In this paper, a time-varying sliding mode control (SMC) strategy and an adaptive enhancement of SMC to achieve state synchronization in unified chaotic systems are presented. A dedicated sliding mode function and its corresponding control law are developed to drive the synchronization error states onto the sliding surface and sustain them there. Once on the surface, complete state synchronization is achieved as the first two error states converge to zero within a finite time, while the third error state asymptotically approaches zero through exponential decay. However, the initial control design exhibits notable chattering in the control input, which is mitigated by introducing an adaptive time-varying SMC scheme that effectively suppresses such a chattering effect. The proposed adaptive approach employs online-updated state feedback gains that adjust in real time in accordance with the designed adaptation laws without requiring prior knowledge of system nonlinearities, lumped uncertainties, or external disturbances. The stability of the closed-loop error system is rigorously proven using Lyapunov stability theory, and numerical simulations are conducted to validate the effectiveness and robustness of the proposed control schemes. The proposed control algorithm has future potential in signal transmission and sensing, such as antipodal chaos shift keying (ACSK) in a spread-spectrum chaotic communication system. The pending problems lie in developing a functional ACSK system by applying the proposed algorithm and facing the limitations such as the degradation of noise mitigation efficacy and the expenses of hardware implementation. Overcoming these points will be future works of this study.