Crystallization induced by stretching

Under all ordinary circumstances the average lengths of terminal chains and of internal chains will be the same, or nearly so hence represents the weight fraction of the structure which is active in deformation. For certain applications, as for example to crystallization induced by stretching, 8a is an appropriate measure of the effective portion of the... 

Flory, P.J. (1947) Thermodynamics of crystallization in high polymers. I. Crystallization induced by stretching, J. Chem. Phys. 15(6), 397-408... 

Intramolecular crystal nucleation does not exclude the fact that intermole-cular crystal nucleation can be quite important in crystallization of very short chains, very rigid chains, or polymer crystallization induced by stretching or polymerization, etc. The competition of these two paths and the resulting crystal morphology are worthy of further studies. 

Similarly, oriented crystallisation can be induced by stretching sheets or films of polymers in two directions simultaneously. The resulting materials have biaxially oriented polymer crystals. Typical examples of such materials are biaxially stretched poly(ethylene terephthalate), poly(vinylidene chloride), and poly (propylene). Since the oriented crystals do not interfere with light waves, such films combine good strength with high clarity, which makes them attractive in a number of applications. 

The polymerization of a,a-disubstituted P-propiolactones leads to polyesters of the general formula (CH2CRiR2COO)n. Several polymers in this series have been found to be semi-crystalline despite their atactic nature and x-ray diffraction studies have revealed two crystal modifications , referred to as the a- and p-forms. In general, unoriented films exhibit the a-fomi and the transformation to the p-form is induced by stretching. 

Only crystallization induced by a tensile type deformation has been discussed here. Other types of deformation such as biaxial extension, shear and torsion should also be considered. Such deformations have been studied and analyzed for amorphous networks. However, there is a paucity of experimental data, as well as analysis, of the equilibrium aspects of crystallization induced by these deformations. In one available report the observed melting temperature of natural rubber networks increased substantially when subject to biaxial deformation.(41) An increase in melting temperature of about 50 °C was found for a biaxial stretching ratio of three. This increase is much larger than that observed for natural rubber when crystallized in simple extension. 

The crystallization induced by orientation consists of stretching polymer chains to form fibrous crystals or fibers [25], The formation of such fiber-like morphology is accompanied by the formation of a typical shish-kebab or bottiebrush morphology [2,4-12,25-27]. A relevant reference for crystallization under orientation during different polymer processing operations has been recently published [28]. 

Probably most of these investigators were studying poly(dichlorophosphazene) in the partially crosslinked state. Most of this was summarized by Allcock (.9). More recently, highly purified, uncrosslinked II has been examined in the solid state (21). The unstressed polymer is amorphous at room temperature, but crystallization can be induced by cooling or stretching techniques. The glass transition temperature, measured by Torsional Braid Analysis, is -66°C (22). 

Polymer films were produced by surface catalysis on clean Ni(100) and Ni(lll) single crystals in a standard UHV vacuum system H2.131. The surfaces were atomically clean as determined from low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Monomer was adsorbed on the nickel surfaces circa 150 K and reaction was induced by raising the temperature. Surface species were characterized by temperature programmed reaction (TPR), reflection infrared spectroscopy, and AES. Molecular orientations were inferred from the surface dipole selection rule of reflection infrared spectroscopy. The selection rule indicates that only molecular vibrations with a dynamic dipole normal to the surface will be infrared active [14.], thus for aromatic molecules the absence of a C=C stretch or a ring vibration mode indicates the ring must be parallel the surface. 

Polymers with rigid, cyclic structures in the polymer chain, as in cellulose and poly(ethy-leneterephthalate), are difficult to crystallize. Moderate crystallization does occur in these cases, as a result of the polar polymer chains. Additional crystallization can be induced by mechanical stretching. Cellulose is interesting in that native cellulose in the form of cotton is much more crystalline than cellulose that is obtained by precipitation of cellulose from... 

A way to stretch or compress metal surface atoms in a controlled way is to deposit them on top of a substrate with similar crystal symmetry, yet with different atomic diameter and lattice constant. Such a single monolayer of a metal supported on another is called an overlayer. Metal overlayers strive to approach the lattice constant of their substrate without fully attaining it hence, they are strained compared to their own bulk state [24, 25]. The choice of suitable metal substrates enables tuning of the strain in the overlayer and of the chemisorption energy of adsorbates. A Pt monolayer on a Cu substrate, for instance, was shown to bind adsorbates much weaker than bulk platinum due to compressive strain induced by the lattice mismatch between Pt and Cu, with Cu being smaller [26]. 

The question may arise whether the same changes in material properties can also be obtained by stretching the filaments after the polymerization has taken place. In our experience the anisotropy was never so great in such cases, whereas the isotropic state was almost completely recovered when the samples were heated above the melting temperature. At this temperature the post-polymerization drawn filaments retained the original dimensions they possessed before stretching. Apparently some strain-induced crystallization yielded a metastable anisotropy which was lost under the combined action of entropy and strained crosslinks when the crystalline areas were melted. 

Finite chain extensibility is the major reason for strain hardening at high elongations (Fig. 7.8). Another source of hardening in some networks is stress-induced crystallization. For example, vulcanized natural rubber (cw-polyisoprene) does not crystallize in the unstretched state at room temperature, but crystallizes rapidly when stretched by a factor of 3 or more. The extent of crystallization increases as the network is stretched more. The amorphous state is fully recovered when the stress is removed. Since the crystals invariably have larger modulus than the surrounding... 

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