Asymmetric block copolymers in neutral good solvents: self‐diffusion through the ordering transition

Abstract

The self-diffusion coefficients of a styrene-isoprene diblock and a styrene-isoprenestyrene triblock copolymer have been measured as functions of concentration in the neutral good solvent toluene. The sample block molecular weights are 1.0 x lo4 and 5.0 x lo4 g/mol for the diblock, and 1.0 x lo4, 1.0 x lo5 and 1.0 x lo4 g/mol for the triblock, and the samples are designated SI-60 and SIS-120, respectively. Synchrotron small-angle X-ray scattering was utilized to locate the ordering transitions, and to identify the morphologies. SI-60 exhibited an order-order transition from a hexagonal to a cubic structure (i.e., from cylinders to spheres), prior to disordering, whereas SIS-120 only exhibited the hexagonal phase. These results were compared to the melt, where both materials exhibit the cylinder-to-sphere transition. The concentration range for diffusion measurements extended into the ordered state: weight fractions up to 0.78 for SI-60 and 0.62 for SIS-120, where the corresponding ordering concentrations are 0.67 and 0.56. Diffusion measurements were performed by forced Rayleigh scattering and by pulsedfield-gradient NMR. The results indicate that substantial concentration fluctuations arise in solution prior to the ordering transition, and that these fluctuations act to retard chain diffusion. However, from comparison of the diblock and triblock mobilities, it appears that the retardation of mobility in the fluctuating and ordered states is due more to the increased monomeric friction in the (cylindrical) styrene-rich microdomains, than to the thermodynamic barrier for escape of the styrene blocks from these microdomains.

Effect of dilution on a block copolymer in the complex phase window

“…The ͌6 : ͌8 peak ratio was observed in previous work on SI diblock copolymers of very similar composition, and through detailed analysis the structure was assigned to the cubic Ia3 d space group, 1,22 also referred to as the "gyroid" morphology based on Schoen's minimal G surface. 23 Figure 1(a) displays a representative diffraction pattern for SI (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21) at 125°C, along with the first 20 allowed reflections for Ia3 d. As demonstrated by Hajduk et al 1 and Förster et al, 22 if the first observed peak is taken as the first possible reflection, then there is excellent agreement between the observed and possible reflections. Due to the numerous allowed reflections of the G phase at large q, it is difficult to assign the peaks to individual reflections, but clearly there is agreement with several groups of closely spaced reflections.…”

“…First, we establish the melt phase behavior of SI (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21) in order to form a firm basis for examining the effect of added solvent. Second, we consider the concentration and temperature-dependent phase behavior in the presence of the neutral solvent DOP, and third, the corresponding response upon the addition of the selective solvent DBP.…”

“…4 -11 The character of the OOTs and order-disorder transitions (ODTs) observed in the melt has generally been preserved, although a progressive relative reduction of the spherical (S) morphology window in the sequence cylinders (C) 3 S 3 disorder (D) has been observed upon dilution. 12,13 The self-consistent mean-field theory 14 -18 captures some of the experimental phenomenology, such as the scaling of the repeat distance with concentration and the slight accumulation of solvent at the interfaces between microdomains. 19 However, it does not anticipate the observed direct transitions lamellar (L) 3 D and C 3 D for asymmetric copolymers, which have been attributed to fluctuation effects.…”

“…, and the copolymer is therefore designated SI (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21). Copolymer composition was calculated by the amount of the added monomer and the determined yield (Ͼ99%).…”

Patterning with block copolymer thin films

“…In the case of lamellar, cylindrical, and gyroid domains, diffusion along an interface occurs by a different mechanism than that perpendicular to the interface. In the perpendicular case as well as in diffusion between spherical nanodomains, the diffusion coefficient is D $ D 0 exp(axN min ), where N min is the shorter of the two blocks [149][150][151][152]. At lower temperatures, the polymer may not diffuse fast enough to allow rearrangement of domains into a structure with long-range order.…”