Restricted Molecular Dynamics of Polymer Chains by Means of NMR Field Cycling Relaxometry

Abstract

Summary: The molecular dynamics of polymer melts with molecular weights above their critical values have been described by a power law dependence of the spin‐lattice relaxation time on the Larmor frequency. Three different regimes of power laws describing the relaxation dispersion of polymers for the chain mode dynamics are distinguishable and have been observed experimentally for several types of polymers. In this study, the influence of chemical and physical constraints on the relaxation dispersion of polymers was investigated. This included the introduction of chemical cross‐links and filler particles to produce elastomers, for which further constraints were imposed by mechanical deformation. As an alternative means of affecting molecular mobility, investigations were also carried out for poly(styrene‐block‐butadiene) diblock and poly(styrene‐block‐butadiene‐block‐styrene) triblock copolymers. Reducing the molecular degree of freedom of the chain, i.e., by introducing cross‐links between different segments, was shown to affect the time scale of motion but left the spectrum of the reorientational dynamics virtually unchanged. Only a small effect was found on the addition of filler particles into the elastomer network. On the other hand, applying macroscopic deformation to natural rubber and butadiene rubber samples is shown to alter also the spectrum of reorientations. Generating chemical fixations to immobile microdomains by copolymerizing a soft butadiene phase with a hard polystyrene phase to produce a biphasic lamellar structure is demonstrated to have a comparable effect. The change in slow molecular motion in the kHz to MHz range is discussed using the corresponding frequency dependence of the NMR spin‐lattice relaxation time, and a common description combining measurements at different temperatures is presented.Master curves generated from relaxation dispersions at different temperatures for a BR1/1 sample filled with 30 phr of carbon black under deformation with λ ≈ 3.5 (compared to an unstretched sample).magnified imageMaster curves generated from relaxation dispersions at different temperatures for a BR1/1 sample filled with 30 phr of carbon black under deformation with λ ≈ 3.5 (compared to an unstretched sample).

Dependence of Order and Dynamics in Polymers and Elastomers under Deformation Revealed by NMR Techniques

“…They have been identified and confirmed experimentally for a number of polymers, among them polybutadiene and polydimethylsiloxane [6,7]. The exponents for polyisoprene were found to be somewhat different [8][9][10], but a power-law was still observed. The same is true for elastomers and networks consisting, for instance, of chains of polyisoprene or polybutadiene [8,9,[11][12][13].…”

“…The exponents for polyisoprene were found to be somewhat different [8][9][10], but a power-law was still observed. The same is true for elastomers and networks consisting, for instance, of chains of polyisoprene or polybutadiene [8,9,[11][12][13]. In semi-dilute solutions, or polymers with molecular weights below the critical value, chains are not affected by entanglements and the frequency dependence changes toward Rouse behavior, i.e.…”

“…, where the constants C 1 , C 2 were determined empirically with T 0 set to 296 K [10]. The curves were reconstructed from original data obtained between 275 K and 333 K [9,11].…”

“…The curves were reconstructed from original data obtained between 275 K and 333 K [9,11]. While γ = 0.16 for the undeformed rubber -a value that is somewhat lower than the one found for other polymers but is still assigned to region II -an increase to λ = 0.24 is observed for stretching at λ ≈ 7, and intermediate slopes for smaller stretching ratios.…”

“…Because of the small available sample volume, an angular dependence of the T 1 dispersion could not be investigated, but deformation parallel and perpendicular to the magnetic field axis revealed indistinguishable results. Likewise, a direct influence of the presence of nanocrystallites to the relaxation behavior could be ruled out because of the dead-time chosen for the receiver, but even indirect influences due to mixing by spin diffusion is unlikely [11], while the reduction in mobility of chains in the amorphous phase by the presence of a crystalline phase might indeed play a role, although the similarity to the effect found for PDMS rubber that does not crystallize speaks against this. Again, the relaxation effect can directly be translated into a change of the shape of the segmental motion's autocorrelation function, as only intramolecular contributions were identified as the responsible mechanism for the observed 1 H relaxation [10].…”