Crystallization rate of PPS

PPS with polyesteramide-type LCP accelerated crystallization rate of PPS Minkova et al., 1992... 

Long et al. (1991) investigated the crystallization behavior in blends of PP with LLDPE. They fotmd the crystallization temperature of the PP matrix, T, to decrease slightly upon the addition of LLDPE. However, the degree of crystallinity, Xc, and the spherulite growth rate, G, were not affected. The authors concluded that the overall crystallization 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 sphemlites 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. 

Nadkami and Jog (1986), Nadkami et al. (1987), and Jog et al. (1993) investigated the crystallization in blends of PPS with three types of HOPE, having a different melt flow index. In contrast to the PPS/PET blends, PPS crystallizes now in a superheated HOPE melt environment. Erom the dynamic cooling experiments, it was found that the presence of the HDPE melt suppresses the crystallization of PPS. The crystal growth rate, G, of PPS was found to remain unchanged, but its nucleation density was reduced as the concentration of HDPE in the blend increased or when the melt viscosity of the HDPE phase decreased. As a consequence, the overall crystallization rate of PPS was found to be retarded. 

The molecular weight of iPP influences crystallizatioa The higher the molecular weight of iPP the higher the concentration of the P-phase. The methyl side groups in PP restrict the movement of PP molecules and the resulting crystallization rate of PP is much slower than that of PE. ... 

In the DSC cooling of PP from the melt, crystallization occurs. The minimum of the exothermic peak defines the crystallization temperature. This temperature is an indication of how rapidly the PP crystallizes. The higher the temperature, the more rapid the crystallization. Nucleating agents added to PP increase the crystallization rate of PP, resulting in a higher crystallization temperature. PP crystallizes such that crystalline structures called spherulites are formed. Nucle-ation results in the formation of smaller spherulites than would otherwise have been formed. This, importantly, results in increased clarity and stiffness but also imparts some possibly undesirable features, such as warpage or brittleness. 

Figures 6.8 and 6.9 are the X(T) - T relationship diagrams of pure PP and 5% whisker-filled PP at different cooling rates. It can be seen that the crystallization rates of both pure PP and 5% whisker-filled PP increase with an increasing cooling rate. That is to say, the higher the cooling rate, the higher the crystallization rate of PP.

As can be seen from Table 6.2, at the same 3> value, ty2 values of 5% whisker-filled PP are all smaller than those of pure PP. Furthermore, the larger the C> value, the smaller are the ty2 values of pure PP and 5% filled PP, indicating that the crystallization rates of PP are improved by the addition of calcium carbonate whiskers, and the larger the cooling rate, the faster is the crystallization rate. 

Figure 6.12 shows the crystalHzation curves of pure PP and 5% filled PP at coohng rates of 5°C/mlnute and 20°C/min-ute. At a 5°C/minute cooling rate, the crystallization rate of whisker-filled PP is obviously faster than that of pure PP, but at a 20°C/minute cooling rate, they are similar, indicating that calcium carbonate whiskers have a greater impact on the crystallization rates of PP at slower cooling rates. 

The isothermal crystallization of PPS in the presence of glass carbon and aramid fibers has been reported by Desio and Rebenfeld [74]. They have used DSC for the crystallization rate measurement. They observed that the presence of aramid and carbon fibers increased the crystallization rate of PPS significantly over a temperature range of 215-245°C, whereas for glass fibers, the effect was observed only at high crystallization temperatures. The differences in the crystallization rates were attributed to the differences in the physical and chemical structure of the fibers. In another study, Desio and Rebenfield [75] have reported that the primary crystallization was sensitive to fiber reinforcement, whereas the secondary crystallization process was independent of the type of the fiber. 

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. 

Shingankuli [1990] studied the crystallization behavior of PP in the presence of solidified PVDF domains. A higher crystallization temperature of the PP matrix phase was observed, indicating an enhanced nucleation in the blends. The degree of crystallinity of PP was found to increase by about 30 to 40% with increasing PVDF content. Isothermal crystallization studies also confirmed the acceleration of the overall crystallization rate in terms of shorter crystallization half-times for PP. 

The crystallization rate of ternary blends is slower than that of the binary blends, and the ternary blends, which include Zn-neutrahzed ionomer, showed slower crystallization rate than ternary blends that included Na-neutrahzed ionomer (35). According to the literature, IPNs that possess physical interlocking at interfaces, strongly restrict crystallization (16,37). This IPN structure is postulated for the dynamically vulcanized EPDM and ionomers, especially for the blends containing Zn-neutralized ionomer. When the ionomer content was 5 wt%, the PP and EPDM blends are incompatible, that is, their phases are separated and the domain of EPDM was peeled off from the continuous matrix of PP (35). For the dynamically vulcanized EPDM and PP/ionomer ternary blends with 15 wt% ionomer, compatibiliza-tion was achieved between the PP and EPDM phases. The Zn-neutralized ionomer showed a much better compatibilizing effect than Na-neutralized ionomer. 

The calorimetric characteristics of iPP phase of the uncompatibilized blends show that the presence of the SBH dispersed phase leads to a slight increase of the PP temperature of crystallization (Fig. 17.8) (37,38). This result can be interpreted by a slightly increased nucleation rate of PP phase in the presence of SBH dispersed particles. As seen in Fig. 17.8, iPP temperature of crystallization increases drastically in the presence of COPP70 and COPP50. The analysis of the X-ray patterns (38) (Fig. 17.9a and b) and calculated parameters (dm values, degree of crystallinity, crystallite size, and intensity ratio) allows the assumption that PP segments of... 

The crystallization behavior and kinetics under isothermal conditions of iPP/SBH and HDPE/SBH blends, compatibilized with PP-g-SBH and PE-g-SBH copolymers, respectively, have been investigated (71). It has been established that the LCP dispersed phase in the blends plays a nucleation role for the polyolefin matrix crystallization. This effect is more pronounced in the polypropylene matrix than in the polyethylene matrix, due to the lower crystallization rate of the former. The addition of PP-g-SBH copolymers (2.5-10 wt%) to 90/10 and 80/20 iPP/SBH blends provokes a drastic increase of the overall crystallization rate of the iPP matrix and of the degree of crystallinity. Table 17.4 collects the isothermal crystallization parameters for uncompatibilized and compatibilized iPP/SBH blends (71). On the contrary, the addition of PE-g-SBH copolymers (COP or COP 120) (2.5-8 wt%) to 80/20 HDPE/SBH blends almost does not change or only slightly decreases the PE overall crystallization rate (71). This is due to some difference in the compatibilization mechanism and efficiency of both types of graft copolymers (PP-g-SBH and PE-g-SBH). The two polyolefin-g-SBH copolymers migrate to blend interfaces and... 

In blends of PPS with hyperbranched PPS, the crystallization temperature of PPS decreases by the addition of the hyperbranched PPS [77]. Also the rate of non-isothermal crystallization decreases as the energy of activation increases. 

Natural fibers also act as nucleating agents and enhance the crystallization rate of polymer as reported for PP. The fiber surface roughness of the fiber and thermal stresses were found to facilitate the growth of transcrystallinity on cotton fibers, whereas bamboo fibers induced significant amount of beta form crystals and transcrystalline growth of maleated PP. 

Thermal properties of PP, neat blend, MFBs and MFCs were studied using a Mettler Toledo DSC 822 at a heating and coohng rate of 5°C/min. The samples were heated up to a maximum temperature of 200°C, held there for 3 min, and then allowed to cool to room temperature to analyze the non-isothermal crystallization behavior of PP component. 

To further study the influences of calcium carbonate whiskers on the crystallization performance of PP, we studied the crystallization dynamics of pure PP and 5% whisker-filled PP. The dynamic DSC curves of these two materials are determined, as shown in Figures 6.4 and 6.5, respectively. The numbers in the figures represent cooling rates. 

FIG. 9 Variation of MW, carbonyl index (a), crystalline structure (b) and rate of crystallization (c) of PP with exposure time [137]. 

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