Crystal Structures of Polymers

Source of the irregularity Polymer Degree of crystallinity (%) (typical) 

Random cis-trans isomerism Foly (trans-1,4-butadiene) 40 

Dipartimento di Scienze Chimiche, Universitd di Napoli Federico II Complesso Monte S.Angelo, Napoli, Italy 

2 Conformation of Polymer Chains in Crystals and Conformational Polymorphism, 33 

4 Disorder in the Stacking of Ordered Layers (Stacking Fault Disorder), 58 

In vinyl polymers, where the two terminals of the monomeric units are not equivalent and a head and a tail can be distinguished (for instance, the case of propylene), the regularity in the chemical constitution implies a regular enchainment of consecutive monomeric units always head to tail. Defects in the regular constitution may arise from the presence of enchainment of consecutive units via head to head or tail to tail. 

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However, defects and distortions are present in crystals of polymers and they play an important role in the Crystal Structures of polymers, X-ray diffraction data provides a lot of information... 

The elucidation of the crystal structures of polymers from their x-ray diffraction patterns is frequently a difficult and laborious task. The work usually proceeds by trial and error methods in which calculated intensities for likely structures are compared with the observed intensities of diffraction spots. Furthermore, x-ray fibre photographs often contain relatively few reflections and it is always possible that more than one structure may give a reasonable fit with the observed intensity data. Additional information which can be obtained from infrared spectra can often provide considerable help with both these difficulties and in particular many trial structures can be eliminated without recourse to time-consuming calculations of x-ray intensities. 

Lotz, B. et al. (1994). An original crystal-structure of polymers with ternary helices Comptes rendus de I academie des sciences serie II, Vol.319. Issue 2, pp.187-192... 

A number of selected crystal structures of polymers are shown in this section in projection along the helix axes. The poly(ethylsilylethylene) of Fig. 5.23 has its silicon atoms marked by solid circles. Neighboring helices are related by a center of symmetry, so that they must be enantiomorphous and also anticlined, i.e., the helix pairs have different handedness (d-RH and f-LH helices) and opposite inclinations of side-groups (up- and down-helices) as discussed in Sect. 5.1.8. The coordination number for the helices is three instead of the expected four because the 31 and 32 screw axes of the 2 3/1 helices match the trigonal lattice symmetry and permit a closer overall packing with CN = 3 rather than 4 (see Fig. 5.21). 

Natta and Carradini [30] postulated three principles for determining the crystal structures of polymers These are ... 

Data on Fig. 5.3 is presented in a scale of relative intensities (intensity of the highest peak is accepted equal 100%). For converrience of corrsideration curves are represented with displacement on an axis of ordinates. The scattering plots without displacement represerrted completely overlapping of diffractogram profiles of composite films with diffractogram of a pure LDPE film, except peaks of crystal silicon, which weren t present on PE diffractogram. It testifies that additives of powders nc-Si practically haven t changed crystal structure of polymer. 

Wide-angle X-ray diffraction (WAXD) scattering has been used widely in studies of the characterisation of crystal structure of polymers (poly(ethylene-2,6-naphthalate) [53], atactic polypropylene [14], poly(co-pentadecalactone) [54], and PP homopolymer [55]). 

As with metals and ceramics, the properties of polymers are intricately related to the structural elements of the material. This chapter explores molecular and crystal structures of polymers Chapter 15 discusses the relationships between structure and some of the physical and chemical properties, along with typical applications and forming methods. 

Corradini has shown that almost all the known crystal structures of polymers are easily rationalizable in terms of these principles [1]. In particular, he suggested that the mode of packing of polymer chains first depends on the outside envelope of the chains [1]. Depending on the conformation, the form of a polymer chain may be approximated by a cylinder of radius r, corresponding to the outside envelope of the atoms of the main chain, bearing a periodic helical relief of radius R,... 

The observation of the lack of symmetry in the ordered structures of polymers is strictly related to the possibility of the direct observation of the local arrangements of chains in the crystals using methods of electron diffraction and solid-state nuclear magnetic resonance (NMR) spectroscopy. The study of the crystal structure of polymers, performed with the traditional X-ray diffraction, generally leads to models of packing that describe the order in the long range. 

The less symmetric model of the structure of sPS of Figure 2.19b is also an example of frustrated crystal structure of polymer [113]. The characteristic feature of frustrated structures is that two motifs maximize their interactions at the expense of the third motif. As a consequence, the third motif is frustrated since it is not in its ideal environment [114]. In the model of Figure 2.19b, the structural motif is the triplet of franx-planar chains. The different orientations of the three triplets of chains included in the unit cell are such that two triplets maximize their interactions at the expense of the third triplet [113]. 

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