Crankshaft motions

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. 

Dyn ic mechanical analysis was discussed in terms of the nodular morphology concept in crossllnked structures. Beta relaxations in all the cured resins were bimodal in appearance. But, vAiile MPD-cured resins shewed a maximum at 25 C with a smaller shoulder at -40 C, TDA and DAEB-cured resins had maxima at -40 C with a less significant peak at 25 C. For DAIPB and DATBB-cured resins the two peaks were approximately equal in magnitude. The two overlapping peaks at -40 and 25 C were attributed to crankshaft motions in the matrix and nodules. 

The cooperative segmental motion in polymer molecules can be considered as a crankshaft motion of six atoms in the polymer chain. According to H. Eyring, the viscosity of a polymer melt decreases exponentially in accordance with the enthalpy of activation AHa instead of the energy of activation Ea as stated in the Arrhenius equation. 

Fig. 3. Illustration of various types of crankshaft motions (cf. text)...

The temperature dependence of AH2 for chloral-PC (Fig. 37) shows that the motions of phenyl rings occur above -80°C. Furthermore, the doublet shape observed for the lH spectrum above room temperature presents a splitting constant of 25 d= 0.2 G, which corresponds to the static interaction between the 2,3 phenyl protons, indicating that the phenyl motions do not affect the dipole-dipole interaction parallel to the 1,4 phenyl axis. A quantitative analysis of the intra- and inter molecular contributions to AH2 leads to the conclusion that the phenyl motions correspond to either isolated or concerted rotations around the 1,4 axis, with little (if any) reorientation of this axis. In addition, it excludes other motions as crankshaft motions, or motion of the phenyl-ethylenic unit as a group. The decrease of AH2 above - 40 °C could be intermolecular in nature. 

FIG. 5.—(a) Three-bond motion (top) and four-bond motion (bottom) of a hydrocarbon chain distributed on a tetrahedral (or diamond) lattice, (b) Crankshaft motion of five bonds around two collinear bonds. P and Q represent the tails of the polymer chain t and g denote the trans and gauche conformations, respectively. [Reproduced with permission from Figs. 2 and 3 of F. Heatley, Progr. 

Early theory propounded the existence of holes in a liquid that accommodated flow, as molecules jumped from hole to hole (Eyring, 1936). Modern theory perceives spaces in a polymer melt originating from randomly distributed segments of the primary structure, whose cooperative bond rotation (crankshaft motion) creates free volume (vp), thus enabling the polymer chain eventually to achieve new positions. For a gram of dispersed solute, ty is the difference between the specific volume of solute (vsp) and vex ... 

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. 17. Crankshaft motion as proposed in the literature for the junction point of crosslinked TGDDM-DDS epoxy...

In crystalline polymers, the principal relaxation process is associated with melting. In polyethylene, a (1-. and 7-transit ions have been identified and, particularly in higb-tlensity polyethylene, the a-transition has been sub-divided into a and a. In ethylene-based polymers, the y-transitions at —120°C is generally associated with the amorphous phase, in particular, with crankshaft motion of methylene sequences. " However, based upon studies of solution grown lamellae, it has also been suggested that this may llien be associated with... 

Motion observable below Tg in solid polymers will vary with the chemical nature of the polymer. Specific motions which have been identified in polymer systems, including rotations of phenyl, methyl, carboxyl, carboxylic ester, nitrile, and keto groups, as well as conformational transformations. At temperatures above Tg the motion of flexible chains becomes very rapid and the conformational mobility appears to be nearly as great as if they were dissolved at high dilution in good solvent (2). However, not all transitions in solid polymers can be identified with specific groups in the chain, for example, those relating to coordinated motion of the backbone atoms such as the so-called crankshaft motion in polyethylene. 

In early work. Hartley and Guillet (25) associated the reduction in < >n in PE-CO at about —40 C with restrictions in conformational mobility associated with the glass transition, Tg. However, measurable values of n were observed down to about — 100°C due to the occurrence of a crankshaft motion of the polyethylene backbone chains which permitted the formation of the cyclic intermediate (Eq. 25) required for reaction within the lifetime of the n-ir excited state of the carbonyl (= 20 ns). The activation energy for )n below —40 C was = 2 kcal moF, which is similar to that of the crankshaft motion. Below — 100°C this motion is frozen out and no further photochemistry is observed. On the other hand, in the absence of quencher the photophysical processes of fluorescence and phosphorescence may be quite efficient. 

Figure 8. Proposed "crankshaft motion" at the junction point of a cross-linked TGDDM-DDS epoxy.

---