Confined crystallization occurrence

It would be quite out of place in a work such as this to describe the detailed features of all, or even of many, of the several thousand crystal structures now known, especially as such descriptions are already available in the invaluable works of reference, cited in appendix i, which crystallographers are fortunate in having at their disposal. Our concern with individual structures lies primarily in the principles which they illuminate, and in so far as detailed descriptions are necessary they will be confined to structures of common occurrence and general interest. Frequently the detailed features of a structure are unimportant or insignificant so that an idealized or simplified description will serve our purpose, and sometimes a purely qualitative account of only certain aspects of a structure will suffice to illustrate the general principles involved. 

Hydroxyapatite crystals are usually confined to areas where there is a need for high mechanical strength, rigidity or hardness as in bone, dentine and enamel. They do not normally occur at all in soft tissues , the only other site of occasional local occurrence being within such epithelial keratins as hard claws and beaks. 

Ferroelectric properties in PVDF and P(VDF-TrFE) copolymers are directly associated with their crystalline phase. Confinement may alter the stmcture of the crystalline phase. We remark that the changes in the ferroelectric behaviour cannot be solely attributed to spatial confinement (finite size effects). The occurrence of peculiar ferroelectric behavior is dictated by the presence of interfaces with solid walls, with whom the polymer chains interacts. To name some of the features discussed in this chapter when PVDF is confined into alumina cylindrical templates, an interfacial layer showing ferroelectric like behavior appears, whereas the bulk phase of this homopolymer is paraelectric under normal processing conditions. However, in P(VDF-TrFE) copolymers confined in alumina templates, due to the the severe confinement conditions, the transition from ferroelectric to paraelectric is inhibited, indicating that the interaction with the confining wall stabilizes de ferroelectric phase. Confinement without interfaces, like in the case of 3D confined nanospheres, does not affect the ferroelectric character of the polymer. The confinement in nanoparticles may decrease the crystallinity of the system, but the crystals, responsible for the ferroelectric character of the polymer, have essentially the same properties as in the bulk. 

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