Publications by authors named "Sissi de Beer"

31 Publications

Chemical Composition and Strain at Interfaces between Different Morphologies in Block Copolymer Thin Films.

Langmuir 2021 Nov 25;37(43):12723-12731. Epub 2021 Oct 25.

Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada.

Transitional composition between two thin-film morphologies of the block copolymer, polystyrene--poly(-butyl acrylate) (PS--PtBuA), was investigated using near-field infrared spectroscopy and atomic force microscopy mechanical measurements. These techniques allowed block identification with nanoscale spatial resolution and elucidated the material's sub-surface composition. PS was found to form coronae around the PtBuA block in spherical valleys on flat areas of the film, and coronae of PtBuA surrounding the PS lamellae were observed at the edge of the polymer film, where parallel lamellae are formed. Furthermore, we found that the peak position and width varied by location, which may be a result of block composition, chain tension, or substrate interaction.
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http://dx.doi.org/10.1021/acs.langmuir.1c02169DOI Listing
November 2021

Polyelectrolytes as Building Blocks for Next-Generation Membranes with Advanced Functionalities.

ACS Appl Polym Mater 2021 Sep 26;3(9):4347-4374. Epub 2021 Aug 26.

Membrane Science and Technology, MESA+ Institute for Nanotechnology, Faculty of Science and Technology, University of Twente, Enschede 7500 AE, The Netherlands.

The global society is in a transition, where dealing with climate change and water scarcity are important challenges. More efficient separations of chemical species are essential to reduce energy consumption and to provide more reliable access to clean water. Here, membranes with advanced functionalities that go beyond standard separation properties can play a key role. This includes relevant functionalities, such as stimuli-responsiveness, fouling control, stability, specific selectivity, sustainability, and antimicrobial activity. Polyelectrolytes and their complexes are an especially promising system to provide advanced membrane functionalities. Here, we have reviewed recent work where advanced membrane properties stem directly from the material properties provided by polyelectrolytes. This work highlights the versatility of polyelectrolyte-based membrane modifications, where polyelectrolytes are not only applied as single layers, including brushes, but also as more complex polyelectrolyte multilayers on both porous membrane supports and dense membranes. Moreover, free-standing membranes can also be produced completely from aqueous polyelectrolyte solutions allowing much more sustainable approaches to membrane fabrication. The Review demonstrates the promise that polyelectrolytes and their complexes hold for next-generation membranes with advanced properties, while it also provides a clear outlook on the future of this promising field.
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http://dx.doi.org/10.1021/acsapm.1c00654DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8438666PMC
September 2021

Vapor sorption in binary polymer brushes: The effect of the polymer-polymer interface.

J Chem Phys 2021 Aug;155(5):054904

Sustainable Polymer Chemistry Group, Department of Molecules and Materials, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands.

Polymer brushes attract vapors that are good solvents for polymers. This is useful in sensing and other technologies that rely on concentrating vapors for optimal performance. It was recently shown that vapor sorption can be enhanced further by incorporating two incompatible types of polymers A and B in the brushes: additional vapor adsorbs at the high-energy polymer-polymer interface in these binary brushes. In this article, we present a model that describes this enhanced sorption in binary brushes of immiscible A-B polymers. To do so, we set up a free-energy model to predict the interfacial area between the different polymer phases in binary brushes. This description is combined with Gibbs adsorption isotherms to determine the adsorption at these interfaces. We validate our model with coarse-grained molecular dynamics simulations. Moreover, based on our results, we propose design parameters (A-B chain fraction, grafting density, vapor, and A-B interaction strength) for optimal vapor absorption in coatings composed of binary brushes.
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http://dx.doi.org/10.1063/5.0057065DOI Listing
August 2021

Autonomous capillary microfluidic devices with constant flow rate and temperature-controlled valving.

Soft Matter 2021 Sep 5;17(33):7781-7791. Epub 2021 Aug 5.

National Center for International Research on Green Optoelectronics & South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China.

In this paper, we report on a capillary microfluidic device with constant flow rate and temperature-triggered stop valve function. It contains a PDMS channel that was grafted by a thermo-responsive polymer poly(N-isopropylacrylamide) (PNIPAm). The channel exhibits a constant capillary filling speed. By locally increasing the temperature in the channel from 20 °C to 37 °C using a microfabricated heater, a change of the surface wettability from hydrophilic to hydrophobic is obtained creating a hydrophobic stop valve. The valve can be reopened by lowering the temperature. The device is simple to fabricate and can be used as an actuatable capillary pump operating around room temperature. To understand the constant capillary filling speed, we performed contact angle measurements, in which we found slow wetting kinetics of PNIPAm-g-PDMS surfaces at temperatures below the lower critical solution temperature (LCST) of PNIPAm and fast wetting kinetics above the LCST. We interpret this as the result of the diffusive hydration process of PNIPAm below the LCST and the absence of hydration on the hydrophobic PNIPAm thin layer above the LCST.
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http://dx.doi.org/10.1039/d1sm00625hDOI Listing
September 2021

Degrafting of Polymer Brushes by Exposure to Humid Air.

ACS Appl Polym Mater 2020 Aug 8;2(8):3039-3043. Epub 2020 Jul 8.

Materials Science and Technology of Polymers, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands.

It is well-known that polymer brushes can degraft in aqueous liquids. Here we show that brushes can deteriorate in humid air too. We observe that the detachment rate of the brushes increases with increasing relative humidity and hydrophilicity of the brushes. We relate this to the increase in water absorption as these parameters are increased. Our results imply that protective measures that are at present being developed for applications of brushes in liquids will also be key in enabling the long-term storage and utilization of hydrophilic brushes in air.
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http://dx.doi.org/10.1021/acsapm.0c00474DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8192051PMC
August 2020

Concentrating Vapor Traces with Binary Brushes of Immiscible Polymers.

ACS Appl Polym Mater 2021 May 23;3(5):2336-2340. Epub 2021 Apr 23.

Sustainable Polymer Chemistry Group, Department of Molecules & Materials, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands.

Vapors in the air around us can provide useful information about our environment, but we need sensitive vapor sensors to access this information, especially because those vapors are often present at very low concentrations. We report molecular dynamics simulations of a concept that can significantly increase the sensitivity of vapor sensors at low concentrations. By coating the sensor surfaces with end-anchored immiscible polymers, surface-bound polymer blends are formed that can concentrate vapors, reaching sorption enhancements of more than one order of magnitude, especially at low vapor concentrations.
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http://dx.doi.org/10.1021/acsapm.1c00321DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8154206PMC
May 2021

Design, construction, and testing of an accurate low-cost humidistat for laboratory-scale applications.

Eur Phys J E Soft Matter 2021 Apr 5;44(4):48. Epub 2021 Apr 5.

Sustainable Polymer Chemistry Group, Department of Molecules and Materials MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE, Enschede, The Netherlands.

Stable and precise control of humidity is imperative for a wide variety of experiments. However, commercially available humidistats (devices that maintain a constant humidity) are often prohibitively expensive. Here, we present a simple yet effective humidistat for laboratory-scale applications that can be easily and affordably (<€250) constructed based on an Arduino Uno as microcontroller, a set of proportional miniature solenoid valves, a gas washing bottle, and a humidity sensor. The microcontroller implements a PID controller that regulates the ratio of a dry and humid airflow. The design and implementation of the device, including a custom driver circuit for the solenoids, are described in detail, and the firmware is freely available online. Finally, we demonstrate its proper operation and performance through step response and long-term stability tests, which shows settling times of approx. 30 s and an attainable relative humidity range of 10-95.
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http://dx.doi.org/10.1140/epje/s10189-021-00062-5DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8021525PMC
April 2021

Designer Core-Shell Nanoparticles as Polymer Foam Cell Nucleating Agents: The Impact of Molecularly Engineered Interfaces.

ACS Appl Mater Interfaces 2021 Apr 30;13(14):17034-17045. Epub 2021 Mar 30.

Materials Science and Technology of Polymers, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500AE Enschede, the Netherlands.

The interface between nucleating agents and polymers plays a pivotal role in heterogeneous cell nucleation in polymer foaming. We describe how interfacial engineering of nucleating particles by polymer shells impacts cell nucleation efficiency in CO blown polymer foams. Core-shell nanoparticles (NPs) with a 80 nm silica core and various polymer shells including polystyrene (PS), poly(dimethylsiloxane) (PDMS), poly(methyl methacrylate) (PMMA), and poly(acrylonitrile) (PAN) are prepared and used as heterogeneous nucleation agents to obtain CO blown PMMA and PS micro- and nanocellular foams. Fourier transform infrared spectroscopy, thermogravimetric analysis, and transmission electron microscopy are employed to confirm the successful synthesis of core-shell NPs. The cell size and cell density are determined by scanning electron microscopy. Silica NPs grafted with a thin PDMS shell layer exhibit the highest nucleation efficiency values, followed by PAN. The nucleation efficiency of PS- and PMMA-grafted NPs are comparable with the untreated particles and are significantly lower when compared to PDMS and PAN shells. Molecular dynamics simulations (MDS) are employed to better understand CO absorption and nucleation, in particular to study the impact of interfacial properties and CO-philicity. The MDS results show that the incompatibility between particle shell layers and the polymer matrix results in immiscibility at the interface area, which leads to a local accumulation of CO at the interfaces. Elevated CO concentrations at the interfaces combined with the high interfacial tension (caused by the immiscibility) induce an energetically favorable cell nucleation process. These findings emphasize the importance of interfacial effects on cell nucleation and provide guidance for designing new, highly efficient nucleation agents in nanocellular polymer foaming.
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http://dx.doi.org/10.1021/acsami.1c00569DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8153546PMC
April 2021

Friends, Foes, and Favorites: Relative Interactions Determine How Polymer Brushes Absorb Vapors of Binary Solvents.

Macromolecules 2020 Dec 4;53(24):10898-10906. Epub 2020 Dec 4.

Sustainable Polymer Chemistry, University of Twente, PO Box 217, 7500 AE Enschede, The Netherlands.

Polymer brushes can absorb vapors from the surrounding atmosphere, which is relevant for many applications such as in sensing and separation technologies. In this article, we report on the absorption of binary mixtures of solvent vapors (A and B) with a thermodynamic mean-field model and with grand-canonical molecular dynamics simulations. Both methods show that the vapor with the strongest vapor-polymer interaction is favored and absorbs preferentially. In addition, the absorption of one vapor (A) influences the absorption of another (B). If the A-B interaction is stronger than the interaction between vapor B and the polymers, the presence of vapor A in the brush can aid the absorption of B: the vapors absorb collaboratively as friends. In contrast, if the A-polymer interaction is stronger than the B-polymer interaction and the brush has reached its maximum sorption capacity, the presence of A can reduce the absorption of B: the vapors absorb competitively as foes.
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http://dx.doi.org/10.1021/acs.macromol.0c02228DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7759003PMC
December 2020

The role of entropy in wetting of polymer brushes.

Soft Matter 2021 Feb;17(5):1368-1375

Physics of Fluids and the MESA+ Institute, University of Twente, PO box 217, 7500 AE Enschede, The Netherlands.

The wetting of polymer brushes exhibits a much richer phenomenology than wetting of normal solid substrates. These brushes allow for three wetting states, which are partial wetting, complete wetting and mixing. Here, we study the transitions between these wetting states for brushes in contact with polymer melts and compare them to predictions using enthalpic arguments based on brush and melt interactions. We show that the transitions are shifted compared to the enthalpic predictions and that the shifts can be positive or negative depending on the length of the melt polymer and the brush grafting density. The reason for this is that these brush and melt parameters can have a positive or negative effect on the entropic contribution to the free energy of the system. Our results highlight the relevance of entropy in predicting the exact wetting transitions, which is important for the design of brush-based coating applications.
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http://dx.doi.org/10.1039/d0sm00156bDOI Listing
February 2021

One-pot, self-catalyzed synthesis of self-adherent hydrogels for photo-thermal, antimicrobial wound treatment.

J Mater Chem B 2021 01 23;9(1):159-169. Epub 2020 Nov 23.

National Engineering Research Center for Biomaterials, Sichuan University, 29 Wangjiang Road, Chengdu 610064, China.

Self-adhering hydrogels are promising materials to be employed as wound dressings, because they can be used for wound healing without the necessity of additional stitching. However, micro-organisms can easily adhere to these hydrogels as well, which usually causes wound infections. Therefore, adhesive hydrogels are often combined with antibiotics. However, this introduces a risk of drug resistance, cytotoxicity and poor cell affinity. Consequently, recently, there has been great interest in developing non-antibiotic, antibacterial adhesive hydrogels. In this article, we present a simple one-pot synthesis procedure to prepare self-adhesive hydrogels composed of poly(acrylamide) (PAM), naturally derived chitosan (CS) and tannic acid/ferric ion chelates ([email protected]). [email protected] enables self-catalysis of the polymerization reaction. In addition, due to its near infrared (NIR) photothermal responsiveness, [email protected] allows for eliminating the bacterial activity with up to 91.6% and 94.7% effectivity against Escherichia coli and Staphylococcus aureus, respectively. Mechanical and adhesion testing shows that the hydrogels are tough as well as flexible and will adhere repeatedly to many types of biological tissues, which can be attributed to the combination of physical and chemical bonding between [email protected] and PAM and CS, respectively. Moreover, in vitro and in vivo tests indicate that the NIR photothermally active hydrogel can effectively prevent bacterial infection and accelerate tissue regeneration, which demonstrates that these hydrogels are promising functional materials for wound healing applications.
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http://dx.doi.org/10.1039/d0tb02160aDOI Listing
January 2021

Sorption Characteristics of Polymer Brushes in Equilibrium with Solvent Vapors.

Macromolecules 2020 Oct 29;53(19):8428-8437. Epub 2020 Sep 29.

Materials Science and Technology of Polymers, University of Twente, Enschede 7522 NB, The Netherlands.

While polymer brushes in contact with liquids have been researched intensively, the characteristics of brushes in equilibrium with vapors have been largely unexplored, despite their relevance for many applications, including sensors and smart adhesives. Here, we use molecular dynamics simulations to show that solvent and polymer density distributions for brushes exposed to vapors are qualitatively different from those of brushes exposed to liquids. Polymer density profiles for vapor-solvated brushes decay more sharply than for liquid-solvated brushes. Moreover, adsorption layers of enhanced solvent density are formed at the brush-vapor interface. Interestingly and despite all of these effects, we find that solvent sorption in the brush is described rather well with a simple mean-field Flory-Huggins model that incorporates an entropic penalty for stretching of the brush polymers, provided that parameters such as the polymer-solvent interaction parameter, grafting density, and relative vapor pressure are varied individually.
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http://dx.doi.org/10.1021/acs.macromol.0c01637DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7558291PMC
October 2020

Swelling of Poly(methyl acrylate) Brushes in Acetone Vapor.

Langmuir 2020 10 30;36(40):12053-12060. Epub 2020 Sep 30.

Materials Science and Technology of Polymers, University of Twente, 7522 NB Enschede, The Netherlands.

Sensor platforms can benefit from the incorporation of polymer brushes since brushes can concentrate the analyte near the sensor surface. Brushes that absorb acetone vapor are of particular interest since acetone is an important marker for biological processes. We present a simple procedure to synthesize acetone-responsive poly(methyl acrylate) brushes. Using spectroscopic ellipsometry, we show that these brushes respond within seconds and swell by more than 30% when exposed to acetone vapor. Moreover, quartz crystal microbalance measurements demonstrate that the brushes can be exploited to increase the acetone detection sensitivity of sensors by more than a factor 6. Surprisingly, we find that the swelling ratio of the brushes in acetone vapor is independent of the grafting density and the degree of polymerization of the polymers in the brush. This is qualitatively different from swelling of the same brushes in liquid environments, where the swelling ratio decreases for increasing grafting densities. Yet, it indicates that the brushes are robust and reproducible candidates for implementation in vapor sensor systems.
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http://dx.doi.org/10.1021/acs.langmuir.0c02510DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7558288PMC
October 2020

Fluorescent Patterns by Selective Grafting of a Telechelic Polymer.

ACS Appl Polym Mater 2019 Feb 18;1(2):136-140. Epub 2019 Jan 18.

Materials Science and Technology of Polymers, MESA+ Institute for Nanotechnology, University of Twente, 7500 AE Enschede, The Netherlands.

The preparation of patterned ultrathin films (sub-10 nm) composed of end-anchored fluorescently labeled poly(methyl methacrylate) (PMMA) is presented. Telechelic PMMA was synthesized utilizing activator regenerated by electron transfer atom transfer radical polymerization and consecutively end-functionalized with alkynylated fluorescein by Cu-catalyzed azide-alkyne cycloaddition (CuAAC) "click" chemistry. The polymers were grafted via the α-carboxyl groups to silica or glass substrates pretreated with (3-aminopropyl)triethoxysilane (APTES). Patterned surfaces were prepared by inkjet printing of APTES onto glass substrates and selectively grafted with fluorescently end-labeled PMMA to obtain emissive arrays on the surface.
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http://dx.doi.org/10.1021/acsapm.8b00180DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6433164PMC
February 2019

Wetting of Polymer Brushes by Polymeric Nanodroplets.

Macromolecules 2019 Mar 20;52(5):2015-2020. Epub 2019 Feb 20.

Physics of Fluids, MESA+ Institute for Nanotechnology, and Materials Science and Technology of Polymers, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands.

End-anchoring polymers to a solid surface to form so-called polymer brushes is a versatile method to prepare robust functional coatings. We show, using molecular dynamics simulations, that these coatings display rich wetting behavior. Depending on the interaction between the brushes and the polymeric droplets as well as on the self-affinity of the brush, we can distinguish between three wetting states: mixing, complete wetting, and partial wetting. We find that transitions between these states are largely captured by enthalpic arguments, while deviations to these can be attributed to the negative excess interfacial entropy for the brush droplet system. Interestingly, we observe that the contact angle strongly increases when the softness of the brush is increased, which is opposite to the case of drops on soft elastomers. Hence, the Young to Neumann transition owing to softness is not universal but depends on the nature of the substrate.
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http://dx.doi.org/10.1021/acs.macromol.8b02409DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6416710PMC
March 2019

The Effect of Adjacent Materials on the Propagation of Phonon Polaritons in Hexagonal Boron Nitride.

J Phys Chem Lett 2017 Jul 14;8(13):2902-2908. Epub 2017 Jun 14.

Department of Chemistry, University of Toronto , 80 St. George Street Toronto, Ontario M5S 3H6, Canada.

In order to apply the ability of hexagonal boron nitride (hBN) to confine energy in the form of hyperbolic phonon polariton (HPhP) modes in photonic-electronic devices, approaches to finely control and leverage the sensitivity of these propagating waves must be investigated. Here, we show that by surrounding hBN with materials of lower/higher dielectric responses, such as air and silicon, lower/higher surface momenta of HPhPs can be achieved. Furthermore, an alternative method for preparing thin hBN crystals with minimum contamination is presented, which provides opportunities to study the sensitivity of the damping mechanism of HPhPs on adsorbed materials. Infrared scanning near-field optical microscopy (IR-SNOM) results suggest that the reflections at the upper and lower hBN interfaces are primary causes of the damping of HPhPs, and that the damping coefficients of propagating waves are highly sensitive to adjacent layers, suggesting opportunities for sensor applications.
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http://dx.doi.org/10.1021/acs.jpclett.7b01048DOI Listing
July 2017

Hexagonal Boron Nitride Self-Launches Hyperbolic Phonon Polaritons.

J Phys Chem Lett 2017 May 1;8(10):2158-2162. Epub 2017 May 1.

Department of Chemistry, University of Toronto , 80 St. George Street, Toronto, Ontario M5S 3H6, Canada.

Hexagonal boron nitride (hBN) is a 2D material that supports traveling waves composed of material vibrations and light, and is attractive for nanoscale optical devices that function in the infrared. However, the only current method of launching these traveling waves requires the use of a metal nanostructure. Here, we show that the polaritonic waves can be launched into the 2D structure by folds within hBN, alone, taking advantage of the intrinsic material properties. Our findings suggest that structural continuity between the fold and hBN crystal is crucial for creating self-launched waves with a constant phase front. This approach offers a single material system to excite the polaritonic modes, and the approach is applicable to a broad range of 2D crystals and thus could be useful in future characterization.
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http://dx.doi.org/10.1021/acs.jpclett.7b00748DOI Listing
May 2017

Dramatic effect of fluid chemistry on cornstarch suspensions: Linking particle interactions to macroscopic rheology.

Phys Rev E 2017 Mar 31;95(3-1):030602. Epub 2017 Mar 31.

Physics of Fluids Group, University of Twente, P.O. Box 217, NL-7500 AE Enschede, The Netherlands.

Suspensions of cornstarch in water exhibit strong dynamic shear thickening. We show that partly replacing water with ethanol strongly alters the suspension rheology. We perform steady and nonsteady rheology measurements combined with atomic force microscopy to investigate the role of fluid chemistry on the macroscopic rheology of the suspensions and its link with the interactions between cornstarch grains. Upon increasing the ethanol content, the suspension goes through a yield-stress fluid state and ultimately becomes a shear-thinning fluid. On the cornstarch grain scale, atomic force microscopy measurements reveal the presence of polymers on the cornstarch surface, which exhibit a cosolvency effect. At intermediate ethanol content, a maximum of polymer solubility induces high microscopic adhesion which we relate to the macroscopic yield stress.
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http://dx.doi.org/10.1103/PhysRevE.95.030602DOI Listing
March 2017

Pick up, move and release of nanoparticles utilizing co-non-solvency of PNIPAM brushes.

Nanoscale 2017 Jan;9(4):1670-1675

Materials Science and Technology of Polymers, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, the Netherlands.

A critical complication in handling nanoparticles is the formation of large aggregates when particles are dried e.g. when they need to be transferred from one liquid to another. The particles in these aggregates need to disperse into the destined liquid medium, which has been proven difficult due to the relatively large interfacial interaction forces between nanoparticles. We present a simple method to capture, move and release nanoparticles without the formation of large aggregates. To do so, we employ the co-non-solvency effect of poly(N-isopropylacrylamide) (PNIPAM) brushes in water-ethanol mixtures. In pure water or ethanol, the densely end-anchored macromolecules in the PNIPAM brush stretch and absorb the solvent. We show that under these conditions, the adherence between the PNIPAM brush and a silicon oxide, gold, polystyrene or poly(methyl methacrylate) colloid attached to an atomic force microscopy cantilever is low. In contrast, when the PNIPAM brushes are in a collapsed state in a 30-70 vol% ethanol-water mixture, the adhesion between the brush and the different counter surfaces is high. For potential application, we demonstrate that this difference in adhesion can be utilized to pick up, move and release 900 silicon oxide nanoparticles of diameter 80 nm using only 10 × 10 μm PNIPAM brush.
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http://dx.doi.org/10.1039/c6nr09245dDOI Listing
January 2017

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

Soft Matter 2015 Nov;11(43):8508-16

Materials Science and Technology of Polymers, MESA + Institute of Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands.

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.
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http://dx.doi.org/10.1039/c5sm01426cDOI Listing
November 2015

Redox-Induced Backbiting of Surface-Tethered Alkylsulfonate Amphiphiles: Reversible Switching of Surface Wettability and Adherence.

Langmuir 2015 Jun 2;31(23):6343-50. Epub 2015 Jun 2.

Materials Science and Technology of Polymers, MESA+ Institute for Nanotechnology, University of Twente, 7500 AE Enschede, The Netherlands.

The synthesis and characterization of electrode-supported poly(ferrocenylsilane) (PFS) films bearing iodopropyl (PFS-I) and undecanesulfonate (PFS-SO3(-)) surface moieties are presented. The redox responsiveness of these PFS films allows for controlled and repeatable switching of the surface energy of the PFS-I and PFS-SO3(-) layers under electrochemical control. Static water/surface contact angle measurements showed a change in contact angle values for PFS-I from 80° (reduced state) to 70° (oxidized state) over repeated cycles. However, an opposite change in wettability was observed for PFS-SO3(-), where the values observed varied from 59° (reduced state) to 77° (oxidized state). Nanoscale adherence was assessed with colloid probe AFM. The adhesive forces between these surfaces and a polystyrene (PS) colloid probe in water alternated between 130 nN (reduced state) and 30 nN (oxidized state) for PFS-I layers and between 75 nN (reduced) and 180 nN (oxidized) for the PFS-SO3(-) films. The reversed response of PFS-I films to oxidation compared to that of PFS-SO3(-), in both contact angles and adhesive forces, suggests a different underlying mechanism for switching. As PFS-I is tuned from the reduced to the oxidized state, positively charged ferrocenium (Fc(+)) centers that formed in the film increase its wettability and reduce its adherence to the hydrophobic colloid probe. For PFS-SO3(-) in the reduced state, the exposed alkanesulfonate moieties increase the hydrophilicity of the surface. When oxidized, the Fc(+) units attract the negatively charged sulfonate groups, which results in a bending of the sulfonate groups toward the PFS surface, exposing the undecyl spacer. This alteration of the surface chemistry reduces the surface energy and increases the adherence between the bent alkyl chains and the hydrophobic PS colloid in water. The attraction of the charged sulfonate group to Fc(+) is in competition with the counterions present in the electrolyte solution. Therefore, the backbiting of the chain can be achieved only in electrolytes where the affinity of Fc(+) for the ions is lower than for the sulfonate group, in agreement with the Hofmeister series.
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http://dx.doi.org/10.1021/acs.langmuir.5b01105DOI Listing
June 2015

Preparation and friction force microscopy measurements of immiscible, opposing polymer brushes.

J Vis Exp 2014 Dec 24(94). Epub 2014 Dec 24.

Materials Science and Technology of Polymer, MESA+ Institute for Nanotechnology, University of Twente;

Solvated polymer brushes are well known to lubricate high-pressure contacts, because they can sustain a positive normal load while maintaining low friction at the interface. Nevertheless, these systems can be sensitive to wear due to interdigitation of the opposing brushes. In a recent publication, we have shown via molecular dynamics simulations and atomic force microscopy experiments, that using an immiscible polymer brush system terminating the substrate and the slider surfaces, respectively, can eliminate such interdigitation. As a consequence, wear in the contacts is reduced. Moreover, the friction force is two orders of magnitude lower compared to traditional miscible polymer brush systems. This newly proposed system therefore holds great potential for application in industry. Here, the methodology to construct an immiscible polymer brush system of two different brushes each solvated by their own preferred solvent is presented. The procedure how to graft poly(N-isopropylacrylamide) (PNIPAM) from a flat surface and poly(methyl methacrylate) (PMMA) from an atomic force microscopy (AFM) colloidal probe is described. PNIPAM is solvated in water and PMMA in acetophenone. Via friction force AFM measurements, it is shown that the friction for this system is indeed reduced by two orders of magnitude compared to the miscible system of PMMA on PMMA solvated in acetophenone.
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http://dx.doi.org/10.3791/52285DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4354440PMC
December 2014

Optical imaging beyond the diffraction limit by SNEM: effects of AFM tip modifications with thiol monolayers on imaging quality.

Ultramicroscopy 2015 Mar 9;150:79-87. Epub 2014 Dec 9.

Materials Science and Technology of Polymers, MESA+ Institute for Nanotechnology, University of Twente, Enschede NL-7500, The Netherlands. Electronic address:

Tip-enhanced nanoscale optical imaging techniques such as apertureless scanning near-field optical microscopy (a-SNOM) and scanning near-field ellipsometric microscopy (SNEM) applications can suffer from a steady degradation in performance due to adhesion of atmospheric contaminants to the metal coated tip. Here, we demonstrate that a self-assembled monolayer (SAM) of ethanethiol (EtSH) is an effective means of protecting gold-coated atomic force microscopy (AFM) probe tips from accumulation of surface contaminants during prolonged exposure to ambient air. The period over which they yield consistent and reproducible results for scanning near-field ellipsometric microscopy (SNEM) imaging is thus extended. SNEM optical images of a microphase separated polystyrene-block-poly (methylmethacrylate) (PS-b-PMMA) diblock copolymer film, which were captured with bare and SAM-protected gold-coated AFM probes, both immediately after coating and following five days of storage in ambient air, were compared. During this period the intensity of the optical signals from the untreated gold tip fell by 66%, while those from the SAM protected tip fell by 14%. Additionally, gold coated AFM probe tips were modified with various lengths of alkanethiols to measure the change in intensity variation in the optical images with SAM layer thickness. The experimental results were compared to point dipole model calculations. While a SAM of 1-dodecanethiol (DoSH) was found to strongly suppress field enhancement we find that it can be locally removed from the tip apex by deforming the molecules under load, restoring SNEM image contrast.
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http://dx.doi.org/10.1016/j.ultramic.2014.12.001DOI Listing
March 2015

Switchable friction using contacts of stimulus-responsive and nonresponding swollen polymer brushes.

Authors:
Sissi de Beer

Langmuir 2014 Jul 30;30(27):8085-90. Epub 2014 Jun 30.

Jülich Supercomputing Centre, Forschungszentrum Jülich , Jülich, Germany.

Stimulus-responsive (SR), solvated polymers can switch between an expanded state and a collapsed state via external stimuli. Using molecular dynamics simulations, I show that such SR polymers can be employed to control the frictional response between two opposing polymer brushes in relative sliding motion. By using a brush composed of SR polymers in contact with a nonresponding solvated polymer brush, the presence of capillaries and the overlap between molecules of the opposing brushes can be switched. When both brushes are solvated, a capillary is formed and polymers of the opposing brushes interdigitate. Interdigitation dominates friction upon shearing flat brush-bearing surfaces, while the breaking and formation of capillaries dominate friction in the low-velocity limit between rough brush-bearing surfaces. Thus, when either rough or flat polymer-bearing surfaces are sheared, friction between two swollen brushes can be high. In contrast, when the SR brush is collapsed, the solvent absorbs only in the brush that does not respond to the external stimulus. The latter circumvents the presence of capillaries and interdigitation of the brushes, which results in a low friction force upon shearing.
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http://dx.doi.org/10.1021/la5013473DOI Listing
July 2014

Solvent-induced immiscibility of polymer brushes eliminates dissipation channels.

Nat Commun 2014 May 14;5:3781. Epub 2014 May 14.

1] Jülich Supercomputing Centre, Institute for Advanced Simulation, Forschungszentrum Jülich, Wilhelm-Johnenstraße, 52428 Jülich, Germany [2] Department of Materials Science and Engineering, Universität des Saarlandes, 66123 Saarbrücken, Germany.

Polymer brushes lead to small friction and wear and thus hold great potential for industrial applications. However, interdigitation of opposing brushes makes them prone to damage. Here we report molecular dynamics simulations revealing that immiscible brush systems can form slick interfaces, in which interdigitation is eliminated and dissipation strongly reduced. We test our findings with friction force microscopy experiments on hydrophilic and hydrophobic brush systems in both symmetric and asymmetric setups. In the symmetric setup both brushes are chemically alike, while the asymmetric system consists of two different brushes that each prefer their own solvent. The trends observed in the experimentally measured force traces and the friction reduction are similar to the simulations and extend to fully immersed contacts. These results reveal that two immiscible brush systems in mechanical contact slide at a fluid-fluid interface while having load-bearing ability. This makes them ideal candidates for tribological applications.
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http://dx.doi.org/10.1038/ncomms4781DOI Listing
May 2014

Atomic force microscopy of confined liquids using the thermal bending fluctuations of the cantilever.

Phys Rev E Stat Nonlin Soft Matter Phys 2013 Jun 21;87(6):062406. Epub 2013 Jun 21.

Physics of Complex Fluids, MESA+Institute for Nanotechnology, University of Twente P. O. Box 217, 7500 AE Enschede, The Netherlands.

We use atomic force microscopy to measure the distance-dependent solvation forces and the dissipation across liquid films of octamethylcyclotetrasiloxane (OMCTS) confined between a silicon tip and a highly oriented pyrolytic graphite substrate without active excitation of the cantilever. By analyzing the thermal bending fluctuations, we minimize possible nonlinearities of the tip-substrate interaction due to finite excitation amplitudes because these fluctuations are smaller than the typical 1 Å, which is much smaller than the characteristic interaction length. Moreover, we avoid the need to determine the phase lag between cantilever excitation and response, which suffers from complications due to hydrodynamic coupling between cantilever and fluid. Consistent results, and especially high-quality dissipation data, are obtained by analyzing the power spectrum and the time autocorrelation of the force fluctuations. We validate our approach by determining the bulk viscosity of OMCTS using tips with a radius of approximately 1 μm at tip-substrate separations >5 nm. For sharp tips we consistently find an exponentially decaying oscillatory tip-substrate interaction stiffness as well as a clearly nonmonotonic variation of the dissipation for tip-substrate distances up to 8 and 6 nm, respectively. Both observations are in line with the results of recent simulations which relate them to distance-dependent transitions of the molecular structure in the liquid.
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http://dx.doi.org/10.1103/PhysRevE.87.062406DOI Listing
June 2013

Confinement-dependent damping in a layered liquid.

J Phys Condens Matter 2011 Mar 3;23(11):112206. Epub 2011 Mar 3.

Physics of Complex Fluids and MESA + Institute for Nanotechnology, Department of Science and Technology, University of Twente, Enschede, The Netherlands.

We present atomic force microscopy (AFM) measurements of the conservative oscillatory solvation forces and the damping in confined films of octamethylcyclotetrasiloxane using small amplitude modulation with magnetic driving. We find distinct maxima in the interaction damping upon probing the discrete molecular layers, supporting earlier observations of the same phenomenon using AFM with an acoustic driving scheme. The maxima in the damping are located at the same tip-surface separation as the maxima in the conservative oscillatory interaction stiffness.
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http://dx.doi.org/10.1088/0953-8984/23/11/112206DOI Listing
March 2011

Do epitaxy and temperature affect oscillatory solvation forces?

Langmuir 2010 Aug;26(16):13245-50

Physics of Complex Fluids and MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands.

We present temperature-dependent atomic force microscope (AFM) measurements in force-distance mode of confined 1-dodecanol. Upon approach of the AFM-tip toward the highly oriented pyrolytic graphite (HOPG) surface, the final liquid film--only a few nanometers thin--is squeezed out in discrete layers. We find that both the force needed to squeeze out these layers and the number of structured layers strongly increase as the freezing temperature is approached. Surprisingly the force increases nonmonotonically and show a local maximum around 3 degrees and a local minimum at 1 degree above the bulk melting point of the liquid. We attribute this result to changes in epitaxial effects between 1-dodecanol and the HOPG surface close to the melting point of the liquid. To test this hypothesis we performed the same measurements in hexadecane, a similar carbon-chain molecule, and octamethylcyclotetrasiloxane (OMCTS), a quasi-spherical molecule. Hexadecane shows the same maximum in the squeeze-out force at 4-5 degrees and a minimum at 1-2 degrees above the freezing temperature of the liquid, while the squeeze-out of OMCTS was found to be independent of temperature.
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http://dx.doi.org/10.1021/la102120hDOI Listing
August 2010

Dissipation and oscillatory solvation forces in confined liquids studied by small-amplitude atomic force spectroscopy.

Nanotechnology 2010 Aug 19;21(32):325703. Epub 2010 Jul 19.

Physics of Complex Fluids, Department of Science and Technology, University of Twente, Enschede, Netherlands.

We determine conservative and dissipative tip-sample interaction forces from the amplitude and phase response of acoustically driven atomic force microscope (AFM) cantilevers using a non-polar model fluid (octamethylcyclotetrasiloxane, which displays strong molecular layering) and atomically flat surfaces of highly ordered pyrolytic graphite. Taking into account the base motion and the frequency-dependent added mass and hydrodynamic damping on the AFM cantilever, we develop a reliable force inversion procedure that allows for extracting tip-sample interaction forces for a wide range of drive frequencies. We systematically eliminate the effect of finite drive amplitudes. Dissipative tip-sample forces are consistent with the bulk viscosity down to a thickness of 2-3 nm. Dissipation measurements far below resonance, which we argue to be the most reliable, indicate the presence of peaks in the damping, corresponding to an enhanced 'effective' viscosity, upon expelling the last and second-last molecular layer.
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http://dx.doi.org/10.1088/0957-4484/21/32/325703DOI Listing
August 2010

Instability of confined water films between elastic surfaces.

Langmuir 2010 Mar;26(5):3280-5

Physics of Complex Fluids and MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands.

We investigated the dynamics of nanometer thin water films at controlled ambient humidity adsorbed onto two atomically smooth mica sheets upon rapidly bringing the surfaces into contact. Using a surface forces apparatus (SFA) in imaging mode, we found that the water films break up into a distribution of drops with a typical thickness of a few nanometers and a characteristic lateral size and spacing of several micrometers. Whereas the characteristic length is found to be independent of the ambient humidity, the characteristic time of the breakup decreases from approximately 1 to 0.01 s with increasing humidity. The existence of characteristic length and time scales shows that this breakup is controlled by an instability rather than a conventional nucleation and growth mechanism for SFA experiments. These findings cannot be explained by a dispersion-driven instability mechanism. In contrast, a model involving the elastic energies for the deformation of both the mica sheets and the underlying glue layer correctly reproduces the scaling of the characteristic length and time with humidity.
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http://dx.doi.org/10.1021/la903051kDOI Listing
March 2010
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