Thin amorphous poly(ethylene terephthalate) (PET) films covered with/without a crystallized surface layer were prepared onto silicon wafers. In the former and latter cases, the surface mobility in the film was depressed and enhanced, respectively. The glass transition temperature (Tg) of the amorphous PET film decreased with the reduction of the film thickness, exhibiting a remarkable nanoconfinement effect. However, once the surface region of the thin film was crystallized, or frozen in terms of the segmental motion, Tg of the films recovered to that of the bulk. Concurrently, the apparent activation energy of the segmental motion in the surface-crystallized film was in good accordance with the bulk value as well. These results make it clear that the mobility in the surface region plays an essential role in the glass transition of the thin films.

The effects of the grafting densities (σp), molecular weights (Mn) and thicknesses of dry polystyrene (PS) brushes on their glass transition temperature (Tbrushg) were investigated by ellipsometry. The results show that Tbrushg strongly depends on the grafting density of the PS brushes. The Tbrushg of the PS brushes with σp > 0.30 increases with decreasing Mn (or brush thickness) and is mainly dominated by entropic effects, in which the grafted chains are highly extended along the film thickness direction resulting in a sharp reduction in configurational entropy. The Tbrushg of PS brushes with σp < 0.30 decreases with decreasing Mn (or brush thickness) which is mainly dominated by surface effects. For intermediate-density brushes (σp = 0.30), Tbrushg becomes independent of Mn or brush thickness. The reason for this grafting density dependence of Tbrushg is attributed to the transition of the PS brush conformation from mushroom-to-brush.

Despite its importance in the processing of nanomaterials, the rheological behavior of thin polymer films is poorly understood, partly due to the inherent measurement challenges. Herein, we have developed a facile method for investigating the rheological behavior of supported thin polymeric films by monitoring the growth of the “wetting ridge”—a microscopic protrusion on the film surface due to the capillary forces exerted by a drop of ionic liquid placed on the film surface. It was found that the growth dynamics of the wetting ridge and the behavior of polystyrene rheology are directly linked. Important rheological properties, such as the flow temperature (Tf), viscosity (η), and terminal relaxation time (τ0) of thin polystyrene films, can be derived by studying the development of the height of the wetting ridge with time and the sample temperature. Rheological studies using the proposed approach for supported thin polystyrene (PS) films with thickness down to 20 nm demonstrate that the PS thin film exhibits facilitated flow, with reduced viscosity and lowered viscous temperature and a shortened rubbery plateau, when SiOx–Si was used as the substrate. However, sluggish flow was observed for the PS film supported by hydrogen-passivated silicon substrates (H–Si). The differences in enthalpic interactions between PS and the substrates are the reason for this divergence in the whole-chain mobility and flow ability of thin PS films deposited on SiOx–Si and H–Si surfaces. These results indicate that this approach could be a reliable rheological probe for supported thin polymeric films with different thicknesses and various substrates.

Acrylate block polymer brushes (Au-PMMA65-b-PODMAy-ec-FMA2) with two 2-perfluorooctylethyl methacrylate units at the brush end were successfully prepared on an Au substrate using a “grafting to” method. Characterization by XPS, contact angle measurement, ellipsometry and nanomechanical measurement showed that a well-ordered and perpendicularly oriented structure of the perfluoroalkyl side chains on the brush surface was enhanced with an increasing degree of polymerization (y) of poly (n-octadecyl methacrylate), and a crystalline structure of perfluoroalkyl side chains formed when y was equal to 24. Protein adsorption studies indicated that the adsorbed mass of fibrinogen decreased with an increasing order of the structure of the perfluoroalkyl side chains at the brush end. When the perfluoroalkyl group on the brush surface formed a crystal structure, there was only trace fibrinogen adsorption on the brush surface. This work demonstrates that the protein-resistant performance was enhanced greatly by constructing polymer brush surfaces with well-ordered and perpendicularly oriented structures of the perfluoroalkyl side chains.

Although poly(vinyl acetate) (PVAc) differs from poly(methyl acrylate) (PMA) only in the reversed position of the ester group, a large difference in the concentration dependence of the casting solution on the corresponding surface structure of the cast films of PVAc, PMA and poly(methyl methacrylate) (PMMA) was observed. The hydrophobicity of both PMA and PMMA films increased with increasing concentration of the corresponding polymer solution, whereas cast PVAc films showed the reverse trend. The surface structure of the cast films prepared with different concentrations of the casting solution, characterized by sum frequency generation (SFG) vibrational spectra, showed that the order of the methylene groups increased while that of the acetyl methyl group decreased on the surface of cast PVAc film with increasing concentration of casting solution. However, the order of the ester methyl group increased and that of methylene groups did not change for cast PMA films with increasing concentration of casting solution. The cast PMMA film showed a reverse trend compared with the corresponding PMA film. It is apparent that well-ordered ester or acetyl methyl groups on the surface, which are oriented away from the polymer film, rather than methylene groups, play an important role in determining surface hydrophobicity, as the latter shield the OCO groups of PVAc, PMA and PMMA film surfaces from being exposed, resulting in low surface free energy. The reason for this difference is attributed to the relatively low energy for ester methyl group reorientation, an ester group structure nearer to the trans state and more regular local configuration of segments in concentrated solutions of PMA and PMMA compared to that of PVAc.

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.

The effect of the concentration of casting solutions on the surface dynamics of the corresponding spin-coated poly(methyl methacrylate) (PMMA) film was investigated by measuring the surface reorganization of fluorine tracer-labeled PMMA. The onset temperature of fluorinated PMMA chain end reorganization (TonsetR) was identified and is shown to depend on the PMMA concentration in the film-forming solution. It was found that the surface TonsetR and relaxation activation energy Ea of the PMMA films prepared from 4.2 wt% PMMA cyclohexanone solution are 70 °C and 260 kJ mol−1, respectively, which are higher than those of the PMMA films prepared from 0.8 wt% PMMA cyclohexanone solution (55 °C and 144 kJ mol−1, respectively). The TonsetR and Ea of PMMA films increased with increasing concentration of casting solutions within the range of 1.8 wt% to 4 wt%. The chain entanglement of PMMA chains is proposed to be the speculative origin for these observed depressed dynamics of poly(methyl methacrylate) chains on the films' surface prepared using casting solutions of various concentrations.

We present an experimental investigation of the segmental relaxation behavior of polystyrene (PS) chains that are confined in a micellar core tethered by a poly(acrylic acid) (PAA) block corona on the dry film surface, along with various core density and molecular weight of PS block from below to well above the entanglement molecular weight. The results show that the onset temperature of PS chain rearrangement (Tonset), which was much lower than the Tbulkg of the corresponding PS block and higher than Tfilmg of ultrathin PS films with corresponding thickness and molecular weights, generally increases with increasing density of the micelle core. It was found that the difference in ΔTonset with increasing relative density ρ/ρmin obtained from PS154-b-PAA49 and PS278-b-PAA47 micelles was large, while these from PS278-b-PAA47 and PS404-b-PAA63 was negligible, suggesting that entanglement has considerable influence on the density dependence of the Tonset of PS chains under confinement in the micelle core on the film surface.

The effects of the properties of casting solution on the surface structure of poly(methyl methacrylate) (PMMA) films were systematically investigated. It was observed that the hydrophobicity of PMMA films increased with increasing viscosity of the corresponding polymer solution regardless of the film-formation techniques that were utilized. The ratio of the C–H symmetric stretching vibrations of methylene groups (hydrophobic components, with a peak at 2910 cm−1) to those of the ester methyl groups (relative hydrophilic components, with a peak at 2955 cm−1) from sum frequency generation (SFG) vibrational spectra, A2910/A2955, was used as a parameter to evaluate the structure on the film surface, which was related to the surface wettability of the films. The results showed that A2910/A2955 of cast PMMA films increased linearly with ηsp0.3 (ηsp, the specific viscosity of the casting solution), whereas that of the corresponding spin-coated films showed a linear relationship defined as ηsp0.3E0.26, where E is the average number of entanglement points per molecule (E = Mw/Me). These results indicate that a relative equilibrium conformation on the PMMA film surface, adopted from the perspective of thermodynamics, was easily achieved during film formation, when the conformation of the polymer chains in the corresponding casting solution was close to that in the bulk. For the spin-coated films, the chain entanglement structure in the casting solution was a more important factor for the resulting film to reach a relative equilibrium state, since this structure was in favor of maintaining the pristine conformation in casting solution under centrifugal force during spin-coating. This work may help to enhance the fundamental understanding of the formation of the film surface structure from polymer solution to the resulting solid film, which will affect not only the corresponding surface properties, but also the dynamics of the resulting thin films.

The dynamic wetting of water droplets on ultrathin poly(vinyl acetate) (PVAc) films supported by substrates with tunable surface hydroxyl group content was investigated at room temperature. The jumping angle of water droplets on the ultrathin PVAc films decreased with increasing hydroxyl content on the substrate surface, while a smooth evolution of the water droplet contacting line emerged for the film supported by a substrate with 100% hydroxyl groups. The critical thickness of ultrathin PAVc films, through which the interfacial effect from substrate chemistry on the dynamic wetting of water droplets on their surfaces will disappear, was measured by this novel method and the results showed that the critical thickness increased from ∼35 nm to ∼75 nm with increasing hydroxyl content from 41% to 100% on the substrate. This relationship between critical thickness and hydroxyl content was attributed to the intensities of the hydrogen bonds, resulting in lower chain mobility at the interface and a higher elastic modulus of the whole ultrathin film. This study demonstrates that the dynamic wetting of a liquid droplet on ultrathin polymer films is very sensitive and effective to access their mechanical properties, relaxation behavior and those properties affected by the substrate surface chemistry.

We investigated the segmental mobility distribution at the poly(methyl methacrylate) film surface within the depth of 9.0 nm, by measuring the surface reorganization of fluorinated tracer-labeled PMMA using three different surface-sensitive techniques with various analytical depth capabilities, including contact angle measurements, sum frequency generation (SFG) vibrational spectra and angle-resolved X-ray photoelectron spectroscopy (XPS). The onset temperature of fluorinated chain end reorganization (Tonset) at various depths was identified and is shown to depend on the sampling depth of the techniques. The Tonset was verified to be related to the segmental relaxation of PMMA. Plotting the Tonset against the sensing depth of the corresponding technique, the distribution of surface Tonset within a 9.0 nm depth was obtained. An approximately 2.4 nm surface layer with depth-independent Tonset was found, below which the Tonset increased with increasing depth. The depth profile of Tonset effectively divides a film into three layers: a “surface layer” with Arrhenius dynamics on the uppermost film surface, a “gradient region” layer underneath the top layer transporting “liquid-like” dynamics into the internal film, and the remaining bulk layer. This observed depth profile of the surface dynamics may suggest that the polymer chains near the surface adopt an oblate conformation and have lowered entanglements.

The effect of chain constraint on the surface dynamic of poly(methyl methacrylate) (PMMA) was investigated in the context of polymer tethering to a micelle core. Film surfaces dominated by either poly(methyl methacrylate) (PMMA) tethered by a poly(2-perfluorooctylethyl methacrylate) (PFMA) micelle core or non-micellized free PMMA chains are fabricated by spin coating a solution of PMMA end-capped with various numbers of FMA units onto silica substrates. By measuring the surface rearrangement kinetics of these films under thermal annealing, the onset temperature of rearrangement (TonsetR) and the activation barrier for relaxation (Ea) of surface PMMA chains are determined. It is found that the TonsetR and Ea of the PMMA micellized chains are 83 °C and 317 kJ mol−1, respectively, which are higher than those of the non-micellized PMMA free chains (70 °C and 164 kJ mol−1). The TonsetR and Ea of PMMA in the corona increase linearly with increasing compactness of the PFMA core. The higher TonsetR and Ea values demonstrate the reduced mobility of surface PMMA segments tethered to a micelle core. The constraint of conformational freedom, reduction of free volume and increment of chain packing density are proposed as the speculative origins for this depressed dynamic of poly(methyl methacrylate) chains in the corona of collapsed dry micelles tethered by fluorinated block core.

A novel method was developed to detect the glass transition of thin and ultrathin 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 glycerol or oligo-poly(ethylene glycol) droplets. The peak temperature (Tjm) obtained from the jumping angle–film temperature curve, in which the jumping angle Δθ was employed to scale the stick–slip behavior, was nearly identical to the corresponding Tg (or Tα) of the PS film. This was confirmed by dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC). The change of the measured Tjm with film thickness and substrate chemistry (SiO2–Si and H–Si) further confirmed that the developed method is very sensitive for detecting the dynamics of ultrathin polymer films.

Acrylate block polymer brushes (Au-PMMA65-b-PODMAy-ec-FMA2) with two 2-perfluorooctylethyl methacrylate units at the brush end were successfully prepared on an Au substrate using a “grafting to” method. Characterization by XPS, contact angle measurement, ellipsometry and nanomechanical measurement showed that a well-ordered and perpendicularly oriented structure of the perfluoroalkyl side chains on the brush surface was enhanced with an increasing degree of polymerization (y) of poly (n-octadecyl methacrylate), and a crystalline structure of perfluoroalkyl side chains formed when y was equal to 24. Protein adsorption studies indicated that the adsorbed mass of fibrinogen decreased with an increasing order of the structure of the perfluoroalkyl side chains at the brush end. When the perfluoroalkyl group on the brush surface formed a crystal structure, there was only trace fibrinogen adsorption on the brush surface. This work demonstrates that the protein-resistant performance was enhanced greatly by constructing polymer brush surfaces with well-ordered and perpendicularly oriented structures of the perfluoroalkyl side chains.