Lower temperature transitions

In the foregoing, attention has been concentrated on the main glass to rubber transition and the P-transition. To these we have ascribed the molecular motions equivalent to full 360° rotation and of libration (partial rotation). Of course many other types of low energy conformation change are possible, giving rise to the y, 6,8 and other transitions. 

It turns out that above the relevant transition temperature this occurs almost as easily in a solid polymer as in liquid cyclohexane. 

These simple conformational processes are thermally activated and follow the simple Arrhenius equation.They are not limited by deficiencies in free volume, as the cyclohexyl case exemplifies. However, since they have a very low activation energy barrier, the Arrhenius plot has a very low slope and the transition temperature does vary significantly with the rate or timescale of the observation or use. 

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The lower-temperature transition is sensitive to structural changes, and such changes usually parallel changes in the material s Tr The T is generally affected much less, compared with the lower-temperature transition, by structural changes. 

In addition, Viswanathan and Miehe (1978) observed a second lower temperature transition at 64°C in Pb3(As04)2 from the b phase to a c polymorph with space group P2 lc. Bismayer et al. (1986) have shown that this b-Xo-c transition is sharply discontinuous for the endmember arsenate (x = 1.0). The first-order nature of this transition is confirmed by a temperature hysteresis of 14°C. However, the transformation becomes continuous with increasing P content, as can be seen from the dependence of birefringence on temperature for different compositions in the Pb3(Pi jcAsjc04)2 series (Fig. 20). 

Lowering the frequency shifts the temperature of a transition to a lower temperature (Fig. 6). At one time, it was suggested that multiple frequencies could be used, and the Tg should then be determined by extrapolation to 0 Hz. This was never really accepted as it represented a fairly large increase in testing time for a small improvement in accuracy. For most polymer systems, for very precise measurements, one uses a DSC. Different types of transitions also have different frequency dependencies. If one looks at the slope of the temperature dependence of transitions against frequency, one sees that in many cases that the primary transitions like and Tg have a different dependence on frequency than the lower temperature transitions. In fact, the activation energies are different for ot, (3, and y transitions because of the different motions required and the transitions can be sorted by this approach. ... 

Nature of the Lower Temperature Transition. The complex nonexponential phosphorescence decays, apparent under the higher time resolution afforded by use of the modified 199 spectrometer are a consequence of interjection by the polymer matrix in the photophysies experienced by the chromophore. Non-exponential decays of triplet naphthalene (and other chromophore) emissions have been observed in PMMA. Horie et al.(18-20) ascribe such effects to dynamic, intermolecular quenching of the excited state by the polymer whereas MacCallum et al(21-23) invoke an energy migrative process within the polymer following quenching of the triplet state of the naphthalene. 

Data of Ref. DSC curves were only registered from 270 to 600 K and lower temperature transitions may have been omitted for example for n = 15 in Ref one can find additional solid-solid transitions at 326 and 346 K with entropy changes of 9.8 and 17.1 J/(K mol)... 

The experimental evidence from diffraction indicates that solid TeFe has two solid phases, a low-temperature phase with an orientationally-ordered, base-centered monoclinic structure and C h symmetry, and a higher-temperature phase with a body-centered cubic structure and cubic Oh symmetry. The experiments with clusters of TeFs, done by electron diffraction, show both of these and at temperatures between the regions of stability of these phases, a monoclinic phase with only partial orientational order as well. Simulations of the clusters show that the transition from body-centered cubic to monoclinic involves both rotational and translational motion of the motion, while the lower-temperature transition, between monoclinic and base-centered monoclinic, requires only coupling of the molecular rotational motions.Furthermore the transition at higher temperature in clusters shows dynamic coexistence of two phases in equilibrium over a range of at least a few degrees of temperature, which immediately implies that this transition involves two local minima in the... 

The DTA results in Rgure 8.12 show 7 at 290 C while the TBA shows first an exothermic, then an endothermic decomposition at 356°C and 400°C, respectively, corresponding to the weight loss shown by the TGA study. Cellulose triacetate is known to have three second-order transitions (37) it is not clear whether the lower temperature transitions associated with the DTA plot represent these or other motions (38). 

As the temperature of a polymer is lowered continuously, the sample may exhibit several second-order transitions. By custom, the glass transition is designated the a transition, and successively lower temperature transitions are called the - - transitions. One important second-order transition appears above Tg, designated the (liquid-liquid) transition. Of course, if the polymer is semicrystalhne, it will also melt at a temperature above Tg. 

C—C bonds). NaCN is also used in the extraction of Ag and Au (see eq. 22.4 and Box 22.2). At 298 K, NaCN and KCN adopt an NaCl-type structure, each [CN] ion freely rotating (or having random orientations) about a fixed point in the lattice and having an effective ionic radius of 190 pm. At lower temperatures, transitions to structures of lower symmetry occur, e.g. NaCN undergoes a cubic to hexagonal transition below 283 K. Crystals of NaCN and KCN are deliquescent, and both salts are soluble in water and are highly toxic. Fusion of KCN and sulfur gives potassium thiocyanate, KSCN. 

The Tg results from the peaks of the DMA loss tangents are compared with the Tg values from DSC measurements in Fig. 6.16. The Tg values obtained from DSC are in fair agreement with the Tgs based on all the lower temperature transitions obtained by DMA. Increasing the sulfonic acid content of the SPTES polymers dramatically decreases the Tg, as indicated by the observed leftward shift of the loss tangent peak with sulfonic acid content, from a value of 223°C for SPTES-50 polymer to 202°C for SPTES-100 polymer. 

As in the case of the lower temperature transitions discussed in the previous section, the glass transition temperature can be determined in these measurements by the onset of the more rapid diffusional motion and the consequent drastic reduction in the fluorescence intensity. 

Note 2—For a relatively narrow cut petroleum wax, the lowest transition will be a solid-solid transition. A narrow cut wax is one obtained by deoiling a single petroleum distillate with a maximum range of 120 F between its S % and 93 % vol in accordance with Test Method D 1160 boiling points (converted to 760 torr). The DSC method cannot differentiate between solid-liquid and solid-solid transitions. Such information must be predetermined by other techniques. In the case of blends, the lower temperature transition may be envelopes of both solid-liquid and solid-solid transitions. 

Dielectric, NMR and low frequency dynamic mechanical studies indicate that the transition detected acoustically at 193 K can be ascribed to the onset of oscillatory motion of the phenyl group. Transitions observed at 383 K and 373 K are attributed to the glass transition and this assignment has been confirmed by dilatometry measurements. The observation of two transitions leads to the suggestion that clustering occurs in the solid phase. The lower temperature transition would therefore correspond to the onset of collective motions of polymer chains in the amorphous regions. The low temperature dependence of the modulus between 373 K and 383 K is believed to be... 

B. Maxwell Yes, I d like to comment on that. I submitted the yield stress data as part of a paper entitled "Stress Relaxation Studies in the Tn Region to a prominent journal about a year and a half ago. When I received the reviews, one of the reviewers asked, "What is this foolishness about this thing called yield stress because there really isn t any yield stress, rather these are viscoelastic plateaus." However, it shows up clearly I don t know if it s the higher or the lower temperature transition that shows up, but "a" Tn shows up beautifully. 

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