Polyethylene segmental mobility

Carbon-13 spin-lattice relaxation times TL (Section 3.3) are relatively insensitive to the chain length of polymers [531]. The influence of local segmental motions predominates, as shown for low-density polyethylenes in which Tx values are one to two seconds for the main chain but up to seven seconds for peripheral side-chain carbon nuclei at 120 C [532] due to segmental mobility (Section 3.3.3.4). To conclude, quantitative evaluation of polymer carbon-13 spectra as necessary for side-chain determination requires the knowledge of spin-lattice relaxation times. 

Klepac extensively studied the effect of uniaxial deformation on the molecular chain segmental mobility of linear low density polyethylene (LLDPE) by ESR spin probe method. The simulated ESR spectra of undeformed and deformed (parallel and perpendicular) LLDPE samples at 0 °C are shown as dotted line in Figure 25.31. The results obtained by simulations indicate that the amount of slow components and corresponding Tr of LLDPE films deformed in both parallel and perpendicular directions are significantly larger than those of the undeformed LLDPE films. However, the amount of fast components decreased in the deformed sample. It was concluded from the results that uniaxial deformation reduces the molecular chain segmental mobility in the amorphous region and increased the amount of crystalline phase of the LLDPE films. 

In this section, the structure and thermophysical properties of isotactic pol q)ropylene (PP) and low-density polyethylene (LDPE) blends were studied by differential scanning calorimetry (DSC) and electron paramagnetic resonance (EPR). The experiment shows non-additive changes of the thermal behavior and segmental mobility in the amorphous regions of immiscible PP/LDPE blends in relation to the properties of PP and LDPE initial homopol5nners. Based on the results of the study, the probable process of the structure formation for PP/LDPE blends of various compositions described. 

As indicated, the 195 K peak had not previously been observed for polyethylene. It is only seen in as-quenched samples it is not seen in rerun samples. It was then suggested that the 195 K peak should be related to Boyer s proposed while the 260 K peak would be 7 - The increase in rigidity at the 195 K peak indicates the development of crystallinity, in agreement with the electron diffraction results. Thus, it is not clear whether the peak in the log decrement ould be considered a relaxation process due to 7, or to the phase transition (ciystallizadon), or a combination thereof. Regardless, there is clearly large scale segmental mobility beginning at about this temperature in samples which were previously wholly amorphous, Lc., the of Boyer would be in the vicinity of this peak. 

A third type of ionic conduction occurs in polymer electrolytes, such as polyethylene oxide. The mechanism of ionic conduction in polymer electrolytes is not entirely understood. However, it is believed to involve rapid polymer segmental motion which creates regions of an elastomeric nature. These elastomeric regions have relaxation times similar to liquids, and, thus, allow a higher ionic mobility than would be concluded from the polymer s macroscopic properties [2]. 

---