Blends crystallization rates

The crystalliza tion resistance of vulcaniza tes can be measured by following hardness or compression set at low temperature over a period of time. The stress in a compression set test accelerates crystallization. Often the curve of compression set with time has an S shape, exhibiting a period of nucleation followed by rapid crystallization (Fig. 3). The mercaptan modified homopolymer, Du Pont Type W, is the fastest crystallizing, a sulfur modified homopolymer, GN, somewhat slower, and a sulfur modified low 2,3-dichlorobutadiene copolymer, GRT, and a mercaptan modified high dichlorobutadiene copolymer, WRT, are the slowest. The test is often mn near the temperature of maximum crystallization rate of —12° C (99). Crystallization is accelerated by polyester plasticizers and delayed with hydrocarbon oil plasticizers. Blending with hydrocarbon diene mbbers may retard crystallization and improve low temperature britdeness (100). 

Conclusively, the calculated Avrami exponents reveal a three-dimensional growth of the crystalline regions for each blend. The rate of crystallization of each blend increased with the decrease in crystallization temperature, and the rate of crystallization of the (PHB80-PET20)/PEN blend was faster than that of the (PHB 80-PET20)/PET blend. 

The overall crystallization behavior was studied by depolarized light intensity (DLI) and DSC (15,16,17). We observed a decrease in overall crystallization rate in the PBT-rich blends when the PET percentage... 

From the IR measurements we have obtained the crystallization halftimes of each component in the blends at the different temperatures they are plotted in Figures 13, 14, 15, 16, and 17. They can be transformed into the crystallization rate vs. temperature of crystallization for the different blends, which are plotted in Figures 18, 19, 20, and 21. From the density—IR correlation we obtained the ultimate degrees of crystallinity of each component in the blends, and their change with blend composition is plotted in Figures 22 and 23. Since the crystallization behavior varies with temperature of crystallization, we will approach its interpretation looking at the behavior at each crystallization temperature separately. 

Crystallization at 200°C. While normally PBT is known to have a faster crystallization rate than PET, at 200°C its undercooling is only 24°C while that of PET is 65°C. Therefore, the crystallization rate at 200°C is higher for PET than for PBT, which was already observed by DLI and now can be verified by IR and density. While the crystallization half-time of PBT in the blends remains quite constant, that of PET increases with PBT content, showing a decrease in the crystallization rate... 

Overall Crystallization Behavior. If instead of plotting the crystallization half times vs. blend composition we plot the crystallization rates for each component in the blends vs. temperature of crystallization, we obtain a series of curves which show a maximum. The right side of these curves is nucleation controlled while the left side is diffusion controlled (Figures 18 and 19). 

Both effects can be accounted for in terms of the change in the Tg of the blends caused by the introduction of the second component. In the PET case, the introduction of PBT in the blends lowers the Tg, causing the increase in crystallization rate and the shift of the maximum in crystallization rate towards lower temperatures. Similarly for the PBT component, the addition of PET increases the Tg which causes a decrease of the rate and a shift in the maximum towards higher temperatures. One can plot the crystallization rate vs. a reduced temperature that would account for the shift of Tg. This reduced temperature is defined as ... 

When doing so, we can observe how the maximum occurs at the same reduced temperature for the polymer and the blends, correcting, therefore, the shift in the crystallization rate curves (Figures 20 and 21). 

Whereas atactic PS is an amorphous polymer with a Tg of 100 CC, syndio-tactic PS is semicrystalline with a Tg similar to aPS and a Tm in the range 255-275 °C. The crystallization rate of sPS is comparable to that of polyethylene terephthalate). sPS exhibits a polymorphic crystalline behavior which is relevant for blend properties. In fact, it can crystallize in four main forms, a, (3, -y and 8. Several studies [8] based on FTIR, Raman and solid-state NMR spectroscopy and WAXD, led the a and (3 forms to be assigned to a trans-planar zig-zag molecular chain having a (TTTT) conformation, whereas the y and 8 forms contain a helical chain with (TTG G )2 or (G+G+TT)2 conformations. In turn, on the basis of WAXD results, the a form is said to comply with a unitary hexagonal cell [9] or with a rhombohedral cell [10]. Furthermore, two distinct modifications called a and a" were devised, and assigned to two limiting disordered and ordered forms, respectively [10]. 

A limited number of patents concern sPS blend which are not compatible. In these cases the properties that are described are functional ones and not related to the poor toughness of sPS. For instance, blends of sPS and partially saponified ethylene-vinyl acetate copolymers exhibit improved gas barrier properties (entry 11) small amounts of sPS added to polyethylene terephthalate) (although the patent actually claims a wide range of compositions) are useful to increase the polyester crystallization rate (entry 5). 

Figure 20.3 Spherulite growth rate (G) for sPS/PPE and sPS/PVME blends as a function of the crystallization temperature Tci ( ) sPS ( ) sPS/PPE 90 10 ( ) sPS/ PPE 80 20 (A) sPS/PVME 80 20 ( ) sPS/PVME 70 30 ( ) sPS/PVME 50 50. Reprinted from Polymer, vol. 34, Cimmino S., Di Pace E., Martuscelli E., Silvestre C., sPS based blends crystallization and phase structure , p. 2799, Copyright 1993, with permission from Elsevier Science.

In the sPS/PPE (75 25 wt%) blend, the crystallization rate vs 7 shows a maximum at 238 °C close to the value for neat sPS, but decreases with increasing the PPE content. Moreover, crystallization starts after an initial delay that increases with increase in T ax (from 290 to 340 CC). 

Gausepohl et ah [31] investigated the behavior of blends between sPS and random styrene-l,l-diphenylethylene copolymers obtained by anionic synthesis. The blends were miscible for copolymer contents of 1,1-diphenylethylene lower than 15 wt% as indicated by the occurrence of a single Tg (114°C). Tm and crystallization rate were not influenced. 

The crystallization rate constant k) is a combination of nucleation and growth rate constants, and is a strong function of temperature (47). The numerical value of k is directly related to the half time of crystallization, ti/2, and therefore, the overall rate of crystallization (50). For example, Herrera et al. (21) analyzed crystallization of milkfat, pure TAG fraction of milkfat, and blends of high- and low-melting milk-fat fractions at temperatures from 10°C to 30°C using the Avrami equation. The n values were found to fall between 2.8 and 3. 0 regardless of the temperature and type of fat used. For temperatures above 25°C, a finite induction time for crystallization was observed, whereas for temperatures below 25°C, no induction time was... 

Quality control systems usually used for judging the quality of oils and fats or oil blends used in margarine production could evaluate color, color stabihty, flavor, flavor stabihty, free fatty acid, peroxide value, active oxygen method (AOM) stabihty, iodine value, shp melting point, fatty acid composition, refractive index, crystallization rate, and sohd fat/temperature relationship (solid fat index) (5, 91, 112, 113). 

There are a number of important factors governing the change of the crystallization rate and semicrystalline stracture of a polymer in blend systems. Those include the degree of miscibility of the constituent polymers, their concentration, their glass-transition and melting temperamre, the phase morphology and the interface structure in the case of immiscible blends, etc. 

The type of added component is also important. Crystallization in the presence of an amorphous component gives rise to segregation. The segregation can occur into three regions interspherulitic, interfibriUar and interlameUar, depending on the ratio of the dilfusion rate of the amorphous component and of the crystallization rate of the crystal-hzable component. In blends of two crystaUizable polymers the phenomena such as separate, concurrent and co-crystaUization may take place. 

The effect of blending on the overall crystallization rate is the net combined effect of the nucleation and sphemlite growth. Martuscelli [1984] observed that in blends of PP with LDPE, crystallized at a T high enough to prevent any LDPE crystallization, the overall rate of crystallization of the PP matrix... 

A different case has also been explored by Martuscelli [1984] for PA-6 blended with an EPR-rubber. As shown in Figure 3.40, the PA-6/EPR blend decreased (faster overall crystallization rate) as the content of the rubbery phase increased, especially at lower concentrations of the EPR phase. 

Long et al. [1991] investigated the crystaUiza-tion behavior in blends of PP with LLDPE. They found the crystallization temperature of the PP matrix, T, to decrease slightly upon the addition of LLDPE. However, the degree of crystaUinity, X, and the spheruUte growth rate, G, were not affected. The authors concluded that the overall crystaUiza-tion rate of PP in the matrix decreased due to a decreasing primary nuclei density. The latter was confirmed in O. M. experiments by the increased size of the PP spherulites upon the addition of LLDPE. However, Zhou and Hay [1993] reported that with the addition of LLDPE to PP, the crystallization rate remained similar as for the PP homopolymer. 

Zhou and Hay [1993] investigated the crystallization behavior in LLDPE/PP blends. The crystallization rate of the LLDPE matrix, measured from isothermal DSC experiments, was not affected by the dispersed PP domains. However, its degree of crystallinity slightly decreased with increasing PP content in the blend. According to the authors, this could be ascribed to the lower degree of perfection of the LLDPE crystals. 

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