Abstract
A series of poly(ethylene oxide) nanocomposites with spherical silica was studied by proton NMR spectroscopy, identifying and characterizing reduced-mobility components arising from either room-temperature lateral adsorption or possibly end-group mediated high-temperature bonding to the silica surface. The study complements earlier neutron-scattering results for some of the samples. The estimated thickness of a layer characterized by significant internal mobility resembling backbone rotation ranges from 2 nm for longer (20 k) chains adsorbed on 42 nm diameter particles to 0.5 nm and below for shorter (2 k) chains on 13 nm particles. In the latter case, even lower adsorbed amounts are found when hydroxy endgroups are replaced by methyl endgroups. Both heating and water addition do not lead to significant changes of the observables, in contrast to other systems such as acrylate polymers adsorbed to silica, where temperature- and solvent-induced softening associated with a glass transition temperature gradient was evidenced. We highlight the actual agreement and complementarity of NMR and neutron scattering results, with the earlier ambiguities mainly arising from different sensitivities to the component fractions and the details of their mobility.
Highlights
Particle-filled polymers, in particular elastomers for, e.g., tire applications, display outstanding mechanical properties as compared to the pure material.1 The unique reinforcement has been explained theoretically2,3 as well as experimentally4–6 by filler networking, where filler-filler contacts are mediated by adsorbed and immobilized polymer species
For polymer components with reduced or even absent mobility, the Free-induction decay (FID) shape is close to a Gaussian, exp −(t/T2,stat
In our previous work on poly(ethyl acrylate) (PEA) networks adsorbed on silica, a significant temperature dependence of both the immobilized fraction and the relaxation parameters was evidenced
Summary
Particle-filled polymers, in particular elastomers for, e.g., tire applications, display outstanding mechanical properties as compared to the pure material. The unique reinforcement has been explained theoretically as well as experimentally by filler networking, where filler-filler contacts are mediated by adsorbed and immobilized polymer species. To highlight the range of phenomena, e.g., PDMS adsorbed on silica in a porous system exhibits a separate Tg as directly observed by DSC, while an acrylate polymer adsorbed on silica spheres was proven to feature a gradient in Tg.. Of the Tg-related step could be quantitatively reproduced by a model based upon a τα profile derived from quantitative NMR data, the latter being sensitive to τα once it is less than 0.1 ms.20 In extending this picture, Napolitano has further advocated the discussion of a lowermost “dead layer,” which remains immobilized and does not devitrify at all accessible temperatures. We address the substantial internal mobility of the adsorption layer, and assess changes arising from changes in temperature, annealing times, and water content Different influencing factors, such as the molecular weight, endgroups, and confinement geometry (spheres of different diameters vs pores), are discussed
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