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 2013

    Cite this: NewJ.Chem.,2013,37, 1142

    Reversibly meltable layered alkylsiloxanes withmelting points controllable by alkyl chain lengths

    Kazuko Fujii,*a Hiroshi Kodama,za Nobuo Iyi,a Taketoshi Fujita,a Kenji Kitamura,aHisako Sato,b Akihiko Yamagishic and Shigenobu Hayashid

    Meltable layered alkylsiloxanes (CnLSiloxanes) were synthesized from tetraethoxysilane and

    alkyltrialkoxysilanes with carbon numbers (n) of 12, 16, and 18 via hydrolysis and polycondensation at

    100 and 150 1C under basic conditions. Differential scanning calorimetric (DSC) measurements revealed

    that 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 that

    an all-trans conformation was dominant. This was supported by the XRD peak corresponding to a d

    value of 0.41 nm. High-resolution solid-state 29Si NMR measurements demonstrated the presence of

    SiO4 and CSiO3 units. A structural model has been proposed for CnLSiloxanes, where siloxane sheets

    consisting of the SiO4 and CSiO3 units are stacked with the ordered interdigitated monolayer of the

    alkyl chains in between and are bonded covalently with the alkyl chains.

    1. Introduction

    Meltable 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 energy

    storage 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 than

    a National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki

    305-0044, Japan. E-mail: FUJII.Kazuko@nims.go.jp; Tel: +81 298 860 4363b Graduate School of Science and Engineering, Ehime University, Matsuyama, Ehime

    790-8577, 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 2013

    DOI: 10.1039/c3nj41008k

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    View Article OnlineView Journal | View Issue

    http://dx.doi.org/10.1039/c3nj41008khttp://pubs.rsc.org/en/journals/journal/NJhttp://pubs.rsc.org/en/journals/journal/NJ?issueid=NJ037004

  • This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013 New J. Chem., 2013, 37, 1142--1149 1143

    100 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.13

    Bourlinos 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. Materials

    CnTRS (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. Synthesis

    Layered 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. Characterization

    DSC 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.

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    http://dx.doi.org/10.1039/c3nj41008k

  • 1144 New J. Chem., 2013, 37, 1142--1149 This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013

    Inductively 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 behavior

    The 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.

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    http://dx.doi.org/10.1039/c3nj41008k

  • This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013 New J. Chem., 2013, 37, 1142--1149 1145

    the melting points range from0.8 to 51.3 1C and are dependenton n. The reversibly meltable CnLSiloxanes a...

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