Grafted Polymer Films Research Articles

A 3D-polyphenylalanine network inside porous alumina: Synthesis and characterization of an inorganic-organic composite membrane.

Surface functionalization of porous materials allows for the introduction of additional functionality coupled with high mechanical stability of functionalized inner pores. Herein, we investigate the surface-initiated ring-opening polymerization (SI-ROP) of phenylalanine-N-carboxyanhydride (PA-NCA) in porous alumina membranes (ALOX-membranes) with respect to different solvent mixtures (tetrahydrofuran (THF) and dichloromethane (DCM)). It was found that increasing the volume fraction of DCM leads to an increasing amount of fibrillar polymer structures within the porous ALOX-membrane. A three-dimensional fibrillar network with intrinsic porosity was formed in DCM, whereas in THF, a dense and smooth polypeptide film was observed. A post-treatment with a mixture of chloroform and dichloroacetic acid leads to rearrangement of the morphology of the grafted polymer films. The analysis by scanning electron microscopy (SEM), near-infrared spectroscopy (NIR) and contact angle measurements (CA) reveals a change in morphology of the grafted polymer films, which is due to the rearrangement of the secondary structure of the polypeptides. No significant loss of the surface-grafted polypeptides was determined by thermogravimetric (TG) measurements, which indicates that the change in morphology of the polymer films is solely a result of a conformational change of the surface-grafted polypeptides. Furthermore, adsorption of a test analyte (chloroanilic acid) was investigated with respect to different polymer functionalization schemes for reversed-phase solid phase extraction applications. The adsorption capability of the functionalized composite membrane was increased from 16.7% to 38.1% compared to the native ALOX-membrane.

Analysis of topographical parameters and interfacial interaction of zinc oxide reinforced poly (vinyl alcohol-g-acrylonitrile) nanocomposite film surfaces using atomic force microscopy

Motivated by the significance of surface morphology and topography of polymer nanocomposite surfaces, the present investigation was undertaken to design zinc oxide reinforced nanocomposites of poly(vinyl alcohol)-g-poly(acrylonitrile) and explore their morphological and topographical features. The grafted polymer matrix was prepared by a free radical polymerization method and zinc oxide nanoparticles were impregnated by adopting a simple sequential dipping method that involves alternate dipping of grafted polymer film in the alkali and the metal ions solutions, respectively to allow in situ precipitation of zinc oxide nanoparticles inside the polymer matrix. The as designed nanocomposite films were investigated for their morphology and topography by the atomic force microscopy (AFM) and transmission electron microscopy techniques. Various roughness parameters were calculated and a correlation was established between the morphology, topography and chemical composition of the nanocomposites. Furthermore, the surface compositional analysis was evaluated by Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) to confirm the results since there usually remains a difference between the nominal and actual concentration of the prepared sample.

Correlating microstructure and performance of PU/(meth)acrylic hybrids as hardwood floor coating

In this work chemically grafted PU/(meth)acrylic hybrid dispersions for wood floor coating applications were synthesized using functional monomers containing hydroxyl groups as well as different vinyl functionalities (acrylic, allylic and methacrylic) to graft the polyurethane and acrylic polymer phases. A complete study of the effect of the type of functional monomer and polymerization mode of (meth)acrylic monomers (batch and semibatch) on the polymer characteristics, particle and film morphology and final properties of the films as wood floor coatings is presented. It is shown that using monomers with lower reactivity during the acrylic polymerization, such as glycidyl mono-allyl ether, limits the formation of a gel fraction, whilst more reactive double bonds, such as the of glycidyl methacrylate, leads to high gel content and significantly higher molar mass of the gel fraction. Using TEM analysis it is demonstrated that more homogeneous particle and film morphologies were obtained in the case of grafted hybrids. The impact of these features on the properties of the coating is discussed. It is shown that grafted polymer films had higher Young´s modulus and strain hardening than the non-grafted ones and this effect is more pronounced in the case of functional monomers that led to hybrids with higher gel content. Finally, it is shown that the wood floor coatings prepared with these PU/(meth)acrylic hybrid dispersions had final properties comparable to a commercially available reference coating.

Role of Grafting on Particle and Film Morphology and Film Properties of Zero VOC Polyurethane/Poly(meth)acrylate Hybrid Dispersions

AbstractHybrid waterborne dispersions of polyurethane (PU)/poly(meth)acrylic are widely applied in coating and adhesive products, as well as in many other applications. The synergetic performance of the two component polymers is highly dependent on the ability to control the PU and (meth)acrylic network structure. In this work, the effect of grafting and the impact of macromolecular architecture on the particle and film morphology as well as on the properties of PU/(meth)acrylic hybrid films is investigated. It is shown that with grafting, the absolute molar mass distribution (MMD) is shifted toward higher values as the low molar mass PU chains become incorporated in the (meth)acrylic polymer network. Furthermore, by using different types of functional monomers, the nature of the MMD can be altered. Using transmission electron microscopy (TEM) analysis, it is demonstrated that more homogeneous particle and film morphologies are obtained in the case of grafted hybrids. Finally, the impact of network structure on tensile strength is outlined highlighting that grafted polymer films have higher Young's modulus and strain hardening than the non‐grafted ones.

Plasmon‐Polariton Induced, “from Surface” RAFT Polymerization, as a Way toward Creation of Grafted Polymer Films with Thickness Precisely Controlled by Self‐Limiting Mechanism

AbstractPlasmon‐induced “from surface” reversible addition‐fragmentation chain‐transfer (RAFT) polymerization is reported for the first time. The gold grating surface, supporting the surface plasmon polariton excitation and propagation, is grafted with RAFT agent, immersed in the solution, containing the NIPAm monomer and AIBN and subsequently illuminated at a wavelength corresponding to plasmon absorption. The grafting of the polymer layer, its thickness, and morphology are characterized by several techniques (including the surface‐enhanced Raman spectroscopy (SERS), scanning electron microscopy and energy‐dispersive X‐ray spectroscopy (SEM‐EDX), X‐ray photoelectron spectroscopy (XPS), nanomechanical atomic force microscopy (AFM) mapping, and goniometry). It is shown that the polymerization efficiently starts only under the surface plasmon‐polariton excitation. The time‐dependent SERS and XPS measurements indicate rather self‐limiting nature of plasmon‐induced “from the surface” PNIPAm growth, namely the reaction takes place up to a certain polymer thickness and is stopped despite a significant excess of polymerization initiator and monomer in the reaction solution. The present results provide the basis for designing further experiments on plasmonic catalysis in general and offer a new way of producing ultrathin polymer films with a defined structural dimension.

Controlling Enzymatic Polymerization from Surfaces with Switchable Bioaffinity.

The affinity of surfaces toward proteins is found to be a key parameter to govern the synthesis of polymer brushes by surface-initiated biocatalytic atom transfer radical polymerization (SI-bioATRP). While the "ATRPase" hemoglobin (Hb) stimulates only a relatively slow growth of protein repellent brushes, the synthesis of thermoresponsive grafts can be regulated by switching the polymer's attraction toward proteins across its lower critical solution temperature (LCST). Poly(N-isopropylacrylamide) (PNIPAM) brushes are synthesized in discrete steps of thickness at temperatures above LCST, while the biocatalyst layer is refreshed at T < LCST. Multistep surface-initiated biocatalytic ATRP demonstrates a high degree of control, results in high chain end group fidelity and enables the synthesis of multiblock copolymer brushes under fully aqueous conditions. The activity of Hb can be further modulated by tuning the accessibility of the heme pocket within the protein. Hence, the multistep polymerization is accelerated at acid pH, where the enzyme undergoes a transition from its native to a molten globule conformation. The controlled synthesis of polymer brushes by multistep SI-bioATRP highlights how a biocatalytic synthesis of grafted polymer films can be precisely controlled through the modulation of the polymer's interfacial physicochemical properties, in particular of the affinity of the surface toward proteins. This is not only of importance to gain a predictive understanding of surface-confined enzymatic polymerizations, but also represents a new way to translate bioadhesion into a controlled functionalization of materials.

Cytocompatibility of polyethylene grafted with triethylenetetramine functionalized carbon nanoparticles

Various carbon nanostructures are widely researched as scaffolds for tissue engineering. We evaluated the surface properties and cell-substrate interactions of carbon nanoparticles functionalized with triethylenetetramine (CNPs) grafted polymer film. Two forms of polyethylene (HDPE, LDPE) were treated in an inert argon plasma discharge and, subsequently, grafted with CNPs. The surface properties were studied using multiple methods, including Raman spectroscopy, goniometry, atomic force microscopy, X-ray photoelectron spectroscopy and electrokinetic analysis. Cell-substrate interactions were determined in vitro by studying adhesion, proliferation and viability of vascular smooth muscle cells (VSMCs) from the aorta of a rat. Cell-substrate interactions on pristine and modified substrates were compared to standard tissue culture polystyrene. Our results show that CNPs affect surface morphology and wettability and therefore adhesion, proliferation and viability of cultured muscle cells.

Review—The Mediating Effect of Chemically Oxidized Silicon Surface on the P4VP Electrografting for Silicon Electrical Insulation

The P4VP electrografting process realized on chemically oxidized p-type silicon surface, instead of traditional oxide free surface, leads to a thicker grafted polymer film. In this paper, we studied the influence of the surface states generated during the chemical oxidation of the silicon surface, on the thickness of the P4VP film. We used electrochemical measurements and Mott-Schottky plot analysis to demonstrate that such an oxidized surface does not only allow the electrografting process to occur, but also increases the accumulation of electrons at the silicon interface. In this condition, the chemical oxidation of silicon surface enhances the electro-initiation of the P4VP electrografting process and promotes the polymer growth.

Covalent Grafting of Tethered Homopolymer Film on p-Si(100).

Covalent grafting of a homopolymer film onto a p-Si(100) surface was performed by use of surface electroinitiated emulsion polymerization. This polymerization method was one-step and cost-effective, and worked in the aqueous dispersed media containing both nitrobenzenediazonium (NBD) tetrafluoroborate and methyl methacrylate (MMA) monomer. NBD was reduced in the vicinity of substrate at the cut-on voltage of the pulsed voltage and was successively grafted on a Si surface, leading to a polynitrophenyl (PNP) layer. The reduction of NBD simultaneously initiated the polymerization of MMA. The resulting PMMA macroradicals in the solution can react with the previously grafted PNP at the cutoff voltage of the pulsed voltage for thickening the polymer film. The prepared polymer layer completely covered the Si surface, as investigated by the scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques. Importantly, chemical components and the thickness of the grafted polymer film were well controlled by the applied voltage and the charge flow passing through the Si electrode. X-ray photoelectron spectroscopy (XPS) measurement further confirmed the Si-C bond between the polymer layer and the Si substrate. The homopolymer film was extremely effective in protecting the underlying Si from atmospheric oxidation.

Preparation of a functionalized glass sensor and its sensing performances for nitroaromatics

A functionalized glass sensor GS-P, which was designed for detecting nitroaromatic compounds (NACs), has been prepared by grafting conjugated polymer P on the surface of a glass slide. For control, three spin-coated films (films I, II and III) with different thicknesses were also prepared. Interestingly, the grafted polymer film showed densely packed conical columns from atomic force microscopy. Quenching efficiency study showed that polymer P exhibited 92% quench toward 2,4-dinitrotoluence (DNT) in solution state while 82% quench toward DNT vapor in film state. Quenching kinetics showed that the fluorescence intensity of GS-P decreased significantly by 80% within first the 100s, followed by gradually decreasing to a quenching equilibrium at 200s. This was much faster than the other two thicker films (films I and II) whose emission intensities decreased only by 37% and 48% within 100s, respectively. The GS-P exhibited similar quenching speed as the ultra-thin spin-coated film (film III) that had the similar thickness to GS-P. Reversibility study showed that GS-P exhibited good repeatability of quenching process after the three cycles, while the ultra-thin spin-coated film hardly showed any reversibility. These great differences indicated remarkable advantages of GS-P over the spin-coated film. Furthermore, the GS-P sensor exhibited good selectivity to the NACs such as DNT, 2,4,6-trinitrotoluence, picric acid, and nitrobenzene.

Thin polymer films grafted to the solid surface with in situ synthesized CdS nanocrystals

Combining two different novel techniques, namely “grafting from” polymerization and in situ sol–gel synthesis, nanocomposites based on CdS nanocrystals embedded into thin polymer films with diverse structure (either poly(acrylic acid) (PAA) homopolymer or mosaic one composed of polystyrene (PS) and PAA chains) grafted to the solid surface have been obtained. The structure of grafted polymer films as well as the composition of obtained on their basis nanocomposite films were studied and characterized by UV-visible spectroscopy, IR-spectroscopy, scanning electron microscopy, and atomic-force microscopy. Two fractions of CdS nanocrystals with the sizes of ~4.5–4.7 and ~5.7–6.0 nm formed in polymer films of both types have been found. It has been revealed that the content ratio of these two fractions mostly depends on the structure of polymer films.

A dynamic duo: pairing click chemistry and postpolymerization modification to design complex surfaces.

Advances in key 21st century technologies such as biosensors, biomedical implants, and organic light-emitting diodes rely heavily on our ability to imagine, design, and understand spatially complex interfaces. Polymer-based thin films provide many advantages in this regard, but the direct synthesis of polymers with incompatible functional groups is extremely difficult. Using postpolymerization modification in conjunction with click chemistry can circumvent this limitation and result in multicomponent surfaces that are otherwise unattainable. The two methods used to form polymer thin films include physisorption and chemisorption. Physisorbed polymers suffer from instability because of the weak intermolecular forces between the film and the substrate, which can lead to dewetting, delamination, desorption, or displacement. Covalent immobilization of polymers to surfaces through either a "grafting to" or "grafting from" approach provides thin films that are more robust and less prone to degradation. The grafting to technique consists of adsorbing a polymer containing at least one reactive group along the backbone to form a covalent bond with a complementary surface functionality. Grafting from involves polymerization directly from the surface, in which the polymer chains deviate from their native conformation in solution and stretch away from the surface because of the high density of chains. Postpolymerization modification (PPM) is a strategy used by our groups over the past several years to immobilize two or more different chemical functionalities onto substrates that contain covalently grafted polymer films. PPM exploits monomers with reactive pendant groups that are stable under the polymerization conditions but are readily modified via covalent attachment of the desired functionality. "Click-like" reactions are the most common type of reactions used for PPM because they are orthogonal, high-yielding, and rapid. Some of these reactions include thiol-based additions, activated ester coupling, azide-alkyne cycloadditions, some Diels-Alder reactions, and non-aldol carbonyl chemistry such as oxime, hydrazone, and amide formation. In this Account, we highlight our research combining PPM and click chemistry to generate complexity in polymer thin films. For the purpose of this Account, we define a complex coating as a polymer film grafted to a planar surface that acts as a template for the patterning of two or more discrete chemical functionalities using PPM. After a brief introduction to grafting, the rest of the review is arranged in terms of the sequence in which PPM is performed. First, we describe sequential functionalization using iterations of the same click-type reaction. Next, we discuss the use of two or more different click-like reactions performed consecutively, and we conclude with examples of self-sorting reactions involving orthogonal chemistries used for one-pot surface patterning.

Stable Dropwise Condensation for Enhancing Heat Transfer via the Initiated Chemical Vapor Deposition (iCVD) of Grafted Polymer Films

Ultra-thin copolymer films are deposited by initiated chemical deposition (iCVD) to investigate their performance under the condensation of water vapor. By forming a grafted interface between the coating and the substrate, the films exhibit stable dropwise condensation even when subjected to 100 °C steam. The applicability of the iCVD to complex substrate geometries is demonstrated on a copper condenser coil.

Viscoelastic phase diagram of fluorinated and grafted polymer films and proton‐exchange membranes for fuel cell applications

ABSTRACTThe influence of temperature and moisture activity on the viscoelastic behavior of fluorinated membranes for fuel cell applications was investigated. Uncrosslinked and crosslinked ethylene tetrafluoroethylene (ETFE)‐based proton‐conducting membranes were prepared by radiation grafting and subsequent sulfonation and their behavior was compared with ETFE base film and commercial Nafion® NR212 membrane. Uniaxial tensile tests and stress relaxation tests at controlled temperature and relative humidity (RH) were carried out at 30 and 50 °C for 10% &lt; RH &lt; 90%. Grafted films were stiffer and exhibited stronger strain hardening when compared with ETFE. Similarly, both uncrosslinked and crosslinked membranes were stiffer and stronger than Nafion®. Yield stress was found to decrease and moisture sensitivity to increase on sulfonation. The viscoelastic relaxation of the grafted films was found to obey a power‐law behavior with exponent equal to −0.04 ± 0.01, a factor of almost 2 lower than ETFE, weakly influenced by moisture and temperature. Moreover, the grafted films presented a higher hygrothermal stability when compared with their membranes counterparts. In the case of membranes, a power‐law behavior at RH &lt; 60% was also observed. However, a markedly different behavior was evident at RH &gt; 60%, with an almost single relaxation time exponential. An exponential decrease of relaxation time with RH from 60 s to 10 s was obtained at RH ≥ 70% and 30 °C. The general behavior of grafted films observed at 30 °C was also obtained at 50 °C. However, an anomalous result was noticed for the membranes, with a higher modulus at 50 °C when compared with 30 °C. This behavior was explained by solvation of the sulfonic acid groups by water absorption creating hydrogen bonding within the clusters. A viscoelastic phase diagram was elaborated to map critical conditions (temperature and RH) for transitions in time‐dependent behavior, from power‐law scaling to exponential scaling. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1139–1148

Morphological control of grafted polymer films via attraction to small nanoparticle inclusions

Control of the morphologies of polymer films and layers by addition of nanosize particles is a novel technique for design of nanomaterials and is also at the core of some important biological processes. In order to facilitate the analysis of experimental data and enable predictive engineering of such systems, solid theoretical understanding is necessary. We study theoretically and computationally the behavior of plane-grafted polymer layers (brushes) in athermal solvent, decorated with small nanoparticle inclusions, using mean field theory and coarse-grained simulations. We show that the morphology of such layers is very sensitive to the interaction between the polymers and the nanoparticles and to the nanoparticle density. In particular, the mean field model shows that for a certain range of parameters, the nanoparticles induce a sharp transition in the layer height, accompanied by a sharp increase in the number of adsorbed nanoparticles. At other parameter values, the layer height depends smoothly on the nanoparticle concentration. Predictions of the theoretical model are verified by Langevin dynamics simulations. The results of the paper are in qualitative agreement with experiments on in vitro models of biological transport and suggest strategies for morphological control of nanocomposite materials.

Grafting of a novel phosphorylcholine-containing vinyl monomer onto poly-carbonateurethane surfaces by ultraviolet radiation grafting polymerization

An original phosphorylcholine (PC) containing vinyl monomer (compound 1) was grafted onto the surface of polycarbonate urethane (PCU) films by ultraviolet (UV) initiated polymerization. The vinyl monomer was synthesized from 2-hydroxyethyl methacrylate (HEMA), hexamethylene diisocyanate (HDI) and L-α-glycerylphosphorylcholine (GPC) by two relative reactions through the coupling of the isocyanate groups in HDI with the hydroxyl groups in HEMA and GPC, respectively. The existence of phosphorylcholine groups on the surface of compound 1-grafted PCU films was verified by X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and water contact angle measurements. The surfaces of the grafted PCU films showed lower water contact angles than did the surfaces of non-grafted PCU films. Furthermore, the surface morphology of a grafted PCU film was observed using scanning electrical microscopy (SEM), which showed that the grafted polymer film exhibits a rough and pinhole-free surface. In addition, the blood compatibility of the compound 1-grafted PCU films was evaluated by platelet adhesion studies using the non-grafted PCU film as a reference. It was found by SEM that fewer platelets adhered to the surfaces of the grafted PCU films compared to the reference. The high PC-grafted PCU film did not show any substantial platelet attachment on its surface. These results indicated that these compound 1-grafted polymers may have potential application as biomaterials for vascular grafts and some subcutaneously implanted devices. Open image in new window

A Novel Characterization Method for Graft‐Polymer Structures Chemically Attached on Thermally Stable Polymer Films

AbstractMost polymer grafts in grafted polymer films obtained by radiation‐induced solid graft polymerization are not analyzed in detail due to difficulties in isolation of the grafts without structural decomposition. Herein, a novel structural and molecular weight characterization method is reported for polymer grafts that are chemically attached to thermally and chemically stable polymer substrates, based on a swelling‐induced graft detachment in hot water. Polymer grafts prepared by the radiation‐induced polymerization of alkyl acrylate into poly(ethylene‐co‐tetrafluoroethylene) (ETFE) followed by a sulfonation reaction are found to have a ternary copolymer structure whose monomer units contain –COOH or –SO3H groups, or both.

Localized grafting through chemical lift-off

In this work we present two techniques that provide localized functionalization of the surface of materials. Both lead to localized grafted thin organic films (10–200 nm). The localization is brought by a chemical lift-off process, which relies on patterned weakly bonded films as sacrificial layers, combined with electrochemical (SEEP) or chemical (GraftFast©) processes which provides the final robust pattern on the surface. Both grafting processes, which were recently described, take advantage of the redox activation of diazonium salts associated with vinylic monomers in aqueous solution, and lead to similar grafted polymer films. Thanks to the high difference in adhesion between the grafted polymer and the patterned sacrificial layer (either an ink or weakly bound self-assembled monolayers), the latter may be easily removed, which unveils uncovered areas of the substrate.

Understanding the Redox-Induced Polymer Grafting Process: A Dual Surface-Solution Analysis

As recently reported, the Graftfast process is a grafting method that provides covalently grafted polymer films. It relies on the chemical reduction of diazonium salts by reducing agents in absence or presence of a vinylic monomer. Contrary to electroinduced methods delivering strongly grafted and stable polymer films such as cathodic electrografting (CE) of vinylic monomers (which requires drastic experimental conditions) or surface electroinitiated emulsion polymerization (SEEP), the Graftfast process provides strongly grafted polymer films on any type of materials (conductors, semiconductors, and insulators). Moreover, it is a fast one-step reaction occurring at atmospheric pressure, ambient air and room temperature in water, which makes it more suitable for applications than the slower ATRP-based methods. This article aims to complete the first paper on this process by giving preliminary answers to the question: How does the Graftfast process work? To achieve this mechanistic study, dual surface-solution analyses were performed. Both spontaneous and redox-induced grafting of polynitrophenylene-like (PNP) films and poly(hydroxyethyl) methacrylate (PHEMA) films were analyzed by infrared-attenuated total reflection (IR-ATR) and X-ray photoelectron spectroscopy (XPS) while the corresponding reactive solutions were studied by electronic paramagnetic resonance, EPR (spin-trapping method using 2-methyl-2-nitrosopropane MNP as spin-trapping agent). The EPR spectra and hyperfine structure of MNP adducts provide evidence of aryl radicals production, growing polymer chains radicals formation, and the existence of critical concentration values, leading to favorable grafting kinetics.

Surface Electroinitiated Emulsion Polymerization (SEEP): A Mechanistic Approach

As recently reported, the SEEP process (surface electroinitiated emulsion polymerization) is a new grafting method that provides covalently grafted polymer films on conducting or semiconducting surfaces by radical polymerization in aqueous dispersed media. It relies on cathodic electroinitiation, which creates radical species able to start a radical polymerization. Contrary to the formerly described cathodic electrografting of vinylic polymers (CE), which also delivers submicrometer-thick and stable polymer films on conducting substrates but requires strictly anhydrous conditions and organic aprotic solvent, SEEP brings a major improvement in switching from a purely anionic mechanism to a radical one by adding an aryldiazonium salt in the reaction mixture, while retaining the same polymer films characteristics. Moreover, SEEP is not restricted to water-soluble monomers but can be performed even with hydrophobic ones, such as n-butyl methacrylate (BMA). In such cases, a surfactant is necessary to stabilize the monomer in water emulsion. From this one-pot electrografting process performed in water at room temperature, in a few minutes, without restrictions on vinylic monomer water solubility, results a polymer coating strongly grafted to the substrate. This article aims at completing our first one and focuses on mechanistic aspects of SEEP to eventually establish a possible “grafting onto” mechanism. To achieve that goal, we have analyzed grafted polymer films obtained by SEEP on gold substrate from BMA in water as a miniemulsion by IR-ATR, X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectroscopy (ToF-SIMS), and atomic force microscopy (AFM).