Jean E. Herz

Polydimethylsiloxane/poly(methyl methacrylate) interpenetrating polymer networks: 2. Synthesis and properties

Abstract The synthesis of full and graft interpenetrating polymer networks (IPNs) based on polydimethylsiloxane and poly(methyl methacrylate) is described. IPNs were obtained by an in-situ sequential synthesis. Most samples were opaque; however, in a few cases, transparency was noted. Other properties like stress-strain behaviour, hardness and glass transition temperature were examined. Owing to the particular behaviour of silicone-containing polymers, the surface properties and the permeability to oxygen of these IPNs were also studied.

Effect of dilution on the orientational order induced in strained polymer networks, as measured by 2H nuclear magnetic resonance

Abstract The orientational order generated in uniaxially strained rubbers is investigated as a function of the degree of swelling. This study is performed on polydimethylsiloxane (PDMS) networks diluted either with conventional solvents (such as chloroform) or with linear PDMS chains. Deuterium ( 2 H) nuclear magnetic resonance is used to monitor selectively the ordering of each component of the solution (the network chains or the diluent molecules). The concentration dependence of the order in the range of network volume fractions larger than 80% is similar for both networks and diluents but depends on the nature of the diluent: the order decreases steeply when chloroform is added whereas it remains unaltered in the presence of free chains. These results mean that the orientational couplings between chain segments (introduced previously by Deloche et al. to account for the free chain orientation) are variably screened according to the nature of the diluent molecules. Introducing these interchain effects in a mean-field description of polymer networks allows one to discuss these observations quantitatively.

Free Polymer Chains Dissolved in a Strained Elastomer: Neutron Scattering Test of the Anisotropy

We have measured, by small-angle neutron scattering, the form factor S(q) of a labelled free chain of polydimethyl-siloxane (PDMS-D) dissolved in a nondeuterated network. The molecular weight between cross-links is the same as the one of the free chain (Mn = 10 000). Using a bidimensional detector, we observe no detectable anisotropy of the free chain in the stretched network (within 3%). At any elongation, S(q) is the same, and is typical of the Gaussian conformation all over the q range, as it is for the PDMS chain inside a melt. DMR data on the same chain, reported in the same issue, and which show an orientation, are taken into account in the discussion.

Polydimethylsiloxane/poly(methylmethacrylate) interpenetrating polymer networks: 1. Efficiency of stannous octoate as catalyst in the formation of polydimethylsiloxane networks in methyl methacrylate

Abstract Deactivation of the polycondensation catalyst in in situ sequential syntheses of polydimethylsiloxane/poly(methyl methacrylate) interpenetrating polymer networks was observed, which impeded the gelation of the elastomeric network. The loss of catalytic activity could be ascribed to the simultaneous presence of radicals and oxygen in the reaction medium. When either oxygen is excluded or a radical inhibitor added, the polycondensation proceeds to gelation at the expected rate.

Orientation of free polymer chains dissolved in a strained elastomer: a deuterium magnetic-resonance study

The H2-NMR technique is used to monitor the orientational behaviour of short, deuterated polydimethylsiloxane (PDMS) chains dissolved in an unlabelled silicone rubber matrix. When the host matrix is uniaxially stretched, an orientational order is induced at the segmental level of the PDMS probe chain (Mn = 10.500). This stress-induced orientation of the probe chain demonstrates that short-range orientational correlations take place in the deformed network. A comparison with previous results obtained on cross-linked chains suggests that these interchain effects may be the dominant contribution to the segmental orientation of the effectively elastic chains.

Competition between polycondensation of α,ω-dihydroxy polydimethylsiloxane and its condensation with alkoxy silane: A kinetic approach

Abstract α,ω-Dihydroxy polydimethylsiloxane (PDMS) undergoes a linear polycondensation, catalysed by stannous octoate, which can yield high molecular weight species. In the presence of monoalkoxysilane, both a polycondensation and a blocking reaction take place simultaneously. Such a condensation between α,ω-difunctional PDMS chains may also occur in the case of crosslinking involving tri- or tetra- alkoxysilane. This implies that, in the so-called model networks thus formed, the average length of the elastic chains is definitely different from that of the precursor chains.

Synthesis and characterization of monofunctional polydimethylsiloxanes with a narrow molecular weight distribution

Polydimethylsiloxanes (PDMSs) with a narrow molecular weight distribution and fitted at one chain end with a hydrogenodimethylsilyl or a dimethylvinylsilyl function were prepared by the anionic ring-opening reaction of hexamethylcyclotrisiloxane in a 50 vol % benzene-tetrahydrofuran solution, followed by endcapping with chlorodimethylsilane or chlorodimethylvinylsilane. The experimental conditions were chosen on the basis of preliminary experiments in order to minimize the contribution of reshuffling reactions and it was checked that the possible formation of difunctional species was negligible. The functionalization was quantitative. The functional polymers were characterized by i.r. spectroscopy and by s.e.c. α-Hydrogenodimethylsilyl and α-dimethylvinylsilyl PDMS were prepared in the molecular mass range between 2500 and 40000 g mol−1.

Contribution to a better knowledge of the crosslinking reaction of polydimethylsiloxane (PDMS) by end-linking : the formation of star-branched PDMS by the hydrosilylation reaction

Monofunctional polydimethylsiloxanes with a dimethylsilyl or a dimethylsilylvinyl end-group were reacted with respectively 1,3,5,7-tetramethyltetravinylcyclotetrasiloxane and 1,3,5,7-tetramethylcycloterasiloxane. The hydrosilylation reaction, carried out in bulk and in the toluene solution, were followed by FTi.r. spectrometry and the star-branched samples formed analysed by SEC-LS. In the second case, for PDMS silylvinylic function at the chain-end, the reaction is noticeably faster and the final conversions reached are higher than in the first case. The principal parameter which limits the maximum conversion is the concentration of functional groups in the medium.

Radical polymerization of styrene initiated by benzopinacolates: kinetics and comments concerning the reaction mechanism

Abstract The present paper reports an investigation on the “pseudo-living” radical chain propagation process in the case of the styrene polymerization initiated by benzopinacolates. Two steps were observed, as in earlier reported investigations. Oligomers were formed first at low monomer conversions, due to the rapid chain-termination by the primary radicals present at a high concentration. Polymerization takes place only after consumption of the major part of these radicals. Experiments carried out on model molecules representative of the oligomer chain-ends yielded evidence that these species are unactive, they do not undergo further thermal cleavage. The in the literature reported insertion process was not confirmed for the styrene polymerization.

Linear polycondensation of α,ω-dihydroxy polydimethylsiloxane, catalyzed by stannous octoate

Abstract In the presence of stannous octoate, high molecular weight polydimethylsiloxane (PDMS) is obtained from an oligomeric α,ω-dihydroxy PDMS. The polycondensation reaction is limited by the presence of oxygen and/or water. Only linear species are found, and network formation has never been observed.