The discovery in the 1980s of the excellent properties of nylon 6/clay composites led to much research on filled polymer composites and a renewed understanding of the role of fillers in polymers. Here’s a look at the effect of fillers on the crystallization behavior of polymers.
Influence of fillers on the crystal morphology of polymers
The addition of fillers affects the crystalline morphology of the polymer：
Crystalline polymers form different crystalline forms under different crystallization conditions, and spherical crystals are one of the most common characteristic forms of polymer crystallization. Under normal molding and processing conditions, crystalline polymers usually form spherical crystalline forms.
The addition of fillers plays a role in heterogeneous nucleation and refinement of grains：
Large and sparse spherical crystals become small and compact, and the refinement of spherical crystals and the improvement of material toughness are beneficial and can be an effective method for toughening polymers.
The compatibility of the filler with the resin matrix also has an effect on the crystalline morphology：
For example, Qiao Fang et al. found that the organicized montmorillonite destroys the spherical crystal structure of nylon 6, mainly because the nylon 6 molecules have a strong coupling effect with the organicized montmorillonite particles, so that the movement of nylon 6 molecular chain segments is hindered in the crystallization process, and it is difficult to grow in a symmetrical and regular arrangement along the radial direction, so that the crystallization growth process is hindered, thus generating incomplete crystals.
During the molding process of fibrous filled polymer composites, the fibers may play a nucleation role and the spherical crystals no longer have the conditions for three-dimensional growth during the growth process, which then takes the form of columnar growth, i.e., oriented one-dimensional growth that produces transverse crystals.
Nucleating agents are a class of substances that can change the crystallization behavior of resins and alter the microscopic morphology of polymers to give them new properties and functions, and are a hot topic of current research at home and abroad.
Mechanism of action: In the molten state, as the nucleating agent provides the required nuclei, the polymer changes from homogeneous nucleation to heterogeneous nucleation, which accelerates the crystallization rate and refines the grain structure, and also improves the rigidity of the product.
Nucleating agents can be mainly divided into inorganic nucleating agents, organic nucleating agents and polymer nucleating agents. Nanoparticle nucleating agents have become a hot spot for nucleating agent research because of their many active centers on the surface, which can be closely bonded with the substrate and have better compatibility, and the particles are not easily detached collectively when subjected to external forces.
Effect of fillers on polymer crystallization kinetics
The polymer crystallization process consists of two stages: nucleation and growth. The Avrami equation is usually used to describe the isothermal crystallization process of polymers, and by finding the Avrami index n and the crystallization rate constant K, the growth mechanism of crystals and the rate of growth can be derived.
Mucha M et al. in their study of the crystallization kinetics of carbon black filled PP found that there was a two-step crystallization phenomenon: in the early stage of crystallization related to the heterogeneous nucleation of PP, nucleation on the surface of carbon black particles, no collision of nuclei occurred in the early stage of crystallization and growth was not limited; followed by a slow crystallization stage where the growth rate was slowed down by mutual collision between crystals. At the early stage of crystallization, n was about 2.0, while at the later stage of crystallization, n was about 3.0. It was also found that the addition of different amounts of carbon black had different effects on the values of K and n. At the same crystallization temperature, the value of K increased with the increase of carbon black addition.
According to the Avrami equation, the n value should be a positive integer less than or equal to 4. In practice, the n value of the crystallization process of polymers is usually a decimal number and appears to be equal to 5, which indicates that the actual crystallization process deviates from the Avrami equation and the phenomenon of secondary crystallization or spherical crystal collision occurs.
Crystalline structure model of filled polymers
In recent decades, theory and experiments have fully demonstrated that there is indeed a transition zone between the crystalline and amorphous regions of crystalline polymers, i.e., an intermediate layer, called the crystalline – amorphous intermediate phase.
The effect of talc on the structure of the crystalline region, amorphous region and intermediate phase of low density polyethylene (LDPE) was analyzed by dynamic two-dimensional infrared spectroscopy. The results show that the addition of talc makes certain changes in the crystalline structure model of LDPE, and it is concluded that talc is closely connected with the crystalline region but not with the amorphous phase, which leads to the inferred structural schematic.
The study of the effect of fillers on the crystallization behavior of polymers can lead to a better understanding of the crystallization mechanism and crystallization model of crystalline polymers. Moreover, the crystallization behavior of the filled polymers can directly affect the properties of the composites, so this should be paid attention to.
Although there has been some progress in the study of the effect of fillers on the crystalline properties of polymers, there is still a relative lack of more in-depth, systematic and detailed studies, and if this aspect can be systematized, it should lead to a higher level of research in material preparation and properties.