Microhardness polymer glasses

Temperature dependence of microhardness in polymer glasses determination of Tg... 

Figure 3.5. Microhardness H as a function of temperature for four amorphous polymers. Glass transition values are denoted by arrows. (From Ania etal, 1989.)...

In the present section it will be shown that microhardness can conveniently detect the glass transition temperature Tg by following // as a function of temperature. We will illustrate the temperature dependence of hardness in case of two amorphous polymers - PMMA and poly(vinyl acetate) (PVAc) - and two semicrystalline... 

Table 3.1. Glass transition temperature Tg as measured from the microhardness and coefficient of thermal softening for the polymers investigated.

It can be concluded that microhardness has been shown to be a promising technique for detecting accurately the glass transition temperature of amorphous and semicrystalline polymers in addition to the techniques commonly used for this purpose (see Section 3.1). 

The dependence of microhardness on the glass transition temperature for some commercial polymers is presented in Fig. 3.11. The polymers used for the plot in Fig. 3.11 are summarized in Table 3.2. Only non-crystallizable polymers have been selected. For the crystallizable ones, for which there are no reliable evidence for the presence of a completely amorphous phase, as is the case for example with polyamide 6 (PA6), the Hg value obtained by extrapolation to zero crystallinity is used. In this way we attempt to avoid the very strong effect on H of even small amounts of the crystalline phase. 

Figure 3.11. The relationship between the microhardness H and the glass transition temperature Tg for the amorphous polymers and copolymers listed in Table 3.2. The open circles correspond to samples 9 and 13 in Table 3.2 (see text). (From Faldrov et al., 1999.)...

Table 3.2. Glass transition temperature Tg and microhardness H of the polymers used for the plot H = f Tg) (Fig. 3.11).

Mishra et al. [1994] and Bajpai et al. [1994] determined the microhardness of PMMA/PVDF and PMMA/PCTFE blends (Table 11.9) made by solution casting from dimethyl formamide solutions. The solutions containing the two polymers were heated at 110°C for 3 h and were poured into an optically plain glass mold to prepare pellets of the blends. The poured material was annealed at 75°C for 3 h. The samples were cut from the slowly cooled (24 h) pellets for this work. 

The microhardness of glassy polymers decreases with increasing temperature because of thermal expansion (9). At the glass-transition temperature Tg, the onset of liquid-like motions takes place. The motions of long segments above Tg require more free volume and lead to a fast decrease of microhardness with temperature. The microhardness of several glassy polymers, measured at room temperature, has been shown to be directly proportional to its glass-transition temperature (10). 

Using the repoited data on the experimentally derived values of glass transition temperature, Tg, degree of crystallinity, Vickers indentation microhardness, H, and blend compositions for homopolymers, block copolymers, blends of polyolefins, or of polycondensates, blends of miscible amorphous polymers and copolymers (some of them with rather complex molecular architecture), all of them containing a soft component and/or phase at room temperature, an attempt is undertaken to look for the reasons for the frequently reported drastic deviations of the experimentally derived H values from the calculated ones by means of the additivity law assuming that the contribution of the soft component and/or phase is negligibly small. 

Ania F, Martiiiez-Salazar J and Balta Calleja F J (1989) Physical aging and glass-transition in amorphous polymers as revealed by microhardness, J Mater Sci 24 2934-2338. 

Jungnickel B J (1996) Poly(vinylidene fluoride) (overview) in Polymeric Materials Encyclopedia, (Ed. Salamone J C) CRC Press, Boca Raton, Vol. 9, pp. 7115-7122. 37. Fakirov S, Balta Calleja F J and Boyanova M (2003) On the derivation of microhardness of amorphous blends of miscible polymers from glass transition temperature values, J Mater Sci Lett 22 1011-1013. 

Yoshida and coworkers discuss densification of glasses caused by indentation [8]. Now consider the finding Bhushan e.a. [9] that microhardness measurements of worn metal samples show a 10-80 % increase of hardness in the worn layer. While behavior of the metals is different from that of polymers since the latter are viscoelastic, a possible connection between the characteristics of groove profiles we have obtained with hardness determination seemed worth pursuing. 

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