All-Conjugated Poly(3-alkylthiophene) Diblock Copolymers:Synthesis, Crystallization, And Microphase Separation

Conjugated polymers have attracted considerable attention due to their promising applications in polymer electronic devices, such as organic field-effect transistors (OFETs), and organic photovoltaic cells (OPVs). It has been known that molecular organization and nanostructures play an important role in developing the performance of these semiconducting polymers. To precisely control the orientation and nanostructures in conjugated polymer films, one elegant way is to develop block copolymers composed of dissimilar conjugated blocks, since block copolymers can self-organize into well-defined microphase-separated domains in nanoscale dimension, driven by factors such as immiscibility or crystallinity difference between the blocks. Hence, all-conjugated block copolymers comprised of dissimilar rod-like blocks have become a focus of interest, due to their ability to form well-defined microphase-separated domains in nanoscale dimension and the maintenance of conjugated block density in the films. In this thesis, we focus on the synthesis, crystallization, and microphase separation of all-conjugated poly(3-alkylthiophene) diblock copolymers. We developed a "two-step" thermal annealing method to significantly enhance their crystallinity, explored the mechanism for their unique enhancement in crystallinity, and studied the impact of varied thermal annealing methods on the crystallinity and nanostructure of diblock copolymers.First, we synthesized a series of all-conjugated diblock copoly(3-alkylthiophene)s with varying alkyl chain length, poly(3-butylthiophene)-b-poly(3-hexylthiophene)(P3BT-b-P3HT), poly(3-butylthiophene)-b-poly(3-dodecylthiophene)(P3BT-b-P3DDT) and poly(3-hexylthiophene)-b-poly(3-dodecylthiophene)(P3HT-b-P3DDT), via modified Grignard metathesis (GRIM) polymerization. The GRIM polymerization is a quasi-living chain-growth synthesis and thereby enables molecular weight to be controlled by changing the molar ratio of monomers to Ni catalyst, and block molar ratio to be modulated by changing the feed molar ratio of respective monomers. The monomers used here were2-bromo-5-iodo-3-alkylthiophnes, which have good selectivity of activation by the Grignard reagent and hence ensure the narrow polydispersity indices (PDIs). All the obtained diblock copoly(3-alkylthiophene)s have well-controlled block ratios and relatively high molecular weights (Mn>13000) with narrow PDI (<1.23).Second, we used the means of DSC, XRD, and AFM to explore the crystallization and microphase separation of these poly(3-alkylthiophene)s diblock copolymers. The poly(3-alkylthiophene)s diblock copolymers comprised by the blocks with the alkyl side-chain length different by two carbon atoms (P3BT-b-P3HT) have the ability of cocrystallizing into an uniform crystal domain, with the mutual interdigitation of the different side-chains of the two blocks into a common interchain lamella. Other poly(3-alkylthiophene)s diblock copolymers with the side-chains different by more than two carbon atoms (P3BT-b-P3DDT and P3HT-b-P3DDT) prefer to microphase separating into two crystal domains formed by the independent crystallization of each block. The thermal analysis of P3BT-b-P3DDT with varied block ratios revealed that P3BT-block crystallized previously followed by the crystallization of P3DDT-block during the cooling procedure. We also observed the melting point depression phenomenon of P3BT-block, and the melting and crystallization points of P3DDT-block comparable to the P3DDT homopolymer’s. The AFM images revealed the formation of clear microphase-separated nanopatterns in microphase-separated systems, including P3BT-b-P3DDT and P3HT-b-P3DDT, whose nanostructures depend on their compositions.Third, based on the crystallization nature of these poly(3-alkylthiophene) diblock copolymers, we rationally designed a "two-step" thermal treatment, which can achieve the significant enhancement of the crystallinity in them compared to the corresponding homopolymers. This "two-step" method includes two procedures:the previous crystallization of P3BT-block with the flexible melting P3DDT-block and the subsequent crystallization of P3DDT-block. The obtained crystallinity in P3BT-b-P3DDTs was at least twice as high as that in the corresponding homopolymers after the "two-step" thermal treatment. The crystalline structure research revealed that both blocks in P3BT-b-P3DDTs independently crystallize into two microphase-separated domains with well-organized lamellar structure after thermal annealing. A microphase separation favored crystallization mechanism was proposed to explain the high crystallinity nature in P3BT-b-P3DDTs treated by the "two-step" thermal treatment, based on the in situ SAXS measurement. After the first step, P3BT-b-P3DDTs self-organized into two microphase-separated domains composed by the crystallinized P3BT-block domain and the collected P3DDT-block domain. The molecular chains of P3DDT-block have order in some extent due to the confinement of the lamellar stacking of the crystallized P3BT-block chains. Hence, the crystallization of P3DDT-block was favored by its collected and aligned state in the second step. The space-charge limited current (SCLC) mobility measurements showed the significant enhancement of carrier mobility in P3BT-b-P3DDTs with high crystallinity after the "two-step" thermal treatment, compared to the corresponding homopolymers.Additionally, a series of different thermal annealing methods were performed to explore their effect on the crystallinity, crystalline structure, and nanostructures in poly(3-alkylthiophene) diblock copolymers. The results demonstrated that their crystallinity and nanostructures can be easily modulated by changing the thermal treatment methods, while the crystalline structure remains the same. Compared to the other thermal treatment methods, the "two-step" method is the most powerful tool to improve the crystallinity in poly(3-alkylthiophene) diblcok copolymers.