Biao Zuo

The architecture of the adsorbed layer at the substrate interface determines the glass transition of supported ultrathin polystyrene films

To elucidate the mechanism underlying the effect of polymer/solid interfacial interactions on the dynamics of thin polymer films, the glass transition of thin end-functionalized polystyrene films supported on SiO2-Si, such as proton-terminated PS (PS-H), α,ω-dicarboxy-terminated PS (PS-COOH), and α,ω-dihydroxyl-terminated PS (PS-OH), was investigated. All the PS films exhibited a substantial depression in Tg with decreasing film thickness, while the extent of such depression was strongly dependent on the chemical structure of the end groups and molecular weights. It was found that T - T of the various PS films increased linearly with increasing hads/Rg, in which hads is the thickness of the interfacial adsorbed layer and Rg is the radius of gyration of PS. The hads/Rg is a direct reflection of the macromolecular chain conformation within the adsorbed layer which was affected by its end groups and molecular weights. These findings are in line with the work of Napolitano, and present direct experimental evidence.

Stepwise crystallization and the layered distribution in crystallization kinetics of ultra-thin poly(ethylene terephthalate) film

Crystallization is an important property of polymeric materials. In conventional viewpoint, the transformation of disordered chains into crystals is usually a spatially homogeneous process (i.e., it occurs simultaneously throughout the sample), that is, the crystallization rate at each local position within the sample is almost the same. Here, we show that crystallization of ultra-thin poly(ethylene terephthalate) (PET) films can occur in the heterogeneous way, exhibiting a stepwise crystallization process. We found that the layered distribution of glass transition dynamics of thin film modifies the corresponding crystallization behavior, giving rise to the layered distribution of the crystallization kinetics of PET films, with an 11-nm-thick surface layer having faster crystallization rate and the underlying layer showing bulk-like behavior. The layered distribution in crystallization kinetics results in a particular stepwise crystallization behavior during heating the sample, with the two cold-crystallization temperatures separated by up to 20 K. Meanwhile, interfacial interaction is crucial for the occurrence of the heterogeneous crystallization, as the thin film crystallizes simultaneously if the interfacial interaction is relatively strong. We anticipate that this mechanism of stepwise crystallization of thin polymeric films will allow new insight into the chain organization in confined environments and permit independent manipulation of localized properties of nanomaterials.

Relationship between Segmental Relaxation of Polystyrene Films and Stick–Slip Behavior during Dynamic Wetting of Liquid Droplets on Their Surfaces

A novel method was previously reported for detecting the glass transition of thin polystyrene (PS) films by correlating the relationships between the temperature-dependent viscoelasticity of the PS films and stick-slip behavior on their surfaces during dynamic wetting of liquid droplets. In the present study, the frequency dependence of the stick-slip behavior is investigated. The results show that the stick-slip behavior of liquid dynamic wetting on PS films is dependent on the contact line velocity, which is related to the deformation frequency of the PS surface during the moving liquid front. The stick-slip behavior was revealed to be determined by a dimensionless parameter (ξ), which is the ratio of the PS segmental relaxation time (τα) and the characteristic time (τc) for PS surface deformation near the droplet contact line. When ξ is close to 1 (τα ≈ τc), the Δθ (jumping angle), a scale of the stick-slip behavior, reaches a maximum. This correlation between Δθ and ξ demonstrates that the stick-slip behavior is related to the energy dissipation caused by the PS α-relaxation process, and the peak temperature (or frequency) in Δθ corresponds to the α-relaxation temperature (time) of the polymer. These results strongly demonstrate that the utilization of the stick-slip behavior is a creditable method, similar to dynamic viscoelastic measurement, for probing the glass transition and segmental relaxation of thin polymer films.

Toward Achieving Highly Ordered Fluorinated Surfaces of Spin-Coated Polymer Thin Films by Optimizing the Air/Liquid Interfacial Structure of the Casting Solutions

Thin polymer films with well-assembled fluorinated groups on their surfaces are not easily achieved via spin-coating film-fabrication methods because the solution solidifies very rapidly during spin-coating, which hinders the fluorinated moieties from segregating and organizing on the film surface. In this contribution, we have proposed a comprehensive strategy toward achieving well-ordered fluorinated thin films surfaces by optimizing the molecular organization at air/liquid interface of the film-formation solutions. To validate such a route, poly(methyl methacrylate) (PMMA) end-capped with several 2-perfluorooctylethyl methacrylate (FMA) units was employed as the model polymer for investigations. The air/solution interfacial structures were optimized by systematically changing the polymer chain structures and properties of the casting solvents. It was found that the polymers that form loosely associated aggregates (e.g., FMA1- ec-PMMA65- ec-FMA1) and a solvent with better solubility to FMA while having not too low surface tension (i.e., toluene) can combine to produce solutions with well-assembled FMA at the interfaces. By spin-coating the solutions with well-organized interfaces, an ultrathin film with perfluorinated groups that were highly oriented toward the film surface was readily achieved, exhibiting surface energies as low as 7.2 mJ/m2, which is among the lowest reported so far for the spin-coated thin films, and a very high F/C ratio (i.e., 0.98).

Progresses in the glass transition and molecular dynamics of thin polymer film

With the advances in nanotechnology, the glass transition and chain relaxation behavior for polymer molecules in a confined geometry have been attracted great attentions. Due to the nano-scale effects, the molecular dynamics of polymer thin film deviates from that in the bulk, and are a function of the dimension of films (i.e. thickness). The studies of the molecular dynamic in polymer thin film are of significant importance both in term of optimizing the physical properties of polymer nanomaterials and also in the developing our understanding of the nature of glass transition for amorphous materials. In this paper, some important results in the field of the dynamics of thin polymer film are reviewed. We summarized the physical mechanisms responsible for the deviation of chain dynamic from that in bulk, and discussed the distribution of chain mobility in polymer thin film, as well, some theoretical models describing the glass transition of thin polymer film was introduced. In the end, the problems in this field and prospects in the further studies were also discussed.