Optical crystallization kinetics

Figure 20.26 Polarized optical micrographs of isothermal crystallized (4h) melt blends (x400) (a) 210°C (b) 230°C (c) 210°C (d) 230°C [44], From Park, J. K Park, Y. H., Kim, D. J. and Kim, S. H., Crystallization kinetics of TLCP with polyester blends, J. Korean Fiber Soc., 37, 69-76 (2000). Reproduced with permission of The Korean Fiber Society...

Resolution of optical isomers via preferential crystallization is outlined in Chapter 7, Example 7-6, as an example of the use of tightly controlled supersaturation in a cooling crystallization. This process is discussed in greater detail in Example 11 -6. The process for resolution of optical isomers utilizes crystallization kinetics, instead of equilibrium solubility, to accomplish the desired isomer separation. It is a proven technique and has been in long-term... 

Crystalhzation studies in blends of iPP/POE reveal that the crystallization process of iPP is affected by the addition of POE and vice versa. It has been demonstrated how the POE promotes the nucleation and crystal growth processes of iPP, this effect being more appreciable at low POE concentration (< 10 wt% POE). Analysis of the crystallization kinetics of the iPP crystals isothermally crystallized at different temperatures in blends of iPP/POE is supported by the morphological observations (lamellae, dendritic, and eventually spherulitic texmres) through optical microscopy. 

The thermal and morphological behaviors of PP/EPDM blends were studied by Da Silva and Coutinho (6) using differential scanning calorimetry (DSC) and polarized optical microscopy (POM), respectively. Crystallization kinetics of PP/ EPDM blends were found similar. Ten to twenty weight percent addition of EPDM resulted in increasing of spherulite size (Fig. 14.3). Heat of fusion and crystallinity degree of PP/EPDM systems decreased when EPDM contents were increased. 

Since comprehensive knowledge of phase equilibria, crystallization phenomena, crystallization kinetics and process controls is required to establish a process to produce optically high purity materials with high yields, preferential crystallization is undoubtedly a challenging topic for those working in the field of industrial crystallization. In this article, the relation between the spontaneous nucleation and the phase equilibrium will be first discussed. A brief survey of spontaneous nucleation phenomena will follow. Then our experimental work on the effect of pretreatment of seed crystals will be discussed. 

Jaffe melted and isothermally recrystallized fibers spun under different stress levels. The bulk crystallization kinetics was measured with both DSC and optical microscopy techniques. Table 3.30 lists the samples studied. The film samples were included to ensure the absence of spurious DSC effects due to fiber packing. The samples were heated from 50°C to the melt temperature at 80°C/min. The melt temperatures ranged from 170 to 230°C. The samples were held in the melt for a specified time and then cooled to the 130°C crystallization temperature at 40°C/min. 

Typically, polymer-grade l-LA with high chemical purity and optical purity (i.e., over 98-99% l-LA and less than 1-2% d-LA) is used for commercial PLA production. When l-LA is dehydrated at high temperature into L-lactide, some l-LA may be converted into d-LA. d-LA mixed with l-LA contributes to meso-lactide (the cyclic dimer of one d-LA and one l-LA) and heteropolymer PLA (with both d-LA and l-LA units). Heteropolymer PLA exhibits slower crystallization kinetics and lower melting points than homopolymer PLA (of pure l-LA units or pure d-LA units). 

Differential thermal analysis (DTA) has also been exploited, mainly to determine polymer crystal melting temperatures but also (less frequently) to determine crystallization kinetics. Crystallite formation also changes the optical... 

In this section experimental results are discussed, concerned with analyses of melting and crystallization kinetics, as well as reversibility of the phase transition. The frame of the discussion is set by Fig. 3.76, which will be supported by experimental data on poly(oxyethylene). The thermal analysis tools involved are TMDSC, optical and atomic-force microscopy, DSC, adiabatic calorimetry, and dilatometry. Most of these techniques are described in more detail in Chap. 4. Results from isothermal crystallization, and reorganization are attempted to be fitted to the Avrami equation. This is followed by a short remark on crystallization regimes and finally some data are presented on the polymerization and crystallization of trioxane crystals. 

A comparison between the polarized optical micrographs taken at temperatures where the PCL block is molten for both PPDX-6-PCL and PLLA-6-PCL diblock copolymers of similar compositions (compare Fig. 13.8c and Fig. 13.10a), lead to the conclusion that when stronger thermodynamic segregation is present (as in PPDX-6-PCL diblock copolymers), the phenomenon of break-out is more difficult. Concurrently, the overall crystallization kinetics is much more strongly depressed at equivalent supercoohngs for the PPDX block than for the PLLA block when in both cases they are covalently bonded to molten PCL blocks. 

Optical microscopy of model films has often been conducted using the hot stage and polarized light. This technique permits the determination of the growth habit and crystallization kinetics of polymer systems from thin films... 

Auer et al. [79] have studied the crystallization kinetics of three different types of PPS by DSC and optical microscopy. It was observed that the storage time in melt affected by crystallization process from melt in terms of the temperature shift and width of the crystallization peak. The different observations in the case of these three samples were attributed to the various molecular processes in the melt. 

Compatibility of additives with polymer, and their effect on polymer crystallinity, were evaluated using differential scanning calorimetry. Scanning electron microscopy was used to follow additive dispersion. WAXS and optical microscopy were used to determine effect of additives on crystallization kinetics and spherulite formation. ... 

Figure 9.6 depicts the optical micrographs showing tiny spherulites of SPS melt-crystallized at (a) 230 and (b) 260 °C, which contained only the a crystal. Both optical microscopy results confirmed that the crystallization kinetics of the a-crystal SPS is of a heterogeneous nucleation, which remained so regardless of the melt crystallization temperature [78]. By contrast. Figure 9.7 shows the optical micrographs for the spherulites of SPS melt-crystalhzed at (a) 230 and (b) 260 °C, which contained only p crystal [78]. 

In this chapter, we take a practical approach to briefly explain how to experimentally determine both spherulitic growth rates by polarized light optical Microscopy (PLOM) and overall isothermal crystallization kinetics by differential scanning calorimetry (DSC). We give examples on how to fit the data using both the Avrami theory and the Lauritzen and Hoffman theory. Both theories provide useful analytical equations that when properly handled represent valuable tools to understand crystallization kinetics and its relationship with morphology. They also have several shortcomings that are pointed out. 

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