Semicrystalline polymers matrix mechanics

Experimental results are presented that show that high doses of electron radiation combined with thermal cycling can significantly change the mechanical and physical properties of graphite fiber-reinforced polymer-matrix composites. Polymeric materials examined have included 121 °C and 177°C cure epoxies, polyimide, amorphous thermoplastic, and semicrystalline thermoplastics. Composite panels fabricated and tested included four-ply unidirectional, four-ply [0,90, 90,0] and eight-ply quasi-isotropic [0/ 45/90]s. Test specimens with fiber orientations of [10] and [45] were cut from the unidirectional panels to determine shear properties. Mechanical and physical property tests were conducted at cold (-157°C), room (24°C) and elevated (121°C) temperatures. 

The addition of a second non-crystallizable component to a crystallizable matrix can cause drastic variations of important morphological and structural parameters of the semicrystalline phase, such as the shape, size, regularity of sphemlites and intersphemlitic boundary regions, lateral dimensions of the lamellae, etc. These factors may greatly influence the mechanical behavior and, in particular, the fracture mechanisms, and thus are of great importance, especially when the toughening of semicrystalline polymer blends is considered. 

Figure 13.3 compiles the main variables and parameters that define a semicrystalline thermoplastic polymer matrix that, in combination with the processing steps, optimize thermophysical and mechanical behavior of the material to be useful for any purpose. This is the well-known structure-processing-properties dynamic triangle. 

For example, a solid polymer electrolyte is a solution of a lithium salt in a PEO matrix the ionic conductivity of such material is due to the mobility of lithium cations and their anions in an electric field. The objective of the electrolyte system is to provide mechanical integrity and ion-conducting properties. PEO is a semicrystalline polymer at room temperature and has an exceptional property to dissolve with high concentration of a wide variety of dopants. 

In spite of some imeertainties in the individual steps of the HAS meehanism in polymer stabilization due to the speerfie effeets of the polymer matrix and the environmental stress, the HAS-based nitroxides are eonsidered the key intermediate in the HAS reaetivity meehanism. Detection and quantification of the formed nitroxides using ESRI spectroscopic technique has been exploited for confirmation of the primary transformation step in HAS mechanism [15, 16, 20], as a consequence of interactions of HAS with oxygenated radical and molecular products of polyolefins (5). Monitoring of the nitroxide development enables tracing of the oxidation process within the polymer matrix. Consequently it is also a tool for marking the heterogeneity of the oxidative transformation of semicrystalline carbon chain polymers [polypropylene (PP), polyethylenes (PE)] or amorphous polymers [copolymers of ethylene with norbomene, polystyrene (PS), high impart polystyrene (HIPS), acrylonitrile-butadiene-styrene polymer (ABS)]. 

Improved dispersion of nanoscaled fillers in polymers is expected to lead to better mechanical properties. The question is if by using a nano-modified matrix instead of a laminate with neat polymer as the matrix, in a fiber-reinforced composite can lead to improve delamination resistance. In the case of semicrystalline polymer matrices, the scenario is even more complicated as the microstructure is not only influenced by the processing but also by the presence of nanofillers. The addition of different types of nanoscaled reinforcements such as carbon nanotubes, nanofibers or... 

The dominant mechanism of deformation depends mainly on the type and properties of the matrix polymer, but can vary also with the test temperature, the strain rate, and the morphology, shape, and size of the modifier particles (Bucknall 1977, 1997, 2000 Michler 2005 Michler and Balta-Calleja 2012 Michler and Starke 1996). Properties of the matrix determine not only the type of the local yield zones but also the critical parameters for toughening. In amorphous polymers with the dominant formation of crazes, the particle diameter, D, is of primary importance, while in some other amorphous and in semicrystalline polymers with the dominant formation of dilatational shear bands or intense shear yielding, the interparticle distance ID, i.e., the thickness of the matrix ligaments between particles, seems to be also an important parameter influencing the efficiency of toughening. This parameter can be adjusted by various combinations of modifier particle volume fraction and particle size. 

The glass-rubber transition temperature, Tg, of cellulose whisker filled polymer composites is an important parameter, which controls different properties of the resulting composite such as its mechanical behavior, matrix chain dynamics, and swelling behavior. Its value depends on the interactions between the polymeric matrix and cellulosic nanoparticles. These interactions are expected to play an important role because of the huge specific area inherent to nanosize particles. For semicrystalline polymers, possible alteration of the crystaUine domains by the cellulosic filler may indirectly affect the value of Tg. 

Toughened Polymers with Semicrystalline Matrix. It is well known that toughness of semicrystalline polymers such as PA (polyamide) and PP can be increased similar to the amorphous poljuners by the addition of relatively small amounts of rubber particles such as EPR or EPDM. As in HIPS and ABS, the modifier particles act as stress concentrators, initiating a plastic deformation of matrix strands between the particles as the main energy absorption step. In impact-modified PA and PP at room temperature, plastic deformation takes place through shear deformation (mechanism of multiple shear deformation). 

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