MAA/MMA Gradient Copolymers: Chain Structure Tailor-making And Properties

Chain microstructures, including molecular weight and its distribution, copolymer composition and its distribution, are crucial to the properties and performances of polymer materials. Gradient copolymers, in which the chemical compositions change along chain backbones, have a unique type of chain microstructures that can be synthesized only via living polymerization processes. In the past, gradient copolymers were synthesized primarily by anion polymerization, so the preparation and properties of gradient copolymers were rarely reported due to the rigorous polymerization condition. Preparation of gradient copolymer having different composition profiles is in urgent need to investigate the relationship between chain microstructures and properties of gradient copolymers.Controlled/living radical polymerization, such as atom transfer radical polymerization (ATRP), provides a powerful tool to precisely control over polymer chain microstructure. Due to different reactivity ratios of comonomers, gradient copolymers can be formed spontaneously via atom transfer radical copolymerization (ATRcoP). However, in a batch ATRcoP process, the gradient composition profile along chain backbone is determined by the monomer composition drifting for a given pair of monomers and its recipe. In this study, we aimed at developing the model-based semibatch ATRcoP to synthesize the gradient copolymer with targeted composition profiles. The solution and bulk properties of those copolymers were then investigated systematically.An ATRcoP model was developed based on terminal model for the ATRcoP of t-butyl methacrylate (tBMA) and methyl methacrylate (MMA). The equilibrium constants in the ATRP of MMA and tBMA at 100℃were estimated to be 1.8×10^-7 and 7.2×10^-7 respectively through the data correlation. The model was verified by batch experiments and was found to give good correlation for the polymerization rate, molecular weight and copolymer composition data.The ATRcoP model coupled with a reactor model was then applied to the semi-batch ATRcoP and was used to calculate comonomer feeding rates for the targeted gradient composition profiles. tBMA/MMA gradient copolymers having uniform, linear, hyperbolic (two types), triblock and diblock composition profiles were synthesized . It was found that the experimental monomer conversion, molecular weight and cumulative copolymer composition were in good agreement with their targeted theoretical values. tBMA/MMA gradient copolymers were then hydrolyzed to obtain the methacrylic acid (MAA) /MMA gradient copolymers with corresponding composition profiles.The aggregate morphologies of MAA/MMA gradient copolymer were investigated. MAA/MMA uniform copolymer could be dissolved in aqueous phase, while MAA/MMA copolymers having linear, triblock and diblock composition profiles aggregating into spherical micelles. MAA/MMA uniform copolymer was in compact coil conformation at low pH and stretched with increasing pH value. The pH range inducing conformational transition was pH=6.11～6.75. Copolymer chains having linear, triblock and diblock composition profiles just took the conformational transition in the hydrophilic shell of micelles and the corresponding pH range triggering conformational transition were pH=5.64～6.63, pH=5.50～6.18 and pH=5.38～6.05 respectively. The copolymers having sharper composition gradient required lower pH to trigger the conformational transition and a narrower pH range to complete the transition.The performances of the MAA/MMA gradient copolymers used as miniemulsion emulsifier were evaluated through measuring the droplet size of dispersed oil. MAA/MMA uniform copolymer was the best emulsifier and it could stabilize the oil droplet with the smallest size. In comparison, MAA/MMA diblock copolymer was the worst emulsifier. At pH>7.0, all MAA/MMA gradient copolymers were low-efficiency emulsifier. Chains conformation of MAA/MMA gradient copolymers was found to be the key parameter determining the droplet size in miniemulsion. The compact coil conformation was propitious to improve the stability of miniemulsion.MAA/MMA gradient copolymers having triblock, hyperbolic (sharper one in composition gradient) and diblock could self-organize into lamellar nanostructures in thin film through solvent-annealing. The lamella grew in the direction perpendicular to the surface of silica wafer and the distances between adjacent lamella were about 28nm. Lamellar at the micrometer level could be fabricated through prolonging the time of solvent-annealing to diminish the defects in films.