| Natural polymer starch is one of the most abundant and inexpensive biomaterials having the characteristics of renewability and bio-degradability, which make it be one of the most competitive raw materials for producing biodegradable materials. However, the hydrophilic nature of starch causes many problems for the practical applications of starch-based materials, to improve the water-resistance of thermoplastic starch (TPS) and reduce the sensitivity of its mechanical properties to the environment humidity have been one of the most important tasks in the thermoplastic starch research area.The surface of TPS films was modified by esterification. Through reactions between the esterification agents and the hydroxyl groups on starch molecules, the hydrophobic groups were induced onto starch molecules to change the hydrophilic character of the material surface. The effects of surface esterification treatment parameters on the moisture absorption behavior and surface hydrophilic nature of the material were investigated. By controlling the esterification conditions, degree of substitution of hydroxyl groups in the surface layer of TPS films was enhanced, the relationships between degree of surface modification and hydrophobic properties of TPS were characterized. It is expected that surface modification of TPS through esterification can provide a scientific basis for developing a technique method to improve the water resistance and reduce the sensitivity to the environment humidity.The surfaces of TPS films were modified through esterification by using dodecenyl succinic anhydride (DDSA). The results of FT/IR transmission analysis and attenuated total reflectance FT/IR (ATR-FT/IR) measurements showed that a new peak located at 1726cm-1 appeared in ATR-FT/IR spectra which was attributed to the C=O stretching of ester groups and indicated that the esterification only occurred in the surface instead of taking place in the bulk of the films. After surface esterification, the films showed a notable lower moisture uptake compared to the control one in the 30%～70% relative humidity (RH) environment. This effect was particularly pronounced in the high RH area, the equilibrium moisture content in the modified films was decreased significantly and the sensitivity to the environment humidity was significantly lowered. The influence of esterification reaction temperature on moisture absorption behavior of surface modified samples was not marked, however, the ones of 6 hours reaction time had a low sensitivity to the environment humidity. The surface esterification carried out on the TPS films gave rise to an enhancement of hydrophilic character of surface and an increase in contact angle with water. The water angle (the control is 57°) was increased up to 98% (113°). The above results indicated that surface wettability of materials was weakened and surface hydrophobilicity was enhanced after surface modification. The ratio of intensities of the C=O stretching vibration of ester groups at 1726cm-1 to the starch CH2 stretching vibration at 2929cm-1 (A1726/A2929) was used to characterize the extent of surface esterification modification. The concentration of DDSA in ethanol was found to be the primary parameter to effect the extent of surface modification, while the concentration of alkaline aqueous solution was the secondary one and followed by environment equilibrium humidity and soaking time in the alkaline aqueous solution. The best treatment conditions for surface esterification were that soaking the samples in 0.7% sodium hydroxide solution for 60 seconds and conditioned at 95% RH for water equilibrium, then dipping them in DDSA diluted with five times (volume) ethanol and heated at 35℃for 6 hours. The extent of surface esterification characterized by ATR FT/IR was not corresponding to the one which actually influence on surface hydrophilic nature and sensitivity to the environment humidity of samples.TPS films modified with octenyl succinic anhydride (OSA) possessed lower sensitivity to the environment humidity, especially in the high RH area, and the equilibrium moisture content decreased significantly. The sample modified at 35℃had a low sensitivity to the environment humidity and the equilibrium moisture content decreased from 55% to 40% at 95% RH, the decrease amplitude was 27%. After surface modification, the surface wettability of materials was reduced and surface hydrophobilicity was enhanced. The samples having higher contact angles with water were those modified by pretreating with 2.0% sodium hydroxide aqueous solution and carrying out the esterification at 35℃. Environment equilibrium humidity was found to be the primary parameter to effect the extent of surface modification, the concentration of OSA in ethanol was the secondary one and followed by concentration of sodium hydroxide aqueous solution and soaking time in the alkaline aqueous solution. The best reaction conditions in the surface esterification process with OSA were as follows: the samples were soaked in 1.0% sodium hydroxide aqueous solution for 10 seconds and conditioned at 95% RH for water equilibrium, then quickly dipped into OSA diluted with three times (volume) of ethanol and heated at 35℃for 6 hours. The ratio of intensities of the C=O stretching vibration of ester groups at 1726cm-1 to the starch CH2 stretching vibration at 2929cm-1 were compared to that of modified with DDSA. It was found that surface modification of TPS with the alkenyl succinic anhydride having shorter chain of n-alkenyl group were more easily to yield a high degree of substitution of hydroxyl groups at the surface. However, on the other hand, an increase of the length of n-alkenyl group in alkenyl succinic anhydride provided a route to decrease hydrophilic character at the same or similar degree of substitution values.Surface modification of TPS with the mixture of DDSA and OSA was more easily to yield a higher degree of substitution of hydroxyl groups than any of them. The concentration of alkaline aqueous solution was found to be the primary parameter to effect the extent of surface modification, while volume ratio of DDSA to OSA was the secondary one and followed by the concentration of anhydride of in ethanol and reaction temperature. The extent of surface modification had little effect on the equilibrium moisture content of films at 15%～75% RH, but in the high RH area the equilibrium moisture content of films showed a trend of decreasing with increasing of the surface modification extent. Surface modification of TPS films through esterification could greatly improve the water contact angle, however, the change of water contact angle with surface modification extent was not marked in the investigated range.
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