Slow motion in [ring-fluoro]polycarbonate by CODEX

Journal of Polymer Science Part B: Polymer Physics (2008) Volume 46 Issue 11, Pages 1062 - 1066

ABSTRACT
Chain dynamics in [ring-fluoro]polycarbonate (an A-B alternating copolymer that has a single fluorine substituent on every fourth main chain ring) have been characterized by centerband only detection of exchange (CODEX) and rotating-frame 13C spin-lattice relaxation. The slow motions detected by CODEX are facilitated by a mechanically active lattice reorganization that permits a flip of the fluorinated ring about its C2 axis. Nonfluorinated rings undergo small-amplitude reorientations and C2 flips, both of which are fast and not CODEX active.

(a) Pulse sequence for centerband only detection of exchange (CODEX). The open rectangles indicate pulses, and the solid rectangles, /2 pulses. The trigger for the /2 pulses was an optical reflection off a timing mark inscribed on the rotor body. (b) 19F pulses coincident with the 13C pulses at tr/2 to remove 13C-19F dipolar coupling from the generation of a CODEX signal.

The 25-MHz 13C CODEX spectra of [ring-fluoro]polycarbonate for a mixing time, tm = 175 ms. The spectra were obtained using the pulse sequence of Figure 1 (N = 8) with (red) and without (black) 19F pulses. The CODEX difference spectra S = S(tm 0, tz) - S(tm, tz 0) are shown at the top of the figure and the full-echo spectra S(tm 0, tz), at the bottom. The reference spectra were obtained with tm = 2.00 ms. Peak assignments are shown by the insets; the carbonyl carbons of [ring-fluoro]polycarbonate also contribute to the 150-ppm peak. Magic-angle spinning was at 4000 Hz. Each spectrum resulted from the accumulation of 16,384 scans.

Pure CODEX signals as a function of mixing time for four of the [ring-fluoro]polycarbonate peaks of Figure 2, using the pulse sequence of Figure 1 (N = 8) without 19F pulses. The red circles are for the 150-ppm peak which includes contributions from the carbonate carbonyl carbon and the nonprotonated aromatic carbons. The solid lines are the calculated fits assuming a stretched-exponential distribution of correlation times.

We conclude that a monofluoro substituent on every fourth ring of polycarbonate lattice adds significantly to the heterogeneity of packing. A single fluorine stiffens the packing of several nearby chains, which shifts the high-temperature component of the transition to even higher temperature. (A symmetrical difluoro-substituted ring has a much less pronounced effect.) The motions associated with this shift are CODEX active. The same fluorine also affects the packing of more distant chains, which shifts the low-temperature component of the transition (T < -100 ¡ÆC at 1 Hz) to higher temperature, and shortens the T1 times of the associated motions. The correlation between such a relatively minor molecular substitution and macroscopic mechanical observables implies the existence of local molecular order.