Phase Transition and Gels in Conjugated Polymer Solutions

Article pubs.acs.org/Macromolecules

Phase Transition and Gels in Conjugated Polymer Solutions Cheng K. Lee,*,† Chi C. Hua,*,‡ and Show A. Chen§ †

Research Center for Applied Sciences, Academia Sinica, Taipei 115, Taiwan, R.O.C Department of Chemical Engineering, National Chung Cheng University, Chia-Yi 621, Taiwan, R.O.C § Department of Chemical Engineering, National Tsing Hua University, Hsin-Chu 30013, Taiwan, R.O.C ‡

S Supporting Information *

ABSTRACT: Highly efficient and prolonged coarse-grained molecular dynamics simulations revealed a previously unknown pathway of phase transition of (poor-solvent) conjugated polymer solutions, evolving from a rarely explored entangled state into elastic microgels characterized by interconnected fibrous (or arm-like) materials with locally regular segmental packing. These microgels are significant in that their dynamic and structural features are rather universal (i.e., independent of the solvents utilized), in stark contrast with the counterpart dilute/semidilute systems previously shown (and also demonstrated in this work) to be dictated by solvent-sensitive aggregate species. The overall findings shed light on the yet-unresolved gelation phenomena of conjugated polymers, reveal a striking similarity with conventional entangled flexible polymers in light of the effects of solvent and concentration, and prompt a different possibility of maneuvering their thinfilm properties in optoelectronics-oriented polymer science and technology.

1. INTRODUCTION Gelation is commonly observable for colloid or polymer solutions, subject to widely varying physics. Mostly driven by interparticle (van der Waals) attractions, energetic colloidal particles in a suspending medium can grow into interconnected clusters and morph into the so-called gel state, which falls somewhere between a viscous liquid and an elastic solid and represents a renowned example of viscoelastic materials.1 Entropic polymer chains, on the other hand, may do so when localized anisotropic interactions, hydrophilic or hydrophobic, lead to selective linkages between different chains that later evolve into similar spatial structures as for the colloids above. Most semiconducting conjugated polymers, like MEH-PPV (poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene), however, possess no strong local interactions in solution state.2,3 Instead, segmental van der Waals (vdW) interactions have been noted to foster interchain aggregates (or clusters) even at large dilution, attributable to the semiflexible and amphiphilic features of such polymers. The aforementioned molecular attributes, in turn, render gel formation thermodynamically unfavorable under typical experimental conditions, as it must demand sufficient chain extension which is to be countered by chain rigidity and, in particular, ubiquitous poorsolvent qualities associated with common conjugated polymer solutions. Occasionally, gel formation has been reported for conjugated polymers under somewhat “harsh” experimental conditions,4−6 yet the underlying physics and the exact pathway of structural evolutions remain elusive. Aside from pure scientific curiosity, © 2013 American Chemical Society

the central motivations for understanding the gel properties of conjugated polymer solutions arise from an imperative need to establish the structural/morphological correlations between their solution and quenching (condensed) state, usually referred to as the “memory effect” which puts stress on the significance of controlling the solution (aggregation) properties in the first place, in order to produce high-performance thin films for the end optoelectronic/photovoltaic applications. In fact, merits of exploiting the gel phase of a conjugated polymer were being unfolded recently.7−12 Still, a deeper understanding of this important class of poor-solvent, concentrated solutions and gels is imperatively demanded through experiment or computer simulation. In particular, knowledge into the detailed structural/morphological evolutions in progressively condensed state is, undoubtedly, of both scientific and technological importance, considering the potentially widespread applications of (solution-processable) polymer semiconductors nowadays and in the near future. In this work, we report computational evidence revealing how a gel-like structure of a standard conjugated polymer may be incubated through a previously unknown pathway of phase transition in concentrated, entangled solutions. The investigative protocol utilized a predictive, coarse-grained molecular dynamics (CGMD) scheme that retains explicit CG solvent particles, while being very efficient in simulating systems large Received: November 13, 2012 Revised: January 16, 2013 Published: February 25, 2013 1932

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Figure 1. Specifications of a few representative bond lengths and angles for the CG model of MEH-PPV, where Bi, Ai, and Ci denote the aromatic backbone, short alkoxy, and long alkoxy side chains in the ith monomer, respectively; a solvent molecule, chloroform (C) or toluene (T), is mapped into a single CG particle.

Figure 2. Schematic illustration of two typical macroscopic gels driven by the thermodynamic pathway of spinodal decomposition, where the dark (concentrated) regions represent the quintessential elastic component being investigated in detail.

2. SIMULATION PROTOCOLS Benefited by a recently advanced, highly efficient software package dedicated to molecular dynamics simulations, we were able to attain the first in-depth evaluation of both effects of concentration and solvent quality on a standard conjugated polymer, thus complementing our previous knowledge on fundamental single-chain and aggregate properties for dilute system. Figure 1 depicts the CG descriptions of the polymer and solvent species under investigation. Among the central considerations in constructing the CG polymer model was to retain the distinction of the two asymmetrical side-chain groups and the backbone (phenyl) unit. In this way, distinct affinities of different solvent molecules when interacting with different parts of the polymer can be captured, as elucidated in prior work13 (see also the Supporting Information). Utilization of this CGMD scheme is crucial as one aims to capture moleculescale, polymer−solvent interactions and, on the other hand, gain practical access to large-scale phase behavior. In this respect, we note that no prior studies have attempted as large a molecular system as achieved in this work. In addition, given that all particle interactions are essentially isotropic in nature at this level of coarse-graining, new physics can be readily unveiled

enough to permit a direct assessment of gel-like microstructure. The unique feature of these polymer microgels lies in that they have been developed in a markedly poor-solvent environment without invoking specific, anisotropic interaction forces other than the universal, isotropic vdW forces. By simultaneously delving into the effects of varying solvent and concentration, we show that the new physics uncovered may complement our conventional knowledge based on flexible polymer solutions with dominantly theta- or good-solvent quality. This paper is organized as follows: Following a brief introduction of the CGMD scheme, we discuss results covering the full range from dilute to concentrated regimes mimicking the essential microscopic composition of a real macroscopic gel. The structural evolution in the concentrated, entangled regime is emphasized in particular and analyzed in detail, and the microgels identified are “mechanically” tested to reveal their elasticity. Accordingly, a comprehensive physical picture is provided to correlate the present findings with yet-unresolved gelation phenomena of conjugated polymers, followed by a summary of some scientific and technological outlooks of the physics disclosed. 1933

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Figure 3. Snapshots contrasting the structural evolutions in MEH-PPV/chloroform solutions at three different concentrations mimicking the composition of a bulk gel as depicted in Figure 2. Initially, all chains were randomly placed in each simulation box, and the results shown correspond to a rescaled (real) time of about 100 ns.

Figure 4. Snapshots illustrating the detailed development of a networking microgel in concentrated MEH-PPV/chloroform solution.

into the previously unexplored, highly intriguing, entangled regime.

that governs the gelation process of a more general class of conjugated polymerscrystalline or amorphous. Briefly, the CG intrachain potentials (i.e., bond lengths and two different types of bond angles) were constructed by Boltzmann inversion of relevant probably distribution functions gathered in atomistic dynamics (AMD) simulation for an oligomer (10-mer) species in vacuum. The nonbonded, interparticle potential governing each pair of CG particles utilized the 12−6 Lennard-Jones potential function, and a typical mixing rule was assumed for unlike CG particles. All CG potentials must be examined against the AMD simulation results for self-consistency. In particular, it had been shown that the interparticle CG potentials so constructed can well produce the radial distribution function for each of the “solvent baths” mixing two like or unlike CG particles at the system temperature and pressure considered. More details can be found in ref 13. For a typical macroscopic gel driven by the thermodynamic pathway of spinodal decomposition, as schematically illustrated in Figure 2, the system may be envisioned to spatially consist of three different concentration regions. Accordingly, we have designed the corresponding microscopic model systems to investigate the structural and dynamic features in the individual regions. This strategy is indispensable as macroscopic gels are practically inaccessible by any of the existing molecular dynamics schemes. The actual concentrations simulated were 2.1−2.8, 11.0−13.8, and 23.0−27.9 wt % (denoted as “dilute”, “transition”, and “concentrated” region, respectively). The simulation box for each system is ca. 70 nm, incorporating up to 1 million CG particles (100−1000 MEH-PPV chains with 300 repeating units for eacha chain length commonly utilized in real experimentsand 999 000−729 000 solvent molecules). The simulation was carried out using the software package NAMD214 on a PC cluster, ALPLS, with 96 cores running in parallel for a total computational time of 2−3 weeks. The ability to tackle this relatively large particle system is crucial to delve

3. RESULTS AND DISCUSSION Three different solvent systems of MEH-PPV, denoted respectively as M/C, M/T, and M/(C + T) (C:T = 1:1 in number density, where “M” denotes MEH-PPV, “T” toluene, and “C” chloroform), have been investigated. These systems differ in the effective solvent quality (in order of decreasing solvent quality: M/(C+T), > M/C > M/T), and considering this particular mixing solvent, M/(C + T) has been motivated by an early observation that it results in an exceptionally better solvent quality than those associated with the two single-solvent systems.13 In the present study, however, we noticed little difference between various solvent systems in light of phase transition and gel structure, and therefore most of the following discussion will focus on results for the M/C system (results not shown here may be found in the Supporting Information). Figure 3 shows representative snapshots for systems corresponding to the three different concentrations noted above, ranging from marginally dilute to concentrated (entangled) state. The main features noticed here are basically independent of the initial conditions, provided that the initial chain conformation bears the usual coil structure in solution. The results could differ, however, as highly extended chain conformation was utilized instead. The previous situation might be encountered under flow conditions, and the significance will be remarked near the end of this article. Upon increasing polymer concentration, one can clearly notice a phase transition from an aggregates-dominated suspension system to the emergence of an interconnected network constituted by fibrous (or arm-like) materials with locally regular segmental packing (see a later discussion). To our knowledge, this gel-like feature for conjugated polymers had not been revealed by any computer simulations prior to this work, and the detailed route of structural evolution may be 1934

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Figure 5. Realizations contrasting single-chain dynamics in dilute or concentrated system in an early (a, b) or later (c, d) stage. Note especially a “tube-like” region as defined by the notably constrained movement in concentrated system. The chain marked in red color in each case represents the realization gathered at the latest record time, and the time interval between any two consecutive realizations is 5 ns.

“entanglement length” might be measured as the effective tube diameter,18 and by visual inspection each chain consists of about four entanglement units. At a later stage when chain interactions lead to aggregate or gel state, as depicted in Figure 5c,d, the same probe chain in concentrated system becomes substantially more extended in conformation and highly confined in motion, in part because of the effect of segmental packing with neighboring chains, as we discuss shortly. For the situation delineated above for entangled conjugated polymers, the semiflexible feature together with the poorsolvent environment must drive the chains to seek for alternative, previously unexplored, route for global energetic stabilization. In this respect, we note that two competing mechanisms would presumably determine the outcome of such quest to lower the overall free energy. As the individual chains assume a relatively extended chain conformation at the cost of increasing the intrachain potentials (or the equivalent chain entropy) and, at the same time, promoting unfavorable polymer−solvent interactions, the corresponding energy reduction via regular segmental packing with neighboring chains could help compensate for the prior energy and entropy penalties. In dilute or semidilute solution, the former (intrachain) factor should play a major role, and therefore, the chains form aggregate clusters with much collapsed chain conformations.19 For entangled chains, however, the later (interchain) factor could prevail over the former because dominantly elongated chain conformationsas seen in Figure 5can greatly facilitate the development of fibrous materials that are especially effective in lowing the overall interchain energy while avoiding some unfavorable polymer−solvent interactions as well (because the interior segments are now largely shielded from the outer solvent medium). Meanwhile, some aggregate clusters survive and were further connected by these fibrous materials, forming a peculiar type of phaseseparated, yet interconnected network as seen in Figure 3 or 4.

perceived from the results shown in Figure 4 where several intermediate stages are provided. Evidently, the solution system has gone through a notably entangled state at early times to its eventual, microscopic phase separation at long times. The effect of entanglementbut not the solvent or its qualityin these poor-solvent media plays a crucial role, as we discuss next. Under dilute or semidilute condition, individual chains in a poor-solvent system may pursue intra- or interchain aggregates so as to reduce contact with the surrounding solvent medium. This tendency, in general, leads to the formation of aggregate clusters as have often been noted with conjugated polymer solutions and discussed extensively in early work.2,3,15−17 Due to the dif f iculty (i.e., generally low solubility) in preparing real concentrated solutions for conjugated polymers, however, early experiments were unable to explore the f ull concentration range as presently investigated. For the concentrated solution considered in Figure 4, for instance, the chains would initially seek for the chance of forming aggregate species, like the usual case in dilute/semidilute solutions. Yet, they could soon find it impossible to do so because the deep entanglement would prevent any individual chains from proceeding in their own ways without reconciling the attributes of the others. The situation is best likened to what underlying entangled flexible polymer solutions, where reducing the intrachain excluded volume must be at the cost of increasing the interchain one, and therefore the individual chain assumes an ideal chain conformation seemingly free of the effects of excluded volume. To find evidence revealing the effect of entanglement as well as the corresponding entangled-chain dynamics for concentrated MEH-PPV solutions, we contrast in Figure 5 timedependent single-chain trajectories in dilute or concentrated system at two different stages. Figure 5a,b shows that, at an early stage, while consecutive single-chain trajectories in dilute solution show no signs of geometric confinement, a “tube-like” region can be clearly identified with the constrained (lateral) movement in concentrated solution. In the latter case, the 1935

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plays an important role of sustaining highly extended chain conformation, which, in turn, greatly facilitates regular segmental packing with surrounding chains. Evidence supporting a significant promotion in local segmental packing for the concentrated system can be found in part by the radial distribution function (RDF) shown in Figure 7a, where the three main peaks indicative of local segmental packing become very prominent at a high concentration. Significantly, it can also be seen that the disparity between various solvent systems faded at a high concentration, suggesting that the structural and even dynamic features become universal for entangled conjugated polymer solutions, akin to the renowned cases of entangled flexible polymers. In contrast, solvent quality had been noted in our early studies to dictate the aggregation properties in dilute/ semidilute solutions, as also reflected in the RDFs of Figure 7a. In Figure 7b, we compare the predicted (local) order parameter,18 which reveals the degree of regular segmental packing as marked in Figure 7c for the concentrated system. Roughly, an order parameter greater than 0.3 is indicative of an ordered (nematic) phase, as solely observed with the concentrated system. It is noteworthy that since the CG polymer beads have embedded only the isotropic vdW forces, the segmental packing presently noted with MEH-PPV molecules obviously requires no explicit (planar) π−π stacking as commonly assigned to crystalline conjugated polymers.7,10,11,20 This observation might imply that entanglement-promoted segmental packing could be a rather universal phenomenon for semiflexible polymers, independent of their detailed chemical or structural attributes. In passing, the gel-like phase so formed may be contrasted with typical polymer networks formed by cross-linked chains (see, for example, refs 21 and 22), as has been noted earlier. To qualitatively examine the gel-like microstructure noted above with the concentrated system, we have conducted “mechanical” test which helps reveal the elasticity of the bulk structure; a more thorough analysis that would further uncover the viscoelastic and kinetic features is open to future work. By the common notion, a gel or its precursor microgels should bear certain elasticity in response to an external deformation. Thus, we performed both extension and compression tests on

To gain support to the above proposals, we have computed the mean end-to-end distance for the three systems shown in Figure 3, and the results were found to be 11.1 ± 3.7, 16.8 ± 5.1, and 19.4 ± 4.8 nm for dilute, transition, and concentrated systems, respectively (cf. the chain contour length is 65.2 nm). Furthermore, Figure 6 shows the corresponding time

Figure 6. Comparison of the time evolutions of mean intrachain or interchain potential energy per chain for MEH-PPV/chloroform solutions at three different concentrations; the interchain potential energy has been shifted upward by a constant magnitude for ease of comparison.

evolutions of intrachain or interchain potential energy per chain at some early times. The results clearly indicated that while chain dynamics in dilute or semidilute solution was primarily driven by the tendency to reduce intrachain potential energy, it is basically governed by seeking a reduction in interchain energy in concentrated system. This major observation, along with the results shown in Figure 5, evidently suggested that polymer entanglement in concentrated system

Figure 7. (a) Interparticle (polymer) RDFs at three different concentrations for M/C, M/T, and M(C + T) solutions at a simulation time of 100 ns. While insignificant segmental packing may be noted with dilute solution, the signature of regular segmental packing becomes very prominent at a high concentration, in addition to a smearing effect of varied solvents. (b) The corresponding (local) order parameters (which utilized the CGbackbone-bond vectors in a representative microdomain with size 5 × 5 × 5 nm3). (c) Snapshot of the local structure of M/C in concentrated system displays the regular segmental packing corresponding to the three main peaks in RDFs. 1936

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Figure 8. External elongation and subsequent relaxation on MEH-PPV/chloroform solutions to demonstrate the gel-like, elastic behavior; similar responses have been noted with the case of compression (see the Supporting Information section SII).

the “microgel” (see the Supporting Information section SII), and the results shown in Figure 8 confirm that the bulk structure under assessment bears the expected elasticity, as it is largely recoverable following an external deformation. Intriguingly, these microgels might be expected to contribute to bulk fluid elasticity as they compose an essential portion of a macroscopic gel as depicted in Figure 2. Given that entangled conjugated polymers are not regularly accessible in experiment, as we noted earlier, other mechanisms that may lead to similar entangled state and gel microstructure are remarked below. To this end, it should be mentioned that the microgels disclosed for concentrated conjugated polymer solutions differ from those of colloids, flexible polymers, or small biological molecules in that the rigidity and connectivity of the polymer chain both play an important role. Thus, once the chains become highly extended via flow deformation5 or through the mediation of certain hybrid solvents that result in unusual local chain extension,6 gelation could become thermodynamically favorable and/or kinetically permissible. Without the aid of such “external” forces, entangled conjugated polymers might also be forged by spontaneous local concentration fluctuations, as might be the case with prolonged aging solutions.4,10 Overall, given that microgels of a conjugated polymer seem not equally accessible by a different, perhaps more common, pathway such as continuous drying of a dilute or semidilute solution, as implied by early experiments and the present simulation (see the Supporting Information section SIII), it appears that incubation of conjugated polymer gels must invoke molecular environment that closely mimics the entangled state as disclosed in this study. In dilute or semidilute solution, for instance, our conjecture is that flow pretreatment could be an ideal means to induce gelation, especially for noncrystalline conjugated polymers, because of its apparent ability to enhance local concentration fluctuations and produce sufficient chain extension. It remains unclear, though, how certain solvents or hybrid solvents may function similarly for specific conjugated polymers. Finally, it is interesting to note that the trademark, bicontinuous, nanoscale phase separation noticed here is analogous to what commonly observed for polymer−fullerene23,24 (or polymer−inorganic)25,26 hybrid systems, where fullerene (or inorganic compound) plays the role of “poor solvent” for the polymer in a condensed state. A similar observation might apply to polymer melt blends that display a stable bicontinuous phase (see, for example, ref 27), where the “poor-solvent” medium formed by one polymer species helps promote chain entanglement and/or packing of the other polymer species. In both cases, like MEH-PPV microgels, no specific (anisotropic) interactions are required to bolster a phase-separated polymer network.

4. CONCLUSIONS In conclusion, this CGMD simulation has revealed a previously unknown pathway of phase transition for poor-solvent, conjugated polymer solutions evolving from a rarely explored entangled state into gel-like microstructure. The present findings have been made possible in part by the current availability of high-performance software packages dedicated to molecular dynamics simulations. In real experiments, the prime challenge has been to prepare a concentrated, entangled solution without encountering first a bulk phase separation. By circumventing these difficulties, the physics so unraveled not only lends crucial insight into yet-unresolved gelation phenomena of conjugated polymers, but it also bears a striking similarity to the universal property well-known for flexible polymer solutions in that the effects of solvent (or solvent quality) become irrelevant in the highly entangled regime. The overall findings help lay the groundwork on which future directions for incubating conjugated polymer microgels or gels may be properly guided, especially in developing new strategy for maneuvering the thin-film properties in optoelectronicsoriented polymer science and technology.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed description of simulation methods/protocols, potential functions/parameters, and supplemental figures. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.K.L.); chmcch@ccu. edu.tw (C.C.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the reviewers for suggestions leading to a general improvement of this work. This work is sponsored by the National Science Council of ROC. Resources provided by the National Center for High-Performance Computing are also acknowledged.

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REFERENCES

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