ARTICLE IN PRESS r2008)1-18 and some other buted to loss of unbound surf al,ca edi a co ch ha mall angle X-ray sca hat e as e 56. 塞 ar monon content in 3.3.Characterization of nanocomposite mopholo relate the perfo rcome re:experimental evalu mages is not in the oper tion of the micros but in uired as in the case I nm thick clay platelets as dark line sis can be used ti时y the strib ion latele ngth [58,84.93,119.Ho ever,it mus t be rem embered that thedi then he pt.Th inte for the hert nulon 6 nan complete ne can see particles co lded su indica ting th to one another as sugg d in Fig.5 93).Thus oids are more 103.118.Hc the pea :mad from rlvole cation relative at of the lay.Th polymers that I lower the n opp ger than Please cite this article in press as:Paul DR,Robeson LM,Polymer(2008).doi:10.1016/j.polymer.2008.04.017
polyurethanes, and some other polar polymers [84,96,101]. It has been found that a small amount of a polyolefin that has been lightly grafted with maleic anhydride, w1% MA by weight is typical, can act as a very effective ‘‘compatibilizer’’ for dispersing the organoclay in the parent polyolefin [84,101,103,106,117,118,143–148]. This does not lead to the high level of exfoliation that can be achieved in polyamides, but this approach has allowed such nanocomposites to move forward in commercial applications, particularly in automotive parts [54,99,106]. In the case of olefin copolymers with polar monomers like vinyl acetate and methacrylic acid (and corresponding ionomers), the degree of exfoliation that can be achieved progressively improves as the polar monomer content increases [113,119]. In all cases, the best exfoliation is achieved when the structure of the surfactant and the process parameters are optimized. 3.3. Characterization of nanocomposite morphology An important issue is to relate the performance of nanocomposites to their morphological structure; experimental evaluation of performance is certainly easier than characterization of their morphology. Wide angle X-ray scattering, WAXS, is frequently used because such analyses are relatively simple to do. However, such analyses can be misleading and are not quantitative [149– 151]. As indicated in Fig. 3, the organoclay has a characteristic peak indicative of the platelet separation or d-spacing; other peaks may be seen resulting from multiple reflections as predicted by Bragg’s law. The presence of the same peak in the nanocomposite is irrefutable evidence that the nanocomposite contains organoclay tactoids as suggested in Fig. 3. However, the absence of such a peak is not conclusive evidence for a highly exfoliated structure as has been repeatedly pointed out in the literature [151]; many factors must be considered to interpret WAXS scans. If the sensitivity, or counting time, of the scan is low, then an existing peak may not be seen. When the tactoids are internally disordered or not well aligned to one another, the peak intensity will be low and may appear to be completely absent. These issues can be well illustrated by analyses of polyolefin nanocomposites, which are never fully exfoliated, that have been injection molded. X-ray scans of the molded surface reveal a peak indicating the presence of tactoids. However, after milling away the surface of these specimens, subsequent scans of the milled surface in the core of the bar may not reveal a peak because the tactoids are more randomly oriented in the interior than near the as-molded surface [103,118]. However, if a more sensitive scan is made, the peak can usually be seen. In some cases, the WAXS scan may reveal a shift in the peak location relative to that of the neat organoclay. The peak may shift to lower angles, or larger d-spacing, and is generally taken as evidence of ‘‘intercalation’’ of polymers (or perhaps other species) into the galleries [48–53,73,79]. However, an opposite shift may also occur, and this is usually attributed to loss of unbound surfactant from the gallery or to surfactant degradation [89,107]. All of these processes may occur simultaneously rendering uncertainty in the interpretation. In any case, intercalation per se does not seem to be a contributor to develop useful nanocomposite performance. Small angle X-ray scattering, SAXS, can be more informative and somewhat quantitative as explained by numerous authors [17,152– 156]. However, this technique has not been widely used except in a few laboratories probably because most laboratories do not have SAXS facilities or experience in interpreting the results. Other techniques like solid-state NMR and neutron scattering have also been used on a limited basis to explore clay dispersion [95,157– 162]. A far more direct way of visualizing nanocomposite morphology is via transmission electron microscopy, TEM; however, this approach requires considerable skill and patience but can be quantitative. Use of TEM is often criticized because it reveals the morphology in such a small region. However, this can be overcome by taking images at different magnifications and from different locations and orientations until a representative picture of the morphology is established. The major obstacle in obtaining good TEM images is not in the operation of the microscope but in microtoming sections that are thin and uniform enough to reveal the morphology. Fortunately, the elemental composition of the clay compared to that of the polymer matrix is such that no staining is required. When exfoliation is essentially complete, as in the case of nylon 6, one can see the w1 nm thick clay platelets as dark lines when the microtome cut is perpendicular to the platelets. Image analysis can be used to quantify the distribution of platelet lengths, but meaningful statistics require analyzing several hundred particles [58,84,93,119]. However, it must be remembered that the dimensions observed reflect a random cut through an irregular platelet and only rarely will the maximum dimension be seen [163,164]. Thus, the aspect ratio distribution seen in this way will lead to smaller values than true dimensions like those given by Fig. 2. Even for the best nylon 6 nanocomposites, exfoliation is generally never complete and one can see particles consisting of two, three or more platelets [58]. In some cases, these platelets may be skewed relative to one another as suggested in Fig. 5 [93]. Thus, some particles may appear to be longer than the platelets really are. These kinds of issues should be kept in mind when interpreting quantitative analyses of particle aspect ratios and in comparison of observed performance with that predicted by composite theory [58]. Nanocomposites made from polyolefins, styrenics, and other polymers that lead to lower degrees of exfoliation reveal particles much thicker than single clay platelets as expected [101,110,111,117,119]. However, the clay particles are also much longer than the individual clay platelets indicated in Fig. 2. As the Length of the whole particle Length of a single platelet a b Length of the whole particle Length of a single platelet ‘Skewed’ agglomerate 50 nm Fig. 5. Examples of skewed platelets such that particles appear longer than platelets of MMT [93]. Reproduced with permission of Elsevier Ltd. 6 D.R. Paul, L.M. Robeson / Polymer xxx (2008) 1–18 ARTICLE IN PRESS Please cite this article in press as: Paul DR, Robeson LM, Polymer (2008), doi:10.1016/j.polymer.2008.04.017
ARTICLE IN PRESS D.R Poul LM.Robeson/Polymer x(200)1-1 Nanocomposite Glass Fibers 30 wt%filler [103,106,117-1201.H nerally the ide thickness de hat the aspect ratio he filler is es not mean tha approxin y thr e m tha MMT me ext ns [120 e ha weight ac e com glas asusually drawn,see Figs.3and4 where the plateletsare all of th ingle axis in th irection of their alig 165).In additio and how these par glass fbsh it owing to nar size of the rsus the 10-15e er of the ntra do with its nanometr de dimens d ely the this wever.the short ans wer is that v ve can ex 3.There iso theoretical uidance on which is the be vithou 31i719 performance or for use in composite o ta The ater d TPO [103 some cases,they are being replaced withTPO nanocomposites.In 30 of dis 26A 。TPO with MMT aspe TPO with Tak nate th can be used t 2.2 and even quar tio be fact 1.8 3.4.Nanocomposite mechanical properties: s for 185)P erly c rsed an 20 Thi omparing the increase in the Filler Content (w 6.relative to the modulus of the neat polvamide matrix.when g品om1 M Please cite this article in press:Paul DR Robeson LM,Polymer (0)
polymer–organoclay affinity is increased by adding a compatibilizer, e.g., PP-g-MA or PE-g-MA, or increasing the content of a polar comonomer, e.g., vinyl acetate, the clay particles not only become thinner (fewer platelets in the stack) but also become shorter [103,106,117–120]. However, generally the particle thickness decreases more rapidly than the length such that the aspect ratio increases; this generally improves performance. The fact that the particles become shorter does not mean that clay platelets are breaking or being attributed during processing, although, this may occur under some extreme conditions [120]. Instead, considerable evidence indicates that the vision of tactoids as usually drawn, see Figs. 3 and 4, where the platelets are all of the same length and in registry with one another is not correct. Fig. 6 shows a more realistic vision of a tactoid where the particle length can be much longer than individual platelets and how these particles evolve as dispersion improves [106,117]. Complications arise when calculating an average aspect ratio of particles when there is a distribution of both length and thickness. First, one can calculate a number average, a weight average, or other weightings of the distribution [58,113,119]. Second, one can average the aspect ratios or average separately the lengths and thickness and calculate an aspect ratio from these averages [113,119]. There is no theoretical guidance on which is the better predictor of performance or for use in composite modeling [113,117,119]. To take full advantage of the reinforcement or tortuosity clay platelets or particles can provide to mechanical and thermal or barrier properties of nanocomposites, they must be oriented in the appropriate direction and not curled or curved. The alignment of particles is affected by the type of processing used to form the test specimen, e.g., extrusion, injection molding, etc. This is a separate issue from the degree of dispersion or exfoliation which is usually determined in the mixing process. Techniques like compression molding usually do not lead to good alignment or straightening of the high aspect ratio particles, and measurements made on such specimens often underestimate the potential performance. TEM can be used to assess and even quantify particle orientation and curvature and this information can, in principle, be factored into appropriate models to ascertain their effect on performance [86,110,131,134,138]. 3.4. Nanocomposite mechanical properties: reinforcement A common reason for adding fillers to polymers is to increase the modulus or stiffness via reinforcement mechanisms described by theories for composites [58,165–185]. Properly dispersed and aligned clay platelets have proven to be very effective for increasing stiffness. This is illustrated in Fig. 7 by comparing the increase in the tensile modulus, E, of injection molded composites based on nylon 6, relative to the modulus of the neat polyamide matrix, Em, when the filler is an organoclay versus glass fibers [58]. In this example, increasing the modulus by a factor of two relative to that of neat nylon 6 requires approximately three times more mass of glass fi- bers than that of montmorillonite, MMT, platelets. Thus, the nanocomposite has a weight advantage over the conventional glass fiber composite. Furthermore, if the platelets are aligned in the plane of the sample, the same reinforcement should be seen in all directions within the plane, whereas fibers reinforce only along a single axis in the direction of their alignment [165]. In addition, the surface finish of the nanocomposite is much better than that of the glass fiber composite owing to nanometer size of the clay platelets versus the 10–15 m diameter of the glass fibers. A central question is whether the greater efficiency of the clay has anything to do with its nanometric dimensions, i.e., a ‘‘nano-effect’’. To answer this requires considering many issues which we will do later in this section; however, the short answer is that we can explain essentially all of the experimental trends using composite theory without invoking any ‘‘nano-effects’’ [58]. Fig. 8 shows an analogous comparison of nanocomposites based on thermoplastic polyolefin or TPO matrix, polypropylene plus an ethylene-based elastomer, with conventional talc-filled TPO [103]. The latter is widely used in automotive applications; however, in some cases, they are being replaced with TPO nanocomposites. In Fig. 6. A more realistic picture of clay tactoids and how they become shorter as the level of dispersion increases. wt % filler 0 10 20 30 40 E / E m 1 2 3 4 Nanocomposites Glass Fibers Fig. 7. Comparison of modulus reinforcement (relative to matrix polymer) increases for nanocomposites based on MMT versus glass fiber (aspect ratio w20) for a nylon 6 matrix [58]. Reproduced by permission of Elsevier Ltd. Filler Content (wt%) 0 5 10 15 20 25 Relative Modulus 1.0 1.4 1.8 2.2 2.6 3.0 TPO with MMT TPO with Talc Fig. 8. Comparison of modulus reinforcement for nanocomposites based on MMT versus talc for a TPO matrix [103]. Reproduced by permission of Elsevier Ltd. D.R. Paul, L.M. Robeson / Polymer xxx (2008) 1–18 7 ARTICLE IN PRESS Please cite this article in press as: Paul DR, Robeson LM, Polymer (2008), doi:10.1016/j.polymer.2008.04.017