Side group relaxation

Molecular Motion in amorphous atactic polystyrene (PS) is more complicated and a number of relaxation processes, a through 5 have been detected by various techniques as reviewed recently by Sillescu74). Of course, motions above and below the glass transition temperature Tg have to be treated separately, as well as chain and side group mobility, respectively. Motion well above Tg as well as phenyl motion in the glassy state, involving rapid 180° jumps around their axes to the backbone has been discussed in detail in Ref.17). Here we will concentrate on chain mobility in the vicinity of the glass transition. 

Figure 3. Energy relaxation pathways in a polymer containing aromatic side groups following absorption of light. (Reproduced with permission from Ref. 21. Copyright 1987 Chemistry in Australia.)...

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. 

To the best of our knowledge, the supercoil conformation of the monoden-dron jacketed polystyrene is one of the first observations of a defined tertiary structure in synthetic polymers. The plectoneme conformation could be caused by underwinding or overwinding of a backbone from its equilibrium state [168]. Quick evaporation of the solvent certainly can cause a residual torsion in the molecule as it contracted in itself. Unlike macroconformations of biomolecules, where the tertiary structures are often stabilized by specific interactions between side groups, the supercoil of the monodendron jacketed polymers is metastable. Eventually, annealing offered a path for the stress relaxation and allowed the structural defects to heal [86]. 

Free Volume Versus Configurational Entropy Descriptions of Glass Formation Isothermal Compressibility, Specific Volume, Shear Modulus, and Jamming Influence of Side Group Size on Glass Formation Temperature Dependence of Structural Relaxation Times Influence of Pressure on Glass Formation... 

Figure 7 presents z as a function of the reduced temperature variable 87a T — 7a /7a- The different curves of Fig. 7 refer to the F-F and F-S polymer classes and the same M as in Fig. 6. Evidently, the calculated z grows much faster with 87a for the F-S polymer class and increases somewhat with M within each polymer class. These trends, taken in conjunction with the AG model, again translate into the prediction that polymer chains with bulky stiff side groups (F-S class) have a stronger dependence of the relaxation time on temperature (i.e., they are more fragile) than flexible chains with...

The sensitivity of the three relaxation times to the molecular dynamics and structure will be discussed in a subsequent section. The general temperature dependence of Tj, T1 and T2 for a typical linear amorphous polymer with one side group attached to a backbone is shown in Fig. 4. 

Fig. 4. Pulsed NMR data for poly(vinyl acetate), as a typical example for polymer relaxation. The glass transition corresponds to the abrupt change in T2 and the high temperature minima in T, and T( curves. Lower temperature minima in T, and Tj curves are attributed to a side group relaxation (reprinted from Ref.11 with permission)...

The relaxation process due to the motion of the side groups is transformed15,146 I48) by diluents into the /9d process in a way formally resembling that operative in the preceding case. The corresponding motional unit probably consists148 of die group R and diluent molecule(s). Data obtained for polymethacrylates, which so far appear to be the most complete, are discussed in Sect. 5.3. 

In order to assess the orientational stability of the poled state, the temperature dependence of the dipole mobility of the side groups was examined through dielectric relaxation measurements. (13) No low temperature relaxation below Tg was observed in the frequency range studied (100 Hz-100 kHz). In addition, the dielectric constant was approximately equal to the square of the refractive index, indicating that below T only electronic and no significant orientational contributions to the dielectric displacement are present. Thus, it was expected that a given orientational state of the ensemble would be stable at temperatures significantly below Tg. 

---