Configurationally Constrained Crystallization of Brush Polymers with

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Configurationally Constrained Crystallization of Brush Polymers with Poly(ethylene oxide) Side Chains Huilou Sun,† Duk Man Yu,§ Shaowei Shi,*,† Qingqing Yuan,‡ So Fujinami,# Xiaoli Sun,† Dong Wang,‡ and Thomas P. Russell*,†,§,∥,⊥

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†

Beijing Advanced Innovation Center for Soft Matter Science and Engineering and ‡State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China § Polymer Science and Engineering Department, University of Massachusetts, Amherst, Massachusetts 01003, United States ∥ Materials Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States ⊥ Advanced Institute of Materials Research (AIMR), Tohoku University, 2-1-1 Katahira, Aoba, Sendai 980-8577, Japan # RIKEN SPring-8 Center, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan S Supporting Information *

ABSTRACT: The influence of physical confinement on the crystallization of poly(ethylene oxide) (PEO) has received much attention in past years. Here, rather than constraining the crystallization of the polymer by a physical or geometric boundaries, the influence of the constraints imposed by the chain architecture on the crystallization of the PEO was investigated, where the PEO chains were anchored to a poly(norbornene) (PNB) backbone. In this brush or comb-type polymer, the crystallizable polymer PEO are side chains comprising the bristles of the brush while the PNB comprises the spine. The brush or comb-type polymers were synthesized by the ring-opening metathesis polymerization (ROMP) of a NB-modified macromonomer. Here, the crystallizable PEO is anchored to the PNB backbone, placing constraints on the PEO during crystallization and annealing. The crystalline morphologies, crystallization kinetics, melting behavior, and crystal structure of the resultant polymers were investigated by polarized optical microscopy (POM), different scanning calorimetry (DSC), and X-ray scattering. Constraining the PEO to the PNB backbone was found to significantly influence the mobility of PEO chains, the degree of crystallinity, the crystal thickness and the equilibrium melting point. Increasing the molecular weight of the PEO or annealing at higher temperature alleviates this constraint to some extent. In addition to crystallization, the influence of annealing on the morphology was also investigated.

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INTRODUCTION The crystallization of linear polymers has been the subject of a tremendous body of research for decades. Much debate has emerged over the growth of the crystalline lamella where chains could adjacently or randomly re-enter the crystalline lamellae that are typically tens of nanometers in thickness with the thickness being dependent on the crystallization temperature, Tc. There is still debate as to how long chain, flexible polymers, with a radii of gyration on the tens of nanometers size scale, are integrated into the crystalline lamellae. What is evident is that the morphology of a semicrystalline polymer can be described by crystalline lamellae, ∼10 nm in thickness, separated by a disordered amorphous phase, also ∼10 nm in thickness, where the trajectory of a single polymer chain is not confined to one lamella but, rather, the chain can pass from one lamella to another.1,2 Among the numerous synthetic polymers, poly(ethylene oxide) (PEO), a water-soluble, biocompatible, and commercially available polymer, which can be purchased or synthesized with a narrow molecular weight distribution, has attracted much attention.3−10 In recent © XXXX American Chemical Society

years, studies on the crystallization of PEO in blends and copolymers or in confined ultrathin layers have appeared,11−19 and the influence of physical confinement on the crystallization of PEO has received much attention, showing the continued interest in the morphological behavior of this rather unique polymer.20−24 Consider now a brush or comb-type polymer where the bristles of the comb can crystallize.25 This chain architecture, where the crystallizable chains are anchored to the polymer backbone, places constraints on the spatial distribution of the polymer chains and the manner in which the crystallizable chains are integrated into growing crystals. Unlike linear polymers, the high steric crowding of the side chains results in a more rigid and extended backbone that decreases chain entanglements and leads to significantly reduced viscosities that can increase the kinetics of phase transformations, like Received: October 22, 2018 Revised: January 1, 2019

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DOI: 10.1021/acs.macromol.8b02265 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules phase separation.26−28 Such transformations, though, do not require the precise ordering of chain segments into a crystalline lattice while being constrained by the physical attachment to the backbone chain. Three strategies, including grafting-through, grafting-onto, and grafting-from, have been developed to generate brush polymers and to characterize the properties in the bulk, in thin films, and in solution.29−47 The crystallization behavior of brush polymers where the crystallizable side chains are attached to a flexible main chain has been investigated in the past decades by various groups, and the generally accepted model shows that the main chain and a portion of side chains in the vicinity of the main chain comprise the disordered, amorphous phase and the side chains are incorporated into crystalline lamella separated by the amorphous domains.48−50 Consequently, the side chains are incorporated into adjacent lamellae. Such a morphological model is rather curious, since it places significant constraints on the spatial distribution of the backbone chain that must be restricted to the center of the amorphous phase, and the amorphous phase must be constrained due to the dense anchoring of the polymer to the main chain. The dense anchoring of the crystallizable polymer chains to the main chain, on the other hand, constrains not only the trajectory of one polymer chain in the crystalline lamella but also multiple chains in adjacent lamellae, since the crystallizable chains are attached to the main chain polymer. Therefore, the growth of one lamella must be coupled with the growth of an adjacent lamella, since polymer chains that crystallize in adjacent lamella are bound to the main chain that, in turn, is subject to its own configurational constraints. A systematic study of the influence of these constraints on the crystallization kinetics and ultimate morphology is limited. In this study, using norbornene (NB)-modified macromonomers (termed NB-PEO) and ring-opening metathesis polymerization (ROMP), we synthesized brush polymers composed of dense PEO side chains (termed brush-PEO) anchored to a poly(norbornene) (PNB) backbone (Scheme 1). The effects of NB end group and the PNB backbone on the

were purchased from Sigma-Aldrich and used without further purification. All anhydrous solvents were bought from Sigma-Aldrich. Synthesis of Norbornene-Terminated PEO (NB-PEO) Macromonomer. Homo-PEO (Mn ≈ 3K, 1.34 g, 0.67 mmol or Mn ≈ 6K, 2.68 g, 0.67 mmol), exo-5-norbornenecarboxylic acid (0.18 g, 1.34 mmol), and DCC (0.33 g, 1.6 mmol) were added into a 50 mL Schlenk flask, followed by the addition of 20 mL of anhydrous dichloromethane (DCM). The resulting solution was put into ice bath for 30 min before adding DMAP (8 mg, 0.067 mmol). The reaction mixture was stirred at room temperature under nitrogen for ∼24 h and then filtered to remove precipitates, and the filtrate was poured into cool diethyl ether to yield a white solid product. Three more precipitations in diethyl ether were performed to remove excess exo-5norbornenecarboxylic acid (Mn = 2730 g mol−1, Mw = 3030 g mol−1, PDI = 1.10 or Mn = 6180 g mol−1, Mw = 6950 g mol−1, PDI = 1.12). 1 H NMR (400 MHz, CDCl3): δ (ppm): 6.15 (s, 2H), 4.26 (s, 2H), 3.89 (s, 2H), 3.66 (t, 4H), 3.39 (s, 2H), 3.06 (s, 2H), 2.93 (s, 2H), 1.94 (s, 2H), 1.55 (d, 4H). Synthesis of Brush-PEO. In a typical experiment, 0.025 mmol of the macromonomer (Mn = 2730 g mol−1, 68.33 mg or Mn = 6180 g mol−1, 154.58 mg) and 2.94 μmol of catalyst (2.6 mg) were added to separate vials. The vials were transferred to a glovebox, and the NBPEO was dissolved in 2 mL of anhydrous DCM while the catalyst was dissolved in 1.00 mL of anhydrous DCM. The catalyst solution (85 μL, 0.25 μmol) was injected into the solution of the NB-PEO to induce polymerization. The mixture was stirred for 1−2 h, then quenched with butyl vinyl ether, and isolated by precipitation into anhydrous methanol three times to yield the brush polymer with ∼3K or ∼6K side chains (termed SMn ≈ 3K or SMn ≈ 6K) as a white solid (Mn = 34600 g mol−1, Mw = 47200 g mol−1, PDI = 1.36, degree of polymerization (DPGPC) = 13 or Mn = 45800 g mol−1, Mw = 60800 g mol−1, PDI = 1.32, DPGPC = 7). 1H NMR (400 MHz, CDCl3): δ (ppm): 5.22 (s, 2H), 4.26 (s, 2H), 3.87 (s, 2H), 3.67 (t, 4H), 3.4 (s, 2H), 3.39 (s, 2H), 3.06 (s, 2H). Size Exclusin Chromatography (SEC) and Proton Nuclear Magnetic Resonance (1H NMR). The apparent number-average molecular weights (Mn) and dispersities (Mw/Mn) were measured by SEC, which was conducted with THF as the eluent. In addition, size exclusion chromatography−multiangle light scattering (SEC-MALS) was employed as well. Polymerization was monitored by 1H NMR spectroscopy using a Bruker Advance 400 MHz NMR spectrmeter with CDCl3 as a solvent. Monomer conversion was calculated from the decrease of the monomer peak area relative to the peak areas of the internal standards. Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectra were obtained on a Shimadzu KOMPACT MALDI II using CHCA as a matrix. Differential Scanning Calorimetry (DSC). The melting temperature Tm and degree of crystallinity, Xc, of homo-PEO, NB-PEO, and brush-PEO were determined by DSC (SETARAM Setsys Evolution). The temperature was increased from 25 to 100 °C at a heating rate of 30 °C min−1, held at 100 °C for 10 min to remove thermal history, and then quenched to 36, 38, 40, and 42 °C and held at that temperature for 30 min to crystallize the polymer. Subsequently, the sample was heated to 100 °C at 5 °C min−1 to determine the thermal characteristics of the polymer. Morphological Observations. The crystallization growth rate was measured by polarized optical microscopy (POM). Films were prepared by melting the sample between a glass slide and a coverslip using a hot stage (Linkam). POM studies were performed using a ZEISS Imager.A2 microscope, making use of λ wave plate to determine the sign of the spherulites. The crystallization of the polymers was observed in situ using POM. The melt-crystallization process of the sample was the same as that of the DSC measurements. Small- and Wide-Angle X-ray Scattering. For in situ smallangle X-ray scattering (SAXS) measurements, polymers were heated from room temperature to 80 °C, and the temperature was kept constant for 30 min to eliminate any thermal history. Samples were then cooled to 40 °C and held at that temperature for 60 min. After that, SAXS was measured for the samples crystallized at different annealing temperatures, TA (45, 50, and 55 °C). In the wide-angle X-

Scheme 1. Chemical Structures of Homo-PEO, NB-PEO, and Brush-PEO

crystallization of PEO were investigated by differential scanning calorimetry (DSC), polarized optical microscopy (POM), and X-ray scattering. The crystallization behavior of the NB-free PEO (termed homo-PEO) is investigated as a control experiment.

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EXPERIMENTAL SECTION

Materials. Homo-PEO (Mn = 2680 g mol−1 or Mn = 5660 g mol−1) purchased from Tokyo Chemical Industry was heated to 120 °C for 4 h under a nitrogen flow to remove water. (H2IMes)(3-Brpy)2-(Cl)2RuCHPh was prepared according to published protocols.51 Second generation Grubbs catalyst, 3-bromopyridine, pentane, exo-5-norbornenecarboxylic acid, N,N′-dicyclohexylcarbodiimide (DCC), 4-(dimethylamino)pyridine (DMAP), and butyl vinyl ether B

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Figure 1. Polarized optical micrographs of homo-PEO (a, Mn ≈ 3K; d, Mn ≈ 6K), NB-PEO (b, Mn ≈ 3K; e, Mn ≈ 6K) and brush-PEO (c, SMn ≈ 3K; f, SMn ≈ 6K) crystallized at 40 °C for 100 s. Scale bar = 100 μm. ray scattering (WAXS) experiments, samples were heated from room temperature to 80 °C, and the temperature was kept constant for 30 min. The samples were then cooled to 40 °C and held at that temperature for 60 min prior to quenching to room temperature. Measurements were performed using a Ganesha SAXS-LAB with Cu Kα radiation. For SAXS, the wavelength of incident X-ray was 0.1542 nm with an incident beam diameter of 0.3 mm under vacuum. SAXS patterns were recorded as a function of the scattering vector (q = (4π/λ) sin θ, where λ is the wavelength and 2θ is the scattering angle.) from 0.05 to 3.0 nm−1 using a 2-dimensional detector (Pilatus 300K).

Figure 2. Dependence of the spherulite growth rate G on the crystallization temperature for homo-PEO (black square), NB-PEO (red circle), and brush-PEO (blue triangle) with different molecular weights: (a) Mn ≈ 3K, SMn ≈ 3K; (b) Mn ≈ 6K, SMn ≈ 6K.

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RESULTS AND DISCUSSION Crystalline Morphologies and Growth Kinetics. The crystalline morphologies of homo-PEO, NB-PEO, and brushPEO at 40 °C were investigated by POM. As shown in Figure 1 and Figure S4, classic Maltese cross-patterns, typical for spherulitic morphologies, are seen for the three different types of polymers. In comparison to NB-PEO and brush-PEO, homo-PEO shows the largest spherulites, characteristic of a lowest nucleation density, indicating the low defect density of homo-PEO. With NB tethered to the end of the PEO chain, the number of crystal nuclei increases with a corresponding decrease in the size of the spherulites, suggesting that NB acts as a defect, lowering the nucleation barrier. This effect is more obvious for brush-PEO, where the crystallizable PEO chains are densely anchored to PNB backbone chain that cannot undergo crystallization, and only small crystals form. The anchoring of the PNB backbone weakens the mobility of the PEO chain, resulting in an increase in the defect density of the PEO crystal. When the molecular weight of the PEO as increased from 3K to 6K, the spherulite size increased, indicating a reduction in the number of nuclei or a reduction in the number density of defects. The radial growth rate of the spherulites as a function of crystallization temperature Tc, from 36 to 42 °C, was measured. In all cases, the spherulite radius r increased linearly with time, the radial growth rate was constant up to the point where the spherulites impinged on one another (Figure S5; the data were collected prior to infringement). The crystallization rates of homo-PEO, NB-PEO, and brush-PEO are compared in Figure 2. Plots of the radial growth rates G as a function of Tc

show the expected decrease in G with increasing Tc. For all crystallization temperatures, Ghomo‑PEO > GNB‑PEO > Gbrush‑PEO, reflecting the increasing constraints placed on the chains due to the NB end group or the physical anchoring of the chain to the PNB backbone. Crystallization Kinetics and Melting Behavior. Based on the isothermal crystallization curves at different temperatures (Figure S6), as measured by DSC, the degree of PEO crystallization, X(t), as a function of time t shows the classical sigmoidal shape (Figure 3). These data can be fit to a modified Avrami equation: ln[− ln(1 − X(t))] = n ln t + ln K, where n is the Avrami exponent characteristic of the nucleation mechanism and geometry of crystal growth and K is the rate constant. In all cases, a linear relationship was found for ln[− ln(1 − X(t))] as a function of ln t at the early stage of crystallization (Figure 3 and Figure S7). Deviations from linearity were observed for longer crystallization times, which can be ascribed to secondary nucleation processes, changes in the nucleation mechanism, or a change in the geometry of crystal growth. The kinetics parameters for homo-PEO, NBPEO, and brush-PEO isothermal crystallization are summarized in Table S1. The Avrami exponent n is in a range of 1.5− 2.6, indicating that the nucleation mode tends to a heterogeneous nucleation.14,16 Also, the crystallization halftime t1/2 of homo-PEO, NB-PEO, and brush-PEO was determined from the time when X(t) = 0.5. As shown in Figure 4, in comparison to homo-PEO and NB-PEO, the C

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Figure 3. Isothermal crystallization X(t)−t curves and Avrami curves of (a, d) homo-PEO, Mn ≈ 3K, (b, e) NB-PEO, Mn ≈ 3K, and (c, f) brushPEO, SMn ≈ 3K under different crystallization temperatures: 36 °C (black curve), 38 °C (red curve), 40 °C (blue curve), and 42 °C (purple curve).

which is in agreement with the value of 64 °C determined by Alfonso and Russell. 52 This also indicates, from the Thompson−Gibbs equation, that the crystal thickness for homo-PEO is the largest, while brush-PEO shows the smallest thickness. The DSC cooling curves are shown in Figure S9, and the PEO supercooling increases by NB end group or by anchoring to the PNB backbone (Figure S10), indicating that the crystallization driving force leads to an increase in the nucleation and growth rates, and the crystal size tends to become smaller. This is agrees with the results obtained for POM where at high supercooling the crystallization is diffusion-controlled.16,54,55 Crystalline Structure. Wide-angle X-ray diffraction profiles for homo-PEO, NB-PEO, and brush-PEO are shown in Figure 6. All samples were crystallized at 40 °C for 1 h and

Figure 4. Half-crystallization time of homo-PEO (black squares), NBPEO (red circles), and brush-PEO (blue triangles) at different crystallization temperatures: (a) Mn ≈ 3K, SMn ≈ 3K; (b) Mn ≈ 6K, SMn ≈ 6K.

brush-PEO shows the slowest crystallization rate and longest t1/2, which is in a good agreement with the results from the POM measurements. Figure S8 shows the DSC melting curves of the crystals obtained by isothermal crystallization at 36, 38, 40, and 42 °C (heating rate 5 °C min−1). The melting temperature Tm increases with increasing Tc, indicating that the lamella thickness increases with increasing crystallization temperature. The equilibrium melting points T0m of homo-PEO, NB-PEO, and brush-PEO can be estimated using the Hoffman−Weeks method, as shown in Figure 5. In all cases T0m of the homoPEO is the highest, followed by NB-PEO, and then T0m of brush-PEO is the lowest. For homo-PEO, T0m = 62.66 °C,

Figure 6. WAXS profiles of homo-PEO (black lines), NB-PEO (red lines), and brush-PEO (blue lines) with different molecular weights: (a) Mn ≈ 3K, SMn ≈ 3K; (b) Mn ≈ 6K, SMn ≈ 6K.

Figure 5. Linear plots of Hoffman−Weeks of homo-PEO (black squares), NB-PEO (red circles), and brush-PEO (blue triangles) by DSC. (a) Mn ≈ 3K, SMn ≈ 3K; (b) Mn ≈ 6K, SMn ≈ 6K.

then observed at room temperature. For homo-PEO sharp reflections are seen at 18.5°, 22.5°, 25.4°, and 26.1°, corresponding to the (120), (032), (024), and (131) reflections and spacings of 0.48, 0.39, 0.35, and 0.34 nm, characteristic of the monoclinic unit cell of PEO with a = 0.805 nm, b = 1.304 nm, c = 1.948 nm, and β = 125.4°.53 For NBPEO or brush-PEO chains, the intensities of all the reflections decrease; the diffraction peaks become broader and move toward a high angle, indicating that the order within the crystals is less, the interplanar spacing becomes smaller, and the crystallinity decreases. Also, an estimate of the weight fraction of crystallinity, Xc, of the PEO can be determined from D

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(L0), crystal thickness Lc, and amorphous phase thickness La. It is clear that with NB or PNB backbone tethered to the end of the PEO chain the crystal shows a tendency toward thinning while the amorphous region becomes larger, which is due to the increase in the constraints on the PEO chains. The intensity, full width at half-maximum (FWHM), and L0 as a function of annealing temperature are summarized in Figure 8. With increasing annealing temperature, L0 of homoPEO (Mn ≈ 3K) increases slightly, indicating a thickening of the lamella structure. However, at a higher annealing temperature of 55 °C, the intensity decreases markedly and the FWHM increases. Similar tendencies are observed in the case of NB-PEO (Mn ≈ 3K). Here the annealing process is more like a selection process, where most small crystals melt away, leading to the decrease in the degree of crystallinity. Completely opposite behavior is seen for the brush-PEO (SMn ≈ 3K) where increasing the annealing temperature results in a significant improvement of the PEO crystal. The intensity increases, the FWHM decreases sharply, and the period increases. This is a clear manifestation of the anchoring to the PNB backbone that prevents significant long-range motion of the PEO, but a localized motion that allows the crystals to improve markedly with increasing annealing temperature. For homo-PEO (Mn ≈ 6K), as the annealing temperature increases, the intensity of q1 and q2 increase while the FWHM decreases, indicating that the crystals are becoming better defined during annealing. This behavior differs from that of lower molecular weight homo-PEO, which can be attributed to the stability of the thicker crystals at the elevated annealing temperatures and the ability for the longer chains to unfold and be incorporated into a thicker crystal. The L0 calculated from q1 is nearly twice that calculated from q2, showing that during annealing the PEO crystals thicken from a twice-folded structure to a once-folded crystal, given the magnitudes of the repeat period, the number of monomers in the PEO (Mn ≈ 6K) chain, and the 7/2-helical conformation of PEO in the crystal. With the NB end-functionalized PEO (Mn ≈ 6K), a slight thickening of L0 and improved order within the lamellae is observed, as evidenced by the slight shift in the SAXS peak position to smaller q, while the intensity of the reflection increased with a decrease in the FWHM. For brush-PEO (SMn

the area under the amorphous halo (Ia) and crystalline reflections (Ic), which is shown in Table 1, giving the clear result that Xc,homo‑PEO > Xc,NB‑PEO > Xc,brush‑PEO. Table 1. Microdomain Parameters Summary of Homo-PEO, NB-PEO, and Brush-PEO When Crystallizing at 40 °C polymer a

homo-PEO NB-PEOa brush-PEOa homo-PEOb NB-PEOb brush-PEOb

qmax (nm−1)

L0 (nm)

Xc

Lc (nm)

La (nm)

0.47 0.44 0.35 0.44 0.42 0.40

13.37 14.28 17.95 14.28 14.96 15.71

0.74 0.71 0.52 0.77 0.72 0.62

9.89 10.14 9.33 10.99 10.77 9.74

3.48 4.14 8.62 3.29 4.19 5.97

Mn ≈ 3K or SMn ≈ 3K. bMn ≈ 6K or SMn ≈ 6K.

a

Figure 7 shows the SAXS results of homo-PEO, NB-PEO, and brush-PEO at different annealing temperatures TA. Detailed measurement conditions can be found in the Experimental Section. It should be noted that homo-PEO shows the sharpest reflections with at least four orders evident over the scattering vector range investigated, indicative of a very well-defined stacking of the lamellae. The scattering peaks of NB-PEO are broader, with up to the third-order reflection being evident. Even though the stacking of the lamellae must be very good to see the third-order reflection, the breadths of the reflections clearly indicate that the terminal NB group has introduced a disorder into the lamellar stacking. Further perturbation of the lamella stacking and a broadening of the crystal/amorphous interface are evident in the SAXS data for the brush-PEO where only the primary and a second-order reflection are observed. Also, it should be noted that in comparison with homo-PEO with molecular weight of 3K, homo-PEO (Mn ≈ 6K) shows two scattering peaks with an increase in the annealing temperature, indicating that the PEO chains undergo a twice-folded to once-folded transition of the polymer stems in the crystal. The corresponding 2D SAXS profiles are shown in Figures S11 and S13. In combination with the degree of crystallinity, the average thicknesses of the crystalline and amorphous domains when crystallizing at 40 °C are calculated and given in Table 1, including the long period

Figure 7. Lorentz-corrected SAXS profiles of homo-PEO (a, Mn ≈ 3K; d, Mn ≈ 6K), NB-PEO (b, Mn ≈ 3K; e, Mn ≈ 6K), and brush-PEO (c, SMn ≈ 3K; f, SMn ≈ 6K) when the annealing time was 60 min at different annealing temperatures. E

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Figure 8. Intensity, FWHM, and L0 as a function of temperature when the annealing time was 60 min: (a) homo-PEO, Mn ≈ 3K; (b) NB-PEO, Mn ≈ 3K; (c) brush-PEO, SMn ≈ 3K.

Figure 9. Intensity, FWHM, and L0 as a function of temperature when the annealing time was 60 min: (a, b) homo-PEO, Mn ≈ 6K; (c) NB-PEO, Mn ≈ 6K; (d) brush-PEO, SMn ≈ 6K.

≈ 6K), a significant increase in the intensity of the reflection and sharp decrease of FWHM are observed, which is similar to the observations with the lower molecular weight brush-PEO. Taken together, the results on the brush polymers clearly indicate that the PNB backbone imposes a frustration on the PEO chains during the crystallization process, since there is a competition between the crystallization process itself and the reorganization of the PNB backbone to accommodate the integration of the PEO chains into the crystal. The kinetically trapped frustrated configuration of the polymer chain can be relieved during the annealing process where sufficient mobility is imparted to the chains to allow the relaxation. From the results shown above we can draw certain conclusions about the morphology. First, the homo-PEO behaves, as would be expected. The spherulites are negative, with the PEO chains in the crystals oriented tangentially or that the lamellae are oriented in a radial direction. The PEO

chain are unencumbered, the nucleation density is low, and the radial growth rate is rapid, leading to the large spherulites observed. The narrow diffraction reflections and the higher order reflections observed by SAXS indicate that the order within the crystals is high and that the lamellar stacking persists over large distances. The absence of any constraints allows the PEO crystals to thicken with annealing, as observed by others. NB-PEO, on the other hand, has the constraint of the NB endfunctionality that cannot be integrated into the PEO crystal. The nucleation density of the NB-PEO, perhaps arising from the presence of the NB, is much higher, giving rise to the smaller size of the spherulites. The NB functionality retards the growth of the crystals, giving rise to a slower growth rate, a less ordered stacking of the lamellae, and the reduced crystallinity. It is interesting to note that the end-functionalization also retards the thickening of the lamellae upon annealing. This is a clear indication that the thickening requires not just one but F

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both ends of the chains to have mobility within the crystalline lattice and that the thickening is a process that requires the concerted motion of multiple adjacently located chains. By anchoring the PEO chains to the PNB backbone the nucleation density is increased further, giving rise to the even smaller spherulites, but the sign of the spherulites does not change. By default, this means that the PNB backbone chains are oriented, on average, radially in the spherulites. The significantly reduced degree of crystallinity and the reduction in the internal ordering of the lamellae stack, as evidenced by the SAXS data, clearly reflect the constraints on integrating the PEO chains on one brush-PEO into the growing adjacent crystalline lamellae. Forcing the PNB backbone into the amorphous phase between two adjacent lamellae places further constraints on the PEO chains anchored to the PNB backbone chain, since more of the PEO chains will have an imposed disorder due to the chain architecture, as reflected in the reduced crystallinity. Yet, by annealing at elevated temperature, sufficient mobility is imparted to the PEO chains so that locally the crystalline order within the lamellae can be significantly improved, as evidenced by the diffraction, but a substantial thickening of the lamellae does not occur, which is consistent with the results on the NB-PEO results. This increase in the constraints on the PEO chains, corresponding to an increase in the disorder within the lamellae, is also reflected in the meting points observed where Tm,homo‑PEO > Tm,NB‑PEO > Tm,brush PEO.

Duk Man Yu: 0000-0003-0682-0870 Shaowei Shi: 0000-0002-9869-4340 So Fujinami: 0000-0001-5151-7008 Xiaoli Sun: 0000-0002-5477-0401 Dong Wang: 0000-0003-2326-0852 Thomas P. Russell: 0000-0001-6384-5826 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Beijing Natural Science Foundation (2194083) and the Air Force Office of Scientific Research under Contract 16RT1602. We also acknowledge the support of the Beijing Advanced Innovation Center for Soft Matter Science and Engineering at the Beijing University of Chemical Technology.

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CONCLUSIONS In summary, brush polymers with PEO side chains were successfully synthesized using NB-modified macromonomers with two different molecular weights and ROMP. The isothermal crystallization behavior, crystal structure, and packing of the crystalline lamellae for linear PEO (homoPEO), PEO end-functionalized with NB (NB-PEO), and brush or comb-type PEO anchored to a PNB backbone were systematically investigated. The introduction of an NB group to one chain end or anchoring the PEO chains to a PNB backbone constrains the ordering of the PEO chains, leading to an increased nucleation rate, decreased growth rates, decreased order, and decreased crystal thickness. Upon annealing, with homo-PEO the polymer chains can easily translate through the crystal, allowing the crystals to thicken with discrete changes in the number of folds. However, for NB-end-functionalized PEO and brush-PEO, upon annealing at temperatures above the crystallization temperature, crystal thickening is arrested where the large end group or the anchoring of the PEO to the PNB backbone prevents the translation of the chain through the crystal. However, a local enhancement in the ordering of the crystals occurs.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b02265. 1 H NMR spectra, SEC traces, POM images, DSC curves, Avrami curves, and SAXS patterns (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. G

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