Study of the molecular motions of polyethylene by line‐shape analysis of broad‐line proton NMR spectra

Abstract

SynopsisThe broad-line proton NMR spectra of polyethylene are separated into three components (broad, medium, and narrow) corresponding to the crystalline and two kinds of amorphous protons, respectively. All amorphous protons are found to be mobile above 210°K. The unusually low molecular mobility in the amorphous regions of polyethylene compared to purely amorphous and other partially crystalline polymers is considered to be adequately described by the network model of Edwards and De Gennes. In this model the chain motions are anisotropic and restricted to small tubular volumes. The medium and the narrow component are believed to result from two different modes of chain motion within these tubes, depending on the free volume available. Two motions in the crystalline regions are observed. One a t temperatures below 110°K involves 2%5% of the protons, depending on the crystallinity of the material, and the other beginning a t 290°K involves all crystalline protons (a-process). Coupling of the amorphous and crystalline motions is found above 290°K. Several line shapes have been tested as representations of the three components. Of these the low-temperature spectrum, the Gauss-Lorentz product curve, and the Lorentz curve proved to be the most suitable shapes for the broad, medium, and narrow component, respectively. Using these line shapes, the best fit of the experimental spectra and the expected agreement of the broad and the crystalline fraction are obtained over a very broad temperature range. Above 310°K the low-temperature spectrum must be replaced by the convolution of a Gauss curve and a rectangular function to take into account the line-shape transition of the a-process. The modulation broadening of all components is considered, and this allows line-shape analysis close to the melting point.It was soon recogni~ed,~-~ however, that the Wilson-Pake method yields crystallinities which deviate greatly from the values obtained from independent measurements and, in addition, are found to depend on temperature in regions where they are expected to be constant. These discrepancies were shown to originate in the greatly simplified assumptions of the Wilson-Pake method.

Molecular dynamics of solid polymers as revealed by deuteron NMR

“…figure 7 of reference [19], one can infer that the average length of the flexible unit increases from 3-5 bonds at room temperature to about 10-15 bonds at 380 K. Our model thus can quantitatively explain the gradual increase of the number of conformations accessible to the chain motion as reflected in the 2H spectra. The earlier 1H wide line data [52] are also in accord with our findings. Our model postulates the existence of long lived topological constraints, which as we will see shortly reflect the presence of the crystallites in such a semicrystalline material.…”

“…Figure 10, therefore, gives the spectra [17] of the mobile fraction alone, spanning a much larger temperature range, however, all the way from the y-transition to the melting point. Comparison of the spectra presented in figures 9 and 10, respectively, demonstrates the efficiency of this separation, which cannot be achieved in conventional 1H broad line NMR [52]. This offers the possibility to get meaningful values for the crystallinity of the sample and its dependence on temperature from the intensity of the respective spectral components [47].…”

Study of Polyethylene by Solid State NMR Relaxation and Spin Diffusion

“…[14][15][16][17][18][19] Also, there usually appears to be a smaller signal having a width between those two values. 16,18,20,21 Crystallinity of polyethylene can be measured with NMR by two approaches.…”

Structure and Dynamics of Drawn Gel‐Spun Ultrahigh‐Molecular‐Weight Polyethylene Fibers by 1H, 13C and 129Xe NMR

“…[9] Proton wide-line NMR spectroscopy and relaxation experiments have been frequently applied to study the effect of the chemical structure, molar mass and temperature on the phase composition and molecular mobility. [10][11][12][13][14][15][16] At temperatures well above T g of the amorphous phase, the T 2 relaxation decay and wide-line NMR spectra for PE can usually be decomposed into three components, which originate from a crystalline phase, a semi-rigid crystalamorphous interphase and a soft fraction of the amorphous phase. Moreover, a recently developed 1 H NMR spindiffusion experiment was applied for measuring the domain sizes in annealed high-density PE (HDPE).…”