Spherulite transparent structure

The properties of a given polymer very much depend on the way in which crystallisation has taken place. A polymer mass with relatively few large spherulitic structures will be very different in its properties to a polymer with far more, but smaller, spherulites. A polymer crystallised under conditions of high nucleation/growth ratios, with smaller structures, is generally more transparent. 

Rubrene thin films were evaporated by HWE on mica. After the formation of an amorphous matrix, spherulite structures start to grow, which finally cover the whole surface. Due to their polycrystalline property the spherulites are resistant against oxidization while the amorphous matrix becomes transparent upon exposure to air, which is a clear sign for oxidation. The mbrene spherulites provide therefore a promising material for the fabrication of OFETs. 

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. 

The process uses crystallizable polymers, of which the most important in PET. The first step is to injection mould (hence the name) a parison, or preform as it is more usually termed here. The preform is closed at the bottom and is considerably shorter and thicker than the final bottle. It is rapidly cooled (quenched) by using chilled water to cool the injection mould and this ensures that it is in its amorphous condition, i.e. no crystalline structure. Next it is reheated with infra-red elements to above its Tg, about 90-100 C for PET and enters the bottle mould and the mould is closed. The blow pin enters and pushes the soft preform downwards almost simultaneously the blow occurs, compressed air blowing the material outwards. The result is biaxial orientation - downwards from the movement of the blow pin, outwards from the action of the expanding air. The orientation induces crystallization, but in the form of lamellar crystals rather than spherulitic ones. This type of crystallization is strain-induced, and is characteristic of synthetic fibres and film, e.g. Melinex. It gives a transparent product with enhanced physical properties, both important for bottling carbonated drinks. The alternative name for the process is the stretch-blow process. Its main feature as a process is the control of the crystallinity of the polymer at its different stages. 

Transparency of PP products can be efficiently improved both with traditional nucleating agents and clarifiers, which are various sorbitol derivatives. The improvement in the optical properties depends on the efficiency in destroying the spherulitic structure of PP or at least decreasing the size of the spherulites below a critical value. Strong nucleation... 

As long as the adhesives used were transparent (no spherulitic structure present as seen by microscopy), the energy absorbed by the laminates was essentially the same. The 1.0 butanediol content polymer was nontacky, indicating some embrittlement. It was also translucent, attributed to isolated spherulites seen in Figures 5 and 6. The lowered impact resistance is in agreement qualitatively with earlier observations on the effect of brittle adhesives. 

It is apparent from Figure 10 that as long as there is no evidence of spherulitic structure and the material stays transparent,... 

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