Crystallization-Driven Self-assembly of Poly(L-lactide)-b-Polydimethylsiloxane-b-Poly(L-lactide) Triblock Copolymers from Solvent

Triblock copolymers poly(L-lactide)-b-polydimethylsiloxane-b-poly(L-lactide) (PLLA-b-PDMS-b-PLLA) were synthesized by ring-opening polymerization of L-lactide (L-LA) initiated with hydroxymethylpolysiloxane (HO-PDMS-OH). The resulted block copolymers contained 74 dimethylsiloxane (DMS) residue units, and variable L-LA block which contains 82, 150, 224 and 280 PLLA units. The self-assemblies of these PLLA-b-PDMS-b-PLLA triblock copolymers were investigated in N, N-dimethylformamide (DMF) in order to avoid the adverse effect of water on the morphologies of the aggregates. The molecular weight of PLLA blocks greatly influences the morphologies of aggregates. Helical nanowires with several tens of micrometer in length were obtained in 10 mg/mL PLLA₁₄₀-b-PDMS₇₄-b-PLLA₁₄₀ solution. While the self-assembly of PLLA₁₁₂-b-PDMS₇₄-b-PLLA₁₁₂ led to hierarchal "eye-like" crystals with about 2.6 μm in long axis and about 1.5 μm in short axis. PLLA₇₅-b-PDMS₇₄-b-PLLA₇₅ could also assemble to "eye-like" crystals but with smaller dimension. No assembly was found with copolymer PLLA₄₁-b-PDMS₇₄-b-PLLA₄₁ under the same condition due to the fact that the PLLA blocks were too short to crystalize in DMF. In addition, the morphology of aggregates was influenced by temperature as well. At -15℃ and 0℃, PLLA₁₁₂-b-PDMS₇₄-b-PLLA₁₁₂ self-assembled to nanowires accompanied with small diamond-shaped crystals and ribbons, respectively. Hierarchal "eye-like" crystals with different dimensions were obtained in the temperature range of 25-40℃. When the temperature was raised to 50℃, only small micelles could be observed in solution. Under higher polymer concentration (20 mg/mL PLLA₁₁₂-b-PDMS₇₄-b-PLLA₁₁₂), "eye-like" crystals could be observed in the temperature range of 30-50℃, and the dimension decreased with the increasing temperature. In conclusion, the aggregate of PLLA-b-PDMS-b-PLLA was mainly controlled by the crystallization of PLLA block. If the PLLA blocks underwent rapid precipitation without sufficient folding for crystallization, the copolymer tended to assemble into helical nanowires (low temperature or high molecular weight). In contrast, mild temperature and suitable PLLA length for folding and crystallization resulted in hierarchal "eye-like" crystals. High temperature and too short PLLA block were not favorable for crystallization, therefore only micelles or nothing could be obtained. This study is a necessary complement to the crystallization-driven self-assembly of amphiphilic triblock copolymers.