ARTICLE pubs.acs.org/Langmuir
Ultrathin Films of Variable Polarity and Crystallinity Obtained from 1,2-Polybutadiene Nanoparticle Dispersions Maria Carmen Morant-Mi~nana* and Brigitte Korthals University of Konstanz, Universit€atsstrase 10, D-78457 Konstanz, Germany
bS Supporting Information ABSTRACT: Characterization of ultrathin films of different polymer nanoparticles obtained at room temperature via spin-coating of aqueous dispersions and their morphology are described. Very small nanoparticles of semicrystalline 1,2-polybutadiene (PB), noncrystalline poly(1-butene) (PH), and poly(1-butenal) (PHF) were prepared via catalytic emulsion polymerization and subsequent hydrogenation or hydroformylation. The prefabricated nanoparticles were used as building blocks. The thin films obtained are continuous and transparent (n = 1.5; k = 0). The properties of these films, formed from different constituents, are analyzed. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) images show that the PB-films are very smooth (rms roughness = 10 nm) and polycrystalline. Recrystallization of these PB films reveals that edge-on lamellae are the constituent units. Films with very low roughness values (rms roughness < 2 nm) are obtained with PH nanoparticles, due to the soft character of the nanoparticles. The AFM profile of the PHF films reveals that the surface remains structured after drying due to the high degree of the internal cross-linking that occurs in the nanoparticles. Quantification of the films’ polarity (I3/I1 = 0.89, 1.3, and 2.1 for PHF, PB, and PH, respectively) agrees well with the previous values obtained for the polymer dispersions. Surfactant molecules are desorbed during the film formation; however, these aggregates can be removed by rinsing with water with no undesirable effects observed on the films.
’ INTRODUCTION Ultrathin polymer films consist of one or more layers of ca. 0.1 μm thickness applied to a substrate. Their applications lie mainly in the field of semiconductor devices (optics and electronics), coatings and packaging. In the past, their structure and properties have been deeply investigated for both scientific and industrial purposes.1 6 Recently, the preparation of crystalline ultrathin films from aqueous dispersions of prefabricated polyethylene (PE) nanocrystals at room temperature has been demonstrated as a novel route.7 This novel principle relies on the interaction of the amorphous parts located at the surface of polymer single crystals, which are sufficient despite their minor volume share due to the very small crystallite size.8 Interestingly, crystalline order does not appear after preparation of the films on a substrate, but the properties present in the nanoparticles are present in the films as confirmed by the analysis of them. Despite these interesting results, the use of PE nanocrystals as building blocks for film formation has been limited, due to the difficult postmodification of the polymer. For instance, modification in the crystallinity of such polymers has been reported but these variations are currently limited to microstructural modifications.9 The development of very small nanoparticles with reactive groups in the polymer backbone, which are amenable to further postpolymerization reactions, would potentially offer a new route to obtaining a wide range of functionalized nanoparticles for the preparation of these polymer thin films. This development is a promising area of research, since great efforts are expended on r 2011 American Chemical Society
understanding the structure properties relationship. For example, very small nanoparticles of syndiotactic 1,2-polybutadiene (PB) with high Mw and 14 nm diameter can be prepared from microemulsion by catalytic polymerization in aqueous media.10 These particles are of particular interest, since they consist of nanocrystals and it is also possible to use the presence of double bonds in each repeating unit for postpolymerization purposes. Examples of catalytic postpolymerization in aqueous dispersions were limited to hydrogenation of the submicrometer particles.11 Recently, we have reported the synthesis of very small polymer nanoparticles with adjustable polarity.12 For example, introduction of aldehyde groups results in nanoparticles which differ from the original nanoparticles in a loss of crystallinity, as well as a change in their polarity due to the complete conversion of their double bonds. Moreover these particles present a high degree of internal cross-linking which affords polymeric microgels in a pure polymer water system, without using monomers, cross-linking agents, or other auxiliary substances. In these microgels, the cross-linking is confined to the particle itself without any additional interaction across the particle boundaries. The presence of these internal bond results in different physical and chemical properties, including fixed shape and a higher resistance to degradation. Received: November 17, 2010 Published: May 18, 2011 7516
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Langmuir Scheme 1. Hydrogenation and Hydroformylation Reaction of PB Nanoparticles
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Table 1. Properties of the As-Prepared Polymer Nanoparticles polymer nanoparticles
size
Tm
degree of
I3/
(nm)a (°C)b crystallinity (%)c I1d
surface tension (mN/m)e
PB PH
14 14
154 n.o.
55 n.o.
1.12 1.38
60 51
PHF
20
n.o.
n.o.
0.68
54
a
Volume average particle size as determined by DLS. b Determined by DSC; n.o. = not observed. c Determined by powder X-ray diffraction of the bulk polymers; n.o. = not observed. d Relative intensity of pyrene I3/I1 bands calculated from their emission spectrum after addition of a water-soluble quencher (dimethylaminoethanol). e Measured on dialyzed dispersions.
thermal transition (cf. Supporting Information). Analysis of the C NMR spectrum confirms indeed that our polymer is syndiotactic. DLS measurements reveal a very small size for PB and PH particles. TEM and AFM images of isolated particles confirmed the DLS values. For PB nanoparticles, it proved difficult to characterize the exact shape of these nanocrystals (see Supporting Information Figure S5). The image displays the PB nanoparticles in diluted aqueous solution as obtained by cryo-TEM. The dispersion consists of a mixture of well-dispersed nanoparticles in the aqueous medium without aggregates. The images revealed that the nanoparticle had a nonspherical shape. The platelets appear as rods when lying parallel to the electron beam and often as circles when orthogonal to the electron beam. This observation is consistent with their crystallinity. The PH particles were revealed as being softer than PB particles. Despite the harsh conditions used in the hydroformylation reaction, colloidal stability of the nanoparticles was retained, the PHF particles appeared to be spherical, and no aggregates were observed.12a In this case, the shape of the nanoparticles is a consequence of the extensive cross-linking brought about by the aldol condensation of the polyaldehydes. The interaction of pyrene with nanoparticles shows different relative intensity ratios of the I3 and I1 band in the pyrene emission spectra between the dispersions (I3/I1 ratio). After removing the contribution of the water-soluble pyrene by the addition of dimethylaminoethanol quencher, the values listed in Table 1 show that the polarity of the surrounding medium is different.9,12a The values of the surface tension demonstrate that all surfactant is absorbed on the particles and no free sodium dodecyl sulfate (SDS) micelles remained as free micelles in the dispersion. Morphology of the Films. Ellipsometric measurements of the continuous films prepared from dispersions with different crystallinity and polarity at 2% wt polymer content revealed an average thickness value over the entire probed area. The values are listed in Table 2. Different regions were measured and similar thickness values were found which implies that spin-coating of the polymer nanoparticles at room temperature produces submicrometer films that are both very smooth and uniform. Mapping of larger areas (534 394 μm2) provides enough information for detecting inhomogeneities in layer quality that can occur in the spin-coating process. The maps show up an overall uniform structure with minor variations in the color scale and total thickness deviation within the sample below 2 nm (cf. Supporting Information). Considering the results determined by ellipsometry, the refractive index (n = 1.5) remains almost
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We present here a full account of the preparation of ultrathin films of variable crystallinity and polarity from PB dispersions and the analysis of their structure, morphology and properties. The effect of the changes in the original dispersions after postpolymerization reaction on the films’ properties is discussed, based on the results obtained from atomic force microscopy (AFM), ellipsometry, and transmission electron microscopy (TEM). Characterization of these unique thin-films, with variable polarity and crystallinity, prepared from very small nanoparticles previously modified is presented in detail.
’ RESULTS AND DISCUSSION Dispersions of PB were prepared by catalytic emulsion polymerization according to literature.10 In brief, dynamic light scattering (DLS) analyses of the latexes show 14 nm nanoparticles of semicrystalline, syndiotactic polybutadiene (97% 1,2and 3% 1,4-cis as determined by IR spectroscopy, Mn = 3.4 104 g mol 1, Mw/Mn = 2.1). Subsequent modification of the original PB nanoparticles was performed using postpolymerization reactions summarized in Scheme 1. Detailed characterization data of these dispersions are listed in Table 1. Hydroformylation of the PB latexes was performed according the procedures described in the literature.12a The obtained dispersions are designated as poly(1-butenal) (PHF) in the following. Polymers of controlled microstructure and different polarity were obtained in the form of particles of 150 °C), especially for those polymers with high syndiotacticity.18 In the case of continuous PB films with thickness ca. 50 nm, polymeric spherulite structures with edge-on lamellae were a preferred crystalline form since they can be better distinguished in the phase image (Figure 1b-2). The recrystallization of the film was also studied using polarization microscopy. At crystallization temperature of 120 °C only small spherulites were observed. The spherulites stop growing (almost immediately after their formation) at a relatively small size of several micrometers. As can be observed in the microscope image in Supporting Information Figure S9, dwarf spherulites induce new dwarf spherulites along their periphery, with the process repeating itself until the apparent end of crystallization. This peculiar morphology of dwarf spherulites was also observed in high molecular weight samples of linear poly(aryl ether ether ketone)19 or long-chain branched 1,4-cis-PB20 where the formation of dwarf spherulites appears related to the suppressed growth rate and the stronger tendency to induce nucleation. Interestingly, as can be seen in the image of Figure 1b-1, aggregation of surfactant after migration from the glass film interface or air interface cannot be observed. However, this fact cannot be excluded because the surfactant molecules can lie in between the roughness limit. No significant changes in roughness values were observed. Distribution of the surfactant molecules desorbed from the nanoparticles during the film-forming process can affect the stability of the nascent films. Static contact angle values obtained for original PB films were around 50° (Table 3). In comparison
to the values reported in the literature for PB films prepared using another method, the angles obtained are very low.21 This difference probably originates from the accumulation of the surfactant on the film during its formation.22 It is well-known that the surfactant adsorbed onto the PB nanoparticles is desorbed during the film formation, with the surfactant molecules forming aggregates which, depending on the SDS and polymer compatibility, may be uniformly distributed within the film or concentrated at the film interfaces.23 As expected, after rinsing the film surface with water and subsequent drying of the film, the contact angle increases to 96° for PB films. Morphology of the Ultrathin Soft Films. As has been mentioned previously, hydrogenation of the semicrystalline 1,2-syndiotactic PB nanoparticles afforded amorphous PH nanoparticles. Spin-coating of these PH nanoparticles prepared in aqueous dispersions on glass substrate achieved PH-films of ca. 100 nm thickness. AFM profiles in Figure 2a are indicative of the soft nature of the constituent nanoparticles of the films. In this case, the nanoparticles fuse, forming a continuous film whose original shape cannot be distinguished in the resulting PH film. Comparison of the PH films and conventional organic solution cast film of poly(1-butene) were performed. Poly(1-butene) was isolated by precipitation from the polymer dispersion. No significant differences in terms of roughness and homogeneity were found (Table 2 and cf. Supporting Information). Figure 2c shows the BF electron micrograph of the PH films. The differences in density can only be attributed to changes in the surface roughness. The electron diffraction patterns only reveal the existence of amorphous halos (Figure 2d) After heating PH films, some brilliant dots with height of 5 nm are observed over the film surface (Figure 2b). A possible explanation is that they are surfactant molecules that form small aggregates on the surface after migration to the film air interface during the heating process, as has been previously observed.17,24 Morphology of the Polar Films. For continuous films with a higher degree of polarity, hydroformylation of the PB-nanoparticles and subsequent spin-coating was performed. PHF-films of ca. 90 nm thickness with roughness values of ca. 5 nm were prepared. Note that in the AFM image (Figure 3a and a-1) of these PHF-films it is possible to distinguish the original building blocks of the films. This observation could be due to the high degree of the internal cross-linking that is observed in the original PHF nanoparticles.25 The surface of the films obtained from cross-linked nanoparticles remains structured, and the spherical contours of the particles can be clearly seen on the surface of the film, in contrast to the flat surface of the film cast from linear polymers. Interestingly, this can also be observed using TEM. In Figure 3b, the TEM image of the film obtained from PHF nanoparticles is shown and it is possible to recognize the shape and size of the original nanoparticles. The film is formed without coalescence of the particles. The areas that appear in dark contrast are indicative of differences in the film roughness (cf. Supporting Information). The electron diffraction pattern (Figure 3c) confirms the amorphous character of the film. After washing, the contact angle values for PHF films increased only to values around 50°, which is indicative of a higher polarity of the PHF films. This was confirmed using pyrene as a probe to examine the polarity of the different films obtained. Pyrene is a hydrophobic probe with a very low solubility in water. For this reason, in the presence of the nanoparticles, pyrene is preferably solubilized in the interior hydrophobic region of these aggregates 7519
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Langmuir
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Figure 2. AFM height and amplitude images of (a) PH films and corresponding cross section (a-1); (b) height of heated PH-films at 120 °C for 10 min; (c) TEM Image of PH film and ED pattern (d) performed on the PH film.
Figure 3. AFM height images of (a) PHF films and corresponding cross section (a-2); (a-1) enlarged height images of PHF films of the areas (0.5 0.5 mm2); (b) TEM image of an ultrathin film of PHF nanoparticles and (c) ED pattern performed on them. 7520
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Figure 4. Staked plot emission spectra of pyrene [2.86 10 3 M] inserted in 2% wt polymer films: PB films (blue line), PHF films (red line), and PH films (black line).
and the intensity ratio I3/I1 of the bands of its emission spectra can be studied as a measure of the polarity of the nanoparticles.26 After spin-coating of polymer nanoparticle dispersions, which contain different volumes of a pyrene stock solution in toluene, the emission spectra of the pyrene inserted in the nanoparticle films were recorded over a wide range of pyrene to polymer ratios. The I3/I1 values were calculated from the Figure 4. Ratios of 0.89, 1.3, and 2.1 were observed for PHF, PB, and PH, respectively. Note that in the case of the films obtained from hydrogenated nanoparticles the values obtained always displayed a pyrene ratio close to 2. In all the cases studied, the intensity ratios I3/I1 show that the polarity of the films can be varied by the postpolymerization reaction of the nanoparticles. The variation of the films can cover a broad range of pyrene values measured for various solvents such as simple polar solvents and hydrocarbon solvents. Although the intensity emission increases with increasing film thickness and pyrene/polymer ratio, I3/I1 was found to be independent of these two parameters. Changes in the humidity conditions of the film environment did not affect the film polarity. The influence of SDS on the I3/I1values can be neglected because similar I3/I1 ratios were obtained before and after rinsing the films with water. Excimer formation of pyrene chromophore is often used as a molecular fluorescence probe for aggregation of chromophores.27,28 No excimer emission (broad band centered at 480 nm) could be detected in the pyrene emission spectra of the film obtained from nanoparticles, which is significant experimental evidence that the pyrene molecules are homogeneously distributed at low concentrations within the bulk of the different polymer films.
’ CONCLUSION The formation of thin and ultrathin films described here is achieved using very small polymer nanoparticles as building blocks. The films are continuous and transparent. By employing different polymer nanoparticles, new properties in terms of crystallinity or polarity can be completely transferred to the films. Different nanoparticles can be synthesized by postpolymerization reactions of the PB nanoparticles. Hydrogenation or hydroformylation of the double bonds results in amorphous polymer nanoparticles, and at the same time the polarity of the nanoparticles is altered. Spin-coating of these aqueous dispersions affords films that are homogeneous in terms of thickness. The films are composed of these prefabricated building blocks interacting via their amorphous surfaces. AFM and TEM provide information regarding the morphology and the crystallinity of the films. PB films present polycrystalline films where the c-axis of the crystallites are oriented
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perpendicular to the substrate. Recrystallization of such films reveals that their morphology consists of edge-on lamellae. For PH films, AFM profiles are indicative of the soft nature of the constituent nanoparticles. Formation of PHF films is believed to occur without the fusing of the nanoparticles due to the high degree of internal cross-linking. A method to quantify the polarity in such films is developed based on the uptake ability of hydrophobic pyrene into the polymer nanoparticles. The pyrene molecules are previously introduced in the dispersions and incorporate into the film after spin-coating. Homogeneous distribution of the pyrene in the films occurs, and PHF films present a lower I3/I1 ratio. Moreover, this value is independent of thickness, pyrene/polymer ratio and surfactant distribution. Extrapolation of I3/I1 ratios of polymer films corresponds well with the I3/I1 values observed for the nanoparticles dispersions. Values for the static contact angle show that the surfactant incorporated into the film can be removed by rinsing with water without any other undesirable effect.
’ EXPERIMENTAL SECTION Materials and General Considerations. Colloidally stable aqueous dispersions of 14 nm nanoparticles of semicrystalline, syndiotactic polybutadiene (97% 1,2- and 3% 1,4-cis as determined by IR spectroscopy, Mn = 3.4 104 g mol 1, Mw/Mn = 2.1) were prepared according to literature.10 The analysis of the polymer particles was performed using DLS on diluted dispersions of a few drops in approximately 1 mL of distilled water. NMR, IR, and DSC measurements were obtained on bulk polymer after the precipitation of an aliquot of dispersion with excess of MeOH and drying in vacuum. Complete hydroformylation of the original latexes was prepared according to the procedures as described in the literature. In brief, to a PB dispersion of typically 2% polymer solid content was added [Rh(CO)2(acac)]/PPh3 (P/Rh 4:1 molar ratio) and the dispersion was exposed to 60 bar of a H2/CO 1:1 mixture and warmed to 80 °C during 20 h. Polymers of controlled microstructure and different polarity were obtained in the form of particles of
