Hierarchical morphology

This book is stmctured as follows Chapter 1 serves as a guide to polyolefin blends introducing this important class of materials, why they are important, typical systems studied, issues of fundamental and applied interest, and current trends. The contributed chapters are divided into two main categories polyolefin/polyolefin blends (Chapters 2-16) and polyolefin/nonpolyolefin blends (Chapters 17-21). Issues covered in these chapters include miscibility, phase behavior, functionalization, compatibilization, microstructure, crystallization, hierarchical morphology, and physical and mechanical properties. Most of the chapters are in the form of review articles. Some original articles are included to capture the latest development in polyolefin blends research. 

J.M. Brown, D.P. Anderson, R.S. Justice, K. Lafdi, M. Belfor, K.L. Strong and D.W. Schaefer, Hierarchical morphology of carbon single-waUed nanotubes during sonication in an aliphatic diamine. Polymer, 46, 10854—10865 (2005). 

Figure 2.12 shows a sehematie representation of the relationship between the twisting degree of the N -LC and the hierarchical morphology of H-PA. In the weakly twisted N -LC, PA fibrils (diameters from 70 to 120mn) are gathered to form fibril bundles (diameters up to 1 pm). Interestingly, the distance between... 

Figure 27 Scanning force microscopy (SFM) phase image of polystyrene-block-poly(3-[triethoxysilyl]propyl isocyanate) film cast on glass and schematic drawing of the evaporation-indued hierarchical morphology. The initial nematic phase reorders into a smectic-layered state and finally crystallizes creating 90° tilt boundaries. (Reprinted from J.-W. Park and E.L Thomas. Adv. Mater. 15 585,2003. Copyright [2003] Wiley-VCH. With permission.)...

As in the case of other material systems, the macroscopic properties of nanocomposites are driven by their micro-/nanoscopic structure. From an electrical insulation perspective, polyethylene (PE) and epoxy resins constitute two technologically important material systems, each of which embodies in very different ways, a great deal of structural complexity. In the case of PE, the constituent molecules are the result of the inherently statistical polymerisation process, which can ultimately result in the formation of a hierarchical morphology in which different molecular fractions become segregated to specific morphological locations. In an epoxy resin, the epoxy monomer chemistry, the hardener and the stoichiometry can all be varied, to affect the network structure that evolves. In the case of nanocomposites, another layer of structural hierarchy is then overlaid upon and interacts with the inherent characteristics of the host matrix. 

The hierarchical morphology of semicrystalline polymers includes structural details in the range from nanometers to millimeters. The smallest ordered structures are crystalline blocks or domains. Figure 2.7 shows a branched PE (LDPE) with many crystalline blocks with defect layers in between. The blocks are arranged in long, narrow lamellar bands. The defect layers between the blocks, as well as the boundary at the lamellae-like structures, appear with an improved, clear contrast (black). 

Fig. 5.32. The hierarchical morphology exhibited by fibers [11], In the case in which rigid, rectilinear polymers are drawn from mesophases, an extended-chain crystal habit is adopted, with the chains ranning parallel to the fiber axis (left inset). In conventional flexible polymers, the semicrystalline folded-chain crystal habit exists (right inset).

The assembly events depend on the length of the crystallizable segments that are either the wax molecule or the ethylene sections of the copolymer. Formation of multilevel hierarchical morphologies is a consequence of one component to crystallize prior to its companion and to template the final overall morphology. Conversely, for the case of well-matched selfassembling properties cooperative co-crystallization is allowed. Typical pinhole SANS results are shown in Fig.l4a,b, which presents under polymer and wax contrast, respectively, scattering patterns from wax-polymer mixed solutions which are typical for the two mechanisms identified (Radulescu et al., 2003, 2004). 

Radulescu, A. Schwahn, D. Monkenbusch, M Fetters, L.J. Richter, D. (2004). Structural Study of the Influence of Partially Crystalline Poly(Ethylene Butene) Random Copolymers on Paraffin Crystallization in Dilute solutions. Journal of Polymer Science Part B - Polymer Physics, Vol.42, No.l7, pp.3113-3132, ISSN 0887-6266 Radulescu, A. Schwahn, D. Stellbrink, J., Kentzinger, E. Heiderich, M. Richter, D. Fetters, L.J. (2006). Wax Crystallization from Solution in Hierarchical Morphology Templated by Random Poly(Ethylene-co-Butene) Self-Assembhes. Macromolecules, Vol.39, No.l8, pp.6142-6151, ISSN 0024-9297... 

It was perhaps Andrew Keller who introduced the hierarchical morphological scheme starting with the lamellar (folded-chain) crystal as the fundamental unit of the lamellar stacks, which in turn build up the various supermolecular structures, of which the spherulite is the most prominent member. These superstructures are polycrystalline. They consist of a great many lamellar crystals and also many lamellar stacks. This section briefly presents the experimental techniques useful in the assessment of supermolecular structures. 

Brown J M, Anderson D P, Justice R S, Lafdi K, Belfbr M, Strong K L and Schaefer D W (2005) Hierarchical morphology of carbon single-walled uanotubes during souica-tion in an aliphatic diamine, Polymer 46 10854-10865. 

Hayashida, K., Saito, N., Arai, S. et al. (2007) Hierarchical morphologies formed by ABC star-shaped terpolymers. Macromolecules, 40,3695-3699. 

Thus, SANS and microscopy observations, in a complementarily fashion, revealed that the aggregates evolve one from another with decreasing temperature and form a hierarchical morphology having multiple sized structures. The micrographs also provided an indirect proof of the thin polymeric rods. The crystallization of C24 wax into one-dimensional objects in the presence of PEB-7.5 is driven by the existence of the primordial rodlike polymer aggregates, which dictate the overall morphology. The polymer rods are decorated by the wax and can be indirectly detected via microscopy (Fig. 62). 

Highly hierarchical morphology of an intercalated epoxy-clay nanocomposite renders application of the multiscale approach very beneficial. In this work, three distinct length scales are distinguished (a) gallery scale, (b) intercalated morphology scale and (c) macroscopic length scale. 

Due to the structure filler hierarchical morphology and surrounding polymer matrix at nanometer length scale, the well-defined concepts in conventional two-phase composites should not be directly applied to polymer nanocomposites. Polymer molecules and nanofillers have equivalent size and the polymer-filer interactions are highly dependent on the local molecular structure and bonding at the interface. Therefore, nanofillers and polymer chains structures cannot be considered as continuous phase at these length scales, and the bulk mechanical properties caimot be determined, for that reason, using traditional continuum-based micromechanical approaches [47,48]. 

Isotactic polystyrene, however, constitutes a system which has none of the problems and limitations described above. Not only do polystyrene lamellae tend to adopt a simple, planar hexagonal habit under a wide range of crystallization conditions, such that complex hierarchical morphologies are not observed, but also the melt is easily quenched into the glassy state. Thus polystyrene constitutes a system which is structurally simple, and, therefore, amenable to investigation. 

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