(267e) Dispersity Stabilized Complex Morphologies in AB Diblock Copolymers

Block copolymers have the ability to self-assemble into a variety of complex microphase separated morphologies. Among the various attainable microdomain structures, complex morphologies such as bicontinuous gyroid (G) and hexagonally perforated lamellae (HPL) are highly desirable due to their inherent structural connectivity. For monodisperse AB diblocks, the G phase is thermodynamically stable, but its window of accessibility is small [1]. The HPL phase is even more difficult to obtain as it is determined to be metastable in nature [2]. It has been suggested by various calculations that such complex morphologies may be stabilized by increasing block dispersity [3-5]. In this study we prepared poly(styrene-b-methyl methacrylate) (PS-b-PMMA) diblock copolymers of varied dispersity (D = M_w / M_n) in both PS and PMMA blocks, then examined the formation of complex morphologies in these highly disperse diblock copolymers.

The PS-b-PMMA diblock copolymers were synthesized by sequential atom transfer radical polymerization (ATRP). The PS block was synthesized first, and its block dispersity was controlled via temporal regulated initiator addition [6]. We selectively prepared two types of PS blocks: narrow-disperse PS (nPS) with D ~ 1.1 and polydisperse PS (pPS) with D ~ 1.5. To control the dispersity of the PMMA block, we utilized the addition of phenylhydrazine (PH) [7]. In the absence of PH, narrow-disperse PMMA (nPMMA) with D ~ 1.2 was obtained. With the addition of PH, polydisperse PMMA (pPMMA) with D > 1.7 was obtained. A total of 9 diblock copolymers were prepared, and they were divided into 3 sets of different block dispersity: nPS-b-nPMMA, pPS-b-nPMMA, and pPS-b-pPMMA. Each set of diblocks with specified block dispersity contained 3 polymer samples covering a range in composition. They were identified as (n/p)PS-b-(n/p)PMMA-X, where X was the percent volume fraction of the PS block (f_PS).

The morphologies of the PS-b-PMMA diblock copolymers with varied block dispersity were investigated by a combination of small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Prior to analysis, the diblocks were annealed in vacuum oven at 230 °C for 6 h. For the nPS-b-nPMMA diblock copolymers, three samples of different PS volume fractions were examined. nPS-b-nPMMA-41 and nPS-b-nPMMA-54 showed lamellar morphology, and nPS-b-nPMMA-61 showed gyroid morphology. These observations were in agreement with expected phase behavior of monodisperse diblock copolymers with moderate segregation strength [8].

The second series of diblock copolymers examined were pPS-b-nPMMA, where the PS block dispersity was controlled to be ~ 1.5, and the PMMA block was relatively monodisperse with estimated dispersity of ~ 1.2. All three diblocks showed SAXS profiles with reflections at integer ratio, thus confirmed their lamellar morphology. When compared to the morphologies of the nPS-b-nPMMA diblocks, a shift in the lamellae phase window towards higher composition in the broad dispersity PS block was confirmed. Similar type of domain boundary shift in asymmetrically polydisperse block copolymers have been reported before [9,10]. Moreover, the characteristic domain spacing for these polymers were calculated based on the position of the primary SAXS peak. When compared to the domain spacing of nPS-b-nPMMA diblocks of similar composition and molecular weight, an increase of up to 42% was observed. This result was consistent with earlier reports on polydisperse block copolymers, where domain dilation was attributed to reduction in the entropic energy for chain stretching and polydispersity induced phase mixing [9,10].

Diblock copolymers where both blocks have broad dispersity have not been examined extensively. In this study, we prepared pPS-b-pPMMA diblock copolymers where dispersity of both blocks were greater than 1.5. This provided an excellent opportunity to examine the morphology of highly disperse diblock copolymers. Sample pPS-b-pPMMA-43 showed SAXS profile with broad reflections at q* and 2q*, indicating a lamellar morphology. In comparison to nPS-b-nPMMA and pPS-b-nPMMA of similar composition, a loss of long-range order was observed. For sample pPS-b-pPMMA-53, its SAXS pattern revealed distinct first-order peak at q* and higher-order peak at 2q*, consistent with an ordered layer structure. Additionally broad but discernable features near 1.75q* and 2.7q* were also observed, characteristic of ABCABC stacking of perforations. TEM analysis of the sample also supported a perforated morphology; however, no clear hexagonal packing of the perforations were seen. We assigned the morphology as perforated lamellae (PL), with no regular arrangement of the perforations. As the diblock composition became even more asymmetric, pPS-b-pPMMA-60 exhibited yet another different SAXS profile. In this case, microphase separation was confirmed by the observation of a strong SAXS primary peak. The second order peak had a broad shoulder over the range of q*-2q*, which has been previous reported and assigned to a disordered bicontinuous (BIC) morphology [11]. The BIC morphology has been previously reported in triblock copolymers, where the polydisperse center block was tethered on both ends [11]. This tethered chain structure was believed to assist in the stabilization of the BIC structure. Our results showed that at sufficiently high block dispersity, BIC structure may also be accessed in simple diblock copolymers.

To further investigate the stability of the observed phase morphologies, especially those of the highly disperse pPS-b-pPMMA diblocks, dissipative particle dynamics (DPD) simulations were performed. The calculated phase morphologies were in agreement with the experimental observations. For pPS-b-pPMMA-43, disordered lamellae was confirmed by simulation results, where we observed large fluctuation at the PS/PMMA interface. For systems pPS-b-pPMMA-53 and pPS-b-pPMMA-60, PL and BIC structures could be clearly seen from the simulated density plots. Collectively, we believe the complex PL and BIC morphologies observed in pPS-b-pPMMA systems were equilibrium morphologies. Our results showed that increasing block dispersity, especially in both blocks, can indeed stabilize highly frustrated complex morphologies even in simple diblock copolymers. As these structures are highly desirable yet difficult to obtain in monodisperse systems, control of block dispersity can be an efficient means to increase their accessibility window.

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