Article pubs.acs.org/Macromolecules
Large, Reversible, and Coherent Domain Spacing Dilation Driven by Crystallization under Soft Lamellar Confinement Adam B. Burns and Richard A. Register* Department of Chemical and Biological Engineering, Princeton University, Princeton, New Jersey 08544, United States S Supporting Information *
ABSTRACT: Crystallization within lamellar block copolymer microdomains, at temperatures above the glass transition of the amorphous blocka form of “soft”, one-dimensional confinementis investigated in a series of hydrogenated diblocks of norbornene and hexafluoroisopropanol-substituted norbornene, abbreviated hPN-hPHFAN. Crystallization results in a large (up to 23%) increase in the lamellar period (d), but in contrast to a multitude of previous reportswhere large changes in d signify breakout of the crystallites from the microdomains, such that the initial and final domain structures and orientations bear no coherent relationship to each otherthe hPN crystallites in hPN-hPHFAN remain confined within the microphase-separated lamellae, as revealed by small-angle X-ray scattering on highly oriented flow-aligned specimens. As the d-spacing dilates, the lamellae contract affinely in the lateral directions (as measured by confocal optical microscopy on a millimeter-scale flow-aligned specimen); dimensional changes in the macroscopic specimen (millimeter scale) precisely track the changes in d (≈50 nm scale). The increase in d, and concomitant changes in the macroscopic specimen dimensions, are fully reversible on melting, over multiple melting−recrystallization cycles. The crystallites are found to be oriented with the b-axis parallel to the microdomain interfaces and the crystal stems tilted with respect to the lamellar normal. In this orientation, the crystal thickness is limited by the thickness of the hPN domains, such that the domain spacing increase is driven by the enhanced thermodynamic stability of thicker hPN crystals.
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INTRODUCTION Block copolymers can self-assemble into a diversity of nanoscale structures, tunable through the chemistries and lengths of the blocks; even when all the blocks are individually amorphous, these nanostructures imbue block copolymers with a range of attractive and useful properties.1 Choosing one or more blocks to be crystallizable further expands the accessible range of block copolymer structures and properties; conversely, attaching an amorphous block to a crystallizable chain can strongly modify its crystallization habit, even in the absence of flow or other external stimuli.2−5 In flexible-chain C−A diblock copolymers, comprising crystallizable C blocks and amorphous A blocks, microphase separation in the melt (above the C block melting temperature, Tm, and the A block glass transition temperature, Tg) gives rise to the aforementioned highly regular nanoscale structures, with the type (e.g., spheres, cylinders, lamellae) of the structure determined primarily by the block volume fractions.1,6 Following crystallization of the C blocks, the solid-state morphology is set by the competition between crystallization and block incompatibility.2−5 If the melt structure is not sufficiently robust, crystallization overwhelms the well-ordered microdomain structure, in favor of alternating crystalline− amorphous lamellae, whose orientation is decoupled from that of the melt microdomains, irrespective of the block fractions (termed “breakout”).2−5,7−12 Crystallization can be confined to © XXXX American Chemical Society
the C-rich microdomains by one of two strategies. First, the crystals can be mechanically confined by choosing A blocks which are glassy at the crystallization conditions; i.e., the glass transition temperature of the A domains lies above the crystallization temperature of the C blocks, Tg > Tccalled hard confinement.11,13−22 The alternative is to choose block chemistries and lengths which yield strong segregation (quantified by the product of the segmental interaction parameter, χ, and the degree of polymerization, N). This latter strategy relies on a large thermodynamic barrier to mixing to prevent the crystallites from propagating through the A domains, enabling soft confinement (Tg < Tc).3−5,9,20−24 In analogy with the hard confinement case, “soft” confined crystallization has been assumed to proceed with no change to the overall domain structure, and the experimental indication of confinement is typically taken to be constancy of the primary small-angle X-ray scattering peak positions between the semicrystalline and melt states.3−5,9,20−23 The present work reports on a series of strongly segregated lamellar crystalline−rubbery diblocks, with hydrogenated polynorbornene25 (hPN, Tc ≈ 115 °C) crystallizable blocks and hydrogenated hexafluoroisopropanol-substituted polynorReceived: July 30, 2017 Revised: September 30, 2017
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DOI: 10.1021/acs.macromol.7b01632 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules Scheme 1. hPN-hPHFAN Diblock Copolymer Synthesisa
a
RuLn denotes the living end.
bornene26 (hPHFAN, with Tg ≈ 50 °C) amorphous blocks. These materials exhibit large, reversible increases in the lamellar period upon crystallization. Though this is commonly interpreted as a signature of breakout crystallization, thorough characterization of the domain structure of flow-aligned hPNhPHFAN specimens reveals that the hPN blocks crystallize under soft confinement within the lamellar morphology established in the melt. The increase in the lamellar period accommodates thicker crystals and is accompanied by contraction of the lamellae in the orthogonal directions. The amorphous block provides a large χ against hPN while remaining rubbery to accommodate the necessary changes in chain conformation upon crystallization and melting. This study demonstrates that crystallization within block copolymer microdomains can reversibly alter the domain periodicity without destroying the structure established in the melt, contrary to the conventional paradigm of confined crystallization.
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calculated using the weight-fraction-weighted dn/dc.27 The elution times for the refractive index trace were calibrated using narrowdistribution polystyrene standards to yield the dispersity (Đ). A representative 1H NMR spectrum and GPC traces are shown in Figures S1 and S2, respectively. Catalytic saturation was performed in an 80/20 v/v mixture of cyclohexane and tetrahydrofuran (polymer concentration 5 g/L) using 5 wt % Pd supported on CaCO3 (2/1 w/w to polymer) at 120 °C under 400 psig of H2. An hPHFAN homopolymer (hPHFAN-47; MLS = 47.1 kg/mol and Đ = 1.16) was prepared in the same manner as the block copolymers. The density of hPHFAN-47 was measured to be ρ = 1.3404 ± 0.0007 g/cm3 (quoted range corresponds to ±1 standard deviation of five specimens) at 23 °C using a density gradient column. A linear gradient was established using aqueous Ca(NO3)2 solutions of different concentrations and was calibrated with glass floats. The thermal expansion coefficients (α) of glassy and rubbery hPHFAN-47 were measured using spectroscopic ellipsometry (J.A. Woollam M-2000) on a film of hPHFAN-47, ≈260 nm thick, supported on a silicon wafer bearing the native oxide layer. The film was prepared by spin-coating from THF and was annealed above Tg overnight prior to the measurement. The thermal expansion coefficients were measured on heating at 2 °C/min (Figure S4) and corrected for mismatch with the substrate;28 in the glassy regime, αg = 2.34 × 10−4 K−1, and in the rubbery regime, αr = 5.98 × 10−4 K−1. Specific volume expressions calculated from the measured density and thermal expansion coefficients can be found in the Supporting Information. Block volume fractions (ϕ) were calculated from the compositions measured by 1H NMR spectroscopy using these expressions and bulk pressure− volume−temperature data for hPN.29 Differential scanning calorimetry (DSC) traces were acquired using a PerkinElmer DSC7 calibrated with mercury and indium; a scan rate of 10 °C/min was used throughout. Oriented specimens were prepared using a lubricated channel die.30,31 Samples were melt-pressed at 160 °C into ≈1 mm thick films. Rectangular sections were cut from the compression-molded films to match the width of the channel die. Approximately 20 films were stacked in the center of a 5 × 115 mm (12 × 203 mm in the case of hPN-hPHFAN-79) aluminum channel die lubricated with 60 000 cSt silicone oil (trimethylsiloxy-terminated, Gelest, Inc.). The die was then loaded into the melt press preheated to 160 °C and allowed to equilibrate for 15 min. Pressure was then applied until the desired thickness of ≈1 mm was achieved and held for an additional 15 min. The pressure was released, and the die was allowed to cool to room temperature. Excess silicone oil was removed by gently washing with soap and water and drying with a paper towel. All X-ray measurements employed Cu Kα radiation (λx = 0.154 18 nm) from PANalytical PW3830 X-ray generators. Two-dimensional (2D) small-angle X-ray scattering (SAXS) measurements were performed using a point-focusing system (Molecular Metrology); collimation was achieved by two bent Si (111) crystals in the “DuMond geometry”.32 The sample-to-detector distance was 1.5 m, and an argon-filled Gabriel-type multiwire detector was used.33 The sample chamber and beam path were evacuated to minimize scattering and absorption by air. Raw data were corrected for detector sensitivity, empty beam scattering, and sample thickness and transmittance (measured by a photodiode positioned on the beam stop). SAXS patterns are plotted versus the magnitude of the momentum transfer vector, q = (4π/λx) sin θ; the scattering angle (2θ) was calibrated with silver behenate.34 2D wide-angle X-ray scattering patterns (WAXS) were acquired in an evacuated Statton pinhole camera (W.H. Warhus), equipped with a graphite monochromator (Huber), using Fuji BAS-IP
EXPERIMENTAL METHODS
Norbornene (Sigma-Aldrich, 99%) was purified by stirring over sodium and degassing by freeze−pump−thaw cycles and vacuum transferred prior to use. The monomer HFAN (2-(bicyclo[2.2.1]hept5-en-2-ylmethyl)-1,1,1,3,3,3-hexafluoropropan-2-ol, >99.5%, Central Glass Co., Ltd. (Japan)) was degassed and stirred over 3 Å molecular sieves. Toluene was purified via an MBraun solvent purification system. Living ring-opening metathesis polymerization (ROMP) was carried out at room temperature in a nitrogen-filled MBraun UNIlab glovebox (
