Styrene - Isoprene (or Butadiene) Block Copolymer Surface Structure Formation And Its Impact Factors

Polymer surface properties depend critically upon the molecular structure at the polymer surface. Hence, to meet the challenge of designing and controlling the structure and properties of block copolymer surfaces at the molecular or nanoscale level, the relationship between the chemical structure of copolymer,film-formation methods surface structures of resulting films must be clearly understanded. Block copolymers composed of styrene and isoprene (butadiene) are thermoplastic elastomeric materials and they are widely used as adhesives and sealants. The chemical structures on these block copolymers surfaces remains largely unstudied and poorly understood because only few surface techniques are effective to probe such copolymer surface at the molecular level.In this paper, polystyrene/polyisoprene (polybutadiene) block copolymers surface structure and properties were investigated by contact angle measurement, surface tension measurements, atomic force microscopy (AFM) and sum frequency generation vibrational spectroscopy (SFG) which has unique ability to probe the molecular spectroscopy and molecular group orientation at submolecular level. The influence of film-forming methods and polymer solution properties on film surface structure formation was explored.It is found that very subtle variation in the polymer concentration of toluene solution can remarkably influence the surface structures of the cast films. At a copolymer concentration of 1% w/v, the PB segment in the polystyrene-block-polybutadiene(SB) diblock copolymer and polystyrene-block- polybutadiene-block-polystyrene(SBS) triblock copolymer were the most apt to segregate in the free surface region. Especially, the film of SB diblock copolymer was covered by pure PB segment. With increasing of the copolymer concentration, surface structures with more PS segment were visualized. During solution concentrating process, ordered microphase-separated surface structures were gradually formed and the final surface structure was affected by molecular association and the activity of polymer chains in the solution. At a relatively low copolymer concentration, the more stretched polymer chains and also the longer presented time of solvent molecular at solution surface give the polymer chain enough time and activity to segregate to the surface, resulting in a pure PB coverage on the surface.For spin-coated films, the PB content is lower on the surface, regardless of polymer concentration. Owe to the fast process for film formation, the evaporation process is too fast for PB component to segregate at the outmost surface. After annealing in toluene vapor, the surface was covered by pure PB, in order to minimize the surface energy.The surface structures of films casted from varing polystyrene/polybutadiene block copolymers solution were studied. The results showed that, the proportion of PB at the outmost surface of films is higher for SB solution cast films than that for SBS solution cast films. When the SB diblock copolymer was cast by cyclohexane solution, a PB-selective solvent, the resultant surface of SB was dominated by PB component. Cast by toluene solution, a good solvent of both PS and PB, but have a preferential affinity to PS, however, surface segregation of PB was restricted and the surface was composed of PS and PB. The SFG results showed that, both toluene and cyclohexane were used as solvents, PB coexist with PS at the outmost surface of SBS cast films.The relationship between the surface viscoelastic properties and the stick-slip of the three-phase line in measurement of dynamic contact angle with water on SIS copolymers was studied by axisymmetric drop shape analysis-profile (ADSA-P) and atomic force microscope (AFM). It is found that, in a certain surface energy range, the abrupt change of water contact angle (Δθ) in the stick-slip pattern has linear relationship with the slope of the contact region in the force curves. All of the evidence strongly suggests that stick-slip phenomenon on SIS films surface was caused by the surface viscoelasticity.