Tensile modulus, liquid crystalline

Poly(/)-phenylenctcrcphthalamiclc) forms a liquid crystalline solution and can be spun into a fiber with a very high orientation these fibers have excellent tensile and thermal properties. These high-modulus fibers are suitable as reinforcing materials in technical applications. 

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. 

Commercial hydroxypropyl cellulose has been melt-spun from a liquid-crystalline melt at 180 °C to produce fibres with an orientation factor (fc) of 0.6 even at zero take-up stress and 0.7 at higher take-up stresses, a tensile strength of 80 MPa, Young s modulus of 3 GPa and extension at break of 2 to 5 % 71). 

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. 

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. 

Highly oriented polymers can exhibit very high tensile moduli and high tensile strengths. Many polymers, both conventional and liquid-crystalline, have been produced in highly oriented form to make use of these properties, so it is important to consider what controls the ultimate modulus available for a particular polymer. It is clear that the highest modulus will be obtained if all the polymer chains are parallel to the tensile axis. The modulus of the material will then depend on the force required to produce unit extension of a chain and on the number of chains passing... 

As shown by Eq. (15) and (17), the tensile elastic behaviom of fibers made from lyotropic polymers is determined by the chain modulus e the shear modulus go and the orientation parameter . As has been discussed in Sect. 2 and 3, the latter is determined by the persistence length, the molecular weight, the polymer concentration and the temperature of the liquid crystalline solution, and furthermore by the spinning and coagulation conditions. 

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. 

Fan and co-workers [6] found that a liquid crystalline PI prepared by copolycondensation of 3,5-diamino-benzoic-4 -biphenyl ester, 4,4 -diamino-biphenyl ether and 3,3, 4,4 -oxydiphthalic dianhydride film had a 40% decrease in expansion coefficient accompanying a 270% improvement in tensile strength and a 300% improvement in modulus. These factors give a polymer with good film processibility. 

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. 

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