Dongshan Zhou

Highly stretchable polymer semiconductor films through the nanoconfinement effect

Trapping polymers to improve flexibility Polymer molecules at a free surface or trapped in thin layers or tubes will show different properties from those of the bulk. Confinement can prevent crystallization and oddly can sometimes give the chains more scope for motion. Xu et al. found that a conducting polymer confined inside an elastomer—a highly stretchable, rubber-like polymer—retained its conductive properties even when subjected to large deformations (see the Perspective by Napolitano). Science, this issue p. 59; see also p. 24 A high-performance conjugated polymer is combined with an elastomer to produce a fully stretchable transistor. Soft and conformable wearable electronics require stretchable semiconductors, but existing ones typically sacrifice charge transport mobility to achieve stretchability. We explore a concept based on the nanoconfinement of polymers to substantially improve the stretchability of polymer semiconductors, without affecting charge transport mobility. The increased polymer chain dynamics under nanoconfinement significantly reduces the modulus of the conjugated polymer and largely delays the onset of crack formation under strain. As a result, our fabricated semiconducting film can be stretched up to 100% strain without affecting mobility, retaining values comparable to that of amorphous silicon. The fully stretchable transistors exhibit high biaxial stretchability with minimal change in on current even when poked with a sharp object. We demonstrate a skinlike finger-wearable driver for a light-emitting diode.

Glass Transitions of Poly(methyl methacrylate) Confined in Nanopores: Conversion of Three- and Two-Layer Models

The glass transitions of poly(methyl methacrylate) (PMMA) oligomer confined in alumina nanopores with diameters much larger than the polymer chain dimension were investigated. Compared with the case of 80 nm nanopores, PMMA oligomer confined in 300 nm nanopores shows three glass transition temperatures (from from low to high, denoted as Tg,lo, Tg,inter, and Tg,hi). Such phenomenon can be interpreted by a three-layer model: there exists an interphase between the adsorbed layer and core volume called the interlayer, which has an intermediate Tg. The behavior of multi-Tg parameters is ascribed to the propagation of the interfacial interaction during vitrifaction process. Besides, because of the nonequilibrium effect in the adsorbed layer, the cooling rate plays an important role in the glass transitions: the fast cooling rate generates a single Tg; the intermediate cooling rate induces three Tg values, while the ultraslow cooling rate results in two Tg values. With decreasing the cooling rate, the thickness of interlayer would continually decrease, while those of the adsorbed layer and core volume gradually increase; meanwhile, the Tg,lo gradually increases, Tg,inter almost stays constant, and the Tg,hi value keeps decreasing. In such a process, the dynamic exchanges between the interlayer and adsorbed layer, core volume should be dominant.

Probing the interfacial molecular packing in TIPS-pentacene organic semiconductors by surface enhanced Raman scattering

In organic thin film transistors (OTFTs), the molecular structure of the first few monolayers at the semiconductor–dielectric interface is crucial to the device performance. The assumption of homogeneous molecular packing throughout the thickness of the film is not always valid considering interfacial effects. However, it remains challenging to unambiguously determine the molecular packing at both the top surface and the buried bottom interface, due to the lack of a nanoscopic tool. Here we show that a combination of Raman spectroscopy and surface enhanced Raman scattering (SERS) provides a means for effective characterization of the interfacial packing in 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) films. We observed that the TIPS-pentacene crystal lattices assume a non-equilibrium packing state near the substrate interface, which gradually relaxes towards equilibrium packing near the top interface. Our investigation suggests the existence of non-equilibrium molecular packing for TIPS-pentacene.

New composite reed vibration method to measure the mechanical spectra of liquids

A new experimental method describing the determination of the mechanical spectra (complex Young’s modulus Y*=Y′+iY″ versus temperature) of materials from the liquid to the glassy state, including the glass transition, is reported. The conventional vibration-reed method developed for solids is extended to composite systems consisting of a reed substrate and a deposited material. Mathematical expressions for the evaluation of the mechanical spectrum of the deposited material are obtained by solving either directly the vibrating equation of the nonuniform reed, or that of an equivalent uniform reed, with new length and stiffness, using a coordinate transformation. The mechanical spectra of glycerol and 1,2-propanediol carbonate covering the liquid and the glassy state are presented as examples in this work. The glass transitions of these two kinds of materials, as well as the recrystallization, melting and, evaporation processes of 1,2-propanediol carbonate, are identified in the respective spectra.

Integration of ultrafast scanning calorimetry with micro-Raman spectroscopy for investigation of metastable materials

A stage-type ultrafast scanning calorimetry (ST-UFSC) with controlled heating and cooling rates up to 10(5) K s(-1) was designed to integrate with microstructural characterization. This enables us to precisely control the evolution of fast transitional states of metastable samples provided by the UFSC platform, and to follow subtle structural changes between intermediate stages. As an example, we collected the Raman spectra of poly(ethylene terephthalate) quenched at different crystallization states obtained by programed rapid cooling and heating processes. Because of the very small sample mass for UFSC measurements, from minimum few nanograms to sub-micrograms, the samples temperature is very sensitive to the perturbation from the laser illumination of the Raman spectrometer. Real time temperature monitoring and compensation was accompanied during the whole process of in situ spectroscopy. The results showed a good agreement of crystallization kinetics obtained from the Raman spectroscopy and from the calorimetric melting enthalpy, given that the sample temperature is well controlled during spectroscopic measurements and that the heating rate for calorimetry is fast enough to suppress structural reorganization during heating scans. We expect that the ST-UFSC is suitable to be integrated with other micro-analysis techniques to investigate the structure and dynamics of metastable states obtained by fast thermal treatments.

A transient polymorph transition of 4-cyano-4′-octyloxybiphenyl (8OCB) revealed by ultrafast differential scanning calorimetry (UFDSC)

In this work, ultrafast differential scanning calorimetry (UFDSC) with heating and cooling rates up to 20 000 K s−1 is used to study the transient polymorph transition of the liquid crystal 8OCB. The square plate form (SP), which was reported to grow only from the solution in binary solvent mixtures at low temperature, is found to be the only form growing from a deeply quenched smectic glass during very rapid heating. If the heating rate is slower than 8000 K s−1, reorganization to the parallelepiped form will start, and if the heating rate is slower than 1000 K s−1, the square plate form will reorganize to the parallelepiped form completely, and only melting of the parallelepiped form is observed. The capacity of the UFDSC to capture the unstable polymorphs of low molar mass organic molecules and to follow their rapid transition is demonstrated in this work.

Tracking the interdiffusion of polymers at a molecular level by 1H dipolar filter solid-state NMR under fast magic angle spinning

In this paper, we characterized the interdiffusion of deuterated polystyrene (PS-D) and hydrogenated poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) by 1H dipolar filter solid-state NMR under fast magic angle spinning (MAS). It is observed that the interdiffusion process of PS-D/PPO is composed of two stages, the wetting stage and the diffusion stage. The characteristic time of the transition from the wetting to diffusion stage is independent of the temperature and PS-D molecular weight. On the other hand, the increment of dipolar filtered 1H NMR signal intensity at the wetting stage depends strongly on the temperature. The PS-D chains can be easily mixed with the PPO chains intimately when the temperature is above the glass transition temperature (Tg) of PPO. However, when the temperature is below the Tg of PPO, the PPO chains are frozen and it is more difficult for the PS-D chains to approach the PPO at a molecular level.

Electrochemical synthesis and properties of polybenzoxazine

A new kind of polybenzoxazine, poly(3-(p-methyl) benzyl-3,4-dihydro-6-methyl-2H-1,3-benzoxazine), has been synthesized by an electrochemical method in acetonitrile/alkali aqueous solution. The obtained film shows good heat resistance properties. The structure of the obtained film is characterized using Fourier Transform Infrared (FTIR) spectroscopy.

Synthesis of Site‐Specific Dye‐Labeled Polymer via Atom Transfer Radical Polymerization (ATRP) for Quantitative Characterization of the Well‐Defined Interchain Distance

Novel difunctional initiators that incorporate Förster/fluorescence resonance energy transfer (FRET) pairs are generated to carry out atom transfer radical polymerization of styrene, methyl methacrylate, and n-butyl methacrylate monomers by an efficient manner. Based on the chemical structures of the initiators, the locations of the fluorophore moiety are dictated to be in the center of the chain with accurately quantified chain functionality (>90% labeling ratio). The site-specific integration of FRET dyes into separate polymer chain centers allows for characterization of the well-defined interchain distance quantitatively based on the response between these fluorescent probes. The reliability of this technique is verified in bulk state, which is in well agreement with the theoretical ones. This well-defined FRET system is expected to be a promising candidate to provide a distinct physical image at a microscopic level regarding scaling chain dimension, chain interpenetration, and polymer compatibility.

The effect of self-nucleation on isothermal crystallization kinetics of poly(butylene succinate) (PBS) investigated by differential fast scanning calorimetry

Differential fast scanning calorimetry (DFSC) was employed on the study of self-nucleation behavior of poly(butylene succinate) (PBS). The ultra-fast cooling ability of DFSC allows investigating the effect of self-nucleation on the isothermal crystallization kinetics over a wide temperature range. Crystallization half-time, instead of crystallization peak temperature, was used to describe the self-nucleation behavior, and the self-nucleation domain for the samples crystallized at different temperatures was determined. Due to the competition between homogenous nucleation and self-nuclei, the effect of self-nucleation was less pronounced at high supercooling than that for the sample isothermally crystallized at higher temperature. An efficiency scale to judge the efficiency of nucleating agents from the crystallization half-time was also introduced in this work.