Crystallization Kinetics Study of Poly(
L-lactic acid)/Carbon
Nanotubes Nanocomposites
YEONG-TARNG SHIEH,1YAWO-KUO TWU,2CHEAN-CHENG SU,1RONG-HSIEN LIN,3GIN-LUNG LIU4 1
Department of Chemical and Materials Engineering, National University of Kaohsiung, Kaohsiung 811, Taiwan 2
Department of Bioindustry Technology, Dayeh University, Dacun, Changhua 51591, Taiwan 3
Department of Chemical and Material Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan 4
Department of Chemical and Materials Engineering, National Yunlin University of Science and Technology, Douliou, Yunlin 640, Taiwan
Received 11 October 2009; revised 20 January 2010; accepted 31 January 2010 DOI: 10.1002/polb.21986
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT:Effects of carbon nanotubes (CNT) on the isother-mal crystallization kinetics of poly(L-lactic acid) (PLLA) were quantitatively investigated using the Avrami equation and the secondary nucleation theory of Lauritzen and Hoffman. CNT via grafting modification with PLLA could well disperse in the PLLA matrix and give significantly enhanced crystallization rate and crystallinity of PLLA as analyzed by differential scanning calorimetry and polarized optical microscopy. Analysis of iso-thermal crystallization kinetics using the Avrami equation demonstrated that CNT significantly enhanced the bulk crystal-lization of PLLA. Analysis of spherulite growth kinetics using the secondary nucleation theory of Lauritzen and Hoffman
found that CNT could expand the temperature range of the crystallization regime III of PLLA. Values of the nucleation con-stant (Kg) in crystallization regimes III and II of PLLA both increased with increasing CNT contents. The KgIII/KgII ratios were found to be close to the theoretical value 2 but were not clearly found to depend on the CNT contents. VC 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 983–989, 2010
KEYWORDS:carbon nanotubes; crystallization kinetics; crystalli-zation; differential scanning calorimetry (DSC); nanocompo-sites; poly(L-lactic acid); polyesters
INTRODUCTION Due to its high aspect ratio and high me-chanical strength, carbon nanotube (CNT) has been investi-gated as a reinforcement for polymers to improve their me-chanical properties. Mechanical properties of several polymers reinforced by CNT have been reported.1–10 Well dispersion of CNT in polymer matrices is critical to achieve such reinforced mechanical properties. Without any modifi-cation, CNT can not effectively disperse in a polymer matrix and, consequently, can not effectively reinforce mechanical property of the polymer. A generally adopted methodology for the modification of CNT by most research groups is to functionalize CNT by strong acids, followed by grafting with a polymer or a low molecular weight compound that is structurally identical to, compatible with, or reactive with the polymer matrix. The grafting modification has been dem-onstrated to be an effective method to significantly increase mechanical strength of polymers in several previous articles.7–10 Among other factors, the enhancement of crys-tallization in crystalline polymer matrices by CNT serving as a nucleating agent is also critical to the improvement of mechanical properties of polymers.
In this study, we thus focus our attention on the crystalliza-tion kinetics, following previous study on effects of CNT on mechanical property, thermal property, and morphology of poly(L-lactic acid) (PLLA),11,12 which is a biodegradable, biocompatible, and crystalline polymer, being popularly used as a biomedical material. Although several articles have reported the preparations and characterizations of PLLA/ CNT composites,11–17 crystallization kinetics was seldom investigated. Only recently, Qiu and coworkers18,19 studied the effect of carboxyl-functionalized CNT on overall crystalli-zation rate and hydrolytic degradation of PLLA. They pre-pared PLLA/CNT nanocomposites via solution blending of a commercial PLLA and a commercial carboxyl-functionalized multiwalled CNT without a further grafting modification. They found that the overall crystallization rate and hydro-lytic degradation of PLLA increased with increasing CNT content. In this study, PLLA is grafted onto the acyl chloride-functionalized CNT to form PLLA-grafted CNT (or CNT-g-PLLA). The CNT-g-PLLA can be seen to well disperse in the PLLA matrix11,12and significantly enhance the nonisothermal melt-crystallization from the melt and the cold-crystallization
Correspondence to: Y.-T. Shieh (E-mail: [email protected])
Journal of Polymer Science: Part B: Polymer Physics, Vol. 48, 983–989 (2010)VC2010 Wiley Periodicals, Inc.
from the glassy state as investigated by a differential scan-ning calorimeter (DSC).11 Following previous study on the melting behavior, morphology, mechanical properties, and crystallization thermodynamics of the PLLA/CNT-g-PLLA composites,11,12 we in this study quantitatively analyze the isothermal crystallization kinetics via the use of the Avrami equation and analyze the spherulite growth kinetics via the use of the secondary nucleation theory of Lauritzen and Hoffman. The effects of CNT on both the bulk crystallization kinetics and the spherulite growth kinetics are emphatically investigated. Study of this kind was not seen in literature for the PLLA/CNT-g-PLLA composites although it was seen ear-lier for the Nylon 6/CNT composites.20
EXPERIMENTAL
Materials and Sample Preparations
Materials and sample preparations of the PLLA/CNT-g-PLLA nanocomposites of 100/0, 100/1, 100/2, and 100/5 were the same as those in a previous study.11 Atomic force micro-scopy and dynamic mechanical analysis demonstrated that, via grafting modification of CNT by PLLA, CNT was well dispersed in PLLA matrix and the resulting nanocomposites were compatible.11,12
DSC Measurements
For analysis of bulk isothermal crystallization kinetics using the Avrami equation, the samples were held in melt at 200 C in an oven to erase all previous crystalline history of PLLA and then quickly moved to differential scanning calo-rimeter (DSC, TA Instruments, Q100) preliminarily set at a temperature [i.e., the crystallization temperature (Tc)] at which the isothermal DSC scan was recorded. The moving of the melt from the oven to the DSC cell was quick enough to ensure that the samples did not crystallize before the iso-thermal crystallization in DSC. The Tc included 110, 115, 120, and 130 C. If the samples were placed in the DSC to erase their previous crystalline history followed by an in situ quick cooling to a Tc for isothermal crystallizations, exother-mic crystallizations could occur during the DSC cooling before thermally equilibrating at the Tc for the isothermal crystallizations. The bulk isothermal crystallization kinetics of samples can be analyzed using the Avrami equation21 as in eq 1:
1 Xt¼ exp½ktn (1)
where Xtis the relative crystallinity at crystallization time t, n is the Avrami exponent, and k is the rate constant. The rel-ative crystallinity Xtat time t can be obtained by calculation using eq 2:
FIGURE 1Relative crystallinity (Xt) of PLLA/CNT-g-PLLA of (*) 100/0, (&) 100/1, (^) 100/2, and () 100/5 isothermally crystal-lized at 110C as a function of crystallization time.
FIGURE 2Plots of ln [ln(1Xt)] versus ln t for PLLA/CNT-g-PLLA of (l) 100/0, (n) 100/1, (^) 100/2, and (~) 100/5 which were isothermally melt-crystallized at 110C.
TABLE 1Avrami Exponent, n, Rate Constant, k (min2n), and Reciprocal Crystallization Half-Time, 1/t1/2(min21), of Pure PLLA and the PLLA/CNT-g-PLLA Composites of 100/1, 100/2, and 100/5 Which were Isothermally Crystallized at Various Temperatures
Tc (C) 100/0 100/1 100/2 100/5 n k 1/t1/2 n k 1/t1/2 n k 1/t1/2 n k 1/t1/2 110 2.0 0.0152 0.1481 1.8 0.0517 0.2364 2.0 0.0633 0.3015 2.1 0.1307 0.4518 115 2.0 0.0399 0.2399 1.8 0.0747 0.2901 2.0 0.0730 0.3245 1.9 0.1547 0.4541 120 2.0 0.0307 0.2105 1.7 0.1168 0.3508 1.9 0.1390 0.4293 1.9 0.2045 0.5260 130 2.0 0.0186 0.1638 1.8 0.0960 0.3335 2.0 0.0870 0.3543 1.8 0.1580 0.4398
JOURNAL OF POLYMER SCIENCE: PART B:POLYMER PHYSICSDOI 10.1002/POLB
of the Lauritzen-Hoffman equation that values of the nuclea-tion constant (Kg) in both crystallization regimes III and II of PLLA increased with increasing CNT contents was consistent with the finding from the analysis of the Avrami equation that CNT could enhance the isothermal crystallization rate. The KgIII/KgII ratios were found to be close to the theoreti-cal value 2 but were not clearly found to depend on the CNT contents.
The authors are grateful for the financial support from National Science Council of Taiwan under contract NSC 962221E390 -033 -MY2.
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POLY(L-LACTIC ACID)/CARBON NANOTUBES NANOCOMPOSITES, SHIEH ET AL. 989
