Effects of Bulky End-Groups on the Crystallization ... - ACS Publications

Sep 15, 2017 - Research Institute for Photofunctionalized Materials, Kanagawa University, 2941. Tsuchiya, Hiratsuka, Kanagawa 259-1293, Japan...
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Effects of Bulky End-Groups on the Crystallization Kinetics of Poly(εcaprolactone) Homopolymers Confined in a Cylindrical Nanodomain Koshun Kawazu, Shintaro Nakagawa, Takashi Ishizone, and Shuichi Nojima* Department of Chemical Science and Engineering, Tokyo Institute of Technology, H-125, 2-12-1 Ookayama Meguro-Ku, Tokyo 152-8552, Japan

Daiki Arai,† Kazuo Yamaguchi,†,‡ and Seiichi Nakahama‡ †

Department of Chemistry, Faculty of Science, and ‡Research Institute for Photofunctionalized Materials, Kanagawa University, 2941 Tsuchiya, Hiratsuka, Kanagawa 259-1293, Japan S Supporting Information *

ABSTRACT: We examined the isothermal crystallization kinetics of poly(ε-caprolactone) (PCL) homopolymers confined in a cylindrical nanodomain (nanocylinder) with 12.9 nm in diameter. The confined PCL homopolymers were prepared using the microphase separation of PCL-block-polystyrene (PCL-b-PS) diblock copolymers with a photocleavable o-nitrobenzyl group (ONB) at block junctions and the subsequent cleavage of ONB with UV light. Several PCL homopolymers with different end-groups, acetyl group (molecular weight MEG = 43 g/ mol, original PCL homopolymer), adamantane (MEG = 163), cholesterol (MEG = 386), methyl 2,3,6-tri-o-benzoyl-α-D-galactopyranoside (MEG = 506), and vitrified PS (MEG = ∞, i.e., PCL blocks in PCL-b-PS), were prepared to gradually change their chain mobility in the nanocylinder. The crystallization rate of corresponding bulk PCL homopolymers (i.e., no spatial confinement imposed) decreased steadily with increasing MEG, suggesting that these end-groups had no extra effect on the crystallization but simply reduced the chain mobility of PCL homopolymers. The time evolution of crystallinity for all the confined PCL chains showed first-order kinetics, indicating the overall crystallization was driven by homogeneous nucleation. The crystallization rate of confined PCL homopolymers with bulky end-groups (163 ≤ MEG ≤ 506) was significantly larger than that of original PCL homopolymers (MEG = 43) and PCL blocks (MEG = ∞). The crystallization kinetics was discussed by considering the effect of chain mobility on the nucleation mechanism of PCL homopolymers confined in the nanocylinder.

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INTRODUCTION The crystallization of polymer chains spatially confined in nanometer-sized domains (nanodomains) is interesting because of their unique crystallization features as compared with those of bulk homopolymers without any spatial confinement. Many experimental studies have been reported on this confined crystallization using various nanodomains provided by micelles or droplets,1−3 anodic aluminum oxides (AAOs),1,3−7 thin films or surfaces,3,8,9 or microdomain structures of block copolymers.5,10−14 Furthermore, the confined crystallization in polymer materials has many practical applications such as thermoplastic elastomers,15 gas-barrier materials,16 or templates for nanolithography.17 Block copolymers form various types of nanodomains in the melt, such as spheres (nanospheres), cylinders (nanocylinders), or lamellae (nanolamellae), according to their composition and segregation strength. Therefore, it is possible to examine the crystallization kinetics and crystal orientation of confined blocks as a function of nanodomain shape, nanodomain size D, or crystallization temperature Tc. Many experimental studies using block copolymers have revealed the characteristics of confined © XXXX American Chemical Society

crystallization. For example, the crystallization behavior of block chains confined in nanospheres or nanocylinders showed first-order kinetics,18−20 indicating the overall crystallization was driven by homogeneous nucleation. Furthermore, the crystal orientation of block chains confined in nanocylinders or nanolamellae was extensively studied as a function of D21−23 or Tc.24−26 In the confined crystallization in various nanodomains (space confinement) provided by the microphase separation of diblock copolymers, one chain-end of crystalline blocks is always tethered at nanodomain interfaces (chain confinement), which will significantly affect the crystallization. In order to examine the effect of chain confinement on the crystallization kinetics and crystal orientation, we prepared poly(ε-caprolactone) (PCL) homopolymers and PCL blocks both confined in an identical nanocylinder27−30 or nanolamella31−33 using PCLblock-polystyrene (PCL-b-PS) with a photocleavable o-nitroReceived: July 21, 2017 Revised: September 3, 2017

A

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Macromolecules benzyl group at block junctions. It was found the chain confinement significantly affected the crystallization of (spatially) confined PCL chains, which also depended considerably on D. For example, PCL homopolymers crystallized extremely faster than PCL blocks in the nanocylinder with D = 13.0 nm (D represents the nanocylinder diameter hereafter) (Figure S-1a), whereas the crystallization rate of PCL blocks was slightly larger than that of PCL homopolymers when D = 17.9 nm (Figure S-1b).29 The crystal orientation of PCL homopolymers and PCL blocks both confined in an identical nanocylinder also depended significantly on D. These characteristic crystallization kinetics and crystal orientation should be ascribed to the difference in chain mobility between confined PCL blocks and PCL homopolymers. It is intuitively supposed that the chain mobility of confined homopolymers is much larger than that of confined blocks due to chain confinement. Therefore, two extremes of chain mobility were examined in our previous studies on the confined crystallization in nanocylinders (Figure 1a,d). In order to

Table 1. Molecular Characterization of Samples

sample PCL-b-PS PCL homopolymer (PCLh) PS oligomerd (PSO)

Mn of PS blocksb (g/mol)

8500 8500

total Mn (g/mol)

Mw/ Mnb

PCL:PSc vol % at l00 °C

28300

36800 8500

1.06 1.05

23:77 100:0

2440

2440

1.01

0:100

a

Determined using 1H NMR. bDetermined using GPC with PS standards. cCalculated from Mn and specific volumes of PCL and PS homopolymers. dPurchased from Scientific Polymer Products, Inc., USA. The chain-end of PCL blocks (in original PCL-b-PS) was replaced with various bulky groups to reduce the chain mobility of confined PCL homopolymers. The end-groups employed are shown in Table 2. The original PCL block actually had an acetyl group with the molecular weight MEG = 43 g/mol (denoted 0-PCL). This acetyl group was replaced with adamantane (MEG = 163, A-PCL), cholesterol (MEG = 386, C-PCL), and methyl 2,3,6-tri-o-benzoyl-α-D-galactopyranoside (MEG = 506, M-PCL), where the 100% introduction of each end-group was confirmed using 1H NMR. These end-groups were carefully chosen by considering specific properties such as the formation of hydrogen bonds or liquid crystals because these properties might accelerate the crystallization. Furthermore, some end-groups which showed the accelerating crystallization of bulk PCL homopolymers were excluded. The PCL block in PCL-b-PS (MEG = ∞, PS-PCL) was also added as a fifth PCL homopolymer with the smallest chain mobility (Table 2). Therefore, it is strongly expected that the chain mobility of PCL homopolymers decreases gradually with increasing MEG. The bulk PCL homopolymers corresponding to each PCL block were also prepared to experimentally confirm that these end-groups did not have any extra effect (for example, possible nucleation effect) on their isothermal crystallization. Sample Preparation. The PCL-b-PS copolymers with various end-groups were completely dissolved in toluene with a concentration of ca. 20 mg/mL, and the solution was dropped on a fluorinecontaining polymer film. The solution was first kept at 50 °C for 24 h to remove toluene and then 130 °C for 3 h to form a regular microdomain structure. The resulting sample film had a diameter of 5 mm and a thickness of 0.05 mm. Subsequently, the block junction (ONB) was cleaved at room temperature by irradiating UV light (Optical Modulex, USH-500SC, USHIO Ltd., Japan) with 1.0 W/cm2 in intensity and 300 nm or longer in wavelength for more than 2 h. The conversion of PCL blocks into PCL homopolymers is schematically illustrated in Figure S-2, together with the chemical reaction of ONB. The photocleavage yield f h to produce PCL homopolymers increased steadily with increasing the irradiation time (Figure S-3), and final f h was ca. 0.87 for all the samples investigated. The introduction of bulky end-groups actually induced a slight change in the nanocylinder diameter D (Table 3), which would significantly affect the crystallization rate of confined PCL homopolymers.36 In order to correct the small difference in D, we prepared the original PCL homopolymer (MEG = 43 g/mol) confined in various D ranging from 12.6 to 13.9 nm to evaluate a relationship between the crystallization rate and D. A small amount of PCL homopolymers (PCLh) or PS oligomers (PSO) (Table 1) was added to change D, as schematically illustrated in Figure S-4. Finally, the crystallization rate of various PCL homopolymers (with bulky end-groups) confined in the nanocylinder with D = 12.9 nm was estimated using this relation assuming that the D dependence of crystallization rates was the same for all confined PCL homopolymers. Synchrotron Small-Angle X-ray Scattering (SR-SAXS) Measurements. The microdomain structure formed in PCL-b-PS copolymers was examined using SR-SAXS, where 15 sample films (ca., 0.75 mm in total thickness) were piled up to obtain sufficient SR-

Figure 1. Schematic illustration showing the crystalline block (a) and various crystalline homopolymers (b−d) spatially confined in an identical nanocylinder. The chain mobility of confined polymers is expected to increase gradually from (a) to (d).

understand a relationship between the chain mobility and crystallization kinetics, it is necessary to gradually change the chain mobility of confined polymers. In this study, we prepare several PCL homopolymers with bulky end-groups all confined in an identical nanocylinder, which are expected to gradually change the chain mobility (Figure 1b,c). The end-groups used are acetyl group (molecular weight MEG = 43 g/mol, original end-group), adamantane (MEG = 163), cholesterol (MEG = 386), methyl 2,3,6-tri-o-benzoyl-α-D-galactopyranoside (MEG = 506), and vitrified PS (MEG = ∞, i.e., PCL-b-PS). The crystallization kinetics of these PCL homopolymers is examined as a function of MEG (or the chain mobility of confined PCL homopolymers) to clarify the characteristics of confined crystallization.

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Mn of PCL blocksa (g/mol)

EXPERIMENTAL SECTION

Samples. The crystalline−amorphous diblock copolymer used in this study is poly(ε-caprolactone)-block-polystyrene (PCL-b-PS) with a photocleavable o-nitrobenzyl group (ONB) between PCL and PS blocks.34,35 The molecular characterization of PCL-b-PS is shown in Table 1. The volume % of PCL blocks is 23% at 100 °C, from which the cylindrical microdomain structure (nanocylinder) is anticipated in the melt. The PCL blocks confined in nanocylinders are subsequently converted into PCL homopolymers by cleaving ONB with UV light. The melting temperature of PCL blocks is lower than 50 °C (Figure 5a) and the glass transition temperature of PS blocks is about 100 °C, so the nanocylinder is completely preserved during melting/ crystallization experiments even after the photocleavage of ONB owing to the vitrification of PS matrices. B

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Macromolecules Table 2. Characterization of End-Groups Used in This Study

chosen by considering an experimentally accessible time scale by DSC measurements.

SAXS intensity. The synchrotron experiment was performed at Photon Factory in High Energy Accelerator Research Organization, Tsukuba, Japan, with a small-angle X-ray equipment installed at BL-10C station. Details of the equipment and instrumentation are already described in our previous studies.37−39 The two-dimensional SR-SAXS pattern was detected using PILATUS3-2M (Dectris Ltd., Switzerland) having 1475 × 1679 pixels with each pixel dimension of 172 × 172 μm2. The accumulation time was 30 s for each measurement. The twodimensional SR-SAXS patterns were azimuthally averaged to derive one-dimensional SR-SAXS curves as a function of wavenumber s (= (2/λ) sin θ, 2θ: scattering angle, λ: X-ray wavelength used (= 0.1488 nm)), where silver behenate was used as a calibration material.40 The background scattering and absorption by the sample were corrected for each SR-SAXS curve. The shape of microdomain structures was obtained from the relative positions of SR-SAXS peaks, and the nanocylinder diameter D was evaluated from the interdomain distance of microdomain structures and the volume fraction of PCL blocks existing in the system. Differential Scanning Calorimetry (DSC) Measurements. The melting temperature Tm and crystallinity XC were examined using DSC (Diamond DSC, PerkinElmer, USA) with a heating rate of 10 °C/min for confined PCL homopolymers isothermally crystallized at −42 °C for 160 min and bulk PCL homopolymers at 50 °C for 360 min. Tm was defined at the peak temperature of DSC endotherms, and XC was calculated from XC =

ΔHPCL ° wPCL ΔHPCL

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RESULTS AND DISCUSSION Crystallization of Bulk PCL Homopolymers. First, the crystallization of bulk PCL homopolymers (i.e., no space confinement imposed) with various end-groups was examined to find how the crystallization features changed with increasing MEG and also to confirm that these end-groups had no specific effect on the crystallization. If the bulky end-group has some extra effect such as an accelerating nucleation, it will affect the crystallization of confined PCL homopolymers to make the effect of space confinement unclear. Figures S-5a,c show the DSC curves during cooling at −10 °C/min from the melt and heating after isothermal crystallization at 50 °C for 360 min. The crystallization temperature during cooling is in a range between 30 and 40 °C, which is typical for the bulk PCL homopolymer and indicates the heterogeneous nucleation and subsequent crystal growth. Figure 2 shows the melting temperature Tm (a) and crystallinity XC (b) plotted against MEG for bulk PCL homopolymers isothermally crystallized at 50 °C, where Tm and XC decrease slightly with increasing MEG. The M n dependence of X C for (regular) bulk PCL homopolymers isothermally crystallized at Tc = 25−40 °C has been reported,42 where XC takes a maximum (XC = 0.60− 0.65) at Mn ∼ 6700 g/mol and decreases monotonically with increasing Mn. This decrease is ascribed to the decelerated chain mobility of PCL homopolymers with increasing Mn. Figure 2b is similar to this result, and the difference in XC between the PCL homopolymers with MEG = 43 and 506 g/ mol roughly corresponds to that between the regular PCL homopolymers with Mn = 6700 and 13 000 g/mol. It is confirmed from Figure 2 that the bulky end-group affects the crystalline morphology (such as the lamella thickness and crystallinity) through a substantial reduction of the chain

(1)

where ΔHPCL is the area under the melting endothermic peak, ΔH°PCL is the melting enthalpy of perfect PCL crystals (ΔHPCL ° = 135 J/g41), and wPCL is the weight fraction of PCL chains existing in the system. Furthermore, the isothermal crystallization behavior of bulk and confined PCL homopolymers was examined using DSC. That is the sample was first annealed at 57 °C (for confined PCL) or 65 °C (bulk PCL) to completely melt PCL crystals, quenched into −35 °C (confined PCL) or 50 °C (bulk PCL), isothermally crystallized there for prescribed crystallization times t, and finally heated at 10 °C/min to derive XC as a function of t. These crystallization temperatures were C

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τ1/2 plotted against MEG, where 1/τ1/2 decreases gradually with increasing MEG , suggesting that the bulky end-groups moderately suppress the nucleation and/or growth processes through the decelerated chain mobility. The Mn dependence of crystallization rates for (regular) bulk PCL homopolymers is also reported for a wide Mn range, where it takes a maximum at Mn = 6000−7000 g/mol and gradually decreases with increasing Mn.42,43 The steady decrease of 1/τ1/2 shown in Figure 3b is comparable to the result reported above. That is the introduction of bulky end-groups corresponds to a slight increase in Mn of (regular) bulk PCL homopolymers from the viewpoint of substantial decrease in their chain mobility. We confirm again from Figure 3b that the bulky end-groups employed in this study (Table 2) have no extra effect on the isothermal crystallization of PCL homopolymers. In summary, it is found from Figures 2 and 3 that every bulky end-group has no extra effect on the overall crystallization of bulk PCL homopolymers and provides a slower crystallization rate through the decelerated chain mobility. Therefore, it is possible to set the chain mobility of confined PCL homopolymers to the values between original PCL homopolymers (MEG = 43 g/mol) and PCL blocks (MEG = ∞). Microdomain Structures of PCL-b-PS with Bulky EndGroups. The microdomain structure formed in PCL-b-PS copolymers with various end-groups was examined using smallangle X-ray scattering with synchrotron radiation (SR-SAXS), and all SR-SAXS curves measured are summarized in Figure S-6 for amorphous and crystallized PCL-b-PS copolymers before (black curves, denoted R-PCL-PS, R = 0, A, C, M) and after (colored curves, R-PCL/PS) irradiating UV light. The electron density of PS chains (332 e/nm3 below 100 °C) is very close to that of amorphous PCL chains (354 e/nm3 at 25 °C), so only the primary SR-SAXS peak is observed for every amorphous sample (broken curves). The crystallized samples, on the other hand, show several SR-SAXS peaks due to an enhanced contrast of electron density between PCL and PS regions, where the PCL regions have a heterogeneous electron density distribution with PCL crystals (393 e/nm3) and amorphous PCL regions. It is found from Figure S-6 that the primary SRSAXS peak position (vertical broken lines) does not change before and after irradiating UV light for each sample, indicating that the microdomain structure is completely preserved after photocleavage, and eventually the PCL homopolymers are

Figure 2. Melting temperature Tm (a) and crystallinity XC (b) of bulk PCL homopolymers with various end-groups plotted against the molecular weight of end-groups MEG. The crystallization temperature Tc is 50 °C.

mobility of PCL homopolymers without any extra effect on the crystallization. The crystallization behavior was further examined for bulk PCL homopolymers with various end-groups. Figure 3a shows XC plotted against crystallization time t for these PCL homopolymers isothermally crystallized at Tc = 50 °C. The time evolution of XC shows a sigmoidal change, which is generally observed in the crystallization of bulk homopolymers and indicates that heterogeneous nucleation and subsequent crystal growth control the overall crystallization. The half-time of crystallization τ1/2, the time necessary to reach to the half of final crystallinity, can be directly evaluated from Figure 3a, and 1/τ1/2 is usually used as a measure of crystallization rates; when 1/τ1/2 is larger, the crystallization is faster. Figure 3b shows 1/

Figure 3. (a) Time evolution of crystallinity XC plotted against crystallization time t for various bulk PCL homopolymers isothermally crystallized at 50 °C: (○) 0-PCL, (△) A-PCL, (▽) C-PCL, (◇) M-PCL. (b) Inverse of half-time of crystallization 1/τ1/2 (proportional to the crystallization rate) for various bulk PCL homopolymers plotted against the molecular weight of end-groups MEG. D

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copolymer/homopolymer blends and triblock copolymer/ homopolymer blends44−46 that L decreases moderately when the molecular weight of added homopolymers is extremely low. This is because the homopolymer is homogeneously mixed with corresponding blocks to yield the relaxation of constant density constraints of confined blocks. Therefore, the small decrease in D observed in this study also might arise from a delicate balance between the volume increase due to the introduction of bulky end-groups and the conformational relaxation of block chains (or the extension of intervals between junction points on nanocylinder surfaces).47 However, it is found from our recent study36 that such a small difference in D yields a significant change in the crystallization rate of PCL blocks confined in the nanocylinder; when D increases from 15.6 to 18.6 nm (3.0 nm increase in D), 1/τ1/2 shows a 140-fold increase. Therefore, it is absolutely necessary to correct the minor change in D when we consider and discuss the effect of bulky end-groups on the crystallization rate of PCL homopolymers confined in nanocylinders. The correction method for different D values is precisely described in the next section. Crystallization of PCL Homopolymers Confined in Nanocylinders. Figures S5-b,d show the DSC curves during cooling at −10 °C/min from the melt and heating at 10 °C/ min after isothermal crystallization at −42 °C for 160 min for various confined PCL homopolymers. The cooling exothermic peak cannot be detected at 30−40 °C, suggesting the confined PCL homopolymers do not crystallize heterogeneously. Figure 5 shows Tm (a) and XC (b) plotted against MEG for confined PCL homopolymers isothermally crystallized at Tc = −42 °C, where the small difference in D (Table 3) is corrected using the D dependence of Tm and XC derived from the original PCL homopolymer (MEG = 43 g/mol) confined in nanocylinders with different D. Namely, Tm and XC at D = 12.9 nm were estimated for all the PCL homopolymers with bulky end-

successfully confined in the nanodomain provided by the microphase separation of block copolymers. The SR-SAXS curves from crystallized samples after irradiating UV light (RPCL/PS) are summarized in Figure 4. Every curve has several

Figure 4. SR-SAXS curves from various crystallized samples indicated on each curve after irradiating UV light. This figure is a summary of Figure S-6.

intensity peaks, the positions of which exactly correspond to a ratio of 1:√3:2:√7, indicating the cylindrical microdomain structure (nanocylinder) is formed, as expected from the vol % of PCL blocks (Table 1). The nanocylinder diameter D can be evaluated from the interdomain distance L evaluated from the primary peak position and the volume fraction of PCL chains φPCL existing in the system through the equation ⎛ 2φPCL ⎞1/2 D = 2L⎜ ⎟ ⎝ 3π ⎠

(2)

The values of D thus evaluated are summarized in Table 3, together with φPCL and L, where D is not constant but depends slightly on MEG, as shown in Figure S-7. That is D takes a minimum at around MEG = 163 g/mol and increases steadily with increasing MEG. It is reported using several binary diblock Table 3. Nanocylinder Diameter D Derived from SR-SAXS Results copolymer

φPCLa

Lb (nm)

Dc (nm)

0-PCL-b-PS A-PCL-b-PS C-PCL-b-PS M-PCL-b-PS

0.229 0.231 0.237 0.240

22.2 21.6 22.5 23.4

12.9 12.6 13.3 13.9

Figure 5. Melting temperature Tm (a) and crystallinity XC (b) plotted against the molecular weight of end-groups M EG for PCL homopolymers confined in the nanocylinder with D = 12.9 nm. Tm and XC are corrected for the small difference in D (see Figure S-8).

a

Volume fraction of PCL blocks. bInterdomain distance evaluated from SR-SAXS curves. cNanocylinder diameter. E

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Macromolecules groups (MEG ≥ 163 g/mol) assuming that the D dependence of Tm (or XC) shown in Figure S-8 holds. The Tm of original confined PCL homopolymers (∼46 °C) is moderately lower than that of corresponding bulk PCL homopolymers (∼62 °C) and decreases slightly with increasing MEG, as is the case with bulk PCL homopolymers (Figure 2). Furthermore, XC of every confined PCL homopolymer is extremely small (= 0.25−0.29) as compared with that of bulk PCL homopolymers (= 0.57− 0.62), suggesting that the crystallization in the nanocylinder with D = 12.9 nm imposes hard limitations on nucleation, crystal growth, and/or lamella thickening, which are usually observed in confined crystallization in various nanodomains.10−14 It should be noted that both Tm and XC of PCL blocks (MEG = ∞) are significantly smaller than those of all the confined PCL homopolymers, indicating that the reduction of chain mobility due to chain confinement is extremely large as compared with the introduction of bulky end-groups. The time evolution of XC for confined PCL homopolymers isothermally crystallized at −35 °C is shown in Figure 6, where

homopolymers are described below after correcting the effect of D. The inverse of half-time of crystallization 1/τ1/2 is semilogarithmically plotted against nanocylinder diameter D in Figure S-9 for confined PCL homopolymers with MEG = 34 g/ mol (0-PCL) (squares) and those with bulky end-groups (circles). The values of 0-PCL make a straight line against D, and it is possible to correct the effect of D on 1/τ1/2 for A-PCL, C-PCL, and M-PCL using this slope to estimate 1/τ1/2 at D = 12.9 nm. This correction for the difference in D might be overestimated because the slope shown in Figure S-9 (∼4.02 nm−1) is considerably larger than that observed for PCL blocks confined in nanocylinders with various D ranging from 15.6 to 18.6 nm (∼1.6 nm−1).36 Furthermore, it is easily found from Figure S-9 that the crystallization rate of all PCL homopolymers with bulky end-groups is moderately larger than that of 0-PCL at corresponding D values. Figure 7 shows 1/τ1/2 plotted against MEG after correcting the difference in D. Surprisingly, 1/τ1/2 of PCL homopolymers with bulky endgroups (i.e., A-PCL, C-PCL, and M-PCL) is significantly larger than that of 0-PCL (with largest chain mobility) and PS-PCL (with smallest chain mobility). That is, Figure 7 shows a sharp contrast with Figure 3b, where the crystallization rate steadily decreases with increasing MEG. It is an unforeseen result and indicates that the crystallization rate of PCL homopolymers confined in nanodomains depends complicatedly on their chain mobility. Therefore, it is necessary to consider the details of homogeneous nucleation for confined PCL homopolymers to clarify the relation between the crystallization rate and MEG (or chain mobility) shown in Figure 7. Crystallization Mechanism of PCL Homopolymers Confined in Nanocylinders. In this section, we discuss the crystallization mechanism of PCL homopolymers confined in nanocylinders on the basis of chain mobility decelerated by bulky groups introduced at PCL chain-ends. The homogeneous nucleation of binary PCL homopolymer/PCL block blends confined in nanocylinders has been already discussed as a function of PCL homopolymer fraction f homo and nanocylinder diameter D in our previous study,30 where the crystallization rate of such systems was successfully explained by the pioneering theory of polymer nucleation.48 Here, our experimental result (Figure 7) is qualitatively explained and discussed using this theory. It is confirmed from Figure 6 that the overall crystallization of confined PCL homopolymers is controlled by the nucleation process, so we discuss the nucleation rate I* (∝ 1/τ1/2) as a function of MEG and D. It is well-known that I* is described48 by

Figure 6. Time evolution of crystallinity XC plotted against crystallization time t for various confined PCL homopolymers isothermally crystallized at −35 °C: (○) 0-PCL, (△) A-PCL, (▽) C-PCL, (◇) M-PCL, (□) PS-PCL.

the crystallization behavior is substantially different from that of bulk PCL homopolymers (Figure 3a); XC increases abruptly at the beginning of crystallization with no detectable induction time, followed by an asymptotic increase at the late stage, indicating that homogeneous nucleation controls the overall crystallization of confined PCL homopolymers. That is the spontaneous nucleation (which does not start from impurities) is the main process for the crystallization because the crystal growth stops instantaneously due to an extremely narrow space. This mechanism is usually observed for the crystallization in isolated nanodomains such as nanospheres or nanocylinders.18−20 The significant difference in the crystallization mechanism between bulk and confined PCL homopolymers is also expected from a large difference in the crystallization temperature (50 °C for bulk PCL and −35 °C for confined PCL). Furthermore, it is found from Figure 6 that the order of crystallization rates of confined PCL homopolymers is not the same as that of bulk PCL homopolymers (Figure 3a), though the confined crystallization is significantly affected by D as well as MEG. Detailed crystallization rates of confined PCL

⎛ ΔGη ⎞ ⎛ ΔG* ⎞ ⎟ exp⎜ − I * ∝ exp⎜ − ⎟ ⎝ kT ⎠ ⎝ kT ⎠

(3)

where ΔG* (>0) is the free energy barrier associated with the formation of a critical nucleus and ΔGη (>0) represents the activation free energy for the short distance diffusion of crystallizing PCL segments. The MEG dependence of ΔG* and ΔGη can be deduced from our previous discussion on the f homo dependence of each free energy.30 It is important to find that the MEG dependence of ΔG* is opposite to that of ΔGη; ΔG* is a thermodynamic factor relating to the probability to find a critical nucleus and increases slightly with increasing temperature within a limited temperature range. Therefore, ΔG* would decrease slightly and monotonously with increasing MEG F

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Figure 7. Inverse of half-time of crystallization 1/τ1/2 (proportional to the crystallization rate) plotted against the molecular weight of end-groups MEG for various PCL homopolymers confined in the nanocylinder with D = 12.9 nm. 1/τ1/2 is corrected for the small difference in D (see Figure S9). The red broken line represents 1/τ1/2 for confined PCL homopolymers with MEG = 43 g/mol (0-PCL).

Figure 8. Illustration showing the MEG dependence of ΔG* (blue), ΔGη (green), and ΔG* + ΔGη (black) for smaller D (a), intermediate D (b), and larger D (c).

at a constant Tc because the PCL homopolymer with larger MEG has a smaller mobility corresponding to slightly lower temperature. On the other hand, ΔGη increases extremely with increasing MEG at smaller D, whereas it changes insignificantly at larger D because it depends critically on the chain mobility of PCL homopolymers determined by the combined effect of MEG and D. The sum of ΔG* and ΔGη is proportional to ln 1/I* (i.e., when ΔG* + ΔGη is larger the nucleation rate is smaller) from eq 3, and it is possible to qualitatively explain the MEG dependence of nucleation rates of confined homopolymers at a constant D (Figure 7). When D is very small, the MEG dependence of ΔGη is predominant over ΔG* (Figure 8a), yielding that the nucleation rate decreases (or ΔG* + ΔGη increases) steadily with increasing MEG; that is, the nucleation rate is in the order 0-PCL > A-PCL > C-PCL > M-PCL > PS-PCL. On the other hand, when D is sufficiently large, the MEG dependence of ΔGη is insignificant, and therefore ΔG* is important for the nucleation rate, resulting in the reverse order (Figure 8c). It is consistent with the crystallization rates of PCL blocks and PCL homopolymers observed in the nanocylinder with D =

17.9 nm (b in Figure S-1). When D is intermediate, a minimum of ΔG* + ΔGη (or maximum of nucleation rates) is expected from the MEG dependence of ΔG* + ΔGη (Figure 8b), and it is possible to qualitatively explain the accelerated crystallization rate observed in the confined PCL homopolymers with intermediate MEG (Figure 7). In summary, the crystallization kinetics of PCL homopolymers confined in nanocylinders depends intimately on their chain mobility, which can be successfully controlled by the introduction of bulky end-groups as well as the nanocylinder diameter D. Therefore, the crystallization of polymer chains confined in various nanodomains is very complicated, and it is necessary to examine the details of confined crystallization by further extensive studies.

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CONCLUSIONS The isothermal crystallization kinetics of poly(ε-caprolactone) (PCL) homopolymers was examined when they were spatially confined in a cylindrical nanodomain (nanocylinder) with a diameter of 12.9 nm. The confined PCL homopolymers were prepared using the microphase separation of PCL-blockG

DOI: 10.1021/acs.macromol.7b01536 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

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polystyrene (PCL-b-PS) diblock copolymers with a photocleavable o-nitrobenzyl group (ONB) at block junctions, followed by photocleaving ONB with UV light. In order to continuously change the chain mobility of confined PCL homopolymers, several bulky groups were introduced to the chain-end of PCL homopolymers; acetyl group (molecular weight MEG = 43 g/mol, original PCL homopolymer), adamantane (MEG = 163), cholesterol (MEG = 386), methyl 2,3,6-tri-o-benzoyl-α-D-galactopyranoside (MEG = 506), and vitrified PS (MEG = ∞, i.e., PCL blocks in PCL-b-PS). It was found that these end-groups had no specific effect on the crystallization of bulk PCL homopolymers (i.e., no space confinement imposed), and eventually the melting temperature, crystallinity, and crystallization rate all decreased with increasing MEG. Therefore, it was expected the chain mobility of confined PCL homopolymers would decrease continuously with increasing MEG. The crystallinity of confined PCL homopolymers increased abruptly at the initial stage of isothermal crystallization, followed by an asymptotic increase at the late stage, indicating homogeneous nucleation controlled the overall crystallization of confined PCL homopolymers. Surprisingly, the crystallization rate of PCL homopolymers with bulky end-groups (i.e., 163 ≤ MEG ≤ 506) was significantly larger than that of original PCL homopolymers (MEG ∼ 43) and PCL blocks (MEG ∼ ∞). These unforeseen experimental results were successfully explained using the pioneering theory of polymer nucleation.

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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.7b01536. Figures S-1 to S-9 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (Shuichi Nojima). ORCID

Shuichi Nojima: 0000-0003-4268-9363 Present Address

Shintaro Nakagawa: Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba Meguro-Ku, Tokyo 1538505, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The SR-SAXS measurement was performed under the approval of Photon Factory Advisory Committee (No. 2016G652). REFERENCES

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DOI: 10.1021/acs.macromol.7b01536 Macromolecules XXXX, XXX, XXX−XXX