Polymer Bulletin 43, 371–378 (1999) * Corresponding author e-mail: menge@physik.uni-halle.de Polymer Bulletin Springer-Verlag 1999 Local order and strain-induced orientation in poly(butadiene) networks studied by 2 H NMR Heike Menge 1,* , Piyasiri Ekanayake 1 , Mike E. Ries 2 , M. G. Brereton 2 , Matthias Findeisen 3 1 Martin-Luther-Universität Halle-Wittenberg, Fachbereich Physik, Friedemann-Bach-Platz 6, D-06108 Halle/Saale, Germany 2 IRC in Polymer Science and Technology, University of Leeds, Leeds LS2 9JT, UK 3 Universität Leipzig, Fakultät für Chemie und Mineralogie, Institut für Analytische Chemie, Lineé-Strasse 3, D-04103 Leipzig, Germany Received: 28 August 1999/Revised version: 6 October 1999/Accepted: 6 October 1999 Summary Deuterium nmr has been employed to determine the average orientation of chain segment in poly(butadiene) networks. The nmr spectrum lineshape reveals the orientation distribution of network vectors due to the crosslinks, whereas the observed splitting gives information about the orientation due to segmental interactions. Both the lineshape and the splitting have been fitted simultaneously for a range of deformed poly(butadiene) networks varied in crosslink density and precursor chain length. From the fitting parameters the separate contribution to the average orientation arising from network constraints and interaction are calculated. These are used to estimate the effective molecular weight between topological constraints and the size of the segmental interaction. It could be shown that the latter remains constant as the crosslink density is varied. This is a strong evidence of the recently published theoretical framework. Introduction Rubbers are important materials for numerous and important applications. Nuclear magnetic resonance (nmr) allows one to relate macroscopic properties (such as elastic moduli) to microscopic behaviour of polymeric chains. One essential nmr property in elastomers is the presence of residual interactions, due to local order related to the constraints resulting from crosslink junctions. 2 H-nmr has been devoted to be a sensitive and powerfull tool to study this anisotropy at a molecular level in strained elastomers. A number of deuterium ( 2 H) nmr studies in elastomers have been already performed (1-6). For undeformed rubbers a single resonance line is observed. Under uniaxial deformation the spectra splits into a well-defined doublet structure (7). A non-interacting Gaussian phantom network has been theoretically shown to generate no splitting under deformation (6,8,9). These results therefore indicate that to model the chain reorientation in a strained elastomer one must introduce chain interactions (6,10,11). In strained rubbers there is a higher degree of anisotropy than that merely induced by the crosslinks. Short-range orientational interactions between segments such as nematic interactions (10), steric interactions (11) and anisotropic junction fluctuations (12) are some of the proposed explanations for this extra degree of anisotropy. A theory by Brereton and Ries (11) attributes the higher degree of anisotropy implied by the splitting to the inter chain steric interactions. In a network a polymer segment interacts with many neighbouring ones. These interactions can effectively described by a mean field. Under deformation the distribution of monomeric segments generates, through their isotropic steric interactions, an anisotropic mean field. All chains within the rubbery matrix experience this perturbed mean field that subsequently causes an induced alignment with the