Controlled ATRP Synthesis of Novel Linear-Dendritic Block Copolymers and Their Directed Self-Assembly in Breath Figure Arrays

Abstract

Herein, we report the formation and characterization of novel amphiphilic linear-dendritic block copolymers (LDBCs) composed of hydrophilic dendritic poly(ether-ester), PEE, blocks and hydrophobic linear poly(styrene), PSt. The LDBCs are synthesized via controlled atom transfer radical polymerization (ATRP) initiated by a PEE macroinitiator. The copolymers formed have narrow molecular mass distributions and are designated as LGn-PSt M_n, in which LG represents the PEE fragment, n denotes the generation of the dendron (n = 1⁻3), and M_n refers to the average molecular mass of the LDBC (M_n = 3.5⁻68 kDa). The obtained LDBCs are utilized to fabricate honeycomb films by a static "breath figure" (BF) technique. The copolymer composition strongly affects the film morphology. LDBCs bearing acetonide dendron end groups produce honeycomb films when the PEE fraction is lower than 20%. Pore uniformity increases as the PEE content decreases. For LDBCs with hydroxyl end groups, only the first generation LDBCs yield BF films, but with a significantly smaller pore size (0.23 μm vs. 1⁻2 μm, respectively). Although higher generation LDBCs with free hydroxyl end groups fail to generate honeycomb films by themselves, the use of a cosolvent or addition of homo PSt leads to BF films with a controllable pore size (3.7⁻0.42 μm), depending on the LDBC content. Palladium complexes within the two triazole groups in each of the dendron's branching moieties can also fine-tune the morphology of the BF films.

Keywords: ATRP; amphiphilic; breath figure; linear-dendritic copolymer; self-assembly.

Scheme 1

Synthesis of LDBCs by LG2 Dendron-Initiated Atom Transfer Polymerization.

Figure 1

(a) Kinetic plots of dendron-initiated ATRP; (b) dependence of molecular mass, M_n (solid symbols), and polydispersity, PDI (hollow symbols), on PSt yield. ATRP conditions: all polymerizations were performed in bulk at 95 °C; [LG1-i]:[St]:[CuBr]:[PMDTEA] = 1:300:2:2; [LG2-i]:[St]:[CuBr]:[PMDTEA] = 1:400:2:2; [LG3-i]:[St]:[CuBr]:[PMDTEA] = 1:1000:4:4.

Figure 2

(a) SEC traces of LG1-PSt copolymers with different lengths of the PSt block, SEC traces of LG2-PSt (b), and LG3-PSt (c) LDBCs with different lengths of the PSt block.

Figure 2

(a) SEC traces of LG1-PSt copolymers with different lengths of the PSt block, SEC traces of LG2-PSt (b), and LG3-PSt (c) LDBCs with different lengths of the PSt block.

Figure 3

SEM images of BF films prepared from first generation LDBCs with acetonide end groups: (a,b) LG1-PSt 11k; (c,d) LG1-PSt 8.5k. SEM images of BF films prepared from first generation LDBCs with acetonide end groups: (e,f) LG1-PSt 4.9k, edge of the film; (g,h) LG1-PSt 3.5k. Scale bars: (a) 20 μm; (c,e) 10 μm; (g) 5 μm, (f) 2 μm; (b,d,h) 1 μm.

Figure 3

SEM images of BF films prepared from first generation LDBCs with acetonide end groups: (a,b) LG1-PSt 11k; (c,d) LG1-PSt 8.5k. SEM images of BF films prepared from first generation LDBCs with acetonide end groups: (e,f) LG1-PSt 4.9k, edge of the film; (g,h) LG1-PSt 3.5k. Scale bars: (a) 20 μm; (c,e) 10 μm; (g) 5 μm, (f) 2 μm; (b,d,h) 1 μm.

Figure 4

SEM images of BF films prepared from third-generation LDBCs with acetonide end groups: (a,b) LG3-PSt 68k, FFT - insert in image (b); (c,d) LG3-PSt 30k; (e,f) LG3-PSt 20k - majority portion of the film; (g,h) edge of LG3-PSt 20k film. Scale bars: (a,c,e) 20 μm; (g) 10 μm; (b) 5 μm; (h) 2 μm; (d,f) 1 μm.

Figure 5

SEM images of BF films produced from first generation LDBCs with hydroxyl end groups: (a, b) deLG1-PSt 11k; (c) deLG1-PSt 8.5k; (d) deLG1-PSt 4.9k. Scale bars: (a) 2 μm; (b, c) 0.5 μm; (d) 10 μm.

Figure 5

SEM images of BF films produced from first generation LDBCs with hydroxyl end groups: (a, b) deLG1-PSt 11k; (c) deLG1-PSt 8.5k; (d) deLG1-PSt 4.9k. Scale bars: (a) 2 μm; (b, c) 0.5 μm; (d) 10 μm.

Figure 6

SEM images of BF films of third-generation LDBCs with hydroxyl end groups from THF/CHCl₃ solutions (1:3 v/v): (a) deLG3-PSt 20k; (b) deLG3-PSt 30k; (c) deLG3-PSt 68k. (d) Plot of average pore size vs. F_PEE.

Figure 7

SEM images of BF films produced from deLG3-PSt 68k with different THF contents in the copolymer chloroform solution: (a) 10%; (b) 25%; (c) 50%; (d) 75%.

Figure 8

SEM images of BF film produced by deLG3-PSt 68k using 1:3 methanol/chloroform mixed solvent.

Figure 9

SEM images of BF films produced from deLG2-PSt 23k and homo PSt 27k at different LDBC concentrations: (a) 0.1 wt%, (b) 0.4 wt%, (c) 0.6 w t%. (d) Plot of average pore diameter vs. deLG2-PSt content.

Scheme 2

Formation of de-LG2 LDBC palladium complexes through ligand exchange.

Figure 10

SEM images of BF films generated by Pd-LDBC complexes. (a,b) Pd-deLG1-PSt 11k, insert: FFT of the image; (c,d): Pd-deLG2-PSt 23k.