Molecular Motion in Ultrathin Polystyrene Films: Dynamic Mechanical Analysis of Surface and Interfacial Effects

Advancing the dynamic mechanical analysis of organic semiconductor materials

Dynamic mechanical analysis (DMA) is a powerful technique for characterizing the mechanical properties of a wide range of materials. However, the importance of DMA in studying organic/polymer semiconductors has not been fully appreciated. In this Highlight, we explore recent advancements in the use of DMA in understanding the viscoelastic and mechanical properties and thermal transitions of organic semiconductor materials. In particular, the insights gained from DMA can serve as new guides for the device optimisation of organic solar cells towards stable operation. Furthermore, we present key findings, challenges, and future directions to advance the application of DMA in organic electronics.

Dynamic Mechanical Analysis of Polymer Thin Films Using a Kirigami-Inspired Support

A method of determining the mechanical relaxation behavior of polymer thin films is presented that employs a kirigami-inspired sample support. The film of interest is placed on the kirigami support and loaded into a dynamic mechanical analyzer. When the composite is placed in tension, the substrate effectively transfers the load to the film of interest. We demonstrate the approach using a number of polymers and conjugated polymer: small molecule blends relevant for organic photovoltaics. The kirigami-inspired method is found to provide an accurate view of thermal relaxation behavior in polymer thin films, including a quantitative assessment of the film storage modulus. The method is particularly valuable in thin films where film morphology is highly dependent on processing conditions. We show that differences in casting conditions have a clear impact on the thermal relaxation of both the neat and blend conjugated polymer films.

Viscoelastic ECAH: Scattering analysis of spherical particles in suspension with viscoelasticity

Ultrasonic scattering method is a promising technique to evaluate the particle size distribution and/or the elastic properties of particle suspended in liquid. Among the wide variety of scattering theories, the ECAH theory proposed by Epstein-Carhart-Allegra-Hawley is one of the most relevant acoustic scattering theories to reproduce the ultrasonic spectroscopy data for the particle suspensions. However, the original theory assumes that the shear contribution is provided for either elastic solid or viscous liquid. Thus, the viscoelastic effect of particle to the frequency spectra of the attenuation coefficient and the phase velocity has gained less attention and is not fully understood yet. In order to minimize the number of unknown parameters for the analysis of viscoelastic suspensions, we employed the multiple echo-reflection ultrasonic spectroscopy (MERUS) technique, which allowed us to determine the material properties, prior to the complicated scattering analysis. In this study, we prepared the particle suspensions of poly(methyl methacrylate) (PMMA), bisphenol-A polycarbonate (PC) and cross-linked poly(dimethyl siloxane) (PDMS) and the acoustic spectra were measured by the ultrasonic spectroscopy. Then, we demonstrated the effect of the particle viscosity on the spectra by using the viscoelastic ECAH model at the wavelength comparable with the particle size.

Direct observation of polymer surface mobility via nanoparticle vibrations

Measuring polymer surface dynamics remains a formidable challenge of critical importance to applications ranging from pressure-sensitive adhesives to nanopatterning, where interfacial mobility is key to performance. Here, we introduce a methodology of Brillouin light spectroscopy to reveal polymer surface mobility via nanoparticle vibrations. By measuring the temperature-dependent vibrational modes of polystyrene nanoparticles, we identify the glass-transition temperature and calculate the elastic modulus of individual nanoparticles as a function of particle size and chemistry. Evidence of surface mobility is inferred from the first observation of a softening temperature, where the temperature dependence of the fundamental vibrational frequency of the nanoparticles reverses slope below the glass-transition temperature. Beyond the fundamental vibrational modes given by the shape and elasticity of the nanoparticles, another mode, termed the interaction-induced mode, was found to be related to the active particle-particle adhesion and dependent on the thermal behavior of nanoparticles.

Reorientation Kinetics of Local Conformation of Polyisoprene at Substrate Interface

The performance of a polymer composite material, in which inorganic fillers are dispersed, is closely related to the aggregation states and dynamics of polymer chains at the interface with the filler. In this study, the local conformation of polyisoprene (PI) at a quartz substrate interface was studied as a model system for the rubber/filler composite material. PI films were prepared from a toluene solution onto quartz substrates by a spin-coating method. Sum-frequency generation spectroscopy revealed that the local conformation of PI chains at the quartz interface depended on the spinning rate. The tilt angle of methyl groups increased with the rotational speed, probably due to the centrifugal force applied to chains and probably also the evaporation rate of the solvent during the solidification process. This result indicates that the interfacial orientation of PI chains can remain even at room temperature, which is 87 K higher than the bulk glass transition temperature (T_g^b). The interfacial orientation disappeared at a temperature approximately 120 K higher than T_g^b.

Why is surface diffusion the same in ultrastable, ordinary, aged, and ultrathin molecular glasses?

The primitive/JG relaxation explains the same surface diffusion coefficient in ordinary, ultrastable and thin film glasses of OTP and TPD.

Relationship between Segmental Relaxation of Polystyrene Films and Stick-Slip Behavior during Dynamic Wetting of Liquid Droplets on Their Surfaces

A novel method was previously reported for detecting the glass transition of thin polystyrene (PS) films by correlating the relationships between the temperature-dependent viscoelasticity of the PS films and stick-slip behavior on their surfaces during dynamic wetting of liquid droplets. In the present study, the frequency dependence of the stick-slip behavior is investigated. The results show that the stick-slip behavior of liquid dynamic wetting on PS films is dependent on the contact line velocity, which is related to the deformation frequency of the PS surface during the moving liquid front. The stick-slip behavior was revealed to be determined by a dimensionless parameter (ξ), which is the ratio of the PS segmental relaxation time (τα) and the characteristic time (τc) for PS surface deformation near the droplet contact line. When ξ is close to 1 (τα ≈ τc), the Δθ (jumping angle), a scale of the stick-slip behavior, reaches a maximum. This correlation between Δθ and ξ demonstrates that the stick-slip behavior is related to the energy dissipation caused by the PS α-relaxation process, and the peak temperature (or frequency) in Δθ corresponds to the α-relaxation temperature (time) of the polymer. These results strongly demonstrate that the utilization of the stick-slip behavior is a creditable method, similar to dynamic viscoelastic measurement, for probing the glass transition and segmental relaxation of thin polymer films.

Effect of free surface roughness on the apparent glass transition temperature in thin polymer films measured by ellipsometry

Ellipsometry is one of the standard methods for observation of glass transition in thin polymer films. This work proposes that sensitivity of the method to surface morphology can complicate manifestation of the transition in a few nm thick samples. Two possible mechanisms of free surface roughening in the vicinity of glass transition are discussed: roughening due to lateral heterogeneity and roughening associated with thermal capillary waves. Both mechanisms imply an onset of surface roughness in the glass transition temperature range, which affects the experimental data in a way that shifts apparent glass transition temperature. Effective medium approximation models are used to introduce surface roughness into optical calculations. The results of the optical modeling for a 5 nm thick polystyrene film on silicon are presented.

Pharmaceutical applications of dynamic mechanical thermal analysis

The successful development of polymeric drug delivery and biomedical devices requires a comprehensive understanding of the viscoleastic properties of polymers as these have been shown to directly affect clinical efficacy. Dynamic mechanical thermal analysis (DMTA) is an accessible and versatile analytical technique in which an oscillating stress or strain is applied to a sample as a function of oscillatory frequency and temperature. Through cyclic application of a non-destructive stress or strain, a comprehensive understanding of the viscoelastic properties of polymers may be obtained. In this review, we provide a concise overview of the theory of DMTA and the basic instrumental/operating principles. Moreover, the application of DMTA for the characterization of solid pharmaceutical and biomedical systems has been discussed in detail. In particular we have described the potential of DMTA to measure and understand relaxation transitions and miscibility in binary and higher-order systems and describe the more recent applications of the technique for this purpose.

Interfacial properties of polystyrene thin films as revealed by neutron reflectivity

We have studied the glass transition temperature (T(g)) and molecular mobility of polystyrene (PS) thin films near the interface between the polymer thin film and substrate with bilayer thin films consisting of surface hydrogenated PS (h-PS) and bottom deuterated PS (d-PS) using neutron reflectivity. With decreasing the thickness of the bottom d-PS layer, T(g) near the interface between the polymer thin film and substrate increased compared to bulk T(g) and a drastic increase of T(g) was observed for the bottom d-PS layer <155 Å thick. The orientation of polymer chains at the interface is supposed to be related to the increase of T(g) near the interface between the polymer and substrate. The polymer chain mobility decreased with thickness even for the bottom d-PS layer with no discernible change of T(g). It is considered that the numerous contacts between polymer chains and substrate are related to the decrease of mobility near the interface between the polymer thin film and substrate.

Anomalous increase in dielectric susceptibility during isothermal aging of ultrathin polymer films

The aging dynamics of poly(2-chlorostyrene) (P2CS) ultrathin films with thicknesses less than 10 nm were investigated using dielectric relaxation spectroscopy. The imaginary part of the dielectric susceptibility ϵ″ for P2CS ultrathin films with a thickness of 3.7 nm increased with an increase in isothermal aging time, while this was not the case for P2CS thin films thicker than 9.0 nm. This anomalous increase in ϵ″ for the ultrathin films is strongly correlated with the presence of a mobile liquidlike layer within the thin films.

Temperature and concentration effects on the equilibrium and dynamic behavior of a Langmuir monolayer: from fluid to gel-like behavior

The equilibrium isotherms of monolayers of poly(4-hydroxystyrene) on the air/water interface have been studied in the 5-60 degrees C range. The results indicate that the interface is a poor solvent for the monolayers over the whole temperature range. For surface pressures within the semidilute regime, the plot of the area occupied by the polymer coils versus temperature at constant surface pressure shows a sharp change of slope near 30 degrees C. Also, the surface excess entropy shows a similar change of slope at the same temperature. The surface shear viscosity can be described by a power law of the surface concentration. Within the semidilute regime, the exponent of the power law changes. The temperature dependence of the viscosity points out a change from independent particle to collective dynamics.

Molecular weight dependence of mean square displacement in ultrathin polymer films as studied by inelastic neutron scattering

In previous papers, we found that the mean square displacement [u(2)] in polystyrene thin films decreased with film thickness and discussed two possible causes for the decrease. One is the hard interface layer between the polymer thin film and the substrate, and the other is the spatial confinement effect of polymer chains. In order to disclose the cause of the decrease of [u(2)] with film thickness, we studied the molecular weight M(w) dependence of [u(2)] in polystyrene thin films. We found that [u(2)] was independent of M(w) within experimental error, concluding that the interface effect is a main reason for the decrease of [u(2)] with film thickness.

Kinetics of Photoinduced E to Z Isomerization of Azobenzene in Polystyrene Films: Thickness, Molecular Weight and Temperature Effects

E to Z (trans-->cis) photoisomerization of azobenzene (Az) chromophores tagged to polystyrene (PS) in thin films was studied as functions of thickness, PS molecular weight, and temperature, using the change in absorption at 336 nm arising from the Az E isomer remaining upon ultraviolet light irradiation at 350+/-5 nm. The photoisomerization in solid films exhibited fast and slow modes. The fractional amount of the fast mode (I1) started to increase with decreasing film thickness once the films were thinner than a threshold value. This was explained in terms of a surface layer in which the photoisomerization reaction proceeds quickly, the effect of which becomes more noticeable with decreasing thickness due to a larger surface to volume ratio. The thickness dependence of the I1 fraction was insensitive to the molecular weight of the PS used. The thickness of the surface layer, estimated through a layer model analysis, increased with rising temperature up to 298 K. Interestingly, the surface layer markedly thickened at temperatures at which the molecular motion of PS is on a relatively small scale, namely, at the gamma and beta relaxation temperatures.

Glass transition and alpha -relaxation dynamics of thin films of labeled polystyrene

The glass transition temperature and relaxation dynamics of the segmental motions of thin films of polystyrene labeled with a dye, 4-[N-ethyl-N-(hydroxyethyl)]amino-4-nitroazobenzene (Disperse Red 1, DR1) are investigated using dielectric measurements. The dielectric relaxation strength of the DR1-labeled polystyrene is approximately 65 times larger than that of the unlabeled polystyrene above the glass transition, while there is almost no difference between them below the glass transition. The glass transition temperature of the DR1-labeled polystyrene can be determined as a crossover temperature at which the temperature coefficient of the electric capacitance changes from the value of the glassy state to that of the liquid state. The glass transition temperature of the DR1-labeled polystyrene decreases with decreasing film thickness in a reasonably similar manner to that of the unlabeled polystyrene thin films. The dielectric relaxation spectrum of the DR1-labeled polystyrene is also investigated. As the thickness decreases, the alpha -relaxation time becomes smaller, and the distribution of the alpha -relaxation times becomes broader. These results show that thin films of DR1-labeled polystyrene are a suitable system for investigating confinement effects of the glass transition dynamics using dielectric relaxation spectroscopy.