From the glassy state to ordered polymer structures: A microhardness study

The review covers the understanding of the nanostructure development in glassy and semicrystalline polymers as revealed by indentation hardness methods. The microhardness of polymer glasses is discussed emphasizing the influence of thermal history and physical ageing. The correlation between hardness and glass transition temperature is brought in. Furthermore, the role played by the lamellar morphology in the case of amorphous blends of a block copolymer and a glassy homopolymer is highlighted. A discussion on the influence of filler structure on the microhardness of polymer glasses is introduced. Indentation hardness is presented as a valuable tool to study the kinetics of crystallization from the glassy state. As an example, distinct results on polymer systems under different confinement conditions are shown. The nanostructure–microindentation hardness correlation in the case of semicrystalline polymers and the influence of degree of crystallinity and crystal thickness for various flexible and semirigid polymer systems are recalled. A comprehensive discussion of the creep properties of polymer materials is offered. Concerning deformation mechanisms, experimental results show that for polymers with low degree of crystallinity and Tg below room temperature, a large deviation from the microhardness additivity law is always found. This is due to a different deformation mechanism with respect to that envisaged for polymer materials with Tg above room temperature. The assumption that microhardness approaches zero for amorphous materials above Tg is experimentally confirmed. In the case of an oriented material, it is shown that indentation hardness is capable to detect the gradual appearance of phases of intermediate order. In addition, the study of the creep properties also yields valuable information on the internal degree of order of the oriented system. Finally, an overview of the future perspectives of the application of depth-sensing indentation to the study of polymer materials is offered.