Branching effects on the segmental dynamics of polyethylene melts

Abstract

13C NMR spin–lattice relaxation time and nuclear Overhauser effect measurements are reported for linear polyethylene, polyethylene with long branches, and linear polyethylene with infrequent short branches at 75 MHz over the temperature range of 400–535 K. A quantitative description of the segmental dynamics of the main‐chain methylene units and the branch points is obtained for linear polyethylene with ethyl branches. At 400 K, the correlation time is 29 ps for the segmental dynamics of the branch point but 5 ps for the segmental dynamics of the main‐chain methylene units. The activation energy for the segmental dynamics of the main‐chain methylene units (4 kcal/mol) is significantly less than the flow activation energy (Eη = 7.2 kcal/mol). In contrast, the activation energy for the segmental dynamics at the branch point (6.4 kcal /mol) is much closer to Eη. This result supports our earlier hypothesis that the lack of side groups in polyethylene allows conformational transitions to happen without substantial intermolecular cooperation. Thus, in contrast to many other polymers, conformational transitions are not the fundamental motion for flow in polyethylene. Branches as small as a methyl group introduce intermolecular cooperation into conformational transitions at the branch point, so conformational transitions at the branch points have a stronger temperature dependence. No difference in local dynamics was found for methylene units in linear polyethylene and polyethylene with long branches. Therefore, local dynamics are not relevant for understanding the factor‐of‐two difference in the flow activation energies of these two polymers. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2634–2643, 2000

Molar Mass Dependence of Polyethylene Chain Dynamics. A Quasi-Elastic Neutron Scattering Investigation

“…Its rheological properties are of considerable practical importance to optimize processing conditions, and numerous dynamic studies of polyethylene melts have been reported. Experimental rheological 1−5 and NMR 6,8,7 measurements have been particularly informative and have been supplemented by computer simulations. 9−21 Systematic investigations have been carried out to understand the rheological behavior as a function of temperature and molar mass, M, and to test molecular theories of polymer melt dynamics, specifically the Rouse and reptation models.…”

“…Its rheological properties are of considerable practical importance to optimize processing conditions, and numerous dynamic studies of polyethylene melts have been reported. Experimental rheological − and NMR ,, measurements have been particularly informative and have been supplemented by computer simulations. − …”

Branch-Induced Heterogeneous Chain Motion in Precision Polyolefins

“…Molecular dynamics and molecular mechanics calculations show that inclusion of short branches such as −CH 3 and −CH 2 CH 3 dramatically increases the intermolecular interaction between adjacent chains. 27,28 This allows the pressure created by conformational changes during creation, propagation, and annihilation of dynamic defects to be efficiently distributed to adjacent chains, allowing the entire crystal lattice respond homogeneously to local conformational stress. Therefore, given sufficient time, and assuming a high concentration of branches, the average conformational space spanned by the C−D vectors at D 2 and D 4 positions will be equivalent, leading to a homogeneous distribution of oscillation amplitudes observed in PE15 and PE21.…”

Effect of Branch Length on 13C NMR Relaxation Properties in Molten Poly[ethylene‐co‐(α‐olefin)] Model Systems

“…[10] Studies of polyethylene-copolymers containing comonomers of different lengths have previously shown that the various branch lengths impart different local segmental mobility. [11] With nuclear relaxation phenomena sensitive to local segmental mobility, resulting from the dominant heteronculear dipolar coupling relaxation pathway, [12] they may be used to probe changes in branch length. The effect of comonomer length on 13 C spin-lattice relaxation time (T C 1 ) has been investigated in both the solution-state [13] and the melt-state.…”