Mobility, molecular

Chain mobility should be considered from both a chemical and a physical standpoint. Chemical reactions require reagents to be physically in contact. The morphology of the interphase organization restricts chain motions which might be considered either a chemical or a physical phenomenon. 

There are three types of chains or chain segments which may be involved in filler-polymer interactions  

A study of SBR rubber showed a difference in the behavior over the temperature range of 20-70°C between the unfilled rubber and rubber filled with carbon black. T2 changes by a few tens percent for filled rubber in this temperature range and it almost doubles in the unfilled rubber. The increased temperature contributes to the increased molecular mobility but this effect is retarded by the bound segments in the filled rubber.  

Monte Carlo simulations show the differences between chains on elonga- 

Dynamic mechanical analysis of filled systems confirms analytical observations. The thickness of the restricted mobility region in carbon filled rubber is proportional to the activity of the carbon black. 

---

Thermal Properties. Thermal properties include heat-deflection temperature (HDT), specific heat, continuous use temperature, thermal conductivity, coefficient of thermal expansion, and flammability ratings. Heat-deflection temperature is a measure of the minimum temperature that results in a specified deformation of a plastic beam under loads of 1.82 or 0.46 N/mm (264 or 67 psi, respectively). Eor an unreinforced plastic, this is typically ca 20°C below the glass-transition temperature, T, at which the molecular mobility is altered. Sometimes confused with HDT is the UL Thermal Index, which Underwriters Laboratories estabflshed as a safe continuous operation temperature for apparatus made of plastics (37). Typically, UL temperature indexes are significantly lower than HDTs. Specific heat and thermal conductivity relate to insulating properties. The coefficient of thermal expansion is an important component of mold shrinkage and must be considered when designing composite stmctures. 

Thermal Properties. Before considering conventional thermal properties such as conductivity it is appropriate to consi r briefly the effect of temperature on the mechanical properties of plastics. It was stated earlier that the properties of plastics are markedly temperature dependent. This is as a result of their molecular structure. Consider first an amorphous plastic in which the molecular chains have a random configuration. Inside the material, even though it is not possible to view them, we loiow that the molecules are in a state of continual motion. As the material is heated up the molecules receive more energy and there is an increase in their relative movement. This makes the material more flexible. Conversely if the material is cooled down then molecular mobility decreases and the material becomes stiffer. 

Gas The state of matter characterized by complete molecular mobility and unlimited expansion at standard temperature and pressure. 

Stein, P. E., and Carrell, R. W., 1995. What do dysfnncUonal serpins tell ns about molecular mobility and disease Nature Structural Biology 2 96-113. 

The observed reversal in the thermal stability of the copolymer at a critical composition, which appears to be between 30 and 40 mol% of ethylene, may be explained on the basis of the emergence of phase-separation between the nonpolar ethylene and polar vinyl chloride blocks. Although crystallization of the ethylene blocks in the copolymer is only observed when more than 70 mol% ethylene units are present, the possibility of phase-separation occurring at lower contents of ethylene units cannot be excluded. Also, round about the critical copolymer composition, the Tg of the copolymer may be reduced to a level that would facilitate separation between the unlike phases by increased molecular mobility within the polymer matrix. As has been discussed earlier, occurrence of phase-separation in the copolymer would not only make the mechanism of stabilization due... 

Holzmiiller, W. Molecular Mobility, Deformation and Relaxation Processes in Polymers. Vol. 26, pp. 1 —62. 

Recombination reactions between two different macroradicals are readily observable in the condensed state where molecular mobility is restricted and the concentration of radicals is high. Its role in flow-induced degradation is probably negligible at the polymer concentration normally used in these experiments (< 100 ppm), the rate of radical formation is extremely small and the radicals are immediately separated by the velocity gradient at the very moment of their formation. Thus there is no cage effect, which otherwise could enhance the recombination efficiency. 

The molecular weight of the polymer can influence the rate of flow-induced degradation in at least two respects first, molecular mobility (characterized by... 

Moreover, the interaction of the surface of the fillter with the matrix is usually a procedure much more complicated than a simple mechanical effect. The presence of a filler actually restricts the segmental and molecular mobility of the polymeric matrix, as adsorption-interaction in polymer surface-layers into filler-particles occurs. It is then obvious that, under these conditions, the quality of adhesion can hardly be quantified and a more thorough investigation is necessary. 

This effect of M can be explained as being due to the crystalline phase in the o semi-crystalline polymer. The presence of this crystalline phase reduces the molecular mobility. The crystalline structure is not something static, but it is perfected on annealing. The longer the reaction at a high temperature, the more perfect the crystalline phase, and the more the molecular mobility is restricted. After melting this starts all over again and the lower the M the faster is this crystallization process, o... 

In semi-crystalline polymers at least two effects play a role in the diffusion of the reactive endgroups. Firstly, the restriction in endgroup movement due to the lowering of the temperature, which usually follows an Arrhenius type equation. Secondly, the restriction of the molecular mobility as a result of the presence of the crystalline phase whose size and structure changes on annealing. 

Reduction of Molecular Mobility Caused by Increasing Solution Viscosity... 

Linear polymers when heated sufficiently undergo transition from solid to liquid, that is, they melt. Liquids are characterised in general by greater disorder than solids and by substantially increased molecular mobility. 

The new interface model and the concept for the carbon black reinforcement proposed by the author fundamentally combine the structure of the carbon gel (bound mbber) with the mechanical behavior of the filled system, based on the stress analysis (FEM). As shown in Figure 18.6, the new model has a double-layer stmcture of bound rubber, consisting of the inner polymer layer of the glassy state (glassy hard or GH layer) and the outer polymer layer (sticky hard or SH layer). Molecular motion is strictly constrained in the GH layer and considerably constrained in the SH layer compared with unfilled rubber vulcanizate. Figure 18.7 is the more detailed representation to show molecular packing in both layers according to their molecular mobility estimated from the pulsed-NMR measurement. 

It is an unfortunate fact that several preexisting theories have tried to explain complicated mechanical phenomena of CB-reinforced rubbery materials but they have not been so successful." " However, a recent report might have a capability of explaining them collectively," when the author accepted the existence of the component whose molecular mobility is different from that of matrix mbber component in addition to the existence of well-known bound rubber component. The report described that this new component might be the most important factor to determine the reinforcement. These mbber components have been verified by spin-spin relaxation time 2 by pulsed nuclear magnetic resonance (NMR) technique, ° while the information obtained by NMR is qualitative and averaged over the sample and, therefore, lacking in the spatial... 

Curing of Polyimlde Resin. Thermoset processing involves a large number of simultaneous and interacting phenomena, notably transient and coupled heat and mass transfer. This makes an empirical approach to process optimization difficult. For instance, it is often difficult to ascertain the time at which pressure should be applied to consolidate the laminate. If the pressure is applied too early, the low resin viscosity will lead to excessive bleed and flash. But if the pressure is applied too late, the diluent vapor pressure will be too high or the resin molecular mobility too low to prevent void formation. This example will outline the utility of our finite element code in providing an analytical model for these cure processes. 

The behaviour, which is not controlled by the topochemical rule but is greatly influenced by non-topochemical factors, is discussed in Section 2 in terms of molecular mobility, stabilization energy by orbital interaction and energy transfer in the crystals. 

Deuterium NMR is used to study the molecular mobility of benzene-de in Na and Cs forms of zeolite X. The systems studied were prepared with loadings in the range 0.7 molecules/supercage to 5.6 and 5.0 mole-cules/supercage for (Na)X and (Cs,Na)X, respectively. 

---