Xiaojuan Hao

Multilayer buildup and biofouling characteristics of PSS-b-PEG containing films

Thin films exhibiting protein resistance are of interest in diverse areas, ranging from low fouling surfaces in biomedicine to marine applications. Herein, we report the preparation of low protein and cell binding multilayer thin films, formed by the alternate deposition of a block copolymer comprising polystyrene sulfonate and poly(poly(ethylene glycol) methyl ether acrylate) (PSS-b-PEG), and polyallylamine hydrochloride (PAH). Film buildup was followed by quartz crystal microgravimetry (QCM), which showed linear growth and a high degree of hydration of the PSS-b-PEG/PAH films. Protein adsorption studies with bovine serum albumin using QCM demonstrated that multilayer films of PSS/PAH with a terminal layer of PSS-b-PEG were up to 5-fold more protein resistant than PSS-terminated films. Protein binding was dependent on the ionic strength at which the terminal layer of PSS-b-PEG was adsorbed, as well as the pH of the protein solution. It was also possible to control the protein resistance of the films by coadsorption of the final layer with another component (PSS), which showed an increase in protein resistance as the proportion of PSS-b-PEG in the adsorption solution was increased. In addition, protein resistance could also be controlled by the location of a single PSS-b-PEG layer within a PSS/PAH film. Finally, the buildup of PSS-b-PEG/PAH films on colloidal particles was demonstrated. PSS-b-PEG-terminated particles exhibited a 6.5-fold enhancement in cell binding resistance compared with PSS-terminated particles. The stability of PSS-b-PEG films combined with their low protein and cell binding characteristics provide opportunities for the use of the films as low fouling coatings in devices and other surfaces requiring limited interaction with biological interfaces.

Dendrimers as Scaffolds for Reversible Addition Fragmentation Chain Transfer (RAFT) Agents: a Route to Star-Shaped Block Copolymers

Star-shaped block copolymers of styrene and n-butyl acrylate having three, six, and twelve pendent arms were successfully synthesized via reversible addition fragmentation chain transfer (RAFT) polymerization. Dendritic cores ( based on 1,1,1-trimethylolpropane) of generation 0, 1, and 2 have been functionalized with 3-benzylsulfanylthiocarbonylsulfanylpropionic ester groups and have subsequently been employed to mediate the polymerization of styrene and n-butyl acrylate to generate macro-star-RAFT agents as starting materials for chain extension. The chain extension of the macro-star-RAFT agents with either styrene or n-butyl acrylate by bulk free radical polymerization at 60 degrees C gives narrowly distributed polymer (final polydispersities close to 1.2) increasing linearly in molecular weight with increasing monomer-to-polymer conversion. However, with an increasing number of arms (i.e., when going from three- to twelve-armed star polymers), the chain extension becomes significantly less efficient. The molecular weight of the generated block copolymers was assessed using H-1 NMR spectroscopy as well as size exclusion chromatography calibrated with linear polystyrene standards. The hydrodynamic radius, R-h, of the star block copolymers as well as the precursor star polymers was determined in tetrahydrofuran by dynamic light scattering (90 degrees) at 25 degrees C. Interestingly, the observed R-h-M-n relationships indicate a stronger dependence of R-h on M-n for poly(butyl acrylate) stars than for the corresponding styrene polymers. R-h increases significantly when the macro-star-RAFT agent is chain extended with either styrene or n-butyl acrylate.

Synthesis, Multilayer Film Assembly, and Capsule Formation of Macromolecularly Engineered Acrylic Acid and Styrene Sulfonate Block Copolymers

We report the use of copolymers synthesized with specific block ratios of weakly and strongly charged groups for the preparation of stable, pH-responsive multilayers. In this study, we utilized reversible addition-fragmentation chain transfer (RAFT) polymerization in the synthesis of novel pH-sensitive copolymers comprising block domains of acrylic acid (AA) and styrene sulfonate (SS) groups. The PAA x- b-SS y copolymers, containing 37%, 55%, and 73% of AA groups by mass (denoted as PAA 37- b-SS 63, PAA 55- b-SS 45, and PAA 73- b-SS 27, respectively), were utilized to perform stepwise multilayer assembly in alternation with poly(allylamine hydrochloride), PAH. The ratio of AA to SS groups, and the effect of the pH of both anionic and cationic adsorption solutions, on multilayer properties, were investigated using ellipsometry and atomic force microscopy. The presence of SS moieties in the PAA x- b-SS y copolymers, regardless of the precise composition, lead to films with a relatively consistent thickness. Exposure of these multilayers to acidic conditions postassembly revealed that these multilayers do not exhibit the characteristic large swelling that occurs with PAA/PAH films. The film stability was attributed to the presence of strongly charged SS groups. PAA x- b-SS y/PAH films were also formed on particle substrates under various adsorption conditions. Microelectrophoresis measurements revealed that the surface charge and isoelectric point of these core-shell particles are dependent on assembly pH and the proportion of AA groups in PAA x- b-SS y. These core-shell particles can be used as precursors to hollow capsules that incorporate weak polyelectrolyte functionality. The role of AA groups in determining the growth profile of these capsules was also examined. The multilayer films prepared may find applications in areas where pH-responsive films are required but large film swelling is unfavorable.