Solution-cast Films Research Articles

A systematic AFM-based method to measure adhesion differences between micron-sized domains in asphalt binders

The adhesion strength of asphalt binders is one of the fundamental properties that affect asphalt pavement’s performance. Atomic force microscopy (AFM) is one of the few methods that are capable of measuring adhesion of asphalt binders at the micro scale. Due to the stickiness and viscoelastic nature of the binder, challenges such as tip contamination are present in the measurement of AFM-based adhesion and the effects of operational parameters (e.g., the maximum compressive force and scanner drive speed) on adhesion measurements are often ignored in published studies. In this paper, a systematic AFM-based adhesion testing procedure was developed to minimize the above limitations, and used to characterize the adhesive force of four different asphalt binders. Results indicated that topographic images of solution-cast binder films displayed two micron-sized domains, with a height difference of a few nanometers between them. The adhesion difference between the raised and recessed areas of the binder films was considered to be statistically significant from t-tests, which might be attributed to the different chemical compositions between these two different domains. Such a rigorous AFM-based testing procedure can benefit adhesion measurements of other sticky polymeric materials. The adhesion difference between different domains should be helpful for building the link between chemical and mechanical properties of asphalt binders.

Photophysical and charge-transporting properties of the copolymer SuperYellow

Conjugated copolymers are important materials for organic light-emitting diodes. Here, an investigation of the photophysical and charge-transporting properties of the prototypical poly(p-phenylene vinylene) based copolymer SuperYellow (SY) is reported. The study also investigated the effect of processing conditions by comparing the properties of spin-coated and solution-cast films. For both types of films, the results of time-resolved fluorescence and photoluminescence quantum yield measurements are similar. The high photoluminescence quantum yield of 60% and its independence of processing conditions shows the effectiveness of the bulky side groups in preventing concentration quenching of fluorescence. Time of flight measurements of charge mobility in both spin-coated and solution-cast films also showed similar results, with mobilities in the range 10−6–10−7cm2/Vs for both films. These results provide important information about a widely used copolymer and show that a good polymer light-emitting diode material can have low mobility.

Electrospun isotactic polypropylene fibers: Self-similar morphology and microstructure

Macroscopically aligned isotactic polypropylene (iPP) fibers with different diameters ranging from 0.7 to 13.2 μm were successfully fabricated by high-temperature solution electrospinning using an infrared (IR) heating system. Polarized optical microscope (POM), scanning electron microscope (SEM), wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC) and polarized Fourier transform infrared (FTIR) were employed to characterize the surface morphology and microstructure of the electrospun fibers. From SEM images, three macroscopically structural models have been found: (1) cylindrical fibers, (2) twisted fibers and (3) beaded fibers. Interestingly, regardless of fiber's shape and size, they (especially for cylindrical fibers) exhibit well-defined self-similar morphology. That is, the tertiary, secondary and primary morphology are similar with each other. Thermally induced phase separation (TIPS) is the most pertinent origin to explain the formation of self-similar morphology. WAXD and DSC results show that α-form crystal prevails in the electrospun iPP fibers. For comparison, the solution-cast film was also prepared under the same conditions as those adopted in electrospinning except that the voltage was 0 kV. Compared with the solution-cast film, lower crystallinity, smaller average crystallite size and higher melting point are achieved in the electrospun iPP fibers. The results of polarized FTIR spectra demonstrate that the macroscopically aligned electrospun fibers are highly oriented along the fiber's longitudinal axis at molecular level. Furthermore, when the electrospun iPP fibers were embed in iPP matrix, surface-induced crystallization is significantly enhanced and finally develops into transcrystalline layer which can be even revived after recrystallization. As a result, it is logical that long-lived thermodynamically stable structure has been formed in the electrospun iPP fiber. Combined with FTIR result, the α-form crystal formed in the fiber is conjectured to be a kind of oriented structure with thermodynamically long-lived stability which is similar to "shish-kebab" structure.

Modification of polymorphisms in polyvinylidene fluoride thin films via water and hydrated salt

In this study, the effects of solvent and magnesium chloride hexahydrate (MgCl2⋅6H2O) on the polymorphism of polyvinylidene fluoride (PVDF) thin films were systematically investigated. Wherein, N,N-dimethylformamide (DMF) and water with different volume ratio were used as mixed solvents to obtain the solution casting films, P series. In addition, MgCl2⋅6H2O was comparatively added to prepare PVDF/MgCl2⋅6H2O hybrid films, P-M series. Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and differential scanning calorimeter (DSC) were utilized to study the influence of the water content in the mixed solvents and the hydrated salt on crystallization behavior of PVDF. Further, the morphologic images from scanning electronic microscopy (SEM) and polarized optical microscopy (POM), as well as the pizoelectirc d33 test also supplies the corresponding evidences. As indicated, the water in the mixed solvent shows different effect on main crystal forms of PVDF. At low water content, the solvents may favor the polar phase (β- and γ-phase) mainly by hydrogen bonds interactions between PVDF and water, together with dipolar interactions between PVDF and DMF. At high water content, the nonsolvent water will impose confinement effect on polymer chain diffusion and crystal growth which facilitate the formation of α-phase PVDF. Moreover, magnesium chloride hexahydrate mainly functioned as the nucleation sites for PVDF crystallization. The result of small-angle X-ray scattering (SAXS) implies the content of water or MgCl2⋅6H2O has little impact on the structure of the long period.

Molecular structure of citric acid cross-linked starch films

The effect of citric acid (CA) on starch films has been examined. A new method to detect cross-linking of starch by CA in solution-cast films by molecular weight measurements is described. Furthermore, we managed to distinguished between free, mono- and di-esterified CA and quantify di-ester content within starch films by using a modification in the method of complexometric titration with copper(II)-sulfate. Cross-linking of starch by CA occurred at low temperature, 70°C, which we assumed is so far the lowest temperature reported where cross-linking reaction occurred. This is essential for starch coating applications within paper industry since no high temperatures for curing will be required. However, curing at 150°C and high CA concentrations, 30pph, increased cross-linking reaction. Furthermore, the physical properties like water solubility, gel content and glass transition temperature, were highly reflected by changes in the molecular structure i.e. cross-linking and hydrolysis, as well as CA content and curing temperature.

Morphology-Driven Sensitivity Enhancement in Organic Nanowire Chemiresistors

Chemiresistors were prepared from the common organic semiconductor, poly(3-hexylthiophene) (P3HT), which is known to respond to atmospheric humidity with an increase of its conductance. Chemiresistors with different P3HT morphologies were compared: a solution-cast film versus an organic nanowire 'mesh.' It was found that both types of chemiresistors show no response for an exposure to less than 50% relative humidity. When humidity exceeds this threshold, the conductance of both chemiresistors increases. The conductance response of the nanowire device is far stronger as compared to that of the solution-cast film. This difference can be justified by the much larger surface area in the nanowire morphology. We conclude that organic nanowires constitute a favourable morphology for organic sensor technology, and recommend their wider employment in organic sensor applications.

Solvent Vapor Annealing in the Molecular Regime Drastically Improves Carrier Transport in Small-Molecule Thin-Film Transistors

We demonstrate a new way to investigate and control the solvent vapor annealing of solution-cast organic semiconductor thin films. Solvent vapor annealing of spin-cast films of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-Pn) is investigated in situ using quartz crystal microbalance with dissipation (QCM-D) capability, allowing us to monitor both solvent mass uptake and changes in the mechanical rigidity of the film. Using time-resolved grazing incidence wide angle X-ray scattering (GIWAXS) and complementary static atomic force microscopy (AFM), we demonstrate that solvent vapor annealing in the molecular regime can cause significant performance improvements in organic thin film transistors (OTFTs), whereas allowing the solvent to percolate and form a liquid phase results in catastrophic reorganization and dewetting of the film, making the process counterproductive. Using these lessons we devise processing conditions which prevent percolation of the adsorbed solvent vapor molecules for extended periods, thus extending the benefits of solvent vapor annealing and improving carrier mobility by nearly two orders of magnitude. Ultimately, it is demonstrated that QCM-D is a very powerful sensor of the state of the adsorbed solvent as well as the thin film, thus making it suitable for process development as well as in-line process monitoring both in laboratory and in future manufacturing settings.

Effect of solvents and thermal annealing on the morphology development of a novel block copolymer ionomer: a case study of sulfonated polystyrene-block-fluorinated polyisoprene

Abstract The morphologies of sulfonated polystyrene-block-fluorinated polyisoprene (s-PS-b-f-PI) were studied as functions of different kinds of solvents and thermal annealing, using transmission electron microscope microscopy (TEM), small angle X-ray scattering (SAXS) and intermediate angle X-ray diffraction (IMAXD). The film of the block copolymer ionomer with an acid form [sulfonated PS-b-fluorinated PI (sH-PS-b-f-PI)], which was cast from anhydrous tetrahydrofuran (THF), developed a well-ordered cylindrical morphology of the f-PI domains in the sH-PS matrix. The morphology was explained by the solubility parameter and the Bjerrum length. The morphology changed to that of a no long-range ordered structure, through thermal annealing. The result was different from the usual behavior of uncharged diblock copolymers and could have been due to the ionic cluster formation. The solution casting films of the block copolymer ionomer with a cesium neutralized form (sCs-PS-b-f-PI) were obtained from the mixed solvent of THF/water (98/2, wt/wt) and DMSO. The ionic cluster formation in the film cast from the mixed solvent was hardly changed by annealing. The promotion of the ionic cluster formation by annealing was clearly observed in the film cast from DMSO. However, hindrance of microphase separation by the promotion of the ionic cluster formation was not confirmed in this study.

Effect of structural anisotropy on electrical and magnetic properties of polyaniline conducting films

It is shown that highly conducting films of polyaniline protonated with di-esters of sulfosuccinic and sulfophthalic acids which contain alkyl- or alkoxy-type substituents exhibit highly anisotropic structural, electrical and magnetic properties. The layered-like structure of these films can be described as consisting of polyaniline chains which are mainly oriented parallel to the plane of the film and form regular out-of-plane stacks. These stacks are separated by bilayers of the dopant anions. Accordingly, the main anisotropy observed for solution cast films implies in-plane and out-of-plane measurements. An electrical anisotropy of about 80 is found for the in-plane and out-of-plane electronic conductivities at 5K. The temperature dependences of the in-plane and out-of-plane conductivities are qualitatively similar and have been fitted as a series combination of variable-range-hopping-type and power law contributions. A maximum is observed in the temperature dependence of the electrical anisotropy at low temperature. The films also show a clear anisotropy of magnetization whose temperature and field characteristics depend on the chemical structure of the dopant anion.

Optical anisotropy in solution-cast film of cellulose triacetate

The out-of-plane birefringence and its wavelength dispersion are studied employing solution-cast films of cellulose triacetate (CTA). In solution-cast process, CTA molecules are induced to align in the film plane. Although refractive index is the lowest in the oriented direction for the CTA films stretched more than 110 %, refractive index is found to be the lowest in the normal direction for the unstretched cast film. Attenuated total reflection measurements reveal that in-plane alignment of the acetyl group which provides strong polarizability anisotropy is responsible for the phenomenon. Furthermore, the out-of-plane birefringence is found to increase with increasing wavelength, i.e. extraordinary wavelength dispersion, whereas a stretched CTA film shows ordinary wavelength dispersion. The level of the out-of-plane birefringence in cast films depends on the preparation conditions, which is predictable considering the evaporation rate. Moreover, it is demonstrated for the first time that the out-of-plane birefringence and its wavelength dispersion can be modified by addition of a certain plasticizer such as tricresyl phosphate (TCP). During the evaporation, TCP molecules orient in the film plane accompanying the orientation of CTA chains by intermolecular orientation correlation, called nematic interaction. This technique will widen the scope of material design of retardation films because there are numerous liquid compounds having strong polarizability anisotropy.

Solvent quality-induced nucleation and growth of parallelepiped nanorods in dilute poly(3-hexylthiophene) (P3HT) solution and the impact on the crystalline morphology of solution-cast thin film

Understanding the chain conformation of conjugated polymers in casting solutions and its impact on the crystalline morphology of solution-cast thin films is crucial for many electronic applications. Using small-angle neutron scattering, we show that well-dissolved poly(3-hexyl thiophene) (P3HT) chains in good solvent (chloroform) form long rectangular parallelepipeds (RPs) via nucleation and growth processes upon increasing the volume fraction of poor solvent (hexane) above a certain critical point. The growth of the RPs is due to the π–π stacking of the P3HT main backbone occurring along the long axis of the RPs. P3HT solutions prepared with different poor solvent volume fractions were drop-cast onto Si-wafers to prepare thin films, which were examined using 2D grazing-incidence X-ray scattering and 1D X-ray diffraction. The results indicate that the RPs grown in solution preferentially orient on the substrate with their two longer axes parallel to the surface after solvent evaporation, and give rise to much improved crystallinity and crystal orientation compared to the disordered chains.

Radially aligned microchannels prepared from ordered arrays of cracks on colloidal films

Arrayed microchannels with dimensions of tens to hundreds of micrometres show great potential for application in microfluidics, microreactor devices and other areas. In this work, we report the formation of ordered arrays of cracks on solution-cast colloidal films, and the preparation of radially aligned microchannels. The polymer film coating enables the colloidal film to be detached from the substrate, retaining the crack patterns on the bottom side. The subsequent chemical corrosion converts the cracks into microchannels. Crack-patterned colloidal films are also used as structured substrates for the preparation of honeycomb films by the breath figure method. Hierarchical honeycomb structures are obtained, giving rise to improved support for colloidal films.

Self-assembly of fully conjugated rod–rod diblock copolymers within nanofibers

We report the fabrication and self-assembly study of electrospun nanofibers of oligothiophene-block-oligophenylenevinylene, a fully conjugated rod–rod diblock copolymer (BCP). X-ray scattering and transmission electron microscopy of microtomed sections of nanofibers and solution-cast films revealed the formation of hierarchical self-assembly, consisting of BCP micro-phase separation at the length scale of 5 nm and molecular ordering within individual conjugated blocks at the length scale of 0.4–1.6 nm. Extensional flow during electrospinning transforms the curved BCP morphology (in films) into a lamellar morphology in nanofibers oriented along the axis. In addition, electrospinning induces a partial inter-digitation of side chains resulting in a unique thiophene phase with two distinct populations of π–π stacking structures at the molecular sub-nanometer level. To our knowledge, this is the first ever study to investigate the self-assembly of fully conjugated rod–rod block copolymers within nanofibers, a material system with the potential to revolutionize the field of wearable photovoltaics.

Evaluation of graphene nanosheets influence on the physical properties of PVDF/PMMA blend

In this work, we report the preparation of PVDF/ PMMA/graphene polymer blend nanocomposites via syn- thesis of PMMA/graphene as a masterbatch through in-situ polymerization. The PMMA/graphene masterbatch com- pounded with PVDF by solution mixing in different ratios. The compounding was followed by solution casting to form polymer blend nanocomposites. Solution cast films were subjected to thermal treatments at three different temper- atures. ATR-FTIR results revealed the specific interaction between PVDF and graphene surface and also increasing the percentage of γ-phase on the surface with rising the anneal- ing temperature. SEM photographs of the top surface of casted films showed visible phase fluctuations with increas- ing the annealing temperature especially for 70:30 which is induced by graphene sheets in the mixture of PVDF/ PMMA. Also, presence of fiber pull-out in the fractured surface of 70:30 was related to the increase in cohesive interface due to existence of PMMA. TGA analysis exposed the improvement in thermal stability of PVDF portion of the mixture due to preferentially adsorption of PVDF chains onto the graphene surface during of the heating. Decreasing in Yong's modulus was observed with addition of PMMA but presence of strong GNPs retrieve this negative effect and increases the tensile modulus. The melt-rheological studies illustrated the high interaction between PVDF, PMMA, GNPs and different status of dispersion in three different samples. As well, measuring MFI showed that PMMA caused to decline in melt flow of the polymer blend while the addition of graphene compensate it to some extent revealed lubrication effect of GNPs.

Fabrication of UV sensor based on electrospun composite fibers

A simple, low-cost and sensitive UV sensor based on electrospun composite fibers was fabricated. Three different types of (PVA/ZnO) composites were used to fabricate the device. The solution-cast film of PVA and ZnO shows lower photocurrent than the other two fiber based devices. A comparative photoresponse study has revealed the high sensitiveness of the in situ derived composite fibers of PVA and ZnO under UV illumination thus making them highly suitable for the fabrication of UV detector. We believe that the coupled effect arising from the presence of PVA and uniform distribution of ZnO are responsible for the sensitivity of UV sensor. Thus we conclude that the response of UV sensor is highly dependent on the morphology of the composites and the preparation method.

Improved mechanical properties of solution-cast silicone film reinforced with electrospun polyurethane nanofiber containing carbon nanotubes

In this study, we describe the enhancing ability of electrospun polyurethane (PU) nanofibers containing carbon nanotubes (CNTs) as nanofillers for silicone film in improving the physico-mechanical properties of the composite material. We prepared the samples combining two simple techniques: solution casting and electrospinning. Neat PU nanofibers alone are good reinforcing materials but the presence of CNTs inside the PU nanofibers has drastically improved the mechanical properties of the silicone composite film. The silicone film increased its tensile strength by 226% and its tensile modulus by more than 14-fold when CNT/PU nanofibers were incorporated.

Relating Hydrogen-Bonding Interactions with the Phase Behavior of Naproxen/PVP K 25 Solid Dispersions: Evaluation of Solution-Cast and Quench-Cooled Films

In this work, we investigated the relationship between various intermolecular hydrogen-bonding (H-bonding) interactions and the miscibility of the model hydrophobic drug naproxen with the hydrophilic polymer polyvinylpyrrolidone (PVP) across an entire composition range of solid dispersions prepared by quasi-equilibrium film casting and nonequilibrium melt quench cooling. The binary phase behavior in solid dispersions exhibited substantial processing method dependence. The solid state solubility of crystalline naproxen in PVP to form amorphous solid dispersions was 35% and 70% w/w naproxen in solution-cast films and quench-cooled films, respectively. However, the presence of a single mixed phase glass transition indicated the amorphous miscibility to be 20% w/w naproxen for the films, beyond which amorphous-amorphous and/or crystalline phase separations were apparent. This was further supported by the solution state interactions data such as PVP globular size distribution and solution infrared spectral profiles. The borderline melt composition showed cooling rate dependence of amorphization. The glass transition and melting point depression profiles of the system were treated with the analytical expressions based on Flory-Huggins mixing theory to interpolate the equilibrium solid solubility. FTIR analysis and subsequent spectral deconvolution revealed composition and miscibility dependent variations in the strength of drug-polymer intermolecular H-bonding. Two types of H-bonded populations were evidenced from 25% w/w and 35% w/w naproxen in solution-cast films and quench-cooled films, respectively, with the higher fraction of strongly H-bonded population in the drug rich domains of phase separated amorphous film compositions and highly drug loaded amorphous quench-cooled dispersions.

Synthesis, characterization and enzymatic degradation of novel biodegradable copolymers of 5-allyloxy-1,3-dioxan-2-one with ε-caprolactone

Ring-opening copolymerization of 5-allyloxy-1,3-dioxan-2-one (ATMC) with ε-caprolactone (CL) was successfully performed for the first time in bulk at 140 °C using Sn(Oct)2 as the catalyst. Novel biodegradable copolymers with different compositions were characterized by 1H NMR, GPC, DSC, X-ray diffraction and static contact angle method. The copolymer composition was relevant to the monomer feed ratio, while the ATMC incorporation in the copolymers was less than that in the feed. With the increase of ATMC molar feed ratio from 25% to 75%, the Mn of the resulting copolymers decreased from 30,500 to 14,300 g/mol, while the static contact angle of the copolymers increased from 92.7° to 98.2°. The relationship between the copolymer glass transition temperature and composition was in agreement with the Fox equation. In vitro cytotoxicity studies using MTT assay demonstrated that the copolymer had low cytotoxicity compared to poly(ε-caprolactone) (PCL). Moreover, solution cast films were allowed to degrade in a pH 7.4 phosphate buffer solution containing Pseudomonas lipase. Weight loss data showed that the copolymers with ATMC content higher than 46% were not degradable and the enzymatic degradation rate increased with CL content. The areas of the diffraction peaks decreased gradually with increasing degradation time according to XRD analysis. Little changes of molecular weight can be detected during enzymatic degradation, in agreement with a surface erosion mechanism shown by SEM.

All-aromatic polyetherimide and polyamide-imide thin films deposited by the infrared laser ablation

The thermo-oxidative stability and molecular weight were determined for polyimide and polyamide-imide thin films, deposited by resonant infrared laser ablation (RIR-LA). RIR-LA films were deposited at various wavelengths and compared with the traditional solution-cast films using thermogravimetric analysis. The RIR-LA films showed slightly lower oxidation temperatures, but the initial weight loss temperatures were greater. Since molecular weight is directly correlated with the physical and mechanical properties of the polymer, multiangle laser light scattering was employed to ascertain the weight-average molecular weight of the polymers before and after LA. The RIR-LA films exhibited slightly reduced weight-average molecular weight. This degradation has three possible sources: bond scission during LA, reduced solvent transfer leading to lower chain extension and hydrolysis caused by the presence of atmospheric water.

Influence of block composition on structural, thermal and mechanical properties of novel aliphatic polyester based triblock copolymers

A series of ABA type triblock copolymers [Poly(lactide)-block-poly(hexamethylene 2,3-O-isopropylidene tartarate)-block-poly(lactide)] PLA-b-PHIT-b-PLA based on renewable monomers l-tartaric acid and l-lactide have been synthesized and the effect of the PLA chain length on the properties of the triblock copolymers has been systematically investigated. The block nature of the copolymers was established by differential scanning calorimetry (DSC) which showed two glass transition temperatures (Tg) corresponding to PHIT and PLA blocks. Solution cast films of these triblock copolymers turned out to be brittle in nature and to overcome this, ε-caprolactone was copolymerized with l-lactide to generate a separate series of triblock copolymers [PLA-ran-PCL]-b-PHIT-b-[PLA-ran-PCL]. Our study systematically demonstrates that the PLA-to-PCL ratio in the outer block composition influences the mechanical properties via a delayed post-yield stress drop phenomenon. The study further elaborates the time-synchronized strain-field analysis of the novel triblocks to be a convincing approach for the characterization of micro-deformation modes.