Crystallization of Green Poly(ε-caprolactone) Nanocomposites with

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Cite This: Ind. Eng. Chem. Res. 2018, 57, 6257−6264

Crystallization of Green Poly(ε-caprolactone) Nanocomposites with Starch Nanocrystal: The Nucleation Role Switching of Starch Nanocrystal with Its Surface Acetylation Guorui Zhang,† Chunjiang Xu,† Defeng Wu,*,†,§ Wenyuan Xie,†,§ and Zhifeng Wang‡ †

School of Chemistry & Chemical Engineering and ‡Testing Center, Yangzhou University, Yangzhou, Jiangsu 225002, China Provincial Key Laboratories of Environmental Engineering & Materials, Yangzhou, Jiangsu 225002, China

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ABSTRACT: Poly(ε-caprolactone) (PCL) was filled with a very small amount of starch nanocrystals (SNC), with the objective being to control its crystallization. Three kinds of SNC particles, including a pristine one and ones acetylated with low- and middle-level substitutions, were used. Their roles during PCL crystallization were studied. The results reveal that pristine SNC acts as the nucleating agent, evidently increasing crystallization temperature and spherulite growth rate. However, nucleation activity of SNC particles reduces with their increasing level of surface acetylation. The middle-level-acetylated SNC particles even act as the antinucleation agent, impeding nucleation and subsequent spherulite growth of PCL. The phase affinity in those systems was then evaluated by the interaction energy parameter, aiming at a better understanding of the mechanism of role alterations of SNC particles. This work provides an interesting way to control PCL crystallization and opens a new door to extend applications of SNC particles. polylactide (PLA),6 poly(butylene succinate) (PBS),7 and poly(β-hydroxyoctanoate) (PHB),8 especially to poly(ε-caprolactone) (PCL)9−11 because PCL is of a low modulus.12 An evident increase of tensile modulus of PCL was reported by Habibi and Dufresne10 in the presence of PCL-grafted SNC particles, and the composites almost had the same levels of strength and elongation as those of neat PCL. In our previous work,11 we found that the presence of a very small amount of acetylated SNC particles (1 wt %) had a significant reinforcement and barrier effect on PCL membrane, reducing oxygen transmission rate by 70% and increasing tearing strength by about 68%. This suggested that incorporation with SNC particles might be an effective way of optimizing final performance of PCL. However, PCL is semicrystalline, which means that its final properties are not only dependent on the filled nanoparticles but also its crystallization histories.12 For example, a high

1. INTRODUCTION Starch is one of the most abundant materials in the world. It is biosynthesized as semicrystalline granules with densely packed polysaccharides and a small amount of water. The crystalline and amorphous phases coexist, forming an onion-like structure in granules.1 The crystalline regions can be isolated by acid hydrolysis or physical treatments, yielding nanometric particles, so-called starch nanocrystal (SNC).2 These SNC particles are commonly platelet-like, although their platelet sizes highly depend on the starch resources.3 The unique platelet-like structure of SNC, together with its high rigidity, make it a promising nanofiller for polymers to improve barrier and mechanical properties. A variety of polymers, including elastomers and plastics and even the starch-based materials themselves, have been suggested for preparation of nanocomposites with SNC up to now.2−5 Among those reported SNC-filled polymer nanosystems, the biodegradable aliphatic polymer-based ones are very attractive because both the matrix and filler particles are biodegradable or bio-originated. This kind of nanocomposites are thus fully green. The reported work revealed that the presence of SNC added good reinforcement to the aliphatic polymers, such as © 2018 American Chemical Society

Received: Revised: Accepted: Published: 6257

February 1, 2018 April 1, 2018 April 18, 2018 April 18, 2018 DOI: 10.1021/acs.iecr.8b00514 Ind. Eng. Chem. Res. 2018, 57, 6257−6264

Article

Industrial & Engineering Chemistry Research

where s is the filler particle loading, and the subscripts, L and M, denote lower and middle-level DSs of SNC, respectively. 2.2. Characterizations. 2.2.1. Differential Scanning Calorimetry. Thermal events for neat PCL and its composites in the heating and cooling processes were detected using the differential scanning calorimeter (Netzsch 204F1, Germany). The sample (about 5 mg) was heated from 0 to 80 °C, held for 5 min, then cooled to 0 °C, and again heated to 80 °C, during which the thermal enthalpies were recorded. To detect heat flow during isothermal crystallization, the sample was cooled from 80 °C to the predetermined temperature with the rate of 60 °C min−1 and held until exothermic event was over. All tests were performed under nitrogen atmosphere. 2.2.2. Polarized Light Microscopy. The growth of spherulites was recorded using a polarized light microscope (Leika DMLP, Germany) on a hot stage (Linkam LTM350, England). The same temperature ramp used for differential scanning calorimetry (DSC) tests was employed here. The sample was heated from 0 to 80 °C at a rate of 10 °C min−1 and held for 5 min to eliminate residual stress and thermal histories, then cooled with at a rate of 50 °C min−1 to a predetermined temperature for the subsequent spherulite growth observation. All sheet specimens almost had the same thickness (about 0.15 μm). 2.2.3. Small-Angle X-ray Scattering. The lamellar structure was detected through small-angle X-ray scattering (SAXS) instrument (Bruker AXS GmbH, NanoStar, Germany) with a Vantec-2000 2D detector. The tests were performed at room temperature, and the incident X-rays of Cu Kα radiation (1.54 Å) were monochromated by a cross-coupled Göbel mirror. The distance between the sample and detector was calibrated using silver behenate, giving the scattering vector q ranging from 0.07 to 2.3 nm−1. The 1D SAXS profiles were collected by integration of 2D pattern, shown as the normalized intensity (I) versus q (q = (4π/λ) sin θ, where λ is the wavelength of the X-rays and 2θ is the scattering angle). The 1D correlation function (K(z)) defined by Strobl and Schneider21 can be plotted by

degree of crystallinity favors improvement of mechanical strengths and barrier properties of PCL but is against its biodegradability and biocompatibility.13 The reported work revealed that the presence of polysaccharide nanocrystals, for instance, cellulose nanocrystal, had a large influence on the crystallization morphology and kinetics of PCL.14,15 This suggests that the presence of SNC particles might also affect the formation of supermolecular structure of PCL. To the best of our knowledge, there are no literature reports on this topic, which is worthy of deep study. Therefore, in this work, the crystallization behavior of the PCL/SNC composite systems were studied in detail, aiming at opening a new window to control PCL crystallization using SNC particles. Three kinds of SNC particles, including a pristine one and ones acetylated with low- and middle-level surface modification, were used as the filler. The different nucleation roles of those SNC particles during crystallization of PCL were detected. The particle− polymer compatibility in these three systems was further evaluated, with the objective to make a deep insight into the relations between nucleation abilities of SNC particles and their affinity to PCL.

2. EXPERIMENTAL SECTION 2.1. Materials and Preparation. Poly(ε-caprolactone) (PCL, Capa6400, with a number-average molar weight of 37 000 g mol−1) was purchased from Solvay Co. Ltd., Belgium. Industrial corn starch (A-type crystalline structure, with the average particle size of 10−20 μm) was purchased from Suchow Ouyang Chemical Technology Co. Ltd., PR China. The SNC suspension were prepared by the way of acid hydrolysis of starch reported by Dufresne et al.16 Details can be found in the previous work.11 As-obtained SNC particles are platelet-like (A-type crystallites), with average platelet sizes of 88.3 ± 7.6 nm, and the average thicknesses are 7.2 ± 2.3 nm. Their transverse modulus is 4.2 ± 0.6 GPa. A continuous route without a freeze-drying step, developed to prepare cellulose nanocrystal filled polylactide nanocomposites,17 was used here to perform the acetylation of SNC and subsequent preparation of PCL/SNC suspensions. The detailed route can be found in the previous work.11 The acid hydrolysis and the acetylation do not affect A-type crystal structure of starch.11 The degree of substitution (DS) of SNC was determined using a method developed by Teramoto and Shibata18 through the nuclear magnetic resonance spectroscopy (NMR) analyses. The DS’s of SNC were controlled within moderate levels in this work because excessive acetylation amorphized polysaccharide nanocrystals severely19,20 and as a result broke down their rigid particle structure.15 Three kinds of SNC particles, including a pristine one and ones acetylated with a lower DS (0.38) and a middle level DS (1.90), were prepared and incorporated with PCL. The acetylated SNC particles with these two DS’s had almost the same particle sizes and thickness as those of the pristine one, with monotonously decreased transverse modulus and surface degree of crystallinity.11 The composites with a thickness of about 200 μm were obtained by casting of suspensions, followed by vacuum-drying to constant mass weight. Good dispersion of those SNC particles in PCL matrix has been detected in the previous work. Surface acetylation could further improve phase affinity between SNC particles and PCL chain, leading to increase of glass transition temperature of PCL, which was further confirmed by FT-IR and rheological tests.11 Hereafter, the samples with pristine SNC and acetylated ones were referred as to PSNs, PSNLs and PSNMs, respectively,

K (z ) =

1 2π 2

∫0

∞

I(q) q2 cos(qz) dq

(1)

where I(q) is the absolute scattering intensity collected from the SAXS tests and z is the direction along which the electron density is measured.

3. RESULTS AND DISCUSSION 3.1. Nucleation Ability of Various SNC Particles to PCL. Figure 1 gives thermograms of PCL and its composite samples with 1 wt % particle loading. The obtained thermal parameter values are summarized in Table 1. It is very interesting that the crystallization temperature (Tc) of PCL decreases monotonously with increasing of DS of SNC particles. The composites with higher particle loadings, 3 and 5 wt %, reveal the same trend (Figure 2). This suggests that the nucleation ability of SNC strongly depends on its surface properties and particle−polymer interactions. Similar dependence has also been reported on the cellulose nanocrystal-filled PCL systems.14,15 The composite with pristine SNC particles (PSN1) shows higher Tc relative to that of neat PCL, which indicates that the presence of pristine SNC provides additional nucleation sites and as a result promotes nucleation of PCL from the melt. Similar heterogeneous nucleation has been widely reported on the PCL composites filled with a variety of 6258

DOI: 10.1021/acs.iecr.8b00514 Ind. Eng. Chem. Res. 2018, 57, 6257−6264

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Figure 2. DSC traces for the PCL composites with (a) 3 wt % and (b) 5 wt % particle loadings with the cooling rate of 5 °C min−1.

Figure 1. DSC traces recorded from (a) cooling and (b) second heating scans with the rate of 5 °C min−1 for neat PCL and the composites with 1 wt % particle loadings.

It is well-known that the homogeneous nucleation of a semicrystalline polymer takes place as a result of random fluctuation of order in the mother phase, which requires a large amount of supercooling.31,32 In the presence of foreign body, the heterogeneous nucleation is the prevailing mode of nucleation in phase transformations because it requires far less supercooling. The heterogeneous nucleating agents are commonly crystalline substances and are insoluble in the polymer melt, with a high-energy surface which favors occurrence of sufficient supercooling at the solid−liquid interface.32,33 The foreign bodies with low-energy surface are commonly ineffective nucleating agents because supercooling at the solid−liquid interface is insufficient to nucleate interfacial region before nucleation occurs in the bulk.31 For the pristine SNC in this work, it is clearly a good nucleating agent during PCL crystallization because its hydrophilic surface is incompatible with the PCL matrix. After acetylation, the surface energy of SNC decreases, accompanied by the formation of some amorphous chain structure.11,15 In this case, the phase compatibility is improved, while the nucleation ability of SNC particles may weaken because the thickened interface is against the occurrence of sufficient supercooling at the solid− liquid interface. For the particles filled polymeric composite system, the interaction energy parameter, B, can be used as a measure of phase compatibility:34

Table 1. Thermal Parameter Values Obtained from the Cooling and the Second Heating DSC Scansa samples

Tc (°C)

Tm1 (°C)

Tm2 (°C)

ΔHm (J/g)

Xc (%)

PCL PSN1 PSNL1 PSNM1

28.3 31.0 30.0 26.0

56.0 56.3 56.0 55.7

57.2 57.2 57.1 57.1

67.4 64.5 65.7 66.3

49.6 47.4 48.3 48.7

Tc, crystallization temperature; Tm, melting temperature; ΔHm, melting enthalpy; Xc, degree of crystallinity (Xc = ΔHm/ΔH0m, ΔH0m = 136 J g−1).22 The scan rate was 5 °C min−1. a

filler particles.23−28 However, the nucleation ability decreases with increasing DSs of SNC, and the composite filled with the acetylated SNC with middle-level DS (PSNM1) shows a Tc even lower than that of neat PCL. This is called crystallization temperature depression15 and has been observed on the PCL composites with the functionalized carbon nanotubes29 or surface-modified cellulose nanocrystals.14,15,30 Thus, the filled acetylated SNC, in this case, becomes an antinucleation agent to PCL, instead of a nucleating one. Clearly, the role of SNC particles during PCL crystallization alters with their surface properties. 6259

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Figure 3. Determination of equilibrium melting points (left) and the plots of T0m − T0mix against (1 − ϕi)2 (right) for the PCL composites with various particle loadings. 0 Tm0 − Tmix =−

T0m

BViu 0 Tm(1 − ϕi)2 ΔHiu

B=ω

(2)

T0mix

where and are the equilibrium melting points of polymer and composite, respectively. ΔHiu/Viu is the heat of fusion of polymer per unit volume, and ϕi is the volume fraction of polymer. T0m can be determined by Hoffman−Weeks method.35 B is a function of Flory−Huggins interaction parameter, χ12, (χ12 = BViu/RT36), which can be obtained from the slope of the plots of T0m − T0mix versus (1 − ϕi)2, as shown in Figure 3. Here the literature-reported value of melt enthalpy of ideal PCL crystal (ΔH0m=136 J g−122) is used for calculation. The acetylated SNC particles with lower DS (0.38) are still incompatible with PCL because their composite (PSNL1) shows a positive value of B. The composite (PSNM1) containing the acetylated SNC particles with middle-level DS (1.90), however, has a negative B value, indicating that the two phases can form the thermodynamically stable system at the temperature above the melting crystallization.36−38 This compatible system can even be read as a “miscible” one in the molten state. In this case, those SNC particles nearly lose their nucleation activity, and the short amorphous chains formed after acetylation on their surface even act in the role of diluent agent,15,39,40 decreasing the probability of thermal nucleation of PCL bulk. As a result, PSNM1 has an even lower Tc relative to that of neat PCL. The nucleation activities of those several kinds of SNC particles can be evaluated by the Dobreva and Gutzow41,42 method. The nucleation activity, Na, is defined as the decreased level of the work of three-dimensional nucleation with the addition of a foreign substrate. Na approaches zero if the foreign substrate is extremely active. For nucleation from melts near the melting temperature, Tm, the cooling rate (ϕ) is related to the degree of supercooling ΔTp (ΔTp = Tm − Tc): log ϕ = A −

B 2.3ΔTp2

σ 3Vm2 3kTmΔSm2n

(4)

where A is constant and B is a parameter related to molar volume of the crystallization polymer (Vm), the entropy of melting (ΔSm), and the specific surface energy (σ). k is the Boltzmann constant, n is the Kolmogorov−Avrami exponent, and ω is the geometrical factor. The nucleation activity is a normalized item: Na = B*/B0

(5)

where B* is the value of B when the polymer is filled and B is the value of B when the polymer is unfilled, which can be obtained from the slope of log ϕ versus 1/ΔT2p. The asobtained values of Na are summarized in Figure 4. Clearly, the Na of PSN1 is only 0.83, confirming that the pristine SNC has good nucleation activity. As for PSNM1, its Na is even higher than 1. In this case, Na may lose its physical meaning,14,30 which 0

Figure 4. Plots of log ϕ against 1/ΔT2p for the SNC-filled PCL composites.

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DOI: 10.1021/acs.iecr.8b00514 Ind. Eng. Chem. Res. 2018, 57, 6257−6264

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Figure 5. Polarized light microscopy images of the final spherulite morphology of all samples. Scale bar of 50 μm.

even different nucleating roles. Besides, the presence of SNC particles does not have large influence on the PCL crystal structure (disclosed by the XRD patterns shown in Figure 7) because PCL only has one crystal form.46 3.3. Spherulite Growth Kinetics of PCL/SNC Composites. However, the growth rates of PCL spherulite are strongly dependent on the roles of SNC particles, as shown in Figure 8a. As nucleating agents, pristine SNC and the low-level-acetylated one promote growth of spherulite. Relative to that of PSNL1, the higher growth rate of PSN1 indicates that pristine SNC has better nucleation activity than that of the low-level-acetylated one. PSNM1 shows a decreased growth rate as compared with that of neat PCL, again confirming that middle-level-acetylated SNC particles can restrain crystallization of PCL in PSNM1 due to their antinucleation role. Further information can be revealed by Lauritzen−Hoffman−Miller equation:47,48

suggests that the middle-level-acetylated SNC is no longer a heterogeneous nucleating agent to PCL and that the presence of those particles even impedes the nucleation of PCL. 3.2. Spherulite Morphology and Lamellar Structure. Actually, the altered nucleation activities of those SNC particles can be indicated by the altered spherulite size of PCL indirectly, as shown in Figure 5. It is seen that the composites with pristine and low-level-acetylated SNC show decreased spherulite size as compared with that of neat PCL, confirming that the presence of those two kinds of SNC particles provides additional nucleation sites.14,23,43 However, PSNM1 shows spherulites with almost the same (42 °C) or even slightly higher size (45 °C) than those of the neat PCL sample. This indicates that the middle-level-acetylated SNC is inert or even an antinucleation agent at higher crystallization temperature. The lamellar structure of PCL in those three systems can be further evaluated by Lorentz-corrected SAXS profiles shown in Figure 6. The long period of lamellae (L) and the average lamellar thickness (L̅ c) can be obtained using the 1D correlation function,44,45 as can be seen in Figure 6b. All samples have almost the same lamellar thickness and long period of lamellae within the error ranges. This indicates that the presence of SNC particles does not evidently affect PCL lamellar structure, despite their different nucleation activities or

G = G0 exp[−U */R(Tc − T∞)] exp[−K g /TcΔTf )]

(6)

where G is the spherulitic growth rate, U* is the activation energy for transportation of segments to the nucleation site, Kg is the nucleation constant, and f = 2Tc/(T0m + Tc). R is the gas constant, and T∞ is the hypothetical temperature where viscous flow ceases. G0 is a pre-exponential factor. The T0m value can be obtained by the Hoffman−Weeks procedure.35 6261

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Figure 8. (a) Spherulite growth rates (44 °C) and (b) Lauritzen− Hoffman−Miller plots for all samples.

Figure 6. (a) SAXS patterns and (b) 1D correlation function curves for neat PCL and its composites.

Table 2. Calculated Values of Kinetic Parameters of Spherulite Growth

Figure 8b gives the plots of ln G + U*/R(Tc − T∞) as a function of 1/(TcΔTf) for all samples. The obtained Kg values are listed in Table 2. PSN1 and PSNL1 have decreased Kg values as compared with that of neat PCL. This confirms a lower barrier for nucleation in the presence of pristine and lowlevel-acetylated SNC particles because Kg is proportional to the energy needed for the formation of nuclei with critical size:49

n ib0σσeTm0 ΔHm0k

105 Kg (K2)

σe (erg/cm2)

PCL PSN1 PSNL1 PSNM1

0.83 0.67 0.72 0.85

85.78 69.55 75.12 88.98

where σ and σe are the lateral surface free energy and folding surface free one, respectively, b is the layer thickness, and k is the Boltzmann constant. The Tc studied in this work is in regime II,50−52 and ni is hence valued as 2. The calculated values of σe are also listed in Table 2. The literature-reported ΔH0m (136 J g−1)22 and b0 (0.438 nm)53 values are used. PSNM1 shows an increased value of folding surface free energy relative to that of neat PCL, indicating a higher energy consumption in this sample as the PCL chain folded on the nuclei surface to form crystals.14,30 This is solid evidence of the antinucleation role of middle-level-acetylated SNC particles. Thus, the three kinds of SNCs, the pristine one and ones acetylated with low- and middle-level DSs, function in fully different roles during PCL crystallization, namely, strong nucleating agent, moderate nucleating one, and even an antinucleation one. In another perspective, the role of SNC particles during crystallization of PCL can be tailored precisely by controlling their surface acetylation level. This opens a new door for the applications of SNC particles in the crystallization control of polymers.

Figure 7. XRD patterns for neat PCL and its composites.

Kg =

samples

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(12) Chandra, R.; Rustgi, R. Biodegradable polymers. Prog. Polym. Sci. 1998, 23, 1273−1335. (13) Castillo, R. V.; Muller, A. J. Crystallization and morphology of biodegradable or biostable single and double crystalline block copolymers. Prog. Polym. Sci. 2009, 34, 516−560. (14) Lv, Q. L.; Xu, C. J.; Wu, D. F.; Wang, Z. F.; Lan, R. Y.; Wu, L. S. The Role of nanocrystalline cellulose during crystallization of poly(εcaprolactone) composites: Nucleation agent or not? Composites, Part A 2017, 92, 17−26. (15) Xu, C. J.; Wu, D. F.; Lv, Q. L.; Yan, L. L. Crystallization temperature as the probe to detect polymer-filler compatibility in the poly(ε-caprolactone) composites with acetylated cellulose nanocrystals. J. Phys. Chem. C 2017, 121, 18615−18624. (16) Angellier, H.; Choisnard, L.; Molina-Boisseau, S.; Ozil, P.; Dufresne, A. Optimization of the preparation of aqueous suspensions of waxy maize starch manocrystals using a response surface methodology. Biomacromolecules 2004, 5, 1545−1551. (17) Xu, C. J.; Chen, J. X.; Wu, D. F.; Chen, Y.; Lv, Q. L.; Wang, M. Q. Polylactide/acetylated nanocrystalline cellulose composites prepared by a continuous route: A phase interface-property study. Carbohydr. Polym. 2016, 146, 58−66. (18) Teramoto, N.; Shibata, M. Synthesis and properties of pullulan acetate. Thermal properties, biodegradability, and a semi-clear gel formation in organic solvents. Carbohydr. Polym. 2006, 63, 476−481. (19) Xu, Y.; Ding, W. Q.; Liu, J.; Li, Y.; Kennedy, J. F.; Gu, Q.; Shao, S. X. Preparation and characterization of organic-soluble acetylated starch nanocrystals. Carbohydr. Polym. 2010, 80, 1078−1084. (20) Tserki, V.; Zafeiropoulos, N. E.; Simon, F.; Panayiotou, C. A study of the effect of acetylation and propionylation surface treatments on natural fibres. Composites, Part A 2005, 36, 1110−1118. (21) Strobl, G. R.; Schneider, M. Model of partial crystallization and melting derived from small-angle X-ray scattering and electron microscopic studies on low-density polyethylene. J. Polym. Sci., Polym. Phys. Ed. 1980, 18, 1343. (22) Crescenzi, V.; Manzini, G.; Calzolari, G.; Borri, C. Thermodynamics of fusion of poly-b-propiolactone and poly(εcaprolactone): Comparative analysis of the melting of aliphatic polylactone and polyester chains. Eur. Polym. J. 1972, 8, 449−463. (23) Lv, Q. L.; Ying, Z. R.; Wu, D. F.; Wang, Z. F.; Zhang, M. Nucleation role of basalt fibers during crystallization of poly(εcaprolactone) composites. Ind. Eng. Chem. Res. 2017, 56, 2746−2753. (24) Wu, D. F.; Wu, L.; Sun, Y. R.; Zhang, M. Rheological properties and crystallization behavior of multi-walled carbon nanotube/poly(εcaprolactone) composites. J. Polym. Sci., Part B: Polym. Phys. 2007, 45, 3137−3147. (25) Siqueira, G.; Fraschini, C.; Bras, J.; Dufresne, A.; Prud'homme, R.; Laborie, M. P. Impact of the nature and shape of cellulosic nanoparticles on the isothermal crystallization kinetics of poly(εcaprolactone). Eur. Polym. J. 2011, 47, 2216−2227. (26) Lv, Q. L.; Wu, D. F.; Qiu, Y. X.; Chen, J. X.; Yao, X.; Ding, K. S.; Wei, N. X. Crystallization of poly(ε-caprolactone) composites with graphite nanoplatelets: relations between nucleation and platelet thickness. Thermochim. Acta 2015, 612, 25−33. (27) Mi, H. Y.; Jing, X.; Peng, J.; Salick, M. R.; Peng, X. F.; Turng, L. S. Poly(ε-caprolactone) (PCL)/cellulose nano-crystal (CNC) nanocomposites and foams. Cellulose 2014, 21, 2727−2741. (28) Wu, D. F.; Lin, D. P.; Zhang, J.; Zhou, W. D.; Zhang, M.; Zhang, Y. S.; Wang, D. M.; Lin, B. L. Selective localization of nanofillers: Effect on morphology and crystallization of PLA/PCL blends. Macromol. Chem. Phys. 2011, 212, 613−626. (29) Perez, R. A.; Lopez, J. V.; Hoskins, J. N.; Zhang, B. Y.; Grayson, S. M.; Casas, M. T.; Puiggalí, J.; Muller, A. J. Nucleation and antinucleation effects of functionalized carbon nanotubes on cyclic and linear poly(ε-caprolactone). Macromolecules 2014, 47, 3553−3566. (30) Chen, J. X.; Wu, D. F.; Tam, K. C.; Pan, K. R.; Zheng, Z. G. Effect of surface modification of cellulose nanocrystal on nonisothermal crystallization of poly(β-hydroxybutyrate) composites. Carbohydr. Polym. 2017, 157, 1821−1829.

4. CONCLUSION Three kinds of SNC particles, including a pristine one and ones acetylated with low- and middle-level DSs, were used as modifier to control the crystallization of PCL. The pristine and low-level-acetylated SNC particles act as the heterogeneous nucleating agent role, increasing the crystallization temperature and spherulite growth rate of PCL, but the former is much stronger than the latter because improved phase adhesion is in opposition to nucleation activity. Therefore, the middle-levelacetylated SNC particles even play the role of antinucleation agent, impeding nucleation and followed spherulite growth of PCL, but the lamellar structure of PCL does not depend on the presence of SNC particles. This work provides an interesting and also effective idea to control crystallization of biodegradable aliphatic polyesters with SNC particles.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-514-87975230. Fax: +86-514-87975244. E-mail: [email protected]. ORCID

Defeng Wu: 0000-0002-8934-7646 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (51573156) and the Jiangsu Provincial Research Innovation Program for Graduates (SJZZ16_0259) for the financial support.

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REFERENCES

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Industrial & Engineering Chemistry Research

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DOI: 10.1021/acs.iecr.8b00514 Ind. Eng. Chem. Res. 2018, 57, 6257−6264