Crystalline polymers symmetry breaking

The high symmetry of the chains can be broken in the limit-ordered crystalline lattice of polymers (symmetry breaking). 

Recently, new interesting phenomena that control the mode of packing of polymers have been found, and it has been shown that the basic principles of polymer crystallography are, in some cases, violated. In particular, (1) an atactic polymer can crystallize this is, for instance, the case of polyacrylonitrile [107] (2) in a crystalline polymer the chains can be nonparallel for instance, the structure of the yform of iPP is characterized by the packing of nearly perpendicular chains [108, 109] (3) the principle of entropy-driven phase formation may be violated and the high local symmetry of the chains is lost in the limit-ordered crystkhne lattice of polymers (symmetry breaking). 

The above examples demonstrate that mirror symmetry breaking by self-assembly of non-chiral molecules into chiral architectures is indeed a feasible process. However, in order to preserve the handedness and amplify the stochastically-generated chirality, it is imperative to couple such chance events with efficient sequential autocatalytic processes. We refer now to several experimental systems that illustrate the occurrence of such scenarios. We shall allude in particular to systems undergoing amplification via non-linear asymmetric catalysis processes, via the formation of 2-D and 3-D crystalline systems and amplification of homochiral bio-like polymers in general and oligopeptides in particular. 

As pointed out by Kleman in early 1985 [24], liquid crystalline polymers (LCPs) and LMWLCs are equivalent from the point of view of topology since they exhibit the same symmetries and the same type of order parameters. Defects are indeed characteristic breaks in local symmetry of the order parameter and can be classified according to their dimensionality and the specific sym-... 

The asymmetric polymerization in crystalline architectures provides an excellent environment to conduct the absolute asymmetric synthesis of polymers, and also provides an effident route for the ampHfication of chirality. Mirror-symmetry breaking might occur either through total asymmetric transformations, either in enantiomorphous crystals that have self-assembled from achiral molecules, or within racemic crystalline architectures which are delineated by chiral rims or surfaces when one of the chiral faces is blocked by an interface. The self-assembly of nonracemic mixtures into a mixture comprising eutectic compositions of a racemic compound and an enantiomorphous assembly, followed by asymmetric transformation, provides a series of thermodynamically controlled, alternative routes for the effident ampHfication of homochirality. 

Keith, C. Reddy, R. A. Tschierske, C. The first example of a hquid crystalline side-chain polymer with bent-core mesogenic units ferroelectric switching and spontaneous achiral symmetry breaking in an achiral polymer. Chem. Commun. 2005, 7, 871-873. 

Piezo/pyroelectric composite (inorganic ferroelectric particles in polymer matrix) Ions in crystalline particles form ionic dipoles upon symmetry breaking Ionic dipoles deform/ rotate with mechanical (electrical) stress Ferroelectric matrix polymer may compensate piezo-or pyroelectric effect... 

A particular state of tacticity is a particular configuration of the molecule and cannot be changed without breaking and reforming bonds and, at ordinary temperatures, there is not enough thermal energy for this to happen. Rotations around bonds produce only different conformations. A vinyl polymer is therefore unlikely to be appreciably crystalline unless it is substantially either isotactic or syndiotactic the atactic chain cannot get into a state in which it has translational symmetry. 

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