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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    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 alum

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