Crystallization, Crystal Structure, and Isothermal Melt Crystallization

Article pubs.acs.org/JPCB

Crystallization, Crystal Structure, and Isothermal Melt Crystallization Kinetics of Novel Polyamide 6/SiO2 Nanocomposites Prepared Using the Sol−Gel Technique Fatima Zohra Rafique and Nadarajah Vasanthan* Department of Chemistry, Long Island University, One University Plaza, Brooklyn, New York 11201, United States ABSTRACT: Polyamide 6/SiO2 (PA6/SiO2) nanocomposites with varying amounts of SiO2 were prepared by using a novel sol−gel technique. These nanocomposites were formed in situ by hydrolysis and through the condensation of tetraethoxysilane (TEOS) using formic acid with a small amount of water as the solvent for PA6. Observations of TGA showed that the thermal stability of PA6 nanocomposite was significantly improved compared to that of neat PA6. Microstructure development during the thermally induced crystallization of PA6/SiO2 nanocomposites was investigated with a combination of differential scanning calorimetry (DSC), FTIR spectroscopy, scanning electron microscopy (SEM), and AFM. FTIR spectroscopy was used to determine the crystal form of these nanocomposites, and it was concluded that SiO2 nanoparticles have the γ-nucleating effect. The crystallinity of nanocomposites decreased with increasing TEOS loading as compared to that for neat PA6. SEM showed a very fine dispersion of nanoscale silica whereas SEM and Zetasizer proved the silica particle size was about 100−200 nm. The isothermal crystallization kinetics of these nanocomposites with increasing SiO2 content were investigated, and it was shown that the amount of SiO2 plays a significant role in crystallization kinetics.

1. INTRODUCTION Polymer-based nanocomposites have received increasing interest due to their unexpected optical, electrical, thermal, and mechanical properties.1−4 The most commonly used nanofillers are carbon nanotubes (one-dimensional), montmorillonite clay (two-dimensional), and SiO2 nanoparticles (three-dimensional).5 Several studies have been conducted on the formation and characterization of polymer nanocomposites with one-dimensional, two-dimensional, or three-dimensional nanoparticles such as polyamide/carbon nanotubes,6 polyamide/clay,7,8 poly(butylene succinate),9 poly(L-lactic acid)/clay,10 poly(trimethylene terephthalate)/silica,11 poly(vinyl acetate)/silica,12 and epoxy/silica13 nanocomposites. The effect of inorganic nanofillers on the crystallization kinetics of semicrystalline polymers has been studied extensively,14−18 and studies have shown that nanoparticles can act either as effective nucleating agents during nucleation, or they can reduce diffusion of the chain during growth. The characteristics of polymer clay nanocomposites are strongly dependent on the type of interaction between nanoparticles, the polymer matrix, and the spatial arrangement of these nanoparticles.19,20 The interface between nanoparticles and the polymer matrix plays an important role in uniformly dispersing the particles.21,22 Despite many efforts in the past, conventional ways of preparing polymer nanocomposites, such as melt blending, intercalation, and in situ intercalative polymerization have their deficiencies.23−25 The sol−gel approach is another technique used to synthesize nanomaterial from atomic or molecular species via chemical © 2014 American Chemical Society

reactions, allowing for the precursor particles to grow in size.26−30 The synthesis of organic−inorganic nanocomposites by the sol−gel process has received considerable interest in recent decades due to its low temperature processing and better control over dispersion. Nylon-66, poly(vinyl acetate), polypropylene, and poly(vinyl alcohol) nanocomposites were prepared using the sol−gel technique.25,31−34 These nanocomposites were fabricated by forming an inorganic phase from the hydrolysis−condensation reaction of alkoxysilanes, such as tetraethoxysilane (TEOS), in a solution containing organic polymers. It has been shown that nanocomposites prepared using this method exhibit good dispersion throughout the polymer matrix. Because PA6 is the most frequently used (and most successful) synthetic polymer, its crystal structure and mechanical properties have been studied extensively.35−40 PA6 crystallizes into two well-known crystal forms: the α and the γ. The α crystalline phase of PA6 is the most commonly observed phase, and it can be obtained by slowly cooling from the melt state. The γ phase can be obtained by fast quenching from the melt state or by treating the α phase with an iodine−potassium iodide aqueous solution, followed by removal of the absorbed iodine by sodium thiosulfate. The γ nucleation effect has been reported for PA6 in the presence of nanofillers such as clay, zinc oxide, and mica.41−45 This Received: May 22, 2014 Revised: July 14, 2014 Published: July 15, 2014 9486

dx.doi.org/10.1021/jp505046v | J. Phys. Chem. B 2014, 118, 9486−9495

The Journal of Physical Chemistry B

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sample. The reported crystallinity values are the average of at least three determinations. Isothermal melt crystallization kinetic studies of PA6 and PA6 nanocomposites were also performed using PerkinElmer DSC 7. The PA6 and PA6 nanocomposite samples were heated from room temperature to 250 °C at 10 °C/min, held for 3 min to erase the prior thermal history, and subsequently cooled to their predetermined crystallization temperature at the cooling rate of 100 °C/min and held for 1 h to complete the melt crystallization. The exothermic scan of heat flow versus time was used for kinetic analysis. 2.5. Microscopy and Nano Zetasizer. Surface topology and dispersion of the nanocomposites within the polymer matrix was determined by an Asylum MFP 3D BIO Atomic Force Microscope (AFM) at the College of Staten Island, City University of New York, New York. The size and distribution of the nanocomposites was determined by SEM in the range 1−20 μm at the material characterization laboratory at the New Jersey Institute of Technology (NJIT). A nano Zetasizer from the Malvern Instrument was used to measure particle size in the range 0.6−6000 nm by Dispersion Technology Software. It was used to determine the particle size of silica nanoparticles using an alternate technique in the range 1−1000 nm. The reported mean values were based on three measurements with 10 cycles for each sample.

study aims to examine the synthesis of PA6/silica nanocomposites using an in situ sol−gel process. This will help determine the role of nanoparticles in isothermal melt crystallization kinetics and help understand the development of various crystalline forms of PA6 in the presence of silica nanoparticles in a series of PA6 silica nanocomposites.

2. EXPERIMENTAL SECTION 2. 1. PA6/Silica Nanocomposite Fabrication. Polyamide 6 (PA6), which has a molecular weight of 80 000, was purchased from Sigma-Aldrich. Eighty-eight percent formic acid, as well as tetraethoxysilane (TEOS), was also acquired from Sigma-Aldrich. Two methods were used to prepare the nanocomposites. A 10 wt % PA6 solution was prepared by dissolving a commercial grade of PA6 in 88% formic acid by heating it for 15−20 min, and subsequently 1, 2, 3, 4, and 5 wt % of tetraethoxysilane (TEOS) samples based on the amount of PA6 were added to a 10 w/v % PA6 solution. The solution was stirred vigorously with a magnetic bar at room temperature. The stirring was carried out for ∼1 h at an ambient temperature. The solutions were kept at room temperature for ∼24 h to allow hydrolysis−condensation reactions of TEOS to continue. The solutions were then poured onto cleaned glass plates and dried at room temperature. Because the resulting nanocomposite films were thick and opaque, they were melt-pressed between Teflon sheets at 250 °C to obtain thinner films. PA6 films without nanoparticles were also prepared in the same manner. 2.2. FTIR Spectroscopy. FTIR spectra were recorded using a Nicolet Magna-IR 760 spectrometer in the spectral range from 4000 to 400 cm−1 at a resolution of 2 cm−1. The OMNIC software was used to determine the peak position and absorbance of various bands. 2.3. Thermogravimetric Analysis. The degradation, thermal stability, and silica content in the PA6 nanocomposite films was determined using PerkinElmer TGA 7. The samples were heated to 800 °C with a heating rate of 10 °C/min for 90 min under a nitrogen atmosphere. The thermal degradation temperature, Td, was calculated by drawing a tangent line and determining the onset value. The wt % silica was calculated using curve plateaus. 2.4. Differential Scanning Calorimetry. DSC experiments were performed with a PerkinElmer DSC 7. The instrument was calibrated for temperature and heat of fusion using standard indium (Tm = 156.6 °C and ΔH = 28.5 J/g). All experiments were performed under a nitrogen atmosphere with a flow rate of 20 mL/min to minimize oxidation. Four to six milligram samples were used for all measurements. The onset values were taken as transition temperatures. Heat of fusion was determined by integrating the area under the melting peak of the DSC curves and was directly proportional to the degree of crystallinity. The heat of fusion of 100% crystalline PA6 was taken as 188 J/g and used to determine the weight percent crystallinity of neat PA6 and PA6 nanocomposites. The degree of crystallinity of nanocomposite χc was determined using the following (eq 1). Xc =

ΔHf × 100% (1 − ϕ)ΔH *

3. RESULTS AND DISCUSSION 3.1. Preliminary Characterization. It is important to assess the dispersion of silica in the PA6 matrix to understand the physical and mechanical properties of nanocomposites and the morphology of PA6-silica nanocomposites prepared by the hydrolysis and condensation of TEOS. These nanocomposites’ morphologies were investigated using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The SEM images of PA6 nanocomposites containing 1%, 3%, and 5% TEOS by weight are shown in Figure 1. These images show uniform dispersion/distribution of nanoscale spherical silica particles within the PA6 matrix up to 3% TEOS. However, the aggregation/agglomeration of nanoparticles was observed for PA6 containing 5% TEOS. Silica particle size estimated using SEM varies between 100−200 nm. Figure 1c shows that the dispersion of silica particles is not uniform for the PA6 film containing 5% TEOS. The surface distribution of silica particles are characterized by atomic force microscopy (AFM). The AFM phase image of neat PA6 and PA6 containing 0% and 3% TEOS ranging from 0−500 μm are shown in Figure 2. It is evident from Figure 2b,c that the surface roughness and average height of the film surface increased with increasing silica content. The distribution of the peaks is uniform, suggesting that homogeneous dispersion is achieved up to 3% TEOS. The size of the SiO 2 nanoparticles was characterized using nano Zetasizer measurements, and the average size of the nanoparticles was found to be within the range 150−200 nm, which is in close agreement with the SEM data. Figure 3 shows the FTIR spectra of neat PA6 and its PA6 nanocomposites in the range of 4000 to 400 cm−1. The neat PA6 films show the characteristic peaks at 3300 cm−1 (N−H stretch), 2938 cm−1 (CH2 stretch), 1640 cm−1 (CO stretch), and 1540 cm (N−H deformation), and the bands at 900−1000 cm−1 are assigned to various amide vibrations.36−38 The bands occurring in the region between 1000 and 1100 cm−1 in the spectrum of nanocomposites are

(1)

where ΔHf is the heat of fusion of PA6 nanocomposite samples, ΔH* is the heat of fusion of 100% crystalline PA6, and ϕ is the weight fraction of TEOS content in the PA6 9487



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increases with increasing SiO2 content. It appears that conformational flexibility of PA6 increases due to the weakening of hydrogen bonding during the formation of nanocomposites. The characteristic IR bands of α and γ crystal forms of pure PA6 have been reported by various research groups. The bands at 960, 930, and 1200 cm−1 are attributed to the α crystalline phase, whereas the band at 973 cm−1 is attributed to the γ-crystalline phase.35−38 The 1170 cm−1 band consists of both the amorphous and crystalline phases of the polymer and are often used as a reference band to characterize the microstructure of PA6.38 The FTIR spectrum of PA6 and its nanocomposite films prepared by fast quenching from the melt in the range from 1200 to 900 cm−1 are shown in Figure 4. It can be observed that the PA6 nanocomposites predominantly crystallize in the γ crystal form, whereas neat PA6 crystallizes predominantly in the α crystal form. However, the PA6 nanocomposite containing 4% and 5% TEOS crystallizes in the α form along with the γ form. The absorbance ratio of the IR band at 973 cm−1 to the band at 1170 cm−1 was calculated, and it was shown that the absorbance ratio increased with increasing SiO2 content. The γ nucleating effect has been reported for various PA6 nanocomposites containing nanofillers such as mica, sepiolite, and zinc oxide, and our observation is consistent with these findings.41−45 It has been shown that the γ crystals appear closer to the particle surfaces than the α crystals. The IR spectra of neat PA6 and PA6 nanocomposites with 3 wt % SiO2 prepared via fast quenching and slow cooling from the melt were compared, and no significant difference in the IR spectra were observed, suggesting that γ crystal formation does not depend on the way PA6 and PA6 nanocomposite films are prepared. 3.2. Thermal Characterization. The thermal stability of materials is usually characterized by TGA, in which the weight loss is monitored as a function of temperature. The TGA measurements of neat PA6 and its nanocomposites with varying amounts of silica were carried out, and the silica content determined in each nanocomposite film from TGA is reported in Table 1, along with theoretical silica content and the temperature at which 10%, 50%, and 90% weight loss occurs. The thermal degradation temperature of PA6 nanocomposites increases significantly compared to that for neat PA6, suggesting that the thermal stability of nanocomposites increased compared to that of neat PA6. The actual silica content determined by TGA curves appears only slightly lower than the theoretical amount given in Table 1. It revealed that the sol−gel reaction was almost complete in all cases. The increase in thermal stability of nanocomposites was probably caused by strong secondary interactions of PA6 chains and silica. The intramolecular hydrogen bonding within the PA6 matrix may decrease due to the strong secondary force between PA6 and silica particles, and this is supported by our FTIR observation. Figure 5 displays the first heating and cooling differential scanning calorimetric (DSC) traces observed at a heating and cooling rate of 10 °C/min for neat PA6 and PA6 nanocomposites. A DSC scan of neat PA6 shows a melting point (Tm) of 217 °C and a nonisothermal melt crystallization temperature (Tc) of 199 °C. The effect of SiO2 nanoparticles on Tm and Tc of PA6 can be clearly seen in the DSC scans of PA6 nanocomposites. A summary of the thermal properties, including the onset of melting and crystallization temper-

Figure 1. SEM images of PA6 nanocomposites with (a) 1 wt %, (b) 3 wt %, and (c) 5 wt % TEOS.

assigned to the asymmetric Si−O−Si stretching vibration and increase in absorbance in this region compared to those for neat PA6, which is attributed to the formation of an Si−O−Si bond. A vibration at 800 cm−1 is assigned to the symmetric Si−O−Si stretching vibration, whereas the band at 1080 cm−1 is attributed to the asymmetric Si−O−Si stretching vibration. A shoulder centered near 3400 cm−1 is assigned to the hydrogen-bonded Si−O−H (silanol) stretching vibrations. It should be noted that the half-width of the band at 3300 cm−1, assigned to the N−H stretching vibration of neat PA6, 9488



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Figure 2. AFM phase images in the range 0−500 μm of (a) PA6, (b) PA6 nanocomposite with 1 wt % SiO2, and (c) PA6 nanocomposite with 3 wt % TEOS loading.

atures, are tabulated in Table 2. Table 2 shows that the Tm of nanocomposites shifts to a lower temperature with increasing silica content as compared to that of neat PA6. A similar trend is also observed for the Tc of nanocomposites compared

to that of the neat PA6. Figure 6 displays the nonisothermal melt crystallization of PA6 and its nanocomposites as a function of time. It can be seen from Figure 6 that the rate of crystallization is lower for PA6 nanocomposites than for neat 9489



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Figure 3. FTIR spectra of (a) pure PA6, (b) 1 wt % TEOS, (c) 3 wt % TEOS loading in the region from 4000 to 800 cm−1.

Figure 4. FTIR spectra of neat PA6 and PA6 nanocomposites in the region from 1200 to 800 cm−1.

Table 1. TGA Data of PA6 Nanocompositesa T90% (°C) T50% (°C) T10% (°C) wt % SiO2 (TGA)

PA6

PA6-1

PA6-3

PA6-5

347 407 448 0

359 432 510 0.3

368 435 511 0.85

369 442 512 1.3

the hydrogen bonding interaction between CO and N−H groups between neighboring PA6 chains. 3.3. Isothermal Melt Crystallization Kinetics. Isothermal melt crystallization kinetics was studied by DSC to see the effect of SiO2 on crystallization kinetics of PA6. Figure 7a shows the development of relative crystallinity with crystallization time for neat PA6 at a temperature ranging from 185 to 200 °C. It is shown in Figure 7a that the rate of crystallization decreases with increasing crystallization temperature. Panels b−d of Figure 7 show the development of the relative crystallinity of PA6 nanocomposites containing 1, 3, and 5 wt % TEOS, respectively. All of them show decreases in crystallization rate with increasing crystallization temperature. Figure 8 shows the relative percent crystallinity of neat PA6 and its nanocomposites at different temperatures as a function of time. It can be seen from Figure 8a that the rate of

a

T10%, T50%, and T90% indicate the temperatures at which 10%, 50%, and 90% weight loss occurs.

PA6. The percent crystallinity of PA6 and its nanocomposites was calculated using eq 1, and the results are also included in Table 2. It can be seen from Table 2 that percent crystallinity decreases with increasing silica content. A similar observation has recently been reported for PA66 and PA6 containing various nanoparticles. The decrease in overall crystallinity with increasing SiO2 nanoparticles may be attributed to weakening 9490



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Figure 5. DSC heating and cooling scans of pure PA6 and its nanocomposites with a heating and cooling rate of 10 °C/min.

200 °C up to 3 wt %. The relative crystallinity development for PA6 film containing 5 wt % TEOS is scattered at different temperatures, indicating again a nonuniform dispersion of SiO2 particles within the PA6 matrix. Isothermal melt crystallization kinetics on PA6 and its nanocomposites can further be studied using the classical Avrami equation, as follows:

Table 2. ΔHm, ΔHc, and % Crystallinity of Neat PA6 and Its Nanocomposites (1% to 5 wt % TEOS) Based on DSC Curves sample (wt %)

ΔHm (J/g)

Tm(onset) (°C)

ΔHc (J/g)

Tc(onset) (°C)

% crystallinity

neat P6 1 2 3 4 5

73.09 54.52 51.78 44.79 52.03 47.11

217.2 216.7 216.3 215.7 215.7 215.3

−67.27 −85.85 −84.56 −83.67 −85.96 −78.59

199.25 192.58 191.26 191.97 191.03 191.62

39 29 28 24 29 27

1 − χt = exp( −kt n)

(2)

where χt is the crystalline fraction at time t; n is the Avrami exponent, which normally ranges between 1 and 4, depending on the nature of nucleation and growth geometry of the crystals; k is the rate constant; and t is the crystallization time. This equation can be further expanded by taking double natural logarithms, as follows:

crystallization decreases with an increasing % of TEOS up to 3 wt % at crystallization temperature of 185 °C. However, the 5 wt % TEOS has a crystallization fairly close to the 1 wt % due to the nonuniform dispersion of the film at this higher concentration. A similar trend is observed at 190, 195, and

ln[− ln(1 − χt )] = n ln t + ln k 9491

(3)



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Figure 6. Nonisothermal crystallization exotherm versus time for pure PA6 and its nanocomposites at a cooling rate of 10 °C/min.

Figure 7. Relative percent of crystallinity of (a) pure PA6, (b) 1 wt % TEOS, (c) 3 wt % TEOS, and (d) 5 wt % TEOS as a function of time at different temperatures.

calculations. The Avrami exponent, n, is dependent on the crystallization mechanism. The values of n obtained for the neat polymer and its nanocomposites ranges from 1.9 to 3.1, which suggest two-dimensional to three-dimensional crystal development. It was seen that silica loading has no significant effect on the “n” value, but that melt crystallization temperature has a significant effect on the “n” value. The “n” value appeared to increase with increasing crystallization temperature from 185 to 200 °C. Crystallization kinetics can be further studied by plotting the half-time of crystallization, t1/2, versus temperatures. The

The Avrami exponent, n, and rate constant, k, for neat PA6 and its nanocomposites were obtained from the plot of ln[−ln(1 − χt)] versus ln t at different crystallization temperatures from 180 to 200 °C. Figure 9 illustrates a sample Avrami plot of neat PA6 and its nanocomposites melt crystallized at 195 °C, and similar plots were observed for other crystallization temperatures (not shown here). A linear regression, y = mx + b, was used to determine the n and k values from Figure 9. The data above 75% crystallinity can lead to a deviation from linearity; therefore, the data between 5% and 75% relative crystallinity were used for the 9492



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Figure 8. Relative percent of crystallinity of neat PA6 and its nanocomposites with different amount of TEOS at (a) 185 °C, (b) 190 °C, (c) 195 °C, and (d) 200 °C.

Figure 9. Avrami plots of ln[−ln(1 − χt)] versus ln t for the neat PA6 and its nanocomposites annealed at 195 °C.

t1/2 is the time at which 50% of the relative crystallinity is obtained. These values were determined from the DSC curves of neat PA6 and its nanocomposites. The t1/2 of PA6 and its nanocomposites are plotted against crystallization temperatures between 185 and 200 °C in Figure 10. It can be observed from the plot that neat PA6 has a lower t1/2 than its nanocomposites at all four temperatures. This observation confirms that neat PA6 has a faster crystallization rate than all of its nanocomposites. The t1/2 values for 5% TEOS are observed to be scattered at all temperatures. This further confirms that PA6 nanocomposites with 5% TEOS are not dispersed uniformly.

lization studies of these nanocomposites were conducted using differential scanning calorimetry (DSC) and FTIR spectroscopy. It was shown that the dispersion of nanoparticles is uniform up to 3% silica content and that nonuniform dispersion was apparent for those containing more than 3% silica. PA6 predominantly forms an α crystal structure during thermally induced crystallization, whereas polyamide 6 nanocomposites crystallize in a γ crystal structure. Tm decreases slightly with increasing silica content as compared to that for neat PA6. The crystallinity of PA6/ silica nanocomposites decreases with increasing silica loading. Isothermal melt crystallization kinetics was carried out using DSC and analyzed using Avrami theory. The Avrami exponent, n, was found to vary from 1.9 to 3 with crystallization temperature, suggesting two- to three-dimensional growth. No significant change in Avrami constant was

4. CONCLUSIONS Polyamide 6/SiO2 nanocomposites were successfully prepared and confirmed using scanning electron microscopy. Crystal9493



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Figure 10. Changes in crystallization half-time, t1/2, as a function of different melt crystallization temperature for neat PA6 and its nanocomposites. (7) Messersmith, P. B.; Ginnelis, E. P. Polymer-Layered Silicate Nanocomposites: In-situ Intercalative Polymerization of ε-Caprolactone in Layered Silicates. Chem. Mater. 1993, 5, 1064−1066. (8) Liu, Y.; Zhang, G.; Feng, M.; Zhang, Y.; Yang, M.; Shen, D. Hydrogen Bonding in Polyamide 66/Clay Nanocomposites. J. Polym. Sci., Part B: Polym. Phys. 2003, 41 (63), 2313−2321. (9) Ray, S. S.; Okamoto, K.; Okamoto, M. Structure-Property Relationship in Biodegradable Poly(butylene succinate)/Layered Silicate Nanocomposites. Macromolecules 2003, 36, 2355−2367. (10) Krikorian, V.; Pochan, D. Unusual Crystallization Behavior of Organoclay Reinforced Poly(L-lactic acid) Nanocomposites. Macromolecules 2004, 37, 6480−6491. (11) Yao, C.; Yang, G. Poly(trimethylene terephthalate)/Silica Nanocomposites Prepared by Dual In-situ Polymerization: Synthesis, Morphology, Crystallization Behavior and Mechanical Properties. Polym. Int. 2010, 59, 492−500. (12) Landry, C. J. T.; Coltrain, B. K.; Landry, M. R.; Fitzgerald, J. J.; Long, V. K. Poly(vinyl acetate)/Silica Filled materials: Material Properties of In-situ vs Fumed Silica Particles. Macromolecules 1993, 26, 3702−3712. (13) Afzal, A.; Siddiqi, H. M. A Comprehensive Study of the Bicontinuous Epoxy-Silica Hybrid Polymers: I. Synthesis, Characterization and Glass Transition. Polymer 2011, 52, 1345−1355. (14) Miri, V.; Elkoun, S.; Peurton, F.; Vanmansart, C.; Lefebvre, J.M.; Krawczak, P.; Seguela, R. Crystallization Kinetics and Crystal Structure of Nylon6-clay Nanocomposites: Combined Effects of Thermomechanical History, Clay Content, and Cooling Conditions. Macromolecules 2008, 41, 9234−9244. (15) Linkoln, D. M.; Vaia, R. A.; Krishnamoorti, R. Isothermal Crystallization of Nylon-6/Montmorillonite Nanocomposites. Macromolecules 2004, 37, 4554−4561. (16) Liu, X.; Wu, Q. Non-isothermal Crystallization Behaviors of Polyamide 6/Clay Nanocomposites. Eur. Polym. J. 2002, 38, 1383− 1389. (17) Fornes, T. D.; Paul, D. R. Crystallization Behavior of Nylon 6 Nanocomposites. Polymer 2003, 44, 3945−3961. (18) Vasanthan, N.; Ly, H.; Ghosh, S. Impact of Nanoclay on Isothermal Cold Crystallization Kinetics and Polymorphism of Poly(L-lactic acid) Nanocomposites. J. Phys. Chem. B 2011, 115, 9556−9562. (19) Rao, Y.; Pochan, J. M. Mechanics of Polymer-Clay Nanocomposites. Macromolecules 2007, 40, 290−296. (20) Bousmina, M. Study of Intercalation and Exfoliation Processes in Polymer Nanocomposites. Macromolecules 2006, 39, 4259−4263.

observed with varying silica content. The crystallization half time, t1/2, increased with increasing silica content, suggesting the rate of crystallization decreases with increasing silica content.

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AUTHOR INFORMATION

Corresponding Author

*N. Vasanthan. E-mail: [email protected]. Phone: 718-246-6328. Fax: 718-488-1465. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support from U.S. Department of Education (ARCC Program). The authors thank Peter Ha for his help with the figures and the College of Staten Island, City University of New York for the use of their microscopy facility.

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REFERENCES

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Crystallization, Crystal Structure, and Isothermal Melt Crystallization

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