Delaminated
and intercalated organically modified montmorillonite in
poly(1,4-cis-isoprene)
matrix. Indications of counterintuitive
dynamic-mechanical
behavior
Maurizio
Galimberti
a,b,⁎
,
Michele Coombs
a,1,
Valeria Cipolletti
a,
Anna Spatola
c, Gaetano
Guerra
c,
Angela Lostritto
d, Luca Giannini
d, Stefano Pandini
e, Theonis Riccò
eaPolitecnico di Milano, Via Mancinelli 7, 20131 Milano (I), Italy
bConsiglio Nazionale delle Ricerche, Istituto per lo Studio delle Macromolecole, Via E. Bassini 15, 20133 Milano (I), Italy cUniversità degli Studi di Salerno, Via Giovanni Paolo II 132, 84084 Fisciano (SA), Italy
dPirelli Tyre, Viale Sarca 222, 20126 Milano, Italy
e Università degli Studi di Brescia, Via Branze 38, 25123 Brescia, Italy
Article history:
Received 19 December 2013 Received in revised form 6 May 2014 Accepted 18 May 2014
Available online 17 June 2014
1. Introduction
Over the last decades, clay minerals and, in particular, montmorillon-ite (Mt) (Bergaya and Lagaly, 2013) have been used for the preparation of clay polymer nanocomposites (CPN) (Alexandre and Dubois, 2000; Bergaya, 2008; Chen et al., 2008; Galimberti, 2011; LeBaron et al., 1999;
Ray and Okamoto, 2003). Clay mineral layers have oxygen atoms and
hy-droxide groups on their surface and they are not compatible with an apolar polymer matrix. Their organic modification is thus performed, preferentially through the exchange reaction of their alkali and alkali-earth cations with organophilic ammonium salts. Alkylammonium ions in the clay mineral interlayer space have arrangements that depend on
layer charge and alkyl chains number and length. It is reported (Lagaly
et al., 2013) that monoalkylammonium ions give rise to mono- and
bi-layers and to pseudo-trimolecular bi-layers, whereas dialkylammoniums have paraffin-type arrangements with different tilting angles. The alkyl chains at the interface with the polymer matrix promote the clay polymer compatibilization. The even dispersion of organically modified clay minerals in the polymer matrix allows the achievement of a substantial improvement of properties such as mechanical reinforcement, reduction of permeability, thermal stability (Paul and Robeson, 2008) and, as a consequence, important applications (Galimberti et al., 2013). It is widely acknowledged that most pronounced properties enhancement of CPN are obtained when clay minerals are exfoliated and individual layers are dispersed in the polymer matrix. To indicate delaminated and exfoliated clay minerals, this manuscript adopts the nomenclature defined in
Bergaya et al. (2013), clay mineral delamination means that an
ap-preciable interaction remains between two successive layers, with some maintained 3D crystallographic order, whereas exfoliation ⁎ Corresponding author at: Politecnico di Milano, Via Mancinelli 7, 20131 Milano, Italy.
Tel.:+39 02 2399 4746; fax: +39 02 2399 3180.
E-mail address:maurizio.galimberti@polimi.it(M. Galimberti).
1
occurs when no interaction is left between isolated layers or stacking of few layers.
The preparation of CPN with extensively delaminated or exfoliated clay minerals is still the subject of large research efforts. To achieve clay mineral delamination in a polymer matrix, most authors rely on polymer intercalation, supposed to be energetically favored by the in-teraction of polymer and alkylammonium chains (Ray and Okamoto,
2003).Galimberti et al. (2007a)reported on the so called delamination
mechanism of pristine clay minerals and organoclays: Mt and OMt were melt blended with natural rubber (NR) and a progressive reduction of the crystallographic order in the direction orthogonal to structural layers was observed for both Mt and OMt in X-ray diffraction (XRD) patterns of CPN prepared with increasing mixing time, without any evidence of the interlayer distance expansion.
The occurring of delamination was attributed to the mixing energy, conveyed to clay minerals by a polymer such as NR. On the basis of the delamination mechanism and of further experimental results
(Galimberti et al., 2009, 2010), the intercalation of polymer chains
was excluded.
In the present work, mechanical energy was used to obtain delaminated Mt and OMt, prior to the mixing with the polymer, and CPNs were then prepared with delaminated OMt. Grinding and milling of clay minerals lead to decrease in particle size, to large increase in sur-face area and to effects such as exfoliation, gliding and folding of the layers. Moreover, the exchange capacity is increased in cationic clay minerals. Some meaningful examples are reported as follows. Mechan-ical treatments were applied to neutral clay minerals such as talc
(Christidis et al., 2004; Sanchez-Soto et al., 1997), pyrophyllite
(Perez-Rodriguez et al., 1988, 2007; Sanchez-Soto et al., 1993;
Stepkowska et al., 2001) and kaolinite (Stepkowska et al., 2001), to cat-ionic clays such as bentonite (Christidis et al., 2004), Ca2 +-Mt
(Dellisanti and Valdre, 2005; Hrachova et al., 2007), Na+-Mt
(Ramadan et al., 2010), beidellite and ripidolite (Sondi et al., 1997). Al-most all of these papers report on the decrease of crystallinity of the clay minerals and, in particular, on their delamination. Effects on particle size and structure were not observed for bentonite (Christidis et al., 2004), but were clearly observed in Ca2 +-Mt (Dellisanti and Valdre,
2005), with also a complete breakdown of the mineral layers
(Hrachova et al., 2007). The (001) reflection disappeared from the
XRD pattern of Na+-Mt, after some hours of ball milling (Ramadan
et al., 2010). Mechanical energy was used for performing the reaction
of clay minerals with alkylamine (Ogawa et al., 1990) and with ammo-nium salts: Mt was allowed to react with dodecyltrimethylammoammo-nium, tetramethylammonium and tetrabutylammonium salts (Ogawa et al., 1995), with cetyl-trimethylammonium bromide (Yang and Zheng, 2003) and with dimethylditalloylammonium chloride (Cipolletti et al., 2014) and vermiculite with hexadecyl trimethylammonium bromide (Wang et al., 2011; Zhang et al., 2008). The ball milling assisted reaction of Mt with dodecyl- and hexadecyl-ammonium ions, in water or kerosene (Lee et al., 2007) was commented to lead to exfoliated OMt. Mechanical energy was applied to modify the structure of Mt modified with octadecyltrimethylammonium salt (Hrachova et al., 2007), reporting that the organo-Mt is more resistant toward mechanical energy than the Ca form. Substantial decrease of the (001) reflection
was observed in the XRD patterns of Mt modified with methyl
di(hydroxyethyl)talloyl ammonium salt (Ramadan et al., 2010). In this work, Mt and OMt, with dimethylditalloyl ammonium (2HT) as the compensating cation, were milled with a planetary ball mill, for different times. The crystalline structure of pristine and milled Mt and OMt was analyzed through XRD analysis, investigating the kinetics of delamination and defining a delamination index. Pristine, unmilled OMt and delaminated OMt were then melt blended with synthetic poly(1,4-cis)isoprene (IR) and dynamic-mechanical properties were determined for uncrosslinked masterbatches and for CPN crosslinked with sulfur based systems. Structure of CPN was studied by means of XRD and Transmission Electron Microscopy (TEM) analysis.
2. Experimental 2.1. Materials
Mt, and O-Mt. Mt sample was Dellite® HPS from Laviosa Chimica Mineraria S.p.A., with CEC equal to 128 meq/100 g. OMt sample was Dellite® 67G from Laviosa Chimica Mineraria S.p.A. The compensation cation was 2HT and the mass % of Mt and of the ammonium moiety was respectively: 55 and 45.
2.1.1. Ingredients for composites' preparation
Synthetic poly(1,4-cis-isoprene) (IR) (Nizhnekamskneftechim Export) with trade name SKI3 and 70 Mooney Units (MU) as Mooney viscosity (ML(1 + 4)100 °C), 3-Octanoylthio-1-propyltriethoxysilane (Sylane NXT) (Momentive), ZnO (Zincol Ossidi), stearic acid (Sogis), N-(1,3-dimetilbutil)-N′-fenil-p-fenilendiammina (6PPD) (Crompton), sulfur (Solfotecnica), N-cyclohexylbenzothiazol-2-sulfenamide (CBS) (Flexsys), N-cyclohexylthiophthalimide (premature vulcanization inhibitor, PVI) (Flexsys), and phtalic anhydride (Aldrich). All the ingre-dients were used as received.
2.2. Ball milling of Mt and OMt
Ball milling of Mt and OMt was performed with a planetary ball mill S100 from Retsch, having the grinding jar moving in a horizontal plane, with a volume of 0.3 l. The grinding jar was loaded with 6 ceramic balls having a diameter of 20 mm. 10 g of either Mt or OMt was put into the jar that was allowed to rotate at 100 rpm, at room temperature, for the following times: 24, 72, 168 and 240 h.
2.3. Preparation of OMt/IR nanocomposites Formulations of CPN are reported inTable 1.
Procedure for composites' preparation is reported as follows. 740 g of IR was introduced into a Banbury® type internal mixer having a vol-ume of 1050 cm3, at 80 °C and was masticated for 30 s, with rotors ro-tating at 75 rpm. OMt (either I-OMt or D-OMt) and the sylane were then added and mixing was carried out for 5 min. The masterbatch was discharged at temperatures in a range from 90 °C to 95 °C and was fed again, after about 16 h, to the mixer, at 50 °C and with rotors rotating at 30 rpm. ZnO, stearic acid and N-(1,3-dimethylbutyl)-N ′-phenyl-p-phenilenediamine (6PPD) were added. The nanocompos-ite was discharged after 120 s at approximately 70 °C. Sulfur, DCBS, phtalic anhydride and PVI werefinally added by performing a further mixing at 50 °C for 180 s. CPNs were further homogenized by passing them 5 times on a two roll mill operating at 50 °C, with the front roll rotating at 30 rpm and the back roll rotating at 38 rpm, with 1 cm as the nip between the rolls. Curing was carried out at 170 °C for 20 min under a pressure of 150 bar.
Table 1
Formulations of CPN with OMt asfiller: either I-OMt or D-OMt.a,b,c
Ingredient Composite
1 2 3
IR 100 100 100
I-OMt 0 8.3 0
D-OMt 0 0 8.3
aI-OMt: intercalated OMt, n-OMt: delaminated O-Mt. b
Amount of ingredients are indicated in php (part per hundred polymer).
c
Further ingredients: sylane NXT 0.8, ZnO 4, stearic acid 2, 6PPD 2, sulfur 2, DCBS 1.8, phthalic anhydride 1, PVI 0.3.
2.4. Characterization of Mt, OMt and nanocomposites 2.4.1. X-ray diffraction (XRD)
Wide-angle X-ray diffraction (WAXD) patterns with nickelfiltered Cu–Kα radiation were obtained in reflection, with an automatic Bruker D8 Advance diffractometer. Patterns were recorded in 2–70° as the 2θ range, being 2θ the reflection angle. For the clay mineral patterns, the in-tensities, after subtraction of the tail of the primary beam, were corrected for polarization and Lorentz factors, by using the following formula: Icor:¼ Iexp:= 1þ cos22θ
=2
h i
hsin2θ cosθ=2i
n o
wherein Icor. is the corrected reflection intensity and Iexp. is the experimental reflection intensity. Moreover, the clay mineral patterns reported below in the text inFigs. 2 and 3were normalized with respect to (060) reflection.
For the composite patterns, reported below in the text inFig. 5, for an easier comparison with most literature data, uncorrected intensities have been reported.
The Dhk‘correlation length of crystals was determined applying the Scherrer equation:
Dhk‘¼ Kλ= βð hk‘cosθhk‘Þ ð1Þ
where: K is the Scherrer constant,λ is the wavelength of the irradiating beam (1.5419 Å, CuKα), βhk‘is the width at half height, andθhk‘is the diffraction angle. The instrumental broadening, b, was also determined by obtaining a WAXD pattern of a standard silicon powder 325 mesh (99%), under the same experimental conditions. For each observed re-flection with βhk‘b 1°, the width at half height, βhk‘= (Bhk‘— b), was corrected by subtracting the unavoidable instrumental broadening of the closest silicon reflection from the experimental width at half height, Bhk‘.
2.4.2. Transmission Electron Microscopy (TEM)
TEM analysis of CPN was carried out with a Zeiss EM 900 microscope, applying an accelerating voltage of 80 kV. Ultrathin sections (about 50 nm thick) were prepared by using a Leica EM FCS cryoultramicrotome equipped with a diamond knife (sample temperature:−130 °C). 2.4.3. Rheometric analysis
Crosslinking reactions were studied at 170 °C with a Monsanto os-cillating disc rheometer MDR 2000 (Alpha Technologies, Swindon, UK), determining the time ts1required to have an increase of the mod-ulus of 1 dN/m, with respect to the minimum value, and the time t90 re-quired to achieve 90% of the maximum modulus.
2.4.4. Shear dynamic-mechanical measurements
Dynamic-mechanical properties were measured by means of a dy-namic mechanical analyzer DMA Q800 (TA Instruments) under shear configuration. Tests were carried out by employing an ad-hoc designed fixture, schematically sketched inFig. 1a and consisting in a symmetric shear sandwich configuration. The fixture was realized by means of alu-minum rods connected to alualu-minum plates (plates dimensions: 8 × 4 × 2 mm), to which rubber specimens, with size of about 8 × 4 × 2 mm
(Fig. 1b), were attached by means of an acrylic glue. Thefixture was
later mounted on the DMA dual cantilever beam clamp, as shown in
Fig. 1c, where, by keeping the extremitiesfixed and subjecting the
cen-tral portion to the alternate motion through the DMA movable arm, it was possible to transfer a dynamic sinusoidal shear strain on the rubber samples.
The specimens were subjected to two main types of dynamic me-chanical shear tests: (i) strain sweep tests; (ii) multifrequency tests at various temperatures. (i) The strain sweep testing methodology consisted in subjecting rubber specimens to progressively higher shear strain amplitudes, on a range between 0.1% and 30%. The tests were carried out at room temperature and at a frequency of 1 Hz on the materials ofTable 1. To evaluate the repeatability of the measure-ment and quantify the standard deviation, three samples were exam-ined for each material family. (ii) Multifrequency testing methodology was performed on the masterbatches containing only the polymer, the silane and either D-OMt or I-OMt, and in absence of any crosslinking. The specimens were subjected to a shear strain amplitude of 0.1%, while cyclically sweeping two decades of frequency (explored frequen-cies: 0.3, 1, 3, 10, 30 Hz) and continuously increasing the temperature from−50 to 100 °C. This allowed one to obtain quasi-isothermal repre-sentations of the frequency dependence of the storage modulus G′. Ac-quired data were elaborated by TA Instrument's Rheology Advantage Data Analysis software, and, by a tentative application of the time –tem-perature superposition principle, it was possible to provide a master curve representations of the G′ dependence on frequency, adopting −45 °C as the reference temperature for the shift of experimental points along the frequency axis.
3. Results and discussion
3.1. Characterization of pristine and milled Mt and OMt
XRD analysis was performed on pristine and milled (in planetary ball mill) Mt and OMt samples. for 24, 72, 168 and 240 h. Very mild mill-ing conditions were adopted, in order to have the kinetics of delamina-tion suitable to compare the structural modifications of Mt and OMt.
Fig. 2shows XRD patterns in the 2θ range 2–70° of Mt samples, pristine and milled for the reported times, with zoom on (001) reflection.
In the pattern of pristine Mt, reflections due to periodicities perpen-dicular to the structural layers are visible: (001), (002) and (004) at 7.2°, 14.6° and 28.6° as 2θ angle values, respectively. From these 2θ values of (00‘) reflections, the d001value can be evaluated as 1.23 nm. The XRD pattern of pristine Mt, displayed inFig. 2, shows as well reflections due to the typical (020), (210) and (060) in-plane Mt periodicities
(Galimberti et al., 2007a). A higher degree of Mt delamination was
ob-tained by increasing the milling time: (002) reflection is not detectable in the pattern of Mt milled for 72 h, (001) reflection is hardly detectable in the patterns of Mt milled for 168 and 240 h. The broadening of (001) and (004) reflections indicates the progressive delamination of Mt as the milling time increases. The correlation length evaluated from (001) reflection decreases from 6 nm, for the unmilled sample, to 2 nm for the sample milled for 168 h, thus approaching the Mt interlayer distance (d001= 1.2 nm). However, the detectability of (001) reflection in the pattern of Mt sample milled for the longest time indicates that Mt exfoliation was not fully achieved with the adopted experimental con-ditions. Reflections due to in-plane Mt periodicities are clearly evident in all the patterns.
Fig. 3shows XRD patterns in the 2θ range 2–70° of OMt samples,
pristine and milled for different times.
A high degree of order in the direction perpendicular to the clay mineral layers is in pristine OMt, as indicated by the presence of some (00‘) reflections: (001), (002), (003) and (004) at 2.5°, 4.7°, 7.2° and 9.8° as 2θ values, respectively. From the 2θ angle value of (001) peak, the d001 value can be evaluated as 3.5 nm. As already reported
(Galimberti et al., 2011; Ray and Okamoto, 2003), the intercalation of
the dialkylammonium leads to the expansion of the clay mineral inter-layer distance. On the basis of the thickness of the clay mineral inter-layer (≈1 nm) and of the length of 2HT (≈2.5 nm) (Osman et al., 2002), the tilting angle of the hydrocarbon chains in the paraffin-type arrange-ment can be evaluated to be not far from 60°. As reported above for pris-tine Mt, well defined reflections are present in the OMt pattern, due to the typical 020, 210 and 060 in-plane Mt periodicities (Galimberti et al.,
2007a). Moreover, a well defined narrow reflection is present at 2θ =
21.7° (100r), with 0.42 nm as the distance between crystallographic planes. This is the distance between long n-alkanes chains in their rota-tor order (Chazhengina et al., 2003; Fu et al., 2011; Ungar and Masic, 1985). Rotator order for long hydrocarbon chains was observed in an-ionic clay minerals (layered double hydroxide, LDH) (Itoh et al., 2003) and in graphite oxide (Mauro et al., 2013) intercalate compounds. The
inspection of XRD patterns reveals that the intensity of the (00‘) reflec-tions is progressively reduced, as the milling time increases. (00‘) re-flections become hardly detectable in OMt samples milled for long times (168 and 240 h), indicating an extensive delamination.
Vice versa, reflections due to in-plane periodicities are not substantial-ly affected by milling. It is worth noting that the reflection corresponding to the rotator order of the long hydrocarbon chains, although reduced in intensity, remains well evident also in the OMt sample milled for 240 h.
The overall examination of experimentalfindings from XRD analysis allows the following comments: delamination occurred to a larger ex-tent in OMt than in Mt and the in plane order was nearly fully retained in milled OMt samples while it was significantly reduced in milled Mt samples. Interaction appears thus stronger between the closest layers in Mt and the milling energy leads to a partial breakdown of the order in the octahedral sheet of Na+-Mt clay mineral, as reported in the liter-ature for Ca2+-Mt (Hrachova et al., 2007). Moreover, the order due to the packing of the long hydrocarbon chains in OMt was maintained also after long-term milling leading to nearly complete exfoliation. In order to substantiate thefirst two comments with a quantitative elabo-ration of XRD data, the areas of (00‘) and (0k0) reflections were mea-sured, for Mt and OMt samples, for every milling time and were normalized with respect to the areas of the same reflections of the pristine samples. (001) and (004) were selected as (00‘) reflections and (020) and (060) as the (0k0) reflections. The obtained ratios (A(hk‘)(t)/A(hk‘)(0)) are reported inFig. 4as a function of the milling time, in a plot that provides the kinetics of delamination.
Curves inFig. 4show that a substantial delamination (about 60%, measured from the (001) reflection) is obtained for OMt with the first milling step, whereas the same delamination level is obtained for Mt with the longest milling time. Milling leads to 90% OMt delamination, hence very close to exfoliation. Milling energy appears to be spent by OMt for the delamination process and, as anticipated above, the in-plane order is not appreciably affected. Vice versa, a reduction of about 40% of the (060) reflection area is observed for Mt, after prolonged milling. Milling energy appears to be spent by OMt essential-ly for the delamination process and by Mt for both delamination and reduction of the in-plane order.
A further quantitative elaboration of XRD data was performed: the area (A) of (001) and (060) reflections (A(001)and A(060)respectively) were used to derive, for all the samples, the ratio R indicated in Eq.(2):
R¼ Að001Þ=Að060Þ: ð2Þ
Fig. 2. XRD patterns in the 2θ range 2–70° of Mt samples, pristine and milled for different times. Zoom on (001) reflection is in the right part of the figure. Milling hours are indicated at the right side of the patterns. The diffraction intensities have been corrected as described in the experimental section.
Values of R ratios were then used to calculate the so called delamina-tion index (DI), expressed by Eq.(3):
DI¼ 1–Rð t=R0Þ 100 ð3Þ
where Rtand R0are the ratios at a given milling time t and at time = 0 (unmilled sample), respectively. This delamination index indicates the efficiency of milling in promoting clay mineral delamination, while preserving the crystalline order inside the structural layers. In this framework, DI = 0 indicates the absence of delamination (or equal re-duction of crystalline order parallel and perpendicular to the structural layers) while DI = 100 indicates complete clay mineral exfoliation. Values of R ratio and DI are reported inTable 2.
Data inTable 2, as derived by the patterns ofFigs. 1 and 2and by the plot ofFig. 3, indicate that both Mt and OMt can be delaminated by milling and that, in particular, milling is an efficient tool for preparing
a largely delaminated, almost exfoliated, OMt, preserving the crystalline order in the structural layers. Thesefindings show thus an easier delam-ination for OMt rather than for Mt. In the prior art (Hrachova et al.,
2007; Ramadan et al., 2010), more pronounced delamination was
instead reported for pristine mineral clay minerals rather than for OMt. However, in these works, OMt contained different compensating cations: octadecyltrimethylammonium (Hrachova et al., 2007) and di(hydroxyethyl)talloyl ammonium (Ramadan et al., 2010). The substi-tution of the nitrogen atom with only one long chain hydrocarbon substituent led to a lower interlayer distance that could account for a more difficult delamination. It is also worth adding that an easier delam-ination of pristine Mt with respect to the OMt with 2HT as the compen-sating cation occurred when such Mt and OMt experienced a prolonged mixing in the NR polymer matrix (Galimberti et al., 2007a).
Two types of OMt become thus available: pristine OMt, intercalated with the ammonium cation and delaminated OMt.
Fig. 3. XRD patterns in the 2θ range 2–70° of OMt samples, pristine and milled for different times. Zoom on (00‘) reflections is in the right part of the figure. Milling hours are indicated at the right side of the patterns. The diffraction intensities have been corrected as described in the experimental section.
Fig. 4. (A(hk‘)(t)/A(hk‘)(0)) ratios versus the milling time for the following reflections: ▲ 020, ▼ 060, ■ 002, ● 001 for OMt sample and △ 020; ▽ 060; ○ 001; □ 004 reflections for Mt
3.2. Clay polymer nanocomposites with I-OMt and D-OMt
CPN were prepared with IR as the polymer matrix and either I-OMt or D-OMt as thefiller. Pristine commercial sample was used as I-OMt and the OMt sample with about 90% as Delamination Index (milled for 168 h) was selected as D-OMt.
In the patent literature, it was reported by some of the authors that D-OMt can be used in elastomeric nanocomposites for dynamic-mechanical applications such as the one in tire compounds (Galimberti et al., 2007b, 2008). An objective of the research activity was thus to in-vestigate the effect of OMt delamination on the dynamic-mechanical properties of elastomeric nanocomposites that were prepared via melt blending, adopting the formulations shown in Table 1, and were crosslinked with a typical sulfur based crosslinking system (Coran, 2005). The ammonium cations present in OMt bring about a noticeable acceleration of sulfur based crosslinking reaction (Avalos et al., 2008; Maiti et al., 2008; Sengupta et al., 2007; Verdejo et al., 2011). The forma-tion of tertiary amines from the ammonium caforma-tions and the increased mobility of sulfur accelerating anionic species are reported to be at the origin of this phenomenon (Giannini et al., 2011). To have smooth crosslinking kinetics, a combination of pre-vulcanization inhibitors, phthalic anhydride and N-cyclohexylthiophtalimide (Giannini et al., 2005) was used in this work. Values determined for the crosslinking pa-rameters allow the comment that CPNs were properly crosslinked. In fact, ts1values were about 3 min, T90values were about 5 min and rever-sion reaction was not observed, for any of the nanocomposites.
CPN structure was investigated by means of XRD and Transmission Electron Microscopy analysis.Fig. 5shows the patterns of CPN containing either I-OMt (Fig. 5a) or D-OMt (Fig. 5b).
It is evident for the higher degree of order, in the direction orthogonal to the clay mineral layers, for I-OMt. In fact, (00‘) reflections are clearly visible in the pattern of nanocomposite with I-OMt, whereas are hardly detectable in the pattern of nanocomposite with D-OMt. For instance, the intensity of the (002) reflection, normalized on the basis of the amorphous rubber halo, is more than 10 times higher for I-OMt than
for D-OMt. This suggests that a delamination close to 90% is maintained for D-OMt also after nanocomposite preparation. It is also worth noting that (002), (003) and (004) reflections of I-OMt are substantially at the same 2θ values in the nanocomposite (Fig. 5a) and in the unmilled OMt (Fig. 2, 0 h milling). Hence, there is no indication of polymer chain intercalation in the OMt interlayer space. To investigate the dispersion of OMt in the polymer matrix, many TEM micrographs were inspected: even dispersion was observed for both I-OMt and D-OMt.Fig. 6shows TEM micrographs representative of CPN containing either I-OMt (Fig. 6a) or D-OMt (Fig. 6b). It is evident that OMt stacks are formed by a higher number of layers in the case of I-OMt whereas smaller aggre-gates are in the nanocomposite with D-OMt. It can be thus commented that CPN with either I-OMt or D-OMt contain evenly dispersed OMt ag-gregates with higher or lower number of clay mineral layers. The lower dimension of crystals in the direction orthogonal to structural layers may be ascribed to the size of the aggregates rather than to the heteroge-neity in the layer organization.
In order to evaluate the effect of OMt structure on the mechanical behavior of IR based nanocomposites, dynamic mechanical analysis was carried out on both crosslinked systems and masterbatches without crosslinks among the polymer chains.
Crosslinked systems ofTable 1were subjected to dynamic mechan-ical analysis by applying the strain sweep methodology described in the
Experimentalpart. Results reported inFig. 6display the dependence of
the storage modulus G′ (Fig. 7a) and of the loss modulus G″ (Fig. 7b) on the strain amplitude for the neat rubber matrix and for CPN with I-OMt and D-OMt.
G′ and G″ data are reported in terms of averaged values, with the corresponding standard deviation error bar. G′ traces inFig. 7a are well separated, while inFig. 7b the G″ curves referring to the neat rub-ber and to the nanocomposite with D-OMt are almost overlapped.
Fig. 7a shows that CPNs have higher G′ values, over the whole span of
strain amplitudes, although only a moderate enhancement is observed, as expected by considering the low content of OMt. The nanocomposite with I-OMt displays the higher enhancement of both G′ and G″, at all the explored strains.
All the composites display a reduction of the storage modulus with the shear strain amplitude, this effect being more pronounced for the system incorporating I-OMt. Such a non linear behavior for the I-OMt based nanocomposite is also reflected by the G″ trace inFig. 7b. D-OMtfilled system exhibits a modest non linear behavior, similar to that shown by the neat rubber that may be ascribed to the presence of additives such as stearic acid, ZnO and the para-phenyldiamine (see
Table 1, note c). This moderate modulus reduction with strain
ampli-tude, associated with the less marked peak of G″, suggests that the de-lamination of OMt may be regarded as a promising strategy to lower Table 2
R ratio and DI delamination index for pristine and milled Mt and OMt samples. Milling time (hours) R DI Mt OMt Mt OMt 0 0.25 0.17 – – 24 0.24 0.060 4 65 72 0.10 0.029 60 83 168 0.09 0.019 63 89 240 0.089 0.013 64 92
the energy dissipation in many dynamic applications of reinforced rubbers, where such a characteristic is demanded.
The decrease of the storage modulus with the applied strain, in poly-mer melts and elastopoly-mers containing reinforcingfillers, is a non linear-ity effect known as Payne effect (Payne and Whittaker, 1971). Such an effect is usually considered as indication of the occurring of the so called filler networking phenomenon and it is ascribed to an agglomeration– de-agglomeration process of thefiller particles above the filler percola-tion threshold (Bohm et al., 2010and refs therein). An alternative mechanism has been proposed, consisting in afiller-matrix bonding and debonding process (Jancar et al., 2010and refs therein). When thefiller content is above the percolation threshold, the agglomera-tion–de-agglomeration process of the filler particles is presumably the prevalent mechanism taking place. The amount of OMt used in the pres-ent work is close to the one needed to have percolation. In fact, in the literature the percolation threshold for the same type of OMt used in the present work was reported to be in a range from about 6 php (Galimberti et al., 2012) to about 8 php (Ramorino et al., 2009) for nano-composites in either IR or NR as the polymer matrix, respectively. On the basis of these considerations, the amount of OMt used in the present work should be considered enough to have an appreciable Payne effect, with both I-OMt and D-OMt. Results here reported show instead larger Payne effect for I-OMtfilled nanocomposite and comparable entity of Payne effect for D-OMtfilled nanocomposite and for neat rubber. It has also to be commented that in the literature works mentioned above (Galimberti et al., 2012; Ramorino et al., 2009), I-OMt was used,
whose XRD pattern was very similar to the one shown inFig. 5a, with-out any evidence of intercalation of polymer chains. It could be thus hy-pothesized an easier occurring of thefiller networking phenomenon with I-OMt.
However, in the interpretation of the reported results, it is not possi-ble to exclude the contribution offiller-matrix bonding–debonding ef-fects. To better investigate the existence of these possible mechanisms, multifrequency dynamic mechanical analysis was carried out on the non-crosslinked masterbatches, as described in theExperimentalpart. These tests were aimed at providing a master-curve representation of G′ as a function of frequency. In fact, previous studies (Sternstein and
Zhu, 2002) have attributed the modulus enhancement to a restriction
of large-scale conformational changes of polymer chains and to a reduc-tion in their conformareduc-tional entropy (entropic hardening). Also in the presence of lowfiller concentrations, this effect exerted by the filler may be evidenced as an extension of the relaxation process to longer times (Sternstein et al., 2010). To obtain this type of information for our systems, tests were performed at small strain amplitudes (0.1%) on the masterbatches containing only the polymer, the silane and either D-OMt or I-OMt, i.e. in absence of any effect arising from the crosslinking network and from additives. By applying frequency sweeps at various temperatures, from below Tgto about 100 °C, it was possible to represent the frequency dependence of the shear storage modulus, G′, as isother-mal curves. From the data, the G′ vs. frequency master curve was built for the various materials, as reported inFig. 8for a reference temperature T0=−45 °C.
Fig. 6. TEM micrographs of CPN based on IR and OMt: (a) I-OMt (b) D-OMt.
It appears that, as expected, the addition of OMt affects the master curve rubbery plateau, which displays a lower slope and a major exten-sion toward low frequencies (i.e. longer relaxation times); A quantita-tive evaluation of the plateau length was performed, as shown in the inset ofFig. 8. It was found that the plateau covers about 5 decades for the neat rubber and extends to almost six decades for thefilled systems. While the plateau slope of CPN is similar, the larger extension of I-OMt curve toward lower frequencies suggests thisfiller structure to be more efficient in restraining the mobility of polymer chains.
The strain sweep and multifrequency tests are thus both confirming that the incorporation of I-OMt leads to a more effective initial modulus enhancement. This result may be regarded as counterintuitive, as it shows that larger reinforcement is obtained with afiller with lower in-terface interaction with the matrix. In fact, separation of nanofillers into individual particles is expected to lead to easierfiller networking, that means to an easier formation offiller particles networks and to a more pronounced interaction with the polymer matrix. To account for these experimental data, it could be hypothesized that I-OMt has higher volume fraction than D-OMt, as the intercalated alkylammonium ions contribute to thefiller volume of I-OMt, whereas they simply act as compatibilizers in the case of D-OMt. Indeed, in this latter system, the amount of effective reinforcingfiller and its structure could be far from achieving percolation, preventing thefiller networking phenome-non and leading to reinforcement effect could be prevalently ascribed to a restriction of the chain mobility.
D-OMt appears to promote a lower non-linearity of dynamic-mechanical properties of CPN based on polymer melts and elastomers and, as commented above, gives rise to a lower dissipation of energy. Thisfinding is of great importance for a large scale application of clay minerals in dynamic-mechanical applications of elastomeric nanocom-posites, such as the one in tire compounds, and it is in line with what reported in the patent literature (Galimberti et al., 2007b, 2008). 4. Conclusions
The preparation of polymer nanocomposites with extensively delaminated clay minerals is still a subject of research activity, both in the academic and industrial laboratories. Efficient delamination of OMt was obtained in this work through ball milling: OMt was delaminated for about 90%, without observing appreciable variations of the in-plane Mt crystalline order. Delamination of Mt was vice versa accompanied by detectable reduction of crystallinity in the structural planes. Polymer nanocomposites were thus prepared with extensively
(90%) delaminated OMt (D-OMtfilled systems) and with OMt with
intercalated ammonium cations (I-OMtfilled systems). Unexpectedly, D-OMt is less effective in promoting networking phenomenon with re-spect to I-OMt intercalated systems and in reducing polymer chains
mobility. The minor volume fraction of D-OMt is hypothesized to be at the origin of thesefindings, as the alkylammonium cations do not con-tribute to OMt volume and essentially act as compatibilizer of OMt with the polymer matrix. These results can be considered as preliminary but stimulating indications of a counterintuitive dynamic mechanical be-havior of nanocomposites with either delaminated or intercalated organoclays. The delamination of OMt appears as a powerful tool to re-duce the non linearity of dynamic-mechanical properties of CPN based on polymer melts and elastomers and, as a consequence, to promote lower dissipation of energy.
Acknowledgments
Dr. Lucia Conzatti (ISMAC-CNR Genova) is gratefully acknowledged for performing TEM analysis.
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