Phase separation in polyfluorene – polymethylmethacrylate blends studied using UV near‐field microscopy

Abstract

Summary In this paper we present a near‐field microscopy study of thin films of a phase‐separated blend of the fluorescent conjugated‐polymer poly(9,9‐dioctylfluorene) [PFO] with the non‐fluorescent polymer polymethylmethacrylate [PMMA]. A scanning near‐field optical microscope (NSOM) was used to generate (blue) fluorescence from the PFO following UV excitation at 362 nm. A range of different concentrations of PFO in PMMA were studied ranging from 1 to 50% PFO in PMMA by mass. By studying both the shear force and fluorescence images we were able accurately to determine the distribution of PFO in the PMMA. We found that phase separation occurs over a number of different length‐scales between 5 µm and 250 nm. We show that at PFO concentrations of 1%, the PFO lies on top of the PMMA. At a PFO relative concentration of 50%, the PMMA phase extends through the whole thickness of the film to the underlying substrate. We use such samples to discuss the resolution of NSOM when imaging thick organic films. Furthermore, we confirm that the length‐scales of phase separation can be modified via control over spin‐casting protocols.

Suppression of Energy‐Transfer Between Conjugated Polymers in a Ternary Blend Identified Using Scanning Near‐Field Optical Microscopy

“…All measurements of far-field emission were made on films held under a vacuum of 10 -6 mbar (1 bar = 100 000 Pa). Time-resolved emission measurements from blend films were made using a time-correlated singlephoton counting system with a frequency-doubled mode-locked Ti:-sapphire laser excitation source as described previously [29]. …”

“…SNOM is one of a number of high-resolution microscopic techniques that have been used to identify the relative distribution of polymers within a blend. [29][30][31][32][33] SNOM is a particularly valuable analytic tool for the study of conjugated polymers [34] as many of the important processes pertinent to their application involve the emission or absorption of a photon. Furthermore, SNOM permits spectroscopic measurements to be made on a surface at a length-scale of ca.…”

Pump‐Probe Spectroscopy in Organic Semiconductors: Monitoring Fundamental Processes of Relevance in Optoelectronics

“…Previous studies on PFO/PMMA blends with scanning near-fi eld optical microscopy by Lidzey and coworkers indicate the presence of vertical segregation, with PFO domains lying on top of embedding PMMA. [ 58 ] The smallest domain sizes observed were of the order of 250 nm. Polli et al performed confocal maps of Δ T/T at fi xed delay (200 fs) monitored at 510 nm.…”

Phase Separation in Poly(9,9‐dioctylfluorene)/Poly(methyl methacrylate) Blends

“…Similar observations have been reported on lateral phase-separated thin-films of PFO/ PMMA blends reported by Lidzey et al in which phase separation is suggested to be influenced by dynamic processes that occur during spin-coating. [21] Indeed, it has been reported that the diffusion of solvent to the surface plays a controlling role in defining the final morphology of muticomponent systems. [22] The solvent THF at the surface evaporates at a faster rate than that in the bulk.…”