Evaluation of Nanoparticle Dispersion in Polypropyleneby Small-Angle X-Ray Scattering

Jonathan Lipp,1 Carina Makarov,1 Rafail L. Khalfin,1 Michael Shuster,2 Larissa Berenstein,3

Ofira Melamed,3 Ayelet Odani,3 Arie Zaban,3 Yachin Cohen1

1Chemical Engineering Department, Technion, Haifa 32000, Israel2Carmel Olefins, Limited, P.O. Box 1468, Haifa 31014, Israel3Chemistry Department, Bar-Ilan University, Ramat-Gan 52900, Israel

Received 3 September 2007; accepted 26 December 2007DOI 10.1002/app.28065Published online 28 March 2008 in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: A small-angle X-ray scattering method hasbeen developed for the quantitative evaluation of the effec-tiveness of nanoparticle dispersion in polymer matrices; itis termed the nanoscale dispersion index. This methodwas applied to dispersions of nanosized TiO2 fillers inpolypropylene. Master batches prepared with lower fillercontents showed better dispersion as evaluated by the

nanoscale dispersion index. The addition of 1,3:2,4-di(3,4-dimethylbenzylidene) sorbitol to the compounds did notaffect the degree of nanoscale dispersion as estimated bythe nanoscale dispersion index. � 2008 Wiley Periodicals, Inc.J Appl Polym Sci 109: 350–354, 2008

Key words: nanotechnology; poly(propylene) (PP); SAXS

INTRODUCTION

Blending additives within polymeric matrices, partic-ularly nanosized ones, can provide beneficial func-tions with minimal disturbance of the polymer’sproperties. However, the adequate dispersion ofnanoparticle agglomerates in the compounded poly-mer is an ongoing challenge. The main obstacle isachieving a uniform dispersion while maintainingthe small dimensions of the dispersed particles andavoiding their tendency to agglomerate into largerstructures. Much effort has been expanded onenhancing the ability to control and direct this dis-persion. Among the methods that have beenemployed toward these goals are solid-state shearpulverization,1 utilization of surface-active agents,2

melt compounding, and so on.3 Dimethyl benzyli-dene sorbitol (DBS), often used as a gelator inorganic liquids and as a nucleator and clarifier forpolypropylene (PP), was reported to interact withdispersed colloidal silica within poly(ethyl methacry-late), so that the interface created between the mol-ten polymer and the DBS fibrillar structure couldenhance and direct the dispersion of the particles.4

An alternative method for nanoparticle dispersionin the polymer matrix is to synthesize the particlewithin the polymer. This may be especially useful inthin polymer films. When bulk applications are of

interest, another approach may be to synthesize theparticles within the voids of a porous polymer solidbefore its melt compounding.

Despite the efforts toward tackling these problems,the ability to correctly evaluate the degree of thenanoscale dispersion remains illusive, and it is oftenbased only on electron microscopy imaging. It istherefore the objective of this article to present asmall-angle X-ray scattering (SAXS) method to eval-uate the degree to which the nanoscale dispersion ofthe particles has been achieved in a compoundedpolymer.

EXPERIMENTAL

Materials

Commercial grades of PP [high-molecular-weightCapilene QB (a random propylene copolymer with4% ethylene), Carmel Olefins, Ltd. (Haifa, Israel),and Accurel PP, Membrana GmbH, Wuppertal, Ger-many] were used. 1,3:2,4-Di(3,4-dimethylbenzyli-dene) sorbitol (DMDBS; Millad 3988) was acquiredfrom Milliken, Inc. (Spartanburg, SC), and used asreceived. TiO2 nanoparticles were in situ preparedby a sol–gel method with titanium tetraisopropoxide(TTIP; 99.99%; Aldrich, Wilmington, DE). TTIP inisopropyl alcohol solutions was impregnated ontothe polymer pellets and within their inner pore sur-face in vacuo. Subsequent exposure to humidityfacilitated TTIP hydrolysis, which was followed bycondensation to TiO2 nanoparticles with a size ofabout 25 nm. Master batches (MBs) at different parti-cle concentrations were made from these pellets by

Correspondence to: Y. Cohen (yachinc@tx.technion.ac.il).

Journal of Applied Polymer Science, Vol. 109, 350–354 (2008)VVC 2008 Wiley Periodicals, Inc.

melt compounding, as described later. Compoundsof desired compositions were made through dryblending at room temperature and subsequentlymelt blending in a Thermo Haake Rheomix batchmelt mixer (Battenfeld Co., Guangdong, China) for10 min at a set point of 1808C at 50 rpm. The operat-ing temperature exceeded the set point by about 20–308C. Polymer plates were prepared from this com-pound either by compression molding with a Collinhydraulic press (Ebersberg, Germany) at 1808C or byinjection molding with a Battenfeld injection-mold-ing machine. Samples were punched from theseplates and used for X-ray scattering measurements.

SAXS

SAXS measurements were conducted with a small-angle diffractometer (KFF CU 2 K-90, Bruker Nano-star, Karlsruhe, Germany) with Cu Ka radiationfrom a sealed tube, two Gobel mirrors, two-pinholecollimation (which resulted in a beam ca. 300 lm indiameter on the sample), and a 10 3 10 cm2 two-dimensional position-sensitive wire detector posi-tioned 65 cm behind the examined sample. The sam-ples were placed in a vacuum chamber perpendicu-lar to the incident beam. Each sample was subjectedto an 18-h exposure time in this apparatus to ensuresufficient data collection and to reduce the noise-to-data ratio. The scattered intensity was recorded inthe interval of 0.13 nm21 < h < 3.20 nm21; the wavevector is defined as h ¼ 4p=k sin u, where 2y is thescattering angle and k is the wavelength. The sam-ples (1.6–2 mm thick) were placed between thin pol-yimide films within a metal holder. The measuredintensities were circularly averaged after azimuthalintegration at a constant value of y to give the scat-tered intensity as appropriate for an isotropic mate-rial. The intensity was then normalized to the time,primary beam intensity, and sample thickness andtransmission. The scattering of the polyimide films(as well as the parasitic scattering and the low elec-tronic noise) was subtracted from the normalized in-tensity to give the normalized and subtracted inten-sity [I(h)]. The sample temperature was maintainedat the desired value (1808C) with a restive heatingsample chamber (KHR, A. Paar, Ltd., Graz, Austria).The variation of the sample thickness (d) with thetemperature (T) was evaluated on the basis of itsmeasured X-ray transmission [Tr(d)]:

TrðdÞ ¼ e�lqðTÞdðTÞ (1)

where l is the mass transmission coefficient and q isthe sample density. l of PP was experimentallyobtained with eq. (1), and the values of Tr, q, and dwere measured at 258C. The T (8C) dependence ofthe melt density was approximated with the follow-ing equation:5

qðTÞ ¼ 0:862 3 expð�6:7 3 104TÞ (2)

Electron microscopy

The instrument used was a Leo 982 high-resolutionscanning electron microscope (Carl Zeiss SMTAG,Oberkochen, Germany) operated at an acceleratingvoltage of 4 kV with a working distance of 3 mm.Rectangular samples with a thickness of about 2 mmwere cut from compression-molded plates. Fromthese, thin slices (� 50 nm thick) were sectionedwith a Reichert-Jung FC 4E cryo-ultramicrotome(Buffalo, NY) at 190 K. The stump was glued to analuminum sample holder and used for high-resolu-tion scanning electron microscopy (HRSEM) imagingafter the sputtering of a thin gold layer.

RESULTS AND DISCUSSION

Nanoscale dispersion index (NSDI)

The assessment of the extent of nanoscale dispersionwithin polymer matrices often uses imaging by elec-tron microscopy. Although this technique providesinformation on the size and shape of particles andindicates their state of agglomeration, it requiresmuch effort for suitable sample preparation andcovers a relatively small sample area, so it oftendoes not provide quantitative and statistically signifi-cant information. Utilization of X-ray scattering, inparticular SAXS, promotes data acquisition from asignificantly larger volume with minimal samplepreparation.

The total scattered intensity from an examinedmaterial, such as the polymer/nanoparticle samplesstudied, is termed the scattering invariant (Q), and itis defined as follows:6

Q ¼Z ‘

0

IðhÞ � h2dh (3)

For a two-phase system of well-defined particleswith sharp interfaces, Q can be calculated from theparticle volume fraction (/F) and the difference inthe electron densities of the filler (qF) and matrix(qm) as follows:6

Qcalc ¼ 2p2ðqF � qmÞ2/Fð1� /FÞ (4)

where Qcalc is the calculated scattering invariant.Data collection in our experiments is limited to scat-tering vectors in the interval of 0.13 nm21 < h < 3.20nm21, as described previously. This poses a limita-tion on the dimensions of the particles that contrib-ute to the measured intensity. This is particularlyimportant for the larger agglomerates that are notdispersed to the desired extent. We make use of thiseffect for comparative evaluation of the extent towhich the particles are dispersed in nanoscaledimensions.

NANOPARTICLE DISPERSION IN PP 351

Journal of Applied Polymer Science DOI 10.1002/app

The experimentally measured invariant (Qexp) isobtained with eq. (3) by numerical integration of thedata in the measurable h interval. The data are extrap-olated beyond the high h limit with Porod’s law:6

IðhÞ ¼ ðDqÞ2 Kh4

þ Ib (5)

where K is Porod’s constant related to the specificinterfacial area and Ib is a background constant.Extrapolation to low values of h uses Guinier’s law:6

IðhÞ ¼ A exp � 1

3R2gh

2

� �(6)

where A is Guinier’s constant and Rg is the radius ofgyration. The intensity of the molten polymer is finallysubtracted from I(h).Thus, we can define NSDI as theratio of Qexp [eq. (3)] to Qcalc [calculated from theknown volume fraction and electron densities of thecomponents; eq. (4)]:

NSDI ¼ 100 3 ðQexp=QcalcÞ (7)

To evaluate the maximal particle size that is accountedfor by this evaluation procedure, model calculationswere performed for various particle sizes and shapes(spheres and ellipsoids of revolution). The simulatedpatterns were cut off at the low and high h intervallimits of the experimental SAXS measurements. NSDIvalues were calculated for the simulated particle scat-tering following the appropriate Guinier and Porodextrapolations. NSDI values of over 90% were esti-mated for the different simulated particles for whichthe largest dimension was 70 nm. These results indi-cate the significance of the NSDI evaluation as a rapid,nondestructive method providing the extent to whichnanoparticles have been successfully dispersed in thepolymer matrix to dimensions smaller than 70 nm.

As described previously, TiO2 nanoparticles weresynthesized by impregnation of precursor solutionsinto granular PP raw materials, which were subse-quently diluted by compounding with the samepolymer grade. These PP grades included a porousgrade (Accurel; specific surface � 13 m2/gr) forwhich a higher particle loading could be achieved,

enabling the preparation of a concentrated MB con-taining over 40% (w/w) filler. NSDI evaluationswere used to compare the effectiveness of the nano-particle dispersion in these polymeric systems. Fur-thermore, PP containing DMDBS was also evaluatedto examine possible effects on the dispersion.

The effect of the MB concentration was examinedwith the Accurel/TiO2 composite polymers. Fourdifferent initial MB concentrations, 4.8, 11, 20, and41% (w/w), were taken and compounded to giveroughly the same TiO2 concentration (2%). As can beseen in Table I, the reduction of the nanoparticleconcentration within the MB (down to ca. 10%) has apronounced effect on particle dispersion, as shownby the NSDI value. Reducing the MB concentrationfrom 41 to 11% more than doubles the amount ofTiO2 that is dispersed on the nanoscale (<70 nm). Itis assumed that the higher concentration of the pre-cursor, required for the preparation of a concen-trated MB, provides more pathways for agglomerateformation during particle synthesis. These agglomer-ates are not dispersed upon subsequent compound-ing, thus reducing the evaluated NSDI.

Table II compares the NSDI estimations for differ-ent particles embedded in different PP grades withand without DMDBS. SAXS measurements of thecompounded samples were performed at 1808C,which is above the melting temperature of PP. Thestatus of the DMDBS fibrils in the samples measuredat this temperature depended on the way in which itwas reached. This was achieved with two proce-dures: (1) heating from room temperature and (2)cooling from 2508C. In the former, the PP crystalsmelted, but the DMDBS fibrils were intact, whereasin the latter, both PP and DMDBS were in a misciblemelt.7 As can be noted, the extent of the nanoscaledispersion seems to be independent of the tempera-ture protocol. The maximum error estimated forthese calculations is about 6%, as evaluated fromseveral measurements of the same system (Accurel/TiO2) and by model calculations. The negligibleeffect of temperature on the evaluated NSDI may bedue to the low signal from DMDBS fibrils with

TABLE INSDI Evaluation for Compounds with About 2% TiO2

Made from MBs of Different Concentrations

TiO2 in theMB (%)

TiO2 in thefinal composition (%) NSDI (%)

4.8 2 6211 2 6120 1.9 5141 1.8 29

TABLE IINSDI and Size Evaluation for Different Compounds

With and Without DMDBS During Heating and Cooling

PP gradeDMDBS

(%)Temperature

protocolTiO2

(%)NSDI(%)

QB 0 Heating 2 44QB 0 Cooling 2 43QB 1 Heating 2 37QB 1 Cooling 2 38Accurel 0 Heating 2 56Accurel 0 Cooling 2 62Accurel 1 Heating 2 58Accurel 1 Cooling 2 59

352 LIPP ET AL.

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respect to that from the dispersed TiO2 because ofthe lower contrast and content. Moreover, the pres-ence of DMDBS does not change the value of NSDI,and this implies no specific interactions eitherbetween these additives or between the DMDBS/PPinterface and the TiO2 particles, which may affectthe dispersion. A comparison of the NSDI values forcompounds with Accurel and QB-grade PP indicatesthat in both cases rather high levels of dispersionwere achieved (ca. 60 and 40%, respectively). Theseresults are encouraging with respect to the use ofthe sol–gel method in PP with some porosity. This iseven somewhat surprising in the case of the QBgrade, which has a specific surface of only 0.4 m2/g.This suggests that in the current method of particlefabrication, the usage of a porous polymer substrateis not of prime importance, although it has someeffect on the ultimate particle dispersion and also onthe initial quantity of titania in the polymer MBs.

HRSEM images obtained from microtomed surfa-ces of compounded samples are shown in Figures 1and 2. Figure 1 shows TiO2 particles of differentsizes, mostly less than 70 nm in diameter, as well as

some larger agglomerates. This is in accord with theSAXS measurements. The micrographs also do notindicate any difference between samples com-pounded with and without DMDBS. In a few cases,HRSEM imaging of DMDBS fibrils could be achievedin the microtomed surfaces, as shown in Figure 2,and this likely occurred when the fibril was nearlyparallel to the sectioning plane. Rarely, particlesattached to the fibrils were observed, as shown inFigure 2(b).

CONCLUSIONS

A SAXS-based method is presented for the compara-tive evaluation of the extent to which dispersions ofnanoscale particles have been successfully achievedin a polymeric matrix. NSDI is evaluated as the ratioof the measured total scattering intensity to that cal-culated from the material composition. Utilization ofthis method was demonstrated for PP/nanofiller sys-tems, some of which included the clarifier DMDBS.Nanoparticle fabrication was performed with precur-sor solutions via a sol–gel method by impregnationof a TTIP/isopropyl alcohol solution into the poly-mer pellets in vacuo. This method demonstrated highdispersion levels: up to 60% of the filler was dis-persed, so the dimensions were below about 70 nm.The NSDI evaluation of porous PP (Accurel) with2% TiO2 compounded from MBs of 4.8–41% indi-cated that above about 10%, the higher the MB con-centration was, the lower the nanoscale dispersionwas that was achieved in the final compound. Thisobservation is attributed to the fact that duringnanoparticle fabrication, usage of a concentratedprecursor solution will enhance the likelihood of

Figure 1 HRSEM micrographs of PP with 2% TiO2 madefrom a 5% MB: (a) without DMDBS and (b) with 1%DMDBS.

Figure 2 HRSEM micrographs of DMDBS fibrils in a PP/DMDBS compound containing nanoparticles.

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agglomeration. As these agglomerates are not bro-ken during master-batching or compounding, thedispersed particles in the final compound will beof larger size. An investigation of similar com-pounds with a relatively nonporous PP indicatedthat a decrease of more than an order of magnitudein the specific surface area of the pellets used toprepare the MBs results in a decrease of NSDI ofonly about 30%. The influence of DMDBS, whichforms a network of nanofibrils in the polymer melt,on the nanoparticle dispersion was also investi-gated. However, the NSDI values remained practi-cally unchanged for samples with and withoutDMDBS.

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