Films spherulites

Fig. 3. Raman spectra of rubrene films spherulites in amorphous matrix (top), and from closed rubrene layer (bottom) 1,2 - the center and close to border of a spherulite 3 - amorphous matrix. Rubrene powder and mica substrate spectra are also shown as reference. AM - labels the signatures of the amorphous matrix.

Fiber Main fiber is made of secondary fibrils Fiber Shish-Kebab Melt cryst. films Spherulitic Soln. grown Lamellae 500 (fibril width) 100 10 (fibril thickness) Low-magnification electron microscopy 25 High-magnification electron microscopy 8 — 12 ... 

Figure 8.1. (a) Spherulites growing in a thin film of isotactic polystyrene, seen by optical microscopy with crossed polars (from Bassett 1981, after Keith 196.3). (b) A common sequence of forms leading to sphertililic growth (after Bassett 1981). The fibres consist of zigzag polymer chains. 

One of the more important uses of OM is the study of crystallization growth rates. K. Cermak constructed an interference microscope with which measurements can be taken to 50° (Ref 31). This app allows for study of the decompn of the solution concentrated in close proximity to the growing crystal of material such as Amm nitrate or K chlorate. In connection with this technique, Stein and Powers (Ref 30) derived equations for growth rate data which allow for correct prediction of the effects of surface nucleation, surface truncation in thin films, and truncation by neighboring spherulites... 

Figure 9.22. Scanning force microscopy images of polyethylene films formed on a model planar chromium polymerization catalyst. The small white stripes are lamellar crystals. These form the well-known spherulite superstructure upon crystallization from the...

Fig. 29 Fractured morphology of spherulitic objects in a thin film of PET crystallized at 220 °C [36]. On the fractured surface many small particles with a diameter of 0.2 0.3 xm are seen while on the spherulite surface there is a fibril structure 0.2 05 xm thick... 

SEM micrographs of two members of these polymers (HB and HBIB-50) are shown in Figure 7 to provide further evidence for superstructure on the micron level within the solution cast films. One can directly observe the surface of the spherulitic structure of the HB homopolymer as well as in that of the copolymer HBIB-50. Clearly, the level of structure (-5 pm) is well above that of the individual domains of either HB or HI and reflects the possible primary nucleation and subsequent growth behavior common to spherulitic semicrystalline polymers. The Hv patterns shown in... 

It should be re-emphasized that although our block copolymers do not display spherulitic morphology when they are compression molded, they are nevertheless crystalline. Hence, this indicates that under this mode of film preparation, aggregation into well developed superstructure is apparently kinetically limited. 

We have already mentioned that depending on composition, semicrystalline triblock copolymers can show some conflict between microphase separation and superstructure formation. In fact, one of the controversial aspects is the question whether block copolymers can or cannot exhibit spherulites. This is a relevant question because spherulitic structures greatly affect the ultimate mechanical properties, and the boundaries between adjacent spherulites are often weak points in mechanical performance. Kim et al. [125] studied the competition between crystallization within microphase-separated regions and reorganization into supermolecu-lar spherulites in semicrystalline PS-b-PB-b-PCL triblock copolymers. These authors found that the formation of spherulites is strongly affected by the thickness of the specimen in such a way that thin films crystallize into... 

Polyolefin foams are easier to model than polyurethane (PU) foams, since the polymer mechanical properties does not change with foam density. An increase in water content decreases the density of PU foams, but increases the hard block content of the PU, hence increasing its Young s modulus. However, the microstructure of semi-crystalline PE and PP in foams is not spherulitic, as in bulk mouldings. Rodriguez-Perez and co-workers (20) showed that the cell faces in PE foams contain oriented crystals. Consequently, their properties are anisotropic. Mechanical data for PE or PP injection mouldings should not be used for modelling foam properties. Ideally the mechanical properties of the PE/PP in the cell faces should be measured. However, as such data is not available, it is possible to use data for blown PE film, since this is also biaxially stretched, and the texture of the crystalline orientation is known to be similar to that in foam faces. 

The morphology of spin-cast film, thickness of 180 nm, from polycaprolactone shows many spherulitic structures with fibrillar nanostructures formed of lamellae lying edge on (about 10 nm thick) and areas with lamellar sheets lying flat on. Different crystalline structures are found when the sample is melted and crystallized as a function of temperature. These two studies reinforce the complex inner relationship between physical treatment and nanostructure. 

In the crystallization of isotactic polypropylene from the melt, the number and size of the spherulites (and hence the rate of crystallization) can be influenced by the addition of certain nucleating agents.The smaller the spherulites, the greater is the transparency of the polypropylene film.The mechanical properties can also be affected in some cases. 

The effect of heterogeneous nucleation on the crystallization of isotactic polypropylene from the melt can be easily established as follows. A small amount of powdered polypropylene is well mixed with about 0.1 wt% of sodium benzoate in a mortar or by means of an analytical mill. Some of the mixture is transferred with a spatula to a microscope slide and melted at about 250 °C on a hot block. A cover slip is pressed on to the melt with a cork to obtain as thin a film as possible.The sample is held at 200-250 °C for some minutes and then allowed to crystallize at about 130 °C on the hot stage of the microscope an unadulterated polypropylene sample is crystallized in the same way. Both samples are observed under a polarizing microscope during crystallization,the difference in spherulite size between nucleated and untreated polypropylene can be seen very clearly. An ordinary microscope can also be used by placing polarizers on the condenser and eyepiece, and adjusting these to give maximum darkness. 

A methoxylated polyamide 78 analogous to Nylon 6 was obtained in several steps fi om D-glucose [61, 63] through the preparation of a dimeric active ester of 6-amino-6-deoxy-2,3,4,5-tetra-(9-methyl-D-gluconic acid (49). This polyamide was highly crystalline, and gave resistant films with a spherulitic texture. 

Figure 5.7. Optical microscope image of a thin film (thickness 2 p.m) of a-p-NPNN grown on a glass substrate (1.6 x 1.0 mm, crossed polarizers). Reprinted from Journal of Crystal Growth, Vol. 209, J. Caro, J. Fraxedas and A. Figueras, Thickness-dependent spherulitic growth observed in thin films of the molecular organic radical p-nitrophenyl nitronyl nitroxide, 146-158, Copyright (2000), with permission from Elsevier.

Figure 5.9 shows the time evolution of the radii of selected 2D spherulites from Fig. 5.8. We observe that the process is non-linear and accelerated, (fR/df > 0. It is also interesting to notice that, at a given time, the radial growth velocity Ur = dR/dt (slope) is nearly the same for all spherulites, which implies that it depends on the deposition time and certainly not on the radius of the spherulites. In the case discussed here the thickness of the film is increasing with time because of continuous exposure to the molecular beam. The non-linearity is more pronounced at the beginning of the experiment (roughly between 250 and 350 s) and the velocity nearly tends towards an asymptotic value, so that 2D spherulites that are formed last show almost linear growth.

Figure 5.10. Schematic cross-sectional view of a thin film during spherulitic crystallization. The perpendicular axis corresponds to a rotation axis conferring cy-clindrical symmetry on the system. The amorphous film is represented by a-film.

In the following derivation we will assume an almost complete wetting of the substrate by the material, in such a way that a continuous amorphous condensed film is formed at a thickness h smaller than the critical size of nucleation. In order to evaluate dG/dN of the process of incorporation of molecules from the amorphous condensed film to the spherulite, that is the ordered phase, we will hypothesize that the thickness of the amorphous film increases linearly with time, h(t) = Uhf, where the velocity is a constant, and that the spherulite has a cylindrical shape of radius R and height h, as illustrated in Fig. 5.10. 

---