Synth esis an d Self- Assembly of Amph iphili c Dendrimer s Based on Aliphatic Polyeth er-Type Dendr itic Cores Byoung -Ki Cho, Anurag J ain, Jo 1 rg Nieberle, † Surbhi Mahajan, and Ulrich Wiesne r* Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853 Sol M. Gruner Department of Physics, Cornell University, Ithaca, New York 14853 Stephan Tu 1 rk and Hans Joachim Ra 1 d e r Max Planck In stitute for Polymer R e search, A cke rman nweg 10, 55128 M ainz, Germany R e ce i v e d N ove m ber 20, 2003; Revised Ma nu script Rece ived February 22, 2004 ABSTRACT: We have prepared a series of amp hiph ilic dendr imers with h ydrophilic alipha tic po lyether- type dend ritic co re a nd hydrophobic do co syl peripheries. All dendrimers from first up to fourt h gener ation 1-4 were synthesized in good yield and high purity by a stepwise convergent approach consisting of a combinat ion of Wil liamson etherification and hydrobo rat ion/oxidation steps. Dif ferential scanning calorimetry (DSC) measurements show that the docosyl peripheries melting temperatures and corre- sponding heats of fusion decrease only slightly with each generation, indicative of a strong segregation between hydrophilic dendritic core and hydrophobic docosyl periphery. The DSC derived docosyl periphery crystallinity shows a gradual reduction with each generation and is corroborated by wide- angle X- ray scattering (WAXS) experiments. Amphiphilic dendrimer self-assembly in the solid state is investigated by small-angle X-ray scattering. Data from 1 , 2 , and 3 suggest a bilayer lamellar structure with interd igitated dend ritic c ore packing. In cont ras t, data from 4 are consistent with a 2-dimensional obl ique columnar assembly with hydrophilic core structure. The observed thermal and morphological behaviors are rationalized in terms of high incompatibility and intrinsic interfacial curvature between hydrophilic dendritic core and hydrophobic docosyl periphery arising from the unique dendritic molecular architecture. Introduction Molecular amphiphiles have great application poten- tial, e. g., as n an ocarriers, 1 as str ucture directing agents fo r nan ostru cture forma tion, 2 or in catalysts. 3 Recently, macromolec ular amphiphil es made from li near-type block copolymers with various molec ular co mpositions have been extensi vely studied due to their simple architecture and facile synthetic accessibility. 2b -f,4 In these systems poly(ethylene oxide) (PEO) has been widely used as the hydrophilic component because of its high polarity, ion-transporting ability, and biocom- patibili ty, resu lting in self -assembli ng materials, 4 ion- co ndu cting polyelectr ol ytes, 5 and drug-delivery 6 appli- cations. Typical examples for the hydrophobic block incompatible wi th PEO ar e polystyrene, 4a,b poly(hexyl methacrylate), 4c polyisoprene, 4f an d poly ethylene (PE ). 4e,7 Besides linear macromolecules, dendrimers with a core-shell architecture are attractive as amphilphiles and self-assembling materials due to notable features like well-defined molecular weight and shape persis- tence. 8 Considering the synthetic approach to an am- phiphili c dendrimer, t he synt hesis of the dendr itic c ore is more difficult than that of the shell due to the branched architecture. This is why only a few chemically distinct dendritic cores of amphiphilic dendrimers have been reported in the literatu re. 9 Among th em, aliphatic amines as pol y(amidoamine) (PAMAM) an d pol y(pro- pyl eneimine) (PPI) a re most common and have been used in combination with hydrophobic coil s in the co nstr uction of nanoassembling mat erials in sol ution, in the bulk state, and at the air -water interface. 9a -e To utilize these aliphatic amines as hydrophilic den- drit ic co res , c hem ic al modifi cations h ave been p erformed to increase th eir h ydrophilici ty, leading t o wel l-defined mic rophase separation. For example, Meijer et al. functionalized hydrophilic carboxylic acid groups onto the surface of PPI or protonated the amine functional groups. 9b,c Crooks et al. introduced ion pairs between the hydrophobic alkyl cha ins an d the PAMA M dendr itic co re t hr ough acid-base chemistr y, which prevents both blocks from mixing. 3c However, pristine a li pha tic amine- based d endritic co res ar e considered to be h ydrophobic rather tha n hydrophilic. For example, Ham mond et al. have used PAMAM as a hydrophobic dendritic block with linear co il- type chains in diblock copolymers wh ich can form ultrathin layered films. 9d,e Meijer et a l. have poi nted out t he h ydrophobic character of amine-termi- nated PPI in their polystyrene-PPI dendritic polymers which did not show any micropha se separ ation between polystyrene and PPI in the bulk. 9c Compa ring linear blo ck co pol ymer a nd dendr itic mac- romolecular systems st rongly suggests the u se of PEO/ polyether-type dendritic cores for the development of the next-generat io n dendritic amph iphil es. Surprisingl y, to the best of o ur knowledge, this ha s n ot been described in the literatu re. 10 In t his paper based on the convergent p olyether -type dendr imer syn th esis devised by Fre ´chet et al., 11 we will describe the synthesis of a series of amphiphilic den- † Present address: Johannes Gutenberg-Universi ty, Saarstrasse 21, 55099 Mainz, Germa ny. * To whom all correspondence should be addressed. E-mail: ubw1 @cornel l.edu . 4227 Macromolecules 2004, 37 , 4227-4234 10.1021/ ma035745e CCC: $27.50 © 2004 American Chemical Soc iety Published on Web 05/05/2004
