Crystallinity transcrystallinity

In semi-crystalline polymers the interaction of the matrix and the tiller changes both the structure and the crystallinity of the interphase. The changes induced by the interaction in bulk properties are reflected by increased nucleation or by the formation of a transcrystalline layer on the surface of anisotropic particles [48]. The structure of the interphase, however, differs drastically from that of the matrix polymer [49,50]. Because of the preferred adsorption of large molecules, the dimensions of crystalline units can change, and usually decrease. Preferential adsorption of large molecules has also been proved by GPC measurements after separation of adsorbed and non-attached molecules of the matrix [49,50]. Decreased mobility of the chains affects also the kinetics of crystallization. Kinetic hindrance leads to the development of small, imperfect crystallites, forming a crystalline phase of low heat of fusion [51]. 

Figure 7.18 shows how crystalline structure is affected by the presence of fiber. Here, bamboo fiber was used for polypropylene reinforcement. A nucleation occurs on the surfaces of fiber. Spherulites grow from the fiber surface. Such growth results in transcrystallinity. The maleation of polypropylene increases interaction because of reactivity with OH groups on the fiber surface. This organization contributes to the reinforcement. 

Crystallization rate, nucleation, size of crystalline units, crystalline structure, crystal modification, transcrystallinity, and crystal orientation are the most relevant characteristics of crystallization behavior in the presence of fillers. Here the discussion is focused on crystallization rate. The other topics are discussed in the following sub-chapters. 

The cooling of polymer melt in the presence of a foreign surface which can nucleate crystalline growth inhibits the lateral growth of spherulites. Crystallization occurs This is called transcrystallinity. It can im-... 

The two-component system—crystal lamellae or blocks alternating with amorphous layers which are reinforced by tie molecules— results in a mechanism of mechanical properties which is drastically different from that of low molecular weight solids. In the latter case it is based on crystal defects and grain boundaries. In the former case it depends primarily on the properties and defects of the supercrystalline lattice of lamellae alternating with amorphous surface layers (in spherulitic, transcrystalline or cylindritic structure) or of microfibrils in fibrous structure, and on the presence, number, conformation and spatial distribution of tie molecules. It matters how taut they are, how well they are fixed in the crystal core of the lamellae or in the crystalline blocks of the microfibrils and how easily they can be pulled out of them. In oriented material the orientation of the amorphous component (/,) is a good indicator of the amount of taut tie molecules present and hence an excellent parameter for the description of mechanical properties. In fibrous structure it directly measures the fraction and strength of microfibrils present and therefore turns out to be almost proportional to elastic modulus and strength in the fibre direction. 

A transcrystalline layer (TCL) is the supermolecular crystalline stmcture, induced by an oriented growth in the presence of the foreign surface. Transcrystallization occurs when the nucleation density of a solid filler that is in contact with melted... 

The crystallinity of PP increased with the addition of coconut fibers. However, the crystallinity also increased because the fiber surface acts as a nucleation site for the crystallization of the polymer, promoting the development and formation of transcrystalline regions around the fiber. 

For semicrystalline polymers, fillers may affect crystallinity, size of crystallites, and direction of crystal growth. Filler surface may provide a large number of nucleation sites, although this also depends on surface functional groups and surface treatments. In certain polymers, fillers may promote transcrystallinity, which can improve adhesion and other properties [10]. 

Neat isotactic polypropylene (iPP) crystallized from melt exhibits spherulitic morphology of the crystalline phase (72,73). In some cases and under very specific conditions, cylindrites, axialites, quadrites, hedrites, and dendrites may be formed of iPP (74). In general, crystallization from quiescent melts results in spherulitic morphology, whereas crystallization fi-om melts subjected to mechanical loads results in cylindrites (75). Crystalline supermolecular structure caused by oriented crystal growth from heterogeneous surfaces is commonly termed transcrystallinity (76). 

Transcrystalline morphology is formed when crystallization takes place on the solid surface of fillers or reinforcements. Transcrystallizafion takes place when the density of the crystal nuclei is substantially greater on the surface of solid inclusions than in the melt bulk (77). Because polyhedral sphemlites cannot develop due to restricted lateral growth on the solid surface, crystallites are allowed to grow only in stacks perpendicularly to the surface plane (78). In the case when only one crystal form occurs in a polymer, Keller (79) confirmed that the microstructure of transcrystalline layer and bulk crystalline phase is identical. For PP, however, the situation is more complicated by the polymorphism so that one crystal form can exist in the transcrystalline layer and another in the polymer bulk. The nature of nucleation of the transcrystalline layer is still somewhat con-... 

There is no doubt that this thin, around 150 nm wide LDPEhlayer, composed of uniformly organized lamellae, is nothing but a transcrystalline layer on the surface of the PET-fibril. The TEM micrograph demonstrates, therefore, very nicely the basic difference between the crystalline mass in the bulk of the matrix and the transcrystalline layers around the fibrils. 

The subject is probably best advanced for polymers in contact with glass fibres and certain platy particles such as sheet silicates. As well as acting as nucleants, this type of material can alter the type of crystal structure present. Thus, it is now well established that glass-fibre surfaces can lead to trans-crystallinity. This is the growth of radially oriented lamellae extending for a few hundred micrometres from the fibre surface. This topic has been discussed in detail by Thomason [88]. Transcrystallinity is also discussed in Chapter 8 of this work. 

---