Superstructural morphology

Polarized light optical micrographs (PLOM) during isothermal crystallization of PLLA a) after 8 min. at 140 °C (modified from Ref. 37), b) after armealingat 160 °C for 2 h followed by slow cooling to 120 °C (modified from Ref. 21). 

PLLA-Z)-PEG diblock copolymers exhibit banded spherulites when iso-thermally crystallized at rc HO Yang et proposed that the ex- 

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Although the superstructural morphology of Hytrel has been studied extensively, little molecular-level information is availaUe in the solid state. In particular, we would like to characterize the domain boundaries and the nature of the phase separation. We would like to understand molecular motion in this polymer, both in the rigid domains and in the moUle regions. Also, we would like to clarify the relationships between molecular motion, chemical composition, and mechanical properties. 

Fig. 13.10. Polarized optical micrographs showing the superstructural morphology of PPDX-fe-PCL diblock copolymer samples during isothermal crystallization. The bar is equivalent to 30 pm. (a) D4o Ceo, Tc = 62° C (b) D4o Ceo, Tc = 40 C (c)...

Regime transition is presented when the data are analyzed with the Lauritzen and Hoffman kinetic theory. Di Lorenzo demonstrated that the discontinuity in the spherulite growth rate is not associated to any change in superstructural morphology. Tsuji et al. and Yuryev et al. also observed this unusual bimodal crystallization behaviour for pure PLLA, while the normal characteristic bell-shaped spherulite growth rate dependence was seen for poly(L/D-lactide) copolymers. 

The lamellae in turn can arrange themselves into a number of su-perstuctures depending on molar mass, the crystallization temperature, whether the growth is confined to a surface or is in bulk and whether the polymer is oriented before or during crystallization. These superstructure morphologies are the third level of hierarchical structure. Considering... 

Several polymer superstructure morphologies can be found in crystallizing a polymer lamellae, rods, sheet-like structures, hedrides or axialites, and spheru-lites. The formation of specific superstructures depends on molecular mass, crystallization condition, and structural regularity of the individual macromolecules. The polymer superstructures most likely to be met are lamellae, spherulites, and hedrites [1-3]. 

It should be re-emphasized that although our block copolymers do not display spherulitic morphology when they are compression molded, they are nevertheless crystalline. Hence, this indicates that under this mode of film preparation, aggregation into well developed superstructure is apparently kinetically limited. 

Myoglobins [442] and iron-oxide particles [447] have also been organized in cast multibilayers. Flexibility and versatility render composite cast multibilayers to be suitable for the construction of functional, ultrathin, polymeric superstructures with molecularly defined morphologies. Cast-multibilayer... 

Mesophases of supermolecular structure do not need a rigid mesogen in the constituent molecules. For many of these materials the cause of the liquid crystalline structure is an amphiphilic structure of the molecules. Different parts of the molecules are incompatible relative to each other and are kept in proximity only because of being linked by covalent chemical bonds. Some typical examples are certain block copolymers50 , soap micelles 51 and lipids52. The overall morphology of these substances is distinctly that of a mesophase, the constituent molecules may have, however, only little or no orientational order. The mesophase order is that of a molecular superstructure. 

Finally, an area that will most likely see an explosive growth over the next few years is the self-assembly of nanoparticles covered with mesogenic and pro-mesogenic capping agents. A number of different approaches have been summarized in this review, and the formation of nematic, smectic-like, cubic, and columnar phases and/or superstructures have been demonstrated. Once more, the possibilities to produce such metamaterials using nanoparticles and liquid crystal motifs are endless, and future research will surely discover other, in part, more complex phase morphologies as well as uniquely tunable nanoscale properties as a result of liquid crystal phase formation. 

The last example we would like to discuss is a lattice of holes formed in stoichiometric hexagonal (h) BN double layers on Rh(lll), see Fig. 5(c) and [99]. The lattice is composed of holes in the BN-bilayer with a diameter of 24 2 A, and an average distance of 32 2 A. The holes in the upper layer are offset with respect to the smaller holes in the lower layer. We note that well-ordered superstructures with a large period have already been observed some time ago by means of LEED for borazine adsorption onto Re(0001) [102], while borazine adsorption onto other close-packed metal surfaces, such as Pt(lll), Pd(lll), and Ni(lll), leads to the self-limiting growth of commensurate ABN monolayers [103,104]. For BN/Rh(lll) it is not clear at present whether the Rh(lll) substrate is exposed at the bottom of the holes. If this was the case the surface would not only be periodic in morphology but also in chemistry, and therefore would constitute a very useful template for the growth of ordered superlattices of metals, semiconductors, and molecules. 

Janus micelles are non-centrosymmetric, surface-compartmentalized nanoparticles, in which a cross-linked core is surrounded by two different corona hemispheres. Their intrinsic amphiphilicity leads to the collapse of one hemisphere in a selective solvent, followed by self-assembly into higher ordered superstructures. Recently, the synthesis of such structures was achieved by crosslinking of the center block of ABC triblock copolymers in the bulk state, using a morphology where the B block forms spheres between lamellae of the A and C blocks [95, 96]. In solution, Janus micelles with polystyrene (PS) and poly(methyl methacrylate) (PMMA) half-coronas around a crosslinked polybutadiene (PB) core aggregate to larger entities with a sharp size distribution, which can be considered as supermicelles (Fig. 20). They coexist with single Janus micelles (unimers) both in THF solution and on silicon and water surfaces [95, 97]. 

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