B. Deloche

Long‐time dynamics in poly(dimethylsiloxane) networks studied by deuterium nuclear magnetic resonance

A deuterium NMR (nuclear magnetic resonance) method is proposed to investigate long‐time correlation of chain orientation in poly(dimethylsiloxane) networks. It is based on the observation of a quadrupolar echo after a Jeener pulse sequence, which is possible owing to the presence of nonzero residual interactions in the system related to local chain orientations. The time evolution of the quadrupolar echo amplitude is calculated in the framework of a slow chain reorientational motion. An experimental result is presented which shows that local orientation is frozen within the time scale investigated, up to 0.3 s.

Analysis of deuterium NMR spectra of probe chains diffusing in a stretched polybutadiene network

Abstract Polybutadiene network was swollen with short perdeuterated network-like chains (PBD). Deuterium NMR spectra of these probe chains are measured as the rubber matrix is uniaxially elongated. Due to the presence of two kinds of deuterons (methylene and ethylene) and two possible configurations of monomer units (cis and trans) in PBD chains, composite NMR spectra are observed and analysed. They evidence for different orientational behaviour of cis and trans units. ‘Rod-like’ motional behaviour of the trans units leads to higher degree of orientational order than for the cis units.

Some aspects of the orientational order distribution of flexible chains in a diblock mesophase

The segmental motions of flexible chains in the lamellar structure of a strongly segregated poly(styrene)-poly(dimethylsiloxane) (PS-PDMS) diblock were investigated over a time scale of a few tens of microseconds. (2)H NMR experiments were performed on the PDMS block, selectively perdeuterated. Transverse relaxation measurements show that the main part of the PDMS repeat units display anisotropic reorientational motions within the diblock lamellae and such a segmental ordering essentially results from interchain steric repulsions. (2)H double quantum-based experiments evidenced a non-uniform local stretching of PDMS chains and enabled the underlying distribution of the orientational order parameter to be determined quantitatively. Besides, a fraction of the PDMS chain segments, about 14%, were found to display isotropic - or nearly isotropic - reorientations, which could be assigned to repeat units located within a thin sublayer (about 1-2 nm) at the lamellae midplane, but also deeper in the lamellae, close to folded parts of the chains. These experimental results were confronted to theoretical descriptions of opposing polymer brushes and, in particular, to the strong-stretching theory (SST) including the entropic contribution of free chain ends.

Orientational diffusion of homopolymer chains in a lamellar structure of diblock copolymer: a deuterium n.m.r. study

Chain dynamics in lamellar microdomains of polystyrene-polybutadiene (PS-PB) diblock copolymer is investigated by deuterium nuclear magnetic resonance (n.m.r.). The experiments are performed on perdeuterated PB homopolymer chains dissolved in the PB sublayers of a macroscopically oriented PS-PB sample. The n.m.r. spectra show that homopolymer chain segments undergo uniaxial reorientations around the direction normal to the layers. This local uniaxial order implies that the PB chains of the diblock are slightly elongated and that the segments between chains are correlated in orientation.

Heterogeneous behavior of free chain-ends in a lamellar diblock copolymer: segmental dynamics and ordering, as probed by 2 H solid-state NMR

The segmental dynamics and orientational ordering for the free chain ends of a strongly segregated diblock copolymer, poly(styrene)-b-poly(dimethylsiloxane) (PS-b-PDMS), self-assembled into a lamellar structure, was investigated with 2H solid-state NMR. The dynamical behavior of the free PDMS chain extremities, selectively deuterated, was probed in a temperature regime intemathrmediate between the PS and PDMS glass transition temperatures. The temperature dependence of the 2H spin-lattice relaxation time indicates a decrease of the characteristic segmental time for such chain ends, compared to the one detemathrmined for all the PDMS repeat units. This faster segmental dynamics was rationalized by a temperature shift estimated to about 8 K. 2H transverse relaxation measurements and double-quantum build-up curves led to a description of the local constraints experienced by the chain ends, within the lamellar domains. More than two thirds of them display anisotropic reorientations over the tens of microseconds time scale. The corresponding distribution of the orientational order parameter was found to be similar to the one displayed for all the PDMS units of the copolymer, suggesting a repartition of the chain ends throughout the entire lamella. This behavior results from the contribution of the entropy related to the chain extremities. The description of this ordering of the free chain ends suggests the occurrence of a significant fraction of folded PDMS chains, within the lamellae.