Crystalline polymers chain orientation

Polarised fluorescence has been used to study the orientation of non-crystalline polymer chains and a fluorescence technique has been developed to quantify the amount of microgel in polyacryl-amide. Time resolved photoluminescence analysis of poly( -phenyl-... 

Melt spun fibers are subjected to a hot drawing process to encourage polymer chain orientation and the development of crystallinity witl the fibers. Electrospun polyester has been produced via melt spinning , but in order to render fimctionality to the resulting material, gelatin was incorporated by subsequent surface modification involving formaldehyde. This in turn improves physical fiber properties, but slows the diffusion of small molecular species within the fiber (thus, for exanqile, imdrawn polyester dyes more readily than drawn material). The shorter diffusion path out of fine fib is responsible for lower color fastness of febrics derived ifom them. 

The scattering patterns of monodomain liquid crystalline polymers represent those from oriented liquid crystalline polymers. Such orientation can be achieved either in the solid state by quenching the oriented polymers or in the melt state by shearing the melt at the mesophase temperatures. Under a force field, such as a tensile force, shear force, or magnetic field, all the domains are aligned in the direction of the external field so that the polydomain liquid crystalline polymers can effectively become monodomain liquid crystalline polymers. For oriented main-chain liquid crystalline polymers, the scattering on the equator is due to interchain correlation and the scatterings on meridian raised from intrachain correlation. 

The first step in a polymer analysis is to identify the specific type of polymer in a given sample. This may be complicated in a formulated sample by the presence of additives. Infrared spectroscopy will usually provide information on both the base polymer(s) and the additive(s) present. The second step, if possible, is to determine details of the chemical and physical characteristics, which define the quality and properties of the polymer. The chemical properties that can be determined are stereo specificity, any irregularities in the addition of monomer (such as 1,2- versus 1,4-addition and head-to-head versus head-to-tail addition), chain branching, any residual unsaturation, and the relative eoncentration of monomers in the case of copolymers. Other important characteristics include specific additives in a formulated product, and the physical properties, which include molecular weight, molecular-weight dispersion, crystallinity, and chain orientation. Some properties such as molecular weight and molecular-weight dispersion are not determined directly by infrared and Raman spectroscopy, except in some special cases. 

Fignre 538. Tensile storage modulus of Vectran liquid crystalline film samples as a function of temperature the number at each curve indicates the angle between the measurement strain direction and the direction of polymer chain orientation (from Menczel et al., 1997a, with permission of Akad miai Kiadb, Hungary). 

Fibrous blends can serve as internal reinforcements or perhaps electrical conductors, depending on the polymer characteristics comprising them. Where very thin fibers can be formed (< 1 [tm), for example, opportunities exist for enhancing crystallinity and tensile properties of the resulting blends especially when extrusions containing them, which themselves can be fibers, are drawn to promote polymer chain orientation [33]. 

The elongations to break in uniaxial stretching among polymers vary considerably. They not only vary among different polymers, but change considerably with factors such as temperature, crystallinity, and polymer chain orientation. 

Stretching a polymer sample tends to orient chain segments and thereby facilitate crystallization. The incorporation of different polymer chains into small patches of crystallinity is equivalent to additional crosslinking and changes the modulus accordingly. Likewise, the presence of finely subdivided solid particles, such as carbon black in rubber, reinforces the polymer in a way that imitates the effect of crystallites. Spontaneous crystal formation and reinforcement... 

The ease of sample handling makes Raman spectroscopy increasingly preferred. Like infrared spectroscopy, Raman scattering can be used to identify functional groups commonly found in polymers, including aromaticity, double bonds, and C bond H stretches. More commonly, the Raman spectmm is used to characterize the degree of crystallinity or the orientation of the polymer chains in such stmctures as tubes, fibers (qv), sheets, powders, and films... 

Structurally the difference between PEN and PET is in the double (naphthenic) ring of the former compared to the single (benzene) ring of the latter. This leads to a stiffer chain so that both and are higher for PEN than for PET (Tg is 124°C for PEN, 75°C for PET is 270-273°C for PEN and 256-265°C for PET). Although PEN crystallises at a slower rate than PET, crystallization is (as with PET) enhanced by biaxial orientation and the barrier properties are much superior to PET with up to a fivefold enhancement in some cases. (As with many crystalline polymers the maximum rate of crystallisation occurs at temperatures about midway between Tg and in the case of both PEN and PET). At the present time PEN is significantly more expensive than PET partly due to the economies of scale and partly due to the fact that the transesterification route used with PEN is inherently more expensive than the direct acid routes now used with PET. This has led to the availability of copolymers and of blends which have intermediate properties. 

MW and MWD are very significant parameters in determining the end use performance of polymers. However, difficulty arises in ascertaining the structural properties relationship, especially for the crystalline polymers, due to the interdependent variables, i.e., crystallinity, orientation, crystal structure, processing conditions, etc., which are influenced by MW and MWD of the material. The presence of chain branches and their distribution in PE cause further complications in establishing this correlation. 

Miscibility or compatibility provided by the compatibilizer or TLCP itself can affect the dimensional stability of in situ composites. The feature of ultra-high modulus and low viscosity melt of a nematic liquid crystalline polymer is suitable to induce greater dimensional stability in the composites. For drawn amorphous polymers, if the formed articles are exposed to sufficiently high temperatures, the extended chains are retracted by the entropic driving force of the stretched backbone, similar to the contraction of the stretched rubber network [61,62]. The presence of filler in the extruded articles significantly reduces the total extent of recoil. This can be attributed to the orientation of the fibers in the direction of drawing, which may act as a constraint for a certain amount of polymeric material surrounding them. 

Two approaches to the attainment of the oriented states of polymer solutions and melts can be distinguished. The first one consists in the orientational crystallization of flexible-chain polymers based on the fixation by subsequent crystallization of the chains obtained as a result of melt extension. This procedure ensures the formation of a highly oriented supramolecular structure in the crystallized material. The second approach is based on the use of solutions of rigid-chain polymers in which the transition to the liquid crystalline state occurs, due to a high anisometry of the macromolecules. This state is characterized by high one-dimensional chain orientation and, as a result, by the anisotropy of the main physical properties of the material. Only slight extensions are required to obtain highly oriented films and fibers from such solutions. 

Hence, the extension of an isotropic unoriented partially crystalline polymer leads to the formation of a highly organized material with a characteristic fibrillar structure. The anisotropy of the sample as a whole is expressed by a higher modulus, tenacity and optical anisotropy. It would seem that the increase in strength in the drawing direction suggests that the oriented samples consist of completely extended chains. However, while the strength of such perfect structure for polyethylene has been evaluated as 13000 MPas), the observed values for an oriented sample are 50 to 30 MPa. 

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