Article pubs.acs.org/Macromolecules
Self-Diffusion and Constraint Release in Isotropic Entangled Rod− Coil Block Copolymers Muzhou Wang, Ksenia Timachova, and Bradley D. Olsen* Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States S Supporting Information *
ABSTRACT: Understanding dynamic relaxation mechanisms in self-diffusion and constraint release processes of rod−coil block copolymers is important for many technological applications that employ neat melts or concentrated solutions. Using a model system composed of poly(alkoxyphenylenevinylene) rods and polyisoprene coils, reptation theories of entangled rod−coil block copolymers are investigated in the isotropic melt state. Self-diffusion was measured by forced Rayleigh scattering using a red laser line and a new blue photoswitchable dye that allow operation above the bandgap of most semiconducting polymers. In contrast to previous tracer studies where the diffusion of rod−coils through a coil homopolymer matrix is slowed relative to coil homopolymers because of a mismatch in the curvature of the rod and coil entanglement tubes, slowed diffusion is only present in self-diffusion measurements above a critical molecular weight. An activated reptation mechanism with constraint release is proposed as a modification to the description of entangled rod−coil block copolymer dynamics, where the slowing occurs when the time scale of rod block reptation is faster than the reorganization of the surrounding entanglement tube. This mechanism is supported by additional tracer diffusion experiments on polyalanine-b-poly(ethylene oxide) diblocks in aqueous entangled poly(ethylene oxide) matrix solutions and Kremer−Grest simulations where the matrix molecular weight is varied. The slowing of tracer diffusion in rod−coil block copolymers relative to coil homopolymers is significantly weaker for smaller matrix polymers, confirming the proposed constraint release effects.
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INTRODUCTION Applications for self-assembled soft materials incorporating functional domains such as semiconducting polymers1,2 and polypeptides2−4 have motivated the study of rod−coil block copolymers, which are hybrid polymers where a functional polymeric component enforces chain rigidity in one block. The self-assembly of these copolymers is fundamentally different from coil−coil block copolymers due to liquid crystalline interactions between rod blocks and differences between rod and coil chain topology.2,5−7 These differences also lead to scientifically rich dynamic phenomena. For example, the dynamic scaling laws for entangled rods8−10 (isotropic selfdiffusion D ∼ M−1, relaxation time τr ∼ M9) are quite different from entangled coil homopolymers9,11−13 (D ∼ M−2.3, τr ∼ M3.4), suggesting that the nature of dynamic entanglement is fundamentally different between rods and coils. A few studies of rod−coil block copolymer dynamics have addressed specific questions, such as identifying order−disorder transitions,14,15 measuring intrinsic viscosities,16 or calculating dynamic structure factors in dilute solution.17 However, a unified theory for the molecular relaxation mechanisms is needed for predicting important dynamic phenomena, such as mechanics, processing, and self-assembly kinetics. Our recent studies of the dynamics of entangled rod−coil block copolymers have combined theory, experiment, and © 2015 American Chemical Society
simulation to show that the tracer diffusion of rod−coils is slower than both rod and coil homopolymers of the same total molecular weight. In these systems, the surrounding entanglements were high molecular weight coil homopolymers, allowing their motion to be neglected. Molecular dynamics simulations using the Kremer−Grest model18 were quantitatively compared with experimental diffusion measurements by forced Rayleigh scattering on model copolymers composed of polyalanine αhelical rods and poly(ethylene oxide) (PEO) coils in entangled PEO coil homopolymer solutions.19 A reptation theory was then introduced which explained the slowed dynamics of rod− coils using the mismatch between the curvatures of the entanglement tubes of the rod and coil blocks. In the small rod limit where the rod is a perturbation on coil motion, the randomly varying curvature of the coil’s tube presents entropic barriers to the reptation of the rod, modifying the unhindered motion of the coil along its tube into an activated reptation process. In the large rod limit where the coil is a perturbation on rod motion, the long rod cannot rotate around the surrounding entanglements so motion is only possible when the coil moves into a straightened entanglement tube in an arm Received: September 21, 2014 Revised: April 5, 2015 Published: April 28, 2015 3121
DOI: 10.1021/ma501954k Macromolecules 2015, 48, 3121−3129
Article
Macromolecules
Figure 1. (a) Synthesis of poly(phenylenevinylene)-b-(isoprene-co-DTE). Vinyl-DTE is copolymerized with isoprene and then terminated with PPV. (b) After synthesis and purification of the rod−coil block copolymers, the DTE photolabel is converted to the blue form by UV light. The red laser in the FRS experiment converts it back to the colorless form. (c) The conversion between the two forms of DTE is a ring-closing and -opening reaction, triggered by UV and visible light.
retraction process. These theories, simulations, and experiments were originally developed for the tracer diffusion of symmetric coil−rod−coil triblock copolymers in high molecular weight coil homopolymers. Further studies then observed the same slowing phenomenon for the tracer diffusion of rod−coil diblock copolymers.20 While the previous work has developed and refined the dynamic relaxation mechanisms of entangled rod−coil block copolymers in the tracer diffusion configuration, most applications for rod−coils are in neat melts or concentrated solutions, where the surrounding entanglements are provided by other copolymers of the same size and composition. Further study is required to understand the new physics of entangled rod−coils for these practical applications where self-diffusion is important. In this work, self-diffusion is studied in rod−coil diblock copolymer melts composed of poly(alkoxyphenylenevinylene) (PPV) rods and cis-1,4-polyisoprene (PI) coils. The study focuses on the isotropic disordered state, avoiding structural inhomogeneities that affect diffusion in assembled systems.21 Unlike the previous studies of polyalanine-bpoly(ethylene oxide) copolymers, the current experimental system is fully organic with extended π-conjugation creating the rodlike conformation and semiconducting properties. This new chemistry expands the scope of diffusion studies to a wider class of rod−coil materials. Using forced Rayleigh scattering (FRS), diffusion measurements are performed on two systems. First, self-diffusion measurements of PPV-b-PI rod−coil diblock copolymers are performed in the melt state (neat polymer). Second, these melt experiments are then compared to
additional tracer diffusion measurements of dilute polyalanine-b-poly(ethylene oxide) diblock copolymers diffusing through a concentrated PEO homopolymer solution, where the molecular weight of the aqueous coil homopolymer solution matrix is varied. Diffusion results are then interpreted using reptation theories of constraint release, with the time scales of reptation relative to the surrounding chains determining the onset of slow diffusion.
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EXPERIMENTAL SECTION
Materials. Cyclohexane was dried over activated alumina and deoxygenated using an Innovative Technology (Newburyport, MA) Pure Solv system. Isoprene was dried over calcium hydride followed by dibutylmagnesium. Monodisperse poly(ethylene oxide)s for tracer diffusion experiments were obtained from Polymer Source (Montreal, QC, Canada). Other reagents and solvents were obtained from Sigma (St. Louis, MO) or VWR (Radnor, PA) and used as received. Synthesis of DTE-Functionalized Poly(alkoxyphenylenevinylene)-b-polyisoprene. As shown in Figure 1a, cis-1,4-polyisoprene was labeled with dye by living anionic copolymerization with a vinyl-functionalized dithienylethene dye (DTE). The synthesis is detailed in the Supporting Information. 160 μL of a 2.5 mg/mL solution of vinyl-DTE in anhydrous cyclohexane (0.4 mg) was added to a flame-dried Schlenk flask, and the solvent was removed under vacuum. 50 mL of anhydrous cyclohexane was added and degassed by three freeze−pump−thaw cycles, and 2 mL of purified isoprene was transferred in by cannula. The polymerization was initiated by secbutyllithium and proceeded at room temperature overnight. For block copolymers, poly(2,5-di(2′-ethylhexyloxy)-1,4-phenylenevinylene) (PPV) was synthesized as previously described (detailed in the Supporting Information).22 A sample of poly(isoprene-co-DTE) was 3122
DOI: 10.1021/ma501954k Macromolecules 2015, 48, 3121−3129
Article
Macromolecules Table 1. PPV-b-PI Block Copolymers rod Mn (kg/mol)
rod length L (nm)
rod PDI
coil Mn (kg/mol)
coil PDI
total Mn (kg/mol)
coil fraction
TODT (°C) by SAXS
TNI (°C) by birefringence
3.47 3.47 3.47 3.47 4.57 4.57 4.57 5.51
6.47 6.47 6.47 6.47 8.51 8.51 8.51 10.3
1.02 1.02 1.02 1.02 1.01 1.01 1.01 1.02
31.7 45.4 72.5 100.4 63.7 72.8 91.7 88.1
1.01 1.01 1.01 1.01 1.01 1.01 1.01 1.01
35.2 48.8 76 103.9 68.2 77.4 96.2 93.6
0.902 0.928 0.952 0.965 0.931 0.939 0.950 0.940
70
