Tao Cai

Biomimetic Anchors for Antifouling and Antibacterial Polymer Brushes on Stainless Steel

Barnacle cement (BC) was beneficially applied on stainless steel (SS) to serve as the initiator anchor for surface-initiated polymerization. The amine and hydroxyl moieties of barnacle cement reacted with 2-bromoisobutyryl bromide to provide the alkyl halide initiator for the surface-initiated atom transfer radical polymerization (ATRP) of 2-hydroxyethyl methacrylate (HEMA). The hydroxyl groups of HEMA polymer (PHEMA) were then converted to carboxyl groups for coupling of chitosan (CS) to impart the SS surface with both antifouling and antibacterial properties. The surface-functionalized SS reduced bovine serum albumin adsorption, bacterial adhesion, and exhibited antibacterial efficacy against Escherichia coli (E. coli). The effectiveness of barnacle cement as an initiator anchor was compared to that of dopamine, a marine mussel inspired biomimetic anchor previously used in surface-initiated polymerization. The results indicate that the barnacle cement is a stable and effective anchor for functional surface coatings and polymer brushes.

Anti-Fouling Behavior of Hyperbranched Polyglycerol-Grafted Poly(ether sulfone) Hollow Fiber Membranes for Osmotic Power Generation

To sustain high performance of osmotic power generation by pressure-retarded osmosis (PRO) processes, fouling on PRO membranes must be mitigated. This is especially true for the porous support of PRO membranes because its porous structure is very prone to fouling by feeding river water. For the first time, we have successfully designed antifouling PRO thin-film composite (TFC) membranes by synthesizing a dendritic hydrophilic polymer with well-controlled grafting sites, hyperbranched polyglycerol (HPG), and then grafting it on poly(ether sulfone) (PES) hollow fiber membrane supports. Compared to the pristine PES membranes, polydopamine modified membranes, and conventional poly(ethylene glycol) (PEG)-grafted membranes, the HPG grafted membranes show much superior fouling resistance against bovine serum albumin (BSA) adsorption, E. coli adhesion, and S. aureus attachment. In high-pressure PRO tests, the PES TFC membranes are badly fouled by model protein foulants, causing a water flux decline of 31%. In comparison, the PES TFC membrane grafted by HPG not only has an inherently higher water flux and a higher power density but also exhibits better flux recovery up to 94% after cleaning and hydraulic pressure impulsion. Clearly, by grafting the properly designed dendritic polymers to the membrane support, one may substantially sustain PRO hollow fiber membranes for power generation.

Barnacle Cement as Surface Anchor for “Clicking” of Antifouling and Antimicrobial Polymer Brushes on Stainless Steel

Barnacle cement (BC) was utilized beneficially as a surface anchor on stainless steel (SS) for coupling of functional polymer brushes via click reactions in both grafting-to and grafting-from processes. Ethylene sulfide (ES), propargyl carbonylimidazole (PPC) and azidoethyl carbonylimidazole (AEC) reacted with amine and/or hydroxyl groups in BC to introduce the corresponding thiol, alkyne, and azide groups on SS surfaces (SS-thiol, SS-alkyne, and SS-azide, respectively). Antifouling zwitterionic SS-PMPC surface was prepared by thiol-ene photopolymerization of 2-methacryloyloxyethyl phosphorylcholine (MPC) from the SS-thiol surface. Protein-resistant SS-PPEGMA and protein-adsorbing SS-PPFS surfaces were prepared by coupling of the respective azide-functionalized poly(poly(ethylene glycol)methyl ether methacrylate) (azido-PPEGMA) and poly(2,3,4,5,6-pentafluorostyrene) (azido-PPFS) polymer brushes in azide-alkyne click reaction. Antifouling alkyne-functionalized poly(N-hydroxyethyl acrylamide) (alkynyl-PHEAA) and antibacterial alkyne-functionalized poly(2-(methacryloyloxy)ethyl trimethylammonium chloride) (alkynyl-PMETA) polymer brushes were clicked on the SS-azide surface. Adsorption of bovine serum albumin and bacteria fouling of Gram-negative Escherichia coli ( E. coli ) and Gram-positive Staphylococcus epidermidis ( S. epidermidis ) were investigated on the polymer-functionalized SS surfaces. The versatile bioanchor and functional polymer brush coatings are stable in an abiotic aqueous environment for over a month.

Surface-functionalized and surface-functionalizable poly(vinylidene fluoride) graft copolymer membranes via click chemistry and atom transfer radical polymerization.

Poly(vinylidene fluoride) (PVDF) with azide-functionalized poly(glycidyl methacrylate) (PGMA) side chains (PVDF-g-P[GMA-(N3)(OH)]) were synthesized via free radical-initiated graft copolymerization of glycidyl methacrylate (GMA) from ozone-pretreated PVDF backbone (PVDF-g-PGMA), followed by reaction of the oxirane rings in the GMA side chains with sodium azide. Alkyne-functionalized poly(N-isopropylacrylamide) (alkynyl-PNIPAM), prepared a priori by atom transfer radical polymerization (ATRP), was used for the click reaction with the azido-containing PGMA side chains of the PVDF-g-P[GMA-(N3)(OH)] copolymer to give rise to the thermoresponsive PVDF-g-P[GMA-click-PNIPAM] copolymer. Both the PVDF-g-P[GMA-(N3)(OH)] and PVDF-g-P[GMA-click-PNIPAM] copolymers can be readily cast into microporous membranes by phase inversion in an aqueous medium. The PVDF-g-P[GMA-(N3)(OH)] microporous membranes with azido-containing surfaces could be further functionalized via surface click reaction with alkyne-terminated PNIPAM of controlled chain lengths to obtain the PVDF-g-P[GMA-click-PNIPAM]surface microporous membranes. The surface composition and morphology of the PVDF-g-P[GMA-click-PNIPAM] membranes can be adjusted by the temperature of casting medium, while the flux through both types of membranes exhibits thermoresponsive behavior.

Preparation of stimuli responsive polycaprolactone membranes of controllable porous morphology via combined atom transfer radical polymerization, ring-opening polymerization and thiol–yne click chemistry

Linear di-block copolymers of poly(N-isopropylacrylamide) (PNIPAM) with a center disulfide linkage were prepared by atom transfer radical polymerization (ATRP) of N-isopropylacrylamide (NIPAM) using a bifunctional disulfide-based initiator. The center disulfide bond was cleaved by reduction with excess DL-1,4-dithiothreitol (DTT) to form thiols. The resulting thiol-terminated PNIPAM chains were conjugated to alkyne-terminated poly(e-caprolactone) (PCL) via UV-initiated thiol–yne click reaction to produce the PCL-click-PNIPAM AB2-type copolymers. The PCL-click-PNIPAM copolymers were cast by phase inversion in an aqueous medium into microporous membranes of well-defined and uniform pores. The PNIPAM content in the PCL-click-PNIPAM copolymers could be used to control the pore size and porosity of the resulting membranes. The PCL-click-PNIPAM-b-PNaSS membrane was prepared via surface-initiated ATRP of sodium 4-styrenesulfonate (NaSS) from the PCL-click-PNIPAM membrane and pore surfaces. The temperature and electrolyte responsive characteristics of the PCL-click-PNIPAM and PCL-click-PNIPAM-b-PNaSS membranes were illustrated in the swelling behavior and controlled glucose transport through the membranes. These stimuli responsive membranes with controllable morphology and low cytotoxicity have potential applications in biomedical engineering, drug delivery and tissue engineering.

Negatively charged hyperbranched polyglycerol grafted membranes for osmotic power generation from municipal wastewater.

Osmotic power holds great promise as a clean, sustainable and largely unexploited energy resource. Recent membrane development for pressure-retarded osmosis (PRO) is making the osmotic power generation more and more realistic. However, severe performance declines have been observed because the porous layer of PRO membranes is fouled by the feed stream. To overcome it, a negatively charged antifouling PRO hollow fiber membrane has been designed and studied in this work. An antifouling polymer, derived from hyperbranched polyglycerol and functionalized by α-lipoic acid and succinic anhydride, was synthesized and grafted onto the polydopamine (PDA) modified poly(ether sulfone) (PES) hollow fiber membranes. In comparison to unmodified membranes, the charged hyperbranched polyglycerol (CHPG) grafted membrane is much less affected by organic deposition, such as bovine serum albumin (BSA) adsorption, and highly resistant to microbial growths, demonstrated by Escherichia coli adhesion and Staphylococcus aureus attachment. CHPG-g-TFC was also examined in PRO tests using a concentrated wastewater as the feed. Comparing to the plain PES-TFC and non-charged HPG-g-TFC, the newly developed membrane exhibits not only the smallest decline in water flux but also the highest recovery rate. When using 0.81xa0M NaCl and wastewater as the feed pair in PRO tests at 15xa0bar, the average power density remains at 5.6xa0W/m(2) in comparison to an average value of 3.6xa0W/m(2) for unmodified membranes after four PRO runs. In summary, osmotic power generation may be sustained by properly designing and anchoring the functional polymers to PRO membranes.

Preparation of jellyfish-shaped amphiphilic block-graft copolymers consisting of a poly(ε-caprolactone)-block-poly(pentafluorostyrene) ring and poly(ethylene glycol) lateral brushes

A versatile synthetic route to jellyfish-shaped amphiphilic block-graft copolymers with a hydrophobic ring of controllable dimensions, consisting of tailored length and number of poly(e-caprolactone) (PCL) and poly(pentafluorostyrene) (PFS) blocks, and hydrophilic poly(ethylene glycol) (PEG) brushes, has been developed via the combination of ring-opening polymerization (ROP), atom transfer radical polymerization (ATRP), alkyne-azide click reaction and thiol-para-fluorine click reaction. The chemical structures and compositions of all the polymers were characterized by nuclear magnetic resonance (NMR) spectroscopy, gel permeation chromatography (GPC), thermogravimetric analyses (TGA), and X-ray photoelectron spectroscopy (XPS). The morphology of micelles from self-assembly of the cyclic-[PCL-b-P(FS-g-PEG)] copolymer in an aqueous medium was investigated by field emission scanning electron microscopy (FESEM).

Hyperbranched polycaprolactone-click-poly(N-vinylcaprolactam) amphiphilic copolymers and their applications as temperature-responsive membranes

Hyperbranched poly(ε-caprolactone) with peripheral terminal alkyne groups (HPCL) was synthesized via thiol-yne click reaction among the AB2-type α-thiol-ω-alkyne-poly(ε-caprolactone) (CH[triple bond, length as m-dash]C-PCL-SH) linear precursors. Azide-terminated poly(N-vinylcaprolactam) (PVCL-N3), prepared a priori via xanthate-mediated reversible addition-fragmentation chain transfer (RAFT) polymerization of N-vinylcaprolactam (VCL), was then linked to HPCL chains through Cu(i)-catalyzed alkyne-azide click reaction. The resultant hyperbranched-linear HPCL-click-PVCL copolymers were cast, by phase inversion in an aqueous medium, into microporous membranes of well-defined and uniform pores. The PVCL content in the HPCL-click-PVCL copolymers could be used to control the pore size and porosity of the resulting membranes. The temperature-responsive characteristics of the HPCL-click-PVCL membranes were illustrated in the swelling behavior and controlled drug transport through the membranes. These stimuli responsive membranes with controllable morphology, improved mechanical properties and negligible cytotoxicity are useful as biomaterials for controlled drug delivery.

Preparation of stimuli-responsive hydrogel networks with threaded β-cyclodextrin end-capped chains via combination of controlled radical polymerization and click chemistry

Hydrogel networks of poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (PEG-b-PPG-b-PEG primary network), with threaded sliding β-cyclodextrin-capped poly(2-(methacryloyloxy)ethyl succinate) (βCD-capped-PMES) chains, are described. The hydrogels were prepared by UV-initiated thiol–ene click reaction of the PEG-b-PPG-b-PEG diacrylate polymer with a crosslink agent, pentaerythritol tetrakis(3-mercaptopropionate) (PETMP), in the presence of βCD end-capped chains, βCD-capped-PMES. The latter was prepared a priori by reversible addition–fragmentation chain transfer (RAFT) polymerization. Due to the controlled character of RAFT polymerization and the quantitative yield of thiol–ene click reaction, the as-synthesized PEG-b-PPG-b-PEG-thread-βCD-capped-PMES hydrogels have a well-defined PEG-b-PPG-b-PEG network and tunable PMES chain length. The length of PMES chains can be regulated by varying the molar ratio of 2-(methacryloyloxy)ethyl succinate (MES) monomer to RAFT agent. The molecular structures and thermal properties of the PEG-b-PPG-b-PEG-thread-βCD-capped-PMES hydrogels were studied by 1H NMR, XPS, TGA and DSC measurements. The polymer hydrogels with threaded sliding rings exhibit both pH- and temperature-dependent equilibrium swelling ratios in aqueous media and have potential applications as biomaterials and biomedical materials.

Multi-functionalization of poly(vinylidene fluoride) membranes via combined “grafting from” and “grafting to” approaches

PVDF-g-[PBIEM-co-PPMA] graft copolymers were first synthesized in a “grafting from” process, involving thermally induced graft copolymerization of two inimers, 2-(2-bromoisobutyryloxy)ethyl methacrylate (BIEM) and propargyl methacrylate (PMA), from ozone-preactivated poly(vinylidene fluoride) (PVDF) chains. Microporous membranes were fabricated from the PVDF-g-[PBIEM-co-PPMA] copolymers by phase inversion in an aqueous medium. The tertiary C–Br groups of BIEM repeat units and the propargyl groups of PMA repeat units on the PVDF-g-[PBIEM-co-PPMA] membrane and pore surfaces provided the respective functionalities for the “grafting from” process involving surface-initiated atom transfer radical polymerization (ATRP) of the viologen-containing monomer, N-benzyl-N′-(4-vinylbenzyl)-4,4′-bipyridium dichloride (BVbpy), and the “grafting to” process involving alkyne–azide click reaction with azido-terminated poly(N-isopropylacrylamide) (PNIPAM-N3), prepared a priori via reversible addition–fragmentation chain transfer (RAFT) polymerization. The resulting PVDF-g-[P(BIEM-g-PBVbpy)-co-P(PMA-click-PNIPAM)] membrane exhibited both redox- and temperature-dependent permeability to aqueous solutions. Alternatively, metal ions, such as Ag, Au or Pt ions, can be immobilized and reduced within the viologen-containing PBVbpy brushes on the PVDF-g-[P(BIEM-g-PBVbpy)-co-PPMA] membrane. The functionalized PVDF-g-[P(BIEM-g-PBVbpy)-co-PPMA]-Ag membrane surfaces were shown to be effective in reducing bacterial adhesion and fouling under continuous-flow conditions.