LDPE, relaxation spectrum

Fig. 13 Storage modulus G and loss modulus G" of unfilled LDPE and highly filled LDPE/LDH nanocomposite (20 wt%) in the frequency sweep experiment. Also shown are the fits of G and G" obtained from conversion of the LDPE relaxation spectrum (see Table 2)...

Initial indications are that a multi-mode version of Eqs. (50) and (51) can be useful at a quantitative level in modelling an extensively-investigated LDPE for all geometries of flow [82]. Matching only the linear viscoelastic relaxation spectrum, and adding a segment priority distribution consistent with that derived... 

The (3 relaxation in polyethylene, which is most prominent in the low-crystallinity LDPE, is associated with the amorphous regions and almost certainly corresponds to what would be a glass transition in an amorphous polymer a diiference in its position in mechanical and dielectric spectra is therefore not surprising. The a relaxation, as discussed in section 7.6.3, is associated with helical jumps in the crystalline regions and, provided that the lamellar thickness is reasonably uniform, might be expected to correspond to a fairly well-defined relaxation time and to a narrow peak in the relaxation spectrum. The dielectric peak is indeed quite narrow, because the rotation of the dipoles in the crystalline regions is the major contribu-... 

Fig. 18.18 (a) Correlation between the abscissas of relaxation spectrum maximum and the zero-shear viscosity for PE/PE Series-I (miscible LPX-30/LLDPE-10) and Series-II (immiscible LPX-30/LDPE). (b) Correlation between the ordinate of relaxation spectrum maximum and the polydispersity factor M /M for PE/PE Series-I (miscible LPX-30/LLDPE-10) and Series-II (immiscible LPX-30/LDPE)... 

In the case of PO blends, compatibilization most frequently aims for improved ductility and/or transparency. The Z-N-LLDPE obtained using multi-sited catalyst constitutes a specific case - the homopolymer may have phase-separated morphology that requires compatibilization. It has been known that addition of 5-20 wt% LDPE needs to be used for improved performance. However, explanation for this is rather recent (Robledo et al. 2009). The relaxation spectrum of the blend may be decomposed into three components (1) Z-N-LLDPE matrix, (2) LDPE dispersed drops, and (3) a thick interphase with its own viscoelastic properties, obtained by interaction between the high-MW linear fraction of the LLDPE and the low-MW linear LDPE macromolecules. 

The methods for calculating the relaxation spectrum were discussed. The calculated spectra were used to determine the MWD by the Mead method as illustrated in the examples polypropylene, LDPE and propylene a-olefin and ethylene-a-olefin copolymers. The rheological method is a quick and accurate process to determine the MWD of homopolymers and copolymers. 

---