Stability And Confined Crystallization In Polymer Blend Films

Due to the wide use of polymer materials, the structure and performance of them, which are mainly controlled by complicated phase behaviors, have been paid much attention in the past decades. As an example, polymer blend films exhibit two kinds of properties:the stability of film and miscibility of polymer blend. Especially, crystallization, as well as phase separation in blend and wetting/dewetting in films, plays an important role in the blend with crystalline polymers. Phase behaviors discussed above and interplay among them not only determine the structures and resultant performance of polymer blend films, but also have been remained as a big challenge in the investigation on them. In this work, therefore, poly(methylmethacrylate)/poly(styrene-ran-acrylonitrile) (PMMA/SAN) films was employed as an example to investigate the stability, vertical phase separation in mono-layer or bi-layer system. Our results clarified that the addition of miscible component and selectivity of adopted annealing solvent played a key role in the interplay between phase separation and dewetting. Furthermore, stability of films, vertical phase separation and confined crystallization in Poly(L-lactide) /polyoxymethylene (PLLA/POM) films prepared by spin-coating blend solution have been investigated. With the help of atomic force microscopy (AFM), transmission electron microscopy (TEM) and drop shape analysis (DSA), the formation mechanism, as well as its temperature, film thickness and composition dependences of banded-spherulite was discussed in detail.(1) Stability of PMMA(SAN) film upon blending miscible component:The novel stability strategy of polymer films incorporated with tiny amount of miscible component has been investigated using AFM by taking PMMA/SAN as an example. Our results indicate that both neat PMMA and neat SAN films wet silicon oxide well. SAN (with 1% or 2% PMMA) films dewet this substrate completely, while PMMA (with tiny amount of SAN) films are stable upon annealing at 145? (in one-phase region in the phase diagram). The stronger interaction between PMMA and silicon oxide plays an important role in the structure evolution. On one hand, it accounts for the composition gradient of PMMA in film thickness direction, which is the reason for the dewetting by means of composition fluctuation; on the other hand, it is also responsible for the difference of the stability between PMMA and SAN films by adding miscible component.(2) Stability of PMMA/SAN bilavers upon solvent annealing:PMMA/SAN bilayers (SAN at the bottom and PMMA on the top) films have been prepared by spin-coating in turn, and studied upon annealing in the atmosphere of various solvents with the help of AFM. The structures of the bilayer film are determined by the selectivity of adopted annealing solvent. Upon annealing in HAc atmosphere (selected solvent for PMMA), the bilayer film turns to form "PMMA droplets on SAN film" structure; when DMF (exhibits much higher solubility for SAN than PMMA) has been used to annealing the samples, the solvent molecules swell the PMMA layer and enter SAN layer. As a result, SAN layer dewets the substrate and produces some droplets on which PMMA is "carried" by the movement of SAN, resulting in the formation of "SAN droplets covered by PMMA layer"; in the case of OBD (selected solvent for SAN), the bilayer remains stable since it is difficult for the solvent molecules to cross the top PMMA layer.(3) Ring-banded spherulites and its thickness dependence in PLLA/POM blend films:PLLA/POM blend films have been taken as an example to investigate the formation mechanism of ring-banded spherulites upon isothermal crystallization with the help of AFM. New methods of "in-situ etching" and "structure reconstruction" have been established to determine the composition distribution in the film thickness direction. Our results show that vertical phase separation takes place and produces "PLLA enrichment-PLLA/POM blend-POM enrichment" tri-layer structure due to the surface/interface effect and the confinement of film thickness. Based on the discussion above, the film thickness dependence of ring-banded spherulite structures has been investigated. Our results indicate that the decrease of film thickness leads to not only the decrease of twisting-radius, but also the increase of period of the ring-band.(4) Temperature and composition dependence of PLLA/POM ring-banded spherulites:The isothermal crystallization dependence has been discussed in detail. It is found that the ring-banded spherulites can be obtained in a large temperature range. With increasing the temperature, the spherulites develop more and more mature, slowing down the twisting of lamellar crystal. This is the reason for the increase of ring-band period. Moreover, the increase of PLLA composition in the blend produces two kinds of effect:on one hand, more and more amorphous PLLA chains enter the space among POM lamellar-crystals, resulting in the decrease of asymmetrical stress. This is the reason for the increase of ring-band period; on the other hand, the decrease of POM content leads to the decrease of PLLA-POM blend layer thickness and consequent twisting-radius.