Glassy polymers microhardness

Microhardness (MH), has been shown to be a convenient additional technique to detect accurately the ferro to paraelectric phase changes in these copolymers. The increase of MH as a function of VF2 polar sequences observed at room temperature is correlated with the contraction of the p-all-trans unit cell On the other hand, the fast exponential decrease of MH with increasing temperature, observed above Tc, is similar to that obtained for glassy polymers above Tg and suggests the existence of a liquid crystalline state in the high temperature paraelectric phase. This phase is characterized by a disordered sequence of conformational isomers (tg-, tg+, tt) as discussed for Condis crystals [109]. 

Rueda et al. (1995), have followed the physical ageing of another glassy polymer. They investigated the variation in microhardness of poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) stored in an ambient atmosphere for different times and after annealing at different temperatures below Tg for different periods of time. This material has stiffer molecules than PET due to the presence of a naphthalene ring instead of the benzene ring in the backbone chain. 

In summary, it can be concluded that using nanoindentation hardness measurements on the crack tip, down to penetration depths of 0.8 pm, it is possible to detect very small craze zones in glassy polymers. The microhardnesses for all investigated samples can be divided into three regions (1) the cracked region, (2) the crazed zone and (3) the bulk material. It was also found that the microhardness of the crazed material is larger than the microhardness of the bulk polymer due to the orientation of the polymer chains within the craze fibrils. 

Mina, M. R, Hague, M. E., Balta CaUeja, P. J., Asano, T., and Alam, M. M. 2004. Microhardness studies of the interphase boundary in rubber-softened glassy polymer blends prepared with/without compatibilizer. Journal of Macromolecular Science B Physics 43(5) 1005-1014. 

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). 

Boyanova M and Fakirov S (2004) Effect of an obstacle during processing on the weld line of injection-molded glassy polystyrene Microhardness study. Polymer 45 2093-2098. 

Garcia Gutierrez M C, Rueda D R, Balta Calleja, F J, Kuehnert I and Mennig G, (1999) Microhardness study across the weld line in doubly injection-molded glassy polymers, J Mater Sd Lett 18 1237-1238. 

Hence, the results stated above have demonstrated that the microhardness of polymers is connected with one of the components of their structure, namely with the loosely packed matrix in which the fluctuation free volume is concentrated. Glassy polymers are characterised hy some quasi-equilihrium state, corresponding to the three-dimensionality of the extra localisation energy. The microhardness is controlled hy the degree of deviation of the polymer structure from this state [87]. 

Let us next illustrate the effect of crystallinity on microhardness for a polymer with Tg above room temperature. For this purpose the correlation of H and the microstructure of PET, a polyester of typically low crystallinity having a Tg value well above room temperature, was examined. PET can be easily prepared in the form of a glassy amorphous material by quenching from the melt. 

F.J. (2009) From the glassy state to ordered polymer structures a microhardness study. Polymer, 50, 729-746. 

The notion was developed earlier that the microhardness of glassy solids is defined by the fluctuation free volume microvoid formation (or collapse) work, ascribed to the microvoid volume unit [1]. Such an approach corresponds to the results in the present section since, as has been shown above, the fluctuation free volume is concentrated in the loosely packed matrix of polymer structure. The relative fraction of the fluctuation free volume can be estimated according to Equation 1.33. As was to be expected, the linear correlation between the values of and calculated according to Equations... 

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