Polybutadiene chemically cross-linked in solution and containing an admixture of deuterated (labeled) molecules has been investigated by small-angle neutron scattering (SANS). The chains exhibit the same dimensions (radius of gyration) in the dry network as in the melt. The structure factor was measured as a function of elongation (Y = L/Lo up to 2. The radius of gyration, R,, obtained from Zimm plots as a function of a was compared with the theoretical predictions of the scattering theory of Ullman for a multilinked deuterium-labeled molecule. The number of cross-links per chain, neglecting loops, was determined from these results to be 4 f 1 for a chain with M, = 13 150 f 3500 g/mol. From the efficiency of the cross-linking reaction, as determined from SANS, it is shown that a large fraction of small loops is present, which do not affect the radius of gyration of primary chains. General OutlineThe conformation of a cross-linked chain in a deformed network has been studied in terms of several macroscopic models, such as the junction-affine model by Kuhnl and the phantom network model as developed by James andGuth.293 More recent theories by Flory4 and Flory and Ermanb7 try to bridge both models by allowing for restrictions or by suppression of the cross-link fluctuations in the phantom network. Apart from macroscopic tests of such networks by means of mechanical analysis (e.g., ref 8), small-angle scattering of neutrons (SANS) became a powerful tool to observe the deformation of cross-linked chains directly on a molecular scale: By replacing a fraction of the polymers by deuterium-labeled material, the static structure factor S(Q) at small scattering vectors Q yields information on the chain conformation as a function of the applied strain cr = L/Lo. More recently, also the crosslink motion and its fluctuation amplitude have been investigated by means of the neutron spin-echo techniquesg SANS investigations on deformed networks have already been carried out in the past on poly(dimethy1siloxane)lO and polybutadiene.ll These experiments were performed on networks consisting of independent and deuteriumlabeled sections between the cross-links, and the mean end-to-end distance in this "end-linked" case was determined as a function of strain. On the other hand, for polystyrene,12J3 labeled chains multilinked by radiation were investigated, which yields the radius of gyration of a k-fold-linked and labeled molecule. Our study is of this kind and refers to a chemically multilinked network of polybutadiene. Ten percent of the chains were perdeuterated for labeling (see section 2). The cross-linking reaction used in this study is exhaustive and leads to four-functional cross-links. This type of reaction has first been applied to cross-linking of polybutadiene in semidilute solution14 and later extended to cross-linking a t any ~0ncentration.l~ In the latter case the cross-linking density can be determined by the amount of the cross-linking agent. Moreover, a uniform molec-0024-9297/91/2224-1269$02.50/0 ular weight of the...

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networks obtained by a 4functional random cross-linking reaction over a broad range of polymer concentration were studied by small angle neutron scattering(SANS),ZH NMR and Monte Carlo(MC) simulation in the isotropic and uniaxially deformed state. The defect structure of the networks has been characterized by MC simulation of the cross-linking reaction. The anisotropy of the radius of gyration in deformed networks determined from SANS has been analyzed by the theory of Ullman. It was found that the number of active cross-links per chain is in agreement with MC and that the chain deformation follows phantom behaviour. The local orientation as measured by 2H NMR is related to the global anisotropy of the network by a MC calculation of oriented chains. The NMR line shape of the deformed network is analyzed in terms of two relaxation processes arising from interior parts of the chains and from segments at chain ends. The mobility of both decrease with strain. It was found that the orientation connected to the first process shows the classical strain dependence of rubber elasticity,whereas the second exhibits a weaker dependence on strain.