The Preparation And Aggregation Behavior Of Sma Comb-like Copolymers

Comb-like amphiphilic copolymers with a poly(styrene-co-maleic anhydride)(SMA) backbone containing hydrophilic poly(ethylene oxide)(PEO) and/or hydrophobic linear alkyl side chains are among the most interesting amphiphilic copolymers. They have been used as model molecule in some investigations across the field including pharmaceuticals, biological simulation and nano-technology because of their wide potential applications.Comb-like amphiphilic copolymer containing poly(styrene-co-maleic anhydride)-g-(poly(ethylene glycol) monomethyl ether)(SMA-M) and its hydrophobically modified products poly(styrene-co-maleic anhydride)-g-(poly(ethylene glycol) monomethyl ether&dodecyl)(SMA-M+C12) and poly(styrene-co-maleic anhydride)-g-(dodecyl)(SMA-C12) were prepared using "Single Batch" method. In this method, the intermediate process of precipitation and purification for SMA was eliminated and the side chain molecules, i.e. MPEG with an average molar molecular weight of1000Da and dodecanol, were transferred into the reaction solution after the end of the copolymerization of St and MA. This method will simplify synthesis process and decrease the wastage of large amounts of solvent.The molecular structures of the comb-like copolymers were confirmed by Fourier transform infrared spectroscopy,1H nuclear magnetic resonance spectroscopy, and gel permeation chromatography. The aggregation behavior of SMA-(M and/or C12) at water/air surface and in aqueous solution was investigated by equilibrium surface tension method, rheological technology, transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The adsorption and aggregation data, such as the saturation surface excess concentration (Γmax), the minimum area (Amin), the adsorption standard free energy△Gads0, and the aggregation standard free energy△Gagg0were evaluated. The pH-induced and temperature dependence adsorption and aggregation behavior of comb-like copolymers was estimated.The surface activity of the SMA-(M and/or C12) was evidently affected by the amphiphilic property of the side chains. As the increase of the hydrophobicity via the conjugation of dodecyl, the surface activity of the macromolecules is improved and the tendency to form aggregates in solution is enhanced also. At neutralization, the samples of SMA-M behaved as Newtonian fluids at all concentrations (from1.0wt%to20.0wt%) indicating that there were no macromolecular chain entanglements or interactions between aggregates in solution. For SMA-M+C12, at concentrations above10.0wt%, the presence of cross-links between aggregates is presumed to be the reason for the viscoelastic behavior. Solid-like elastic behavior could occur at low concentration (5.0wt%) of SMA-C12suggesting the formation of networks by inter-chain aggregation of the hydrophobic C12moiety.The pH-induced aggregation behavior was observed in all three copolymers. The increase of expansion degree of backbone because of the increasing dissociation degree (a) leads to the transformation of the macromolecular configuration at water/air surface an in solution. The surface activity of SMA-M decreased with increasing pH. The critical aggregation concentration (cac) and surface tension at cac (γcac) of SMA-C12increased with pH when a was less than0.3, then the shift of cac and ycac above pH=6.0was not observed. The variety of cac and γcac of SMA-M+C12gave the inflexion at pH=6.0, which gives the clue of the formation of the helical configuration and burying the C12moiety inside the helix at lower pH.With increasing pH, the macroscopic appearance of the concentrated aqueous solution of SMA-M+C12and SMA-C12transformed from fluid solution to jelly like, and returned to a solution (sol-gel-sol). The viscoelastic property was shown by the jelly-like samples of SMA-M+C12. However, the elastic gel behavior of SMA-C12gel samples was identified by rheological experiments.The morphology of the aggregate of SMA-M transformed from micelle-like aggregates connected by "necklace" to isolated swollen micelle-like aggregates, then to belt-like aggregates with increasing pH. The sol-gel-sol transition of SMA-M+C12and SMA-C12could be attributed to the transformation of the conformation of aggregate confirmed by the TEM and SEM. At low pH, micelle-like aggregates formed and became swollen as the ionization degree increased. Then micelle-like aggregates were substituted by a sponginess network due to the aggregation of the hydrophobic side chains and hydrogen bonding. The transformation of these networks to the orientation pipelike aggregates as pH exceeded the neutralization was the reason that the solid-like elastic behavior changed to a liquid-like system with viscoelastic properties.The temperature dependence of the physiochemical property of SMA-M and SMA-M+C12was attributed to the containing of MPEG chains. The enhanced hydrophobicity of MPEG chains with increasing pH improved the surface activity of these two copolymers. However, the gel point of jelly-like SMA-M+C12samples shifted to the higher frequency.The results of the investigation demonstrate that the properties of these comb-like amphiphilic copolymers can be tailored through appropriate molecular design.