We directly measure the equilibrium length scale of dynamic heterogeneities close to the glass transition by means of a new multidimensional NMR experiment. The spatial information is gained from a proton spin diffusion experiment combined with two 2D 13 C exchange sequences via appropriate back and forth transfer of magnetization between 13 C and 1 H spins. For poly(vinyl acetate) at 10 K above the glass transition we detected a length scale of 3 6 1 nm. [S0031-9007(98)07244-5] PACS numbers: 64.70.Pf, 76.60. -k Supercooled amorphous systems exhibit a complex dynamic behavior (for a recent review, see Ref. [1], and references therein). For most glass-forming systems this results in highly nonexponential a relaxation. Recent experiments have demonstrated that the nonexponential time behavior is related to a superposition of relaxation processes, each characterized by an individual rate hence giving rise to the notion of dynamic heterogeneities [2-9], composed of slow and fast relaxators. Similar results have also been observed in polymer simulations [10].Hence dynamic heterogeneities are an essential ingredient of the glass transition. Two characteristic properties of dynamic heterogeneities-time scale of fluctuations of rates and length scale j het of temporary clusters of slow segments-are sketched in Fig. 1.Information about the time scale of fluctuations within the distribution of reorientation rates has already been obtained from reduced four-dimensional solid state exchange NMR (4D experiment) [3,6,7,11] and from photobleaching [5]. Application of the 4D experiment to a variety of polymers [3,6,11] and low-molar glass formers [7] has clearly shown that the fluctuations within the heterogeneous rate distribution occur on the same time scale as the relaxation process itself.The length scale j het of the dynamic heterogeneities may be related to that of cooperatively rearranging regions (CRRs), first postulated by Adam and Gibbs [12,13]. It is reasonable to assume that j het is an upper limit for the length scale of CRRs. As j het is a fundamental parameter of the glass transition, several attempts to determine its value have been reported. Most approaches, however, involve perturbations of the system itself. Either the liquids are confined to pores [14][15][16] or probe molecules are added to the sample [17][18][19]. Information about the characteristic length scale is derived from the dependence of the a-relaxation properties on the size of the confining structure or the probe molecule. Interpretation of these experiments has to take into account that, e.g., the relaxation in pores strongly depends on the surface properties of the host [15,16] and that large probe molecules may change the local dynamics. Within the model of cooperatively rearranging regions Donth has estimated the length scale of these regions to be close to 2 nm [13,20].Obviously it is desirable to have a method which allows one to directly measure the length scale of dynamic heterogeneities of the glass transition in a nonperturbative way. ...

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For the first time, new catalysts for olefin polymerization have been discovered through the application of fully integrated high-throughput primary and secondary screening techniques supported by rapid polymer characterization methods. Microscale 1-octene primary screening polymerization experiments combining arrays of ligands with reactive metal complexes M(CH(2)Ph)(4) (M = Zr, Hf) and multiple activation conditions represent a new high-throughput technique for discovering novel group (IV) polymerization catalysts. The primary screening methods described here have been validated using a commercially relevant polyolefin catalyst, and implemented rapidly to discover the new amide-ether based hafnium catalyst [eta(2)-(N,O)[bond](2-MeO[bond]C(6)H(4))(2,4,6-Me(3)C(6)H(2))N]Hf(CH(2)Ph)(3) (1), which is capable of polymerizing 1-octene to high conversion. The molecular structure of 1 has been determined by X-ray diffraction. Larger scale secondary screening experiments performed on a focused 96-member amine-ether library demonstrated the versatile high temperature ethylene-1-octene copolymerization capabilities of this catalyst class, and led to significant performance improvements over the initial primary screening discovery. Conventional one gallon batch reactor copolymerizations performed using selected amide-ether hafnium compounds confirmed the performance features of this new catalyst class, serving to fully validate the experimental results from the high-throughput approaches described herein.

A study of the reorientational segmental dynamics in supercooled poly(vinyl acetate) is presented, yielding detailed information about geometry and time scale of the motion close to the glass transition. The geometry information is derived from systematic variation of the evolution time in 13C 2D echo NMR measurements. The dynamics can be described as a superposition of angular jumps of approximately 10° and rotational diffusional processes. Both processes are related to the macroscopic α-relaxation. On the time scale of one jump process the orientation of a segment changes by about 2° via small step diffusion (<0.6°). Furthermore the temperature dependence of this reorientatinal scenario is analyzed within the limits imposed by the experiment. All results are compared with previous 2H 2D NMR measurements on low-molecular glass formers. In both cases the loss of correlation, as described by conventional correlation times, results from a sequence of many distinct reorientational steps.

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Site specific 13C labeling of anhydrous glucose is used to study the time scale and geometry of reorientational motion of the exocyclic CH2OH group in relation to the main glucose ring. By comparison of 2D echo decay NMR experiments with Monte Carlo simulations a bimodal distribution of jump angles, a 75% fraction of 1−2° jumps and a 25% of 7−8° jumps, is found to describe the geometry of the reorientational processes of the main ring. For the CH2OH group the average jump angle of the larger jump process is somewhat larger. The jump rates for both the CH2OH group and the ring are similar. The apparent activation energy determined for the rotational motion of the CH2OH group and the ring is 480 ± 40 kJ/mol, which is very similar to an earlier determination using viscometry. It is concluded that the glucose ring and the exocyclic CH2OH-group mobility are strongly correlated and that the rotational freedom of the CH2OH group should not be used to explain the faster β-relaxation process also found for glucose.

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