ISSN:
0887-6266
Keywords:
multiple quantum NMR
;
polymer dynamics
;
polycarbonate
;
Chemistry
;
Polymer and Materials Science
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
,
Physics
Notes:
Proton multiple quantum (MQ) nuclear magnetic resonance (NMR) is applied for the first time to probe chain dynamics in the polycarbonate of 1,1-dichloro-2,2-bis(4-hydroxyphenyl)ethylene (chloral) over a temperature range of 195-280 K. Since chloral polycarbonate contains only one essentially indistinguishable proton, any dynamical information is associated with the phenylene group motion, the dominant sub-Tg motion. First, the fractional multiple quantum (fMQ) signal which can be refocused at a fixed MQ preparation time decreases with increase in temperature from 195 to 200 K, associated with the onset of the phenylene π flips. Since π flips do not modulate dipolar couplings between phenylene protons at positions 2 and 3 on the same ring, this decrease in fMQ can be attributed to modulation of the remaining couplings on the time scale of the MQ experiment. Next, over the range 200-240 K, the fMQ maintains a reduced, nonfluctuating value. This suggests a broad distribution of correlation times exists for the π flips, since at any one temperature in this intermediate range, only a small portion of the phenylene groups undergo motion at the precise kHz frequency required to refocus the MQ signal. The increase in the signal strength as the temperature rises above 240 K is due to the majority of π flips occurring in the fast motional limit. These observations are in agreement with previous single quantum proton NMR studies on phenylene group rotation in choral polycarbonate. Furthermore these fMQ measurements show several similarities to those for PTFE (poly(tetrafluoroethylene)), measured by 19F MQ-NMR. One notable difference is that in as-polymerized PTFE at intermediate temperatures (200-220 K) rapid oscillations of fMQ were noted as the temperature was changed by a few degrees. Such behavior is not observed in the chloral polycarbonate. © 1994 John Wiley & Sons, Inc.
Additional Material:
5 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/polb.1994.090321310
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