Orientation of monomer units

A characteristic of interest in connection with PVOH and PVAc is the consistency of orientation of monomer units along the chain. In the formula given above, it is assumed... 

Mattern and co-workers [8] carried out laser mass spectrometry on polytetrafluoro-ethylenes. They found a fragmentation mechanism common to each fluoropolymer yields structurally relevant ions indicative of the orientation of monomer units w ithin the polymer chain. A unique set of structural fragments distinguished the positive ion spectra of each homopolymer, allowing identification. 

Birefringence provides a measure of the local orientation of a material (i.e., the mean orientation of monomer units). The relation between orientation and birefringence was known from early studies of polystyrene filaments, which described both the theory and measurement [23, 25]. They showed that the orientation was greater at the surface than in the core. Mechanical properties, such as tensile strength and elongation at break, have been shown to... 

From this equation, it is clear that concentration of the solvent, S, influences a number of sites on the template which are occupied by the monomer, M. As the result of monomer units association with the template, the orientation of the substrate takes place and some special type of structure can be created. The structures, in which the monomer is aligned in a regular manner on the polymer template, were described by Chapiro in the case of polymerization of acrylic acid and acrylonitrile and details are described below. The ordered structure increases concentration of monomer at the reaction site, affects distances between pre-oriented monomer molecules, and changes a steric hindrance. This change in structure leads to the change in the kinetics of the polymerization reaction and it is responsible for stereo-control of the propagation step. 

However, this mechanism of motion does not provide any great contribution to the Kerr effect since the dispersion curves of EB fall to virtually zero (Fig. 60). This difference may be interpreted by the proportionality of the orientational EB of a ripd-chain polymer to the square of the number of monomer units in segment whereas increment Ae/c related to the orientational mechanism is proportional to S (see Sects. 5.8 and 5.9). Hence, in the case of dielectric polarization the part played by the deformational mechanism as cmnpared to the orientational mechanian can be more important than in the case of EB. 

This can be explained by the fact that in a polymer molecule (Fig. 78b) the longitudinal components of monomer unit dipoles mh are mutually compensated and the main part in the observed EB is played by normal components of monomer unit dipoles, mi, which can be parallel to the main chain of the macromolecule owing to its comb-like structure. In other words, in molecules of comb-like polymers containing mesogenic side chains, the orientations of the Mi components of the side group dipoles are correlated with each other. As a result, the macromolecule as a whole or part of it can exhibit a considerable dipole moment m in the direction of the main chain L (Fig, 78b). The existence of this dipole accounts for the orientation of the main chain in the field direction leading to negative EB. 

In addition to lipid-like molecules, polymers can also form self-assembling particulates [8]. As expected, the amphiphilic polymers do form polymeric micelles with polar part orienting towards the surface of the particle and the hydrophobic part orienting towards the core of the particle. Also, in this case, different complicated self-assembled structures can be tailored by using block copolymers with regular blocks of monomer units but, only the simpler ones have been investigated so far in the context of drug delivery. 

Most HiPIPs have been assumed to function as monomers. However, the HiPIP from Rhodospirillum salinarum has been shown to exist as a tetramer or hexamer 17). Other evidence points to aggregation as a common phenomenon for HiPIPs, with possible functional implications. Dunham et al. interpreted EPR features observed in concentrated solutions of C. vinosum HiPIP as arising from dimerization (27), whereas E. halophila, E. vacuolota, andi . tenuis HiPIPs crystallize as dimers 113). Although the same general face is involved in this dimerization, it was noted that the orientations of protein units were distinct for each. Nevertheless, it is possible that this hydro-phobic face may be involved in physiological interactions with redox partners. 

Table 5 lists the number of monomer units in a segment s =A/x, where X is the length of projection of the monomer unit on the axis of the molecule. For polyacrylic chains, X is 2.5 A. In the series presented in the Table, the value of optical anisotropy increases sharply thirty-fold. As shown previously, this reflects the generation of orientational order in the molecule. In this case, attention is drawn to the fact that the increase in... 

In comparison, four different orientations for the isomer units can be derived by synthetic polymerization of isoprene c/s-1,4-, trans-, A-, trans-3,4-, and trans-1,2- additions. The last two addition orientations produce asymmetric carbon atoms. An identical configuration of asymmetric carbon atoms is referred to as isotactic, regular alternating is syndiotactic, and random is atactic. Another consideration is the successive addition of monomer units in head-to-head, head-to-tail, or tail-to-tail configurations, though head-to-tail is preferred due to steric hindrances. The microstructure of Hevea rubber is predominantly c/s-1,4- units, eliminating the tacticity concern. Because of its biochemical origin, the addition of isoprene units is almost entirely in a head-to-tail orientation. 

I. Valvassori, A. Zambelli, A. Copolymers of ethylene, higher a-olefins, and monocyclomonool-efins or aUcyl derivatives thereof. U.S. Patent 3,505,301 (Montecatini Edison S.p.A.), April 7, 1970. (b) Zambelli, A. Tosi, C. Sacchi, C. Polymerization of propylene to syndiotactic polymer. VI. Monomer insertion. Macromolecules 1972, 5, 649-654. (c) LocateUi, R Immirzi, A. Zambelli, A. Palumbo, R. Maglio, G. Orientation of propylene units in polypropylene and ethylene/propylene copolymers. Makromol. Chem. 1975,176, 1121-1128. (d) Carman, C. J. Harrington, R. A. Wilkes, C. E. Monomer sequence distribution in ethylene-propylene rubber measured by C NMR. 3. Use of reaction probability mode. Macromolecules 1977, 10, 536-544. (e) Zucchini, U. Dall Occo, T. Resconi, L. Ziegler-Natta catalysis for the polyolefin industry Present status and perspectives. Indian... 

In conclusion, it must be emphasized that although it is convenient for the purpose of constructing models to assume a composite that comprises distinct crystalline and non-crystalline components, on the molecular level a gradual transition must occur, extending over a number of monomer units, between the well-orientated and ordered crystallites and the bulk of the remaining material. 

As follows from eqn [4], the characteristic dimensions of the chain scale proportionally to the square root of the number of monomer units N irrespective of the correlations in orientation of consecutive segments. This enables an equivalent freely jointed chain of the same contour length L to be introduced but with renormalized number of the effeaive statistical segments of length I tl. 

Thus, although LC polymers contain rather flexible main chains, they can form all three main types of LC stmctures (Section f.fO.2), namely the nematic stmcture, characterized only by the orientational order (Figure 14(a)) the smectic stmcture accompanied by development of the layered stmcture (Figures 14(a) and 14(b)) and finally, the so-called cholesteric helical stmcture, which appears when chiral moieties in the form of monomer units (red fragments in Figure 14(c)) exist in the macromolecules. 

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