Temperature and Pressure dependent Phase Behavior of Weakly Interacting Block Copolymer Characterized by FTIR Spectroscopy

Abstract

Block copolymers have been widely investigated due to their self-assembled properties. The segregation of block components due to thermodynamic incompatibility and a connectivity of two chains in a block copolymer can produce various nano-scale (10 ~ 100 nm) microdomains, such as lamellar, cylinder, sphere, and gyroid microdomains. The microphase-separation of a block copolymer can be driven by the enthalpy of repulsive interactions or by the entropy due to a negative volume change on mixing. Recently, we have reported, by using SAXS, rheometry, and depolarized light scattering, that the polystyrene-block-poly(n-pentyl methacrylate) [PS-PnPMA] copolymer exhibited a closed-loop phase behavior bounded by the lower critical disordered-to-ordered transition (LDOT) and the upper critical ordered-to-disordered transition (UODT). However, the exact mechanism of the closed-loop type phase behavior of PS-PnPMA was not known at that time. To understand this mechanism based on the molecular level of each chain, we employed the temperature-dependent Fourier transform infrared (FTIR) spectra of PS-PnPMA.We determined the phase transition temperatures of PS-PnPMA by temperature-dependent FTIR spectroscopy. Even though there are no strong segmental interactions in PS-PnPMA, it is still possible to assess changes in the absorption spectrum for a weakly interacting system undergoing transitions. Furthermore, we demonstrate that the conformation of the C-C-O group in the disordered state at temperatures below the LDOT is qualitatively different from that in the disordered state observed at temperatures above the UODT. Also, we have performed two-dimensional correlation spectroscopy (2DCOS) of FTIR spectroscopy as well as two-dimensional (2D) Wide angle X-ray scattering (WAXS) and FTIR spectroscopy hetero spectral correlation spectroscopy to investigate in detail the phase behavior of PS-PnPMA. The 2D WAXS-IR hetero spectral correlation analysis has not yet been investigated for any polymer blends or block copolymer. We showed that the synchronous 2D hetero correlation spectrum of the ordered state was completely different from that in the two disordered states. The CH group of the main chains of PS and PnPMA did not contribute to the cluster formation in the two disordered states, indicating that the main chains of PS and PnPMA blocks were uniformly distributed at two disordered states. However, we showed that only the C=C group in the PS block contributed to the cluster with a size of 1-2 nm at a disordered state below the LDOT, whereas both the C-C-O group in PnPMA and the phenyl ring and C=C group in PS contributed to the cluster formation at another disordered state above the UODT. Thus, the probability that PS (or PnPMA) chains were located at their own neighboring chains in one disordered state above the UODT is larger than that in another disordered state below the LDOT.It is important to understand the detailed mechanisms related to the pressure (P) dependence on the phase transitions, since the phase transitions of PS-PnPMA are significantly affected by P. The transition temperatures and their pressure dependence are easily determined by using small-angle neutron scattering and birefringence methods

however, these methods cannot provide information on the transition mechanisms at the molecular level of the block chains. Temperature-dependent FTIR spectra at constant P and pressure-dependent spectra at constant temperature were obtained from this pressure cell and analyzed by using 2DCOS techniques. We found that at lower temperatures and as P increases, the PS main chains move ahead of PnPMA main chains. The mobility of the PnPMA chains is prohibited by the cluster formation of the alkyl side chains. On the other hand, at higher temperatures the PnPMA block chains are more mobile than the PS block chains due to larger specific volume.Finally, Gas pressure effect on the phase transitions of two block copolymers (PS- PnPMA and deuterated polystyrene-block-poly(n-hexyl methacrylate) copolymer (dPS-PHMA) was studied by birefringence and sorption measurements. When nitrogen gas was used, the LDOT of PS-PnPMA decreased, while the UODT increased with increasing pressure. Also, the ODT of dPS-PHMA increased. Thus, the miscibility was decreased with increasing nitrogen gas pressure. This is very interesting phenomenon because nitrogen gas has been long time regarded as an inert gas. If nitrogen gas would have been indeed as an inert gas, the miscibility of PS-PnPMA and dPS-PHMA should be enhanced, not decreased! This interesting behavior was explained by the fact that nitrogen gas could be adsorbed onto both PS and PnPMA chains, even though the amount was small. Since the adsorption amount on PnPMA block is larger than that on PS block, the compressibility difference between two blocks becomes increased with increasing nitrogen pressure, resulting in the decrease of the miscibility. On the other hand, we found that helium gas acts indeed as an inert gas for the block copolymers. Thus, with increasing helium gas pressure, the miscibility of the block copolymer was enhanced, similar to the hydrostatic pressure driven by hydraulic oil pressure.