Issue 33

A. Winkler et alii, Frattura ed Integrità Strutturale, 33 (2015) 262-288; DOI: 10.3221/IGF-ESIS.33.32 264  The type and arrangement of the atomic chains  The average molecular mass along with its distribution function  The sequence length of the basic building blocks  The number, distribution and length of the arborisations, i.e. the branching behaviour  The type of substituents and end groups, i.e. what is being attached along the chains, apart from hydrogen atoms  The sterical arrangement of the substituents, i.e. what is the three-dimensional layout of the above groups Apart from the polymer molecules themselves, for the material to be fully chemically characterised, information about the kind and quantity of additives (e.g. fillers, softeners, stabilisers, lubricants and flame retardants) is necessary. The chemical structure will to the largest extent remain unchanged during mechanical processing. However, excessive thermal loading may damage the polymer, manifested by e.g. decomposition, depolymerisation and chain degradation. The physical structure in a polymer contains (depending on the generic morphological form [9] contains the amorphous state, the crystalline state and the mixing zone of the two states 5 . The crystalline state is considered to be that volumetric part of the plastic that contains a repetitively ordered structure of any form, whereas the amorphous state is that volumetric part which has no recognisable structure. The mixing zone is clearly the boundary layer between structured and unstructured domains. We would like to emphasize, that the crystalline state is not really a crystal, but likened to a crystal because it has repetitive substructures, which we dub crystallinity. Key parameters governing the physical structure are:  The state of orientation of the molecules/molecular groups  The energy elastic residual stress  The short range order  The degree of cross-linking  The degree of crystallinity and its distribution  The size and distribution of spherulites, which is an observed crystalline substructure. In essence, each macroscopic property of a plastic is governed by a different set of parameters; this prompts a selective influence of the physical structure on the properties. One particularly interesting and revealing structural parameter is the degree of crystallinity [10], from which the quality of the material processing can be deduced. The crystallinity distribution allows for the homogeneity of the structure to be assessed. Crystallinity directly influences the following properties (as an example): chemical durability, tribological fitness, thermal resilience, strength, transparency, ductility and toughness [11]. The relation between structure and property can be quantified using the following significant structural parameters [7]: Amorphous Crystalline Orientation Degree of crystallinity and the resulting supramolecular structure Residual stress Characteristic of the amorphous phase Short range order Table 1 : Significant structural parameters for amorphous and partially crystalline plastics. As an example, the UTS for a mostly amorphous plastic is going to be influenced by the orientation, the residual stress and the short range order. If on the other hand, the plastic is mostly crystalline, that same property is going to be governed by the degree of crystallinity and its behaviour, and then to a lesser extent the behaviour in the amorphous phase we just discussed. The degree of crystallinity in a plastic is always limited, for example PA66 is between 35% and 45%. Whereas POM has a typical degree of crystallinity between 70% and 80%, as does HDPE. We are going to consider in this article non-reinforced (partially) crystalline plastics. These contain multiple states or phases, with clearly identifiable geometrical substructures, which on a micro-scale level indicate a certain amount of deterministic structure. The following is simplified account of the formation of such entities (crystalline growth) in a quiescent melt. More detailed treatises can be found in [12-15]. 5 Usually, the amorphous phase tends to be synonymous with entropy elasticity (rubber elasticity) and high ductility, whereas the crystalline phase is described by energy elasticity and high strength.