Bassel Haidar

1H multiple-quantum nuclear magnetic resonance investigations of molecular order distributions in poly(dimethylsiloxane) networks: Evidence for a linear mixing law in bimodal systems

We present proton–proton multiple-quantum investigations on a series of monomodal and strongly bimodal end-linked poly(dimethylsiloxane) model networks. A robust pulse sequence characterized by a well-defined double-quantum Hamiltonian along with a specific normalization approach is used to obtain double-quantum build-up curves. These curves are analyzed in terms of the spin dynamics of a local subsystem of monomer-fixed spins, where analytical fitting functions yielding residual dipole–dipole coupling constants are derived on the basis of exact solutions provided by simulations. Further employing the novel experimental strategy of double-quantum preselection of elastically active network chains, it is shown that the network response is purely heterogeneous, and that the data can be analyzed in terms of distributions of local dynamic order parameters using different models. The results yield consistent proof that local chain order in bimodal networks obeys a linear mixing law of short- and long-chain comp...

High resolution solid-state n.m.r. investigation of the filler-rubber interaction: 1. High speed 1H magic-angle spinning n.m.r. spectroscopy in carbon black filled styrene-butadiene rubber

Abstract This present work, dealing with filler/rubber interactions, provides a new approach for investigating the behaviour of elastomeric chains (styrene-butadiene rubber, SBR) in the close vicinity of carbon black surfaces. Pulsed nuclear magnetic resonance measurements have been carried out on pure SBR, and on carbon gels obtained by solvent extraction of the carbon black filled elastomers. The initial concentration of carbon black in the filler/rubber mixtures was varied between 20 and 100 parts per hundred parts by weight (phr). The high resolution proton spin-spin relaxation time, T 2 , has been measured for each of the individual resonance species belonging to the SBR chain. High resolution was achieved by rapidly spinning the sample at the magic angle. It has been observed that a very high magic-angle spinning (MAS) rate (> 18 kHz) is necessary to achieve a fully resolved spectrum of SBR. However, at a spinning rate of ∼ 15 kHz it is possible to avoid all of the spinning side bands and to deconvolute, unambiguously, the different resonance peaks that are present in the spectrum. In the unfilled elastomer, at temperatures much higher than the T g , the chain segmental motions are anisotropic and deviate from true liquid-like behaviour. The adsorbed rubber chains are found to consist of loosely and tightly immobilized segments. The relative immobilization of the different protons has evidenced the methine 1 H to be much more immobilized than the aromatic or methylene species. Therefore, the olefinic part of the butadiene segment of the elastomeric chain appears to be the most affected by the carbon black surface. Moreover, T 2 is found to be independent of the filler concentration in the 30–80 phr range, and the relative concentration of the tightly bound rubber in the composite shows a maximum at a filler content of ∼ 50 phr, where maximum reinforcement is normally observed. As far as the molecular dynamics is concerned, highly filled systems (> 80 phr carbon black) behave differently from low and medium filled systems (

High-Resolution Solid State NMR Investigation of the Filler-Rubber Interaction: Part III. Investigation on the Structure and Formation Mechanism of Carbon Gel in the Carbon Black-Filled Styrene—Butadiene Rubber

Abstract This investigation describes the elastomer—filler interaction and its formation mechanism using solid state high-resolution, high-speed 1H magic-angle spinning nuclear magnetic resonance (NMR) spectroscopy. Pulsed NMR measurements were carried out on pure styrene-butadiene rubber (SBR), and solvent extracted carbon gels from freshly prepared and storage matured master batches. The effects of filler loading, storage maturation, severity of extraction and experimental temperature on the elastomer-filler interaction were examined and discussed. High resolution in NMR was achieved by higher temperature/low spinning rate, room temperature/high spinning and combined rotation and multiple-pulse spectroscopy (CRAMPS) techniques. High-speed magic-angle spinning (MAS) was found to be the most suitable method to achieve high resolution. Proton spin—spin relaxation time, T2, was measured successfully for each of the principal resonance species present in the samples. These measurements reveal an insight into...

Photocrosslinking of elastomer systems—III Ultraviolet light induced reactions in EPDM and EPR blended with or modified by grafting of derivatives of benzophenone

Abstract Photocrosslinking experiments were performed in elastomeric polymer films (EPR or EPDM) that either contained benzophenone or some of its derivatives, or had been previously modified (EPDM) by grafting of benzophenone. This latter modification was performed by a series of Friedel-Crafts reactions. The resulting materials were characterized by i.r. and u.v. spectroscopy in conjunction with physical property measurements. The results allowed relationships between the properties of the photosensitizers in their excited states and their efficiency in the crosslinking processes to be established. The results suggest that each coupled polymer-photoinitiator exhibits a particular behaviour. Thus it appeared that, in the presence of oxygen, the hexadiene branches of the terpolymer can undergo various types of reactions. Moreover, the grafting of the photosensitizer onto the elastomer backbone was shown to enhance considerably the crosslinking reaction. It is possible to conclude that oxygen plays a substantial role in the photocrosslinking. Indeed it always enhances crosslink formation. It can be assumed that the photooxidation of the polymer backbone yields alkoxy and hydroperoxy radicals which can act as crosslink sites.

Filled Elastomers: Characteristics and Properties of Interfaces and Interphases, and Their Role in Reinforcement Processes

The dispersion of fillers in polymers is generally associated with an improvement of at least some of the properties (mechanical, thermal, electrical, etc.) of the corresponding macromolecular material. These phenomena are known to result from the interactions, which take place at the molecular level between the polymer and the surface of the filler and result in the formation of a so‐called interphase. It is, thus, of the utmost importance to be in a position to assess the characteristics of the latter. In this article, we will report on the particular case of elastomers interacting with reinforcing fillers—such as carbon black or silica—the information which may be provided by approaching the segmental mobility of a polymer in close proximity to a solid surface. In this respect, we will discuss the effect of compounding conditions on the adsorption of rubber chains (amount of adsorbed polymer, possible chain conformations, intensity of adsorption, filler/polymer networking) and examine the thermodynamic state of the corresponding elastomer by assessing the molecular mobility of the macromolecular chains through solid‐state NMR spectrometry, physical aging processes, and so on.

Adsorption of monodisperse polybutadienes on carbon black, 3 Displacement of pre-adsorbed molecules by the same polymer. Conformational and molecular weight effects

Abstract Displacement of pre-adsorbed macromolecules by the same polymer, polybutadiene, of the same or of different molecular weight was studied in solution and in the bulk. The effect of polymer concentration on pre-adsorption and displacement processes was determined. Displacement was investigated by gel permeation chromatography and by determination of the amount of bound polymer before and after displacement. A conformational factor was established as a major driving force - besides molecular weight - in the displacement process. Polymer chains adsorbed in flat conformation had the highest adsorption stability and could not be displaced by any other molecular weight of the same polymer.

Adsorption of monodisperse polybutadienes on carbon black, 2 Bulk adsorption

Abstract The adsorption on carbon black (CB) of a series of monodisperse polybutadienes (PBs) with different molecular weights (Mn in the range of 3000 to 120 000) but identical microstructure (1,2-PB content ≈ 80%) was studied in the bulk for different polymer/CB ratios (τ, in the range of 2 to 10 g/g). In this respect we particularly studied the evolution of the amount of polymer, which cannot be extracted from the polymer/filler system upon blending, the so-called bound polymer, and attempted to investigate the conformation of the resulting adsorbed molecules as well as the conditions that control the formation of the polymer/filler network. The conformational transition turned out to depend exclusively on molecular weight, and the networking transition to occur at constant Mn1/2/τ, essentially through an entanglement process of the adsorbed chains.

Adsorption of monodisperse polybutadienes on carbon black, 1 Adsorption from dilute and semi-dilute solutions

Abstract A series of polybutadienes (PBs) of different molecular weights (in the range of 3000 to 120 000) but of identical microstructure (1,2-PB content ≈ 80%) was synthesized via anionic polymerization. The adsorption in solution of the various monodisperse PBs on carbon black (CB) was studied for different polymer/ CB ratios as well as for polymer and CB concentrations in the range of 0.1 to 100 g/l. The amount of adsorbed polymer was followed and was found to be essentially governed by the polymer concentration, which determines also the conformation of the polymer chain on the filler surface. CB concentration didn’t prove to be a pertinent parameter, whereas the polymer/filler ratio was found to control the formation of the three-dimensional polymer-filler network on the basis of a percolation process. Results are discussed with respect to the available theories.