Effect of Alkyl Side Chain Length on Relaxation Behaviors in Poly(n-alkyl Acrylates) and Poly(n-alkyl Methacrylates)

Abstract

Dynamic mechanical analysis (DMA) is used to investigate the effect of alkyl side chain length on the relaxation behavior of poly(n-alkyl acrylates) (PnAA) and poly(n-alkyl methacrylates) (PnAMA) above the glass transition temperature (T_g). Master curves and shift factors (log a_T) were obtained using the time–temperature superposition (TTS) principle. The log a_T curves of PnAA and PnAMA exhibit a dynamic crossover from one Vogel–Fulcher–Tammann–Hesse (VFTH) equation to another above T_g. The corresponding temperature was designated as the dynamic crossover temperature (T_c). It is found that T_c/T_g and the apparent activation energy (E_g) increases, e whereas the fragility index (m) decreases with increasing alkyl side chain length. Further analysis shows that m ∝ T_g, E_g∝ , and E_g∝ m² for both PnAA and PnAMA.

Keywords:

• dynamic crossover temperature
• dynamic mechanical analysis
• fragility index
• poly(n-alkyl acrylates)
• poly(n-alkyl methacrylates)
• relaxation behavior

Acknowledgment

This work was financially supported by the National Natural Science Foundation of China (Grant no. 50873070).

Notes

^aFitting coefficients of the single VFTH equation.

^bFitting coefficients of VFTH equation in the low-temperature ranges.

^cFitting coefficients of VFTH equation in the high-temperature ranges.

^a T _g is defined as the intersection point of the Arrhenius fitting line and the VFTH fitting curve on the low-temperature side.

^b m is calculated from the fitting coefficients of the single VFTH equation.

^c E _g is determined by Eq. (6).

^d T _c corresponds to the intersection point of the two VFTH fitting curves.

^eThe data of PpenMA, which represent polymers of poly(n-pentyl methacrylate), are compiled from [32] and [33].

^fThe data of PHMA, which represent polymers of poly(hexyl methacrylate), are compiled from [32] and [33].