Monomer packing

Fig. 9 Possible monomer packing structures in Cyan J3iPr J-aggregate formed in the poly(dA)-poly(dT) groove, built in HyperChem 6.03...

A surfactant at low concentration in aqueous solution exists as monomers (free or unassociated surfactant molecules). These monomers pack together at the interface, form monolayer and contribute to surface and interfacial tension lowering. Although this phenomenon is highly dynamic (surfactant molecules arrive and leave the interface on a very rapid timescale), molecules at the interface interact with the neighbouring molecules very strongly which enables measurement of the rheological properties of the monolayer. 

We present here some computer simulation results for the model described in Section I.E that illustrate the important role of monomer packing fraction rj, Eq. (7), or equivalently of the reduced density p, Eq. (8), on the response of the model to a constant volume extension X. In these simulations, 2=1.6. Simulations were mn for T = kT/ C/j in the range 1.4 < T < 5, and for p in the range 0.8 < p < 1.2. The system consists of Nc = 40 chains, each with... 

The symmetry of the monomer packing determines the symmetry of the products, e.g. cinnamic acids will form only one of the various possible dimers, the symmetry of which is present in the monomer crystal Even absolute asymmetric syntheses are possible utilizing the symmetry relationships of topochemical reactions... 

The experimental data qualitatively confirm the geometrical model presented in Fig. 4. The reactivity is controlled by the monomer packing and not by the chemical nature of the substituents. In many cases different modifications of a specific monomer can be obtained which exhibit drastic differences in reactivity However, it should be emphasized at this point that the packing parameters give no absolute scale for... 

It appears that the reaction mechanism and the intermediates involved in the solid-state polymerization of diacetylenes are reasonably well understood. However, experimental results obtained with special monomers should not be generalized. It is not possible to design a monomer with desired properties. Inspection of Table 1 shows that on the basis of the crystallographic data and the monomer packing the absolute reactivity and the polymerization kinetics caimot be quantitatively predicted, e.g. it is not possible, to date, to explain why certain diacetylenes can be polymerized thermally whereas others with equal packing are thermally inactive. A more realistic kinetic model should include the various energy transport processes and the complex side group motions which are connected to the reaction. 

FIGURE 4 Diagram of monomer packing in a side-polar filament. This model suggests that the antiparallel overlap between molecules is 14.3 nm. For clarity, only one of myosin s two heads is shown. Reprinted from Cross et al., Embo. /, 10, 747-756, 1991, by permission of Oxford University Press. 

When a polymerization is accompanied by phase transition, the overall thermodynamic parameters are the sum of parameters of the chemical reaction and phase transition (cf. p. 11). Thus, for instance, thermodynamics of polymerization in the crystalline state from a liquid monomer will be given by the thermodynamics of formation of the amorphous (condensed) polymer and polymer crystallization, provided, that polymerization proceeds in the solid state with monomer packing in the crystalline state simultaneously with propagation. 

Fig. 4. Myosin filament structure. Dit rams of myosin monomer packing in non-helical side-polar and helical bipolar filaments. For simplicity, only one myosin head per monomer is shown. A bare zone is observed at the center of the bi-polar filament, and at each end of the side-polar filament...

Usually the PDA polymer has a deep blue color but it may also be red or yellow. The color of PDA is a result of the energy of electronic excitations. This energy is affected by the state of the monomers packing and the polymer s exposure to environmental variables, for example, heat (thermochormism), mechanical stress (mechan-... 

The effect of monomer packing in reaction rates was first determined decades ago studying reactions in mono-layers [39]. The rate of lactonization of y-hydroxystearic acid (HSA) was determined in substrate monolayers as a function of applied lateral pressure ( r). As n increases the film of HSA passes from an expanded to a condensed state and at the transition the rate of acid catalyzed lactonization of HSA decrease sharply. The rate decrease was attributed to a change in the accessibility factor related to the alteration of the fluidity of the monolayer, preventing the approach of hydroxyl and carboxylate groups [39]. 

Figure 36. Mean-squared radius-of-gyration of / X 100 stars as a function of monomer packing fraction and the indicated number of arms. The curves are an empirical power law fit given by 35/ 17...

Then the parameters of the FENE potential of the bonded monomers and the parameters and Va are chosen in a way that a melt of such chains had the same (R ), R ), pressure and Rouse friction as the LJ melt. For p = 0.S5cr the parameters are = 2.22e,= l.Ocr and Ro = 1.75cr (in the FENE Potential). The amplitude of the FENE was also reduced by a factor of 0.175. Monomers can cross each other with a penalty of a few ksT. Since the macroscopic properties are not altered, these two potentials can be used to identify the contributions from the noncrossability of the chains. The general monomer packing constraints are only weakly affected, as the pressure and the temperature remain the same. 

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