Influence of Polymer Chain Concentration and Molecular Weight on Deformation-Induced 2 H NMR Line Splitting P. Ekanayake* ,† and H. Menge Martin-Luther-University Halle-Wittenberg, Department of Physics, Friedemann-Bach-Platz 6, D-06108 Halle/Saale, Germany M. E. Ries and M. G. Brereton IRC in Polymer Science and Technology, Department of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK Received September 25, 2001 ABSTRACT: Deuterium nuclear magnetic resonance line splitting arising from deformed cis-1,4-poly- (butadiene) network, of which the network chain concentration is varied by incorporating protonated free chains of similar kind, is investigated. The results are discussed according to the theoretical framework, introduced by Brereton and Ries, 1 which dealt with network vectors which were treated as quenched variables, to show explicitly how they collectively determine the anisotropy in the mean field. The influence of the network chain concentration on the NMR line splitting and molecular weight dependence of Flory interaction parameter  and the possibility of using the deuterium NMR line splitting to investigate  parameter of a system are discussed. Introduction When a deuterated network is stretched, an oscilla- tion, corresponding to a splitting in frequency space, is produced in the transverse deuterium NMR decay 2 (see Figure 1). This indicates an anisotropic orientation of the chain segments translated through the junction points. How- ever, Brereton 3 and Sotta and Deloche 4 showed that for a noninteracting (phantom) network the oscillations from individual chain segments, when averaged over all network chain orientations, give rise to a nonoscillating signal and consequently a single (broadened) line shape. The doublet line shape seen in the frequency domain was therefore indicative of a higher degree of anisotropy than that induced merely by the cross-link points. This is further confirmed by the interesting observation that deuterated free chains within a protonated deformed polymer network exhibit approximately the same line splitting as a deuterated deformed network. The situ- ation is schematically represented in Figure 2. An oscillation in the free chain signal reveals that the splitting does not depend explicitly on the presence of cross-links; i.e., the constraint arising from the network due to cross-links is not responsible for the line splitting. Similar observations have been reported in the litera- ture from deuterated solvent molecules within a proto- nated deformed poly(dimethylsiloxane) (PDMS) net- work. 5-7 Sotta et al. demonstrated that when oligomers of deuterated PDMS were dissolved into a uniaxially deformed PDMS network, they also showed the char- acteristic doublet. 8 Further, these oligomers displayed the usual orientational dependence (3 cos 2 θ - 1) of their splitting on the angle θ between the applied strain and the magnetic field. This clearly revealed that all the chains in the sample, both network and free chains, were aligned along the strain direction. Previously, Sotta and Deloche 8 introduced nematic interactions occurring between neighboring segments to explain this phenomenon. These nematic interactions would both enhance the anisotropy of the network segments and generate it in any dissolved free chains. In a later work Brereton 3 showed that it was sufficient to include only excluded-volume interactions in order to account for the observed line splitting. For the network these were treated as the mean field level, and it was shown that an anisotropic mean field arises when the network is deformed. Subsequently, numerical simulations 9,10 on deformed one-component systems have demonstrated the ability of excluded-volume in- teractions to produce the experimentally observed split- ting. A general analytic result that includes the effect of anisotropic mean field and network constraint was derived in our previous work. 11 It was shown, from analyzing a range of deformed network sample signals, † Present address: Department of Physics, University of Per- adeniya, Peradeniya, Sri Lanka. E-mail:piyasiri2001@yahoo.co.uk; Fax:+94 (0)8 388018. * Corresponding author. Figure 1. NMR response in time domain and after Fourier transformation: (a) free induction decay (FID) without oscil- lation corresponds to a single line while (b) FID with oscillation corresponds to a splitting indicating that the deuterium NMR line splitting is solely due to the oscillations in FID. 4343 Macromolecules 2002, 35, 4343-4346 10.1021/ma011677p CCC: $22.00 © 2002 American Chemical Society Published on Web 04/17/2002