Effect of Substrate Surface on Dewetting Behavior and Chain Orientation of Semicrystalline Block Copolymer Thin Films

Langmuir and Langmuir-Blodgett films of multifunctional, amphiphilic polyethers with cholesterol moieties

Langmuir films of multifunctional, hydrophilic polyethers containing a hydrophobic cholesterol group (Ch) were studied by surface pressure-mean molecular area (π-mmA) measurements and Brewster angle microscopy (BAM). The polyethers were either homopolymers or diblock copolymers of linear poly(glycerol) (lPG), linear poly(glyceryl glycidyl ether) (lPGG), linear poly(ethylene glycol) (lPEG), or hyperbranched poly(glycerol) (hbPG). Surface pressure measurements revealed that the homopolymers lPG and hbPG did not stay at the water surface after spreading and solvent evaporation, in contrast to lPEG. Because of the incorporation of the Ch group in the polymer structure, stable Langmuir films were formed by Ch-lPG(n), Ch-lPGG(n), and Ch-hbPG(n). The Ch-hbPG(n), Ch-lPEG(n), Ch-lPEG(n)-b-lPG(m), Ch-lPEG(n)-b-lPGG(m), and Ch-lPEG(n)-b-hbPG(m) systems showed an extended plateau region assigned to a phase transition involving the Ch groups. Typical hierarchically ordered morphologies of the LB films on hydrophilic substrates were observed for all Ch-initiated polymers. All LB films showed that Ch of the Ch-initiated homopolymers is able to crystallize. This strong tendency of self-aggregation then triggers further dewetting effects of the respective polyether entities. Fingerlike morphologies are observed for Ch-lPEG(69), since the lPEG(69) entity is able to undergo crystallization after transfer onto the silicon substrate.

Crystal growth transition from flat-on to edge-on induced by solvent evaporation in ultrathin films of polystyrene-b-poly(ethylene oxide)

The transition of lamellar crystal orientation from flat-on to edge-on in ultrathin films of polystyrene-b-poly(ethylene oxide) (PS-b-PEO) via solvent vapor (toluene) treatment was investigated. When the as-prepared film was treated in saturated solvent vapor, breakout crystals could form quickly, and then they transformed from square single crystals (flat-on lamellae) to dendrites and finally to nanowire crystals (edge-on lamellae). Initially, heterogeneous nucleation at the polymer/substrate interface dominated the structure evolution, leading to flat-on lamellar crystals orientation. And the transition from faceted habits to dendrites indicated a transition of underlying mechanism from nucleation-controlled to diffusion-limited growth. As the solvent molecules gradually diffused into the polymer/substrate interface, it will subsequently weaken the polymer-substrate interaction. In this case, lower free energy lamellae of an edge-on orientation could be obtained by the gain of free volume of the soluble PS block.

Effect of substrate and molecular weight on the stability of thin films of semicrystalline block copolymers

The thermal stability of the thin film morphology of two symmetric oxyethylene/oxybutylene block copolymers (E76B38 and E114B56) on mica and silicon was investigated via atomic force microscopy (AFM). It is found that morphological transition of EmBn thin films during melting is strongly dependent on the molecular weight of the diblock copolymers and their interaction with the substrate. For E76B38 on mica, a single-layered structure transforms into a double-layered structure upon melting, but the same polymer on silicon retains a single-layered structure after melting and spreads quickly to wet-out the silicon surface. Conversely a longer polymer, E114B56, has a thin film on mica that does not change much after melting of the crystalline E block. A mechanism was proposed to explain the relative stability of E76B38 and E114B56 thin films upon melting. Internal stress is produced during melting and can be released along two directions. The release along the vertical direction is restricted by the energy barrier related to the segregation strength, and the release along the horizontal direction is dependent on the mobility of block copolymer related to the interaction between the block copolymer and the substrate. Domain size affects the release rate of the internal stress along the horizontal direction and thus the thermal stability of EmBn thin films. Switching between horizontal and vertical releases can be realized by controlling the domain size of the thin films.