A fibre composite model of drawn crystalline polymers

The two-phase fibre composite model proposed by us elsewhere [Barham and Arridge (1977)] is used to explain the behaviour of polyethylene and polypropylene on drawing to very high draw ratios. The basis of the theory is that, after necking, there exists a uniform distribution of aligned needle-like elements of near perfect crystallinity in a less-perfect matrix. On further drawing these reinforcing elements are supposed to elongate in a homogeneous deformation while contracting laterally. The resultant change in their aspect ratio is sufficient, using short fibre reinforcement theory, to account for the increase in the modulus on drawing. The Young's modulus of the drawn fibre is given by the expression:

$\text{E}{}_{\mathrm{f}}\text{=cE}{}_{\mathrm{c}}\text{1?}\text{tanh x}\text{x}\text{+(1?C)E}{}_{\mathrm{m}}$

where c is the concentration of elements, E_c the modulus of the crystalline elements and E_m that of the matrix while x, derived from shear-lag theory, depends upon the aspect ratio of the elements and the moduli of the two phases. For both polyethylene and polypropylene it is found that x ∝ $\text{t}{}^{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$, where t is the post-neck draw ratio. The model is applied (1) to the explanation of the observed relation between Young's modulus and draw ratio, (2) to explain the temperature dependence of drawing behaviour, (3) to postulate a mechanism far non-linear viscoelasticity and creep behaviour and (4) to explain the self-stiffening after annealing under constraint [Arridge, Barham and Keller (1977)].     

Annealing of drawn monofilaments of liquid crystalline polymer vectra/vapor grown carbon fiber nanocomposites

The annealing treatment of drawn LCP nanocomposite monofilaments is discussed here. Upon annealing, an unstable structure is generated which presents a strong dependence on the annealing time sequence and temperature as observed by DSC. Moreover, it seems that increasing the draw down of the fiber restricts the formation of this new (unstable) structure. Based on dynamic mechanical analysis and on solid state ¹³C NMR, the nature of the newly-formed structure is elucidated, and based on sequence ordering within the non-oriented amorphous phase in the copolymer, a 10% change in the monomer proportion in the ordered phase is observed after annealing.     

Thermotropic liquid crystalline polymers

Summary Electro-optical behaviour of a liquid crystalline polymer- poly{1-[6-[4-N-(4-cyanophenyl)iminomethylphenoxy] hexyloxycarbonyl] ethylene}- is studied A sharp change of the optical properties measufred in crossed polarizers under the cooling of the preliminary oriented sample in an electric field is observed. The effect is explained from the viewpoint of the electric field induced cooperative structure transition.Part 6. V.P. Shibaev, S.G. Kostromin, N.A. Platé: European Polym. J., in press     

Thermotropic liquid crystalline polymers

Summary Liquid crystalline comb-like polymers with spontaneous polarization are described.     

Thermotropic liquid crystalline polymers

Rigid chain molecules have the tendency to display fluid liquid crystalline phases within a limited temperature range. This range depends on the particular chemical structure of the chain molecules. The chain rigidity as well as the intrinsic dynamical and elastic properties of the liquid crystalline melt can successfully be used to obtain high strength, high stiffness polymers, which can be melt processed. The rigid chain polymers display unusual thermodynamical, dynamical, structural, rheological and mechanical properties.     

Thermotropic liquid crystalline polymers

Summary A new nematic monomer — p-( -acryloyloxycaproyloxy)p-methoxyphenylbenzoate — was synthesized. A homepolymer of this monomer and its copolymer with cholesteryl--acryloyloxycapronate were prepared by radical polymerization. The homopolymer forms the mesophase of nematic type. The copolymer forms the cholesteric mesophase that shows selective light reflection. The maximum wavelength of the reflected light is equal to 495–500 nm and is not changed on temperature.Part 7. R. V. Talroze, V. V. Sinitzyn, V. P. Shibaev, N. A. Plate Polymer Bull. 6, 309 (1982)     

Plastic deformation of crystalline polymers

Draw ratios have been measured for samples of polyethylene and trons-polyisoprene, crystallized at various temperatures and at various degrees of orientation. The values obtained range from unity, i. e., no drawing is observed, up to values of about 15X for materials crystallized in the oriented state and then drawn in a perpendicular direction. The results are in rough accord with a simple molecular network model in which network strands are incorporated into crystallites with a number of reversals of direction (folds), and the remainder of a strand between network junctions is randomly arranged. The reduction in draw ratio with increasing temperature of crystallization and with increasing orientation at the time of crystallization is then accounted for in terms of a reduction in the number of reversals (folds) per molecular strand. Differences in natural draw ratio for different polymers are attributed to variations in characteristic sequence length within a crystallite and in the number of folds per network strand.     

Molecular orientation in drawn smectic and crystalline isotactic polypropylene

The drawing behavior of two polypropylene films of different structures was analyzed. The two films differ as a consequence of different quenching conditions. At low temperature, a biphasic smectic-amorphous system was obtained, while quenching at 100°C produced a biphasic crystalline-amorphous system. The drawing of samples was carried out at 110°C at which temperature the smectic phase is not stable and is transformed into the crystalline α-form. The initial structure affects the drawing behavior and the properties of the drawn samples. The mechanical, optical, and X-ray analyses clearly show that high molecular orientation is achieved at lower deformations in the initially smectic sample. In particular, the amorphous phase is highly oriented, inducing higher axial elastic modulus.     

Plastic deformation of crystalline polymers

Under uniaxial tensile load, the plastic deformation of unoriented crystalline polymers first transforms the lamellae into a fibrous structure. Usually the drawing is inhomogeneous with a neck propagating through the sample. The higher the draw ratio, the higher the axial elastic modulus as a consequence of the larger fraction of taut tie molecules in amorphous layers connecting the crystalline blocks of each microfibril. As a consequence of the almost 1/(1 ? α) times higher strain of amorphous layers under tensile load, the taut tie molecules are much more strained than the chains in crystal blocks. Hence, their contribution to elastic modulus is substantially higher than one would guess from their fraction β. This is more so in polyethylene with higher crystallinity (α = 0.8) than in nylon 6 with low crystallinity (α = 0.5). Even for the highest modulus polyethylene E = 70 GPa ～ 0.3 × E_c, one needs less than 7.5 percent of taut tie molecules. The plastic deformation of the fibrous structure markedly enhances the number of interfibrillar tie molecules in nylon 6 and to a lesser extent in polyethylene and polypropylene. Homogeneous drawing without a neck transforms the whole sample into a fibrous structure rather uniformly so that for a long while one has the lamellar and fibrillar morphology side by side. The end effect on the structure obtained does not differ appreciably from inhomogeneous drawing with neck propagation. The drawing of polymers with a liquid crystal structure yields a highly aligned fibrous structure with very few chain folds and an exceptionally high elastic modulus and strength. But the axial connection of individual highly oriented and ordered domains is affected by a relatively small fiaction of tie molecules, and this is responsible for reduction of the elastic modulus below the value of the ideal crystal lattice.     

Surface morphology and crystalline orientation of uniaxially drawn polytetrafluoroethylene films

For polytetrafluoroethylene (PTFE) films drawn uniaxially at 100, 150, 250, and 350°C, the morphology and degree of crystalline orientation were investigated by electron scanning microscopy and wide-angle X-ray diffraction, respectively. On the surface of the PTFE films which were heat-treated at 350°C for 2 h before the drawing, granules with a diameter of about 2 μm and bands with a width of about 0.3 μm were observed. By the drawing below the melting point, the bands are transformed into fibrils and a porous structure is formed by the drawing above the draw ratio (λ) of about 1.3. By drawing above the melting point, striations appear perpendicularly to the draw direction, and the collapse of the granules is observed. The orientation of the crystallites increases rapidly with increasing the draw ratio and approaches a plateau at λ ≈ 1.5 for the films drawn above the melting point and at λ ≈ 2.0 for that below it.     

结晶聚合物冷却定型的小建议

在生产塑料制品过程中,如果略微的冷却对制件的品质有好处,则按理说加大冷却程度对制件的效果会更好。但是,对于由结晶聚合物为原料制得的厚壁挤出产品而言这未必是正确的。因为冷却往往只发生在厚壁制件的某一侧。管材、片材、中空成型件和空心异型材是厚壁产品的典型代表。     

Structure and properties of crystalline polymers

Starting with a structural study of the crystallization behaviour of poly(vinyl alcohol), the author has been analysing the crystal and molecular structures of crystalline polymers for the past 35 years. One of the characteristic points of the methods used is the co-operative use of X-ray diffraction and infrared and Raman spectroscopy (normal coordinate treatment). There are many examples of the application of this technique to a series of polyethers, polythioethers, polyesters, polymer complexes etc. Furthermore, the intra- and intermolecular energy calculations have succeeded in accelerating the structural analyses of important but complicated polymer materials and in revealing the factors governing the stable crystal structure and molecular conformation of a polymer. Poly(ethylene oxygenzoate) x form and double-stranded helices of isotactic poly(methyl methacrylate), the first double helix ever found for synthetic polymers, are used as typical for the application of this method. The structural interpretation of the mechanism of optical compensation in racemic polymers is also a good example. The energy calculations have developed to the stage where the stability of two crystal forms of polyethylene can be discussed in terms of free energy. Utilizing the structural data thus accumulated and the spectroscopically obtained interaction parameters, the structure-property relationship has been clarified quantitatively by lattice dynamical theory. The calculated crystallite moduli of polymer chains agree well with the observed ones for many polymers such as poly-p-phenylene terephthalamide, etc. The study has been advanced by a new method of calculation of the three-dimensional elastic constant tensor and its application to polyethylene, poly(vinyl alcohol), nylon 6 etc. As an extension the general method of calculating the piezoelectric constant tensor has also been derived and successfully applied to poly(vinylidene fluoride) form I, resulting in the interpretation of the origin of macroscopic piezoelectricity of this polymer.     

Electrorheological effect of liquid crystalline polymers

Large increases in shear stress upon application of a 2.0 kV/mm electric field were observed in homogeneous fluids composed of polysiloxane-based liquid crystalline polymers (LCPs) in dimethyl silicone at a shear rate of 200 s^?1. The increase was largest (about 3,000 Pa at 50°C) with LCP consisting of a polysiloxane bearing mesogenic groups as side chains. With LCP having the mesogenic groups within the main chain, the maximum increase was about 1,300 Pa at 90°C. It was about 400 Pa at 30°C with LCP having the mesogenic groups at both ends only (biterminal), and several Pa at 30°C with LCP having the mesogenic group at one end only (monoterminal). The increases were smaller with mesogenic groups of lower positive dielectric anisotropy in the side chain LCP. The side chain, biterminal, and monoterminal LCPs exhibited Newtonian flow in the electric field and shear stress yield at low shear rates in its absence. The complex dynamic modulus and viscosity of the side chain LCP in the electric field showed no dependence on strain at deformation displacements approaching 5°, but in its absence were generally strain-dependent, and suggest the strong electrorheological effect of these homogeneous LCP fluids is related to a flexible-chain linkage between their crystalline domains. © 1995 John Wiley & Sons, Inc.     

Microheterogeneous structure of liquid crystalline polymers

Liquid crystalline polymers with phenyl benzoate side groups with various lengths of flexible chain-end were synthesized. Small-angle X-ray scattering studies were carried out. Layer structures in the liquid crystalline polymers were characterized and the microheterogeneous structure was investigated. It was concluded that the microstructure is dependent on the length of the flexible mesogenic chain-end.     

On shish-kebab morphologies in crystalline polymers

Shish-kebabs or row structures consist of transverse lamellae growing from a central linear thread all with a common chain axis direction parallel to the central nucleus. Attention is drawn to the fact that adjacent lamellae are not in contact; in melt-crystallized -polypropylene the separation is generally 3–4 nm. Moreover, lamellae in asymmetric environments, such as the ends of rows, are often elastically curved outwards. Both observations support proposals concerning the importance of cilia for melt-crystallized growth and the development of spherulites but would have been difficult to establish without the advantage of oriented nucleation. The former leads to values for the effective dimensions of cilia, the latter to values of the pressure they exert on an adjacent lamella. The advantages of using linear rather than point nuclei in evaluating melt-crystallized morphologies are emphasized.     

Chiral liquid crystalline side chain polymers

Summary Optically active liquid crystalline side chain polymers with dif ferent flexible spacer groups (n=2–11) and different main chains (acry lates, methacrylates and α chloroacrylates) are described. (+)-2-methyl butyloxyphenyl esters of benzoic acid were chosen as the mesogenic group.     

Liquid crystalline epoxy based thermosetting polymers

In recent years many examples of liquid crystalline thermosetting polymers have been reported in the literature. The main reason for the development of this new class of materials comes from both technological and theoretical implications. Among the different anisotropic thermosets, epoxy resins represent a family of polymers whose properties make them primary candidates in different fields ranging from high performance matrices in advanced composites to polymers for optical applications. The chemistry of curing process of amine hardened conventional epoxy resins is well known from the literature; however some substantial differences arise during liquid crystalline epoxy monomers crosslinking. The level of order of the cured resin can be strongly affected by the nature of the hardener, as well as the physical properties of the cured material. This review will present the results of synthesis and physico-chemical characterization of liquid crystalline epoxy resins in relation to their applications in some specific fields.     

Stability of blends of thermotropic liquid crystalline polymers with thermoplastic polymers

Breakup of fibers of a thermotropic liquid crystalline polymer (TLCP) above the melting temperature in various ordinary polymers has been studied by capillary instability experiments on single TLCP fibers and by annealing experiments on extruded TLCP/thermoplast blends. The TLCP was an aromatic copolyester, Vectra A900, the matrix polymers were PP, PS, PC, PEL PES, and PEBT. Both types of experiments show that the fiber/matrix morphology is, in general, highly unstable in the molten state. The TLCP fibers break up into droplets by a combination of Rayleigh distortions, end-pinching and retraction, depending on the system and shape of the fiber. Fibers of a thickness of ～1 μm can break up in a few seconds. Breakup times of fibrous blends and individual fibers are in agreement provided size effects are accounted for. Rayleigh distortions develop exponentially in time up to relative distortions of 0.5 to 0.6. Breakup occurs within a range of wave numbers rather than at one distinct dominant wave number, which is shown to be the consequence of relatively large initial distortions. Apparent values for the interfacial tensions calculated with Tomotika's theory turned out to be of the correct order of magnitude, ranging from 7 mN/m for Vectra/PES to 24 mN/m for Vectra/PP and to yield correct values of the interfacial tensions of PP/PS, PP/PC, and PS/PC using Antonow's rule.