Self-Assembly of Low-Molecular-Weight Asymmetric Linear Triblock Terpolymers: How Low Can We Go?

The synthesis of two (2) novel triblock terpolymers of the ABC type and one (1) of the BAC type, where A, B and C are chemically different segments, such as polystyrene (PS), poly(butadiene) (PB1,4) and poly(dimethylsiloxane) (PDMS), is reported; moreover, their corresponding molecular and bulk characterizations were performed. Very low dimensions are evident from the characterization in bulk from transmission electron microscopy studies, verified by small-angle X-ray data, since sub-16 nm domains are evident in all three cases. The self-assembly results justify the assumptions that the high Flory–Huggins parameter, χ, even in low molecular weights, leads to significantly well-ordered structures, despite the complexity of the systems studied. Furthermore, it is the first time that a structure/properties relationship was studied for such systems in bulk, potentially leading to prominent applications in nanotechnology and nanopatterning, for as low as sub-10 nm thin-film manipulations.

Subsequently, the 1,3-butadiene (purity 99+%) was added to the solution and the reaction was kept at room temperature for 24h until all quantity of the second monomer was consumed. Following the specific synthetic procedure leads to high -1,4 microstructure (~92%) while a small percentage (~8%) is attributed to the vinyl content (-1,2 microstructure). In the third step thoroughly purified hexamethylcyclotrisiloxane (D3) (purity 98%) was introduced to the solution and after 18h the ring opening of the D3 took place. An equal quantity of a polar solvent (tetrahydrofuran, purity 99.9%) relative to that of the benzene, was added to the solution and after 4h at room temperature the solution was placed in -20 o C for seven days under continuous stirring in order to successfully polymerize the D3 with 100% yield as required in anionic polymerization. After completion of the polymerization the final triblock terpolymer was precipitated in a non-solvent (methanol, purity 99%) for all blocks and was vacuum-dried. Aliquots from the solution were taken during the three different polymerization steps, for molecular characterization purposes, in order to verify the successful synthesis of the homopolymer, the diblock and the final terpolymer. The reaction procedure for the PS-b-PB1,4-b-PDMS type linear triblock terpolymers is illustrated in Scheme S1a (all reagents were purchased from Sigma-Aldrich Co., St Louis, MO, USA).

Synthesis of PB1,4-b-PS-b-PDMS
In order to synthesize the triblock terpolymer of PB1,4-b-PS-b-PDMS sequence a different synthetic route was adopted from that described above. The reaction of 1,3-butadiene with sec-BuLi was accomplished after 24h in room temperature using benzene as a solvent. A small quantity of THF was then added to the solution, in order to alter the aggregation degree, prior to the addition of the styrene and also to increase the initiation vs. propagation rate of the styrenic monomers from the PB (-) Li (+) macroinitiator.
Subsequently, the styrene was introduced to the solution and after 18h the complete conversion of the second monomer was achieved. Finally, the third monomer (D3) was also introduced to the solution following the same reaction route as already discussed previously. The synthetic route employed is given in Scheme S1b.

B). SEC and 1 H-NMR Molecular Characterization Results
In the following SEC chromatographs the initial block, the intermediate precursors and the final products are presented (Figures S1-S3). It is straightforward that the final terpolymers exhibit increased molecular and compositional homogeneity since the dispersity index (Đ) is well below 1.1 for all three cases.
For the third sample, in order to receive a purified final product, fractionation of the unpurified triblock was employed. Firstly, the unfractionated triblock terpolymer is dissolved to a good solvent for all blocks (toluene), creating a solution usually 1% w/v and the non-solvent (methanol) is added carefully to the diluted polymer mixture. Afterwards, the mixture is warmed up and then is added to a separation flask, until higher molecular weight macromolecules precipitate first, creating a lower layer and the by-products form an overlaying phase.
Finally, it is evident in the homopolymer and the diblock intermediates (for all three samples) that a small peak appears on the left of the main SEC peak corresponding to dimer product values resulting from the small quantity of oxygen incorporated in the aliquots during testing of the synthetic procedure process. This small peak is not evident in the final terpolymers since the termination reaction with methanol is conducted exclusively under high vacuum environment.

C). DSC Thermal Characterization Results
In Figures Table S2.
For sample 2 ( Figure S8 amorphous. This fact can be attributed to the slightly increased average molecular weight of the PDMS block in comparison to sample 1, despite the fact that the average molecular weight of PDMS also in this sample is lower than the Me (Table S2).
Similarly, for sample 3 ( Figure S9), two distinct Tgs at -121 o C and 65 o C for the PDMS and PS blocks respectively, are observed. The crystallization (Tc=-88 o C) and two melting points (Tm=-46 o C,-39 o C) of the PDMS block are evident. In all cases the glass transition temperature for the PB1,4 segments is absent, probably attributed to the very low average molecular weight, which is smaller than the corresponding Me (Table S2). In Table S2
(2) = 4 5 . are exhibited in Table 2. In Table S3, the interaction parameter for known segment-pairs is also given.