Plastics phenomenon

Long-term depression (LTD) is a synaptic plasticity phenomenon that corresponds to a decrease in the synaptic strength (decrease in the post-synaptic response observed for the same stimulation of the pre-synaptic... 

Long-term potentiation (LTP) is a synaptic plasticity phenomenon that corresponds to an increase in the synaptic strength (increase in the post-synaptic response observed for the same stimulation of the presynaptic terminals) observed after a high frequency stimulation (tetanus) of the afferent fibres. This increased response is still observed hours and even days after the tetanus. The phenomenon is often observed at glutamatergic synapses and involves, in most cases, the activation of the V-methyl D-aspartate (NMDA) subtype of ionotropic glutamate receptors. 

It is now well established that TCP acts as an antiplasticizer of PVC at low concentrations and as a plasticizer at high concentrations. The observed variations in tensile modulus, tensile strength, impact strength, ultimate elongation, and thermal expansion coefficient of PVC with additive concentration have been attributed to the antiplasticization-plasticization phenomenon (T7, 18, 19). Antiplasticization of PVC also results in a decrease in gas permeability, and plasticization in an increase in gas permeability (17). 

Ruigrok TJH, De Zeeuw Cl, Voogd J (1990) Hypertrophy of inferior olivary neurons a degenerative, regenerative or plasticity phenomenon. Eur. J. Morphol, 28, 224-239. 

Lopes da Silva, F. H., Kamphuis, W., and Wadman, W J (1992) Epileptogenesis as a plastic phenomenon of the brain, a short review. Acta Neurol. Scan. 86, 34-40. 

Society of Plastics Engineers See World of Plastics Reviews The Plastics Phenomenon. 

A. F. IsmaU, W. Loma, Penetrant-induced plasticization phenomenon in glassy polymers for gas separation membrane, Sep. Pur. Tech., 27, 173-194 (2002). 

In order to understand the phenomena behind resistance variations in the conductive polymer composites (CPCs) due to solvent diffiision. Feller et al. [71] have performed sorption exjjeriments with poly (ethylene-co-ethyl acrylate)-carbon black (EEA-CB) and EEA films in the presence of toluene. One main point here was to determine the influence of CB in the diffiision process. The first results showed that, whichever polymer was used, the diffiision coefificiem increased with toluene activity, which indicated plasticization of the material by the solvent. However, it was interesting to note that the plasticization phenomenon was reduced when fillers were introduced into the polymer matrix. It also appeared that the toluene diflfiisivity was about twofold lower in EEA-CB than in EEA, which was certainly due to a hindrance effect of the carbon black particles. In other words, the decrease in toluene solubility was the result of a tortuosity effect due to the morphology of CPC with a dispersion of CB particles, which acted as barrier components and increased the path for toluene molecules inside the composite. 

In addition to the test above, the adhesive sensitivity as a function of the moisture take-up is evaluated. It is well known that plasticization phenomenon occurs in epoxy polymers which affects the overall characteristics but also the bonding properties. 

The observed quasi-plasticity phenomenon in case o-f granite and basalt concretes, was mainly caused by cleavable character o-f splits o-f the gravel grains in -failuring process and also by arising o-f characteristic terraces with cle-fts. 

In EP 0778077A3, gas separation polymer membranes were prepared from mixtures of polysulfone, Udel P-1700, and an aromatic polyimide, Matrimid 5218. The two polymers were proven to be completely miscible as confirmed by optical microscopy, glass transition temperature values and spectroscopy analysis of the prepared mixtures. This complete misdbility allowed for the preparation of both symmetric and asymmetric blend membranes in any proportion from 1 to 99 wt. % of polysulfone and polyimide. The blend membranes showed significant permeability improvements, compared to pure polyimides, with a minor change in their selectivity. Blend membranes were also more resistant to the plasticization phenomenon compared with pure polyimides. This work showed the use of polysulfone-polyimide blends for the preparation of gas separation membranes for application in the separation of industrial gases. 

The iatroduction of a plasticizer, which is a molecule of lower molecular weight than the resia, has the abiUty to impart a greater free volume per volume of material because there is an iucrease iu the proportion of end groups and the plasticizer has a glass-transition temperature, T, lower than that of the resia itself A detailed mathematical treatment (2) of this phenomenon can be carried out to explain the success of some plasticizers and the failure of others. Clearly, the use of a given plasticizer iu a certain appHcation is a compromise between the above ideas and physical properties such as volatiUty, compatibihty, high and low temperature performance, viscosity, etc. This choice is appHcation dependent, ie, there is no ideal plasticizer for every appHcation. 

Resistance to Chemical Environments and Solubility. As a rule, amorphous plastics are susceptible, to various degrees, to cracking by certain chemical environments when the plastic material is placed under stress. The phenomenon is referred to as environmental stress cracking (ESC) and the resistance of the polymer to failure by this mode is known as environmental stress cracking resistance (ESCR). The tendency of a polymer to undergo ESC depends on several factors, the most important of which are appHed stress, temperature, and the concentration of the aggressive species. 

Measurements of stress relaxation on tempering indicate that, in a plain carbon steel, residual stresses are significantly lowered by heating to temperatures as low as 150°C, but that temperatures of 480°C and above are required to reduce these stresses to adequately low values. The times and temperatures required for stress reUef depend on the high temperature yield strength of the steel, because stress reUef results from the localized plastic flow that occurs when the steel is heated to a temperature where its yield strength is less than the internal stress. This phenomenon may be affected markedly by composition, and particularly by alloy additions. 

Not all fracture is by crack propagation. Highly ductile materials stressed at high temperature will eventually break by the growth, through absorption of lattice vacancies, of plastic voids. This shades into the phenomenon of superplasticity, which was examined in Section 4.2.5. 

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