Yen-Hao
Lin
a,
Kevin G.
Yager
b,
Bridget
Stewart
a and
Rafael
Verduzco
*a
aDepartment of Chemical and Biomolecular Engineering, MS-362, Rice University, 6100 Main Street, Houston, Texas 77005, USA. E-mail: rafaelv@rice.edu
bCenter for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, USA
First published on 31st March 2014
All-conjugated block copolymers are an emerging class of polymeric materials promising for organic electronic applications, but further progress requires a better understanding of their microstructure including crystallinity and self-assembly through micro-phase segregation. Here, we demonstrate remarkable changes in the thin film structure of a model series of all-conjugated block copolymers with varying processing conditions. Under thermal annealing, poly(3-hexylthiophene)-b-poly(9′,9′-dioctylfluorene) (P3HT-b-PF) all-conjugated block copolymers exhibit crystalline features of P3HT or PF, depending on the block ratio, and poor π–π stacking. Under chloroform solvent annealing, the block copolymers exhibit lamellar ordering, as evidenced by multiple reflections in grazing incidence wide- and small-angle X-ray scattering (GIWAXS and GISAXS), including an in-plane reflection indicative of order along the π–π stacking direction for both P3HT and PF blocks. The lamellae have a characteristic domain size of 4.2 nm, and this domain size is found to be independent of block copolymer molecular weight and block ratio. This suggests that lamellar self-assembly arises due to a combination of polymer block segregation and π–π stacking of both P3HT and PF polymer blocks. Strategies for predicting the microstructure of all-conjugated block copolymers must take into account intermolecular π–π stacking and liquid crystalline interactions not typically found in flexible coil block copolymers.
Comprehensive theoretical and experimental studies of rod–coil diblock copolymers (containing one conjugated polymer block and one flexible coil polymer block) have revealed an interplay between relative block size, polymer crystallinity, liquid crystal ordering, and micro-phase segregation.6–11 Processing conditions have been demonstrated to give some control over the microstructure of rod–coil block copolymers, including aligned domains through the application of a magnetic field12–15 or the formation of micellar assemblies using a selective solvent.16 This work has led to the development of a quantitative model to describe the phase behavior of rod–coil block copolymers, including self-assembly and liquid crystal ordering.17 Comparable predictive models are unavailable for all-conjugated block copolymers due in part to limited experimental studies on structure–processing relationships. Previous work with P3HT-based all-conjugated block copolymers have found that P3HT crystallization dominates the film morphology, suppressing micro-phase segregation and crystallization of the second polymer block.18,19 Evidence for micro-phase segregation in rod–rod all-conjugated block copolymer has been inconclusive, potentially due to polymer crystallization, slow dynamics, stiff polymer backbones, and/or low enthalpic driving force for micro-phase segregation.20 Improved control over the microstructure of all-conjugated block copolymers and a broader understanding of structure–processing–property relationships could benefit their development for and use in organic electronic devices and applications.
Here, we report remarkable changes in the thin film structure of a model series of all-conjugated block copolymers under different annealing conditions. Thermally annealed poly(3-hexylthiophene)-b-poly(9′,9′-dioctylfluorene) (P3HT-b-PF) (Scheme 1) all-conjugated block copolymers exhibit crystalline features characteristic of P3HT or PF, but poor order along the π–π stacking direction. The same block copolymers, under solvent annealing, self-assemble into a lamellar phase, as evidenced by multiple reflections in grazing incidence wide- and small-angle X-ray scattering (GIWAXS and GISAXS). The degree of face-to-face π–π stacking substantially increases in solvent-annealed compared with unannealed films. We present a detailed analysis of the structure of P3HT-b-PF block copolymer films and the self-assembled lamellar phase, including paracrystallinity measurement, spectroscopic properties, and phase behaviour with temperature. These results indicate that all-conjugated block copolymers have a rich, processing-dependent microstructure determined by a combination of π–π stacking, crystallization, and micro-phase segregation of the polymer blocks.
Polymers | P3HTaMw (PDI) | BCPaMw (PDI) | DP ratiosb (P3HT wt%) |
---|---|---|---|
a M w (kg mol−1) and PDI for P3HT and block copolymers determined by comparison to a set of monodisperse polystyrene standards. Head-to-tail regioregularity of P3HT is greater than 93% for all samples as determined from 1H NMR. b DP ratios and P3HT content were determined by 1H NMR via comparison of the integrated intensity of P3HT aromatic peak (6.9 ppm) and fluorene alkyl peaks (2.2 ppm). | |||
P3HT36-b-PF100 | 6.1 (1.16) | 48.4 (1.86) | 36:100 (13%) |
P3HT81-b-PF105 | 13.5 (1.32) | 60.6 (1.87) | 81:105 (25%) |
P3HT84-b-PF80 | 14.1 (1.10) | 42.5 (1.58) | 84:80 (33%) |
P3HT84-b-PF13 | 14.1 (1.10) | 22.8 (1.39) | 84:13 (74%) |
As detailed by prior work, the microstructure of P3HT and PF homopolymers is influenced by processing. Regioregular P3HT organizes into lamellar crystalline domains with face-to-face π–π stacking between chains and lamellar stacking through the hexyl side-chains.25 In solution processed thin films, the preferred orientation of P3HT crystallites can be dictated to some extent by varying processing conditions.26 PF exhibits crystalline, liquid crystalline, or amorphous phases depending on its processing history.27–29
To investigate the role of processing and annealing conditions, the microstructure of a series of P3HT-b-PF films with varying P3HT contents after solvent and/or thermal annealing were analyzed by grazing-incidence X-ray scattering, microscopy, UV-VIS absorbance measurements, and differential scanning calorimetry (DSC). P3HT-b-PF all-conjugated block copolymers with varying block ratios were synthesized via a combination of Grignard metathesis and Suzuki–Miyaura polycondensation. The P3HT-b-PF block copolymers studied have varying compositions ranging from 13 wt% of P3HT to 74 wt% of P3HT, as shown in Table 1. Samples films were drop cast from chloroform and analyzed by grazing-incidence wide- and small-angle scattering (GIWAXS and GISAXS) without annealing (as-cast), after thermal annealing, and with long-time solvent annealing (5 days at room temperature, CHCl3).
GIWAXS measurements of P3HT-b-PF films are shown in Fig. 1 and ESI S2 and S3.† As-cast P3HT-b-PF films are amorphous and exhibit a weak in-plane reflection at q ∼ 1.45 Å−1 (see ESI Fig. S2 and S3†). This in-plane reflection matches the π–π stacking reflection observed in PF homopolymer films,31 and we therefore conclude this reflection indicates some π–π stacking of the PF polymer block in as-cast films. P3HT84-b-PF13 additionally exhibits weak scattering peaks characteristic of P3HT crystallites: an in-plane π–π stacking (010) peak at q ∼ 1.65 Å−1 and out-of-plane (100), (200), and (300) peaks.4,25,32
After thermal annealing at 220 °C – beyond the crystal melting temperatures for both blocks – and cooling to 100 °C, the films exhibit characteristics of pure P3HT or PF crystallites, or both, depending on the block ratio (Fig. 1a). Thermally annealed P3HT36-b-PF100 films exhibit characteristics of PF α-phase crystals33 while thermally annealed P3HT84-b-PF13 films show (100), (200), and (300) P3HT crystal reflections. Block copolymers with intermediate P3HT compositions, P3HT81-b-PF105 and P3HT84-b-PF80, show primarily reflections consistent with PF α-phase and some crystallization of the P3HT block. In contrast to thermally annealed P3HT homopolymer films, reflections corresponding to π–π stacking (q ∼ 1.45 Å−1 and 1.65 Å−1 for PF and P3HT, respectively) are absent for all thermally annealed block copolymer films.
Block copolymer films subjected to 5 days of solvent annealing in a good solvent for both blocks (chloroform) exhibit multiple out-of-plane reflections indicative of long-range, lamellar ordering (Fig. 1b, 2 and 3). A primary GISAXS reflection is observed at ∼0.15 Å−1, which corresponds to a spacing between lamellae of approximately 4.2 nm (Fig. 2). As shown in Fig. 3, the observed peaks are separated by integral multiples of q. These reflections reveal long-range lamellar ordering in solvent annealed P3HT-b-PF films not present in solvent or thermally annealed pristine P3HT or PF films. By contrast, solvent annealed P3HT homopolymer exhibits a (100) crystal peak while solvent-annealed PF films exhibit a metastable liquid crystal β mesophase34 (see ESI Fig. S4, S5, and S6†).
Fig. 2 GISAXS measurements for solvent-annealed P3HT36-b-PF100, P3HT81-b-PF105, P3HT84-b-PF80, P3HT84-b-PF13. Samples were analyzed at room temperature. |
Fig. 3 Linecuts of GISAXS (imbedded figure) and GIWAXS measurements along qxy ∼ 0 Å−1 for solvent annealed samples. |
Solvent-annealed block copolymer films also display strong in-plane reflections at q ∼ 1.45 Å−1 and q ∼ 1.65 Å−1 (see ESI Fig. S7†). These reflections are characteristic of face-to-face π–π stacking for PF and P3HT, respectively. Other crystal reflections characteristic of PF or P3HT homopolymers are absent in solvent annealed films, except for P3HT84-b-PF13. A peak corresponding to the (100) P3HT crystal spacing (q ∼ 0.39 Å−1) is clearly evident in solvent-annealed P3HT84-b-PF13 (see linecut in Fig. 3). A quantitative analysis of crystalline disorder through measurements of a paracrystallinity parameter is provided below.
On heating the solvent-annealed films from room temperature to 150 °C, lamellar GISAXS and GIWAXS reflections disappear and P3HT-b-PF films, with the exception of P3HT84-b-PF13, show only diffuse scattering, with no sharp reflections or peaks. This is indicative of an amorphous or liquid crystal nematic phase (Fig. 1c). Note that 150 °C is below the crystal melting temperatures for both P3HT and PF blocks.
The presence of nematic liquid crystal ordering is confirmed by inspection of the films by polarized optical microscopy (POM), which reveals a birefringent Schlieren texture characteristic of a nematic phase for P3HT36-b-PF100, P3HT81-b-PF105, and P3HT84-b-PF80 (Fig. 4a–c). The block copolymer with the highest P3HT content, P3HT84-b-PF13, (comprised of 85 wt% P3HT) does not exhibit a nematic phase. Instead, as shown in Fig. 1c, some P3HT crystallization is evident. POM analysis of all solvent-annealed and solvent plus thermal-annealed block copolymer films are provided in the ESI Fig. S8.†
Fig. 4 Polarized optical microscopy images of P3HT-b-PF block copolymer thin films subjected to solvent-annealing followed by thermal annealing at 150 °C. Images were acquired at room temperature. |
P3HT-b-PF films, with the exception of P3HT84-b-PF13, thus form a nematic phase after solvent annealing followed by heating to 150 °C and slowly cooling to room temperature. PF homopolymers exhibit a nematic phase only at temperatures higher than the crystal melting temperature35 or at room temperature after rapid quenching.28,36 The enhanced stability of the nematic phase in block copolymers may be due to poor chain mobility resulting from the rigidity of polymer chains or close π–π stacking in the self-assembled lamellar phase.37 The results also suggest that the lamellae of P3HT-b-PF is metastable and can be affected by elevated temperature as well as the crystallization of P3HT as shown in P3HT84-b-PF13.
P3HT-b-PF films thus exhibit remarkable changes depending on the processing history. Thermally annealed films show characteristics of P3HT or PF crystallites, but with poor order along the π–π stacking direction. Solvent-annealed films exhibit a self-assembled lamellar phase along with ordering along the π–π stacking direction. Solvent-annealed films subsequently heated to 150 °C transition to a nematic liquid crystal phase, which is stable on cooling back down to room temperature. This phase behaviour is distinct from that of P3HT or PF homopolymers. Below, we provide a proposed structure of the lamellar phase, detail the optical and thermal properties of the materials, and quantify disorder in solvent-annealed films through quantitative analysis of peak spacings and widths.
Fig. 5 Representative GISAXS and GIWAXS reflections and schematic for lamellar ordering in solvent-annealed P3HT-b-PF all-conjugated block copolymers. |
The size of the lamellar domains formed after solvent annealing is invariant with block copolymer molecular weight and composition. All solvent-annealed block copolymers exhibit lamellar ordering with a characteristic domain spacing of 4.2 nm. The underlying mechanism for the formation of lamellar domains cannot be attributed to conventional micro-phase segregation as in coil–coil block copolymers, in which the domain spacing is dependent on the block copolymer molecular weight.38 Instead, we propose that self-assembled lamellae arise due to a combination of polymer block segregation and π–π interactions of both P3HT and PF polymer blocks. The lamellae are expected to consist of both ordered and amorphous regions, consistent with the structure of P3HT and other semicrystalline, conjugated polymers.39 Prior work has shown that P3HT polymer chains can be folded in amorphous regions and between crystalline domains, even with very sharp turns and folds of the P3HT backbone.40 This allows for a single chain to occupy both ordered and amorphous domains and pass in and out of relatively small domains. As a result, the lamellar domain size (4.2 nm) is determined primarily by π–π stacking associations and is relatively unaffected by block copolymer molecular weight and composition.
An alternative possibility for the structure of solvent-annealed block copolymer films is shown schematically in the ESI Fig. S9.† In this proposed schematic, polymer backbones are oriented in-plane rather than out-of-plane. As a result, the domain spacing is determined by the fully extended length of the alkyl side-chains. While this structure is consistent with the features observed by GISAXS and GIWAXS, we believe this type of ordering is unlikely due to the relatively large spacing between polymers, 4.2 nm. This would require stretching of the P3HT and PF alkyl side-chains. Additionally, the hypothetical ordering is not volume conserving due to the large space (4.2 nm) between polymer chains and close π–π stacking. As a result, the structure in Fig. 5 is the more likely possibility, but we acknowledge the potential for an alternative configuration shown in ESI Fig. S9.†
After solvent annealing, UV-VIS measurements reveal peaks at λ ∼ 522, 560 and 603 nm in the absorption spectra, reflecting P3HT π–π stacking in all solvent annealed films. Regioregular P3HT homopolymer, ordered in the π–π stacking direction, exhibits similar peaks by UV-VIS analysis.41–43 UV-VIS analysis also reveals peaks at λ ∼ 400 and 435 nm in the absorption spectra for lamellar block copolymer films, similar to what is seen for the PF β phase.31 This suggests that face-to-face π–π of PF and P3HT polymer blocks is enhanced in solvent-annealed films, compared with thermally annealed films.
After heating to 150 °C resulting in nematic liquid crystal ordering, sharp absorbance features characteristic of π–π stacking disappear. Nematic block copolymer films show only broad peaks in the absorption spectra at λ ∼ 390 and 500 nm, similar to thermally annealed block copolymer films and corresponding to poor face-to-face ordering of PF and P3HT blocks. The observed transformations in the UV-VIS spectra are thus consistent with GIWAXS measurements that indicate poor π–π stacking in crystalline and nematic block copolymer films and enhanced π–π stacking in the lamellar block copolymer phase.
Differential scanning calorimetry (DSC) reveals differences in the crystal melting temperatures for samples with different processing histories. With the exception of P3HT84-b-PF13, DSC measurements detect a phase transition at 145 °C in solvent-annealed block copolymer on the first heating cycle (Fig. 7a). This transition temperature is consistent with the transition to the nematic liquid crystal phase observed by GIWAXS.
Fig. 7 DSC analysis of P3HT-b-PF block copolymers after (a) long-term solvent annealing and (b) thermal annealing above 220 °C. |
For thermally annealed samples, melting transitions at roughly 157 °C and 215 °C are detected. These temperatures correspond approximately to the crystal melting temperatures of PF and P3HT, respectively.
DSC measurements thus indicate that the self-assembled lamellar phase melts near 150 °C, while thermally annealed samples exhibit crystal melting temperatures comparable to the constituent homopolymers. Similar results for thermally annealed films have been previously reported for P3HT-b-PF4,21 and other all-conjugated block copolymers.3,18,44–46 For P3HT84-b-PF13, only one thermal transition near 220 °C (near the crystal melting temperature of P3HT) is detected by DSC on both first and second heating cycles, suggesting that PF crystallization is suppressed in this sample due to the much higher content of P3HT in the block copolymer.
(1) |
As shown in Table 2, the paracrystallinity disorder parameter g measured for P3HT π–π stacking in thermally-annealed films is in agreement with literature values (g ∼ 6–8%39). The disorder for π–π stacking of the P3HT block is slightly higher for solvent-annealed P3HT-b-PF compared with thermally annealed P3HT homopolymer. This is expected due to connectivity to a PF block which can increase disorder in the P3HT π–π stacking.
Spacingsa | Samples | g (%) |
---|---|---|
a π–π stacking of P3HT and PF was analyzed on GIWAXS patterns whereas 1st lamellar ordering was analyzed on GISAXS patterns. b Commercially available P3HT homopolymer has Mw ∼ 60 kg mol−1. | ||
π–π stacking, P3HT | P3HT36-b-PF100 | 9.68 |
P3HT81-b-PF105 | 7.58 | |
P3HT84-b-PF80 | 9.54 | |
P3HT84-b-PF13 | 8.42 | |
Solvent ann. P3HTb | 8.82 | |
Thermal ann. P3HTb | 8.19 | |
π–π stacking, PF | P3HT36-b-PF100 | 8.23 |
P3HT81-b-PF105 | 5.18 | |
P3HT84-b-PF80 | 8.81 | |
P3HT84-b-PF13 | 7.59 | |
Solvent ann. PF | 17.84 | |
Primary lamellar reflection | P3HT36-b-PF100 | 9.38 |
P3HT81-b-PF105 | 9.05 | |
P3HT84-b-PF80 | 9.44 | |
P3HT84-b-PF13 | 8.89 |
The paracrystallinity disorder parameter has similar values, roughly 8–10%, for PF π–π stacking and for ordering of the self-assembled lamellae. We also note that PF is more ordered along π–π direction in solvent-annealed BCPs compared with solvent annealed PF homopolymer β phase (g ∼ 17.84%).
Quantitative paracrystallinity disorder analysis thus indicates that ordering along the π–π stacking direction is substantially improved for solvent-annealed films compared with thermally-annealed block copolymers.
Footnote |
† Electronic supplementary information (ESI) available: NMR spectra, additional GIWAXS and GISAXS images, and POM images of films. See DOI: 10.1039/c3sm53090f |
This journal is © The Royal Society of Chemistry 2014 |