Crystalline structures occuring with polymer processing

As is well-known, polymer processing always means that the molten material is deformed in a mold filling process or another process, in which flow plays a predominant role. Solidification is achieved in a cooling process, in which polymers, possessing a regular molecular structures, are crystallizing. In this way a great variety of inhomogeneous structures are formed, which determine the quality of the product. In fact, flow rates and cooling speeds are always larger close to the mold wall than in the center of the mold. And it is also well-known that the influence of flow on the obtained morphologies is enormous. There have been two main routes for the investigation of the flow induced kinetics of crystallization. According to one of these routes, which has been invented in Linz, short term flow is applied. In this process heavy shearing or stretching is applied for rather short times at a variety of chosen temperatures. The success of this method is based on the fact that the microstructures, as apparently formed during the period of flow, (nuclei or thread-like precursors: shishs) are quite stable. At the applied temperatures their relaxation times are many orders longer than those of the free or entangled molecules. This means that after cessation of flow secondary structures, which are characteristic for the previous nucleation process and which are visible under the microscope, can grow without a hurry. In the second route flow is continued up to the moment, when the molten material shows an upswing in its viscosity (gelation etc.). Because of experimental restrictions only relatively slow flows, which are not very characteristic for polymer processing, have been applied so far in quantitative experiments. Melt spinning would be a good example. However, melt spinning is a fast and not an isothermal process. The goal of the present investigation is to conciliate the apparently contradictory results obtained with the aid of the mentioned two routes. In particular, the morphologies, as obtained with the second route, deserve a further elucidation.

The goal of project P 21228-N14 certainly was to create a basis for the improvement of the quality of plastics parts. There are two branches of industry, which are interested: The chemical industry, which produces polymers (= macromolecules, long molecular chains). But there are also the firms, which make processing machines. One has injection molding machines, extruders, equipment for film blowing, vacuum forming and fiber spinning. In all cases one has to do with a cooling process leading to the fixation of a shape. The mention of all these machines shows, how versatile plastics are. There are no other materials, which are comparable in this respect. From garden chairs to pipes of all sizes and to miniature parts for the electronic industry one has all kinds of applications. In this connection the corrosion-resistance and the capability of isolation in electric applications must be mentioned. For large parts also the low weight (compared with steel or concrete) and the toughness are of importance. In this connection one has to consider the rough conditions, which are unavoidable, when pipes of a large diameter have to be laid into the ground. One should also not forget the importance of food package. However, what is the reason, why crystalline structures are mentioned in this connection? The answer is simple: most of the usual polymers crystallize during the cooling process. But the obtained structures very much depend on the course of this process. These structures are almost always strongly oriented and have a considerable influence on the properties of the products. One obtains a high toughness in the previous flow direction. In the perpendicular direction the obtained parts are mostly brittle. In principle these features also reduce the stability of the shape. Warping is a well-known consequence. Also the size of the grains, which is important for the impact strength, is strongly influenced. However, in contrast to metals the tendency of polymers to crystallize is rather low. This fact has to do with the statistical shape of the clews, which are formed by the chain molecules. In this state chain molecules hardly find partner chain molecules for a local fit. Mostly it happens that the mutual state of rotation around the chemical bonds is inappropriate. But an appropriate fit is the prerequisite for the click into a crystal lattice, even if this lattice is still rudimentary. During the shearing of a melt macromolecules, which move in neighboring layers, are touching each other regularly. But apparently, the majority of the encounters remains without effect. Only a few of them lead to clicks. This is shown by our experiments, where with unchanged (low) speed of homogeneous shearing (low "rate of shear") the duration of the shearing is essential. Apparently, only the number of clicks counts. With prolonged shearing one passes through two regions. In the region of relatively short shearing so-called point-like nuclei are formed. With extended shearing, however, one enters a region, where thread-like nuclei are created. Apparently, the low density of the cross-links, which are formed by clicks, suffices locally for rudimentary networks, which can easily be extended. It goes without saying that the fraction of the longest macromolecules, as contained in the sample, is particularly effective in this respect. If these threads are formed at high temperatures (close to 200C in industrial polypropylene), they relax after the cessation of the flow. Only after an immediate quench highly oriented structures are formed by secondary crystallization, filling up the space. This picture has been developed on the basis of previous experiences and of experiments, which are enabled by a new apparatus, which has recently been developed during more than four years at Linz University. With this apparatus homogeneous total shears up to unheard homogeneous total shears of 3200 became possible in samples of reasonable thickness.