and ingredients, the method of isolation, and the thermal/mechanical history during isolation and subsequent processing. 

The problem of analyzing all the data on H entry rate is that this rate, even in a pure metal, depends on many variables the nature of the metal, its thermal-mechanical history, the surface conditions (especially on iron, surface states are not easily reproducible due to the difficulty of removing oxide films on the electrodes), composition of the electrolyte, cathodic current density or electrode potential, temperature, etc. The determining factors in the kinetics of the H cathodic reaction on bare metal surfaces are the cathodic overpotential and the surface variables, which are the density of sites for H adsorption and the binding eneigy of the adsorbed H atoms, both dependent on the stmcture and the chemical composition of the surface. 

In the introduction of this communication we pointed out the similarities between the pressure dependence, the activation energy, and the annealing effects of the (Tp< Tg) and the (T/ > Tg) transitions, which strongly suggests that these transitions are the manifestation of the same relaxation process, probably issued from the complexity of the kinetic mechanism responsible for the glass transition temperature itself. In the second part of this communication we present and reanalyze DSC studies of atactic Rheomolded polystyrene specimens treated with various thermal-mechanical histories. The effect of frequency, amplitude of vibration, and annealing time... 

Other PUS components, such as nozzles, plenums, piping, pumps, heat exchangers, valves, expansion joints, etc., have design characteristics that affect their susceptibility to in-service degradation. Their susceptibility is also affected by the chemical composition and thermal-mechanical history of the steel and the stresses (both applied and residual) acting on the steel. 

If the temperature dependence of a-j- is known, the value of the tensile modulus may be calculated from the master curve for any temperature, time, and strain. The implication is that the modulus may be estimated for a rubber in the internal mixer at a specified location and at a specified time, provided that its thermal-mechanical history is known. 

Relaxations of a-PVDF have been investigated by various methods including dielectric, dynamic mechanical, nmr, dilatometric, and piezoelectric and reviewed (3). Significant relaxation ranges are seen in the loss-modulus curve of the dynamic mechanical spectmm for a-PVDF at about 100°C (a ), 50°C (a ), —38° C (P), and —70° C (y). PVDF relaxation temperatures are rather complex because the behavior of PVDF varies with thermal or mechanical history and with the testing methodology (131). 

The glass transition temperature can be measured in a variety of ways (DSC, dynamic mechanical analysis, thermal mechanical analysis), not all of which yield the same value [3,8,9,24,29], This results from the kinetic, rather than thermodynamic, nature of the transition [40,41], Tg depends on the heating rate of the experiment and the thermal history of the specimen [3,8,9], Also, any molecular parameter affecting chain mobility effects the T% [3,8], Table 16.2 provides a summary of molecular parameters that influence the T. From the point of view of DSC measurements, an increase in heat capacity occurs at Tg due to the onset of these additional molecular motions, which shows up as an endothermic response with a shift in the baseline [9,24]. 

Rietveld (g.c.) analysis of the neutron diffraction data on isotactic polypropylene is still in progress. It has afforded the interesting result, already discussed, that the profiles are better approximated by Cauchy than by Gaussian functions. The structural analysis is now restricted to the fourth model (P2 /c, Immirzi), which gives an excellent agreement between observation and calculation, but with the fraction of reversed helices close to 50% instead of 25% and with less chain symmetry. The other models will be tested for a more complete comparison with x-ray results. We cannot exclude, however, the possibility that the two samples used, which have different chemical, thermal and mechanical history, can really have different structures. 

In conclusion, it can be seen that thermal analysis is able to make a considerable contribution to forensic science. Because of its capability to differentiate between manufacturing lots, it has for years been employed in quality control laboratories to monitor production of polymeric products. Its capability of differentiating between materials of identical chemical composition on the basis of differences in molecular weight distribution and thermal or mechanical history should be a capability quite unique and useful to forensic science. With the advent of second-generation instrumentation, this technique can be usefully extended to the realm of submilligram level analysis. 

The properties of an alloy are primarily determined by those of its constituent alloy phases however, as stated, the microstmcture, that is, the phases grain size, perfection, distribution, and so on, which is due to the thermal and mechanical history of the alloy, plays a major, modifying role. It is in this area that modem methods of alloy processing have had their most visible results, as will be seen from the review of specific types of alloys in Sections 3-5. 

Relaxation in Insulating Crystals. The low-frequency relaxation of ionic materials consists typically of a set of simple decays arising from the motion of defects. - Some of these have freedom of extensive motion and contribute to the conduction current others are constrained to a few ndghbouring sites. Except at high temperature, the defects are remnants of the previous thermal and mechanical history of the sample. Their movement is a thermally activated process and relaxation times normally vary with temperature as exp AjkT). The e qierimental picture may be much complicated by the motion of electrons loosely bound to crystal defects. 

The melt viscosity of LCPs is sensitive to thermal and mechanical histories. Quite often, instrumental influences are important in the value of viscosity measured. For example, the viscosity of HBA/HNA copolyesters are dependent on the die diameter in capillary flow (59). LCP melts or solutions are very efficiently oriented in extensional flows, and as a result, the influence of the extensional stresses at the entrance to a capillary influence the shear flow in the capillary to a much greater extent than is usually found with non-LC polymers. 

TA results can be significantly affected by experimental parameters such as sample dimension and mass, heating (or cooling) rates, atmosphere surrounding the sample and even thermal and mechanical history of the sample. Reproducible results of a chemical species... 

Sample dimension and mass should be small. For most TA techniques, samples in the form of a powder with sample mass less than 10 mg are preferred. Heat transfer between the sample and atmosphere will be faster in such a sample than in a lump thus, thermal equilibrium is more likely to be achieved between sample and atmosphere during analysis. Samples to be analyzed should have the same thermal and mechanical history. Thermal events are affected by such history and different results for the same chemical species are likely to be generated if the samples have different histories. The main reason for this is that thermal measurement is affected by the internal energy of samples, and the internal energy can be changed by thermal and mechanical processes. 

By carefully selecting the thermal and mechanical history of a material, it is possible to tailor a number of different features of that material. Low-temperature anneals can induce precipitate reactions that increase the yield stress. Cold working changes the dislocation density, and this too alters the yield stress. If the material... 

As a first experimental example. Fig. 9.4 depicts the spatial distributions of sound velocity v and sound attenuation F across a blown film of poly(4-methyl-1-pentene) (P4MP-1, thickness 180 pm). There are two maxima at the surfaces and two adjacent local minima within the film. Then, the sound velocity decreases from left to right inside the fihn, indicating a different thermal or mechanical history in different parts of the P4MP-1. The sound velocity maxima... 

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