Uniaxial mechanical deformation

An uniaxial mechanical deformation provokes drastic changes in the identation pattern of drawn polymers. Some typical results illustrating the dependence of MH on draw ratio for plastically deformed PE are shown in Fig. 19 a. The quoted experiments 12) refer to a linear PE sample (Mw 80.000) prepared in the usual dumbbell form drawn at a rate of 0.5 cm/min at atmospheric pressure. Identations were performed longitudinally along the orientation axis. Before the neck (A = 1), the... 

NMR spectroscopy has been applied to investigate the behavior under uniaxially mechanical deformation. A study of drawn fibers prepared from an isotactic polypropylene modified by an ethylene-aminoalkyl acrylate copo-l)uner has been done using the broad line of H NMR. NMR spectra were measured on the set of fibers prepared with a draw ratio X from q to 5.5 at two temperatures, one of them corresponding to the onset of segmental motion and the other one is the middle of the temperature interval as determined by decrease of the second moment 2D time-domain H NMR was used to... 

Uniaxial mechanical deformation produces a conspicuous anisotropic shape of the residual indentation (4). The anisotropy depends on the orientation of the diagonals of indentation relative to the axial direction. Two well-defined hardness values emerge. One value (maximum for a Vickers indenter) can be derived from the indentation diagonal parallel to the fiber axis, d. The second one (minimum) is deduced from the diagonal perpendicular to it, d . The former value responds to an instant elastic recovery of the fibrous network in the draw direction. The latter value defines the plastic component of the oriented material. It is useful to define the indentation anisotropy as AH = 1 - (d /d ). 

Fig. 6 Schematic representation of the conical layer distribution of polydomain Sc elastomers exposed to uniaxial mechanical deformation (a) and corresponding X-ray pattern (b). 0 (denoted < > in this figure) is the Sc tilt angle, d the smectic layer spacing, and / the length of the mesogenic units. The layer normal k is conically distributed around the stress axis z. Reprinted with permission from [87]. Copyright (2008) American Chemical Society...

Fig. 16 Smectic-C elastomer with uniform director orientation but conical layer distribution subjected to a second uniaxial mechanical deformation under an angle of 0 90° with respect to the first deformation axis (0 is the Sc tilt angle) [80, 116]...

Another technique used for obtaining macroscopically polar films involves mechanical extension of the material. Uniaxial plastic deformation induces a destruction of the original spherulitic structure into an array of crystallites in which the molecules are oriented in the deformation direction. In case of PVF2 when such deformation takes place below 90 °C the original tg+ tg chains are forced into their most extended possible conformation which is all-trans [32]. 

Lin et al. [66] have exploited this variation in specific volume of the RAF to control the barrier properties of polyester films. An attempt to correlate the mechanical deformation of PET with the amount of RAF present in these films has been made recently. Moreover, the observations have been that a sample with a larger amount of RAF, on uniaxial compression, shows considerable loss in crystallinity compared to a sample having a lower amount of RAF. These findings have been reported in a recent publication [67]. 

Cross-linked liquid crystalline polymers with the optical axis being macroscopically and uniformly aligned are called liquid single crystalline elastomers (LSCE). Without an external field cross-linking of linear liquid crystalline polymers result in macroscopically non-ordered polydomain samples with an isotropic director orientation. The networks behave like crystal powder with respect to their optical properties. Applying a uniaxial strain to the polydomain network causes a reorientation process and the director of liquid crystalline elastomers becomes macroscopically aligned by the mechanical deformation. The samples become optically transparent (Figure 9.7). This process, however, does not lead to a permanent orientation of the director. 

The modulus is the most important small-strain mechanical property. It is the key indicator of the "stiffness" or "rigidity" of specimens made from a material. It quantifies the resistance of specimens to mechanical deformation, in the limit of infinitesimally small deformation. There are three major types of moduli. The bulk modulus B is the resistance of a specimen to isotropic compression (pressure). The Young s modulus E is its resistance to uniaxial tension (being stretched). The shear modulus G is its resistance to simple shear deformation (being twisted). 

The volumetric properties (Chapter 3) and the mechanical properties are interrelated since the mechanical properties are defined in terms of the response of the volume and the shape of a specimen to an applied mechanical deformation. Each type of modulus is defined in terms of the stress a required to deform a specimen by a strain of e, in the limit of an infinitesimally small deformation of the type quantified by that modulus. For example, Young s modulus is defined by Equation 11.1, in the limit of —>0 under uniaxial tension. This equation shows that the stress cr required to achieve a small strain of under uniaxial tension is proportional to E. 

Orientational ordering may be observed even without explicit mechanical deformation. Processing procedures as routine as simply casting a film or electrochemical synthesis at an interface can lead to pronounced anisotropy in the molecular-level order and orientation of the polymer chains [70]. This anisotropy, in combination with uniaxial stretching, often leads to a 3D texturing of the observed structure [65,419]. Interaction of CPs at interfaces is another important area of study. The structural anisotropy of CP and P3AT thin films [420,421] has been studied by a number of groups especially in... 

Smectic elastomers, due to their layered structure, exhibit distinct anisotropic mechanical properties and mechanical deformation processes that are parallel or perpendicular to the normal orientation of the smectic layer. Such elastomers are important due to their optical and ferroelectric properties. Networks with a macroscopic uniformly ordered direction and a conical distribution of the smectic layer normal with respect to the normal smetic direction are mechanically deformed by uniaxial and shear deformations. Under uniaxial deformations two processes were observed [53] parallel to the direction of the mechanical field directly couples to the smectic tilt angle and perpendicular to the director while a reorientation process takes place. This process is reversible for shear deformation perpendicular and irreversible by applying the shear force parallel to the smetic direction. This is illustrated in Fig. 2.14. 

The structures of TPNR and their composites with carbon black were investigated by SAXS, and small-angle neutron scattering (SANS). Thomas et al. studied the deformation of cylindrical PS domains of a near single crystal styrene-isoprene-styrene (SIS) triblock copolymer by SAXS. The structural changes in the hexagonal lattice of cylinders on uniaxial tensile deformation were related to the macroscopic mechanical behaviour. 

However, it is well known that a mechanical deformation of a conventional, isotropic polymer network causes anisotropy. Under deformation the chain segments become oriented according to the symmetry of the external field and the state of order of the network can be characterized by an order parameter similar to that of nematic liquid crystals. Very early mechanical experiments on nematic polydomain elastomers actually demonstrate that a uniaxial deformation of a nematic elastomer converts the polydomain structure into a macroscopically xmi-formly ordered monodomain network [44]. This is shown in Fig. 2, where the opaque polydomain becomes optically transparent and converts into a monodomain... 

Figure 2.22 Typical micromechanical mechanisms in p-iPP with the characteristic lamellar morphology after uniaxial tensile deformation at room temperature left orientation of the lamellae...

The general Landau theory, which was developed by de Gennes to describe critical phenomena in MFCs, has been appUed to elastic networks comprised of PLCs [66]. The Landau formalism also allows one to make contact with the theory used to describe conventional orientation phenomena in nonmesogenic polymer networks. In particular, a mechanical deformation via its associated stress field a influences g (and therefore Tc and 5c) analogously to external magnetic or electric fields. For a small (uniaxial) extension ratio X = e —, where e is the strain, the form of g in Eq. (5.16) is modified by the additional terms as follows ... 

FIGURE 13.1 T3fpes of mechanical deformation (a) unstressed (b) uniaxial tension (c) pure shear, and (d) isotropic compression. 

---