Toughening of semi-crystalline polymers

The aim of this chapter is to describe the micro-mechanical processes that occur close to an interface during adhesive or cohesive failure of polymers. Emphasis will be placed on both the nature of the processes that occur and the micromechanical models that have been proposed to describe these processes. The main concern will be processes that occur at size scales ranging from nanometres (molecular dimensions) to a few micrometres. Failure is most commonly controlled by mechanical process that occur within this size range as it is these small scale processes that apply stress on the chain and cause the chain scission or pull-out that is often the basic process of fracture. The situation for elastomeric adhesives on substrates such as skin, glassy polymers or steel is different and will not be considered here but is described in a chapter on tack . Multiphase materials, such as rubber-toughened or semi-crystalline polymers, will not be considered much here as they show a whole range of different micro-mechanical processes initiated by the modulus mismatch between the phases. 

Mechanical properties of semi-crystalline thermoplastics polymers can be improved by incorporating various modifier particles with different physical properties [1]. Particulate mineral fillers generally enhance the stiffness but reduce the fi acture strength and toughness, while toughening rubbery inclusions reduce stiffiiess [2, 3]. However, it is possible to improve... 

PPE has been blended with semi-crystalline polymers, PA in particular, in order to improve chemical resistance. The success of PPE/PA blends depends on adequate compatibilization. Because the PPE/PA blends show low value of notched impact strength, toughening by addition of an... 

Many semi-crystalline polymers have remarkable toughness in uniaxial tension at room temperature but show brittleness at low temperatures, under high strain rates and in notched impact loading. Since toughening of HDPE and of Nylon-6 and -66 is of primary interest, their baseline response is considered first. 

A.S. Argon, Z. Bartczak, R.E. Cohen, O.K. Muratoglu, Novel mechanism of toughcming semi -crystalline polymers, in Toughening of Plastics Advances in Modelling and Experiments, ed. by R.A. Pearson, H.-J. Sue, A.F. Yee. ACS Symposium Series, vol. 759 (Oxford University Press, London, 2000), pp. 98-124... 

One can regard the semi-crystalline polymers as behaving like the rubber-toughened ones, in that they carry their own in situ rubber domains. They do have a Tg, usually well below room temperature, and below it they become brittle for example, polypropylene articles become brittle below about 0 C, which is the Tg of amorphous polypropylene. 

Non toughened semi-crystalline PET is a very brittle polymer whatever the loading conditions are i.e., un-notched and notched tensile tests, dart test (impact of a hemispherical striker against a clamped plaque) and izod test (Fig. 1. to Fig.4.). Amorphous PET exhibits a more ductile behaviour except when notched. In such a case even amorphous PET is a brittle material at room temperature (Fig. 3. and 5.). 

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