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Spontaneous elongation

Rusakov 107 108) recently proposed a simple model of a nematic network in which the chains between crosslinks are approximated by persistent threads. Orientional intermolecular interactions are taken into account using the mean field approximation and the deformation behaviour of the network is described in terms of the Gaussian statistical theory of rubber elasticity. Making use of the methods of statistical physics, the stress-strain equations of the network with its macroscopic orientation are obtained. The theory predicts a number of effects which should accompany deformation of nematic networks such as the temperature-induced orientational phase transitions. The transition is affected by the intermolecular interaction, the rigidity of macromolecules and the degree of crosslinking of the network. The transition into the liquid crystalline state is accompanied by appearence of internal stresses at constant strain or spontaneous elongation at constant force. [Pg.68]

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. [Pg.98]

Thioglutamic acid was entrapped within DDAB vesicles, together with Glu,o, acting as a primer. Spontaneous elongation occurs. [Pg.476]

As an example, we consider oriented samples of side-chain polysUoxane nematic elastomers [66] that show spontaneous elongations up to X = 1.6 upon cooling through the clearing point (Fig. 6). This corresponds to a rather large step... [Pg.199]

Rg///Rg = (///// l) = Am of about 8. This number is consistent with the characteristic values quoted above for main-chain polymers (Sect. 2.3). Such materials are a prime candidate for use as artificial muscles or mechanical actuators. These examples correspond to a prolate backbone anisotropy, which translates into a spontaneous elongation along n. The case of an oblate structure is much less common but has been observed in some side-chain nematic elastomers [53, 54, 69, 70]. [Pg.200]

Thermoelastic measurements on such samples reveal a spontaneous elongation along n at the transition to the smectic phase, indicating a prolate polymer backbone conformation in the smectic elastomer [137]. On another hand, SANS results for end-on side-chain polymers in the smectic phase indicate an oblate chain conformation, with the backbone preferentially confined in the plane of the layers (Sect. 2.2). Thus, the chain distribution and macroscopic shape of the smectic elastomer change their sign if crosslinking is made under uniaxial mechanical stress in the isotropic and/or nematic phase. This result is remarkable and indicates that the oblate chain conformation of a smectic end-on polymer can be easily turned into prolate by a low uniaxial extension during solvent evaporation. [Pg.214]

In an analogous strip of rubber, the corresponding situation would be the spontaneous elongation of the strip (a rubber band stretching itself). On a molecular level, such motions are spontaneous, but because of the low probabilities involved, are but momentary. Again, the phenomenon is possible but unlikely. Instead of a pressure P to hold the gas in the volume V, a stress <7 (force per unit area) will be required to keep the elastomer stretched from Lq to L (a=LILo). [Pg.441]

Asymmetric fission is observed in the spontaneous decomposition of sCf1Ji(15M and other very heavy nuclei. We may ask when the transition to symmetric fission would begin. The next elongated core, in the series represented in Figs. 11 and 12, would contain 31 spherons, and the transition to it should occur for 28 spherons in the core of the undistorted nucleus, that is, at N = 163 (calculated with use of Eq. 1). We conclude that lftf,Lw,(i,20 and adjacent nuclei should show both asymmetric and symmetric fission. [Pg.824]

In a 300-ml. Claisen flask, whose side neck is elongated by a 10-cm. indented section, 102 g. (1 mole) of acetic anhydride and 0.1 ml. (Note 1) of concentrated sulfuric acid are mixed by hand swirling. The mixture is cooled to 10° by swirling in an ice bath, then there is added, during about 10 minutes, 96 g. (1 mole) of recently distilled furfural (Note 2). The temperature is maintained at 10-20°. After addition is complete and the contents of the flask have been well mixed by swirling, the cooling bath is removed and the reaction allowed to warm up spontaneously. A maximum temperature of about 35° is usually reached in about 5 minutes. After the temperature has dropped to that of the room (20-30 minutes), 0.4 g. (Note 1) of anhydrous sodium... [Pg.80]

After elongation has proceeded for a sufficient period of time, the protomer and polymer will reach concentrations corresponding to the equilibrium constant Ko, which from our earlier discussion may be represented as k-/k+. This constant represents the critical concentration for polymer assembly, and protomer concentrations which fall below the magnitude of this constant will not spontaneously assemble. In this sec-... [Pg.182]

Consider the case where the protein consists of o /3 dimers exclusively at the very beginning of an assembly experiment. Suppose further that spontaneous nucleation is sufficiently infrequent as the polymerization reaction reaches 5-10% of its maximal amplitude achieved over the remaining course of elongation. In this case, a reduction of the protomer concentration from about 20 to 18 JU.M would reduce the apparent extent of nucleation by a factor of about 10-20, such that the polymer number concentration remains fixed throughout the ensuing elongation phase. If nucleation were viewed as a one-step cooperative event, then the rate of nucleation would be proportional to the ith power of the protomer concentration if /protomers cooperatively form the polymerization nucleus ... [Pg.470]

Thus, the (R)-glycidol (R)-897 was transformed to ethyl (S)-6-benzyloxy-3-methyl-4(E)-hexenoate (S)-899 via addition of acetylide followed by spontaneous isomerization, stereoselective reduction, and Claisen-Johnson rearrangement. The chiral ester (S)-899 was converted to (R)-4-methyl-6-phenylthiohexanol (R)-902. The primary alcohol (R)-902 was then transformed to the terminal acetylene (R)-904, a common intermediate for the synthesis of carbazoquinocins A (272) and D (275). Chain elongation of (R)-904 by two carbon atoms led to (R)-905, the chiral precursor for carbazoquinocin D (275) (639) (Scheme 5.116). [Pg.267]

The spontaneous formation of piperazine-2,5-diones occurs mainly during N-deprotection or the acylation step to dipeptide esters (usually unhindered esters such as Me, Et, Bzl, and Pac esters) that contain an TV-alkyl amino acid especially at the C-terminusJ152 In some cases the formation of piperazine-2,5-diones becomes the major reaction product and thus prevents peptide elongation by the [1+2] or [1+3] segment condensation strategy in solution synthesis or elongation of the peptide from the C-terminus in SPPS. Piperazine-2,5-dione formation... [Pg.247]

Figure 8.8 Nucleation and growth in a polymer chain (numerical simulation with Brownian dynamics Sakaue et al., unpublished). After staying for a long time in an elongated state, a nucleation centre appears spontaneously on a chain. Then, the densely packed region grows quickly to form a toroid. Essentially the same process has been observed in an experiment with single DNA observations (Yoshikawa and Matsuzawa, 1995,1996). Figure 8.8 Nucleation and growth in a polymer chain (numerical simulation with Brownian dynamics Sakaue et al., unpublished). After staying for a long time in an elongated state, a nucleation centre appears spontaneously on a chain. Then, the densely packed region grows quickly to form a toroid. Essentially the same process has been observed in an experiment with single DNA observations (Yoshikawa and Matsuzawa, 1995,1996).

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