Reversibly meltable layered alkylsiloxanes with melting points controllable by alkyl chain lengths

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1142 New J. Chem., 2013, 37, 1142--1149 This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013Cite this: NewJ.Chem.,2013,37, 1142Reversibly meltable layered alkylsiloxanes withmelting points controllable by alkyl chain lengthsKazuko Fujii,*a Hiroshi Kodama,za Nobuo Iyi,a Taketoshi Fujita,a Kenji Kitamura,aHisako Sato,b Akihiko Yamagishic and Shigenobu HayashidMeltable layered alkylsiloxanes (CnLSiloxanes) were synthesized from tetraethoxysilane andalkyltrialkoxysilanes with carbon numbers (n) of 12, 16, and 18 via hydrolysis and polycondensation at100 and 150 1C under basic conditions. Differential scanning calorimetric (DSC) measurements revealedthat CnLSiloxanes melted reversibly at 0.8 to 51.3 1C, the melting points being dependent on n.Scanning electron microscope (SEM) images showed that thin plates were stacked. X-ray diffraction(XRD) peaks were observed at angles corresponding to distances (d) of 2.1, 2.5, and 2.8 nm for C12-,C16-, and C18LSiloxanes, respectively. High-resolution solid-state13C nuclear magnetic resonance (NMR)measurements showed that the organic moieties were alkylsilyl groups with long alkyl chains and thatan all-trans conformation was dominant. This was supported by the XRD peak corresponding to a dvalue of 0.41 nm. High-resolution solid-state 29Si NMR measurements demonstrated the presence ofSiO4 and CSiO3 units. A structural model has been proposed for CnLSiloxanes, where siloxane sheetsconsisting of the SiO4 and CSiO3 units are stacked with the ordered interdigitated monolayer of thealkyl chains in between and are bonded covalently with the alkyl chains.1. IntroductionMeltable layered inorganicorganic hybrids have attractedconsiderable attention19 because of their potential applicationsas fillers dispersed into polymers, and as coating reagents. If theinorganic and organic moieties are covalently bonded to eachother, then they do not separate from each other even afterexfoliation by procedures involving their mixing with meltpolymers, e.g., kneading. Therefore, they disperse into polymers,keeping their miscibility. Reversibly meltable layered hybrids canbe applied more extensively, because they can absorb heat whentemperature increases and release heat when temperaturedecreases, around their melting points. Reversibly meltablelayered inorganicorganic hybrids find applications in energystorage materials, materials for the adjustment of temperatures,and temperature-sensing elements.It is well known that many of one-dimensional polymers (i.e.chain polymers such as polystyrene) are thermoplastic, whereasthree-dimensional organic polymers (i.e. cross-linked polymerssuch as phenol resin) do not melt until they decompose.Phyllosilicates are sometimes referred to as two-dimensionalinorganic polymers.10 These materials do not melt until theyare metamorphosed to another phase at temperatures higherthan 600 1C. It has been reported that almost all of two-dimensional inorganicorganic hybrids do not melt.1 For example,we have reported that an inorganicorganic hybrid, which consistsof Mg-phyllosilicate/alkylammonium and alkylammoniumsiloxaneparts, shows endothermic peaks around 55 and 72 1C without anymass loss.2 However, the hybrid does not melt. It can be speculatedthat the endothermic peaks result from the transition from orderedto disordered arrangements of the long alkyl chains.Hence, we can assume that two-dimensional inorganic/organic hybrids melt reversibly when the influence of theordered organic part is intensified on the thermal behavior. Alayered alkylsiloxane with a high organic/inorganic ratio and alow degree of polycondensation is one of these candidates.Unfortunately, however, inorganicorganic layered materialsthat are synthesized from alkyltrialkoxysilanes at room tempera-ture under acidic conditions liquefy at temperatures higher thana National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki305-0044, Japan. E-mail: FUJII.Kazuko@nims.go.jp; Tel: +81 298 860 4363b Graduate School of Science and Engineering, Ehime University, Matsuyama, Ehime790-8577, Japanc Faculty of Science, Toho University, Funabashi, Chiba 274-8510, Japand National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba,Ibaraki 305-8565, Japan Electronic supplementary information (ESI) available: Additional discussion onthe structural model. See DOI: 10.1039/c3nj41008k Deceased.Received (in Montpellier, France)8th November 2012,Accepted 15th January 2013DOI: 10.1039/c3nj41008kwww.rsc.org/njcNJCPAPERPublished on 16 January 2013. Downloaded by University of California - Santa Cruz on 31/10/2014 08:54:51. View Article OnlineView Journal | View Issuehttp://dx.doi.org/10.1039/c3nj41008khttp://pubs.rsc.org/en/journals/journal/NJhttp://pubs.rsc.org/en/journals/journal/NJ?issueid=NJ037004This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013 New J. Chem., 2013, 37, 1142--1149 1143100 1C, but subsequently become amorphous on cooling, asreported in an early study by Shimojima et al.3 These resultssuggest that the layered alkylsiloxane melts irreversibly if thedegree of polycondensation is too low.To date, many interesting investigations have beenreported about the polycondensation of long-alkyltrifunctionalsilanes.59,1114 Some of these reports have described interestingthermal behaviors of the monolayers and layered alkylsiloxanes onthe substrates57 and on the particles.11 Jung et al.12 have reportedthe interesting reversible melt of a spherical alkylsiloxane. Otherinteresting articles have also reported layered alkylsiloxaneswithout reporting any results concerning their melting behaviors.13Bourlinos et al.8 have reported a meltable layered octadecylsiloxanebut they have not reported on the control of the melting point.Thus, even today, remarkable attention is still being focused on thesynthesis of reversibly meltable layered alkylsiloxanes and thecontrol of the melting points.We reported preliminary results for the synthesis of reversiblymeltable layered alkylsiloxanes.4 Although we succeeded inthe synthesis of reversibly meltable layered alkylsiloxanes, thestructural model was proposed only from the XRD results and wedid not attempt to control the melting points.In this paper, we report reversibly meltable layered inorganicorganic hybrid materials with a variety of different melting points.At first we attempted to synthesize the reversibly meltable layeredinorganicorganic hybrid materials, layered alkylsiloxanes.Relatively high reaction temperatures were adopted as opposedto the use of room temperature (RT), and basic conditions wereused instead of acidic conditions. Furthermore, tetraethoxysilane(TEOS) was used as a starting material together with alkyltrialk-oxysilanes (CnTRS) to provide siloxane networks with suitabledegrees of polycondensation for the reversible melt. The ratiosof the starting reagents were selected to regulate the ratio oforganic/inorganic materials to be high. These synthetic condi-tions were selected based on the postulation that reversiblymeltable layered alkylsiloxanes can be obtained by reducing theinfluence of the inorganic moiety on the thermal properties ofthe layered alkylsiloxanes, whilst simultaneously regulating thedegree of polycondensation of the siloxane networks. Thedegree of polycondensation should be regulated at relativelylow levels, but should not be too low in order to avoid theirreversible change accompanying the melting. Attempts werealso made to control the melting points by changing the alkylchain lengths. Furthermore, a discussion on the structureand mechanisms associated with the synthesis of the layeredalkylsiloxanes has been provided.2. Experimental2.1. MaterialsCnTRS (CnH2n+1Si(OR)3, n = 12, 16, and 18, and R = CH3 andC2H5) were purchased from Shin-Etsu Chemical Co., Ltd.(C12- and C18TRS, R = C2H5) and Gelest Co., Ltd. (C16TRS, R =CH3). TEOS, ethyl alcohol (EtOH), and ammonium hydroxide(NH4OH) were purchased from Wako chemical Co., Ltd. Allreagents were of reagent grade and used as purchased.2.2. SynthesisLayered alkylsiloxanes were synthesized as follows: TEOS wasdissolved in EtOH. CnTRS was added to the ethanol solution ofTEOS. The molar ratio of CnTRS to TEOS was set at 1 : 1 tocontrol the lateral intermolecular distance between the nearestlong alkyl chains. After stirring for 30 min, H2O was added tothe mixtures with a molar ratio of 7 with respect to CnTRS. Theconcentrations of the total silanes (TEOS and CnTRS) wereadjusted to 40 wt% in the starting mixtures. After an additionof NH4OH, the starting mixtures were kept for 1 day at 150 1Cfor n of 16 and 18 and at 100 1C for n of 12 to facilitate thehydrolysis reaction and to prevent the CnTRS from separatingout. The reaction mixtures were washed with H2O and filtered.The samples were then dried under reduced pressure. The finalproducts were denoted as CnLSiloxanes. Another synthesiscondition was also tested for reference, where the reactiontemperature was set to RT, with the remaining conditions beingthe same.2.3. CharacterizationDSC measurements were performed using a MAC ScienceDSC3100S with repeated heatingcooling cycles with ramps of3 and 5 1C min1, respectively. DSC measurements wererecorded over a temperature range of 40 to 200 1C. Thesamples were placed in aluminum specimen containers. Thesamples were then packed by crimping the specimen containerswith aluminum covers. In our preliminary letter,4 some measure-ments were performed without the covers.Thermogravimetric-differential thermal analysis (TG-DTA)was carried out using a MAC Science TG-DTA 2000 under anAr flow with a ramp of 10 1C min1. The samples were placed inopen platinum specimen containers.The synthesized samples were observed under a light micro-scope (Nikon Eclipse E400 and Olympus BX51). They wereplaced on slide glasses, and cover glasses were then placedover the samples. The specimens were observed with heatingand cooling.SEM images were obtained on a HITACHI S-5000 at anaccelerating voltage of 10 kV. The samples were placed on aconductive carbon tape stuck onto metal SEM specimenmounts. The specimens were then coated with a thin layer ofplatinum to prevent charging.XRD measurements were performed using a Rigaku Rint2000S diffractometer equipped with a humidity/temperaturecontrollable chamber15 using graphite-monochromatized CuKa radiation (1.54 ) at RT under an ambient atmosphere. Thefollowing conditions were used: 1/21 scattering, 1/21 divergence,0.3 mm receiving slits, and a scan rate of the diffraction angle(2y) of 1.01 min1. For C12LSiloxane, a specimen was frozen at atemperature below its melting point. The XRD analysis was thenimmediately conducted at RT for the frozen specimen. For theother CnLSiloxanes, the XRD analyses were conducted at RT forpowder specimens without any cooling beforehand.Elemental analyses were performed using combustion-infraredabsorption to estimate the amounts of carbon and oxygen.Paper NJCPublished on 16 January 2013. Downloaded by University of California - Santa Cruz on 31/10/2014 08:54:51. View Article Onlinehttp://dx.doi.org/10.1039/c3nj41008k1144 New J. Chem., 2013, 37, 1142--1149 This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013Inductively coupled plasma (ICP) atomic absorption was usedto estimate the silicon content.High-resolution solid-state 13C and 29Si NMR spectra wereobtained on a Bruker ASX400 spectrometer at RT. The Larmorfrequencies of 13C and 29Si were 100.61 and 79.49 MHz,respectively. The ordinary cross-polarization (CP) pulsesequence was used together with a magic angle spinning(MAS) of the samples. Contact times were set at 1 and 5 msand recycle times were 5 and 6 s for 13C and 29Si, respectively.The samples were packed into a 7 mm rotor, and the spinningrate was 4.5 kHz. The chemical shifts of 13C and 29Si were bothreferenced to neat tetramethylsilane.3. Results and discussion3.1. Thermal behaviorThe synthesized C16- and C18LSiloxanes are white powders.C12LSiloxane is liquid at RT even after the drying process.Fig. 1 shows the DSC diagrams with repeated heatingcoolingcycles for C12- and C16LSiloxanes. C12LSiloxane has an endothermicpeak at 0.8 1C when the temperature is increased, while anexothermic peak is observed at 8.2 1C in the cooling process, asshown in Fig. 1(a). No other peaks are observed. Repeated heatingcooling cycles do not lead to any change in the DSC diagrams,demonstrating that the endothermic and exothermic eventsoccurred reversibly. For C16LSiloxane, the DSC diagram (Fig. 1(b))shows that the endothermic event initiates at around 34 1C andhas a peak at 40.1 1C in the heating process and that anexothermic event initiates at around 37 1C, accompanied by apeak at 34.2 1C in the cooling process. The DSC curve measuredwith the aluminum cover for C18LSiloxane was reported in ourpreliminary letter.4 The endothermic event starts at 45.0 1C andhas a peak at 51.3 1C.Fig. 2 shows the results of the TG-DTA measurements.C16LSiloxane shows no mass loss in the temperature rangestudied (up to 200 1C). C18LSiloxane shows no mass loss up toabout 350 1C and then starts to decompose.Fig. 3(a) and (b) show images of C16LSiloxane in a test tubewith heating in a water bath from RT to 47 1C. Fig. 3(a) showsthat C16LSiloxane is a white powder at RT. Fig. 3(b) shows thatC16LSiloxane becomes liquid at temperatures above theendothermic peak. The liquid specimen is solidified uponcooling to RT. Fig. 3(c,d) shows images of C18LSiloxaneobserved under a light microscope as the temperature isincreased. Fig. 3(c) shows that C18LSiloxane is still a solidpowder at 45 1C. When C18LSiloxane is heated above thetemperature of the endothermic peak (51.3 1C), it becomes adrop, as observed at 70 1C (Fig. 3(d)), showing fluidity. Thismelted specimen is solidified upon cooling to RT. Theseexperimental results demonstrate that the endothermic eventsin the DSC diagrams correspond to melting and that theexothermic events correspond to the solidifying of the liquidstate. Conclusively, CnLSiloxanes melt reversibly. Furthermore,Fig. 1 DSC curves of (a) C12- and (b) C16LSiloxanes with repeated heatingcooling cycles (TT; 1st cycle and ---; 2nd cycle). The heatingcooling ramps were(a) 3 and (b) 5 1C min1. Fig. 2 TG-DTA curves in Ar flow for (a) C16- and (b) C18LSiloxanes.NJC PaperPublished on 16 January 2013. Downloaded by University of California - Santa Cruz on 31/10/2014 08:54:51. View Article Onlinehttp://dx.doi.org/10.1039/c3nj41008kThis journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013 New J. Chem., 2013, 37, 1142--1149 1145the melting points range from0.8 to 51.3 1C and are dependenton n. The reversibly meltable CnLSiloxanes absorb heat duringthe increase in temperature and release heat upon cooling at thevariable melting points within the range of temperatures weexperience in our daily life. The reversibly meltable CnLSiloxanestherefore have the potential for application in daily life tomaintain temperatures of bodies, food and houses in a suitablerange, and as fillers for polymers, and coating reagents.3.2. Structures of synthesized layered alkylsiloxanesFig. 4 shows SEM images for C18LSiloxane before (Fig. 4(a)) andafter (Fig. 4(b) and (c)) the meltingsolidification process.Fig. 4(a) shows the layered morphologies consisting of stackingthin sheets. The layered stacking is more ordered for C18LSiloxaneafter the meltingsolidification process (Fig. 4(b)) rather than beforethe process. Fig. 4(c) shows a magnified SEM image of Fig. 4(b).Fig. 4(a) and (c) demonstrate that the thin sheets consist of smallthin leaves with sizes of about 300 nm. The leaves aggregate withordering in the stacking direction and without any order in thelateral directions, resulting in the layered assemblies. Transmissionelectron microscope (TEM) images showed thin leaves with sizes ofabout 300 nm for C18LSiloxane in our preliminary letter.4 Thesethin leaves could be stacked into the layered morphologiesobserved by SEM.XRDmeasurements showed the diffraction peaks correspondingto d-values of 2.06, 2.54, and 2.81 nm for C12-, C16-, andC18LSiloxanes, respectively, as shown in Fig. 5(a, c and e).4The results suggest that the stacking leaves observed bySEM and TEM have layer spacings of 2.06, 2.54, and 2.81 nmfor C12-, C16-, and C18LSiloxanes, respectively. The XRDpeaks are observed even after the meltingsolidification process,as shown in Fig. 5(b and d). These SEM and XRD resultsdemonstrate that the layered structures are recovered aftercooling down without any decomposition during the meltingprocess.Elemental analyses provide mass% results of C, 55.1; O,18.4; and Si, 16.7 for C16LSiloxane. The mass% results forC18LSiloxane are C, 58.5; O, 19.0; and Si, 15.2, as reported inour preliminary letter.4 These results indicate that large quan-tities of organic components are contained in CnLSiloxanes.The organic components of CnLSiloxanes are characterizedby high-resolution solid-state 13C NMR spectroscopy. Fig. 6shows the spectra for C16- and C18LSiloxanes as-synthesized(i.e. before themeltingsolidification process) together with theirproposed assignment. All signals are assigned to carbons in thealkylsilyl groups (i.e. CnH2n+1SiR). For example, the signal at15 ppm is assigned to the terminal methyl group. The 13C NMRchemical shifts for all the carbons agree before and after themeltingsolidification process.Fig. 3 Photographs for (a, b) C16LSiloxane in a test tube heated in a water bath from (a) RT to (b) 47 1C and for (c, d) C18LSiloxane observed under the lightmicroscope with elevating temperature from (c) 45 1C to (d) 70 1C.Fig. 4 SEM images of C18LSiloxane (a) before and (b and c) after the meltingsolidification process. (c) is a magnified image of (b).Fig. 5 XRD patterns of (a) C12-, (b and c) C16-, and (d and e) C18LSiloxanesbefore (a, c, and e) and after (b and d) themeltingsolidification process. The XRDpatterns of C16- and C18LSiloxanes before the meltingsolidification process(c and e) have been cited from our preliminary letter.4Paper NJCPublished on 16 January 2013. Downloaded by University of California - Santa Cruz on 31/10/2014 08:54:51. View Article Onlinehttp://dx.doi.org/10.1039/c3nj41008k1146 New J. Chem., 2013, 37, 1142--1149 This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013The most intense signals at 33 ppm are assigned to the innermethylene groups with the trans conformation. The 13C NMRchemical shifts are around 33 ppm for the crystalline compoundswith an all-trans zigzag conformation, such as crystalline poly-ethylene16 and n-paraffin.7,11 On the other hand, the signals at30.5 ppm are assigned to the inner methylene groups with thetrans-gauche conformation.17 The spectra demonstrate that themajority of the long alkyl chains are packed in the orderedarrangement with the all-trans conformations, although the alkylchains with the trans-gauche conformation also exist.Fig. 7 shows the XRD patterns in the middle angle range forC16- and C18LSiloxanes as-synthesized. The XRD peaks corres-ponding to a d value of 0.41 nm are considered to be caused bythe ordered alkyl chain packing with the all-trans conforma-tion. It is known that XRD peaks are observed at positionscorresponding to a d value of about 0.4 nm for the crystallinecompounds with the all-trans zigzag conformation, such as thecrystalline n-alkane phases,18 polyethylene,16 and denselypacked alkyl chains.3,8,12,19,20 It has been reported for thesecrystalline compounds that the lateral interchain spacing isabout 0.4 nm.1820 Thus, the XRD results are consistent withthose of the high-resolution solid-state 13C NMR. The XRDpeaks were recovered after the meltingsolidification process,as reported in part in our preliminary letter.4 These 13C NMRand XRD data reveal that the ordered arrangement with theall-trans conformation is recovered after themeltingsolidificationprocess.To confirm the environments of the silicon, high-resolutionsolid-state 29Si NMR spectra were measured. Fig. 8 shows thespectra for C16- and C18LSiloxanes as-synthesized. The signalsin the range of 40 to 80 ppm are assigned to C(OSi)k(OZ)3k(Z is H or R (CH3 or C2H5) and k = 03), i.e., Si atoms with oneSiC bond and k SiOSi bonds and 3 k SiOZ bonds.21,22These Si species are expressed as Tk in the present work. Thesignals in the range of 80 to 120 ppm are assigned toFig. 6 High-resolution solid-state 13C CP/MAS NMR spectra for (a) C16- and (b)C18LSiloxanes as-synthesized (before the meltingsolidification process). Aproposed assignment set is indicated.Fig. 7 XRD patterns around 2y of 20 1 for (a) C16- and (b) C18LSiloxanes as-synthesized.Fig. 8 High-resolution solid-state 29Si CP/MAS NMR spectra for (a) C16- and(b) C18LSiloxanes as-synthesized. Spinning sidebands are marked with ss.NJC PaperPublished on 16 January 2013. Downloaded by University of California - Santa Cruz on 31/10/2014 08:54:51. View Article Onlinehttp://dx.doi.org/10.1039/c3nj41008kThis journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013 New J. Chem., 2013, 37, 1142--1149 1147Qm.21,22 Qm is (OSi)m(OZ)4m (m = 04). The signals at 59,66, 102, and 111ppm are assigned to T2, T3, Q3, andQ4.13,7,9,16,19,2325 The results reveal the presence of CSiO3(Scheme 1(a)) and SiO4 units (Scheme 1(b)). The positions ofthe 29Si NMR signals do not change following the meltingsolidification process.Integration of the results above allows for a structural modelto be proposed for CnLSiloxanes as follows: CnLSiloxanes con-sist of stacking siloxane layers and ordered interdigitatedmonolayers of long alkyl chains in between, as presented inScheme 2(a). The siloxane sheets and the long alkyl chains arebonded covalently to each other. The siloxane sheets consist ofthe SiO4 and CSiO3 units. The long alkyl chains form anordered arrangement with the all-trans conformation. Thelengths of the C12H25SiO, C16H33SiO, and C18H37SiO units arecalculated to be 1.9, 2.4, and 2.7 nm, respectively, assumingfully extended long alkyl chains with the all-trans conformation.Therefore, the siloxane sheets are expected to be spaced atabout 1.9, 2.4, and 2.7 nm for the structural model of C12-, C16-,and C18LSiloxanes, respectively. These predicted spaces are ingood agreement with the XRD results described above. Theoccurrence of a small discrepancy of 0.1 nm can be understoodin terms of the suggestion that the terminal methyl groups ofthe long alkyl chains are apart slightly from the surfaces of thesiloxane sheets, as shown in Scheme 2.The results of the high-resolution solid-state 13C and 29SiNMR spectroscopy demonstrate that the long alkyl chains bondto the Si atoms through a SiC covalent bond. The 29Si NMRreveals that the Si atoms are present as CSiO3 and SiO4 units.The 13C NMR demonstrates that the organic moieties arealkylsilyl groups. Accordingly, the siloxane sheets consist ofCnH2n+1SiO3 and SiO4 units. The molar ratios of silicon tocarbon atoms are calculated based on the results of theelemental analyses as about 0.13 and 0.11 for C16- and C18LSi-loxanes, respectively. Therefore, the molar ratios of Si to thelong alkyl chains are 2.1 and 2.0 for C16- and C18LSiloxanes,respectively. Compositional formulae are described asC16H33Si2.1Ox(OZ)y and C18H37Si2Ox(OZ)y for C16- and C18LSi-loxanes, respectively. Thus, the molar ratio of SiO4 toCnH2n+1SiO3 is about 1. The existence of the SiO4 unit withthe above molar ratio in the siloxane sheets allows the inter-digitated monolayer packing of the long alkyl chains. Thelateral interchain spacing is considered to be 0.4 nm for thelong alkyl chains, as described above. The SiOSi distance isestimated as at most 0.32 nm.13,2628 The interdigitated mono-layer packing is difficult if all Si atoms bond to the long alkylchains. The long alkyl chains can be packed in the interdigi-tated monolayer arrangement (Scheme 2(a)), if one SiO4 unit isinserted between the nearest CnH2n+1SiO3 units in the siloxanesheet, as presented in Scheme 2(b). The nearest long alkylchains bonded to the Si atoms in the same siloxane sheetshould extend to the opposite side from the siloxane sheet(Scheme 2) to form the interdigitated monolayer packing (ESI).When CnLSiloxanes are heated, the trans-gauche transforma-tion should be allowed and the arrangement of the long alkylchains should become disordered. The layered structures arenot conserved without the ordered arrangement of the alkylchains because of the relatively low degrees of polycondensa-tion. Consequently, CnLSiloxanes melt at their melting points.During cooling, the all-trans conformation becomes dominantand the arrangement of the long alkyl chains becomes ordered.The layered structures are then recovered.3.3. Design of the synthesis conditionsDuring the design of the synthetic conditions, we speculatedthat the important keys of any prospective synthesis of rever-sibly meltable layered inorganicorganic hybrids would be (1)high ratios of the organic to inorganic moieties; and (2)relatively low degrees of polycondensation of the siloxanes.When the organic/inorganic ratio is low, the property of theinorganic moiety influences significantly the overall thermalproperties. In this case, the layered inorganicorganic hybridsdo not melt until they are decomposed or metamorphosed.1,2,23For example, the hybrid with the Mg-phyllosilicate moiety (i.e. alower organic/inorganic ratio than CnLSiloxanes) did not meltand ultimately decomposed around 240 1C, although it showedthe endothermic peaks around 55 and 72 1C without a massloss, as reported previously.2 When the degree of polycondensa-tion is very high, the hybrids do not melt.2 However, if the degreeof polycondensation is too low, the hybrids melt irreversibly.3These layered alkylsiloxanes are derived via hydrolysis andpolycondensation of only alkyltrialkoxysilane at RT under acidicconditions.3Scheme 1 Schematic representation for environments of silicon atoms.Scheme 2 Schematic representation of (a) the proposed structural model forCnLSiloxanes and (b) a part of siloxane sheet.Paper NJCPublished on 16 January 2013. Downloaded by University of California - Santa Cruz on 31/10/2014 08:54:51. View Article Onlinehttp://dx.doi.org/10.1039/c3nj41008k1148 New J. Chem., 2013, 37, 1142--1149 This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013Consequently, we attempted a synthesis using only CnTRSand TEOS as the starting compounds and in the absence of anyother inorganic reagents, such as magnesium hydroxide, toregulate the organic/inorganic ratio to a high value. To controlthe degree of polycondensation to relatively low but not to toolow, we adopted the other reaction conditions as follows: thereaction temperatures of 150 1C for n = 18 and 16, and 100 1Cfor n = 12 rather than RT, basic conditions rather than acidicconditions, and the high concentration of the silanes.The molar ratio of TEOS/CnTRS was controlled to adjust thelateral interchain spacing between the long alkyl chains as wellas to keep the high organic/inorganic ratio. The long alkylchains were predicted to aggregate in the monolayer arrange-ment rather than the bilayer arrangement3,9,19 because of therelatively high reaction temperatures (100 and 150 1C), asdescribed in the next section. The interdigitated monolayerarrangement is not allowed, if every Si atom bonds to the alkylchain, as described above. When CnH2n+1SiO3 and SiO4 unitscross-link alternatively, the lateral interchain spacing is fit forthe interdigitated monolayer arrangement. If the molar ratio ofSiO4 to CnH2n+1SiO3 units is too high, the arrangement of thelong alkyl chains may be disordered because of too long lateralinterchain spacing. Therefore, the molar ratio of TEOS/CnTRSwas adjusted to 1 in the starting mixture.3.4. Proposed formation mechanismAs a proposed formation mechanism, the polycondensationbetween the hydrolyzed CnTRS and TEOS is considered toproceed almost simultaneously with the hydrolysis, followingthe aggregation of the long alkyl chains under the reactionconditions (i.e. at relatively high temperatures and under basicconditions),29 although the additional polycondensation isconsidered to proceed even during the drying process afterthe reaction at 100 or 150 1C. In contrast, a three-step process,hydrolysis as the first step followed by the self-assembly andslow polycondensation, has been proposed as the mechanismof the formation of layered alkylsiloxanes at RT and underacidic conditions.6,7,13,19,23,30,31The long alkyl chains are considered to aggregate in the reactionmedia with the relatively hydrophilic solvent (EtOH) because of vander Waals force and hydrophobic interactions. The aggregation isconsidered to form the monolayer assemblies6,19 rather than thebilayer arrangement. The trans-gauche transformation should beallowed for the long alkyl chains under the reaction conditionsbecause of the relatively high temperatures. It is considered that theall-trans conformation becomes dominant and the interdigitatedmonolayer arrangement becomes ordered during the cooling to RTafter the reaction.The reaction temperature is considered to be the mostimportant factor for determining the arrangement type (i.e.monolayer or bilayer) of the long alkyl chains, although the alkylchain length and the molar ratio of TEOS/CnTRS also affect thearrangement. The self-assembly of the long alkyl chains into abilayer arrangement has been reported for the layered alkylsilox-anes synthesized below 60 1C and with TEOS/CnTRS ratios below1.3,9,19 The monolayer arrangement has been reported for thelayered alkylsiloxanes synthesized at temperatures higher thanor equal to 110 1C.4,8 For syntheses at medium temperatures (i.e.around 50 1C) and with TEOS/CnTRS ratios of 4, the monolayerarrangement has been reported for n Z 14, although the bilayerarrangement has been reported for n from 8 to 12.23 There havebeen no reports for any layered alkylsiloxane synthesized ataround 100 1C. The monolayer arrangement was formed forC12LSiloxane synthesized at 100 1C in the current study.The hydrolysis has been considered to proceed via thenucleophilic attack of hydroxide ions on Si under basic condi-tions.29 Although the hydrolysis is a slow reaction, once one ofthe alkoxide groups is hydrolyzed, hydrolysis of the otheralkoxide groups and condensation of the hydrolyzed silanesoccur immediately.32 Therefore, three-dimensional SiOSi net-works with high degrees of condensation are formed.29 Incontrast, it has been understood that the hydrolysis is startedby an electrophilic attack under acidic conditions.33 The rela-tively high reaction temperatures are speculated to acceleratethe slow hydrolysis and prevent CnTRS from separating out. Infact, a pasty product without any layered structure was obtainedby the reference reaction at RT, keeping the other reactionconditions the same as in the synthesis of CnLSiloxanes.Because the hydrolysis is slow at RT under basic conditions,32polycondensation is considered to proceed only a part andCnTRS is considered to separate out. Therefore, the obtainedproduct was a mere mixture of polycondensed siloxanes andunreacted starting reagents for this reference reaction.The silanol group is the terminal group in the hydrolyzedCnTRS, and thus it locates at the edge plane of the monolayerassemblies of the long alkyl chains. The polycondensation istherefore speculated to occur within confined two-dimensionalspaces between the monolayer assemblies. The formed SiOSinetworks are considered to develop extensively because of basicconditions though confined within the two-dimensional spaces.The hydrolyzed CnTRS would undergo polycondensationreactions by inserting one hydrolyzed TEOS between them.This speculation is consistent with the resulting molar ratios(2/n) of Si/C. Therefore, the lateral interchain spacing betweenthe nearest long alkyl chains was adjusted and the interdigi-tated monolayer packing was formed as described above.The most intense signal of the Tk signals was the T3 signal andthe Q4 signal was more intense than the Q3 signal in the high-resolution solid-state 29Si NMR spectra (Fig. 8). These results implythat the degrees of polycondensation were not too low about thesiloxane networks of CnLSiloxanes. The T2 signal is the mostintense signal in the solid-state 29Si NMR spectrum for the irrever-sibly meltable layered alkylsiloxanes.3 We speculate that the highorganic/inorganic ratios, which were revealed by the elemental and13C NMR analyses, made CnLSiloxanes meltable and that thesuitable degrees of polycondensation, which were demonstratedby the 29Si NMR, realized reversible melt in this study.4. ConclusionThe reversibly meltable layered alkylsiloxanes, CnLSiloxaneswith several alkyl chain lengths (n = 12, 16, and 18), wereNJC PaperPublished on 16 January 2013. Downloaded by University of California - Santa Cruz on 31/10/2014 08:54:51. View Article Onlinehttp://dx.doi.org/10.1039/c3nj41008kThis journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013 New J. Chem., 2013, 37, 1142--1149 1149successfully synthesized from CnTRS and TEOS at 100(for n = 12) and 150 1C (for n = 16 and 18) under basicconditions. The reversible melting is confirmed by DSC andTG-DTA. The melting point varies from 0.8 to 51.3 1C,depending on the alkyl chain length.A structural model was proposed for CnLSiloxanes, based onthe results of the SEM, TEM, XRD, elemental, and high-resolu-tion solid-state 13C and 29Si NMR analyses. Siloxane sheetsconsisting of the SiO4 and CSiO3 units are stacked with theordered interdigitated monolayer of the alkyl chains in betweenand are bonded covalently with the alkyl chains. The majorityof the long alkyl chains are packed in ordered arrangementswith the all-trans conformations. The important keys to synthe-size the reversibly meltable layered inorganicorganic hybridsare (1) high ratios of the organic to inorganic moieties and (2)relatively low degrees of polycondensation in the siloxanes. Wesucceeded in regulating the organic/inorganic ratios to highlevels and the degrees of polycondensation to relatively lowlevels by controlling the synthetic conditions. The high organic/inorganic ratios made CnLSiloxanes meltable and the suitabledegrees of polycondensation realized reversible melt.AcknowledgementsWe are grateful to Mr. K. Kosuda, NIMS, for the SEM observa-tion, Dr T. Nakanishi, NIMS, for the observation by the lightmicroscope, Dr H. Hashizume and Dr T. Sasaki, NIMS, for theXRD measurement, and Mr. S. Takenouchi, NIMS, for theelemental analyses.References1 e.g., K. Fujii and S. Hayashi, Appl. Clay Sci., 2005, 29, 235;M. Jabor, J. Miehe-Brendle, M. Roux, J. Denzer, R. L. Dredand J.-L. Guth, New J. Chem., 2002, 26, 1597.2 K. Fujii, S. Hayashi and H. Kodama, Chem. Mater., 2003,15, 1189.3 A. Shimojima, Y. Sugahara and K. Kuroda, Bull. Chem. Soc.Jpn., 1997, 70, 2847.4 K. Fujii, T. Fujita, N. Iyi, H. Kodama, K. Kitamura andA. Yamagishi, J. Mater. Sci. Lett., 2003, 22, 1459.5 Q. Ke, G. Li, Y. Liu, T. He and X. Li, Langmuir, 2010,26, 3579.6 Q. Lu, T. Hao, Q. Ke, W. Wang, T. He and X. Li, Appl. Surf.Sci., 2011, 257, 2080.7 A. Chemtob, L. 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