Phase behavior of poly(ε-caprolactone)-b-poly(tert-butyl acrylate) block copolymer at the air/water interface

Thermodynamic phase behavior of poly(ε-caprolactone)-b-poly(tert-butyl acrylate) (PCL-b-PtBA) block copolymers at the Air/Water (A/W) interface was studied by analyzing surface pressure-area isotherms. Optical microscopy and atomic force microscopy were utilized to reveal the morphological features of three-dimensional domains grown in the Langmuir films. A series of PCL-b-PtBA block copolymers synthesized for this study share the same PCL block with molecular weight of M_n = 7.5 kg•mol⁻¹, while the length of PtBA blocks varies. Homogeneous Langmuir monolayers were formed at the A/W interface before reaching the dynamic collapse pressure of PCL blocks. The growth of PCL flat-on crystalline domains predominately took place during the plateau regime after the collapse transition, though nucleation process occurred prior to the collapse transition. The tethered PtBA blocks remain as a monolayer at the A/W interface prior to the second collapse pressure at ∼ 24 mN•m⁻¹, which is comparable to the collapse pressure of PtBA homopolymer monolayer. Depending on the length of PtBA blocks, the block copolymer crystals exhibit morphological features from distorted hexagonal shape, truncated parallelogram, dendrites with defects, stripes, and nano-scale domains, suggesting that the tethered amorphous PtBA block significantly reduced the mobility and packing efficiency of PCL blocks during the crystallization of PCL-b-PtBA copolymers at the A/W interface.