Study On Thin Film Morphology Of Semicrystalline Diblock Copolymers

This dissertation focuses on thin film morphology of two kinds of semicrystalline diblock copolymers, including crystalline/glassy block copolymers (T_ODT>T_c≈T_g) (polystyrene-b-poly(ethylene-co-butylene), PSt-b-PEB) and crystalline/rubbery block copolymers (T_ODT＞T_c>T_g) (poly(oxyethylene)-b-poly(oxybutylene), PEO-b-PBO). In the thin films of semicrystalline block copolymers there exist three driving forces: the interaction between substrate and block copolymer, crystallization of crystallizable component and microphase segregation between unlike blocks. The relative strength of these three driving forces can be regulated by various methods. For example, the interaction between substrate and block copolymer is determined by surface free energy of the substrate, crystallization is dependent on crystallization temperature and annealing temperature, and microphase segregation strength varies with chain length of the block copolymer. In this dissertation, we found that the thin film morphology of semicrystalline block copolymers is tunable by regulating the relative strength of these three driving forces.Firstly, a series of polystyrene-b-poly(ethylene-co-butylene) diblock copolymers were synthesized by anionic living polymerization of styrene and butadiene monomers and subsequently selective hydrogenation. The composition and microstructure of PSt-b-PEB diblock copolymers were characterized by GPC, ¹H-NMR, ¹³C-NMR, FTIR, DSC and XRD. The isothermal and non-isothermal crystallization behaviors of PSt-b-PEB were investigated for determination of necessary data for thin film morphology. The results show that crystallization behavior of PSt-b-PEB diblock copolymers is strongly dependent on the compositionand crystallization temperature.This dissertation puts emphasis on the thin film morphologies and morphological re-arrangement of crystalline/glassy PSt-b-PEB and crystalline/rubbery PEO-b-PBO diblock copolymers represented. It is found that orientation of diblock copolymer domains depends on molar mass. For the diblock copolymers with lower molar mass, the orientation of microdomains is parallel to the substrate. For higher molar mass diblock copolymers, microdomains are perpendicular to the substrate. This dependence of orientation of microdomains on molar mass is observed for both types of block copolymers.Annealing at the temperature well above T_g of the amorphous component leads to formation of stable structures. The lamellae perpendicular to substrate in S_xE_y thin films transform into the lamellae parallel to the substrate.The morphology of S_xE_y thin film depends on the evaporation temperature of solvent. When solvent is evaporated at 90℃, the lamellae are parallel to the substrate, whereas the S_xE_y thin films exhibit two-dimension spherulites with lamellae perpendicular to the substrate when solvent is evaporated at 50℃. These spherulites are unstable and transform into lamellar structure after annealing at 90 ℃.The morphology of S_xE_y thin films also depends on solution concentration from which thin films are prepared. The thin films prepared from thinner solution consist of half polymer layer parallel to the substrate, exhibiting asymmetric wetting. This structure is very unstable, and morphological re-arrangement occurs after annealing at temperature higher than T_g (90℃).l/2Lo polymer layer is observed at polymer/air interface of E_mB_n thin film, indicating the crystalline domains of PEO consists of double-layer, which provide a direct evidence for determination the structure of the crystalline domains in semicrystalline block copolymers. The unstable l/2Lo polymer layers at the upper layers of E_mB_n thin films disappear after annealing.Dewetting behavior and orientation of the crystalline PEO chains are related to the surface property of the substrate. Common dewetting occurs for low molar mass E_mB_n on hydrophobic modified silicon wafers, but autophobic dewetting on silicon wafersand mica takes place. PEO chains align parallel to mica surface due to the strong interaction between mica and the E segments in E_mB_n block copolymers.Annealing leads to morphological transformation from multiple-layered lamellar structure into dense branches for E_mB_n/mica thin films when the concentration of the polymer solution for spin-coating is low. The polymer branches with large width is formed initially, and split into thinner branches accompanying crystallization of PEO during annealing. The branches can be used to analyze the front speed of crystal growth. It is found that the front speed of crystal growth decreases drastically with the increase of molar mass and the front speed of crystal growth of E₂₂₄B₁₁₃ is only half of thatof E₇₆B₃₈.Molar mass and substrate also exert influences on thermal stability of E_mB_n thin films. For the high molar mass diblock copolymers and the substrate adsorbing macromolecules strongly, the thin films exhibit good thermal stability. Small microphase segregation strength may lead to vertical release of internal stress and formation of double-layer lamellar structures. A weak interaction between substrate and block copolymers leads to rapid spread of diblock copolymer on substrate. A competition mechanism of internal stress release along the vertical direction and horizontal direction is proposed to explain the experimental results.