As has been explained several times in previous blogs, polymers like silicones are macromolecular structures which are created through polymerization processes such as addition (chain growth) or condensation (radical initiated) polymerization [1]. Those chains are held together through van der Waals forces. The lengths of the different chains are not the same, so it is possible to find very short and very long chains in the material. When the length of a group of chains is short, its arrangement is random; that is, the zone is not organized and it can be said that the zone is amorphous. However, when the chain length is long enough, organized zones can be created called crystalline zones. The process of obtaining those organized structures in polymers is not easy or simple, and it always depends on the time and temperature. First, the chains fold together to create an ordered region called lamellae, which are fine layers of the chains form the structure. When a group of lamellae is big enough, another morphological structure is created: a spherulite. This structure can be seen by optical microscopy and it directly affects the mechanical properties of the material. Also, another special crystalline arrangement is found at the same level as the spherulites. Shish-Kebab structures are formed by crystals in the shape of circular plates and whiskers. These appear when a shear deformation occurs during solidification. With that, the arrangement of the polymer structure, called the morphology, in a conventional polymer can include a highly crystalline structure found just next to an amorphous zone. Figure 1 depicts the possible structures in a polymer.
Figure 1. Schematic diagram of polymer structure (from random to highly organized structure) [1]
Up to this point, the theory is equal for all polymers, but some differences occur from polymer to polymer. For silicone rubber, two scenarios must be analyzed: silicone rubber in the unvulcanized state and silicone rubber in the vulcanizated state. In the unvulcanized state, the statements described above apply completely and additional states appear; due to regularity of the chemical structure of silicone rubber, its arrangement is periodic and crystalline regions and zones with preferential orientation can be found that don’t necessarily correspond to a crystalline zone [2].
VULCANIZATION
But what happens when the vulcanization occurs? It is well known that during vulcanization some bonds are broken to create the characteristic tridimensional network that improves the mechanical, chemical and physical properties of the silicone rubber. Those reactions are exothermic and irreversible processes. It creates a morphology where small and a few randomly oriented crystalline domains are embedded in a “matrix” of randomly-oriented amorphous material. Although the silicone structure is very regular, the temperature at which the vulcanization occurs creates a “disturbance” where the chains have enough energy to lose their order. If the processing conditions are also considered, a quick cooling of the material always occurs after the vulcanization, which avoids the formation of as many crystalline domains as existed in the uncured state. That is the reason why amorphous domains dominate silicone rubber morphology.
But how much do the processing conditions affect the orientation or the crystallinity obtained in the liquid silicone rubber? Similar to all polymers, the physical mechanical properties of the liquid silicone rubber products and the dimensional stability after the processing are strongly linked to the molecular orientation obtained during the processing. Despite the effect of the vulcanization, in some zones of the material, during extrusion or injection a preferential orientation in the flow direction in the die or the mold, respectively, always occurs. In extrusion, processing conditions such as extrusion temperature, die configuration, and even the liquid silicone employed influence the orientation and quality of the part. With injection molding, injection speed, mold temperature, and cooling time are the main parameters. Similar to the above, during processing the orientation can be analyzed when the liquid silicone rubber is still unvulcanized (mixing, metering and injection) and when the liquid silicone rubber is vulcanized and cooled into the mold.
During the mixing, metering, and injection of the liquid silicone rubber, the molecular chains are straightened, and the simple chemical structure of liquid silicone rubber especially favors this orientation. When the liquid silicone rubber enters the mold, two different mechanisms lead to orientation in the silicone rubber part: fountain flow effect and radial flow. However, due the fact that silicone rubber curing starts at the moment in which the material enters in the mold, their effect is not as radical as for thermoplastics. In that precise moment, the molecular orientation created during the flow through the injection machine largely disappears due to the crosslinking reaction. The chain energy is high enough during the crosslink formation that the new chemical bonds inhibit the movement of the macromolecule which prevents a rearrangement; on the other hand, the crosslinking density in silicone rubber is low, so it could be possible to find some oriented structures which may affect the final properties of the product in some way. On some occasions, when the silicone rubber is filled with fibers or particles, the orientation is refocused onto the position of these fillers. The final mechanical properties will be better in the direction in which the filler is oriented. Nowadays, computer simulations are used to predict the fillers orientation using models that includes mold filling, thermal history, residual stresses, and warpage.
The final application of the silicone rubber parts can also lead to changes in the orientation and crystallinity due to external parameters, with temperature and deformation being the most critical. During applications at high temperatures, it is possible that the few crystalline zones disappear, causing an almost imperceptible change in size or shape in the silicone rubber part; but, depending on the rate of heat removal, the crystalline zones can be reformed or can remain a completely amorphous structure. When the application is at low temperatures, things are somewhat different. It is well known that the glass transition temperature (Tg, approximately -125°C for silicone rubber) is a lower limit below which the material loses its elastic properties and becomes brittle. This happens because the molecules are frozen; that is, they do not have sufficient energy to be able to move relative to each other. But what can happen to the morphology when the temperature decreases below this point? The silicone rubber will be more and more rigid, which is caused by crystallization and second order transitions [3]. A second order transition occurs when the heat capacity changes without latent heat, for example, the glass transition temperature.
As the temperature decreases, a great decrease in the dimensions of the silicone rubber part can be seen, which can be considered a demonstration of crystallization. Weir et al. [3] conducted a study to analyze silicone rubber at low temperatures and found that a cold crystallization occurs between -60°C and -67°C and the degree of that crystallization depends on the rate at which the part is cooled. The higher the cooling rate, the lower the crystallinity after the cooling [4]. This behavior was also found in natural rubber, but it occurs more quickly for silicone rubber, possibly due to the presence of inorganic atoms in the chemical structure of the silicone rubber. At temperatures above -60°C, the changes in silicone rubber properties are negligible, and it can be said that radical changes will occur only when the temperature is above room temperature. Now, with regards to the second order transitions, silicone rubber has one of the lowest glass transition temperature and it is not affected in presence of fillers and additives. Here the crystallization of supercooled materials occurs. If this behavior is added in a graph of expansion as a function of temperature (Figure 2), the change in the crystallinity and the effect of the glass transition temperature can be seen.
Figure 2. Expansion vs. temperature (and change in crystallinity) of silicone rubber
References
- Menges, G. Osswald, T.A. Materials Science of Polymers for Engineers. Carl Hanser Verlag, 2012.
- Ohlberg, S.M., Alexander, L.E. Crystallinity and orientation in silicone rubber. I. X-ray studies. Journal of Polymer Science, 27, 1-17, 1958.
- Weir, C.E., Leser, W.H., Wood, L.A. Crystallization and second-order transitions in silicone rubbers. Journal of Research of the National Bureau of Standards, Volume 44, 367-372, 1950.
- https://www.mt.com/be/en/home/supportive_content/matchar_apps/MatChar_HB436.html