Lamellar polymers

Fig. 31. 2H spectra of a polymer model membrane, cf. Fig. 27b), methyl deuterated at the head group. The spectra are compared for the monomer as well as the polymer lamellar phases at the same temperatures, respectively... 

Additional neutron scattering studies on different polymer systems could prove very important. Strobl [31,32,47,103] provides evidence that, for some polymers, lamellar crystallization is preceded by pre-ordering of the melt followed by formation of planar arrays of blocks. Investigating crystallization from the melt, Kaji and coworkers [25] find pre-ordering phenomena relating to orientational fluctuations of stiff polymer segments which, under appropriate conditions, determine phase separation prior to crystallization. 

Sect. 4.1). In polymers, lamellar thickness is kinetically determined by the restricted upward fluctuation in stem length on the one hand and, on the other hand, by the high detachment rate of shorter stems with l -> /mjn = 2ae/(A(p) (downward fluctuation). This problem has been studied analytically [62,74] and by simulation [71,75]. 

Other typically observed morphologies are co-continuity of phases and dispersion of lamellar domains. Co-continuity might allow to draw on properties of both constituents, yielding for example a stiffness/ductility balance not achievable with pure polymers. Lamellar morphologies will obviously yield improved barrier performance when the proper constituents are selected. 

Disregarding any detailed morphological models proposed for the lamellar crystals in semicrystalline polymers in the past half centiny, crystal imit cells in semicrys-talUne poisoners exactly obey the rules of classical crystallography, which consist of 7 crystal systems and 14 Bravais lattices (more precisely, there are only 6 crystal systems in polymer crystals, and the cubic system does not exist). Most of the unit cell determinations of the semicrystalline polymers rely on the wide-angle x-ray diffraction (waxd) experiments on oriented poisoner fibers and films. This is be-canse of small crystal sizes in polymers, which lead to a difficult experimental task to obtain single-crystal waxd results on semicrystalfine polymers. It is also possible to nse electron diffraction (ed) method in transmission electron microscopy (tern) to determine polymer lamellar crystal unit cell structures, dimensions, and symmetries. 

Crystallization of cis—1,4-polyisoprene from solution at -65 C has been carried out it is therefore possible that block copolymer preparation by epoxidation, bromination or some other reaction could be accomplished with lamellas of this polymer. Lamellar crystallization of cellulose, of amylose and of polyacrylic acid have been reported substitution reactions such as acetylation or ether formation with the hydroxyl groups and esterfication of the acid groups are possible reactions to carry out with lamellas of those polymers. The use of nonaqueous systems may be better suited to prevent swelling, and therefore, attack of the crystalline regions. It should also be possible to react poly(vinylalcohol) lamellas in suspension with acids or anhydrides to form vinyl-alcohol-vinyl ester block copolymers or with phosgene to obtain chloroformate groups which can undergo further reactions. 

Figure 3 Packing of comblike polymers bearing polymethylene side chains, (a) Scheme of the hexagonal packing of polymethylene chains as viewed down the chain axis, (b) One-layer (left) and two-layer (right) models for the comblike polymer lamellar structure.

Polymer Lamellar Crystals Grown from Solution. Similarly to the regular array of polymer chains exposed at extended-chain fibrillar crystals, the folded sections of polymer molecules at the fold plane of solution-grown single crystals can cause anisotropy of friction (Figures 6). This has been observed for a variety of materials, such as POM, PE, and poly(4-methyl-l-pentene) (11 -13). The explanation proposed for this observation is based on the presence of oriented folds at the surface of the fold plane. 

We have recently proposed (1) a simulation model of polymer crystallization through a dynamic Monte Carlo approach in a 2-dimensional lattice. Similar results are obtained in a three dimensional cubic lattice (2). We have found that this method can predict the compaction of polymer chains as folded molecules as they are found in polymer lamellar crystals. The model correctly predicts a gradual thickening of the crystals as the temperature is increased. In this paper we want to test further our model in order to determine if it is able to predict other experimental features of polymer ciystallization such as ... 

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