Crystalline stems

Fig. 11 Monomer distributions of 32-mers with Ef/Ec = 0.1 at Ec/k /T =0.174 vs. variable crystalline-stem lengths changing with time during isothermal crystallization at a specific temperature. The evolution time is denoted by the numbers (times 1000 Monte Carlo cycles) near the curves. The curves are shifted vertically with an interval of 300 for clarity. We can see that with time the peak shifts from one third to half of the chain length [56]...

Fig. 23 The bonds that constitute crystalline domains must lie nearly parallel to the jy-axis with an angle 6 of less than 20°. Furthermore, the bonds must have at least three neighbors that satisfy 0.7a < Jr + r < 1.3a and ry < r0/2. Note that the crystalline stems deep inside the crystal (black spheres) have six neighbors, while those on the free sin-faces (hatched spheres) have four neighbors. The stems at the half-crystal site, or at the kink site, (white sphere) have three neighbors. Stems attached on the free surface, and those floating in the melt phase have less than three neighbors...

Fig. 28 Detailed step propagations at 330 K, from 6.4 ns in every 12.8 ps. Frequent attachment and detachment of the crystalline stems (black circles) are evident...

In any state preceding the onset of crystallization at T < To we assume that bundle stability is favored by localized attractive interactions between contacting (short) stems, some enthalpy advantage being balanced by a corresponding entropy loss (see Fig. 3). Depending upon the core structure of the crystalline stems, various bundle models were examined [8,9]. In the present... 

In view of this disagreement, as well as of evidence from polymer mesophases and MD simulations, we also propose an alternative model, based on the concept that the attractive interactions are so short-lived as to be effectively delocalized. As a consequence, bridges separating consecutive bundles are also taken into account in the evaluation of the average stem length of the growing crystal, in addition to the crystalline stems and to the loops... 

Fig. 4 Left Two bundles with the same lamellar thickness L, separated by a bridge. Right After aggregation, in principle the bundles may either retain the previous thickness L while the bridge produces new crystalline stems (old model), or the bridge is reeled in with an increase of L to U (new model)...

The orientation of crystalline stems with respect to the lamellar interface in block copolymers is a subject of ongoing interest and controversy. In contrast to homopolymers, where folding of chains occurs such that stems are perpendicular to the lamellar interface, the parallel orientation has been observed for block copolymers crystallized from the heterogeneous melt. It is not yet clear whether this is always the preferred orientation, or whether chains can crystallize perpendicular to the lamellar plane, for example when crystallization occurs from the homogeneous melt or from solution. 

Provided the crystalline stem length t,c is known and a stacked lamellar structure is assumed, the thicknesses of the interphase and amorphous phase can be evaluated from these data by the Eqs. (6) and (7) ... 

As a continuation of the earlier and very lively forum experience (that included the two mentioned papers and two commentaries [11,12]), the present contribution develops some further arguments based on helical hand of crystalline stems in favor of a nucleation and growth crystallization scheme. Specifically, the crystallization process is analyzed by taking two additional approaches ... 

Fig. 11. Perpendicular crystalline stem orientation with respect to cylinders...

Figure 8. Orientation of the lamellae and the molecular chains with respect to the extrusion direction. The angles uand cji represent tilting of the lamellar normal and the chains from the extrusion direction, respectively. L and L show the crystalline stem length and the lamellar long period, respectively.

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