Confined Nucleation and Crystallization Kinetics in Lamellar

Mar 12, 2015 - The nucleation and crystallization kinetics of lamellar crystalline–amorphous diblock copolymer poly(ε-caprolactone)-b-poly(4-vinylp...
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Confined Nucleation and Crystallization Kinetics in Lamellar Crystalline−Amorphous Diblock Copolymer Poly(εcaprolactone)‑b‑poly(4-vinylpyridine) Lanlan Chen,† Jing Jiang,† Lai Wei,†,‡ Xiaoliang Wang,† Gi Xue,† and Dongshan Zhou*,†,‡ †

Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Key Laboratory of High Performance Polymer Materials and Technology, MOE, State Key Laboratory of Co-ordination Chemistry,Nanjing University, Nanjing 210093, P. R. China ‡ School of Physical Science and Technology, Xinjiang Laboratory of Phase Transitions and Microstructures in Condensed Matters, Yili Normal University, Yining 835000, P. R. China ABSTRACT: The nucleation and crystallization kinetics of lamellar crystalline−amorphous diblock copolymer poly(ε-caprolactone)-b-poly(4-vinylpyridine) (PCL−P4VP) was investigated by ultrafast differential scanning calorimetry (UFDSC) with temperature scanning rates up to 10 000 K/s and compared with that of poly(ε-caprolactone) (PCL) homopolymer. We found that the critical cooling rate (ccr) to get the fully amorphous PCL is 1 order of magnitude slower than that for PCL homopolymer with the similar molecular weight. Isothermal nucleation and crystallization of PCL block in the PCL−P4VP copolymer and PCL homopolymer were studied covering times from 10−2 to 103 s and temperatures from 200 K (10 K below the glass transition temperature of PCL) to 300 K (about 40 K below the equilibrium melt temperature of PCL). It was found that the PCL block in PCL−P4VP copolymer experienced a slower homogeneous nucleation rate as well as crystallization rate than PCL homopolymer, indicating that even the local nucleation events of PCL chains is affected by the long-range glassy P4VP in copolymer. The confinement also hinders the long-range diffusion of PCL chains and becomes more effective once the chains get the mobility from the glassy state at crystallization temperatures above the Tg. Another effect of the confinement is the lower Avrami index in copolymer than that in homopolymer attributed to the restricted growth dimension under confinement. The results reported here might enhance the understanding of confinement effect on crystallization and give new details on the nucleation kinetics under nanoscale confinement.

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INTRODUCTION During the past few decades, many efforts have been devoted to explore the crystallization behavior of polymers under confinements.1−4 There have been several approaches to investigate the effect of confinement on polymeric crystallization.5−9 Diblock and triblock copolymers are widely used because their self-assembled microdomains provide a convenient way to achieve the confinement under nanoscales.10−13 When crystallizable phase is confined in a large number of microdomains, the nucleation and crystallization kinetics is dramatically different from that of homopolymer.14−17 Typical homopolymers show a sigmoidal kinetics with Avrami index ranging from 2 to 4, reflecting from one-dimensional to threedimensional growth from homogeneous or heterogeneous nuclei.18 There are two extremes in copolymers. At one extreme, the total crystallization process from the generation of one nucleus to the fully crystallization occurs in subdivided nanoscale domains. At the other extreme, the high degree of interconnectivity in polygrain structure of copolymer results in the crystal growth from one grain to another; kinetics © 2015 American Chemical Society

resembling that of homopolymer is observed. Loo et al. showed the first-order (an Avrami index of n = 1) crystallization kinetics of polyethylene block confined in 25 nm spheres.16 They also found the greater Avrami index value and sigmoidal crystallization kinetics in E/VCH diblock copolymer caused by the interconnectivity of crystallizable domains.17 They claim the first-order kinetics implies that crystallization is isolated within individual microdomains, resulting in a crystallization rate that is proportional to the fraction of material not yet crystallized, i.e., that the crystal growth must be essentially instantaneous within microdomains and homogeneous nucleation is the ratedetermining step in the crystallization. This assumption is based on the fact that the free-energy barrier for homogeneous nucleation is much higher than that associated with crystal growth. Received: December 25, 2014 Revised: February 5, 2015 Published: March 12, 2015 1804

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Macromolecules The nucleation process can be detected only indirectly due to the small size of the nuclei. In polarized optical microscopy, the number of stabilized nuclei can be linked to the number of observed crystals.19−21 In calorimetry, the cold crystallization enthalpy on heating or at isothermal conditions at low temperature is found to be dependent on the number of previously formed nuclei.22,23 Studying homogeneous nucleation kinetics requires a starting situation without such nuclei. Differential scanning calorimetry (DSC) is routinely used to study crystallization behavior of polymers.24,25 However, the cooling rate of standard DSC is limited to prevent nucleation on cooling for rapidly crystallizing polymers. Nowadays, the development of ultrafast differential scanning calorimetry (UFDSC) improves the ability to separately study both nucleation and crystallization at certain crystallization temperature over a wide temperature range, by applying much faster controlled cooling and heating rates to avoid the formation of homogeneous nuclei on cooling and heating processes.26−30 Schick et al. designed a serial of experiments to systematically demonstrate the nucleation and crystallization kinetics of PCL: homogeneous nucleation started with overcoming a pertinent critical free energy barrier then crystallization occurred with a growth of small to large crystals following by the nucleation.31 They also found that the critical rate for keeping the sample amorphous on cooling decreased slightly with increasing molar mass.32 In this work, we center on the crystallization− amorphous diblock copolymer PCL−P4VP, which is a hard confinement system. Ho et al. found the PCL chains in PCL− P4VP copolymer were homogeneous nucleation when they were confined in the critical size in both one-dimensional confined system33 and two-dimensional confined system.34 Nevertheless, most investigations on the homogeneous nucleation and crystallization kinetics in copolymers are carried out at a certain crystallization temperature or in a limited crystallization temperature range. In the present study, with the ultrafast cooling ability of UFDSC, we have investigated isothermal nucleation and crystallization kinetics of PCL block confined in lamellar PCL−P4VP block copolymer, covering times from 10−2 to 103 s and temperatures from 200 K (10 K below the Tg of PCL) to 300 K, which is 40 K below the equilibrium melting temperature of PCL, and compared with that of PCL homopolymer. Ultimately, we construct a full nucleation and crystallization map in time and temperature for PCL chains confined in amorphous P4VP phase. From these results, we try to clarify the development of the confinement effect in a wide temperature range on both the nucleation and crystallization kinetics of PCL chains in the lamellar copolymer.

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Scheme 1. Synthesis of PCL Homopolymer and PCL−P4VP Block Copolymer

Instruments. 1H NMR spectra were obtained on a DRX-500 NMR spectrometer (Bruker BioSpin, Coventy, UK) in deuterated chloroform (CDCl3) as solvent. The molar masses and dispersities were determined by gel permeation chromatography (GPC) using a PLGPC 120 chromatograph equipped with a refractive index detector. A combination of two polystyrene gel columns of PL gel-MIXED C was used, with tetrahydrofuran as a solvent at a flow rate of 1.0 mL/min and a temperature of 313 K. The columns were calibrated using polystyrene standards. Standard DSC experiments were carried out on Mettler-Toledo DSC1STARe DSC to characterize the PCL homopolymer and the PCL−P4VP copolymer. Samples were heated from ambient temperature to 473 K and held there for 3 min to erase the thermal history, cooled to 223 K at 10 K/min, and then reheated to 473 k at 10 K/min. Small-angle X-ray scattering (SAXS) measurements were conducted on the synchrotron X-ray beamline BL16B1 in Shanghai Synchrotron Radiation Facility (SSRF). The wavelength of the X-ray beam is 1.239 Å. The scattering vector q is calibrated using silver behenate. A Linkam heating stage with N2 protection environment and a MarCCD detector were used. Ultrafast Differential Scanning Calorimetry (UFDSC). UFDSC was applied for the detailed nucleation and crystallization of PCL− P4VP diblock copolymer and PCL homopolymer. The details of the device are reported elsewhere.27 The thin film chip sensors XI-395 of Xensor Integration were used for experiments. A small piece of the polymer was cut and placed directly on the heated area of the sensor. The temperature signal of the thermopile of the sensor was calibrated by the melting temperatures of indium. Operating the UFDSC with liquid nitrogen as the coolant allows cooling and heating below the glass transition temperature at 210 K and above the equilibrium melting temperature at about 340 K, at rates up to 10 000 K/s. All crystallization experiments on the sample of PCL−P4VP copolymer were carried out in the temperature range of 120 to 370 K, in which the P4VP block kept amorphous.

EXPERIMENTAL SECTION

Synthesis and Characterization of PCL and PCL−P4VP. Materials. PCL−P4VP block copolymer was synthesized via living ring-opening polymerization (ROP) and atom transfer radical polymerization (ATRP) in sequence,35,36 as shown in Scheme 1. The homopolymer PCL-OH was prepared by ROP in the first step. Then a bromine-terminated PCL was prepared as a macroinitiator, further used for the controlled polymerization of 4-vinylpyridine. Sample Preparation. In order to ensure the consistency of the phase behavior, uniform sample preparation procedure and thermal history were necessary. The copolymer sample was cast from a 10% (w/v) dichloromethane solution. To eliminate possible effects of PCL crystallization and residual solvent, we annealed the sample at 420 K for 12 h. The sample was studied using different experimental techniques including standard DSC, SAXS, and UFDSC.

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RESULTS AND DISCUSSION General Characterization. PCL is characterized by gel permeation chromatography (GPC) using polystyrene standard calibration, and has a Mn of 11.6K and a polydispersity of 1.12. The Mn of the P4VP block is determined by proton nuclear magnetic resonance (1H NMR) to be 12K. The final diblock copolymer has a polydispersity of 1.43, which is determined by GPC. The synthesized diblock copolymer can be expressed as PCL101−P4VP114, of which the ratio of the volume fraction of PCL block to that of P4VP block is 0.47:0.53. 1805

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Macromolecules The result of SAXS at 353 K (above the Tm of the PCL block) in Figure 1 reveals the lamellar microphase morphology

P4VP block is located at 409 K, which is 87 K higher than the melting temperature of PCL. The constraint of the glassy P4VP block decreases the mobility of PCL chains in copolymer; less ordered crystals formed on cooling results in the lower Tm of PCL block in copolymer than that of PCL homopolymer at the same cooling rate.19 On the basis of self-consistent field theory for a symmetric diblock copolymer37 and Sun’s experimental observations,33 the order−disorder temperature TODT values of PCL−P4VP are far above 473 K. Because TODT is greater than the glass transition of amorphous phase (Tg (P4VP)) greater than the crystallization temperature of crystallizable temperature (Tc (PCL)), PCL−P4VP bock copolymer is a hard confinement system, and crystallization of PCL block occurs within preexisting lamellar microdomains, confirmed by results of SAXS. Cooling Rate Dependence of Nucleation and Crystallization. The ultrafast differential scanning calorimetry (UFDSC) makes it possible to cool sufficiently fast to prevent formation of homogeneous nucleation as well as crystals growth. Reheating these “clean” samples with different protocols enables to obtain additional information about crystallization, nucleation and stability of the objects formed on controlled cooling. The protocol for investigation of nonisothermal nucleation and crystallization in PCL homopolymer and PCL block in PCL−P4VP copolymer is shown in Figure 3. The samples were heated to the temperature which is

Figure 1. 1D SAXS profiles of PCL−P4VP copolymer at different temperatures.

of PCL−P4VP in melt, with the scattering peaks being at q ratio of 1:2:3. At 273 K, which is below the crystallization temperature of PCL block, the peaks are broadened and keep essentially the same positions as in the melt, indicating that the lamellar morphology is essentially preserved after crystallization. The peak broadening may be caused by the fact that the electron density in the PCL domains is less uniform after the PCL block crystallization than in the melt. The long period of microphase-separated lamellae is calculated according to an expression dlam = 2π/q. Combining the long period and the volume fraction of PCL block, the PCL is confined in 12.5 nm lamellae. Figure 2 shows standard DSC cooling and reheating scans of PCL−P4VP block copolymer and PCL homopolymer. An exothermic crystallization temperature Tc peak appears at about 309 K on cooling, and an endothermic melt temperature Tm peak occurs at about 329 K on heating in PCL homopolymer. In the case of PCL−P4VP, Tc of PCL block is about 30 K shift to the low temperature compared with the PCL homopolymer. Tm of PCL block is about 322 K, which is 7 K lower than that of PCL homopolymer. The glass transition temperature Tg of

Figure 3. Time−temperature profile of the experiments for investigation of nonisothermal nucleation and crystallization in PCL homopolymer and PCL block in PCL−P4VP copolymer.

Figure 2. Standard DSC (a) cooling and (b) reheating scans of the PCL−P4VP copolymer and the PCL homopolymer at a rate of 10 K/min. 1806

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Figure 4. Heating curves of (a) PCL homopolymer and (b) PCL block in PCL−P4VP copolymer from UFDSC on heating with 10 000 and 5000 K/s, respectively, after cooling with different rates.

Figure 5. Time−temperature profile of the experiments for investigation of isothermal nucleation and crystallization in PCL homopolymer and PCL block in PCL−P4VP copolymer.

range of possible crystallization.28 PCL block in PCL−P4VP shows almost no cold crystallization at cooling rates from 10 to 500 K/s. PCL block becomes amorphous at lower cooling rate than PCL homopolymer, since the melting peak is almost diminished at a cooling rate of 300 K/s, while for PCL homopolymer the critical cooling rate is 2000 K/s. This can be explained by the fact that the constraint of the glassy P4VP block slows down the crystallization rate of PCL block, as shown in Figure 8a. Results of Isothermal Crystallization. With the ability of cooling fast enough to prevent the formation of nuclei and crystals growth, reheating after isothermal annealing covering times from 10−2 to 103 s and temperatures from 10 K below the glass transition up to 300 K, which is 30 K below the equilibrium melt temperature of PCL, allows a detailed studying of nucleation and crystallization kinetics. The protocol for isothermal experiments is shown in Figure 5. We select three annealing temperatures to demonstrate the nuclei and crystal developments of PCL block in copolymer at low, intermediate, and high annealing temperature regime, as given in Figure 6. At low annealing temperature of 210 K (see Figure 6a,b), for PCL block in PCL−P4VP, a tiny peak of cold crystallization followed by a just formed crystal melting peak is detected for 1 s. The total enthalpy change (ΔHtotal = cold crystallization exotherm plus melting endotherm) is zero. For

above the Tm of PCL and below the Tg of P4VP and cooled with different rate to below the Tg of PCL. The final measurement on reheating for analysis was performed at a fixed rate. The reheating rate must be fast enough to avoid formation of homogeneous nuclei on heating; meanwhile, the reheating rate must be slow enough for allowing the growth of nuclei formed prior heating (Figure 6). To fulfill these two conditions, the reheating rates of 10 000 and 5000 K/s for PCL homopolymer and PCL block in copolymer, respectively, were chosen for the experiments. A series of heating scans after cooling at different rates are shown in Figure 4. Both samples show a relatively small and broadened glass transition step and a large melting peak at slow cooling rates. With increasing cooling rate the glass transition step becomes sharper and the melting peak decreases due to the less crystallization on cooling. A cold crystallization peak is observed for PCL homopolymer at a certain cooling rate and increases with increasing cooling rate (see the inset of Figure 4a). With further increasing cooling rate cold crystallization reaches a maximum at 400 K/s and decreases at higher cooling rates because the number of homogeneous nuclei formed on cooling is decreasing. It is consistent with Sanchez’s work38 on the effect of the cooling rate on the nucleation kinetics. At very high cooling rate, both nucleation and crystallization are suppressed because of the very short time passing though the temperature 1807

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Figure 6. Heating curves of PCL homopolymer and PCL block in PCL−P4VP on heating with 10 000 and 5000 K/s, respectively, after annealing for different times at 210 K (a, b), 230 K (c, d), and 270 K (e, f).

slightly longer time, such as 3 s, the enthalpy of cold crystallization increases due to the increased homogeneous nucleation with increasing annealing time. Meanwhile, the melting peak is also increasing. The total enthalpy change remains zero. For even longer time, such as from 10 to 7200 s, crystallization has occurred during annealing; the glass transition is broadened and shifted to higher temperatures in the semicrystalline structure. The melting peak of the poorly ordered crystals appears at just above the annealing temperature. For partially crystallized samples, such as observed after 30 s, the low-temperature melting peak is followed by cold

crystallization or cold recrystallization of the just melted crystals and final melting. The shift of the low-temperature melting peak from 230 K after annealing for 10 s to 250 K after annealing for 7200 s is the result of the internal stabilization of the isothermal formed crystals.20 However, the final melting temperature shows no dependence of the annealing time because heating at the same rate provides the same recrystallization conditions, resulting in an identical final melting temperature.26 Compared with PCL homopolymer, PCL block in PCL−P4VP presents a small cold crystallization 1808

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Figure 7. Total enthalpy change and cold crystallization enthalpy change of (a) PCL homopolymer and (b) PCL block in PCL−P4VP on heating with 10 000 and 5000 K/s, respectively, after annealing for different times at different annealing temperatures.

parametrize the curves in Figure 7. The overall crystallization enthalpies of samples were analyzed with the Avrami equation modified by the secondary crystallization:31,39

peak and needs long annealing time to observe any peaks on heating. At intermediate annealing temperature of 230 K (see Figure 6c,d), PCL block in PCL−P4VP copolymer shows only melting−recrystallization−remelting process without cold crystallization. The shapes of the PCL block in PCL−P4VP melting curves indicate the two processes of crystallization at the annealing temperature: initially growing crystals melting at the low-temperature side of the melting peak and perfection crystals melting shifted to the high-temperature side of the melting peak. Cold crystallization can be observed in PCL homopolymer for short annealing time at 230 K, such as from 0.3 to 3 s. For even longer annealing time, the situation is similar as that of PCL block in PCL−P4VP. At high annealing temperatures of 270 K (see Figure 6e,f), no cold crystallization and only melting can be detected during the analysis in both samples, because PCL chains have directly crystallized during annealing and these seed crystals do not act as nuclei on heating.32 Reheating curves as shown in Figure 6 allow calculating the total enthalpy change ΔHtotal between the glass transition temperature and final melting temperature, as a measure of crystallinity, and the cold crystallization enthalpy ΔHcc as a measure of the number of previously formed active nuclei. The calculated results of these two enthalpies for (a) PCL homopolymer and (b) PCL block in PCL−P4VP on heating for each annealing temperature and annealing time are shown in Figure 7. With the annealing temperature increasing, the curves of the total enthalpy change shift to shorter time and back to longer time; that is, the crystallization rate increases to the maximum and then decreases. The cold crystallization enthalpy change has the same trend as the total enthalpy change. The nucleation rate reaches a maximum at low temperature close to the Tg of PCL. Confinement Effect on Nucleation and Crystallization Kinetics. To obtain the characteristic time and index of nucleation and crystallization, an Avrami function was used to

⎛ ⎛ ⎛ t ⎞ n ⎞⎞ ⎜ ΔHc(t ) = Hc ∞⎜1 − exp⎜⎜ −ln 2⎜ ⎟ ⎟⎟⎟⎟ + A 2 (ln t − ln τc) ⎝ τc ⎠ ⎠⎠ ⎝ ⎝ ⎛ 1 ⎛ |t − τ | ⎞⎞ c × ⎜⎜ ⎜ + 1⎟⎟⎟ ⎠⎠ ⎝ 2 ⎝ t − τc

(1)

where ΔHc∞ is the final enthalpy of primary crystallization, τc is the half-crystallization time, n is the Avrami index, t is the annealing time, and A2 is the secondary parameter. The development of the cold crystallization enthalpy in Figure 7 contains two parts: the kinetics of nuclei formation, the increasing of the peak at short annealing times, and the crystallization during annealing, the decreasing of the peak at long annealing times, which is the same as for the overall enthalpy change. The homogeneous nucleation process was expressed as31 ⎛ ⎛ ⎛ t ⎞ 2 ⎞⎞ ⎜ ⎜ ΔHn(t ) = ΔHnhom ∞⎜1 − exp⎜ −ln 2⎜ ⎟ ⎟⎟⎟⎟ ⎝ τn ⎠ ⎠⎠ ⎝ ⎝

(2)

where ΔHnhom∞ is the limiting cold crystallization enthalpy caused by homogeneous nuclei and τn is the half-nucleation time. An Avrami index of 2 was used for the nucleation. In general, nucleation process is commonly accepted as a local event and is insensitive to the chain length.40 Interestingly, the nucleation is hindered under nanoscale confinement, reflecting in the slower nucleation rate in copolymer (see Figure 8a). According to the classical theories of the primary homogeneous nucleation rate I, the temperature dependence of I can be described as41,42 I(T ) ∝ exp( −ΔG*/kT ) exp(−ΔGη /kT ) 1809

(3)

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Figure 8. (a) Half-crystallization time, half-nucleation time and (b) Avrami index of PCL−P4VP copolymer and PCL homopolymer as a function of crystallization temperature.

The first term, exp(−ΔG*/kT), is a thermodynamic factor and proportional to the probability for finding a critical nucleus, which is formed by the random segmental aggregation through thermal fluctuations. To enter the thermodynamically stable state, the nuclei have to overcome critical size lc*. Nuclei smaller than l*c will dissolve spontaneously whereas nuclei larger than l*c will grow spontaneously. Measurement of lc* presents a significant challenge because of the minute sizes of nuclei and the absence of suitable characterization. The calculated lc* is obtained from the Gibbs−Thomson equation for the longitudinal dimension of the critical nucleus size:43,44 lc* =

4σeTm° ° ΔHf (Tm° − Tc)

nucleation rate in copolymer arises from the energy barrier ΔG* to create a critical nucleus or from the local mobility factor ΔGη or combined both factors. Nevertheless, it is rather difficult to obtain experimental confirmation of the confinement effect separately on ΔG* or ΔGη because the nucleation rate obtained reflects the average nucleation kinetics. Confinement effect on long-range diffusion of PCL results in a slower crystallization rate than PCL homopolymer with similar molecular weight (Figure 8a), which is already known in other reports. 1,33 Taking the advantage of the wide crystallization temperature range operated by UFDSC, we capture the development of confinement effect degree from the high surpercoolings to low surpercoolings. At low crystallization temperatures nearby the Tg of PCL, where the motion of PCL chains themselves is very slow at such large supercoolings, the effect of glassy P4VP block confinement is not so obvious, reflecting in the small decrement in the half-crystallization time of copolymer compared with PCL homopolymer. Once the diffusion ability of PCL chains increases at crystallization temperatures beyond Tg, the role of the confinement becomes prominent and the crystallization rate decreases dramatically in copolymer. The thickness of crystalline lamellae formed in semicrystalline polymers is typically tens of nanometers, which is comparable to the nanoscale confinement size. Hence, the growth of lamellar crystals is greatly hampered, reflecting in not only the growth rate42 but also the growth dimension.19,49 The Avrami index n is related to the mechanism of the growth geometry as well as nucleation. Balsamo et al.50 considered the Avrami index is given by the addition of two terms:

(4)

where Tm ° and ΔHf° are the equilibrium melting temperature and heat of fusion per unit volume, respectively. σe is the fold surface free energy. Tc is the crystallization temperature. The calculations for lc* of PCL make use of σe = 8.71 J/cm2,45 Tm °= 347.7 K,46 and ΔHf° = 163 J/cm3.47 At low Tc in the range from 200 to 220 K, where homogeneous dominates at such large supercoolings, the estimated l*c varies from 5.0 to 5.8 nm, which is comparable to the confinement lamellae size 12.5 nm. That is, the confinement possibly affects the critical nuclei formation, resulting in larger ΔG*/kT in copolymer. This speculation can be used to explain the fact that the larger decrement is observed in nucleation rate with the higher Tc shown Figure 8a because lc* is crystallization temperature-dependent: the higher Tc, the larger critical nucleus is needed to create and the confinement is more effective. The second term, exp(−ΔGη/kT), is a dynamic factor and related to the local diffusion of crystallizing segments toward the nucleus surface. Because of the intramolecular junctions in the copolymer, the tethered PCL chains decreases the local mobility. The value of −ΔGη/kT is larger in copolymer than that in homopolymer because the local rate of arrival of new segments at the nucleus surface is largely restricted due to the combined effect of chain tethering and limited dimension of chain conformation. Nakagawa et al.42 found the nucleation rate of PCL block decreased with the increasing tethered PCL chains in PCL−PS copolymers. Slower nucleation rate in copolymer than homopolymer was also observed by Organ for PHB−HV copolymers.48 The discussion above can qualitatively explain the experimental results on nucleation rate shown in Figure 8a. That is, the slower

n = nn + ngd

(5)

where nn is the fraction of the index related to nucleation (0 corresponds to instantaneous, athermal nucleation and 1 to sporadic, thermal nucleation) and ngd that related to growth dimensionality (values from 1 to 3 for one-dimensional to three-dimensional growth). The noninteger values reflect spherulitic growth from a mixture of “instantaneous” with “sporadic” nuclei. Figure 8b shows the Avrami index plotted against crystallization temperature. PCL block in the PCL−P4VP copolymer has always a lower Avrami index value than PCL homopolymer at a specific crystallization temperature. Both 1810

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observed slower nucleation rate in copolymer gives new insight into the nucleation kinetics under nanoscale confinement.

samples have extremely low Avrami index at crystallization temperature below the Tg of PCL due to the constraint from glassy PCL matrix. The case of n < 1 was found by Lotz et al. in the PS−PEO diblock copolymer with crystallization at a very large supercoolings.51 In the case of PCL block in copolymer, in crystallization temperature range from 210 to 230 K, where homogeneous nucleation is prevalent at such large supercoolings, the values of n increase with the crystallization temperature increasing because the rate of homogeneous nucleation is decreased, each nucleus grows more PCL, inevitably including the dimensionality growth.10 At crystallization temperatures above 230 K, the Avrami index exhibits a slightly decrease trend as Ta increases, which is caused by changes in nucleation mechanism from homogeneous to heterogeneous. Similar nucleation mechanism change in crystallization kinetics was also observed in lamellar PE-bPVCH diblock copolymer,17 PCL containing multiwalled carbon nanotubes (MWCNT)39 and iPP.52

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (D.S.Z). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of China (973 Program, 2012 CB 821503) and also by the National Natural Science Foundation of China (No. 21274059, 21474049, 51133002, and 21274060). The authors gratefully acknowledge the instrument subsidy from the Alexander von Humboldt Foundation and Prof. Christoph Schick at the University of Rostock for his help to build the UFDSC instruments in Nanjing University and extensive discussions in related work.

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CONCLUSIONS Confined nucleation and crystallization kinetics in lamellar PCL−P4VP diblock copolymer was systematically investigated by ultrafast differential scanning calorimetry (UFDSC). The advantage of the technique is the ultrafast cooling ability allowing one following the development of nuclei formation and crystals growth at annealing in a wide range of temperature on time scales starting from 0.01 s. We aimed to find details of the confinement effect in a wide temperature range on both nucleation and crystallization kinetics of PCL block confined in the lamellar copolymer. The SAXS results reveal the lamellar morphology is preserved after the crystallization of PCL block in the hard confinement PCL−P4VP copolymer. The critical cooling rate to obtain the fully amorphous PCL in the PCL− P4VP diblock copolymer is about 1 order of magnitude slower than that for PCL homopolymer. The dependence of nucleation rate and overall crystallization rate on crystallization temperature was investigated in a wide range from 200 to 300 K. The PCL block in PCL−P4VP exhibits a slow nucleation rate as well as crystallization rate comparing with that of PCL homopolymer, indicating that even the local nucleation events of PCL chains is affected by the long-range glassy P4VP matrix. The homogeneous nucleation was discussed from the viewpoint of the free energy barrier to create a critical nucleus and the local molecular mobility necessary for nucleation. The estimated homogeneous critical nuclei size is comparable to the lamellae confinement thickness in this study. Interamolecular junctions and the tethered PCL chains decrease the local mobility in the copolymer, resulting in the slow nucleation rate. The confinement effect of amorphous P4VP block in copolymer becomes prominent on the crystallization rate of PCL block with increasing diffusion ability at high crystallization temperatures. PCL block in copolymer has always a lower Avrami index than PCL homopolymer at a specific crystallization temperature. Both the PCL block in copolymer and the homopolymer have relatively low Avrami values at crystallization temperatures below Tg of PCL. In the case of PCL block in copolymer, the Avrami index increases with increasing crystallization temperature in the range from 210 to 230 K, where the homogeneous nucleation rate decreases, allowing each nucleus time to grow to high dimension. At crystallization temperatures above 230 K, the Avrami index slightly decreases because of the nucleation mechanism change from homogeneous to heterogeneous. The

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