Crankshaft-like motion

These results seem Incompatible with the assumption of a "crankshaft-like motion". We believe that they can be understood if we asstime that the transition from the cis to the trans form does not take place In a single step but rather by a large number of oscillations around the bond angle by which the transition state Is approached. If we then Impede these oscillations by Incorporating the azobenzene group into a polymer chain, we reduce equally the rate at which the transition state Is approached and the rate at which a strained bond relaxes to its initial shape. 

Figure 1. Schematic of a conformaticmal transition in a polymer chain. Top, rotation arourtd a single bond bottom, correlated rotations around two bonds in a crankshaft-like motion.

Figure 1. Sctiematic represeiitat ion of conformatioiia] transitions in a flexible chain molecule. (a) A single hindered rotation.

The y-transition is a broad relaxation in the temperature- or frequency-domain, interpreted as a localized crankshaft-like motion of the backbone of the chain. This interpretation of the DMA result [51] agrees well with the calorimetry. The calorimetric results were interpreted already in 1962 based on an energy estimation as a local relaxation of gauche conformations in the amorphous phase [53,54]. The broad increases in the heat capacity beyond the vibrational Cp of amorphous PE seen in Fig. 2.46 were interpreted as a gradual, local unfreezing of the gauche-trans equilibrium. 

The Tg of polymers is very much a function of chain structure. Repeat units which introduce stiffness into the backbone chain or bulky side groups pendant from the backbone tend to increase the observed Tg. Strong interchain forces, such as hydrogen bonding, also reduce the crankshaft-like motion of the polymer backbone at any given temperature and tend to increase the Tg. The glass transition temperature is often termed the a-transition. If the temperature is lowered below that of the Tg, temperatures are reached where the rotation of side groups - and eventually, at even lower temperatures, the vibrational behaviour of bonds - are frozen . These lower transition temperatures are known as j - and y-transitions respectively. 

Extensive theoretical and experimental works were carried out on local dynamics of polymers in solution and bulk to elucidate the mechanism of conformational transitions [106]. Formerly, it was believed that the most reasonable mechanism for the conformational transitions was a crankshaft-like motion such as the Schatzki crankshaft [117] or three-bond motions [118,119] in which two bonds in a main chain rotate simultaneously. However, recent computer simulations [ 120-128] have revealed many interesting features of conformational transitions of a polymer chain in solutions and melts. 

The movements which are executed by a polymer chain wiU resemble those depicted as a crankshaft-like motion in Figure 3.11. 

Finally, the y-relaxation at low temperature again occurs in the amorphous phase. It is a broad relaxation in the frequenty or time domain. The molecular interpretation links this relaxation with a localized crankshaft-like motion of the backbone of the chain. Again, it may be possible that the slow increase of heat capacity of amorphous polyethylene above about 100 K, as shown in Fig. 5.17, is an indication of this motion. 

The observation that the Ty relaxation also occurs in ethylene copolymers, containing at least 3-5 consecutive — (CH2)— sequences confirms both the above statement of Wunderlich [39] and the assumption of Reding et al. [60] that this relaxation characterizes crankshaft like motions during the development of the trans gauche equilibrium. 

ABSTRACT. The concept of "crankshaft-like motion" in the backbones of chain molecules implies much slower conformational transitions than in small molecules if two energy barriers have to be surmounted simultaneously. Studies of the rates of hindered rotation around the amide bonds in solutions of piperazine polyamides by NMR and of the photochemical and dark isomerization of polyamides with azobenzene residues in the chain backbone revealed no difference between these rates in the polymers and their analogs. Polyoxyethylene with a dibenzylacetamide residue in the middle of the chain exhibited similar excimer emission as N,N -di(p-methylbenzyl)acetamide. These observations imply that only one energy barrier is surmounted in conformational transitions of polymer backbones. 

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