Main chain crankshaft

Figure 5.5. Sources of free volume for plasticization A, chain end motion B, side chain motion, C, main chain Crankshaft , D, external plasticizer motion. [Adapted, by permission, Kern Sears J, Darby J R, The Technology of Plasticizers, John Wiley ...

Below Tg, in the glassy state the main dynamic process is the secondary relaxation or the )0-process, also called Johari-Goldstein relaxation [116]. Again, this process has been well known for many years in polymer physics [111], and its features have been estabhshed from studies using relaxation techniques. This relaxation occurs independently of the existence of side groups in the polymer. It has traditionally been attributed to local relaxation of flexible parts (e.g. side groups) and, in main chain polymers, to twisting or crankshaft motion in the main chain [116]. Two well-estabhshed features characterize the secondary relaxation. 

The mechanical dispersion peaks in low-Tg epoxies such as Bisphenol-A based resin (Epon 828, products from Shell Development Company) have been the subject of numerous studies 143,145148,152 "155, l59>. The alpha-dispersion peak related to the glass transition can undoubtly be attributed to the large-scale cooperative segmental motion of the macromolecules. The eta-relaxation near —55 °C, however, has been the subject of much controversy 146,153). One postulated origin of the dispersion peak is the crankshaft mechanism at the junction point of the network epoxies (Fig. 17). The crankshaft motion for linear macromolecules was first propos-ed 163 166> as the molecular origin for secondary relaxations which involved restricted motion of the main chain requiring at least 5 and as many as 7 bonds 167>. This kind of... 

FIG. 13.31 Crankshaft models of ratability of main chain according to (a) the Schatzki model with rotation around the first and seventh bond (b) the Boyer model with rotation around the first and fifth bond (c) the Wunderlich helix model with rotation around the first and sixth bond. From Haward (1973). Courtesy Chapmann Hall. 

First is the crankshaft mechanism of Schatzki.16 It has been observed for many polymers containing linear (CH2) sequences with n = 4 or greater, that a secondary relaxation occurs at about -120 °C at 1 Hz. This seems to be true regardless of whether the CH2 sequences occur in the main chain or in the side groups. Thus both polyethylene and poly-n-butyl methacrylate exhibit this relaxation. The mechanism proposed by Schatzki is shown in Figure 5-14. 

Introduction of kinks in the main chain, such as by using meta substituted monomers (Figure 5.7c) or a crankshaft monomer (e.g., 6-hydroxy-2-naphthoic acid) (Figure 5.7d). 

The y transition is a result of crankshaft rotation of short methylene main chain segments and can influence the low temperature impact stability of PE. 

Local movements of the main chain, these being weaker cooperative movements than at the glass transition, e.g., the crankshaft movement of the CH2 sequence. 

Many polymers undergo second order transitions at temperatures below T, [224] the transitions are caused by rotations of side groups about an axis which is perpendicular to the chain, by the motion of sequences of 3 — 5 methylenic groups from the backbone or by motion of chain portions in polymers containing heteroatoms in the main chain [162, 226]. The second kind of motion, noted by Schatzki [226] in ethene copolymers and in many homopolymers, is referred to as the crankshaft effect and it occurs immediately below T,. Because it is almost insensitive to the... 

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. 

Figure 17.11. Examples of small-scale molecule motions in polymer fibers (A) crankshaft motion of main chain, and (B) rotation and oscillation of side groups.

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