PMMA, creep moduli

Fig. 3.3-34 Poly(methyl methacrylate), PMMA creep modulus versus time, at 23 °C...

PMMA has medium moduli that involve medium strains for moderate loading. Consequently, creep moduli are also in an intermediate range at room temperature. When the temperature rises moderately, the creep modulus decreases significantly, as we can see in Figure 4.74 the difference is slightly more than 40% for a temperature increase from 20°C to 50°C. 

Figure 4.74. PMMA examples of creep modulus (GPa) versus time (h) under 3.5 or 7 MPa at23°C or50°C...

Craze growth at the crack tip has been qualitatively interpreted as a cooperative effect between the inhomogeneous stress field at the crack tip and the viscoelastic material behavior of PMMA, the latter leading to a decrease of creep modulus and yield stress with loading time. If a constant stress on the whole craze is assumed then time dependent material parameters can be derived by the aid of the Dugdale model. An averaged curve of the creep modulus E(t) is shown in Fig. 13 as a function of time, whilst the craze stress is shown in Fig. 24. 

Fig. 3.25. Material data of the microregion around the propagating crack tip as derived by the application of the Dugdale model to measured craze sizes in HMW PMMA (Fig. 3.22) craze stress and creep modulus E...

Many viscoelastic parameters of polymers can be used to obtain master curves such as creep compliance relaxation modulus E, E", C, G" and loss factor tan d. These viscoelastic variables of polymers are dependent on temperature, frequency and relaxation time. As an example. Figure 4.65 shows the experimental results and Figure 4.66 shows the master curve of PMMA. The parameter chosen is the storage moduli. The following procedure is recommended for the master curve measurement. 

The other acrylate bone cement is based on poly-ethylmethacrylate (PEMA) and n-butylmethacrylate ( -BMA) monomer [61], Comparing to PMMA cement, less heat is produced during polymerization of the PEMA-n-BMA cement, and the polymer has a relatively low modulus and high ductility to reduce the issue of fracture. The biocompatibility of the PEMA-n-BMA cement has been excellent [62]. But these bone cements have been found to be susceptible to creep. To improve creep resistance, bioactive HA particles were incorporated [63]. Although HA improved bioactivity and creep behavior of the cement, the cement failed at lower number of cycles. 

Wu [49] proposed estimating G° as the storage modulus at the frequency where tan 5= minimum, but often there is no minimum in the data. Using this method, Fuchs et al. [38] reported a modest effect of the tacticity of PMMA on the plateau modulus. They also found that Eq. 5.4 fitted their viscosity data with a = 3.4 but that the constant K depended on tacticity. Plazek et al. [50] studied the effect of tacticity on the creep behavior of PMMA and noted that absorbed water is always a problem in making measurements on polar polymers. Wu [51] later proposed an empirical equation between the ratio (G Gq) of the plateau modulus to the crossover modulus, G, (where G = G") and the polydispersity index. 

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