Polymer crystals defects

Wunderlich, B. The melting of defect polymer crystals. Polymer 5, 611... 

The following factors are required to calculate the states of the defect polymer crystal with known, ideal lattice structure ... 

In fact, a more complicated pattern of interaction is likely to give rise to different types of short-range order. The symmetries of the Bragg-Williams approximation can be readily broken if, instead of the chain molecules, the pairs of chains corresponding to the interaction described in Eq. (2.21) are regarded as statistical units. This leads to the Bethe-Peierls approximation or to the quasi-chemical approximation of Fowler and Guggenheim defect polymer crystal appears then to be a sub-... 

Wunderlich B (1964) A Thermodynamic Description of the Defect Solid State of Linear High Polymers. Polymer 5 125-134 The Melting of Defect Polymer Crystals. Ibid 5 611-624. 

The discussion of metastable, semicrystalline phases of polymers and their irreversible melting is based on the two early papers Wunderlich B (1964) A Thermodynamic Description of the Defect Solid State of Linear High Polymers. Polymer 5 125-134 and The Melting of Defect Polymer Crystals. Polymer 5 611-624. A later review and expansion is given in Wunderlich B (1997) Metastable Mesophases. Macromol Symp 113 51-65. 

In order for a model to be a comprehensive representation of the defect polymer crystal, it should satisfy a number of rather rigid requirements ... 

Hellmuth, Wunderlich, and Rankin (1966), for polyoxymethylene by Jaffe and Wunderlich (1967), and for pol3 aprolactam by Liberti and Wunderlich (1968). From this series of investigations the following conclusions have been drawn based on the general description of melting of defect polymer crystals by Wimderlich (1964). 

Secondly, the ultimate properties of polymers are of continuous interest. Ultimate properties are the properties of ideal, defect free, structures. So far, for polymer crystals the ultimate elastic modulus and the ultimate tensile strength have not been calculated at an appropriate level. In particular, convergence as a function of basis set size has not been demonstrated, and most calculations have been applied to a single isolated chain rather than a three-dimensional polymer crystal. Using the Car-Parrinello method, we have been able to achieve basis set convergence for the elastic modulus of a three-dimensional infinite polyethylene crystal. These results will also be fliscussed. 

A very similar technique is atomic force microscope (AFM) [38] where the force between the tip and the surface is measured. The interaction is usually much less localized and the lateral resolution with polymers is mostly of the order of 0.5 nm or worse. In some cases of polymer crystals atomic resolution is reported [39], The big advantage for polymers is, however, that non-conducting surfaces can be investigated. Chemical recognition by the use of specific tips is possible and by dynamic techniques a distinction between forces of different types (van der Waals, electrostatic, magnetic etc.) can be made. The resolution of AFM does not, at this moment, reach the atomic resolution of STM and, in particular, defects and localized structures on the atomic scale are difficult to see by AFM. The technique, however, will be developed further and one can expect a large potential for polymer applications. 

We will then examine other flexible polymer crystallization instances which may be interpreted, at least qualitatively, in terms of the bundle model. We will concentrate on crystallization occurring through metastable mesophases which develop by quenching polymers like isotactic polypropylene, syndiotactic polypropylene etc. In principle also hexagonal crystallization of highly defective polymers, and order developing in some microphase-separated copolymer systems could be discussed in a similar perspective but these two areas will be treated in future work. A comparison between the bundle approach and pertinent results of selected molecular simulation approaches follows. 

In most cases, however, polymers crystallize neither completely nor perfectly. Instead, they give semicrystalline materials, containing crystalline regions separated by adjacent amorphous phases. Moreover, the ordered crystalline regions may be disturbed to some extent by lattice defects. The crystalline regions thus embedded in an amorphous matrix typically extend over average distances of 10-40 nm. The fraction of crystalline material is termed the degree of crystallinity. This is an important parameter of semicrystalline materials. 

Gerasimov et al. have reported that poly-p-PDA Et is obtained quantitatively at 170 - 4.2 K and that the activation energy is 1600 300 eal/mol at 170 - 100 K and close to zero (<20 cal/mol) at 90 — 4.2 K, respectively. From the outstanding reactivity of p-PDA Et at an extremely low temperature, the barrier to the reaction in the monomer crystals has been attributed to the force of the crystal lattice and classified into the region of negative values of the potential energy. In addition the observed induction period at 4.2 K has been attributed to the growth period of crystal defects (see Sect. IV.a.) In the case of DSP, quantitative conversion of monomer to polymer crystals has been achieved by photoirradiation at — 60°C26). 

In Fig. 21 DCH crystals are shown before polymerization and at an intermediate conversion. It is typical for the thermal reaction that more perfect monomer crystals require longer reaction times than defect-rich crystals. There is evidence that in the radiation polymerization of DCH the polymer crystal perfection increases with decreasing temperature, i.e., the nucleation process requires a rather high thermal activation energy. 

The usual and therefore most important situation where polymers crystallize is in melts eooled below the point of the fusion of a crystallite of infinite dimensions. Then, crystallization occurs by the nucleation and growth of spherulites. Another crystallization process is sometimes encountered in oriented melts and glasses. In such systems, the crystallization seems to occur at once in the whole sample and not at the interface between the growing crystallites and the amorphous matrix. Despite munerous studies, the crystallization process is not fully understood. Scattering measurements suggest a preliminary spinodal decomposition of the undercooled isotropic melt in phases with and without chain ends and chain defects before the formation of the crystallites [32]. 

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