Polymers, chain type orientation

Pulsed deuteron NMR is described, which has recently been developed to become a powerftd tool for studying molectdar order and dynamics in solid polymers. In drawn fibres the complete orientational distribution function for the polymer chains can be determined from the analysis of deuteron NMR line shapes. By analyzing the line shapes of 2H absorption spectra and spectra obtained via solid echo and spin alignment, respectively, both type and timescale of rotational motions can be determined over an extraordinary wide range of characteristic frequencies, approximately 10 MHz to 1 Hz. In addition, motional heterogeneities can be detected and the resulting distribution of correlation times can directly be determined. 

The parameters K1/ K2/ and K3 are defined by the refractive indices of the crystal and sample and by the incidence angle [32]. If the sample has uniaxial symmetry, only two polarized spectra are necessary to characterize the orientation. If the optical axis is along the plane of the sample, such as for stretched polymer films, only the two s-polarized spectra are needed to determine kz and kx. These are then used to calculate a dichroic ratio or a P2) value with Equation (25) (replacing absorbance with absorption index). In contrast, a uniaxial sample with its optical axis perpendicular to the crystal surface requires the acquisition of spectra with both p- and s-polarizations, but the Z- and X-axes are now equivalent. This approach was used, through dichroic ratio measurements, to monitor the orientation of polymer chains at various depths during the drying of latex [33]. This type of symmetry is often encountered in non-polymeric samples, for instance, in ultrathin films of lipids or self-assembled monolayers. 

A few examples of the moduli of systems with simple symmetry will be discussed. Figure 1A illustrates one type of anisotropic system, known as uniaxial orthotropic. The lines in the figure could represent oriented segments of polymer chains, or they could be fibers in a composite material. This uniaxially oriented system has five independent elastic moduli if the lines (or fibers) ara randomly spaced when viewed from the end. Uniaxial systems have six moduli if the ends of the fibers arc packed in a pattern such as cubic or hexagonal packing. The five engineering moduli are il-... 

Material response is typically studied using either direct (constant) applied voltage (DC) or alternating applied voltage (AC). The AC response as a function of frequency is characteristic of a material. In the future, such electric spectra may be used as a product identification tool, much like IR spectroscopy. Factors such as current strength, duration of measurement, specimen shape, temperature, and applied pressure affect the electric responses of materials. The response may be delayed because of a number of factors including the interaction between polymer chains, the presence within the chain of specific molecular groupings, and effects related to interactions in the specific atoms themselves. A number of properties, such as relaxation time, power loss, dissipation factor, and power factor are measures of this lag. The movement of dipoles (related to the dipole polarization (P) within a polymer can be divided into two types an orientation polarization (P ) and a dislocation or induced polarization. 

By building - in combinations of aromatic rings into the polymer chains, chemists are able to produce polymer chains with very low chain flexibility. In the limit they reach rigid-rod-type op polymers. Such polymers show substantial temperature - pressure -concentration regions in which the stiff polymer chains arrange in some form of orientation. This phase behaviour gave them the name Liquid Crystalline Polymers (LCP) and LCP have unique properties. 

The 1.4-cis polymerization of 1.3-pentadiene offers a second type of steric control. The methyl group of the new monomer can be sterically oriented by the methyl groups at the end of the growing polymer chain. Isotactic cis polymers can be obtained by a planar six membered ring... 

In coordinated polymerizations with alkyl metal and Ziegler-type catalysts, vacant p- or d-orbitals of a metal component coordinate with the jr-electrons of olefins, diolefins and non-polar monomers. When the polymer chain end is fixed in position and partially stabilized by its metal-containing gegen-ion, repetitive insertion of the polarized and oriented monomer between the chain end and gegen-ion yields stereoregular polymers. Of the various factors which affect polymer stereoregularity, the most important appears to be the gegen-ion structure and its ability to coordinate and orient the monomer. 

Figure 6.8 Stereospecific propylene insertion in a metallocene catalyst of the type 6.22. For clarity the coordination of Zr to the second indene ring (broken line) is not shown, (a) Preferred orientation of the growing polymer chain. Note the trans orientation of the methyl group (above the plane of paper) and (below the plane of paper), (b) Rotation along the Zr—C bond may make and CH3 cis to each other, (c) Agostic interaction that prevents rotation around the Zr—C bond and keeps and CH3 away from each other.

Figure 12 (a) Optical absorption of trans-PA and a related oligomer. Solid lines polarized absorptions of an oriented PA film (precursor route), calculated from the reflection spectrum (from Ref. 113). The polymer chains are parallel to c. Dashed line absorption of the hexamer all-trans dodecahexaene, in solution in hexane (from Ref. 114). Dotted line hexamer emission in hexane, (b) Optical absorption and reflectivity of unoriented cis- and trans- A films (Shirakawa type) (from Ref. 116). 

The type of carbosilane core and the length of siloxane arms play an important role in this process. For high siloxane arms the more flexible core (B) causes large (AH = 14 J/g for B-R-4 versus AH =34 J/g for R-4), but slightly less pronounced decrease of transition enthalpy than compact ones [A, C (AH =4J/g for C-R-4)] (Fig. 5). It implies easier orientation of polymer chains on heating. The opposite was found for lower polymers (AH =3 J/g for B-R-2 and AH =8 J/g for C-R-2). 

The vacant sixth coordination site of these Ti centres can take up an olefin molecule to form the reaction complex required for the initiation and subsequent growth of polyolefin chains. Due to their octahedral dichelate-type structure, these Ti(III) centres are chiral and thus able to steer each incoming molecule into a preferred enantiofacial orientation. The stereospecificity with which subsequent propylene units insert into the growing polymer chain is most likely based on a mechanism analogous to that determined for soluble polymerization catalysts (Section 7.4.3). 

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