Tensile modulus, liquid crystalline polymers

Mechanical Properties. To reveal the reinforcing effect of liquid crystalline polymer microfibrils on the mechanical properties of the films both their dynamic torsional moduli and dynamic tensile moduli have been studied as a function of temperature using a Rheometrics Mechanical Spectrometer (RMS 800) and a Rheometrics Solids Analyzer (RSA II), respectively. For comparison purpose the modulus of neat matrix polymers and, in some cases, the modulus of carbon fiber and Kevelar fiber reinforced composites has also been measured. 

Thermotropic liquid crystalline polymers exhibit some degree of molecular order in their liquid crystalline phases, or so-called mesophases, that denotes the intermediate states between crystalline solids, or amorphous solids in the case of non-crystalline polymers, and isotropic liquids. Due to the molecular order in their mesophases, liquid crystalline polymers provide us with an effective way to achieve high tensile strength and high modulus materials. In the past three decades, researches in the area of liquid crystalline polymers have attracted much attention. 

For copolymer fibers, the tensile strength and modulus were found to be much higher than those of the PI fibers. Furthermore, Figure 8 shows that tensile strength and modulus of PBTA/PI molecular composite fibers increase with an increase of the PBTA content in block copolymers. It is evident that introdudng a liquid-crystalline polymer in molecular architecture makes considerable reinforcing effects in molecular composites. 

The aggregate model also has been used with success to describe the mechanical anisoti opy of several liquid crystalline polymers. Ward and co-workers [56] examined the dynamic mechanical behaviour of several thermotropic polyesters in tension and shear over a wide temperature range, and used the single-phase aggregate model to relate quantitatively the fall in tensile modulus with temperature to the corresponding fall in shear modulus. 

The primary motivation for industrial participation in the development of liquid crystalline polymers was the search for high performance tensile properties, i.e., fiber tensile strength and modulus. As it later turned out, the wholly aromatic, thermotropic polyesters were found to offer a great many more useful properties than just their now well-known tensile capabilities. 

Fig. 5.119. The model suggests there are structures on the scale from 500 nm to < 50 nm, specific to the liquid crystalline polymers. The key element is the microfibril, the same micro-structural unit basic to melt spun and drawn flexible polymers. The orientation of the microfibrils is along the fiber or elongational axis and this results in extremely high tensile modulus values for these materials. However, on a local scale it is clear that the microfibrils meander along the path of the director and are not literally rigid rods.

Tensile-fractured as-spun fibers of /Bu-HQ/Cl-PEC and Ph-HQ/Cl-PEC no longer exhibited fibrils, with such phenomena being different to those of the fBu-HQ/PEC and Ph-HQ/PEC systems. Thus, in order to obtain high-modulus as-spun fibers, the stability of the liquid crystalline state and the rigidity of the polymer chain are both assumed to be influential factors. 

It is not excluded that this mechanism is observed during the formation of fibres from X-500 The authors of this work pointed out that when the fibre was heated to 250-300 °C, its spontaneous elongation took place. Note that to attain higher orientation of a polymer in a fibre, it is necessary not only to transfer it to the liquid crystalline state but also to orient the liquid crystalline domains formed along the axis of the fibre. This orientation of the domain in which the macromolecules have been already mutually ordered requires not too high a draw ratio (the theoretical value must be <2). Indeed, experiments have shown that at the draw ratio of 1.53 to 1,70 the modulus (E) and the tensile strength (a) of the fibre at thermal treatment increase, which can be seen from the table compiled according to the results of this work. 

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