Thermoresponsive Polymer Chains Research Articles

Modelling thermoresponsive polymer brush by mesoscale computer simulations

We consider a functional surface comprising thermoresponsive polymer chains, the material that has found numerous technological and biomedical applications. However, to achieve the required time and length scales for computer modelling of such applications, one is compelled to use coarse-grained mesoscopic modelling approaches. The model used here is based on the previous work [Soto-Figueroa et al., Soft Matter, 8, 1871 (2012)], and it mimics the principal feature of the poly(N-iso-propylacrylamide) (PNIPAM), namely, the rapid change of its hydrophilicity at the lower critical solution temperature (LCST). For the case of an isolated chain, we discuss scaling properties of the radius of gyration, end-to-end distance, various distribution functions, and the density profile of monomers below and above the LCST. For the case of the model thermoresposive brush, we search for the optimum grafting density at which the change in the brush height, upon crossing the LCST, reaches its maximum value. The interpretation of the thermoresponse, in terms of the Alexander-de Gennes blobs and the level of solvation of polymer chains in a brush, is provided.

Nanostructure-assisted optical tweezers for microspectroscopic polymer analysis

Raman/fluorescence microspectroscopic analysis of individual polymer chains and nanobeads dissolved in solution will become a powerful analytical method to study their molecular structure and characteristics. With this motivation, we focused on the use of Raman microspectroscopy for optically trapped soft matter. A tightly focused near-infrared laser beam formed a microassembly of thermoresponsive polymer chains such as poly(N-isopropylacrylamide) due to a local photothermal effect and optical force. By using this method, we developed a technique for determining the polymer concentration in a polymer microassembly. Furthermore, we demonstrated a molecular condensation and detection technique based on microassembly on plasmonic nanostructures. For this molecular condensation and detection process, localized surface plasmons play an essential role in the optical force enhancement and local temperature increase around the plasmonic nanostructures. Finally, aiming toward novel manipulation methods of smaller soft nanomaterials, nanostructured semiconductor-assisted (NASSCA) optical tweezers are introduced. In this paper, we reviewed the optical manipulation methods of polymer chains and nanobeads and their applications in analytical chemistry. We reviewed our recent studies on optical manipulation techniques for microspectroscopic analysis of optically trapped polymers. A focused laser beam exerts an optical force on polymer chains, leading to the microassembly formation of them. We applied the conventional optical tweezers for the concentration determination techniques of phase-separated thermoresponsive polymer chains by combining with microspectroscopies. To overcome the limitations of the conventional optical tweezers, localized surface plasmon has attracted much attention. Finally, we introduced our original manipulation technique based on nanostructured semiconductor-assisted optical tweezers; NASSCA optical tweezers.

Design Principles for Thermoresponsive Core-Shell Nanoparticles: Controlling Thermal Transitions by Brush Morphology.

In this feature article, we summarize our recent work on understanding and controlling the thermal behavior of nanoparticles grafted with thermoresponsive polymer shells. Precision synthesis of monodisperse superparamagnetic iron oxide nanocrystals was combined with irreversible dense grafting of nitrodopamide-anchored thermoresponsive polymer chains. We provide an overview of how the dense and stable grafting of biomedically relevant polymers, including poly(ethylene glycol), poly(N-isopropylacrylamide), polysarcosin, and polyoxazolines, can be achieved. This platform has made it possible for us to demonstrate that the polymer brush geometry, as defined by the nanoparticle core and relative polymer brush size, determines the thermal transitions of the polymer brush. We furthermore summarize our work on how the polymer shell transitions and nanoparticle aggregation can be tuned. With the independent variation of the core and the shell, we can optimize and precisely control the thermally controlled solubility of our system. Finally, our feature article gives examples relevant to current and future applications. We show how the thermal response of the shell influences the nanoparticle performance in biological fluids and interactions with proteins and cells, also under purely magnetic actuation of the nanoparticles through the superparamagnetic iron oxide core.

Thermo-responsive polysulfone membranes with good anti-fouling property modified by grafting random copolymers via surface-initiated eATRP

• Block copolymer chains are constructed on polyethersulfone membrane surface via SI-eATRP. • Thermo-responsive segment of PNIPPAm/hydrophilic segments of PAm and PGMA are incorporated. • The modified membrane shows thermo-responsive property and the their LCST is mediated. • Both permeation and anti-fouling properties of fabricated membranes are improved. We graft thermo-responsive polymer chains of PNIPAm, random polymer chains of P(NIPAm- co -Am), and P(NIPAm- co -GMA) on the surface of polyethersophone membrane via SI-eATRP method. The polymer chains of PNIPAm are thermo-responsive segment, and PAm and PGMA are hydrophilic and hydrophobic segment, respectively, which endow the modified membrane thermo-responsive property and adjust the lower critical solution temperature (LCST) of the modified membrane. The influence of temperature on membrane permeation and fouling process are investigated. The LCST is determined by filtration experiment at different temperature. The results show that all modified membranes have thermo-responsive behavior, and LCST of M-PNIPAm- co -PAm increased to 38 °C compared with M-PNIPAm; but M-PNIPAm- co -PGMA has the same LCST with M-PNIPAm. The modified membranes have higher flux than that of pristine polyethersulphone membrane whether feeding with pure water or BSA solution, which mainly owing to the hydrophilic surface and rich porosity of modified membranes.

Switching of Macromolecular Ligand Display by Thermoresponsive Polymers Mediates Endocytosis of Multiconjugate Nanoparticles

Ligand-mediated targeting and internalization of plasma membrane receptors is central to cellular function. These types of receptors have accordingly been investigated as targets to facilitate entry of diagnostic and therapeutic constructs into cells. However, there remains a need to characterize how receptor targeting agents on nanoparticles interact at surface receptors and whether it is possible to control these interactions via exogenous stimuli. Here, we describe the switchable display of the iron-transporting protein, transferrin (Tf), at the surface of thermoresponsive polymer-coated gold nanoparticles and show that internalization of the coated nanoparticles into target cells changes across temperature ranges over which transferrin is expected to be sterically "hidden" by an extended polymer chain and then "revealed" by polymer chain collapse. The switching process is dependent on the numbers of transferrin molecules and thermoresponsive polymer chains attached and whether the assay temperature is above or below the transition temperatures of the responsive polymers at the nanoparticle surfaces. Significantly, however, the control of internalization is critically reliant on overall nanoparticle colloidal stability while the thermoresponsive component of the surface undergoes conformational change. The data show that the cell entry function of complex and large biomolecule ligands can be modulated by polymer-induced accessibility change but that a simple "hide and reveal" mechanism for ligand display following polymer chain collapse is insufficient to account for nanoparticle uptake and subsequent intracellular trafficking.

Temperature and pH Controlled Self‐Assembly of a Protein–Polymer Biohybrid

AbstractA novel pH and temperature dual‐responsive bioconjugate is prepared by grafting thermoresponsive polymer chains from a pH‐responsive protein amelogenin via atom transfer radical polymerization. To the best of our knowledge, this is the first time that amelogenin is exploited to prepare a hybrid biomaterial with new stimuli‐responsive property. In both basic and acidic solutions the protein–polymer bioconjugate is able to self‐assemble into uniform and stable nanoparticles when heated to above the lower critical solution temperature of the polymer. The amelogenin‐based stimuli‐responsive bioconjugate may be of great use in the fields of bioseparation and drug/gene delivery, and the synthetic approach reported here should provide a convenient means to preparing amelogenin‐based functional biohybrid materials.

Sensitive Western-Blot Analysis of Azide-Tagged Protein Post Translational Modifications Using Thermoresponsive Polymer Self-Assembly.

Western-blot (WB) is a powerful analytical technique for protein identification in complex biological samples and has been widely used in biological studies for decades. Detection specificity and sensitivity of WB largely relies on quality of the antibodies and performance of the conjugated HRP. However, the application of WB analysis for the detection of protein post-translational modifications (PTMs) is hampered by the low abundance of protein PTMs and by the limited availability of antibodies that specifically differentiate various kinds of PTMs from their protein substrates. Therefore, new recognition mechanisms and signal amplification strategies for WB analysis of protein PTMs is in high demand. In this work, we prepared a soluble polymer that detects various azide-tagged PTM proteins in WB analysis using triarylphosphine and HRP modified thermoresponsive polymer. Specific and efficient detection of azide-tagged PTM protein is achieved via the bioorthogonal reaction between azide and triarylphosphine. More importantly, the chemiluminiscent signal in the WB analysis is largely amplified by the temperature induced self-assembly of numerous thermoresponsive polymer chains carrying multiple HRPs. As a result, approximately 100 times more sensitive detection than commercial antibodies is achieved by this method using standard PTM proteins. Though, this new reagent does not directly detect native PTMs in cell, tissue or blood samples, it still has important application potential in protein PTM studies, considering the wide availability of azide-tagging techniques to a variety of PTMs.

Poly(glycidyl ether)-Based Monolayers on Gold Surfaces: Control of Grafting Density and Chain Conformation by Grafting Procedure, Surface Anchor, and Molecular Weight.

For a meaningful correlation of surface coatings with their respective biological response reproducible coating procedures, well-defined surface coatings, and thorough surface characterization with respect to layer thickness and grafting density are indispensable. The same applies to polymeric monolayer coatings which are intended to be used for, e.g., fundamental studies on the volume phase transition of surface end-tethered thermoresponsive polymer chains. Planar gold surfaces are frequently used as model substrates, since they allow a variety of straightforward surface characterization methods. Herein we present reproducible grafting-to procedures performed with thermoresponsive poly(glycidyl ether) copolymers composed of glycidyl methyl ether (GME) and ethyl glycidyl ether (EGE). The copolymers feature different molecular weights (2 kDa, 9 kDa, 24 kDa) and are equipped with varying sulfur-containing anchor groups in order to achieve adjustable grafting densities on gold surfaces and hence control the tethered polymers' chain conformation. We determined "wet" and "dry" thicknesses of these coatings by QCM-D and ellipsometry measurements and deduced anchor distances and degrees of chain overlap of the polymer chains assembled on gold. Grafting under cloud point conditions allowed for higher degrees of chain overlap compared to grafting from a good solvent like ethanol, independent of the used sulfur-containing anchor group for polymers with low (2 kDa) and medium (9 kDa) molecular weights. By contrast, the achieved grafting densities and thus chain overlaps of surface-tethered polymers with high (24 kDa) molecular weights were identical for both grafting methods. Monolayers prepared from an ethanolic solution of poly(glycidyl ether)s equipped with sterically demanding disulfide-containing anchors revealed the lowest degrees of chain overlap. The ratio of the radius of gyration to the anchor distance (2 Rg/l) of the latter coating was found to be lower than 1.4, indicating that the assembly was rather in the mushroom-like than in the brush regime. Polymer chains with thiol-containing anchors of different alkyl chain lengths (C11SH vs C4SH) formed assemblies with comparable degrees of chain overlap with 2 Rg/l values above 1.4 and are thus in the brush regime. Molecular weights influenced the achievable degree of chain overlap on the surface. Coatings prepared with the medium molecular weight polymer (9 kDa) resulted in the highest chain packing density. Control of grafting density and thus chain overlap in different regimes (brush vs mushroom) on planar gold substrates are attainable for monolayer coatings with poly(GME-ran-EGE) by adjusting the polymer's molecular weight and anchor group as well as the conditions for the grafting-to procedure.

Surface design of antibody-immobilized thermoresponsive cell culture dishes for recovering intact cells by low-temperature treatment

Antibody-immobilized thermoresponsive poly(N-isopropylacrylamide-co-2-carboxyisopropylacrylamide) [poly(IPAAm-co-CIPAAm)]-grafted cell culture surfaces were designed to enhance both the initial adhesion of weakly adhering cells and the ability of cells to detach in response to low temperature through the regulation of affinity binding between immobilized antibodies and antigens on the cellular surface. Ty-82 cells and neonatal normal human dermal fibroblasts (NHDFs), which express CD90 on the cell surface, adhered to anti-CD90 antibody-immobilized thermoresponsive surfaces at 37°C, a condition at which the grafted thermoresponsive polymer chains shrank. Adherent Ty-82 cells were detached from the surfaces by lowering the temperature to 20°C and applying external forces, such as pipetting, whereas cultured NHDF sheets spontaneously detached themselves from the surface in response to reduced temperature alone. When the temperature was decreased to 20°C, the swelling of grafted thermoresponsive polymer chains weakened the affinity binding between immobilized antibody and antigen on the cells due to the increasing steric hindrance of the polymer chains around the antigen-recognition site of the immobilized antibodies. No contamination was detected on cells harvested from covalently immobilized antibodies on the culture surfaces by low-temperature treatment, whereas a carryover of the antibody and avidin from the avidin-biotin binding surface was observed. Furthermore, the initial adhesion of adipose tissue-derived cells, which adhere weakly to PIPAAm-grafted surfaces, was enhanced on the antibody-immobilized thermoresponsive surfaces.

Selective Cell Adhesion and Detachment on Antibody-Immobilized Thermoresponsive Surfaces by Temperature Changes

Anti-CD90 antibody-immobilized thermoresponsive (AIT) surfaces were prepared for obtaining temperature-triggered switching of the selective adhesion and detachment of CD90-expressed cells. Thymic carcinoma cells (Ty-82) expressing CD90 molecules on the cellular surface were unable to adhere to isotype AIT surfaces and aggregated. In contrast, Ty-82 cells selectively adhered to anti-CD90 AIT surfaces at 37°C. These results indicate that Ty-82 cells adhered to CD90 antibody-immobilized surfaces through affinity interaction, not through nonspecific interactions when grafted thermoresponsive polymer chains shrunk at 37°C. Adhered cells were detached from surfaces by lowering temperature to 20°C with pipetting. Although affinity interaction between cells and immobilized antibodies was decreased by the dynamic swelling of grafted thermoresponsive polymer chains by lowering temperature to 20°C, the application of additional force such as pipetting was required to completely detach adhered cells. Through temperature-induced changes in affinity interaction, the condensation of CD90-positive Ty-82 cells was carried out by using anti-CD90 AIT surfaces. AIT surfaces for regulating selective cell adhesion and detachment were then successfully prepared. A novel bioassembler technology using AIT surfaces could thus be useful for temperature-dependent selective cell adhesion/detachment such as cell separation.

Molecular dynamics studies of poly(N-isopropylacrylamide) endgrafted on the surfaces of model slab pores

We report first results of a systematic study of the properties of thermo-responsive polymer chains of poly(N-isopropylacrylamide) (PNIPAM), which are endgrafted onto the inner surfaces of a slab pore of approximately 9nm width. We have systematically varied the strength of the PNIPAM–surface interaction energy to estimate the variation of the extent of the thermo-responsive effect on different surfaces. For weak to intermediate PNIPAM–surface interactions, the MD simulations show thermo-responsive behavior as characteristic changes of the radius of gyration and other measures of the polymer structure and polymer–water interactions, when comparing simulations below (at 280K) and above (at 320K) the lower critical solution temperature of PNIPAM, which is at 305K. When the PNIPAM–surface interactions become stronger, the polymer loses its thermo-responsive behavior and is adsorbed flatly on the pore walls at both investigated temperatures.

A versatile method for the preparation of poly-acrylamide derivative functionalized thermo-responsive gold nanoparticles.

In this work, we present a facile in situ strategy for synthesis of thermo-responsive gold nanoparticles (GNPs) at room temperature. By using the RAFT polymerization methods, thermo-responsive polymer chains possessing trithiocarbonate groups have been synthesized which can be split into two chains and can produce a thiol end-group by NaOH. Then the thiol end-group polymer chains can act as anchors to capture the Au(iii) ions in aqueous solution to form thermo-responsive GNPs. The thermo-responsive GNPs synthesized by the proposed method exhibited a reversible, clear-opaque transition in solution between 35 °C and 60 °C. It is conjectured that this strategy possesses the virtues including in situ, "green", universal and tailored without a tedious reducing procedure using any toxic reducing agents. Further, the thermo-responsive GNPs have been applied for the effective analysis of cysteine in urine samples.

Thermoresponsive Macroporous Scaffolds Prepared by Emulsion Templating

A versatile method to prepare non-covalently crosslinked polyHIPEs hydrogels from oil-in-water high internal phase emulsions (HIPEs) whose aqueous phase contained thermo-responsive linear polymers is described. The interconnected pore structure of the polyHIPEs is maintained by reversible physical aggregation of thermo-responsive polymer chains in an aqueous environment. This method to prepare interconnected porous hydrogels using a thermal trigger in the guise of thermo-responsive polymers by emulsion templating requires no chemical reaction during solidification of the template. This particular feature could provide a safer route to injectable scaffolds as issues of polymerisation/crosslinking chemistry and residual initiator fragments or monomers do not arise.

Thermally controlled permeation of ionic molecules through synthetic nanopores functionalized with amine-terminated polymer brushes

We present temperature-dependent ionic transport through an array of nanopores (cylindrical and conical) and a single conical nanopore functionalized with amine-terminated poly(N-isopropylacrylamide) [PNIPAAM-NH2] brushes. For this purpose, nanopores are fabricated in heavy ion irradiated polyethylene terephthlate (PET) membranes by a controlled chemical track-etching technique, which leads to the generation of carboxyl (COOH) groups on the pore surface. End-functionalized polymer chains are immobilized onto the inner pore walls via a ‘grafting-to’ approach through the covalent linkage of surface COOH moieties with the terminal amine groups of the PNIPAAM molecules by using carbodiimide coupling chemistry. The success of the chemical modification reaction is corroborated by measuring the permeation flux of charged analytes across the multipore membranes in an aqueous solution, and for the case of single conical pore by measuring the current–voltage (I–V) characteristics, which are dictated by the electrostatic interaction of the charged pore surface with the mobile ions in an electrolyte solution. The effective nanopore diameter is tuned by manipulating the environmental temperature due to the swelling/shrinking behaviour of polymer brushes attached to the inner nanopore walls, leading to a decrease/increase in the ionic transport across the membrane. This process should permit the thermal gating and controlled release of ionic drug molecules through the nanopores modified with thermoresponsive polymer chains across the membrane.

Design and Optical Properties of Active Polymer-Coated Plasmonic Nanostructures.

The grafting of stimuli-responsive polymer brushes on plasmonic structures provides a perfectly controlled two-dimensional active device with optical properties that can be modified through external stimuli. Herein, we demonstrate thermally induced modifications of the plasmonic response of lithographic gold nanoparticles functionalized by thermosensitive polymer brushes of (poly(N-isopropylacrylamide), PNIPAM). Optical modifications result from refractive local index changes due to a phase transition from a hydrophilic state (swollen regime) to a hydrophobic state (collapsed regime) of the polymer chains occurring in a very small range of temperatures. The refractive index of the polymer in aqueous solution is estimated in both states, deduced from the discrete dipole approximation (DDA) method. The combination of lithographic gold NPs and thermoresponsive polymer chains leads to a new generation of perfectly calibrated and dynamically controlled hybrid gold/polymer system for real-time nanosensors.

Fluorescent water-soluble responsive polymers site-specifically labeled with FRET dyes possessing pH- and thermo-modulated multicolor fluorescence emissions as dual ratiometric probes

We reported on the synthesis of well-defined thermoresponsive polymers labeled with fluorescence resonance energy transfer (FRET) pairs at chain middle and terminals, which can act as single chain-based dual ratiometric fluorescent probes for pH and temperature under extremely dilute conditions. Starting from difunctional initiator containing a 7-nitro-2,1,3-benzoxadiazole (NBD) moiety, the atom transfer radical polymerization (ATRP) of oligo(ethylene glycol) monomethyl ether methacrylate (OEGMA) and di(ethylene glycol) monomethyl ether methacrylate (DEGMA), and the subsequent terminal group functionalization with Rhodamine B (RhB)-ethylenediamine derivative afforded thermoresponsive NBD-P(OEGMA-co-DEGMA)-RhB2, which were labeled with FRET donor (NBD) and acceptor moieties (RhB) at the chain middle and terminals. The fluorescence emission of terminal RhB functionalities is highly pH-dependent, i.e, non-fluorescent in neutral or alkaline media (spirolactam form) and highly fluorescent in acidic media (ring-opened acyclic form), thus the off/on switching of FRET process can be facilely modulated by solution pH. Moreover, at acidic pH and highly dilute conditions, the thermo-induced chain collapse and extension of NBD-P(OEGMA-co-DEGMA)-RhB2 can effectively modulate the spatial distance between FRET donor and acceptor moieties, leading to prominent changes in FRET efficiencies. The site-specific incorporation of one FRET donor and two pH-switchable acceptors at the chain middle and terminals of thermoresponsive polymers allows for the effective off/on switching and the modulation of efficiency of FRET processes by dually playing with solution pH and temperatures. This work represents the first report of single thermoresponsive polymer chains acting as dual ratiometric fluorescent probes under highly dilute conditions.

Thermally Controlled Intracellular Uptake System of Polymeric Micelles Possessing Poly(N-isopropylacrylamide)-Based Outer Coronas

Temperature-induced intracellular uptake mechanism of thermoresponsive polymeric micelles comprising poly(N-isopropylacrylamide-co-N,N-dimethylacrylamide)-b-poly(d,l-lactide) (P(IPAAm-DMAAm)-b-PLA) inside cultured bovine carotid endothelial cells is investigated by flow cytometry and confocal laser scanning microscopy. Hydrodynamic sizes of P(IPAAm-DMAAm)-b-PLA micelles are approximately 20 nm below the lower critical solution temperature (LCST) of 39.4 degrees C, and their sizes increased to ca. 600 nm above the LCST due to the aggregation of micelles. Intracellular uptake of P(IPAAm-DMAAm)-b-PLA micelles is significantly limited at a temperature below the micellar LCST, 37 degrees C. Of great interest, the P(IPAAm-DMAAm)-b-PLA micelles are internalized into the cells above the micellar LCST (42 degrees C), being dependent on polymer concentration, time, and temperature. By contrast, no intracellular uptake of polyethylene glycol-b-PLA micelles is observed regardless of temperature changes. Enhanced intracellular micelle uptake is probably due to the enhanced interactions between the micelles and cell membranes through the dehydration of corona-forming thermoresponsive polymer chains. Internalization of submicrometer-scale micellar aggregates inside the cells is probably due to their various endocytosis mechanisms. P(IPAAm-DMAAm)-b-PLA micelles localize at the Golgi apparatus and endoplasmic reticulum, but not inside lysosomes. These results indicate that the thermoresponsive polymeric micelles are greatly promising as intracellular delivery tools of drugs, nucleic acids, and peptides/protein without lysosomal decomposition in conjunction with applied heating.

Formation of Rodlike Silica Aggregates Directed by Adsorbed Thermoresponsive Polymer Chains

This study deals with the fine-tuning of the interactions between silica nanoparticles and a LCST polymer in order to build permanent rigid linear aggregates. LCST polymers become hydrophobic and collapse above a critical temperature. The collapse of the polymer chains at the surface of the silica particles generates an attractive potential that can overcome the repulsive electrostatic forces between the silica particles under certain circumstances. The combined use of the thermoresponsiveness of poly(ethylene oxide) and of the chemical condensation properties of silica enables us to build permanent rigid aggregates displaying rodlike shapes just by increasing the temperature. These aggregates have been characterized using two complementary techniques: transmission electron microscopy and small angle neutron scattering. For low curing time, it appears that small linear aggregates are obtained when the electrostatic surface potential (pH = 8.5) is high and the initial ionic strength is low (I approximately = 10(-3) M). For higher heating time these objects aggregate further leading to some branching and ultimately to 3D gels which phase separate.

The effect of extensible PEG tethers on shielding between grafted thermo-responsive polymer chains and integrin–RGD binding

The affinity control of integrin–RGD (Arg-Gly-Asp) binding by a thermal “on–off” switch has been achieved using newly designed surfaces presenting grafted temperature-responsive poly( N-isopropylacrylamide- co-2-carboxyisopropylacrylamide) copolymers functionalized with synthetic peptides. The prepared surface was designed to expose the tethered peptides available for cell binding at active “on” state above the lower critical solution temperature (LCST). The fully extended chains, on the other hand, masked the peptides completely and the cells started to detach from the surfaces at inactive “off” sate below the LCST. This paper elucidates the shielding effect of the grafted polymer chains on the dissociation of integrin–RGD binding below the LCST. To assess the ability of the polymer-shielding, extensible poly(ethylene glycol) (PEG) tethers were introduced between peptides and the grafted polymers. PEG chains allow peptides to be tethered to surfaces via functional PEG end-groups, leading to active “on” state even below the LCST. The time required to release cells from the surface was found to be longer when peptides were coupled to an extensible tether ends, suggesting that the surfaces can engender cell attachment through adhesive moieties covalently bound to the free ends of PEG chains. These results indicate that architectural changes on the nanometer length scale are crucial for controlling integrin–RGD binding and one of the main factors causing cell detachment is the shielding effect of the grafted polymer chains.

Track etched membranes with thermo-adjustable porosity and separation properties by surface immobilization of poly( N-vinylcaprolactam)

The surface of poly(ethylene terephthalate) (PET) track etched membranes (TMs) was covered with thermo-responsive poly( N-vinylcaprolactam) (PVCL) by a photochemical immobilization method. First, a photo-reactive azidophenyl group was incorporated into the thermo-responsive PVCL chains. Secondly, the polymers were cast on the PET membrane followed by irradiation with UV light. Characterization by X-ray photoelectron spectroscopy and FT-ATR-IR demonstrated that the thermo-responsive polymer chains are effectively grafted on the membrane surface. Scanning electron microscopy and atomic force microscopy on the dry composite membranes demonstrated that the pore size diameter decreases and the surface roughness increases with increasing amount of grafted polymer chains on the TM. The water permeation through the modified TM drastically increases when the cloud point ( T cp) of the grafted PVCL-chains is reached. The permeability of a mixture of dextrane molecules is not only controlled by the molecular weight of the individual macromolecules but also strongly by temperature. This last observation opens the possibility to use the same membranes for the separation of macromolecular mixtures over a broad molecular weight range.