Characterization of polymer networks using the dipolar correlation effect on the stimulated echo and field-cycling nuclear-magnetic resonance relaxometry

The heterogeneity of segmental dynamics of filled EPDM by (1)H transverse relaxation NMR

Residual second moment of dipolar interactions M(2) and correlation time segmental dynamics distributions were measured by Hahn-echo decays in combination with inverse Laplace transform for a series of unfilled and filled EPDM samples as functions of carbon-black N683 filler content. The fillers-polymer chain interactions which dramatically restrict the mobility of bound rubber modify the dynamics of mobile chains. These changes depend on the filler content and can be evaluated from distributions of M(2). A dipolar filter was applied to eliminate the contribution of bound rubber. In the first approach the Hahn-echo decays were fitted with a theoretical relationship to obtain the average values of the (1)H residual second moment <M(2)> and correlation time <τ(c)>. For the mobile EPDM segments the power-law distribution of correlation function was compared to the exponential correlation function and found inadequate in the long-time regime. In the second approach a log-Gauss distribution for the correlation time was assumed. Furthermore, using an averaged value of the correlation time, the distributions of the residual second moment were determined using an inverse Laplace transform for the entire series of measured samples. The unfilled EPDM sample shows a bimodal distribution of residual second moments, which can be associated to the mobile polymer sub-chains (M(2) ≅ 6.1 rad (2) s(-2)) and the second one associated to the dangling chains M(2) ≅ 5.4 rad(2) s(-2)). By restraining the mobility of bound rubber, the carbon-black fillers induce diversity in the segmental dynamics like the apparition of a distinct mobile component and changes in the distribution of mobile and free-end polymer segments.

Structure of Poly(vinyl alcohol) Cryo-Hydrogels as Studied by Proton Low-Field NMR Spectroscopy

The network structure of poly(vinyl alcohol) (PVA) hydrogels obtained by freezing-thawing cycles was investigated by solid-state (1)H low-field NMR spectroscopy. By the application of multiple-quantum NMR experiments, we obtain information about the segmental order parameter, which is directly related to the restrictions on chain motion (cross-links) formed upon gelation. These measurements indicate that the network mesh size as well as the relative amount of nonelastic defects (i.e., non-cross-linked chains, dangling chains, loops) decrease with the number of freezing-thawing cycles but are independent of the polymer concentration. The formation of the PVA network is accompanied by an increasing fraction of polymer with fast magnetization decay ( approximately 20 mus). The quantitative study of this rigid phase with a specific refocusing pulse sequence shows that it is composed of a primary crystalline polymer phase ( approximately 5%), which constitutes the main support of the network structure and determines the mesh size, and a secondary population of more imperfect crystallites, which increase the number of elastic chain segments in the polymer gel but do not affect the average network mesh size appreciably. Correspondingly, progressive melting of the secondary crystallites with increasing temperature does not affect the network mesh size but only the amount of network defects, and melting of the main PVA crystallites at approximately 80 degrees C leads to the destruction of the network gel and the formation of an isotropic PVA solution.

Segmental order in end-linked polymer networks: a Monte Carlo study

Segmental order in end-linked monomodal and bimodal polymer networks is investigated by means of bond-fluctuation Monte Carlo simulations. The tensor order parameter, which is a central observable in NMR experiments, is not uniquely related to simple vectorial order. The relaxation of NMR-detected tensorial interactions towards their finite long-time limit is best described by a power law and occurs over much longer time scales than the relaxation of vectorial order. The well-known prediction for the segmental order of Gaussian chains as a simple function of the segment number between constraints is not straightforwardly obeyed, neither in dry nor in swollen networks. Excluded-volume interactions tend to significantly reduce the tensorial order, as is clearly observed in single-chain simulations. A distribution extends along the chain, where order is increased in a region of 30-40 bonds around the cross-links in networks. The dominating contribution to the order parameter distribution arises from the frozen-in distribution of end-to-end separations. We find strong deviations from the Gamma distribution, which has so far been implicitly used in most NMR works, as it is a straightforward consequence of a Gaussian distribution of end separations. Specifically, we find narrower distributions, as small values of the tensor order parameter are strongly suppressed, most probably as a result of trapped entanglements. The markedly subaffine behavior of the average order parameter and the changes in its distribution on swelling are assigned to orientation processes of strands which compensate for the non-affine local deformation. Our central observations and interpretations are well supported by our previous experimental and theoretical work.

Constant time steady gradient NMR diffusometry using the secondary stimulated echo

A pulse sequence producing a second stimulated echo is suggested for the compensation of relaxation and residual dipolar interaction effects in steady gradient spin echo diffusometry. Steady field gradients of considerable strength exist in the fringe field of NMR magnets, for instance. While the absolute echo time of the second stimulated echo is kept constant throughout the experiment, the interval between the first two radiofrequency pulses is augmented leading to a modulation of the amplitude of that second stimulated echo by self-diffusion only. The unique feature of this technique is that it is of a single-scan/single-echo-signal nature. That is, no reference signals neither of the same pulse sequence nor of separate experiments are needed. The new method was tested with poly(ethylene oxide) melts and proved to provide reliable data for (time dependent) self-diffusion coefficients down to the physical limit (D approximately 10(-15)m(2)/s) when flip-flop spin diffusion starts to become effective.

1H multiple-quantum nuclear magnetic resonance investigations of molecular order in polymer networks. II. Intensity decay and restricted slow dynamics

We present an approach towards the analysis of the intensity decay in proton multiple-quantum experiments on polymeric networks in terms of slow fluctuations of the residual dipole-dipole coupling tensor. Solutions for individual spin pairs as well as the three-spin system of methyl groups are derived, and the influence of the cycle time of the multiple-quantum pulse sequence is evaluated. The multiple-quantum strategy discussed herein features the advantage that the magnitude of the fluctuating part of the residual dipole-dipole coupling constant and the correlation time of the slow process can be determined independently of the integral residual coupling constant as well as its distribution. The theory is applied to experiments on end-linked poly(dimethylsiloxane) model networks with mono- and bimodal chain length distributions, where it is found that, for all samples, correlation times of the slow processes average to about 1 ms, and that the magnitude of the fluctuating part of the dipole-dipole coupling is significantly smaller than the average dipole-dipole coupling constant. This observation is interpreted in terms of considerably restricted reorientations of topological constraints.

NMR study of molecular dynamics in selected hydrophilic polymers

The temperature dependencies of the 1H spin-lattice relaxation times T1 and of the proton NMR second moment M2 in the temperature range from about 90 to 420 K were measured for methyl cellulose, hydroxypropyl cellulose and hydroxypropyl methyl cellulose. The proton spin-lattice relaxation measurements reveal two minima due to the C3 reorientation of the methyl groups of the methoxy, methylenemethoxy or propylene oxide groups and the restricted motion of the segment of the polymer chain. The activation energy barriers for these motions were calculated.

A new contrast parameter for visualization of the cross-link density in rubber based on the dipolar-correlation effect

A new parameter for NMR mapping is suggested on the basis of the mean squared dipolar fluctuation (MSDF). The MSDF characterizes the relaxation mechanism due to ultra-slow dipolar fluctuations in liquids subject to local anisotropy of molecular motions. These fluctuations can be monitored on the time scale exceeding a few microseconds. In rubber materials, the MSDF is a function of the density of chemical cross-links strongly affecting (anisotropic) mesh chain fluctuations. Experimentally, the MSDF is determined from the attenuation curves of the quotient of the amplitudes of the stimulated and the primary echoes produced by the three 90 degrees radio-frequency pulse sequence. In order to evaluate the MSDF maps, the latter sequence was combined with the standard scheme of the magnetic field gradients providing a spatial resolution. The pixel values of the MSDF are "visualized" using grey shades related to the equidistant intervals covering the whole range of the measured values. The MSDF maps are demonstrated for the two composite samples. The first sample consists of a water filled tube in the middle part surrounded by high molecular mass polyisoprene (PI) in the outer part. The relaxation weighted spin density image of this sample is dominated by a water signal with PI producing a much weaker intensity. The MSDF map, on the contrary, enhances the relative intensity of the outer, PI, part while scaling the middle, water, part down to the level of noise. The second sample consists of the four rubber pieces with different cross-link density. This sample thus models an inhomogeneous rubber object. The MSDF map produces clear contrast for the relevant regions. The advantages of employing this kind of NMR mapping for a characterization of materials are discussed.

Characterisation of crosslinked elastomeric materials by 1H NMR relaxation time distributions

One of the most critical structural parameters in elastomeric materials is the density of cross-linking between the polymeric chains. This chemical feature greatly affects chain motions and is determinant in controlling mechanical properties of the final product. NMR techniques are widely and efficiently applied to investigation of such materials. In this study we have measured both transverse and longitudinal 1H relaxation times of a series of polybutadiene rubber samples with increasing crosslink density induced by chemical treatment. This approach allowed the observation of T(1) and T(2) decrease with the increase of crosslink density in the samples examined. The data obtained have been analyzed and compared to theoretical models.

Proton residual dipolar couplings by NMR magnetization exchange in cross-linked elastomers: determination and imaging

Proton nuclear magnetic resonance (NMR) magnetization exchange is used to investigate residual dipolar couplings in a series of cross-linked poly(styrene-cobutadiene) elastomers. A new model for the dipolar unit is used for the evaluation of the signal decay in magnetization exchange experiments. It takes into account an extended residual dipolar coupling network along the polymer chain. It is shown that in the regime of short mixing times, information about the residual dipolar coupling between methine and methylene protons can be obtained which is not affected by other inter- and intramolecular dipolar couplings. The dynamic order parameter of methine-methylene protons is measured and correlated with cross-link density. This study certifies the quality of a filter for magnetization from residual dipolar couplings which exploit magnetization exchange. The filter can be used to generate contrast in NMR images of heterogeneous elastomers. The first proton NMR parameter image of a dynamic order parameter is presented for a phantom made from poly(styrene-cobutadiene) samples with different cross-link densities.