Storage modulus crystallization

The effect of the minor components was kinetic, rather than thermodynamic. Although the crystallization kinetics were altered, the structure and mechanical properties of milk fat were the same with or without the minor lipids. The samples reached the same SFC value and had a similar microstructure as observed visually and as characterized by the fractal dimension. The rheological properties of the fats were also similar. Neither the storage modulus nor the yield force was affected by removal of the minor components (Wright and Marangoni, 2003). Large changes in the... 

A secondary stereocomplex gel with the ratio [iso-PMMA]/[synd-PMMA] = 1/1 obtained from o-xylene solution showed an endothermic peak at 110 °C which was attributed to the melting point of the stereocomplex. The degree of crystallization was about 5-6%. Since this gel contained over 80% solvent, micro-Brownian motions of PMMA chains in the amorphous region were relatively unhindered. When the PMMA gel was cooled rapidly after melting above its Tm (i.e. above 145°C), first the loss modulus (c") changed relative to the viscosity of the system whereas the storage modulus began to increase after some time. This result confirms the assumption that first the polymer chains cannot be cross-linked but are weakly associated with one other, and then a... 

Above the -relaxation process, the 2,4-TDI/PTMO polymer displayed a short rubbery plateau at a storage modulus of about 5 MPa while 2,6-TDI/PTMO was capable of crystallization, as evidenced by the ac-loss process. This difference in dynamic mechanical properties demonstrates the effect of a symmetric diisocyanate structure upon soft-segment properties. As previously discussed, single urethane links can sometimes be incorporated into the soft-segment phase. The introduction of only one of these diisocyanate molecules between two long PTMO chains inhibits crystallization if the diisocyanate is asymmetric. In the case of a symmetric diisocyanate, soft-segment crystallization above Tg can readily occur. The crystals formed were found to melt about 30°C below the reported melting point for PTMO homopolymer, 37°-43°C (19), possibly because of disruption of the crystal structure by the bulky diisocyanate units. 

In practice, the fractal dunensions of fat crystal networks, are measured by determining the shear storage modulus, G, of the fat crystal networks within the LVR measured by small deformation rheology. The fractal dimension of the fat crystal network may be calculated from the slope of the curve as > = 3-1/slope of logarithm of G at different SFC, O. 

Fig. 5 Cold crystallization in PET caused a large increase in the storage modulus, E, above the Tg. A DSC scan of the same material is included. (From Ref. l) (View this art in color at www.dekker.com.)...

Figure 2. Mechanical loss tangent (tan 5) and elastic storage modulus at 102 c.p.s. and spin-lattice relaxation time (20) vs. temperature for poly (4-methyl-l-pentene) crystallized from dilute solution...

As the mix is frozen and aerated its viscosity increases both because the temperature is decreased and also because ice crystals and air bubbles are formed. The viscosity of an aerated mix increases according to Einstein s equation (equation 2.11) for low air volume fractions. Figure 7.22 shows what happens as an aerated ice cream mix is frozen. The storage modulus, G starts to rise at about — 0.5 C when the first ice crystals form, and continues to increase as the temperature decreases. This data was obtained by using a scraped surface heat exchanger that is simultaneously a rheometer G was calculated from the torque on the dasher. 

Elastomeric systems are quite commonly monitored by thermal techniques. Mohler [5] has discussed how DSC is used to characterize the quality of blends of elastomers. Changes in polymer Tg can point to miscible phases that might otherwise require a microscopic techmque to establish. The cold crystallization of a polymer in a blend can also be examined using the relaxation enthalpy of the sample. Vacuum TG can look at plasticizer content separate from polymer degradation. Dynamic mechanical analysis (DMA) is always a powerful technique with elastomers. Dimensional stability, storage modulus, loss modulus and loss factor are all important for this class of materials and are... 

For the above reasons, it is probably preferable that the samples be totally quenched to obtain a fully amorphous sample, and then to anneal the samples to achieve the correct crystallization profile. This is of course possible only for a polymer that can be quenched to its fully amorphous state. In some cases, the rate of crystallization for the polymers may be so high that some amount of crystallization is unavoidable. Most thermal analysis methods used for measuring crystallinity, such as the DSC or DMA methods, preclude the availability of data relating the heat of fusion or the storage modulus against the crystallinity [2]. Such data may not be always available especially for polymers that either do not fully crystallize or cannot be fully amorphous. An alternative method to determine the absolute crystallinity value would be to use the X-ray diffraction technique [. ]. 

Li et al. synthesized a PMMA-PEG semi-IPN by radical polymerization and cross-linking of PMMA in the presence of linear PEG, which exhibits two independent shape memory effects at two transition temperatures, the of the PEG crystal and the Tg of the semi-IPN [39]. In the IPN, a single Tg appeared due to the miscibility of the amorphous phase of the two polymers. Based on a reversible order-disorder transition of the crystals below and above the of PEG, and the large difference in storage modulus below and above the Tg of the semi-IPN, the polymer has a recovery ratio of 91 and 99%, respectively For the shape-memory behavior at the of PEG crystals, the fixing phase was the PMMA network and the reversible phase was PEG crystals. For the shape memory behavior at the Tg of the semi-IPNs, the fixing phase was the chemical cross-linked point, while the reversible phase was the PMMA-PEG complex phase. 

A considerable secondary crystallization, typical for PEEK, is seen in Fig. 6.99. Only about 50% of the total crystallinity can be assigned to primary crystallization. Secondary crystallization starts at about 28 h ( 10 s). At that time, the reversing amplitude of the storage modulus is almost constant. At later times, it decreases slowly, indicating very different processes for primary and secondary crystallization. 

An increase in the crystallinity [77,133,137] or interfacial crystallinity have been considered other parameters for improving the mechanical response of polymers [170]. Oxidized NDs effectively enhanced the crystallization ability of PVA, indicating the crystalline regions may act as physical crosslinks or filler particles. The increased crystallinity and reinforcing effect of oxidized NDs increased the storage modulus of all the PVA nanocomposites to a higher degree than that of neat PVA [48]. Sahoo... 

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