Microhardness technique

The present review shows how the microhardness technique can be used to elucidate the dependence of a variety of local deformational processes upon polymer texture and morphology. Microhardness is a rather elusive quantity, that is really a combination of other mechanical properties. It is most suitably defined in terms of the pyramid indentation test. Hardness is primarily taken as a measure of the irreversible deformation mechanisms which characterize a polymeric material, though it also involves elastic and time dependent effects which depend on microstructural details. In isotropic lamellar polymers a hardness depression from ideal values, due to the finite crystal thickness, occurs. The interlamellar non-crystalline layer introduces an additional weak component which contributes further to a lowering of the hardness value. Annealing effects and chemical etching are shown to produce, on the contrary, a significant hardening of the material. The prevalent mechanisms for plastic deformation are proposed. Anisotropy behaviour for several oriented materials is critically discussed. 

Nowadays, the microhardness technique, being an elegant, non-destructive sensitive and relatively simple method, enjoys wide application, as can be concluded from the publications on the topic that have appeared during just the last five years - they number more than 100, as is shown by a routine computer-aided literature search. In addition to some methodological contributions to the technique, the microhardness method has also been successfully used to gain a deeper understanding of the microhardness-structure correlation of polymers, copolymers, polymer blends and composites. A very attractive feature of this technique is that it can be used for the micromechanical characterization of some components, phases or morphological entities that are otherwise not accessible for direct determination of their microhardness. 

Muller (1970) described the application of the microhardness technique using small loads, employing the Vickers approach. The effect of various factors on the microhardness of a wide range of polymers by means of the same approach was reported by Eyerer Lang (1972). These authors reported that the diagonals of the impression did not change after the removal of the load. 

Both thermoset and thermoplastic resins and CF composites have been examined using microhardness techniques. The thermoset resin used was an epoxy, both with and without PA6 particles that served as a toughening agent. 

In Sections 6.2.1 and 6.2.2 it has been demonstrated that the microhardness technique is very sensitive for detecting structural changes including polymorphic transitions in crystalline homopolymers and copolymers. 

In summary, it can be concluded that the microhardness technique is sensitive enough to detect strain-induced polymorphic transitions in polymers. The results in this chapter reveal that in materials characterized by a high and reversible deformation ability it is possible to observe reversible microhardness provided the strain-induced structural changes are reversible too. 

Application of microhardness techniques to the characterization of polymer materials... 

In the previous chapters the main fields of application of the microhardness technique in polymer physics have been highlighted. The emphasis has been mostly on solving structural problems, looking for relationships between the structures of polymers and their properties (initially mechanical ones) or on studying the factors which determine the microhardness behaviour of various polymeric systems. 

The correlation of microhardness and morphology for injection-moulded PET will be highlighted in this section as a second example of the application of the microhardness technique. 

During the past few years the microhardness technique has frequently been applied to the characterization of super-hard-surfaced polymers obtained by ion implantation and to plasma-deposited hard amorphous carbon films (Balta Calleja Fakirov, 1997). These products represent an entirely new class of materials that are lightweight and have the flexibility of polymers combined with a surface microhardness and wear resistance greater than those of metallic alloys (Lee et al., 1996). 

The microhardness technique is used when the specimen size is small or when a spatial map of the mechanical properties of the material within the... 

It is to be noted that is intimately related to the packing of the chains in the crystals (4). Since the crystal hardness reflects the response of the inter-molecular forces holding the chains within the lattice, it has been shown that the microhardness technique permits to distinguish between polymorphic modifications of the same polymer (20,21). Indeed, the study of the transition from the a to the form in iPP confirmed that changes in H were directly related to the different crystal hardness values of each phase (20). More recently, the microhardness technique has been successfully applied to follow the reversible strain-induced poljmiorphic a p transition occurring on PBT (21). 

The use of the microhardness technique in blends of condensation polymers [PET/PEN and PET/polycarbonate (PC)] evidences the formation of copolymer sequences within the blends (30). 

The microhardness technique is used when the specimen size is small or when a spatial map of the mechanical properties of the material within the micron range is required. Forces of 0.05-2 N are usually applied, yielding indentation depths in the micron range. While microhardness determined from the residual indentation is associated with the permanent plastic deformation induced in the material (see section on Basic Aspects of Indentation), microindentation testing can also provide information about the elastic properties. Indeed, the hardness to Young s modulus ratio HIE has been shown to be directly proportional to the relative depth recovery of the impression in ceramics and metals (2). Moreover, a correlation between the impression dimensions of a rhombus-based pyramidal indentation and the HIE ratio has been found for a wide variety of isotropic poljuneric materials (3). In oriented polymers, the extent of elastic recovery of the imprint along the fiber axis has been correlated to Young s modulus values (4). 

Polymorphic transitions in semicrystalliae polymers and the microhardness technique. When polymers are crystallized from the melt or from solution, their crystalline region may exhibit various types of polymorphic modifications, depending on the cooling rate, evaporation rate of the solvent, temperature, and other factors. These modifications differ in their molecular and crystal structures, as well as in their physical properties. Many types of crystalline modifications have been reported [40]. 

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