A simple model of quintessential inflation with the modified exponential potential $e^{-alpha phi} [A+(phi-phi_0)^2]$ is analyzed in the braneworld context. Considering reheating via instant preheating, we conclude that the model exhibits transient acceleration at late times for $0.96 lesssim A alpha^2 lesssim 1.26$ and $271 lesssim phi_0 alpha lesssim 273$, while permanent acceleration is obtained for $2.3times10^{-8} lesssim A alpha^2 lesssim 0.98$ and $255 lesssim phi_0 alpha lesssim 273$. The steep parameter $alpha$ is constrained to be in the range $5.3 lesssim alpha lesssim 10.8$.
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