Crystallization, Crystal Structure, and Isothermal Melt CrystallizationKinetics of Novel Polyamide 6/SiO2 Nanocomposites Prepared Usingthe Sol−Gel TechniqueFatima 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 varyingamounts of SiO2 were prepared by using a novel sol−gel technique. Thesenanocomposites were formed in situ by hydrolysis and through thecondensation of tetraethoxysilane (TEOS) using formic acid with a smallamount of water as the solvent for PA6. Observations of TGA showed that thethermal stability of PA6 nanocomposite was significantly improved comparedto that of neat PA6. Microstructure development during the thermally inducedcrystallization of PA6/SiO2 nanocomposites was investigated with acombination of differential scanning calorimetry (DSC), FTIR spectroscopy,scanning electron microscopy (SEM), and AFM. FTIR spectroscopy was usedto determine the crystal form of these nanocomposites, and it was concludedthat SiO2 nanoparticles have the γ-nucleating effect. The crystallinity ofnanocomposites decreased with increasing TEOS loading as compared to thatfor neat PA6. SEM showed a very fine dispersion of nanoscale silica whereas SEM and Zetasizer proved the silica particle size wasabout 100−200 nm. The isothermal crystallization kinetics of these nanocomposites with increasing SiO2 content wereinvestigated, and it was shown that the amount of SiO2 plays a significant role in crystallization kinetics.

1. INTRODUCTION

Polymer-based nanocomposites have received increasinginterest due to their unexpected optical, electrical, thermal,and mechanical properties.1−4 The most commonly usednanofillers are carbon nanotubes (one-dimensional), mont-morillonite clay (two-dimensional), and SiO2 nanoparticles(three-dimensional).5 Several studies have been conducted onthe formation and characterization of polymer nanocompo-sites with one-dimensional, two-dimensional, or three-dimen-sional nanoparticles such as polyamide/carbon nanotubes,6

polyamide/clay,7,8 poly(butylene succinate),9 poly(L-lacticacid)/clay,10 poly(trimethylene terephthalate)/silica,11 poly-(vinyl acetate)/silica,12 and epoxy/silica13 nanocomposites.The effect of inorganic nanofillers on the crystallizationkinetics of semicrystalline polymers has been studiedextensively,14−18 and studies have shown that nanoparticlescan 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 betweennanoparticles, the polymer matrix, and the spatial arrangementof these nanoparticles.19,20 The interface between nano-particles and the polymer matrix plays an important role inuniformly dispersing the particles.21,22 Despite many efforts inthe past, conventional ways of preparing polymer nano-composites, such as melt blending, intercalation, and in situintercalative polymerization have their deficiencies.23−25 Thesol−gel approach is another technique used to synthesizenanomaterial from atomic or molecular species via chemical

reactions, allowing for the precursor particles to grow insize.26−30 The synthesis of organic−inorganic nanocompositesby the sol−gel process has received considerable interest inrecent decades due to its low temperature processing andbetter control over dispersion. Nylon-66, poly(vinyl acetate),polypropylene, and poly(vinyl alcohol) nanocomposites wereprepared using the sol−gel technique.25,31−34 These nano-composites were fabricated by forming an inorganic phasefrom the hydrolysis−condensation reaction of alkoxysilanes,such as tetraethoxysilane (TEOS), in a solution containingorganic polymers. It has been shown that nanocompositesprepared using this method exhibit good dispersionthroughout the polymer matrix.Because PA6 is the most frequently used (and most

successful) synthetic polymer, its crystal structure andmechanical properties have been studied extensively.35−40

PA6 crystallizes into two well-known crystal forms: the α andthe γ. The α crystalline phase of PA6 is the most commonlyobserved phase, and it can be obtained by slowly cooling fromthe melt state. The γ phase can be obtained by fast quenchingfrom the melt state or by treating the α phase with aniodine−potassium iodide aqueous solution, followed byremoval of the absorbed iodine by sodium thiosulfate. Theγ nucleation effect has been reported for PA6 in the presenceof nanofillers such as clay, zinc oxide, and mica.41−45 This

Received: May 22, 2014Revised: July 14, 2014Published: July 15, 2014

Article

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study aims to examine the synthesis of PA6/silica nano-composites using an in situ sol−gel process. This will helpdetermine the role of nanoparticles in isothermal meltcrystallization kinetics and help understand the developmentof various crystalline forms of PA6 in the presence of silicananoparticles in a series of PA6 silica nanocomposites.

2. EXPERIMENTAL SECTION

2. 1. PA6/Silica Nanocomposite Fabrication. Poly-amide 6 (PA6), which has a molecular weight of 80 000, waspurchased from Sigma-Aldrich. Eighty-eight percent formicacid, as well as tetraethoxysilane (TEOS), was also acquiredfrom Sigma-Aldrich. Two methods were used to prepare thenanocomposites. A 10 wt % PA6 solution was prepared bydissolving a commercial grade of PA6 in 88% formic acid byheating it for 15−20 min, and subsequently 1, 2, 3, 4, and 5wt % of tetraethoxysilane (TEOS) samples based on theamount of PA6 were added to a 10 w/v % PA6 solution. Thesolution was stirred vigorously with a magnetic bar at roomtemperature. The stirring was carried out for ∼1 h at anambient temperature. The solutions were kept at roomtemperature for ∼24 h to allow hydrolysis−condensationreactions of TEOS to continue. The solutions were thenpoured onto cleaned glass plates and dried at roomtemperature. Because the resulting nanocomposite filmswere thick and opaque, they were melt-pressed betweenTeflon sheets at 250 °C to obtain thinner films. PA6 filmswithout 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 spectralrange from 4000 to 400 cm−1 at a resolution of 2 cm−1. TheOMNIC software was used to determine the peak positionand absorbance of various bands.2.3. Thermogravimetric Analysis. The degradation,

thermal stability, and silica content in the PA6 nanocompositefilms was determined using PerkinElmer TGA 7. The sampleswere heated to 800 °C with a heating rate of 10 °C/min for90 min under a nitrogen atmosphere. The thermaldegradation temperature, Td, was calculated by drawing atangent line and determining the onset value. The wt % silicawas calculated using curve plateaus.2.4. Differential Scanning Calorimetry. DSC experi-

ments were performed with a PerkinElmer DSC 7. Theinstrument was calibrated for temperature and heat of fusionusing standard indium (Tm = 156.6 °C and ΔH = 28.5 J/g).All experiments were performed under a nitrogen atmospherewith a flow rate of 20 mL/min to minimize oxidation. Four tosix milligram samples were used for all measurements. Theonset values were taken as transition temperatures. Heat offusion was determined by integrating the area under themelting peak of the DSC curves and was directly proportionalto the degree of crystallinity. The heat of fusion of 100%crystalline PA6 was taken as 188 J/g and used to determinethe weight percent crystallinity of neat PA6 and PA6nanocomposites. The degree of crystallinity of nanocompositeχc was determined using the following (eq 1).

ϕ=

Δ− Δ *

×XH

H(1 )100%c

f

(1)

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

sample. The reported crystallinity values are the average of atleast three determinations.Isothermal melt crystallization kinetic studies of PA6 and

PA6 nanocomposites were also performed using PerkinElmerDSC 7. The PA6 and PA6 nanocomposite samples wereheated from room temperature to 250 °C at 10 °C/min, heldfor 3 min to erase the prior thermal history, and subsequentlycooled to their predetermined crystallization temperature atthe cooling rate of 100 °C/min and held for 1 h to completethe melt crystallization. The exothermic scan of heat flowversus time was used for kinetic analysis.

2.5. Microscopy and Nano Zetasizer. Surface topologyand dispersion of the nanocomposites within the polymermatrix was determined by an Asylum MFP 3D BIO AtomicForce Microscope (AFM) at the College of Staten Island,City University of New York, New York. The size anddistribution of the nanocomposites was determined by SEMin the range 1−20 μm at the material characterizationlaboratory at the New Jersey Institute of Technology (NJIT).A nano Zetasizer from the Malvern Instrument was used tomeasure particle size in the range 0.6−6000 nm by DispersionTechnology Software. It was used to determine the particlesize of silica nanoparticles using an alternate technique in therange 1−1000 nm. The reported mean values were based onthree measurements with 10 cycles for each sample.

3. RESULTS AND DISCUSSION3.1. Preliminary Characterization. It is important to

assess the dispersion of silica in the PA6 matrix to understandthe physical and mechanical properties of nanocomposites andthe morphology of PA6-silica nanocomposites prepared by thehydrolysis and condensation of TEOS. These nanocomposites’morphologies were investigated using scanning electronmicroscopy (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 imagesshow uniform dispersion/distribution of nanoscale sphericalsilica particles within the PA6 matrix up to 3% TEOS.However, the aggregation/agglomeration of nanoparticles wasobserved for PA6 containing 5% TEOS. Silica particle sizeestimated using SEM varies between 100−200 nm. Figure 1cshows that the dispersion of silica particles is not uniform forthe PA6 film containing 5% TEOS. The surface distribution ofsilica particles are characterized by atomic force microscopy(AFM). The AFM phase image of neat PA6 and PA6containing 0% and 3% TEOS ranging from 0−500 μm areshown in Figure 2. It is evident from Figure 2b,c that thesurface roughness and average height of the film surfaceincreased with increasing silica content. The distribution ofthe peaks is uniform, suggesting that homogeneous dispersionis achieved up to 3% TEOS. The size of the SiO2nanoparticles was characterized using nano Zetasizer measure-ments, and the average size of the nanoparticles was found tobe within the range 150−200 nm, which is in close agreementwith 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 neatPA6 films show the characteristic peaks at 3300 cm−1 (N−Hstretch), 2938 cm−1 (CH2 stretch), 1640 cm−1 (COstretch), and 1540 cm (N−H deformation), and the bandsat 900−1000 cm−1 are assigned to various amide vibra-tions.36−38 The bands occurring in the region between 1000and 1100 cm−1 in the spectrum of nanocomposites are

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assigned to the asymmetric Si−O−Si stretching vibration andincrease in absorbance in this region compared to those forneat PA6, which is attributed to the formation of an Si−O−Sibond. A vibration at 800 cm−1 is assigned to the symmetricSi−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 thehydrogen-bonded Si−O−H (silanol) stretching vibrations. Itshould be noted that the half-width of the band at 3300 cm−1,assigned to the N−H stretching vibration of neat PA6,

increases with increasing SiO2 content. It appears thatconformational flexibility of PA6 increases due to theweakening of hydrogen bonding during the formation ofnanocomposites.The characteristic IR bands of α and γ crystal forms of pure

PA6 have been reported by various research groups. Thebands at 960, 930, and 1200 cm−1 are attributed to the αcrystalline phase, whereas the band at 973 cm−1 is attributedto the γ-crystalline phase.35−38 The 1170 cm−1 band consistsof both the amorphous and crystalline phases of the polymerand are often used as a reference band to characterize themicrostructure of PA6.38 The FTIR spectrum of PA6 and itsnanocomposite films prepared by fast quenching from themelt in the range from 1200 to 900 cm−1 are shown in Figure4. It can be observed that the PA6 nanocompositespredominantly crystallize in the γ crystal form, whereas neatPA6 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. Theabsorbance ratio of the IR band at 973 cm−1 to the band at1170 cm−1 was calculated, and it was shown that theabsorbance ratio increased with increasing SiO2 content. Theγ nucleating effect has been reported for various PA6nanocomposites containing nanofillers such as mica, sepiolite,and zinc oxide, and our observation is consistent with thesefindings.41−45 It has been shown that the γ crystals appearcloser to the particle surfaces than the α crystals. The IRspectra of neat PA6 and PA6 nanocomposites with 3 wt %SiO2 prepared via fast quenching and slow cooling from themelt were compared, and no significant difference in the IRspectra were observed, suggesting that γ crystal formationdoes not depend on the way PA6 and PA6 nanocompositefilms are prepared.

3.2. Thermal Characterization. The thermal stability ofmaterials is usually characterized by TGA, in which the weightloss is monitored as a function of temperature. The TGAmeasurements of neat PA6 and its nanocomposites withvarying amounts of silica were carried out, and the silicacontent determined in each nanocomposite film from TGA isreported in Table 1, along with theoretical silica content andthe temperature at which 10%, 50%, and 90% weight lossoccurs. The thermal degradation temperature of PA6nanocomposites increases significantly compared to that forneat PA6, suggesting that the thermal stability of nano-composites increased compared to that of neat PA6. Theactual silica content determined by TGA curves appears onlyslightly lower than the theoretical amount given in Table 1. Itrevealed that the sol−gel reaction was almost complete in allcases. The increase in thermal stability of nanocomposites wasprobably caused by strong secondary interactions of PA6chains and silica. The intramolecular hydrogen bonding withinthe PA6 matrix may decrease due to the strong secondaryforce between PA6 and silica particles, and this is supportedby our FTIR observation.Figure 5 displays the first heating and cooling differential

scanning calorimetric (DSC) traces observed at a heating andcooling rate of 10 °C/min for neat PA6 and PA6nanocomposites. A DSC scan of neat PA6 shows a meltingpoint (Tm) of 217 °C and a nonisothermal melt crystallizationtemperature (Tc) of 199 °C. The effect of SiO2 nanoparticleson Tm and Tc of PA6 can be clearly seen in the DSC scans ofPA6 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.

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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

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 3wt % TEOS loading.

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PA6. The percent crystallinity of PA6 and its nanocompositeswas calculated using eq 1, and the results are also included inTable 2. It can be seen from Table 2 that percent crystallinitydecreases with increasing silica content. A similar observationhas recently been reported for PA66 and PA6 containingvarious nanoparticles. The decrease in overall crystallinity withincreasing SiO2 nanoparticles may be attributed to weakening

the hydrogen bonding interaction between CO and N−Hgroups between neighboring PA6 chains.

3.3. Isothermal Melt Crystallization Kinetics. Iso-thermal melt crystallization kinetics was studied by DSC tosee the effect of SiO2 on crystallization kinetics of PA6. Figure7a shows the development of relative crystallinity withcrystallization time for neat PA6 at a temperature rangingfrom 185 to 200 °C. It is shown in Figure 7a that the rate ofcrystallization decreases with increasing crystallization temper-ature. Panels b−d of Figure 7 show the development of therelative crystallinity of PA6 nanocomposites containing 1, 3,and 5 wt % TEOS, respectively. All of them show decreases incrystallization rate with increasing crystallization temperature.Figure 8 shows the relative percent crystallinity of neat PA6and its nanocomposites at different temperatures as a functionof time. It can be seen from Figure 8a that the rate of

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

PA6 PA6-1 PA6-3 PA6-5

T90% (°C) 347 359 368 369T50% (°C) 407 432 435 442T10% (°C) 448 510 511 512wt % SiO2 (TGA) 0 0.3 0.85 1.3

aT10%, T50%, and T90% indicate the temperatures at which 10%, 50%,and 90% weight loss occurs.

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crystallization decreases with an increasing % of TEOS up to3 wt % at crystallization temperature of 185 °C. However, the5 wt % TEOS has a crystallization fairly close to the 1 wt %due to the nonuniform dispersion of the film at this higherconcentration. A similar trend is observed at 190, 195, and

200 °C up to 3 wt %. The relative crystallinity developmentfor PA6 film containing 5 wt % TEOS is scattered at differenttemperatures, indicating again a nonuniform dispersion ofSiO2 particles within the PA6 matrix.Isothermal melt crystallization kinetics on PA6 and its

nanocomposites can further be studied using the classicalAvrami equation, as follows:

χ− = −kt1 exp( )tn

(2)

where χt is the crystalline fraction at time t; n is the Avramiexponent, which normally ranges between 1 and 4, dependingon the nature of nucleation and growth geometry of thecrystals; k is the rate constant; and t is the crystallization time.This equation can be further expanded by taking doublenatural logarithms, as follows:

χ− − = +n t kln[ ln(1 )] ln lnt (3)

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

Table 2. ΔHm, ΔHc, and % Crystallinity of Neat PA6 andIts Nanocomposites (1% to 5 wt % TEOS) Based on DSCCurves

sample (wt%)

ΔHm(J/g)

Tm(onset)(°C)

ΔHc(J/g)

Tc(onset)(°C)

%crystallinity

neat P6 73.09 217.2 −67.27 199.25 391 54.52 216.7 −85.85 192.58 292 51.78 216.3 −84.56 191.26 283 44.79 215.7 −83.67 191.97 244 52.03 215.7 −85.96 191.03 295 47.11 215.3 −78.59 191.62 27

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The Avrami exponent, n, and rate constant, k, for neat PA6and its nanocomposites were obtained from the plot ofln[−ln(1 − χt)] versus ln t at different crystallizationtemperatures from 180 to 200 °C. Figure 9 illustrates asample Avrami plot of neat PA6 and its nanocomposites meltcrystallized at 195 °C, and similar plots were observed forother crystallization temperatures (not shown here). A linearregression, y = mx + b, was used to determine the n and kvalues from Figure 9. The data above 75% crystallinity canlead to a deviation from linearity; therefore, the data between5% and 75% relative crystallinity were used for the

calculations. The Avrami exponent, n, is dependent on thecrystallization mechanism. The values of n obtained for theneat polymer and its nanocomposites ranges from 1.9 to 3.1,which suggest two-dimensional to three-dimensional crystaldevelopment. It was seen that silica loading has no significanteffect on the “n” value, but that melt crystallizationtemperature has a significant effect on the “n” value. The“n” value appeared to increase with increasing crystallizationtemperature from 185 to 200 °C.Crystallization kinetics can be further studied by plotting

the half-time of crystallization, t1/2, versus temperatures. The

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 atdifferent temperatures.

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t1/2 is the time at which 50% of the relative crystallinity isobtained. These values were determined from the DSC curvesof neat PA6 and its nanocomposites. The t1/2 of PA6 and itsnanocomposites are plotted against crystallization temper-atures between 185 and 200 °C in Figure 10. It can beobserved from the plot that neat PA6 has a lower t1/2 than itsnanocomposites at all four temperatures. This observationconfirms that neat PA6 has a faster crystallization rate than allof its nanocomposites. The t1/2 values for 5% TEOS areobserved to be scattered at all temperatures. This furtherconfirms that PA6 nanocomposites with 5% TEOS are notdispersed uniformly.

4. CONCLUSIONS

Polyamide 6/SiO2 nanocomposites were successfully preparedand confirmed using scanning electron microscopy. Crystal-

lization studies of these nanocomposites were conductedusing differential scanning calorimetry (DSC) and FTIRspectroscopy. It was shown that the dispersion of nano-particles is uniform up to 3% silica content and thatnonuniform dispersion was apparent for those containingmore than 3% silica. PA6 predominantly forms an α crystalstructure during thermally induced crystallization, whereaspolyamide 6 nanocomposites crystallize in a γ crystalstructure. Tm decreases slightly with increasing silica contentas compared to that for neat PA6. The crystallinity of PA6/silica nanocomposites decreases with increasing silica loading.Isothermal melt crystallization kinetics was carried out usingDSC and analyzed using Avrami theory. The Avramiexponent, n, was found to vary from 1.9 to 3 withcrystallization temperature, suggesting two- to three-dimen-sional growth. No significant change in Avrami constant was

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.

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observed with varying silica content. The crystallization halftime, t1/2, increased with increasing silica content, suggestingthe rate of crystallization decreases with increasing silicacontent.

■ AUTHOR INFORMATIONCorresponding Author*N. Vasanthan. E-mail: nadarajah.vasanthan@liu.edu. Phone:718-246-6328. Fax: 718-488-1465.NotesThe authors declare no competing financial interest.

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

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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 itsnanocomposites.

The Journal of Physical Chemistry B Article

dx.doi.org/10.1021/jp505046v | J. Phys. Chem. B 2014, 118, 9486−94959494

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The Journal of Physical Chemistry B Article

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