Thick Polystyrene Films Research Articles

Viscoelastic Properties of Electrospray‐Deposited Polymer Shells via Quartz Crystal Microbalance With Dissipation (QCM‐D)

AbstractMultilayer polymer films are extensively used in multiphase separation. Electrospray deposition (ESD) is an important technique for fabricating such films with tunable morphology. Viscoelastic properties of polystyrene (PS) nanoshell coatings produced by ESD on gold and spin‐coated PS surfaces are evaluated using Quartz Crystal Microbalance with Dissipation (QCM‐D). The thickness of PS films on gold increases with flow rate from ∼200 nm at 0.5 to ∼400 nm at 1.5 mL h−1, accompanied by an order‐of‐magnitude increase in dissipation due to larger particle sizes from shorter droplet flight times. This effect is absent on spin–coated PS films, suggesting the onset of the self‐limiting effect of charges. Although the shear moduli for ESD films calculated from Voigt models is only 0.08%–0.20% of the bulk PS modulus, the stiffness ratio of spray‐coated PS to a single shell is (5.00–13.3) × 103 m−1, due to shell–shell and shell–substrate interactions. These are novel results related to the interparticle friction obtained using QCM‐D for the first time. This work demonstrates that mechanical properties of particulate viscoelastic films with potential applications in high surface area sensors, such as size‐selective membranes for protein or electrolyte adsorption, can be evalauted by QCM‐D with nanograms of material.

Solvothermal vapor annealing setup for thin film treatment: A compact design with in situ solvent vapor concentration probe

A compact setup for in situ solvothermal vapor annealing of thin polymer films in the temperature range from room temperature to 70 °C is presented. The design is optimized to avoid solvent condensation in tubes and other components. The setup consists of a chamber, a gas delivery system, a commercial film-thickness measuring device, and a solvent vapor concentration sensor. The chamber is equipped with heaters and thermometers and is thermally insulated. It has windows to allow for penetration of xrays together with a sample table with a gas distributer to ensure homogeneous swelling of the polymer film. The computer-controlled gas delivery system has two different components, which can be used separately or in parallel (for mixed gas solvent annealing): (a) mass flow controllers and a bubbler system and (b) a commercial “controlled evaporation and mixing” system. The film-swelling ratio is determined in situ using optical reflectometry, and the solvent concentration in the exhaust gas is continuously monitored by UV-absorption. Test results are presented where the setup is used for swelling of ∼100 nm thick polystyrene and polyisoprene homopolymer films with toluene and acetone, respectively.

8Li Spin Relaxation as a Probe of the Modification of Molecular Dynamics by Inelastic Deformation of Glassy Polystyrene

The molecular dynamics of inelastic strain in glasses remains poorly understood, in contrast to the plasticity of crystalline materials that is well-characterized by measurements of dislocation activity. We report initial results on a 300 nm thick atactic polystyrene film undergoing plastic strain in its glassy state. This physical modification was applied by nanoimprint stamping with a 1 mm ultra-smooth spherical die to induce a stress exceeding mechanical yield (0.8% residual strain). Using 8Li implanted-ion βNMR, we monitor the spin-lattice relaxation to infer depth-resolved rates of molecular dynamics. We find a significant change in the bulk molecular dynamics of the imprinted film (away from the surface) compared to an identically prepared control film. The relaxation is ∼ 20% slower in the film left densified by imprinting. We expect this relaxation to be coupled to the motion of the phenyl side rings; wherein slower dynamics due to densification is reasonable, as tighter packing should increase the energy barrier to molecular motion. In addition, we see an increase in the apparent thickness of a nanometric mobile surface layer, but this may be an artefact of surface roughening caused by imprinting.

Contact line motion in dewetted polymer droplets on anisotropic nanopatterned surfaces.

Thin polymer films can become unstable and dewet on a non-wettable substrate leading to the formation of an array of droplets. This instability-mediated drop formation lacks spatial order on flat substrates, but it can be ordered for better combinatorial studies using patterned substrates. In this work, we studied the process of dewetting polystyrene (PS) films on grating patterned substrates upon solvent vapor exposure. The PS film thickness was commensurate with the grating pattern height (hP). Our findings show that the initial dewetting stages follow the direction of the underlying grating pattern with the formation of directional holes in the film. During the later stages of dewetting, there was a lateral coalescence of polymer threads across the grating stripes. The final morphology comprised smaller droplets or threads confined within the pattern grooves and anisotropic large drops covering several pattern stripes. Furthermore, the larger drops show a unique behavior of shape change from anisotropic to isotropic as a function of solvent vapor concentration (Cn) inside the dewetting chamber. The drop regained its anisotropic shape with an increase in Cn, and this transition continued, with the movement of the three-phase contact line (TPCL). While the capillary flow of the polymer causes anisotropy during high Cn, the local orientation of the contact line and a mismatch in the value of the equilibrium contact angle can drive the drop back into an isotropic shape as Cn reduces and capillary forces weaken. We also observed that the extent of anisotropy quantified as droplet distortion ratio (Dr) not only depends on the Cn during dewetting but also on solvent type and hP. This new-found dynamic behavior of dewetted polymeric drops can be studied in greater detail and potentially leveraged for applications in sensing and microfluidics.

Controlling Lengthscales in Water-Solvent Induced Self-Organized Dewetting of Thin Polystyrene Films by Modulating the Surface Properties of the Substrate

We report here a facile method of modulating the lengthscales of self-organized dewetting of polystyrene thin films by varying the extent of silane grafting on the substrate. The self-assembled monolayers (SAM) of octadecyltrichlorosilane (OTS) were grafted on glass substrates by exposing them to different doses of OTS vapors in a controlled, low pressure environment. Measurement of water contact angles on the modified glass surfaces showed that the contact angles could be continuously increased from 40° to 70° by increasing the time of exposure to the OTS vapors from a few minutes to several hours. Polystyrene (PS) thin films were spin coated on the modified glass substrates and water-solvent mixture induced dewetting of the PS thin films was carried out, where the dewetting kinetics depended on the spreadability of water on the surface. We observed a reduction in the length scales of the dewetting, i.e. the wavelength of instability (λ) and droplet size (d), by increasing the extent of OTS exposure. The λ for a 50 nm thick PS film decreased from 12 to 4 µm while the droplet diameter (d) also decreased, from 2.5 to 1.6 µm.

Enhanced Glass Transition Temperature of Thin Polystyrene Films Having an Underneath Cross-Linked Layer.

Due to the importance of the interface in the segmental dynamics of supported macromolecule ultrathin films, the glass transition temperature (Tg) of polystyrene (PS) ultrathin films upon solid substrates modified with a cross-linked PS (CLPS) layer has been investigated. The results showed that the Tg of the thin PS films on a silica surface with a ∼5 nm cross-linked layer increased with reducing film thickness. Meanwhile, the increase in Tg of the thin PS films became more pronounced with increasing the cross-linking density of the layer. For example, a 20 nm thick PS film supported on CLPS with 1.8 kDa of cross-linking degree exhibited a ∼35 and ∼50 K increase in Tg compared to its bulk and that on neat SiO2 substrate, respectively. Such a large Tg elevation for the ultrathin PS films was attributed to the interfacial aggregation states in which chains diffused through nanolevel voids formed in the cross-linked layer to the SiO2-Si surface. In such a situation, the chains were topologically constrained in the cross-linked layer with less mobility. These results offer us the opportunity to tailor interfacial effects by changing the degree of cross-linking, which has great potential application in many polymer nanocomposites.

Statistical strength of a self-bonded incompatible polymer–polymer interface

The thick bulk films of amorphous polystyrene (PS) and poly(ethylene terephthalate) (PET) were held in contact for various periods of healing time (t) of 5 min to 15 h at 3 constant healing temperatures T = 74 °C, 89 °C, and 108 °C ranging from below to above the bulk glass transition temperatures of the 2 polymers involved. As a result, the PS–PET single lap-shear adhesive joints which can bear the mechanical load were formed. As-formed PS–PET adhesive joints (or self-bonded asymmetric incompatible PS–PET interfaces) were shear-fractured in tension, and their lap-shear strength (σ) developed due to physical self-healing of the interfaces was measured as a function of T and t. The distributions of the measured σ values (totally, 16 data sets) have been analyzed in the framework of the Weibull statistical model, and the Weibull parameters (Weibull modulus and scale factor) were estimated and compared with those reported recently for other partially self-healed interfaces of amorphous and semi-crystalline polymers. The Weibull behavior of the polymer–polymer interfaces differing in the chain chemical structure, compatibility, crystallizability, and crystallinity has been discussed.

Tensile testing of freestanding polymer thin films with non-standard geometries

While an increasing requirement from both fundamental and application, a direct tensile testing of freestanding polymer thin films in the air remains a challenge, mainly due to their extreme fragility. This study aims to establish a facile and reliable testing system, and investigate the effect of specimen geometry on the testing. First, a thin film transfer and adhesive guide frame gripping technique is introduced. With low length-to-width ratio specimens, tensile testing of rectangle non-standard polystyrene films with thickness down to 45 nm is achieved on a commercial universal testing machine. By changing specimen width and gauge length individually, a scale law between system compliance and specimen width is then verified. After corrected by compliance, a constant modulus of ~3.15 GPa for 45–4319 nm thick polystyrene films can be obtained. This study provides a potential strategy to overcome operational difficulties in performing tensile testing of freestanding polymer thin films.

Morphological and Elastic Transition of Polystyrene Adsorbed Layers on Silicon Oxide.

Herein we present a study on the formation of irreversibly adsorbed layer of polystyrene molecules on silicon oxide surfaces. Various scanning probe microscopy techniques have been employed to study both the morphology and the mechanical properties of these self-assembled thin polymeric layers. More in detail, standard contact mode, force versus distance spectroscopy and ultrasonic force microscopy have been employed to obtain spatially-resolved maps and, thus, observe the physisorption of polystyrene on native silicon oxide substrate in function of time. Thick films, spin coated from a toluene solution, have been annealed at a temperature above the glass transition for increasing time intervals, and finally thoroughly rinsed in toluene. We have found that isolated islands of adsorbed chains are already present after an annealing time of half an hour. Prolonged annealing determines a progressive increase of the covered areas, whereas the formation of a complete flat layer requires 24 h. The pattern observed is in line with expected evolution of an unstable system, corresponding to the phenomenon of spinodal dewetting. Adhesion measurements show that the films present a reduced snap-off and the formation of a meniscus between tip and surface for annealing time up to 8 h. On the other hand, elastic measurements allow us to observe a progressive increase of the elastic modulus, with a complete transition for annealing time above 20 h. This is indication that a dense packing of the polystyrene molecules occurs, in line with the predictions of current models on the kinetics of irreversible adsorption. LAY DESCRIPTION: Herein we present a study on the formation of irreversibly adsorbed layer of polystyrene molecules on silicon oxide surfaces. Various scanning probe microscopy techniques have been employed to study both the morphology and the mechanical properties of these self-assembled thin polymeric layers. Thick polystyrene films, spin coated from a toluene solution, have been thermally annealed at a temperature above the glass transition for increasing time intervals, and finally thoroughly rinsed in toluene. We have found that isolated islands of adsorbed chains are already present after an annealing time of half an hour. Prolonged annealing determines a progressive increase of the covered areas, whereas the formation of a complete flat layer requires twenty-four hours. The adsorption pattern observed is in line with expected evolution of an unstable system, corresponding to the phenomenon of spinodal dewetting. Adhesion and elastic measurements have allowed us to observe a progressive increase of the packing density of the polystyrene molecules, in agreement with the predictions of current models on the kinetics of irreversible adsorption.

Intensified dewetting of polystyrene thin film under water-solvent mixture: role of solvent composition

Intensified dewetting of ultrathin polystyrene (PS) films induced by immersing in a homogeneous mixture of good solvent and water was previously reported to push the limits of dewetting to the sub-100 nm scale. Here, we systematically analyse the role of dewetting mixture composition, i.e. solvent to water ratio, on the length scales of instability. The effect of the solvent concentration in the dewetting mixture on the instability wavelength ( $$\lambda $$ ) and droplet diameter (d) are determined. The instability wavelength ( $$\lambda $$ ) for 50 nm thick PS film was found to be decreasing from nearly 17 to 7 $$\upmu \hbox {m}$$ as the solvent concentration is decreased from 95 to 35% in the dewetting mixture. This is significantly smaller than the instability wavelength in air, which is nearly $$50~\upmu \hbox {m}$$ for the same PS film. The solubility of PS in the dewetting mixture is also examined and the mechanism of observed variation in the length scale is proposed.

X-ray ptychographic mode of self-assembled CdSe/CdS octapod-shaped nanocrystals in thick polymers

This work describes the application of X-ray ptychography for the inspection of complex assemblies of highly anisotropic nanocrystals embedded in a thick polymer matrix. More specifically, this case deals with CdSe/CdS octapods, with pod length L = 39 ± 2 nm and pod diameter D = 12 ± 2 nm, dispersed in free-standing thick films (24 ± 4 µm) of polymethyl methacrylate and polystyrene, with different molecular weights. Ptychography is the only imaging method available to date that can be used to study architectures made by these types of nanocrystals in thick polymeric films, as any other alternative direct method, such as scanning/transmission electron microscopy, can be definitively ruled out as a result of the large thickness of the free-standing films. The electron density maps of the investigated samples are reconstructed by combining iterative difference map algorithms and a maximum likelihood optimization algorithm. In addition, post image processing techniques are applied to both reduce noise and provide a better visualization of the material morphological details. Through this process, at a final resolution of 27 nm, the reconstructed maps allow us to visualize the intricate network of octapods inside the polymeric matrices.

Investigation of processing parameters in micro-thermoforming of micro-structured polystyrene film

In this study, a thermoforming process for manufacturing micro-structured polystyrene (PS) films is investigated to characterize the effects of significant processing parameters. The present micro-thermoforming process utilizes a mold core with five concave rectangular grooves (each of width 0.4-1.2 mm and a depth of 1 mm). Two types of PS films (thicknesses of 50 and 190 μm) are employed to examine the effect of film thickness. Three main processing parameters namely heating temperature, heating time, and mold core temperature are analyzed. The results show that as the width of the groove in the mold core decreases, the forming ratio (depth-to-width ratio of the thermoformed micro-feature) slightly reduces, consequently indicating poor thermoforming. Both thin and thick PS films exhibit similar forming results under favorable conditions. However, when the processing conditions are not suitably applied, the thick film shows the worst result. From the design of experiment analysis using a normalized forming ratio, the mold core temperature is found to be the most influential factor in the thermoforming process of manufacturing micro-structured PS films.

Optical-force-directed single-particle-based track etching in polystyrene films

The establishment of optical trapping theory by A Ashkin almost half a century ago led to the trapping and manipulation of micro-/nano-objects and atoms by laser beams, which is now applied in many fields. However, in a complex system where multi-physical effects interact synergistically, light–matter interaction becomes dynamic and complicated and is still poorly understood. Here, by utilising plasmonic heating, nanolithography of polystyrene (PS) films and nanomanipulation of gold nanoparticles are realised. We find that laser power and PS film thickness as well as particle shape strongly affect the etching behaviour of the PS films, which is a synergistic effect of photothermal ablation and optical forces. Theoretical calculations and simulations rationalise the proposed mechanism, which is also verified by experimental observation. This understanding not only sheds light on how the synergistic effect of photo-ablation and optical forces acts on the particles and polymer films, but also provides a guideline for light-directed single-particle-based nanolithography and nanomanipulation.

Characterization of Ultrathin Polymer Films Using p-Polarized ATR-FTIR and Its Comparison with XPS.

We report on the chemical analysis of ultrathin (10 nm) polymer films using the attenuated total reflectance-Fourier transform infrared (ATR-FTIR) technique based on p-polarized infrared light and two types of enhancing substrates, that is, metallic (Au) and dielectric (Si). We selected low-temperature plasma-treated ∼10 nm thick polystyrene films as a test case for demonstrating the capability of the p-polarized ATR-FTIR, whose performance was further compared with the conventional X-ray photoelectron spectroscopy (XPS) techniques. Although ATR-FTIR cannot be used for quantitatively determining elemental compositions in polymers at which XPS excels, it is able to be operated under nonvacuum conditions and allows the study of hydrogen-containing moieties. By correcting the contact condition between the polymer surface and the ATR prism, the relative concentration of the chemical bonds from different samples can be compared. Because ATR-FTIR and XPS provide complementary information on chemical bonds, their combination provides a powerful approach for studying the chemicalcomposition of polymers.

Violet Emission of ALD‐Grown ZnO Nanostructures on Confined Polymer Films: Defect Origins and Emission Control via Interface Engineering Based on Confinement of the Bottom Polymer Template

AbstractZnO thin films on polymer templates (polystyrene, PST) show strong influence of the interface via degree of confinement of the bottom polymer template in introducing the controlled defect levels in the ZnO nanostructures and consequently yield significant variation at room temperature photoluminescence (PL). The thickness of the polymer thin film in terms of its radius of gyration (R g) defines the degree of confinement of PST. Simultaneous growth of ZnO thin films by atomic layer deposition (ALD) with 150 cycles at 35 °C on spin‐coated polystyrene (PS) films of different thicknesses forms sets of ZnO/PST hybrid systems. The ZnO thin film exhibits a distinct/unusual violet emission (2.95 eV) along with the common near‐band‐edge and green emissions, when it grows on 6.5R g PST. Such defect‐induced emissions disappear with increasing degree of confinement of the bottom PST from 6.5R g to 1.5R g, and sharper UV peak dominates the PL spectra along with a prominent redshift in near‐band‐edge. Furthermore, with increasing film thickness of the bottom PST template from 1.5R g to 6.5R g, the ALD growth rate of ZnO increases by 25%. The study offers new insights into the research on opto‐electronic applications of ZnO thin films in the emerging field of organic electronics.

MEH-PPV photophysics: insights from the influence of a nearby 2D quencher

The effect of 2D quenching on single chain photophysics was investigated by spin coating 13 nm thick films of polystyrene lightly doped with MEH-PPV onto CVD grown graphene and observing the changes in several photoluminescent (PL) observables. With 99% of the PL quenched, we found a 60% drop in the PL lifetime, along with a significant blue-shift of the PL emission due to the preferential quenching of emission at longer wavelengths. During photo-bleaching, the blue spectral shift observed for isolated polymers was eliminated in the presence of the quencher up until 70% of the polymer was photo-bleached. Results were interpreted using a static disorder induced conjugation length distribution model. The quencher, by opening up a new non-radiative decay channel, ensures that excitons do not have sufficient time to migrate to nearby lower energy chromophores. The reduction of energy transfer into the lowest-energy chromophores thus reduces their rate of photo-bleaching. Finally, the difference between the quenched and non-quenched spectra allows the rate of energy transfer along the polymer backbone to be estimated at ∼2 ns−1.

Ellipsometry-based combination of isothermal sorption-desorption measurement and temperature programmed desorption technique: A probe for interaction of thin polymer films with solvent vapor.

An environmental chamber equipped with an in situ spectroscopic ellipsometer, programmatic vapor pressure control, and variable temperature substrate holder has been designed for studying polymer coating behavior during an exposure to a solvent vapor and also for probing the residual solvent in the film afterwards. Both sorption-desorption cycle at a constant temperature and temperature programmed desorption (TPD) of the residual solvent manifest themselves as a change of the film thickness. Monitoring of ellipsometric angles of the coating allows us to determine the thickness as a function of the vapor pressure or sample temperature. The solvent vapor pressure is precisely regulated by a computer-controlled pneumatics. TPD spectra are recorded during heating of the film in an oil-free vacuum. The vapor pressure control system is described in detail. The system has been tested on 6-170 nm thick polystyrene, poly(methyl methacrylate), and poly(2-vinyl pyridine) films deposited on silicon substrates. Liquid toluene, water, ethanol, isopropanol, cyclohexane, 1,2-dichloroethane, and chlorobenzene were used to create a vapor atmosphere. Typical sorption-desorption and TPD curves are shown. The instrument achieves sub-monolayer sensitivity for adsorption studies on flat surfaces. Polymer-solvent vapor systems with strong interaction demonstrate characteristic absorption-desorption hysteresis spanning from vacuum to the glass transition pressure. Features on the TPD curves can be classified as either glass transition related film contraction or low temperature broad contraction peak. Typical absorption-desorption and TPD dependencies recorded for the 6 nm thick polystyrene film demonstrate the possibility to apply the presented technique for probing size effects in extremely thin coatings.

Tuning relaxation dynamics and mechanical properties of polymer films of identical thickness.

Using dewetting as a characterization tool, we demonstrate that physical properties of thin polymer films can be regulated and tuned by employing variable processing conditions. For different molecular weights, the variable behavior of polystyrene films of identical thickness, prepared along systematically altered pathways, became predictable through a single parameter P, defined as the ratio of time required over time available for the equilibration of polymers. In particular, preparation-induced residual stresses, the corresponding relaxation times as well as the rupture probability of such films (of identical thickness) varied by orders of magnitude following scaling relations with P. Our experimental findings suggest that we can predictably enhance properties and hence maximize the performance of thin polymer films via appropriately chosen processing conditions.

Weibull statistics of lap-shear strength development at partially self-healed polymer−polymer interfaces: a long-term contact

The distribution of lap-shear strength (σ) developed at self-healed interfaces between the contacting thick (bulk) films of amorphous polystyrene (PS) and/or poly(methyl methacrylate) (PMMA) at healing temperatures (T) in the surroundings or well below the bulk glass transition temperature (T g bulk), down to T = T g bulk − 60 K, as a result of a long-term (for 24 h) contact, has been analysed in the frameworks of the Weibull statistical model. The Weibull modulus (m) and the scale parameter (σ 0) have been estimated over a broad interval of T, from 54 to 104°C, at both compatible PS−PS and PMMA−PMMA and incompatible PS−PMMA interfaces. The variations in m and σ 0 with T have been discussed and compared with those reported earlier for a short-term 1-h contact.

Structural and Optical Properties of Gold/Silica “Mushroom” Particles Prepared by Interfacial Templating

Complex shaped nanoparticles featuring structural or surface chemical patchiness are of special interest in both fundamental and applied research areas. This study reports the preparation and optical properties of gold/silica “mushroom” nanoparticles, where a gold particle is only partially covered by the silica cap. The synthetic approach allows precise control over the particle structure. The interfacial preparation method relies on partially embedding the gold particles in a polystyrene layer that masks the immersed part of the gold particle during silica shell growth from an aqueous solution. By adjusting sacrificial polystyrene film thickness and silica growth time, precise control over the coverage and cap thickness can be achieved. Correlative electron microscopy and single particle scattering spectroscopy measurements underline the high precision and reproducibility of the method. The good agreement between the measured and simulated single particle spectra supported by near‐field calculations indicates that the observed changes in the dipolar plasmon resonance are influenced by the extent of coverage of the gold core by the silica cap. The straightforward methods readily available for gold and silica surface modification using range of different (bio)molecules make these well‐defined nanoscale objects excellent candidates to study fundamental processes of programmed self‐assembly or application as theranostic agents.