Transition behavior

The previous sections have described the structural units and molecular weights of high molecular weight polymer chains. We have not, however, discussed the thermal and rheological behavior of polymers. Many polymers crystallize into well-ordered structures. Others do not crystallize and, at low temperatures, form high modulus disordered glasses. 

At high temperatures both crystalline and glassy thermoplastics transform into unstructured amorphous polymer melts. This occurs abruptly at melting temperature for crystalline thermoplastics and gradually above the glass temperature for glassy thermoplastics. 

There are other important commercial thermoplastics beyond polyolefins. There are the various vinyl polymers. Both atactic polystyrene and syndiotactic polystyrene have a Tg of 100 C. Syndiotactic polystyrene has a crystalline melting point of 270 °C. Poly(vinyl chloride) has both atactic (-85%) and syndiotactic (-15%) sections of chains depending upon polymerization conditions. Its Tg is 65 C and is higher than 200 °C. In addition to vinyl polymers, there is poly(methyl methacrylate), which is atactic and has Tg about 110 °C. 

The commercial polydienes are elastomers. Q s-1,4 polybutadiene has a Tg of -100 °C and has a crystalline melting point of less than 0 °C. Q s-1,4 polyisoprene has a Tg of -70 °C and has a crystalline melting point of 35 C. Both polymers crystallize rather slowly. Trans-1,4 polybutadiene and polyisoprenes are crystalline thermoplastics at room temperature. They are not, however, used commercially because of their poor aging characteristics relative to polyolefins. This is associated with the double bonds in their backbones. Polybutadienes with high atactic 1,2-contents have been widely used in the tire industry. Their Tg is about -15 °C. Isotactic and syndiotactic 1,2-polybutadienes are high melting crystalline thermoplastics, but age poorly compared to polyolefins. The 1,2-polybutadienes have been used as packaging for additives in the rubber industry. 

Aliphatic polyesters have crystaUine melting points lower than polyethylene. Polycaprolactone has a melting point of 50 °C. Placing phenyl rings in the backbone increases both and Tg significantly. Poly(ethylene terephthalate) with = 265 C and Tg = 65 °C and polyfbutylene terephthalate) with T = 210 °C and Tg = 35 C are both commercial thermoplastics (Formula 1.8). Closely related to these polymers is poly(bisphenol-A carbonate) (Formula 1.9) with Tg = 150 °C. This polymer usually does not crystallize. 

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DMPPO and polystyrene form compatible blends. The two components are miscible in all proportions (59). Reported dynamic—mechanical results that indicate the presence of two phases in some blends apparendy are caused by incomplete mixing (60). Transition behavior of thoroughly mixed blends indicates that the polymers are truly compatible on a segmental level (61). CompatibiUty may be attributed to a %— % interaction between the aromatic rings of the two polymers sufficient to produce a negative heat of mixing. However, the forces are very small, ie, = ca40 J/mol (9.6 cal/g), and any... 

On the other hand, polymeric materials show universal aspects of glass transition behavior, just like other materials. For instance, the classical Vogel-Fulcher behavior... 

Fig. 3.44 Example of the range deperident behavior of rule T20 the system evolves as class-2 for r=l, class-4 for r = 2 and class-3 for r 3. The various possible intermediate (or transitional) behaviors can be studied by successively applying we.ll-defined minimal topological deformations to the initial lattice.

Despite this apparent simplicity, however, we nonetheless observe a rich variety of transitional behaviors. Compare the very smooth decay of in figure 3.46-... 

Maynes and Webb (2002) presented pressure drop, velocity and rms profile data for water flowing in a tube 0.705 mm in diameter, in the range of Re = 500-5,000. The velocity distribution in the cross-section of the tube was obtained using the molecular tagging velocimetry technique. The profiles for Re = 550,700,1,240, and 1,600 showed excellent agreement with laminar flow theory, as presented in Fig. 3.2. The profiles showed transitional behavior at Re > 2,100. In the range Re = 550-2,100 the Poiseuille number was Po = 64. 

Allcock HR, Mang MN, Dembek AA, et al. Poly[(aryloxy)phosphaz enes] with phenylphenoxy and related bulky side groups, synthesis, thermal transition behavior, and optical properties [J]. Macromolecules, 1989, 22, 4179 190. 

After expression of poly(VPGXG) genes, the biopolymer can easily be purified from a cellular lysate via a simple centrifugation procedure, because of the inverse temperature transition behavior. This causes the ELPs to undergo a reversible phase transition from being soluble to insoluble upon raising the temperature above the and then back to soluble by lowering the temperature below Tt (Fig. 9). The insoluble form can be induced via addition of salt [27]. The inverse transition can... 

In a subsequent study, the effect of reducing the ELP molecular weight on the expression and purification of a fusion protein was investigated. Two ELPs, ELP [V-20] and ELP[VsA2G3-90], both with a transition temperature at 40°C in phosphate-buffered saline (PBS) containing 1 M NaCl, were applied for the purification of thioredoxin. Similar yields were observed for both fusion proteins, resulting in a higher thioredoxin yield for the ELP[V-20] fusion, since the ELP fraction was smaller. However, a more complex phase transition behavior was observed for this ELP and therefore a selection of an appropriate combination of salt concentration and solution temperature was required [39]. 

Fig. 9.13 Time evolution of the NFS intensity for various temperatures around the HS-LS transition of [Fe(tpa)(NCS)2]. The measurements were performed at 1D18, ESRF in hybrid-bunch mode. The left-hand side shows measurements in the transition region performed with decreasing temperature and the right-hand side with increasing temperature. (The spectral patterns at comparable temperatures do not match due to hysteresis in the spin-transition behavior). The points give the measured data and the curves are results from calculations performed with CONUSS [9, 10]. The dashed line drawn in the 133 K spectmm represents dynamical beating. (Taken from [41])...

With respect to the screening of hydrodynamic interactions, one is confronted with the occurrence of a multiple-transition behavior. Instead of the expected crossover from ordinary (unscreened) Zimm to enhanced Rouse relaxation, one observes, at increasing concentrations, additional transitions from enhanced Rouse to screened Zimm and from screened Zimm to enhanced Rouse relaxation. This sequence of crossover effects are highly indicative of an incomplete screening of hydrodynamic interactions. 

The glass transition involves additional phenomena which strongly affect the rheology (1) Short-time and long-time relaxation modes were found to shift with different temperature shift factors [93]. (2) The thermally introduced glass transition leads to a non-equilibrium state of the polymer [10]. Because of these, the gelation framework might be too simple to describe the transition behavior. 

The dynamic mechanical behavior of the block copolymers of HB and HI are typified by the results obtained for the HIBI series which are given in Figure 15A and B which display spectra for different composition ratios. The transition behavior of the HBIB series is so similar that it will not be repeated here. The samples used for this study were compression molded and they all had been stored at room temperature between one to two months before use. The experiments were run at 110 Hz. The behavior of HB, represented by HIBI 100, is similar to that given in Figure 14B. 

On the other hand, when associated with [Ni(dmit)2]-, [Fe(sal2-trien)]+ exhibits a cooperative spin transition behavior with a wide hysteresis loop (30 K) around... 

Cheng, S. Z. D., Pan, R. and Wunderlich, B Thermal analysis of polybutylene terephthalate for heat capacity, rigid-amorphous content, and transition behavior, Makromol. Chem., 189, 2443-2458 (1988). 

Simon, S.A. and T.J. McIntosh. 1984. Interdigitated hydrocarbon chain packing causes the biphasic transition behavior in lipid/alcohol suspensions. Biochim Biophys Acta 773 169-172. 

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