Polymer crystallization process, characterization

The details of the polymer crystallization process can be quite complicated. Practically, one may not care about the details of crystal nucleation and the linear crystal growth rates, but just want to characterize the overall crystallization kinetics. The degree of crystallization process can be roughly defined as crystallinity, regardless of their detailed crystal morphologies. The conventional methods to characterize the crystallinity include DSC, X-ray diffraction and dilatometer. Depending on the measured quantity, crystallinity is also separated into the weight crystallinity... 

The primary crystallization process is characterized by three parameters. These are the rate of radial growth of the spherulite, G, the time constant for nucleation, t , and the time constant for the primary crystallization process, Tc, which is determined from the Avrami equation. All three parameters seem to depend on the stereoregularity of the polymer, but the nucleation rate seems to depend most strongly. 

Morphological changes, which are classified into four groups, have been correlated to the degree of topotactic control. Thermal analysis has been studied on DSP poly-DSP in some detail. Two main endothermic peaks of as-polymerized poly-DSP crystals are characterized as thermal depolymerization in the crystalline state and crystal melting point followed by thermal depolymerization in the molten state. From the results of the studies on the heat treatment of as-polymerized polymer crystals, a reversible topochemical processe has been established. 

Mesoscopic non-equilibrium thermodynamics provides a description of activated processes. In the case considered here, when crystallization proceeds by the formation of spherical clusters, the process can be characterized by a coordinate y, which may represent for instance the number of monomers in a cluster, its radius or even a global-order parameter indicating the degree of crystallinity. Polymer crystallization can be viewed as a diffusion process through the free energy barrier that separates the melted phase from the crystalline phase. From mesoscopic non-equilibrium thermodynamics we can analyze the kinetic of the process. Before proceeding to discuss this point, we will illustrate how the theory applies to the study of general activated processes. 

Recently, we have developed a real time pulsed NMR(RTPNMR) system(2) controlled by a microcomputer as shown in Figure 3 to study dynamics of structure formation in polymer systems. It has been successfully applied to study crystallization process (3), gelation process(4) and so on. In this paper, application of RTPNMR for characterization of curing process of epoxy resins, polymerization crosslinking process of diacetylenes, and orientation process of liquid crystalline polymers(LCPs) under magnetic field are described. Papers related to the detailed analysis of these data will appear in the near future. EXPERIMENTAL... 

Hu WB (2005) Molecular segregation in polymer melt crystallization simulation evidence and unified-scheme interpretation. Macromolecules 38 8712-8718 Hu WB, Cai T (2008) Regime transitions of polymer crystal growth rates molecular simulations and interpretation beyond Lauritzen-Hoffman model. Macromolecules 41 2049-2061 Jeziomy A (1971) Parameters characterizing the kinetics of the non-isothermal crystallization of poly(ethylene terephthalate) determined by DSC. Polymer 12 150-158 Johnson WA, Mehl RT (1939) Reaction kinetics in processes of nucleation and growth. Trans Am Inst Min Pet Eng 135 416-441... 

Another side benefit that accompanies with the use of certain nucleants is improved clarity. Since clarity or transparency is evidently related to the crystalline structure of the polymer and the structure is determined by the conditions of crystallization, parameters characterizing crystallization must be also connected with the optical properties of a PP product. The peak temperature of crystallization (Tc) is one of the quantities often used for the characterization of the crystallization process and efficiency of nucleating agents. With increased crystallization temperature, the thickness of the lamellae increases well. Higher efficiency and concentration of nucleating agent lead to an increase of Tc (as determined by DSC) and decrease of the size of the spherulites. 

Differential scanning calorimetry (DSC) is one of the routine methods used in polymer characterization and improves the knowledge of the microphase structure with other complementary methods. Lu et al. [149] investigated nonisothermal crystallization processes of Nylon/EVM (ethylene-vinyl acetate rubbers) blend using DSC and they found out that EVM rubber could act as heterogeneous nuclei acting more effective in Nylon/... 

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