Chain conformation in linear polyethylene and paraffins defined by four NMR‐components

A systematic investigation of a series of nuclear magnetic resonance spectra (NMR) of differently treated polyethylene (PE) and paraffin samples, indicates that all observed spectra can be analysed unequivocally as the sum of four standard functions which are attributed to four components, α, β 1 , β 2 , and γ, of increasing proton mobility. In some spectra most of the components are clearly separated from each other. The shape of their derivatives is therefore directly measurable and symmetric for the α‐component. For β 1 the derivative is that of a Gaussian and for γ that of a Lorentzian function. β 2 alone is not directly observable but appears as a remainder in the spectra of melt‐crystallized PE. It exhibits the shape of the derivative of a Lorentzian function, too. The basic hypothesis is that each of these four components maintains his characteristic shape even if they overlap each other. The line widths Δ H are: 13,2 to 17,9 Gauß (1 Gauß = 10 −4 Vs m −2 ), 3,0 to 6,0 Gauß, 0,85 to 1,2 Gauß, and 0,04 to 0,18 Gauß, depending on the history of the sample. The α‐component is assigned to protons in the microparacrystalline phase (regular backfolding included), fixed chain ends and tight tie‐molecules. In hotstretched PE the latter enlarge the α‐component by 4% without changing the “crystallinity” measured by the density. γ is assigned to longer chain ends undergoing micro‐Brownian motion. The more regular the lamella surface, the smaller is the contribution to the β 1 and β 2 component. So‐called single crystals of PE e.g. have only 0,02% γ, which increases during melt‐crystallization to ca. 0,1% (sample MPE 5). Its extremely high value of 0,4% in all drawn samples (DPE) is attributed to an enlarged number of longer mobile endsegments generated by chain scission. β 1 is attributed to less mobile chain ends, chains in lateral grain boundaries, defects within the paracrystals, chainfolds and tie‐molecules in the “amorphous” parts which are of great importance for the mechanical properties. If the component β 1 e.g. is smaller than 15%, melt‐crystallized PE (MPE) is always brittle. When the melt is stirred, β 1 in the MPE reaches values up to 19%. The tensile strength then is larger. In extended chain crystals (ECC) β 1 is reduced to about 10% of the MPE‐value. This may indicate that about 10% of the ECC still have the folded structure. This is underlined by the fact that MPE alone has a remarkable β 2 ‐component from segments more mobile than β 1 which is reduced also to about 10% in ECC. The β 2 ‐component is attributed to loose loops and loose connecting molecules, which connect neighbouring paracrystals.