Dominant lamellae

The twisting lamellar structure of banded spherulites has been debated for decades without obtaining any satisfactory answer until recently. The nature of the isochiral (certain uniform handedness) lamellar twisting and the synchronic character of the twisting of a group of adjacent dominant lamellae both require an explanation. The permanganic etching technique provided... 

At high crystallisation temperatures, the high molar mass polymer crystallised alone. Data for the fold surface free energy obtained from linear growth rate data supported the view that the nature of the fold surface of the dominant lamellae was related only to the molar mass of the crystallising component and was not affected by the composition of the melt. 

At low temperatures, partial co-crystallisation was indicated by transmission electron microscopy and differential scanning calorimetry [156, 157]. Both electron microscopy of stained sections and optical microscopy showed that the segregated low molar mass material was present as small domains between the stacks of dominant lamellae within the spherulites/ax-ialites [115, 157, 158],... 

Cavitation is often a precursor to craze formation [20], an example of which is shown in Fig. 5 for bulk HDPE deformed at room temperature. It may be inferred from the micrograph that interlamellar cavitation occurs ahead of the craze tip, followed by simultaneous breakdown of the interlamellar material and separation and stretching of fibrils emanating from the dominant lamellae visible in the undeformed regions. The result is an interconnected network of cavities and craze fibrils with diameters of the order of 10 nm. This is at odds with the notion that craze fibrils in semicrystalline polymers deformed above Tg are coarser than in glassy polymers [20, 28], as well as with models for craze formation in which lamellar fragmentation constitutes an intermediate step [20, 29] but, as will be seen, it is difficult to generalise and a variety of mechanisms and structures is possible. 

Much effort has been devoted to investigating the detailed architectures and the construction of spherulites. Early investigations of the crystallization of polymers through optical microscopy (OM) [7,8] posited that polymer spherulites consisted of radiating fibrous crystals with dense branches to fill space. Later, when electron microscopy (EM) became available, spherulites were shown to be comprised of layer-like crystallites [9,10], which were named lamellae. The lamellae are separated by disordered materials. In the center of the spherulites, the lamellae are stacked almost in parallel [5,6,11-15]. Away from the center, the stacked lamellae splay apart and branch, forming a sheaf-like structure [11,13-15]. It was also found that the thicknesses of lamellae are different [5,6,11,12]. The thicker ones are believed to be dominant lamellae while the thinner ones are subsidiary lamellae. 

In the initial stage of the crystallization, the formation of a skeleton of dominant lamellae of equal widths separated by the melt is clearly visible as shown in Fig. 12a. The onset of branching is also visible in Fig. 12a. As the crystal grows, the hedrite becomes more asymmetrical with respect to the central dominant lamellae because it is tilted with respect to the surface (c.f., Fig. 12b-d). The dynamics of this space filling can clearly be observed in Fig. 12c,d. The subsidiary lamellae originating from the edge of the skeleton eventually develop a dominant character. 

If we take the radiation-growing axialites at high temperatures as the dominant lamellae, and the empty spacing is filled with the subsidiary lamellae grown at low temperatures, we can obtain the sphere-like crystals with dense filling, often called the type-I spherulites. Besides this kind of spherulites obtained by sequential formation of dominant and subsidiary lamellae during cooling, there exists another... 

Separation of small molecules into diflferent phases is a well-known phenomenon." Polymers that have a diflferent molar mass have sufficient free energy difference to have a tendency to separate and differences in their melting points will allow fractionation into different crystalline species. Lower molar mass materials will crystallize at low temperatures into separate lamellae often located between the dominant lamellae or at the spherulite boundaries. 

Blends of linear and branched polyethylene normally crystallize in two stages. The components crystallize separately provided that they are of similar molar mass. Linear polyethylene will crystallizes at the highest temperatures, forming regular shaped crystal lamellae. Branched polymers crystallize at lower temperatures in finer, S-shaped lamellae located between the stacks of the dominant lamellae. Although linear and branched polyethylenes are chemically very similar they can phase separate in the molten state. A characteristic of phase separated behaviour is the observation of a dominant lamella structure (Figure 6.14). ... 

Fig. 9. Initial crystallization at 125°C for 3 min of a linear polyethylene within a quenched matrix. Note the S-shaped dominant lamellae. Replica of an...

The conditions under which the different profiles of dominant lamellae occur are as follows (59). At crystallization temperatures > 127°C, melt-crystallized lamellae of linear pol5uner have 201 fold surfaces, the same basal surface typical of n-alkanes, allowing folds more surface area in a subcell with a comparatively small cross-sectional area per fold stem. Their profile viewed down b, the growth direction, has lamellar normals inclined at 35° to c, indicating 201 surfaces. For higher mass materials, lamellae are planar, but for molecular masses of 30,000, 201 inclinations alternate in ridged lamellae (Fig. 3a). At immediately lower temperatures, the corresponding profile of dominant lamellae differs it is then... 

Fig. 3. Profiles of dominant lamellae of linear polyethylene viewed down the growth direction b. (a) For slower growth, at temperatures >127°C, lamellae grow with 201 facets, (b) For faster growth, at lower temperatures, dominant lamellae display S- or C-profiles. From Ref. 18.

---