Cross-relaxation isotactic

The initial relaxation results on PP over a temperature range from 24°C to -195°C are summarized in Figure 13. The T3 data were collected using a pulse sequence developed by Torchia (44) which allows cross-polarization enhancement of the signals. The T] p data were determined at 58 kHz using Tj p methodology of Schaefer et al. (1). As indicated in the figure, each of the carbons display individual relaxation rates in both types of experiments. The CH and CH2 carbons have a T minimum at ca. -110°C, nearly the same temperature as that reported by McBrierty et al.(45) for the proton Ti minimum in isotactic PP. 

H and 13C NMR relaxation studies in concentrated chloroform solution307 308 and 13C relaxation studies of cross-linked gels in benzene- 309 were carried out on PMMA. The dependence of the segmental mobility on stereosequence was not affected by the presence of covalent cross-links in the chain. The isotactic sequences are more mobile than the syndiotactic sequences.309... 

C NMR spin-lattice relaxations in solid PMMAs of different tacticity were studied by CP/MAS NMR spectroscopy.324-328 The relaxation proceeds more slowly in isotactic PMMA than in syndiotactic PMMA even in the solid state this is because of the faster reorientation of the a-CH3 group in the isotactic chain.135 The nC-T times of the a-methyl carbons of PMMAs with different tacticities were measured over a wide range of temperatures by cross polarization, magic-angle spinning, dipole dephasing (CP/DD)... 

These points can be illustrated by comparing the elastic relaxation modulus ,(0 for crystalline (isotactic), amorphous, and chemically cross-hnked (atactic) polystyrene samples, as shown in Figure 15.9. Crystalhnity has little effect below Tg, but as the molecular motion increases above T, the modulus of the amorphous polymCT drops more sharply. The value of E t) remains high for the crystalline polymer throughout this range until the rapid decrease at the melting temperature is recorded. The cross-hnked sample maintains its modulus level at this temperature as the crosslinks are not thermally labile and do not melt. 

To illustrate the effect of temperature on mechanical properties, it is sometimes preferable to plot the property vs. temperature for constant values of time. For example, data of the type shown in Fig. 18.21 may be cross-plotted as (10) (the 10-second relaxation modulus) vs. T, Such a plot is given in Fig. 18.23 for several polystyrene samples," The five regions of viscoelastic behavior are evident in the linear, amorphous (atactic) samples (A) and (C) along with the effect of molecular weight in the flow region. The drop in modulus in the vicinity of Tg (100°C) is dearly seen. The crystalline (isotactic) sample maintains a fairly high modulus all the way up to (a 235 "C). Given values of one can convert data in the form vs, t at constant T (a master curve) to vs. T at constant t and vice versa. 

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