Bijal B.
Patel
a,
Dylan J.
Walsh
a,
Kush
Patel
a,
Do Hoon
Kim
a,
Justin J.
Kwok
b,
Damien
Guironnet
a and
Ying
Diao
*a
aDepartment of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign, 600 South Mathews Avenue, Urbana, IL 61801, USA. E-mail: yingdiao@illinois.edu
bDepartment of Materials Science and Engineering, University of Illinois at Urbana–Champaign, 1304 W. Green Street, Urbana, Illinois 61801, USA
First published on 2nd February 2022
Favorable polymer-substrate interactions induce surface orientation fields in block copolymer (BCP) melts. In linear BCP processed near equilibrium, alignment of domains generally persists for a small number of periods (∼4–6 D0) before randomization of domain orientation. Bottlebrush BCP are an emerging class of materials with distinct chain dynamics stemming from substantial molecular rigidity, enabling rapid assembly at ultrahigh (>100 nm) domain periodicities with strong photonic properties (structural color). This work assesses interface-induced ordering in PS-b-PLA bottlebrush diblock copolymer films during thermal annealing between planar surfaces. To clearly observe the decay in orientational order from surface to bulk, we choose to study micron-scale films spanning greater than 200 lamellar periods. In situ optical microscopy and transmission UV-Vis spectroscopy are used to monitor photonic properties during annealing and paired with ex situ UV-Vis reflection measurement, cross-sectional scanning electron microscopy (SEM), and small-angle X-ray scattering (SAXS) to probe the evolution of domain microstructure. Photonic properties were observed to saturate within minutes of annealing at 150 °C, with distinct variation in transmission response as a function of film thickness. The depth of the highly aligned surface region was found to vary stochastically in the range of 30–100 lamellar periods, with the sharpness of the orientation gradient decreasing substantially with increasing film thickness. This observation suggests a competition between growth of aligned, heterogeneously nucleated, grains at the surface and orientationally isotropic, homogeneously nucleated, grains throughout the bulk. This work demonstrates the high potential of bottlebrush block copolymers in rapid fabrication workflows and provides a point of comparison for future application of directed self-assembly to BBCP ordering.
A complementary and growing approach towards expanding the processability of BCP films has been to modify the architecture of constituent polymeric blocks by introducing branching at one or more points along the contour to create nonlinear block copolymers.12 Of these, molecular ‘bottlebrush’ block copolymers (BBCP) have emerged as strong candidates for rapid self-assembly. BBCP exhibit a hierarchical structure of densely grafted linear block copolymer arms attached to a common backbone.13 With increasing side chain length and grafting density, steric repulsion between adjacent side chains of the same molecule induces increased molecular rigidity and causes the overall molecule to behave as a semiflexible filament with substantially reduced entanglements in the melt14,15 and limited interpenetration of flexible side chains16 compared to the well-studied coil-like conformation of linear BCP. The distinct shape-persistence of BBCP is anticipated to have a pronounced impact on both dynamic and equilibrium self-assembly behavior.
Studies over the past decade have revealed much about the fundamentals of self-assembly for lamellae-forming BBCP prepared by melt annealing and slow solution casting. Notably, it is well established that the equilibrium scaling exponent of domain size (D0) with backbone length (Nbb) is substantially higher for bottlebrush polymers (α ≈ 0.8–0.9)13,17 than for strongly segregated linear block copolymers (α ≈ 2/3),18 suggesting that molecular backbones indeed adopt more extended conformations than linear analogues. More recent theoretical and experimental works have clarified that BBCP in melt do, however, retain significant flexibility, with the Kuhn length of the backbone approximately equal to the overall molecular diameter.19–21
One very useful attribute of BBCP with domain sizes on the order of 100 nm and suitable refractive index contrast is the formation of a photonic band gap upon self-assembly (causing structural color).22–28 Studies have demonstrated modulation of the photonic band gap across the visible to infrared spectrum by varying bottlebrush molecular weight25 blend composition with bottlebrush,29 homopolymer,30 or nanoparticle additives,31 and by tuning processing conditions during solution casting.26 The direct connection between microstructure and optical properties (primarily reflection) has been used to evaluate ordering in photonic BBCP films by comparison predicted wavelength and reflectance of a 1D Bragg Stack.22 More recently, confined self-assembly of BBCP within emulsified microdroplets has been reported as a route towards preparation of photonic crystals with reduced angular wavelength dependence,24 and switchable color,32 with interfacial effects strongly modifying domain orientation.
Interestingly, although many researchers have commented on the potential for bottlebrush block copolymers to exhibit substantially faster assembly into large well-ordered structures compared to their linear analogues, there are few published works that have investigated the evolution of BBCP microstructure over time or have quantified the extent of ordering compared to linear BCP. Using in situ SAXS, Gu, Russell, et al.,17 monitored the self-assembly of bulk (∼mm thickness) lamellae-forming PS-b-PLA at 130 °C and observed higher-order reflections forming at timescales of 5 minutes (Mn = 105 kg mol−1) to 1 hour (Mn = 529 kg mol−1), depending on the overall size of the molecule. Song, Watkins, et al.33 in 2015 also use SAXS to demonstrate rapid ordering of large (Mn = 1850 kg mol−1) BBCP in the bulk (1 mm thickness) after 5 minutes of annealing at 110 °C for low-Tg poly(tert-butyl acrylate)-b-poly(ethylene oxide) [PtBA-b-PEO,], and use a rastering SAXS approach to identify ordered, mm-scale grains.
The orientation of anisotropic BCP domains is substantially affected by the relative strength of interactions between each block and the confining surfaces. When the surface-polymer interactions for each block are balanced (‘neutral’), BCP can adopt surface-perpendicular domain orientation, a behavior frequently achieved by grafting random copolymers of matching chemistry to the substrate. Preferential (‘non-neutral’) surfaces, as used here, instead induce surface-parallel domain orientation, where one block chemistry preferentially wets the substrate interface.34,35 The penetration depth of surface-induced orientation fields has been found to be very system dependent, with reports ranging from 4–6 domain periods36,37 to over 40 periods.38 Several works have systematically varied the strength of substrate-polymer interactions to demonstrate control the propagation depth of surface oriented microdomains, generally concluding that increasing the polymer-substrate interfacial energy difference increases propagation depth.36,37,39,40 Decreasing BCP molecular weight41 or increasing annealing temperature42 have also been reported to increase propagation depth by increasing molecular mobility to allow domain reorganization.
In this work, we consider interfacial ordering and confinement effects during rapid thermal annealing of a large (Mn = 1800 kg mol−1) bottlebrush block copolymer system confined between hard interfaces at thicknesses in between the typical thin-film (submicron) and bulk (> mm) scale. These intermediate length scales (order of tens to hundreds of μm, or ∼270–1700 lamellar periods) are of substantial relevance for both micro-extrusion based additive manufacturing26 and common rheometric measurement geometries, and this method parallels the most commonly reported approach for preparing thermally annealed BBCP for photonic property characterization (compression between clamped glass slides). Thus, we believe that characterization of the spontaneous surface-induced ordering at these length scales provides a vital point of comparison for further work on direct self-assembly of bottlebrush block copolymers and a deeper understanding of prior work on large photonic-crystal forming BBCP.
Quantitative analysis of film color over time was performed via a MATLAB image processing script. To approximate the change in reflected color as a single parameter, the measured red, green, and blue intensity values for each pixel were converted to the HSV (hue, saturation, and value) scale. In this color model, the hue parameter predominately captures the spectral composition of the color, while saturation and value reflect the color purity and brightness, respectively.55 Hue values were averaged frame by frame and tracked over the 10 minutes annealing time (Fig. 1c). For all samples, hue versus time curves evolve in two stages. First, an initial, rapid decrease in hue occurs over the course of less than 30 s from onset, followed by a more gradual decrease to the final stable value. In all cases, the rate of change of hue levels off well before 10 minutes has occurred. We find no systematic variation in inductance time before the transition.
In a complementary in situ experiment, we probed the transmission properties for a series of films spanning the same thickness range (Fig. 1d). Data were collected by taking sequential UV-Vis transmission measurements over the course of annealing. Although these data are expressed in terms of absorbance, both polystyrene and poly lactic acid are transparent within the visible spectrum, allowing us to attribute the measured ‘absorbance’ to deflection of light out of the beam path by reflection from internal BBCP domain interfaces. Beginning with the 47.0 μm thick sample, we observe the initial absence of a clear absorption peak, followed by appearance and progressive redshift of a peak over the course of annealing. Peak intensity does not substantially vary over time, although the width does gradually narrow. In thicker films (118.3 μm and 296.7 μm), we observe a similar redshift of the primary peak, however there is a region of broad absorption at all wavelengths below the primary peak. For the 296.7 μm film, this region between 350 nm and 550 nm is nearly totally flat.
The optical properties of the films after annealing are presented in Fig. 2, beginning with images of samples annealed under the microscope camera (Fig. 2a). Considering the color of each sample individually, there are generally three distinct regions. Near the center of the film, a weakly colored spot appears at the locations where the original BBCP powder fused together. This is surrounded by a broad region of much more uniform yellow/orange coloration, although there are minor patches streaks of color/brightness variation. The thickest film considered (304 μm), was found to exhibit substantial variation in color on the top surface of the film, although the bottom surface appearance (inset in Fig. 2a) was more consistent with the rest of the series.
Normalized reflection and transmission spectra, along with the results of peak fitting are shown in Fig. 2b–d. The measurement area for reflection spectra encompassed the entire film, and raw reflectivity values at the band gap center are as high as 80% versus a Spectralon (PTFE) standard (ESI,† Section 6). Transmission spectra probe a 1.5 mm × 1 mm spot contained within the uniform region of each sample. For all samples, both reflection and transmission spectra contain a primary Gaussian peak centered between 560–570 nm, with a broad plateau at lower wavelengths. The full-width half-max (FWHM) of the primary peak is in all cases is quite large (greater than 60 nm). In reflection mode there is an apparent minimum at intermediate thicknesses, while in transmission mode the FWHM monotonically increases with film thicknesses. In both reflection and transmission, as film thickness increased, the plateau at low wavelength increased substantially. Particularly for the 296.7 μm thick sample, the absorbance profile below the peak wavelength is nearly flat, with broad filtering of all wavelengths in the range of 350 nm to 580 nm.
Quantitative assessment of the film structure by image analysis was performed to map domain orientation and calculate an overall 2D orientational order parameter (S2D, eqn (1)) for each SEM image.
S2D = 2〈cos2θn〉–1 | (1) |
Here, S2D is calculated after first binarizing and skeletonizing domains as individual fibers. The angle (θn) is computed as the difference between the local director vector calculated at each fiber pixel (n) and the average director of the entire image. S2D ranges from 0 to 1, corresponding to a totally isotropic population of lamellae and perfect uniaxial alignment, respectively.51 In Fig. 3b, we show representative SEM images taken within the surface, transition, and bulk regions of the 69.0 μm film; the analyzed fibers were superimposed and colorized by angle, showing high accuracy of the domain mapping approach. As highlighted by the pole figures, this method captures the distinctly different domain orientation distributions in each region. S2Dvs. depth data from two non-overlapping trajectories of SEM images traversing the entire film thickness of 69.0 μm are consistent (Fig. 3c), supporting the robustness of this approach. There is a clear quantitative difference between the orientation parameter in the regions visually identified as the interfacially aligned, transition, and bulk regions (arrows in Fig. 3c correspond to images in Fig. 3b). The same image analysis routine was applied to SEM image trajectories for films of varying thicknesses (Fig. 3d and ESI,† Videos) and reveals qualitatively similar behavior. In all cases, the orientation parameter near the substrate is quite high (more than 0.8), begins to abruptly decrease in the transition region, and substantially decreases near the film center to values less than 0.3. The clearest trend across the thin-thick samples is that the steepness of the transition between highly oriented regions and the orientationally disordered bulk substantially decreases with increasing film thickness (Fig. 3d). The propagation depth of the highly oriented region varies stochastically across the samples, but in all cases exceeds 60 layers (30 lamellar periods), and in the case of the 69.0 μm sample even spans ∼200 layers (100 lamellar periods).
As expected for BCP films prepared on a non-neutral substrate, a clear interfacially oriented region forms near both interfaces during annealing, which transitions to an orientationally disordered bulk near the film center. A detailed perspective on the local microstructure can be drawn from the cross-sectional SEM images, such as Fig. 3a and b and the ESI,† Videos, which contain composited colorized SEM image trajectories for each film thickness. As shown in the colorized images in Fig. 3b, the grain size in the interfacially oriented region is substantially larger than in the orientationally disordered bulk, with lateral dimensions exceeding several microns. While the interfacial region does exhibit strong alignment, the film is still substantially defective, with frequent dislocations, meandering domains, and the occasional presence of large (≈ 4*D0), substantially misaligned grains (tilted up to ∼30°) occurring even within a few periods of the substrate interface.
The decay of domain alignment with distance from the interface is quite complex. The profile is nearly symmetric about the film center (Fig. 3c), as expected for alignment induced by identical substrates. When comparing across several samples of different film thickness (Fig. 3d), in all cases the highly oriented interfacial region (S2D > 0.8) propagates for greater than 30 lamellar periods before decaying to a substantially orientationally disordered bulk (S2D < 0.3). The propagation depth of the interfacial region ranges from approximately 30 lamellar periods for the thickest film (304 μm) to a high of nearly 100 lamellar periods for one of the thinnest (69 μm), although there is no consistent trend with film thickness, likely due to the stochastic nature of lamella nucleation. There is a clear change in the sharpness of the transition, with thicker films exhibiting a much more gradual decay in orientation parameter than thinner films. We discuss this point further below.
SAXS data allow us to evaluate microstructural parameters averaged over a much larger volume of the film. Here we have obtained scattering data using a beam sized to probe the entire film thickness. All samples exhibiting regularly spaced peaks in intensity versus q (Fig. 4a and b) indicative of formation of lamellar domains. Evaluation of domain period from vertical linecuts gives an average value of 187.1 ± 2.1 nm across all film thicknesses (Fig. 4c). The narrow standard deviation is expected, as these film thicknesses are large enough that confinement effects56 are not expected to lead to meaningful chain compression or stretching. The observed azimuthal variation in scattering intensity (Fig. 4d) is consistent with the local picture from SEM cross-sections. Peak intensity is spread over a substantial azimuthal range but is sharply enhanced along the vertical (qz) axis, reflecting the larger proportion of lamellae oriented parallel to the substrate. The increased number of diffraction orders visible along the vertical axis also provides evidence for the larger size of substrate-parallel grains.
Because the structural color reflected by high molecular weight BBCP arises from the interaction between incident photons and the periodic modulation of refractive index within the film, analysis of the film's photonic properties provides a useful probe of microstructure. To inform the following discussion we first briefly summarize the relationship between optical properties and film characteristics by comparison to an ideal 1D lamellar photonic crystal (Bragg stack22,57). Eqn (2), (3),58 and eqn (4) relate the peak reflected wavelength (λ), bandwidth (Δλ), and the reflectivity (R), respectively, to the refractive index (n), layer thickness (d), and number of layers (N).
λ = 2(n1d1 + n2d2) | (2) |
(3) |
(4) |
The simplified forms shown above are derived for normally incident light and (for eqn (3)) assume only small differences between n1 and n2 and d1 and d2. We judge this to be a good approximation in this case, based on the literature values for refractive index of polystyrene (1.592) and polylactic acid (1.461) at 560 nm, and calculation of the volume fraction of PS as ϕPS = 0.56 (ESI,† Section 7). We note that the bandwidth specified in eqn (3) represents the minimum bandwidth for a perfect lamellar crystal. Peak broadening beyond this minimum is commonly attributed to different types of disorder such as block thickness variation, defects, or overall lamellar tilt. Lamellar tilt can also can substantially shift the reflectivity band to higher frequencies (lower wavelength).57
The measured photonic properties of the films after annealing are in excellent agreement with the ex situ microstructural characterization. From the cross-sectional scanning electron micrographs, the highly oriented interfacial region propagates for > 30 domain periods. From eqn (4), a perfect 1-dimensional photonic crystal comprising 60 layers would reflect over 99.9% of normally incident light. Thus, we can expect the reflection properties of films to be dominated by the domains near the surface. In fact, evaluating eqn (2) with the average spacing determined by SAXS results in a predicted band gap centered on 574.3 nm, in excellent agreement with the measured peak reflection wavelengths of 561.1 to 569.4 nm (Fig. 2d). The increase in off-peak absorbance is also consistent with the observed film microstructure. Because transmission-mode measurements probe the entire film thickness, and thicker films contain more of the orientationally disordered region, increasing thickness means that incident photons of all wavelengths below the peak wavelength are much more likely to encounter domains tilted at angles that prohibit wave propagation, and are thus reflected out of the path of the detector. Finally, the photonic bandwidth measurements confirm that all films contain substantial disorder, with measured full-width half-max values ranging from 67.3 nm to 104.7 nm, far greater than the predicted bandwidth for a perfect 1D photonic crystal (≈ 31 nm by eqn (3)).
We now discuss the rapid evolution of photonic properties early in the annealing process in the context of eqn (2)–(4) and propose a mechanism for the formation of the observed microstructure (Fig. 5). At the start of annealing, all films appear to be faintly blue in reflection mode and lack a clear absorption peak in transmission mode. These weak photonic properties can be directly linked to the disordered microstructure of the films prior to annealing, as indicated by cross-sectional scanning electron micrographs (ESI,† Section 8). As previously described, the wavelength (color) of reflected light is primarily governed by domain thickness (eqn (2)) and orientation,57 while the reflected intensity is additionally impacted by refractive index contrast and the number of layers (eqn (4)). Block copolymers in the disordered state are not compositionally uniform and are instead characterized by short range compositional fluctuations with broad interfacial width.1 For this BBCP system, the PS/PLA compositional fluctuations are of sufficient length scale as to lead to reflection of blue wavelengths. The low reflected intensity prior to annealing is likely due to a combination of two factors: (1) the suppression of constructive interference of reflected light due to the randomness of PS/PLA interfacial orientation and (2) poor refractive index contrast due to the large interfacial widths.
Within minutes of heating beyond Tg of both blocks, the samples’ reflected color rapidly red-shifts and reflected intensity increases, before stabilizing and changing little for the remainder of the annealing time (Fig. 1c). Here, the increase in the wavelength of reflected light has contributions from both (1) the formation of lamellar domains whose domain size increases to minimize interfacial area, and (2) the re-alignment of domains to be more parallel to the interface. The increased reflected intensity can be linked to both the incorporation of additional layers into the photonic crystal, and to a lesser extent by the increase in domain purity (refractive index contrast) that occurs during microphase separation.
With these links between evolving photonic properties and microstructure established, we can propose the mechanism for the observed final microstructure that is consistent with established theory on microphase separation and grain coarsening during annealing of block copolymer films from an initial quenched disordered state.4,59 Because annealing occurs at a temperature well below the order–disorder transition temperature, the driving force for domain nucleation and growth is high, and in the early stages of annealing, grains can grow rapidly by incorporating nearby chains from the surrounding disordered regions. Once the volume is saturated with domains, further grain coarsening slows considerably, and is achieved by defect elimination. As discussed earlier, the reflection-mode measurement of hue is primarily surface-sensitive, and so we infer that the rapid color change observed for all samples (Fig. 1b and c) is dominated by heterogeneous nucleation of substrate-parallel lamellae at the polymer-glass interface, and their rapid growth and coarsening into larger grains. By contrast, the slower formation and shift of the broad, thickness-dependent absorption feature is attributed to the formation, coarsening, and realignment of the randomly oriented grains that nucleate homogeneously within the bulk of the polymer films.
The full microstructure of the films is complex, with grain size and orientation varying as a function of film depth, along with a change in the sharpness of the decay in the orientational order parameter S2D with increase film thickness (shown in Fig. 3d). In all cases, there is a transition between large, substrate-aligned grains near the glass-polymer interface and small, isotropically oriented grains near the film center. In thinner films, the transition is sharp, while in thicker films it occurs gradually, across a large portion of the film. A possible mechanism for the formation of this morphology can be proposed by considering the nucleation and growth process. We suspect that the very early stages of annealing, where the film transitions from disordered to ordered play a major role in determining the observed microstructure.
We infer that the microstructure observed is a result of competing heterogenous nucleation at the substrate interfaces vs. homogenous nucleation in the bulk of the film. Heterogeneous nucleation of substrate-parallel domains is expected to occur first, given the lower free energy barrier for surface-induced nucleation. The thus-formed lamella domains can then rapidly propagate inwards through the disordered melt in the bulk. As the phase front propagates, homogeneous nucleation may occur throughout the remaining bulk disordered phase, leading to formation of isotropically oriented domains. Because homogeneous nucleation is volume dependent, it occurs later in the annealing process, with fewer nucleation sites for thinner films, leading to a sharp transition between the heterogeneously aligned grains and the isotropic grains (upper row of Fig. 5). In thicker films, multiple homogeneous nucleation events are likely to occur earlier in the annealing process. The homogeneous grains that nucleate earlier can themselves grow into the disordered phase, interfering with the propagation of substrate-parallel domains and leading to more gradual decay of orientation parameter (lower row of Fig. 5).
In summary, the key conclusions of this manuscript are as follows:
1. During conventional thermal annealing of thick films of large (Nbb = 400) bottlebrush polymers, interface-directed assembly promotes extremely rapid (∼minutes) formation of well-aligned grains which extends for up to 100 lamellar periods, far beyond the penetration of typical surface fields in linear block copolymers.
2. The degree of orientational order in thermally annealed thick films decays towards the film center, eventually reaching an orientationally isotropic bulk, and that the sharpness of the orientation gradient is strongly affected by the overall film thickness in the 10 s–100 s of micron regime.
3. The formation of the interfacially-aligned regions gives rise to the previously reported strong reflective properties of BBCP PC, while the differing width of the bulk isotropic region leads to variation of transmittance and leads to broadband filtering response for very thick (>100 micron) BBCP films.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1sm01634b |
This journal is © The Royal Society of Chemistry 2022 |