Low-Field NMR Investigations of Nanocomposites: Polymer Dynamics and Network Effects

Abstract

The dynamics of the polymer matrix in filled rubbers is modified by the presence of solid particles. We used low-field proton NMR to investigate model filled samples consisting of a dispersion of grafted silica particles into an elastomeric matrix. Exploiting magic-sandwich echo experiments, we were able to determine the fraction of polymer with slower dynamics and to correlate it to the silica specific surface. The presence of immobilized polymer;most probably due to a gradient of glass transition temperature around the solid particles;is detected whether there is a covalent bond between the filler and the matrix or not. Moreover, the fraction of immobilized polymer decreases in similar ways with either an increase of the temperature or the addition of solvent. In the case of covalent bonds between the silica and the polymer, multiple-quantum experiments reveal that the cross-link density of the elastomer matrix is locally increased in the vicinity of the particles. This is an observation that was not made in any conventional filled elastomer system and it can be attributed to the good particle dispersion and the covalent links in our model samples.

Reduced-mobility layers with high internal mobility in poly(ethylene oxide)–silica nanocomposites

“…For example, if the jump process is nearly isotropic (as is the case for the α relaxation associated with the glass transition), heating (i.e., decreasing τ c = τ α ) affords a progressive reduction of the coupling strength and a smooth transition to a more exponential decay with a decay time constant (T * 2 ) reaching at least the millisecond range. 20 Note that NMR probes the α relaxation in the high frequency range far above T g , due to the specific value of T 2,stat .…”

“…Bare oxide fillers such as silica and alumina, displaying surface hydroxy groups, interact favorably with polymers capable of acting as hydrogen-bond acceptor, such as poly(dimethyl siloxane), PDMS, [8][9][10] poly(ethylene oxide), PEO, [11][12][13][14][15] poly(2-vinyl pyridine), P2VP, 16,17 or different acrylate polymers. 5,[18][19][20][21][22] The amount of dynamically modified polymer, and with this the nominal thickness of an assumed-to-be contiguous reduced-mobility layer, can be estimated on the basis of results from different techniques. Such results are often not compatible with each other even for virtually the same polymer-surface a) Electronic address: kay.saalwaechter@physik.uni-halle.de.…”

“…URL: www.physik.uni-halle.de/nmr system and the same method. [12][13][14] In this context DSC, 10,16,19 dielectric spectroscopy, 23,24 NMR spectroscopy, 13,18,20,21 and static 22 as well as dynamic 9,11,12,14,15 neutron scattering techniques have been used. Apart from purely structural (=density) information obtained by static scattering, 22 most other techniques are sensitive to dynamic properties.…”

“…For example, the notion of a "glassy layer" has emerged 1,4,13,18,20 but is subject to ambiguity as this implies literally that the reduced-mobility components are below their T g , i.e., have a cooperative segmental relaxation time (τ α ) of at least 100 s at the temperature of measurement, and may exhibit only rather localized faster motions ( β and higher relaxations). Therefore, conclusions based upon neutron-scattering observations at comparably high temperatures, stating the absence of a "glassy" layer 9 or even claiming "contradiction" 14 to earlier NMR results, 13 originate partially from the simplistic nomenclature and, specifically, a lack of a more detailed discussion of the layer properties in Ref.…”

“…21 In the latter case, DSC observations of a broadening of the T g -related step could be quantitatively reproduced by a model based upon a τ α profile derived from quantitative NMR data, the latter being sensitive to τ α once it is less than 0.1 ms. 20 In extending this picture, Napolitano has further advocated the discussion of a lowermost "dead layer," 23 which remains immobilized and does not devitrify 19 at all accessible temperatures.…”