Low-temperature molecular motion

If full 360° internal rotation is not possible, what then are the motions that give rise to these subsidiary transitions There are two of importance. 

A second molecular motion that might be able to take place at low temperatures is rotation of, or inside, the substituent side group on the chain. Very often the libration and side group rotation are coupled, so that both start up together 

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A DIELECTRIC STUDY OF THE LOW TEMPERATURE MOLECULAR MOTIONS IN POLYSTYRENE AND THE POLY /0-. M-. OR P-CHLOROSTYRENES/. PH. D. THESIS. 

As the alkyl chains assume an ordered arrangement with weak intermolecular forces, the thermal liberation of rotational freedom around the chains takes place at a relatively low temperature. Molecular motion within the chain increases gradually as the temperature increases until, at characteristic temperatures, there is a considerable increase in the molecular motion, causing the formation of various polymorphs. Polymorphic crystals may be defined as crystals that are formed from the same molecule and have the same composition but are different in crystal structure. During a phase transition the crystal that exists at low temperatures may be tfansformed on heating into a different structure. Two different kinds of polymorphs exist equilibrium and metastable [14,15]. A form that has a range of temperature over which it is stable with respect to other polymorphs is said to be an equilibrium polymorph. An equilibrium polymorph exhibits thermodynamically reversible isothermal phase fiansitions. Metastable polymorphs are kinetically stable states whose existence depends on the presence of a kinetic barrier to the attainment of equilibrium polymorphs. The fiansformation of a metastable polymorph to the corresponding equilibrium polymorph is an irreversible process. 

Many reactions create free radicals, which can recombine into less favorable compositions and also cause depolymerization. At very low temperatures, molecular motions are frozen. Radiation damage is therefore much more severe at cryogenic temperatures than at room temperature. 

In order to obtain stable amorphous states of organic materials, the glass transition temperature (Tg) of the materials should be higher than room temperature. Molecular motion is frozen at temperatures below Tg, and crystal growth is suppressed. The Tg of la was found to be lower than room temperature. Although several photochromic amorphous materials containing azobenzene unit have been synthesized, the practical use of amorphous azobenzenes is limited because of thermal instability of the cis-isomer and low reactivity in the solid state. Figure 17.3 shows some molecular structures... 

From the NMR data of the polymers and low-molecular models, it was inferred that the central C—H carbons in the aliphatic chain in polymer A undergo motions which do not involve the OCH2 carbons to a great extent. At ambiet temperatures, the chemical shift anisotropy of the 0(CH2)4 carbons of polymer A are partially averaged by molecular motion and move between lattice positions at a rate which is fast compared to the methylene chemical shift interaction. 

Cyanobacteria, prokaryotic algae that perform oxygenic photosynthesis, respond to a decrease in ambient growth temperature by desaturating the fatty acids of membrane lipids to compensate for the decrease in the molecular motion of the membrane lipids at low temperatures. During low-temperature acclimation of cyanobacterial cells, the desaturation of fatty acids occurs without de novo synthesis of fatty acids [110, 111]. All known cyanobacterial desaturases are intrinsic membrane proteins that act on acyl-Hpid substrates. 

In this section we continue to explore the consequences of the existence of the low temperature excitations in amorphous substances, which, as argued in Section III, are really resonances that arise from residual molecular motions otherwise representative of the molecular rearrangements in the material at the temperature of vitrification. We were able to see why these degrees of freedom should exist in glasses and explain their number density and the nearly flat energy spectrum, as well as the universal nature of phonon scattering off these excitations at low T < 1 K). 

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