Spin probing polymer lattices

There has been extensive effort in recent years to use coordinated experimental and simulation studies of polymer melts to better understand the connection between polymer motion and conformational dynamics. Although no experimental method directly measures conformational dynamics, several experimental probes of molecular motion are spatially local or are sensitive to local motions in polymers. Coordinated simulation and experimental studies of local motion in polymers have been conducted for dielectric relaxation,152-158 dynamic neutron scattering,157,159-164 and NMR spin-lattice relaxation.17,152,165-168 A particularly important outcome of these studies is the improved understanding of the relationship between the probed motions of the polymer chains and the underlying conformational dynamics that leads to observed motions. In the following discussion, we will focus on the... 

These traps, (Fig. 6) and similar effects in the motion of holes and other charges through polymers, would eventually be correlated also with such structural probes as positron lifetimes in macromolecular solids. Extensive recent studies of positron lifetime are based on positronium decay. In this, the lifetime of o-positronium (bound positron-electron pair with total spin one) is reduced from about 140 nanoseconds to a few nanoseconds by "pick-off annihilation" in which some unpaired electron spins in the medium cause conversion quenching of orthopositronium to para-positronium. The speed of the t2 effect is supposed, among other things, to represent by pick-off annihilation the presence of defects in the crystalline lattice. In any case, what amounts to empty space between molecules can then be occupied by orthopositronium.(14,15,16) It is now found in linear polyethylene, by T. T. Wang and his co-workers of Bell Laboratories(17) that there is marked shift in positron lifetimes over the temperature range of 80°K to 300°K. For... 

Solid-state NMR spectroscopy has been used to study polymer electrolytes [27, 259-266]. Among the various nuclei that have been used as probes 7Li and 23Na have received maximum attention. However, the quadrupolar nature of these nuclei compounded with the solid state of the sample result in considerable line broadening of the NMR signal. Thus, a lot of valuable information pertaining to the structure of polymer electrolytes is not accessible. However, an analysis of the linewidths and the spin lattice relaxation times of the nuclei affords considerable information on the nature of the ions present in a polymer electrolyte. 

Radiofrequency pulses are also utilized to measure relaxation times. Three relaxation times have been measured in TPEs, and each is sensitive to different phenomena. Ti, the spin-lattice relaxation time in the laboratory frame, is the relaxation from the nonequilibrium population distribution created by the pulse to the equilibrium Boltzmann distribution. Ti is sensitive to molecular motions that rate in the range of 10 -10 Hz. T2, the spin-spin relaxation time, is the relaxation caused by the establishment of equilibrium between nuclear spins within the system. Spin-spin relaxation measurements also probe motions with rates in the range of 10M0 Hz however, low frequency motions (lOMtPHz) also affect T2. Generally,T2 is one to three orders of magnitude smaller than Ti in solid polymers. Tip, the spin-lattice relaxation time in the rotating frame, probes motions with rates on the order of lO -KfHz. Cross polarization is usually used in Tip measurements. 

Figure 16 shows typical proton spin-lattice relaxation dispersion data for polyethylene melts as an illustration of the three-component behavior of polymer melts. For comparison with model theories the chain-mode regime represented by component B is suited best and will be discussed in detail. It will be shown that the NMR relaxometry frequency window of typically 10 Hz< V <10 Hz (for proton resonance) almost exclusively probes the influence of chain modes represented by component B (compare Fig. 5). That is, the correlation function experimentally relevant for spin-lattice relaxation dispersion may be identified with component B according to...

The total proton frequency range that can be probed by NMR spin-lattice relaxation techniques is 10 Hz < v < lO Hz. For the present application, deuteron resonance selectively applied to perdeuterated polymers confined to the pores is superior to proton resonance which, unlike the situation in field-gradient experiments, is affected by signals from the matrix and flip-flop spin diffusion across the matrix. The deuteron fi equency range is shifted by a factor of 0.15 to lower frequencies. This frequency window largely matches the time scale of chain modes of polymers with medium molecular masses. 

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