Stretching direction

According to Hosemann-Bonart s model8), an oriented polymeric material consists of plate-like more or less curved folded lamellae extended mostly in the direction normal to that of the sample orientation so that the chain orientation in these crystalline formations coincides with the stretching direction. These lamellae are connected with each other by some amount of tie chains, but most chains emerge from the crystal bend and return to the same crystal-forming folds. If this model adequately describes the structure of oriented systems, the mechanical properties in the longitudinal direction are expected to be mainly determined by the number and properties of tie chains in the amorphous regions that are the weak spots of the oriented system (as compared to the crystallite)9). 

The model describes the characteristic stress softening via the prestrain-dependent amplification factor X in Equation 22.22. It also considers the hysteresis behavior of reinforced mbbers, since the sum in Equation 22.23 has taken over the stretching directions with ds/dt > 0, only, implying that up and down cycles are described differently. An example showing a fit of various hysteresis cycles of silica-filled ethylene-propylene-diene monomer (EPDM) mbber in the medium-strain regime up to 50% is depicted in Figure 22.12. It must be noted that the topological constraint modulus Gg has... 

A non-electrochemical technique which has been employed to alter the physical characteristics of a number of polymers is that of stress orientation [26, 27], in which the material is stressed whilst being converted to the desired form. This has the effect of aligning the polymer chains and increasing the degree of order in the material, and is obviously most applicable to materials which can be produced via a precursor polymer. With Durham polyacetylene (Section 4.2.1) increases in length in excess of a factor of twenty have been achieved, with concomitant increases in order, as shown by X-ray diffraction and by measurements of the anisotropy of the electrical conductivity perpendicular and parallel to the stretch direction. 

Stretching a polymer in two perpendicular directions, either successively or by blowing a bubble of molten material, leads to its biaxial orientation, which strongly improves mechanical properties in the stretching directions and/or gas permeability (e.g., biaxial orientation of polypropylene leads to BOPP (for biaxially oriented polypropylene) or biaxial orientation of poly(ethylene terephthalate) gives CC>2-impermeable bottles for carbonated beverages.) (Characterisation methods for determining molecular orientation are considered in Chapter 8.)... 

Figure 9. SANS measurements of R /Rt° and RL/R ° for stretched radiation cross-linked polystyrene. is determined by measurements in which the neutron is parallel (iso) and perpendicular (aniso) to the stretching direction. Mc is molecular weight between crosslinks. Theoretical curves 2 and 3 are drawn for tetrafunctional networks. Data from Ref. 21.

Moreover, in developing and testing the theory, biaxial stress-relaxation experiments were carried out. That is, square sheets were stretched in both directions but in unequal amounts. In all cases, the stress in the major stretch direction relaxed at the same relative rate as that in the minor... 

Fig. 7 X-ray diffraction patterns of a the as-cast film, b the stretched film of the DNA-lipid complex in water moisture, and c the stretched DNA-lipid film in the dry state. Open, two-headed arrows show the stretching direction of the film. Closed arrows show the incident beam of the X-rays. X-ray photographs of a and c were enlarged to show diffraction more clearly...

When the stretched film was dried in air (simply applying a hair-dryer for 2 min), only diffraction on the equator as two spots of 41 A was observed but not diffraction on the meridian for base-pairs (Fig. 7c). This suggests that the orientation of the base pairs is not perpendicular to the stretched direction, although the DNA strands are aligned parallel to the stretched direction. Figure 8 shows CD spectra of the cast film of the DNA-lipid complex in wet (a) and dry states (b). The CD spectrum of the wet film was consistent with a B-form structure of native DNA in fibers obtained from aqueous solution [1-3]. On the contrary, the CD spectrum in the dry state resembles the A-form of DNA in which the base pairs slant to the axis of the strands [2,3]. These results are consistent with CD spectra of the DNA-lipid complex in an organic solution DNA exists in the B-form in the presence of water and the C-form in the ab-... 

When a polarized spectrum of the film was taken in the dry state, a small dichroic ratio was observed (A L/A// = 1.3, see Fig. 9c). This is due to the slant structures of base pairs and intercalated dyes to the axis of the strands, since the DNA strands were confirmed to be aligned along the stretching direction even in the dry film (see Fig. 7c). These changes of polarized absorption spectra dependent on water moisture were reversible at least 10 times. 

An interesting experiment on a DNA-based network embedded in a cast film had already been done by Okahata et al. in 1998 [49]. In this pioneering experiment the DNA molecules were embedded (with side groups) in a polymer matrix that was stretched between electrodes (see Fig. 12). It was found that the conductivity parallel to the stretching direction (along the DNA) was 4.5 orders of magnitude larger than the perpendicular conductivity. 

Another apparatus for general biaxial extension testing, illustrated in Fig. 8, was built at our laboratory27). A square sheet of rubber, 11.5 cm long and 1 mm thick, is clamped at the edges by several pieces of movable chucks and stretched biaxially by means of the two bars that can be displaced separately bade and forth by a servo-meachnism. Tensile forces acting on the sheet in the stretching directions are transmitted to the bars and measured by two transducers mounted at the ends of each bar. After many modifications and improvements on the mechanical and electric parts,... 

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