Crystallinic fibrils

The materials used in most current research are irregular mats of highly crystalline fibrils with diameters of around 10 nm, so that the films are characterised by a very high surface area (around 60 m2 g-1), a problem in some potential applications and an asset in others. The morphology of polyacetylene is sensitive to the conditions of preparation and to ageing and was the subject of much heated discussion in the early development of polyacetylene. 

The a-chitin nanofibrils had an average width and length of 6.2 1.1 and 250 140 nm, respectively (ratio ca. 40). Because conversion to nanofibrils was achieved in water at pH 3, the protonation of the amino groups on the crystalline fibril surface, which means reciprocal repulsion, allows the individualization of a-chitin fibrils to be maintained for lasting periods [44]. 

In woody materials, the long crystalline fibrils of cellulose are bound into a composite structure by lignin, a macromolecule based on polyphenols. Lignin, which is present to the extent of 25-30% in most woods, is a cross-linked polymer rather similar to man-made phenol-formaldehyde resins and may be looked upon as a glue which gives wood its permanent form (Figure 1.1). 

The structure of chitin is very analogous to cellulose (Fig. 3.2). They both are supporting materials for living bodies i.e., for both plants and animals, with sizes increasing firom simple molecules and highly crystalline fibrils at the nanometer level to composites at the micron level upward [26]. Thus, they intrinsically have... 

Models of fringed micelles (a) and fringed fibrils (b). C, crystalline fibril A, amorphous domain. 

Within the crystalline fibrils (see Sect. 4) the chains crystallise in lamellae as extended chains in the planar zig-zag conformation [11,12], akin to that in crys-... 

Crystallisation of polymers such as PCL, which crystallise to give spherulitic structures, starts from a nucleus which subdivides at the growth surface to generate a series of very thin (typically 10 nm thick) crystalline lamellae. The lamellae continue to grow and sub-divide to establish the spherically-symmetric structures (spherulites) which consist of a series of crystalline fibrils, bundles of lamellar crystals, extending from the nucleus in all directions, with a constant... 

Fig. 6. Schematic representation of the internal structure of a spherulite in a partially crystalline polymer, showing the arrangement of the crystalline fibrils, and of the internal structure of the fibrils showing the arrangement of polymer chains in the lamellae and several local structural features as described in the text...

Fig. 2.8. A polymer spherulite growing into the melt. In polyethylene the crystalline fibrils are thin lamellae. The molecules crystallize most rapidly on to the (010) plane the b axis is therefore the direction of most rapid growth and is parallel to the spherulite radius R. The a and c axes are randomly distributed around R. If the solidification is isothermal, the lamellae are all of the same thickness. In order to fill space the radiating lamellae must branch and give birth to daughter lamellae as they grow out into the melt. Amorphous polymer is left trapped between the crystals.

Under certain conditions, the lamellae of the TPEEs organize into a spherulitic structure, which is characteristic structure of the common semi-crystalline polymers. Based on the results of different methods, it was concluded that crystallization occurs by chain folding through which a spherulitic superstructure is formed. A well developed spherulitic crystalline superstructures, with diameters of about 5-20 pm, can be formed depending on the crystallization conditions [33]. Also, the soft, amorphous phase is embedded between radial crystalline fibrils of the hard segment spherulites. Under some conditions, other structures such as dendrites are developed. 

This section is followed by another extensive discussion of starch, the other major commercial polysaccharide. It is amazing how much difference a simple stereochemical variation can make in the structure and properties of a macromolecule. Even more fascinating patterns are observed in native starch grains. The crystalline fibrils are arranged in a fractal-like pattern called trichites (Fig. 3.14). 

The interface between the crystalline fibrils and the interstitial amorphous regions has a sharp boundary, giving rise to the power law with the exponent Odi = 4.0 as predicted by the Porod law. The exponent suggests the corresponding surface fractal dimension of Dj = 2 from eqn [5], indicating a smooth and flat interface at the relevant length scale. The upper and... 

---