The design and synthesis of new linear-dendritic diblock copolymers with a linear PEO block and a dendrimeric PAMAM block are described. Two series with PEO molecular weights of 2000 and 5000 were synthesized. DSC results on the effect of diblock composition on the microphase segregation behavior of the diblocks are reported. Glass transition temperatures of the diblocks depend on the end group functionality of the dendrimer. The aqueous solution behavior of the hybrid copolymers at 30°C studied using intrinsic viscosity and GPC techniques is influenced by the length of the PEO block and the end group functionality of the dendrimer. The intrinsic viscosity trends exhibited by the diblocks with the shorter PEO chain length did not deviate greatly from that of linear polymers, and those exhibited by the diblocks with the longer PEO chain length indicate the formation of unimolecular micelles. IntroductionDendrimers have been of particular interest to the polymer science community in the past 20 years. These materials present a new, well-defined architecture and a large degree of functionality in a single macromolecule. A wide variety of dendrimers with different chemistries have been synthesized; 1-3 however, commercial applications of these materials have only recently begun to be realized. Many of the potential applications under investigation make use of two important properties of dendrimers, both of which are a direct consequence of their architecture: namely, the large number of end groups and the nanoporous nature of the interior at higher generations. 4 The interior regions may be used to sequester ions, to host molecules, or to act as catalytic sites for reaction of small molecules. When appropriately configured in consistent thin films, dendrimeric materials could prove to be useful as membranes for separations applications. Dendrimericlinear hybrid diblock copolymers present material systems which are suitable for the formation of selfassembled films in the bulk state, on surfaces, and at the air-water interface. Dendrimeric diblock copolymers with one linear block and one dendritic block have been synthesized by a number of different groups. Examples include PEO-poly(L-lysine) dendrimer, 5 PSpoly(propyleneimine) dendrimer, 6 PEO/PS-poly(benzyl ether) dendrimer, 7,8 and poly(oxazoline)-PAMAM dendrimer. 9 In some cases the aqueous solubility of the two blocks was dissimilar and the resulting diblocks have been shown to be amphiphilic. 5,6,9 We have chosen to investigate an amphiphilic dendrimeric diblock copolymer with a hydrophobic dendron unit and a water soluble linear tail.Thus far, limited information is available on the dilute solution behavior of dendrimeric diblocks. Deviations from traditional linear behavior of polymers has been observed for dendrimeric homopolymers and block copolymers by other researchers. 1,10 The exponential increase of molecular weight in dendrimeric homopolymers, when compared to a linear incremental increase of molecular diameter with generation, results in large variatio...

The dendrimer end group modification of a series of PEO−PAMAM linear−dendritic diblock copolymers to various chemical functionalities is described. The molecular weight of the PEO block was 2000, and the dendrimer generations used were one to four. The amphiphilic behavior of the modified diblocks was studied by spreading monolayers of the material at the air−water interface of a Langmuir trough and recording the pressure−area isotherm. Stearate-terminated diblocks were found to give stable monolayers which formed condensed phases on compression. The limiting area per molecule in the condensed phase measured from the pressure−area isotherm suggests interesting effects of dendrimer morphology, curvature, and size on the organization of the diblock monolayer at the air−water interface. Physisorbed films of the diblocks with third and fourth generation dendrimer blocks were made by transferring the diblock monolayer onto hydrophobically functionalized surfaces and are reported here for the first time. Films made by transferring at high surface pressures were continuous, free of holes, and relatively smooth. Z-type multilayer films were formed for both of the diblocks studied.

Transmission electron microscopy and X-ray scattering have been used to characterize the microphase segregation of linear−dendritic diblock copolymers in the bulk state. The dendritic block copolymers consisted of a linear poly(ethylene oxide) (PEO) block of 2000 molecular weight attached covalently to a polyamidoamine (PAMAM) dendron. The morphology and temperature dependence of diblocks containing dendrons of generations 1.0−4.0 was characterized. In addition, the dendritic end groups were functionalized with stearic acid to make amphiphilic linear−dendritic diblock copolymers. The morphology and temperature dependence of these materials was also characterized. Results indicate that the unfunctionalized diblocks exhibit a segregated melt state above the PEO melting point and that the PEO block undergoes confined crystallization below its melting point. The glass transition of the PAMAM block is below room temperature such that the PEO crystallinity is weakly confined by the PAMAM domains. The stearate functionalized diblocks also exhibit a segregated melt state at high temperature. However, at low temperatures both the stearate and PEO are crystalline and crystallization is strongly confined within lamellar domains.

Specular neutron reflectivity has been used to investigate the structure of monolayers formed from linear−dendritic diblock copolymers at an air−water interface. The dendritic block copolymers consist of a linear poly(ethylene oxide) (PEO) block of 2000 molecular weight linked to a third or fourth generation polyamidoamine (PAMAM) dendron. The dendritic end groups were functionalized with deuterated stearic acid to make the dendritic block hydrophobic, resulting in a macroamphiphile. Results indicate that stable monolayers are formed with PEO resting on the subphase and stearate groups extending into the air. In general, at low surface concentrations the PEO block intermixes with the PAMAM dendron, whereas at high surface concentrations the PAMAM forms a distinct layer above the PEO. The ordering of the stearate groups functionalized on the dendrimer was dependent on generation. Stearate groups form a distinct ordered layer which is separate from the third generation PAMAM dendron, whereas the stearate groups are intermixed with fourth generation PAMAM segments due to the curvature of the higher generation dendron. The PEO block becomes intermixed with the water subphase if the monolayer is held at constant area for at least 10 h. These finding are consistent with earlier published studies of pressure−area isotherms of these systems on the Langmuir−Blodgett trough.

A model polymer colloid with simple "recognition" ability was synthesized by dispersion polymerization using comb copolymer stabilizers that incorporate both 23-unit poly(ethylene oxide) and C18H37 side chains. Force interactions between beads of the resulting latex were examined using colloid-probe atomic force microscopy (CAFM) and Langmuir compression (LC). CAFM measurements showed an initial repulsion between beads on close approach that gives way to a strong bridging attraction and a minimum in the force-distance profile at bead surface separations comparable to twice the hydrocarbon side chain dimensions. By contrast, latex beads stabilized with combs that incorporate only the (EO)23 side chains exhibited purely repulsive behavior. LC experiments on the same systems produced pressure-area isotherms qualitatively similar to the CAFM data; however, significant forces were registered at monolayer bead densities corresponding to surface separation distances 1-2 orders of magnitude larger than the side chain dimensions and comparable to the bead size (∼1 µm). A simple model was developed to connect the data from these two experiments into a unified picture of how particle surface chemistry is expressed in the multiparticle behavior of complex colloidal dispersions.