Conformation of End-Tethered PNIPAM Chains in Water and in Acetone by Neutron Reflectivity

On the foundation of thermal "Switching": The culture substrate governs the phase transition mechanism of thermoresponsive brushes and their performance in cell sheet fabrication

Thermally "switchable" poly(glycidyl ether) (PGE) brushes constitute effective coatings for the temperature-triggered harvest of confluent cell sheets. Based on a simple "grafting-to" approach, such coatings can be tethered to various applied plastic culture substrate materials. Herein, we elucidate the self-assembly of PGE brushes with tunable grafting densities up to 0.12 and 0.22 chains nm^-2 on polystyrene (PS) and tissue culture PS (TCPS), respectively. In terms of temperature-dependent wettability and protein adsorption, we found that brushes exhibit distinct grafting density-dependent properties which correlate with their cell sheet fabrication performance. In addition, temperature-ramped quartz-crystal microbalance with dissipation (QCM-D) measurements revealed marked substrate-specific PGE phase transitions which allowed us to deduce comprehensive switching mechanisms. Thus, we demonstrate that brushes tethered to hydrophilic TCPS (contact angle (CA) ∼ 60°) undergo a "cushioned" transition comprising a non-switchable, hydrated basal layer as well as a switchable top layer which regulates cell sheet detachment. In contrast, PGE brushes tethered to PS undergo a "grounded" transition which is substantially influenced by the dehydrating effect of the less hydrophilic PS substrate (CA ∼ 90°). These divergent phase transition mechanisms give rise to a broad scope in cell sheet fabrication performance, yielding staggered detachment times within a 30 min to 3 h range. Hence, we emphasize the importance of a detailed knowledge on the effect of applied culture substrates on the thermal switchability and phase transition characteristics of thermoresponsive brush coatings to accomplish an optimized design for functional cell culture dishes. STATEMENT OF SIGNIFICANCE: As the first comparative study of its kind, we elucidate the substrate-dependent thermal switchability of thermoresponsive brush coatings and evaluate their grafting density-dependent phase transition mechanism and its effect on cell sheet fabrication performance.

Weakly Ionically Bound Thermosensitive Hyperbranched Polymers

We synthesized novel amphiphilic hyperbranched polymers (HBPs) with variable contents of weakly ionically tethered thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) macrocations in contrast to traditional covalent linking. Their assembling behavior was studied below and above the lower critical solution temperature (LCST). The HBPs underwent a morphological transition under changing temperature and ionic strength due to the LCST transition of PNIPAM and the reduction in the ionization degree of terminal ionic groups, respectively. We suggest that, in contrast to traditional branched polymers, ionically linked PNIPAM macrocations can reversibly disassociate from the sulfonate groups and form mobile coronas, endowing the dynamic micellar morphologies. In addition, assembly at the air-water interface confined PNIPAM macrocations and resulted in the formation of heterogeneous Langmuir-Blodgett (LB) monolayers with diverse surface morphologies for different peripheral compositions with circular domains formed in the condensed state. The HBPs with 25% PNIPAM showed larger and more stable circular domains that were partially preserved at high compression than those of HBPs with 50% PNIPAM. Moreover, the LB monolayers showed variable surface mechanical and surface charge distribution, which can be attributed to net dipole redistribution caused by the behavior of mobile PNIPAM macrocations and core sulfonate groups.

Thermoresponsive Gigaporous Microspheres Facilitate the Efficient Refolding of Recombinant Nitrilase Inclusion Bodies

In order to assist the refolding of recombinant nitrilase inclusion bodies, a series of thermoresponsive media were prepared by grafting poly(N-isopropylacrylamide-co-butyl-methacrylate) [P(NIPAM-co-BMA)] brushes onto PS microspheres with various particles and pore sizes via an atom transfer radical polymerization (ATRP) method. The effects of particle sizes, pore sizes, and brush grafting amounts of thermoresponsive microspheres on nitrilase refolding were investigated preliminarily. The results showed that the PS-P(NIPAM-co-BMA) microspheres with the medium particle size (74 μm), gigapore size (320 nm), and high grafting amount (35.6 mg/m²) were the most effective candidates. The final nitrilase activity yield could be up to 84.5% with a high initial protein concentration (1 mg/mL) at 30 °C, which was 52.5% higher than that of a simple dilution refolding method at the initial protein concentration (0.1 mg/mL). After the refolding process, the PS-P(NIPAM-co-BMA) microspheres can be easily separated by self-precipitation, and the activity yield of nitrilase still reached 74.5% after being reused for five batches. These results indicated that the thermoresponsive gigaporous medium was an ideal alternative as an artificial chaperone.

Responsive Adsorption of N-Isopropylacrylamide Based Copolymers on Polymer Brushes

We investigate the adsorption of pH- or temperature-responsive polymer systems by ellipsometry and neutron reflectivity. To this end, temperature-responsive poly (N-isopropylacrylamide) (PNIPAM) brushes and pH-responsive poly (acrylic acid) (PAA) brushes have been prepared using the "grafting onto" method to investigate the adsorption process of polymers and its reversibility under controlled environment. To that purpose, macromolecular brushes were designed with various chain lengths and a wide range of grafting density. Below the transition temperature (LCST), the characterization of PNIPAM brushes by neutron reflectivity shows that the swelling behavior of brushes is in good agreement with the scaling models before they collapse above the LCST. The reversible adsorption on PNIPAM brushes was carried out with linear copolymers of N-isopropylacrylamide and acrylic acid, P(NIPAM-co-AA). While these copolymers remain fully soluble in water over the whole range of temperature investigated, a quantitative adsorption driven by solvophobic interactions was shown to proceed only above the LCST of the brush and to be totally reversible upon cooling. Similarly, the pH-responsive adsorption driven by electrostatic interactions on PAA brushes was studied with copolymers of NIPAM and N,N-dimethylaminopropylmethacrylamide, P(NIPAM-co-MADAP). In this case, the adsorption of weak polycations was shown to increase with the ionization of the PAA brush with interactions mainly located in the upper part of the brush at pH 7 and more deeply adsorbed within the brush at pH 9.

Synthesis of acrylamide-based block-copolymer brushes under flow: monitoring real-time growth and surface restructuring upon drying

Block-copolymer brushes of water-soluble acrylamides have been synthesised by SI-ATRP under continuous flow and their growth monitored in situ by means of a quartz-crystal microbalance with dissipation (QCM-D).

Direct Nanoscopic Measurement of Laminar Slip Flow Penetration of Deformable Polymer Brush Surfaces: Synergistic Effect of Grafting Density and Solvent Quality

A detailed quantitative nanoscopic description of soft surfaces under dynamic flow is lacking, despite its importance. To better understand the role of surface texture in nanoscopic mass transport in complex media, we used Förster resonance energy transfer in combination with total internal reflectance fluorescence microscopy (FRET-TIRFM) to directly measure laminar slip flow penetration depth (slip length) on poly(N-isopropylacrylamide) (pNIPAM) thin films (50-110 nm) of different grafting densities (0.60, 0.38, and 0.27 chain/nm²) in solvents of different qualities created via cononsolvency in situ. Nontrivial synergistic interplay of grafting density and solvent quality on slip length was observed. Slip lengths are typically tens of nm (40-100 nm), increasing and then reaching a plateau with applied linear flow velocity (192-2,952 μm/s) regardless of experimental system. Slip length was systematically larger for lower density films, but the effect of grafting density was more significant in a good solvent than a poor solvent. Interestingly, however, the stagnant film thickness (polymer swollen thickness minus the slip length) collapsed to almost a singular value for a given grafting density regardless of solvent quality, likely suggesting a large gradient of segmental mobility at nonequilibrium. Moreover, we found that slip flow penetrates into soft pNIPAM surfaces more deeply in a good solvent than in a poor solvent and that this behavior was general and independent of grafting density. This behavior is counter to the notion that less interaction between a fluid (probe) and a solid surface promotes slip.

Switch It Inside-Out: "Schizophrenic" Behavior of All Thermoresponsive UCST-LCST Diblock Copolymers

This feature article reviews our recent advancements on the synthesis, phase behavior, and micellar structures of diblock copolymers consisting of oppositely thermoresponsive blocks in aqueous environments. These copolymers combine a nonionic block, which shows lower critical solution temperature (LCST) behavior, with a zwitterionic block that exhibits an upper critical solution temperature (UCST). The transition temperature of the latter class of polymers is strongly controlled by its molar mass and by the salt concentration, in contrast to the rather invariant transition of nonionic polymers with type II LCST behavior such as poly(N-isopropylacrylamide) or poly(N-isopropyl methacrylamide). This allows for implementing the sequence of the UCST and LCST transitions of the polymers at will by adjusting either molecular or, alternatively, physical parameters. Depending on the location of the transition temperatures of both blocks, different switching scenarios are realized from micelles to inverse micelles, namely via the molecularly dissolved state, the aggregated state, or directly. In addition to studies of (semi)dilute aqueous solutions, highly concentrated systems have also been explored, namely water-swollen thin films. Concerning applications, we discuss the possible use of the diblock copolymers as "smart" nanocarriers.

Direct Measurement of Water Permeation in Submerged Alkyl Thiol Self-Assembled Monolayers on Gold Surfaces Revealed by Neutron Reflectometry

Self-assembled monolayers (SAMs) of alkyl thiols are frequently used to chemically functionalize gold surfaces for applications throughout materials chemistry, electrochemistry, and biotechnology. Despite this, a detailed understanding of the structure of the SAM-water interface generated from both formation and use of the SAM in an aqueous environment is elusive, and analytical measurements of the structure and chemistry of the SAM-water interface are an ongoing experimental challenge. To address this, we used neutron reflectometry (NR) to measure water association with both hydrophobic and hydrophilic SAMs under both wet and dry conditions. SAMs used for this study were made from hydrophobic decanethiol mixed with hydrophilic 11-azido-1-undecanethiol with compositions of 0-100% of the azide-terminated thiol. All SAMs were formed by conventional solution incubation of a Au substrate immersed in ethanol. Each SAM was characterized by grazing incidence angle reflection-absorption Fourier transfer infrared spectroscopy, contact angle goniometry, and electrochemical methods to confirm it was a completely formed monolayer with evidence of extensive crystalline-like domains. NR measured significant absorption of water into each SAM, ranging from 1.6 to 5.7 water molecules per alkyl thiol, when SAMs were immersed in water. Water infiltration was independent of SAM composition and terminal group hydrophilicity. These results demonstrate that water accesses defects, fluid regions, and heterogeneous domains inherent to even well-formed SAMs.

Adsorbed Polyzwitterion Copolymer Layers Designed for Protein Repellency and Interfacial Retention

Poly(2-methacryloyloxyethyl phosphorylcholine) (pMPC), when end-tethered to surfaces by the adsorption of copolymeric cationic segments, forms adsorbed layers that substantially reduce protein adsorption. This study examined variations in the molecular architecture of copolymers containing cationic poly(trimethylammonium ethyl methacrylate (pTMAEMA) anchor blocks that adsorbed strongly to negative surfaces. With appropriate copolymer design, the pTMAEMA blocks were shielded, by pMPC tethers, from solution-phase proteins. The most protein-resistant copolymer layers, eliminating fibrinogen and lysozyme adsorption within detectible limits of 0.01 mg/m², had metrics (the amount of pMPC at the surface and the reduced tether footprint) consistent with the formation of an interfacial polymer brush. The p(TMAEMA-b-MPC) copolymer layers substantially outperformed the protein resistance of surface-polymerized pMPC layers when compared on a per-polyzwitterion-mass basis or on the basis of the scaled tether area. Additionally, p(TMAEMA-b-MPC) copolymer layers offered advantages over the much-studied cationically anchored poly(ethylene glycol) (PEG) graft copolymer system, which forms PEG brushes by the adsorption of a poly l-lysine (PLL) backbone. Although the optimized p(TMAEMA-b-MPC) and PLL-PEG copolymers were similarly fibrinogen-resistant, the cationic protein lysozyme was repelled by pMPC but adhered to the PEG brush via PEG-lysozyme attractions. Additionally, the adsorbed p(TMAEMA-b-MPC) copolymers were not displaced by poly l-lysine homopolymers, which completely displaced the PLL-PEG copolymer to expose a protein-adhesive surface. Thus, the p(TMAEMA-b-MPC) copolymer system comprises a scalable means to produce protein-repellent surfaces, free of the complexities of surface-initiated polymerization and with the advantages of polyzwitterions.

Effect of Solvent Quality on Laminar Slip Flow Penetration of Poly(N-isopropylacrylamide) Films with an Exploration of the Mass Transport Mechanism

The effect of solvent quality on the slip flow penetration of polymer films was evaluated by monitoring small-molecule mass transport under varying laminar flow rates using Förster resonance energy transfer in combination with total internal reflectance fluorescence microscopy (FRET-TIRFM). For thin films of poly(N-isopropylacrylamide) (pNIPAM), solvents with solvent quality ranging from good to poor were studied. The solvents used were composed of varying mole ratios of methanol and water in order to take advantage of the unique cononsolvency phenomenon of pNIPAM such that differences in the physicochemical properties of these solvents were insignificant for fluorescence detection. FRET quenching of a donor fluorophore covalently tethered on the substrate surface at the bottom of the pNIPAM film by a solution-confined acceptor was monitored as a function of time. Quenching curves were fit to a combined Taylor-Aris-Fickian mass transport model for the acceptor, rhodamine B (RhB) or 2-nitrobenzylaclohol (2-NBA), allowing apparent diffusion coefficients to be determined and used to assess slip flow penetration into the polymer film. An increase in the apparent diffusion coefficient of tracer molecules was observed with increasing laminar flow rate for all solvents, indicating that mass transport processes in the pNIPAM film are significantly perturbed by laminar slip flow penetration. In going from poor solvents, 31 mol % MeOH/H₂O and 20 mol % MeOH/H₂O, to the theta solvent, 13 mol % MeOH/H₂O, and finally to a good solvent, 100% methanol, the slip length increases from 25 to 37 to 70 to 128 nm, with the corresponding percentage of the film penetrated by slip flow increasing from 19 to 27 to 42 to 57%, respectively. The apparent diffusion coefficients of the two acceptors, RhB and 2-NBA, which differ substantially in size and charge, in pNIPAM films under identical conditions were found to be of the same order of magnitude, albeit with a small difference (∼10%) due to inherently different diffusive properties. Therefore, the dominant mechanism for the mass transport of small molecules in densely grafted thin pNIPAM brush films is suggested to be linear Fickian diffusion under the chosen laminar flow conditions with linear flow velocities ranging from 192 to 2952 μm/s. High-quality fits to a Taylor-Aris-Fickian diffusion model of the experimental breakthrough curves obtained with both acceptor molecules further substantiate the proper use of this model and the validity of the FRET-TIRFM method.

Preferential adsorption of the additive is not a prerequisite for cononsolvency in water-rich mixtures

NMR studies reveal the distinct molecular interactions accounting for cononsolvency.

Polyelectrolyte brushes: theory, modelling, synthesis and applications

Polyelectrolyte (PE) brushes are a special class of polymer brushes (PBs) containing charges. Polymer chains attain "brush"-like configuration when they are grafted or get localized at an interface (solid-fluid or liquid-fluid) with sufficiently close proximity between two-adjacent grafted polymer chains - such a proximity triggers a particular nature of interaction between the adjacent polymer molecules forcing them to stretch orthogonally to the grafting interface, instead of random-coil arrangement. In this review, we discuss the theory, synthesis, and applications of PE brushes. The theoretical discussion starts with the standard scaling concepts for polymer and PE brushes; following that, we shed light on the state of the art in continuum modelling approaches for polymer and PE brushes directed towards analysis beyond the scaling calculations. A special emphasis is laid in pinpointing the cases for which the PE electrostatic effects can be de-coupled from the PE entropic and excluded volume effects; such de-coupling is necessary to appropriately probe the complicated electrostatic effects arising from pH-dependent charging of the PE brushes and the use of these effects for driving liquid and ion transport at the interfaces covered with PE brushes. We also discuss the atomistic simulation approaches for polymer and PE brushes. Next we provide a detailed review of the existing approaches for the synthesis of polymer and PE brushes on interfaces, nanoparticles, and nanochannels, including mixed brushes and patterned brushes. Finally, we discuss some of the possible applications and future developments of polymer and PE brushes grafted on a variety of interfaces.

Stretching of collapsed polymers causes an enhanced dissipative response of PNIPAM brushes near their LCST

Poly(N-isopropyl acrylamide) (PNIPAM) is a stimulus-responsive polymer that can switch in water from an expanded state below the lower critical solution temperature (LCST) of 32 °C to a globular state above the LCST. It was recently shown that, as a consequence of this conformational transition, the interfacial and (tribo-)mechanical properties of polymeric systems composed of PNIPAM can be switched between two states. Here we show that the tribo-mechanical properties of a particular type of PNIPAM system, which is the PNIPAM brush, do not just change between two states, but instead evolve continuously and non-monotonically upon increasing/decreasing temperature. To do so, we present atomic force microscopy experiments in which we measure the adhesion hysteresis and the friction upon bringing a gold colloid in relative motion with PNIPAM brushes at temperatures around the LCST. Both the friction and the adhesion hysteresis display a pronounced maximum exactly at the LCST. The force vs. distance data captured at these temperatures show a long-ranged adhesive interaction upon moving the colloid away from the original point of contact, which indicates that during this retraction the partly collapsed polymers in the brush become strongly stretched.

Submicrometric Films of Surface-Attached Polymer Network with Temperature-Responsive Properties

Temperature-responsive properties of surface-attached poly(N-isopropylacrylamide) (PNIPAM) network films with well-controlled chemistry are investigated. The synthesis consists of cross-linking and grafting preformed ene-reactive polymer chains through thiol-ene click chemistry. The formation of surface-attached and cross-linked polymer films has the advantage of being well-controlled without any caution of no-oxygen atmosphere or addition of initiators. PNIPAM hydrogel films with same cross-link density are synthesized on a wide range of thickness, from nanometers to micrometers. The swelling-collapse transition with temperature is studied by using ellipsometry, neutron reflectivity, and atomic force microscopy as complementary surface-probing techniques. Sharp and high amplitude temperature-induced phase transition is observed for all submicrometric PNIPAM hydrogel films. For temperature above LCST, surface-attached PNIPAM hydrogels collapse similarly but without complete expulsion of water. For temperature below LCST, the swelling of PNIPAM hydrogels depends on the film thickness. It is shown that the swelling is strongly affected by the surface attachment for ultrathin films below ∼150 nm. For thicker films above 150 nm (to micrometers), surface-attached polymer networks with the same cross-link density swell equally. The density profile of the hydrogel films in the direction normal to the substrate is confronted with in-plane topography of the free surface. It results that the free interface width is much larger than the roughness of the hydrogel film, suggesting pendant chains at the free surface.

Poly(N-isopropylacrylamide)-Based Mixed Brushes: A Computer Simulation Study

Temperature-sensitive poly(N-isopropylacrylamide) (PNIPAM) polymer brushes of fixed molecular weight and grafting density are modeled in the framework of a coarse-grained model with soft, nonbonded interactions and an implicit solvent. This model has been developed to address experimentally relevant, large invariant degrees of polymerization, and nonbonded interactions are expressed via a third-order (virial) expansion of the equation of state. The choice of interaction parameters is intended to mimic the swelling behavior of PNIPAM in water as the temperature increases toward the lower critical solution temperature (T(LCST)). Results of molecular dynamics simulations for one component brushes are compared to experimental data. Mixed brushes incorporating small and large amounts of grafted poly(ethylene glycol) polymers are then considered. The effects of mixing polymer components on the response of the mixed brushes to temperature changes are monitored, and the results are compared to experimental data. In the end, two design principles for biomolecule triggering using temperature-sensitive mixed polymer brushes with functional and switchable end-groups are proposed and studied. This work is in favor of establishing qualitative rules for the design, optimization, and comprehension of binary polymer brushes for bioengineering purposes.

Buffers more than buffering agent: introducing a new class of stabilizers for the protein BSA

In this study, we have analyzed the influence of four biological buffers on the thermal stability of bovine serum albumin (BSA) using dynamic light scattering (DLS).

Thermal aggregation properties of nanoparticles modified with temperature sensitive copolymers

In this paper, we describe the use of a temperature responsive polymer to reversibly assemble gold nanoparticles of various sizes. Temperature responsive, low critical solution temperature (LCST) pNIPAAm-co-pAAm polymers, with transition temperatures (T(C)) of 51 and 65 °C, were synthesized with a thiol modification, and grafted to the surface of 11 and 51 nm gold nanoparticles (AuNPs). The thermal-responsive behavior of the polymer allowed for the reversible aggregation of the nanoparticles, where at T < T(C) the polymers were hydrophilic and extended between particles. In contrast, at T > T(C), the polymer shell undergoes a hydrophilic to hydrophobic phase transition and collapses, decreasing interparticle distances between particles, allowing aggregation to occur. The AuNP morphology and polymer conjugation were probed by TEM, FTIR, and (1)H NMR. The thermal response was probed by UV-vis and DLS. The structure of the assembled aggregates at T > T(C) was studied via in situ small-angle X-ray scattering, which revealed interparticle distances defined by polymer conformation.

Using temperature-sensitive smart polymers to regulate DNA-mediated nanoassembly and encoded nanocarrier drug release

In this paper we describe the use of a temperature-responsive polymer to regulate DNA interactions in both a DNA-mediated assembly system and a DNA-encoded drug delivery system. A thermoresponsive pNIPAAm-co-pAAm polymer, with a transition temperature (TC) of 51 °C, was synthesized with thiol modification and grafted onto gold nanoparticles (Au NPs) also containing single-stranded oligonucleotides (ssDNA). The thermoresponsive behavior of the polymer regulated the accessibility of the sequence-specific hybridization between complementary DNA-functionalized Au NPs. At T < TC, the polymer was hydrophilic and extended, blocking interaction between the complementary sequences at the periphery of the hydrodynamic diameter. In contrast, at T > TC, the polymer shell undergoes a hydrophilic to -phobic phase transition and collapses, shrinking below the outer ssDNA, allowing for the sequence-specific hybridization to occur. The potential application of this dynamic interface for drug delivery is shown, in which the chemotherapy drug doxorubicin (DOX) is bound to double-stranded DNA (dsDNA)-functionalized Au NPs whose sequences are known to be high-affinity intercalation points for it. The presence of the polymer capping is shown to decrease drug release kinetics and equilibrium at T < TC, but increase release at T > TC, thus improving the cytotoxicity of the encoded nanocarrier design.

Poly(N-isopropyl acrylamide) brush topography: dependence on grafting conditions and temperature

The topography of poly (N-isopropyl acrylamide) brushes end-grafted from initiator-terminated monolayers was imaged by atomic force microscopy, as a function of the area per chain and of solvent quality. Measurements were done in air and in water, below and above the lower critical solution temperature. At low grafting densities and molecular weights, area-averaged ellipsometry measurements did not detect changes in the volume of water-swollen, end-grafted polymer films above the lower critical solution temperature. However, atomic force microscopy images revealed surface features that suggest the formation of lateral aggregates or "octopus micelles". At high grafting densities and molecular weights, the films collapsed uniformly, as detected by both AFM imaging and ellipsometry. These findings reconcile in part prior results suggesting that some poly(N-isopropyl acrylamide) chains do not collapse in poor solvent, and they also reveal more complex collapse behavior above the lower critical solution temperature than is commonly assumed. This behavior would influence the ability to tune the functional properties of poly(N-isopropyl acrylamide) coatings.

Loading and distribution of a model small molecule drug in poly(N-isopropylacrylamide) brushes: a neutron reflectometry and AFM study

The structure of a hydrated poly(N-isopropylacrylamide) brush loaded with 5 vol % Isoniazid is studied as a function of temperature using neutron reflectometry (NR) and atomic force microscopy (AFM). NR measurements show that Isoniazid increases the thickness of the brush before, during and after the polymer collapse, and it is retained inside the brush at all measured temperatures. The Isoniazid concentration in the expanded brush is ~14% higher than in the bulk solution, and the concentration nearly doubles in the collapsed polymer, suggesting stronger binding between Isoniazid and the polymer compared to water, even at temperatures below the lower critical solution temperature (LCST) where the polymer is hydrophilic. Typically, additives that bind strongly to the polymer backbone and increase the hydrophilicity of the polymer will delay the onset of the LCST, which is suggested by AFM and NR measurements. The extent of small-molecule loading and distribution throughout a thermo-responsive polymer brush, such as pNIPAAm, will have important consequences for applications such as drug delivery and gating.

Porosity and surface properites of SBA-15 with grafted PNIPAAM: a water sorption calorimetry study

Mesoporous silica SBA-15 was modified in a three-step process to obtain a material with poly-N-isopropylacrylamide (PNIPAAM) grafted onto the inner pore surface. Water sorption calorimetry was implemented to characterize the materials obtained after each step regarding the porosity and surface properties. The modification process was carried out by (i) increasing the number of surface silanol groups, (ii) grafting 1-(trichlorosilyl)-2-(m-/p-(chloromethylphenyl) ethane, acting as an anchor for (iii) the polymerization of N-isopropylacrylamide. Water sorption isotherms and the enthalpy of hydration are presented. Pore size distributions were calculated on the basis of the water sorption isotherms by applying the BJH model. Complementary measurements with nitrogen sorption and small-angle X-ray diffraction are presented. The increase in the number of surface silanol groups occurs mainly in the intrawall pores, the anchor is mainly located in the intrawall pores, and the intrawall pore volume is absent after the surface grafting of PNIPAAM. Hence, PNIPAAM seals off the intrawall pores. Water sorption isotherms directly detect the presence of intrawall porosity. Pore size distributions can be calculated from the isotherms. Furthermore, the technique provides information regarding the hydration capability (i.e., wettability of different chemical surfaces) and thermodynamic information.

Dynamic perspective on the function of thermoresponsive nanopores from in situ AFM and ATR-IR investigations

This article describes the morphological and chemical characterization of stimuli-responsive functionalized silicon surfaces provided in parallel by atomic force spectroscopy (AFM) and Fourier transform infrared spectroscopy (FT-IR) enhanced by the single-beam sample reference attenuated total reflection method (SBSR-ATR). The stimuli-responsive behavior of the surfaces was obtained by grafting-to in melt carboxyl-terminated poly-N-isopropylacryl amides (PNIPAAM) with different degree of polymerization (DP) on epoxide-functionalized silicon substrates. The unprecedented real time and in situ physicochemical insight into the temperature-triggered response of the densely packed superficial brushes allowed for the selection of a PNIPAAM with a specific DP as a suitable polymer for the fabrication of silicon membranes exhibiting switchable nanopores. The fabrication process combines the manufacture of nanoporous silicon surfaces and their subsequent chemical functionalization by the grafting-to in melt of the selected polymer. Then, relevant information was obtained in what concerns the chemical modifications behind the topographical changes that drive the functioning of PNIPAAM-based hybrid nanovalves as well as the timescale on which the opening and closing of the nanopores occur.

Understanding the force-vs-distance profiles of terminally attached poly(N-isopropyl acrylamide) thin films

In this work, we examine the interaction between thin films composed of terminally anchored poly(N-isopropyl acrylamide) (PNIPAAm) immersed in water and test surfaces. Understanding this force of interaction can be important when using PNIPAAm surfaces in biotechnological applications such as biological cell cultures. The two novel contributions that are presented here are (1) the use of a recently developed self-consistent field (SCF) theory to predict the force-vs-distance profiles, and (2) the use of a modified polymer scaling theory to estimate the wet film thickness from experimental force-vs-distance profiles. SCF theory was employed to model the equilibrium structure of the uncompressed PNIPAAm chains, and the force between a compressed polymer film and a test surface as a function of wall separation distance. The parameters that were varied include temperature, polymer molecular weight, and surface coverage. The force-vs-distance profiles obtained at low and high temperatures with the SCF theory were in qualitative agreement with the experimentally measured profiles reported in the literature. We also compared the results of our SCF theory to the Alexander de Gennes scaling theory and found agreement at large separation distance. We also propose a method to estimate the wet polymer film thickness from a force-vs-distance profile obtained from an atomic force microscope measurement. The main novelties of this approach are that we employed a density functional theory corrected version of scaling theory proposed by McCoy et al. [McCoy, J. D.; Curro, J. G. J. Chem. Phys. 2005, 122, 164905], and we provide equations to account for various geometries of AFM tips.

Chain conformation of a new class of PEG-based thermoresponsive polymer brushes grafted on silicon as determined by neutron reflectometry

The thermoresponsive PEG-based copolymer poly[2-(2-methoxyethoxy)ethyl methacrylate-co-oligo(ethylene glycol) methacrylate) (P(MEO(2)MA-co-OEGMA)] was grafted onto a silicon wafer, and its chain conformation in aqueous solution was studied by neutron reflectometry. The effects of temperature and salt concentration on the polymer's conformation were evaluated. With increasing temperature, it was found that the polymer brushes underwent a transition from an extended state to a compressed state, and eventually a collapsed state above the lower critical solution temperature. The presence of salt significantly affected the well-extended brushes but had little effect on compressed and collapsed brushes. This PEG-based thermoresponsive surface exhibited good protein adsorption resistance. Interestingly, extended and collapsed brushes showed the same level of protein repulsion, something that was not expected.

Thermal response of poly(ethoxyethyl glycidyl ether) grafted on gold surfaces probed on the basis of temperature-dependent water wettability

Two series of thiol-modified poly(ethoxyethyl glycidyl ether) with different chain-end groups and molecular weights (PT-PEEGE-SH and Bu-PEEGE-SH), which undergo lower critical solution temperature (LCST)-type phase separation in an aqueous milieu, are grafted onto gold substrates through Au-S bonding. The water wettability of the resultant polymer-tethered surface discontinuously varies with temperature, and this alteration of wettability is reversible according to the variation in temperature of the environment. For all the polymers examined, the transition temperature on surface, TC(surf), the temperature at which half the discontinuous change in surface wettability occurs, increases with the number-average molecular weight (M(n)). This tendency does not necessarily agree with the relationship between M(n) and Tc(soln), the phase separation temperature in solution, thereby suggesting that the different factors contribute toward the determination of the Tc(surf) and Tc(soln) values. For both series of thermoresponsive polymers, the increase in crowding of the polymer chains at the surface causes the value of Tc(surf) to increase due to an increase in the interchain interaction in the outermost region of the tethered polymer chains and reduction in the chain mobility. The greater interactions between neighboring chains at the surface explain the larger dependency of Tc(surf) on M(n) as compared to that of Tc(soln).

Poly(N-isopropylacrylamide) thin films densely grafted onto gold surface: preparation, characterization, and dynamic AFM study of temperature-induced chain conformational changes

Thermally responsive poly(N-isopropylacrylamide) (PNIPAM) films are attracting considerable attention since they offer the possibility to achieve reversible control over surface wettability and biocompatibility. In this paper, we first report a new and simple method for the grafting under melt of amine-terminated PNIPAM chains onto gold surfaces modified with a self-assembled monolayer (SAM) of reactive thiols. The formation of homogeneous tethered PNIPAM films, whose thickness can be tuned by adjusting polymer molecular weight or SAM reactivity, is evidenced by using the combination of ellipsometry, X-ray photon spectroscopy, infrared spectroscopy (PM-IRRAS), and atomic force microscopy. The calculation of grafting parameters from experimental measurements indicated the synthesis of densely grafted PNIPAM films and allowed us to predict a "brushlike" regime for the chains in good solvent. In a second part, the temperature-induced responsive properties are studied in situ by conducting dynamic AFM measurements using the amplitude modulation technique. Imaging in water environment first revealed the reversible modification of surface morphology below and above the theoretical lower critical solution temperature (LCST) of PNIPAM. Then, the determination of amplitude and phase approach curves at various temperatures provided direct measurement of the evolution of the damping factor, or similarly the dissipated energy, as a function of the probe indentation into the PNIPAM film. Most interestingly, we clearly showed the subtle and progressive thermally induced chain conformational change occurring at the scale of several nanometers around the expected LCST.

Synthetic mimic of selective transport through the nuclear pore complex

The nuclear pore complex is a large protein channel present universally in eukaryotic cells. It generates an essential macromolecular separation between the nucleus and cytoplasm. The transport mechanism relies on recognition of molecular cargos by receptor proteins, and on specific interaction between the receptors and the pores. We present a chemical mimic of this "receptor-mediated" transport using modified nanoporous membrane filters, polyisopropylacrylamide as the carrier molecule, or receptor, and single-stranded DNA as the cargo. We show that a complex of ssDNA and polyisopropylacrylamide diffuses faster through the modified pores than does the bare ssDNA, in spite of the larger size of the complex. The mobile polymer thus acts as a soluble receptor to usher a macromolecular cargo specifically through the pores.