Crystallites fringed micelle

In this study more attention was paid to the possible formation of fringed micelle crystallites by rigid segments of RIM formulations. These crystallites could be considered to be an important factor inhibiting the mobility of the reactive groups and consequently causing the deviations from the straight line dependence of the second order reaction kinetic plot. 

The fringed micelle crystallites would be an organic part of the total network of the polymeric RIM system. The gel particles or mic-roheterogeneous systems could be considered as separate phases suspended in the matrix of the polymer system. 

The fringed micelle crystallites formed by aggregation of rigid segments would not only restrict the mobility of the reactive groups, but would also introduce physically bonded crosslinks into the amorphous polymer matrix (4,5,6). 

Formed by the physical intercohesion forces among the rigid segments, the fringed micelle crystallites as the crosslinks would be weaker than the chemical covalent crosslinks. The physical crosslinks could be partially or completely decrystallized at the softening point. Under these conditions the chemical covalent bonds would not be broken. 

If exposed to an external stress, cured RIM elastomers containing fringed micelle crystallites as crosslinks would show a faster decay of the physical crosslinks even at a moderately elevated temperature (7). The two factors, temperature and stress, according to literature data, could be very effective in decrystallization of the fringed micelle nuclei (7,8). ... 

The determination of crosslink densities, covalent, and physical, at various temperatures by the stress relaxation method could help to elucidate to what degree and at what conversion level the fringed micelle crystallites would cause and affect the deviation from the straight line dependence. 

Incorporating a plasticizer into the RIM formulations as a nonvolatile residual solvent should at least partially inhibit the formation of the fringed micelle crystallites and thus depress the build-up of physical crosslinks by die aggregation of crystallites. 

The urethane formulation presented in the paper was based on 1,6 hexamethylene diisocyanate (HDI), an aliphatic diisocyanate, and on tripropylene glycol (TPG) prepolymer, cured with a medium molecular size triol with an equivalent weight per OH of 180 (mol. wt. 540). This formulation did not have rigid segments for formation of fringed micelle crystallites as physical crosslinks. Therefore, the kinetic plot of the adiabatic reaction data resulted in a straight line without any deviation point. 

The crystalline aggregates, probably the fringed micelle crystallites, act as physical crosslinks and decrystallize at elevated temperatures. RIM elastomers containing a higher content of rigid crystallizing segments show more resistance to thermal decay. 

Figure 3.6. Two-dimensional representation of molecules in a crystalline polymer according to the fringed micelle theory showing ordered regions (crystallites) embedded in an amorphous matrix.

The traditional model used to explain the properties of the (partly) crystalline polymers is the "fringed micelle model" of Hermann et al. (1930). While the coexistence of small crystallites and amorphous regions in this model is assumed to be such that polymer chains are perfectly ordered over distances corresponding to the dimensions of the crystallites, the same polymer chains include also disordered segments belonging to the amorphous regions, which lead to a composite single-phase structure (Fig. 2.10). 

Whatever the cause of this small amount of crystalline phase, the dimensions of a crystallite (ca. 100 A) are much smaller than a chain length, and it is likely that a given chain will go through two or more crystallites, which will then be connected by one or more covalent links. This situation is similar to the one known in polymer physics as the fringed micelle model (see Ref. 12, p. 187), and is sketched on Fig. 9. This has consequences on the behavior of films upon stretching (see Section II.D.3). 

If the fringed micelle model (see Section II.C.5.a) is applicable, large values of, which would correspond to distances between crystallites greater than the length of the chain connecting them, imply breaking of covalent bonds this may place the limit for fracture of the sample. 

Figure 3.6). This theory known as the fringed micelle theory or fringed crystallite theory helped to explain many properties of crystalline polymers but it was difficult to explain the formation of certain larger structures such as spherulites which could possess a diameter as large as 0.1 mm.

Figure 2.17 Schematic representation of (a) fofd plane showing regular" chain folding, (b) ideal stacking of lamellar crystals, (c) interlamellar amorphous model, and (d) fringed micelle model of randomly distributed crystallites.

---