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  • ptapy

    etal M

    Received 3 November 2011Received in revised form 22 February 2012Accepted 4 March 2012Available online 11 March 2012

    Keywords:Poly(N-isopropylacrylamide)Polyhedral oligomeric silsesquioxaneOrganicinorganic amphiphiles

    (POSS) was synthesized via the copper-catalyzed Huisgen 1,3-cycloaddition (i.e., click

    of which is reactive (Scheme 1). POSS-containing hybridnanocomposites have been becoming the focus of manystudies due to their excellent thermomechanical

    chanical properties of POSS-containing nanocomposites.Poly(N-isopropylacrylamide) (PNIPAAm) is a kind of

    interesting thermoresponsive polymer. In aqueous solu-tion PNIPAAm can display a lower critical solution temper-ature (LCST) behavior at ca. 32 C [1923]. Below thistemperature, individual PNIPAAm chains adopt a random

    0014-3057/$ - see front matter 2012 Elsevier Ltd. All rights reserved.

    Corresponding author. Tel.: +86 21 54743278; fax: +86 21 54741297.E-mail address: szheng@sjtu.edu.cn (S. Zheng).

    European Polymer Journal 48 (2012) 945955

    Contents lists available at SciVerse ScienceDirect

    European Poly

    else

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    GYhttp://dx.doi.org.10.1016/j.eurpolymj.2012.03.0071. Introduction

    Polyhedral oligomeric silsesquioxanes (POSS) are a classof important nanosized cage-like molecules, derived fromhydrolysis and condensation of trifunctional organosilanes.In the past years, POSS reagents, monomers and polymershave been emerging as a new chemical technology for pre-paring the organicinorganic hybrids [18]. A typical POSSmolecule possesses a structure of cube-octameric frame-work represented by the formula (R8Si8O12) with an inor-ganic silica-like core (Si8O12) (0.53 nm in diameter)surrounded by eight organic corner groups, one (or more)

    properties [18]. Recently, POSS cages have been incorpo-rated into organic polymers to optimize the functionalproperties of the materials via the formation of specicmorphological structures [9,10]. For instance, POSS cageswere incorporated into conjugated luminescent polymerssuch as polyuorene, polyphenylene and polythiopheneto reduce the formation of aggregates and to increase ther-mal stability [1113]. It was reported that the incorpora-tion of POSS can enhance deswelling and reswelling ratesof poly(N-isopropylacrylamide) hydrogels [1418]. How-ever, such an investigation remains largely unexploredvis--vis the studies on the improvement of thermome-Self-assemblychemistry). With this initiator, the atom transfer radical polymerization (ATRP) of N-iso-propylacrylamide (NIPAAm) was carried out to afford the POSS-capped PNIPAAm. Theorganicinorganic amphiphiles were characterized by means of nuclear magnetic reso-nance spectroscopy (NMR) and gel permeation chromatography (GPC). Atomic forcemicroscopy (AFM) showed that the POSS-capped PNIPAAm amphiphiles in bulk displayedmicrophase-separated morphologies. In aqueous solutions, the POSS-capped PNIPAAmamphiphiles were self-assembled into micelle-like aggregates as evidenced by dynamiclight scattering (DLS) and transmission election microscopy (TEM). It was found that thesizes of the self-organized nanoobjects decreased with increasing the lengths of PNIPAAmchains. By means of UVvis spectroscopy, the lower critical solution temperature (LCST)behavior of the organicinorganic amphiphiles in aqueous solution was investigated andthe LCSTs of the organicinorganic amphiphiles decreased with increasing the percentageof POSS termini. It is noted that the self-assembly behavior of the POSS-capped PNIPAAm inaqueous solutions exerted the signicant restriction on the macromolecular conformationalteration of PNIPAAm chains while the coil-to-globule collapse occurred.

    2012 Elsevier Ltd. All rights reserved.Article history: In this work, a novel initiator bearing heptaphenyl polyhedral oligomeric silsesquioxaneMacromolecular Nanotechnology

    Synthesis and characterization of hesilsesquioxane-capped poly(N-isopro

    Yaochen Zheng, Lei Wang, Sixun Zheng Department of Polymer Science and Engineering and State Key Laboratory of M

    a r t i c l e i n f o a b s t r a c t

    journal homepage: www.phenyl polyhedral oligomericlacrylamide)s

    atrix Composites, Shanghai Jiao Tong University, Shanghai 200240, PR China

    mer Journal

    vier .com/locate /europol j

  • 946 Y. Zheng et al. / European Polymer Journal 48 (2012) 945955

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    GYOCH3coil conformation. While the solution is heated up to 32 Cor higher, the random coils will collapse into globules. It isproposed that in the former case, the intermolecularhydrogen bonding interactions between amide groups of

    Si OCH3OCH3

    NaN3

    Si O SiO

    SiO

    Si O SiO

    SiO

    O

    O

    O OOO

    Si

    SiN3

    Si O SiO

    SiOSi O Si

    OSiO

    O

    O

    O

    OO

    OH

    + ClCl

    O

    Si O SiOSiO

    Si O SiO

    SiO

    O

    O

    O OOO

    Si

    SiN3

    CuBr / PMDETA

    NIPAAm

    CuCl / Me6TRENSi O Si

    OSiO

    Si O SiO

    SiO

    O

    O

    O OOO

    S

    S

    Scheme 1. Synthesis of POSi O SiO

    SiO

    O-O- O-PNIPAAm and water promote the solubility of PNIPAAmwith water. In the latter cases, the intermolecular hydro-gen bonding interactions are interrupted owing to the con-formational changes of PNIPAAm chains. As a consequence,

    Si O SiO

    SiO O

    OO

    Si .

    SiCl

    ClCl Br

    Si O SiO

    SiO

    Si O SiO

    SiO

    O

    O

    O OOO

    Si

    SiBr

    3Na+

    OSi

    Si N

    OCl

    O

    NN

    O

    OCl

    i

    i N NN

    O

    O

    HNO

    n

    SS-capped PNIPAAm.

  • ylamine (1.3 ml, 8.8 mmol) were charged to a askequipped with a magnetic stirrer and then anhydrous

    Y. Zheng et al. / European Polymer Journal 48 (2012) 945955 947

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    GYthe hydrophobic association among the collapsed PNI-PAAm chains takes place. During the past decades, theLCST behaviors of PNIPAAm in aqueous solutions havebeen extensively investigated [1930]. It is identied thatsome structural factors such as co-monomer, tacticity,crosslinking, grafting, topology of macromolecules, molec-ular weights and end groups all affect the LCST behavior ofPNIPAAm in aqueous solutions [19,3137]. Recently, thePNIPAAm amphiphiles containing hydrophobic end groupsor polymer chains are of interest since the specic archi-tectures can signicantly affect the LCST behaviors of PNI-PAAm in aqueous solutions [20,3841]. Winnik et al.investigated the effect of the hydrophobic n-octadecyl ter-mini on the merging of mesoglobules of PNIPAAm and pro-posed that the rigidity and partial vitrication ofmesoglobules may be the prevalent cause of their stabilityagainst aggregations [20]. Zhang et al. [40] found that theower-like aggregates resulting from PS-b-PNIPAAm-b-PStriblock copolymers are much less collapsed than thestar-like aggregates derived from the corresponding ow-er-liked aggregates.

    POSS-capped telechelic polymers are a class of novel or-ganicinorganic hybrids owing to their specic topologiesand self-assembly behavior [16,4246]. Depending on thechemical strategies used, one (or two) end of a linear or-ganic polymer chain can be bonded with POSS cage to con-stitute so-called semi-telechelics (or telechelics). Recently,several POSS-capped polymer telechelics such as poly(eht-ylene oxide) [16,45], poly(e-caprolactone) [47,48], poly(acrylic acid) [49], polystyrene [46] and poly(hydroxyetherof bisphenol A) [50] have been reported by various invesit-gators; these POSS-capped telechelic polymers can displaysome interesting morphologies and properties. Wang et al.[17] reported synthesis of POSS-capped PNIPAAm teleche-lics with a POSS-spreading strategy. In this approach, aPOSS-capped trithiocarbonate was synthesized and usedas a chain transfer agent, with which the reversible addi-tion-fragmentation chain transfer (RAFT) polymerizationof NIPAAm was carried out and thus two ends of PNIPAAmchain were capped with hepta(3,3,3-triuropropyl) POSS.Owing to the highly hydrophobic of the POSS end groups,the POSS-capped PNIPAAm telechelics can form physicalhydrogels in aqueous solutions. It was found that suchphysical hydrogels displayed fast deswelling and reswell-ing properties compared to traditional chemical hydrogels.

    In this contribution, we reported the synthesis of hepta-phenyl POSS-capped PNIPAAm, a semi-telechelic PNIPAAmvia atom transfer radical polymerization (ATRP). Firstly, aninitiator bearing heptaphenyl POSS was synthesized via thecopper-catalyzed Huisgen 1,3-dipolar cycloaddition (i.e.,click chemistry). Thereafter, the atom transfer radical poly-merization (ATRP) of N-isopropylacrylamide (NIPAAm)was carried out with the initiator bearing POSS to affordthe POSS-capped semi-telechelic PNIPAAm. It should bepointed out that Zhang et al. [51] ever reported the synthe-sis of a semi-telechelic PNIPAAm (i.e., heptaisobutyl POSS-capped PNIPAAm) via RAFT polymerization. The RAFTagent was prepared via the direct reaction between3-aminopropylheptaisobutyl POSS and 3-benzylsulfanyl-thiocarbonylsufanylpropionic chloride. In this work, theself-assembly behavior of the organicinorganic amphi-THF (250 ml) were added with vigorous stirring. The askwas immersed into an ice-water bath and purged withhighly pure nitrogen for 1 h. After that, 3-bromopropyltri-chlorosilane (3.11 g, 12.12 mmol) dissolved in 20 ml anhy-drous THF were slowly dropped within 30 min. Thephiles was investigated by means of atomic force micros-copy (AFM), transmission electron microscopy (TEM) anddynamic laser scattering (DLS). The lower critical solutiontemperature behavior in aqueous solutions was addressedaccordi

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