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Temperature- and thickness-dependent elastic moduli of polymer thin films.

Ao Z, Li S - Nanoscale Res Lett (2011)

Bottom Line: The mechanical properties of polymer ultrathin films are usually different from those of their counterparts in bulk.Understanding the effect of thickness on the mechanical properties of these films is crucial for their applications.However, it is a great challenge to measure their elastic modulus experimentally with in situ heating.

View Article: PubMed Central - HTML - PubMed

Affiliation: School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia. zhimin.ao@unsw.edu.au.

ABSTRACT
The mechanical properties of polymer ultrathin films are usually different from those of their counterparts in bulk. Understanding the effect of thickness on the mechanical properties of these films is crucial for their applications. However, it is a great challenge to measure their elastic modulus experimentally with in situ heating. In this study, a thermodynamic model for temperature- (T) and thickness (h)-dependent elastic moduli of polymer thin films Ef(T,h) is developed with verification by the reported experimental data on polystyrene (PS) thin films. For the PS thin films on a passivated substrate, Ef(T,h) decreases with the decreasing film thickness, when h is less than 60 nm at ambient temperature. However, the onset thickness (h*), at which thickness Ef(T,h) deviates from the bulk value, can be modulated by T. h* becomes larger at higher T because of the depression of the quenching depth, which determines the thickness of the surface layer δ.

No MeSH data available.


Related in: MedlinePlus

The thickness-dependent elastic modulus of PS thin films at different temperatures. The inset enlarges the encircled region to show the good match of Tg(h)/Tgb and Ef(T,h)/Ef(T,∞).
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Figure 3: The thickness-dependent elastic modulus of PS thin films at different temperatures. The inset enlarges the encircled region to show the good match of Tg(h)/Tgb and Ef(T,h)/Ef(T,∞).

Mentions: Most recently, the deviation of the elastic modulus from its bulk value was studied for thin glassy polymer films with different glass-transition temperatures at ambient temperature. These results suggested that the deviations are significantly influenced by the quench depth into the glass (Tgb - T) [8]. To induce the different quench depths, the temperature T is varied in this study. Figure 3 plots the thickness-dependent elastic moduli of PS thin films at different temperatures. For clearly demonstrating the size effect, a relative value of E(T,h)/E(T,∞) is taken as the ordinate in the figure. From Figure 3, it is found that the size effect is more significant at high temperatures. Recent research has reported that the elastic modulus and glass-transition temperature deviate from the corresponding bulk values at the same thicknesses for poly(n-propyl methacrylate) (PnPMA) thin film on poly(dimethylsiloxane) (PDMS) substrate [8]. In Figure 3, Tg(h)/Tgb for PS thin films on a passivated substrate is shown by the dashed curve, which is obtained by Equation 4. We find that the functions Tg(h)/Tgb and Ef(150,h)/E(150,∞) behave in a similar manner as a function of h. Also, the thicknesses at which the two functions deviate from the corresponding bulk values are almost the same, as in the case of the PnPMA/PDMS system. Considering a criterion that the deviation of the modulus starts when E(T,h)/E(T,∞) ≈ 0.96, the critical film thickness h*, at which the deviation starts for different temperature, is shown in Figure 4. Note that h* increases as T increases. In other words, at high temperatures, the size effect is more important, and it can be tuned by the application temperature. Therefore, in actual applications, to avoid decreasing the strength of thin films, one should make sure that the film thickness is larger than h* for a given temperature. The temperature dependence of h* is the consequence of the depression of quench depth when the temperature is near the glass-transition region.


Temperature- and thickness-dependent elastic moduli of polymer thin films.

Ao Z, Li S - Nanoscale Res Lett (2011)

The thickness-dependent elastic modulus of PS thin films at different temperatures. The inset enlarges the encircled region to show the good match of Tg(h)/Tgb and Ef(T,h)/Ef(T,∞).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211304&req=5

Figure 3: The thickness-dependent elastic modulus of PS thin films at different temperatures. The inset enlarges the encircled region to show the good match of Tg(h)/Tgb and Ef(T,h)/Ef(T,∞).
Mentions: Most recently, the deviation of the elastic modulus from its bulk value was studied for thin glassy polymer films with different glass-transition temperatures at ambient temperature. These results suggested that the deviations are significantly influenced by the quench depth into the glass (Tgb - T) [8]. To induce the different quench depths, the temperature T is varied in this study. Figure 3 plots the thickness-dependent elastic moduli of PS thin films at different temperatures. For clearly demonstrating the size effect, a relative value of E(T,h)/E(T,∞) is taken as the ordinate in the figure. From Figure 3, it is found that the size effect is more significant at high temperatures. Recent research has reported that the elastic modulus and glass-transition temperature deviate from the corresponding bulk values at the same thicknesses for poly(n-propyl methacrylate) (PnPMA) thin film on poly(dimethylsiloxane) (PDMS) substrate [8]. In Figure 3, Tg(h)/Tgb for PS thin films on a passivated substrate is shown by the dashed curve, which is obtained by Equation 4. We find that the functions Tg(h)/Tgb and Ef(150,h)/E(150,∞) behave in a similar manner as a function of h. Also, the thicknesses at which the two functions deviate from the corresponding bulk values are almost the same, as in the case of the PnPMA/PDMS system. Considering a criterion that the deviation of the modulus starts when E(T,h)/E(T,∞) ≈ 0.96, the critical film thickness h*, at which the deviation starts for different temperature, is shown in Figure 4. Note that h* increases as T increases. In other words, at high temperatures, the size effect is more important, and it can be tuned by the application temperature. Therefore, in actual applications, to avoid decreasing the strength of thin films, one should make sure that the film thickness is larger than h* for a given temperature. The temperature dependence of h* is the consequence of the depression of quench depth when the temperature is near the glass-transition region.

Bottom Line: The mechanical properties of polymer ultrathin films are usually different from those of their counterparts in bulk.Understanding the effect of thickness on the mechanical properties of these films is crucial for their applications.However, it is a great challenge to measure their elastic modulus experimentally with in situ heating.

View Article: PubMed Central - HTML - PubMed

Affiliation: School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia. zhimin.ao@unsw.edu.au.

ABSTRACT
The mechanical properties of polymer ultrathin films are usually different from those of their counterparts in bulk. Understanding the effect of thickness on the mechanical properties of these films is crucial for their applications. However, it is a great challenge to measure their elastic modulus experimentally with in situ heating. In this study, a thermodynamic model for temperature- (T) and thickness (h)-dependent elastic moduli of polymer thin films Ef(T,h) is developed with verification by the reported experimental data on polystyrene (PS) thin films. For the PS thin films on a passivated substrate, Ef(T,h) decreases with the decreasing film thickness, when h is less than 60 nm at ambient temperature. However, the onset thickness (h*), at which thickness Ef(T,h) deviates from the bulk value, can be modulated by T. h* becomes larger at higher T because of the depression of the quenching depth, which determines the thickness of the surface layer δ.

No MeSH data available.


Related in: MedlinePlus