Experimental evidence of ultrathin polymer film stratification by AFM force spectroscopy

Dynamics and Structure Formation of Confined Polymer Thin Films Supported on Solid Substrates

The stability/instability behavior of polystyrene (PS) films with tunable thickness ranging from higher as-cast to lower residual made on Si substrates with and without native oxide layer was studied in this paper. For further extraction of residual PS thin film (hresi) and to investigate the polymer-substrate interaction, Guiselin's method was used by decomposing the polymer thin films in different solvents. The solvents for removing loosely adsorbed chains and extracting the strongly adsorbed irreversible chains were selected based on their relative desorption energy difference with polymer. The PS thin films rinsed in chloroform with higher polarity than that of toluene showed a higher decrease in the residual film thickness but exhibited earlier growth of holes and dewetting in the film. The un-annealed samples with a higher oxide film thickness showed a higher decrease in the PS residual film thickness. The effective viscosity of PS thin films spin-coated on H-Si substrates increased because of more resistance to flow dynamics due to the stronger polymer-substrate interaction as compared to that of Si-SiOx substrates. By decreasing the film thickness, the overall effective mobility of the film increased and led to the decrease in the effective viscosity, with matching results of the film morphology from atomic force microscopy (AFM). The polymer film maintained low viscosity until a certain period of time, whereupon further annealing occurred, and the formation of holes in the film grew, which ultimately dewetted the film. The residual film decrement, growth of holes in the film, and dewetting of the polymer-confined thin film showed dependence on the effective viscosity, the strength of solvent used, and various involved interactions on the surface of substrates.

Glass transition temperature of single-chain polystyrene particles end-grafted to oxide-coated silicon

A method based on the PeakForce QNM atomic force microscopic (AFM) adhesion measurement is employed to investigate the glassy dynamics of polystyrene (PS) single-chain particles end-grafted to SiO₂-Si substrates with different diameters, D₀, of 3.4 nm-8.8 nm and molar masses, M_n, of 8-123 kg/mol. As temperature was increased, the adhesion force, F_ad, experienced by the AFM tip on pulling off the single chains after loading demonstrated a stepwise increase at an elevated temperature, which we identified to be T_g based on previous works. Our result shows that T_g of our grafted single chains increases with M_n in a manner consistent with the Fox-Flory equation, but the coefficient quantifying the M_n dependence of T_g is only (36 ± 6)% the value of bulk PS. In addition, the value of T_g in the M_n → ∞ limit is about 25 °C below the bulk T_g but more than 15 °C above that of (untethered) PS nanoparticles with D₀ ≈ 100 nm suspended in a solution. Our findings are consistent with T_g of our single chains being dominated by simultaneous effects of the interfaces, which depress T_g, and end-grafting, which enhances T_g. The latter is believed to exert its influence on the glass transition dynamics by a mechanism reliant on chain connectivity and does not vary with chain length.

Nanoparticle-polymer interfacial layer properties tune fragility and dynamic heterogeneity of athermal polymer nanocomposite films

Enthalpic interactions at the interface between nanoparticles and matrix polymers are known to influence various properties of the resultant polymer nanocomposites (PNC). For athermal PNCs, consisting of grafted nanoparticles embedded in chemically identical polymers, the role and extent of the interface layer (IL) interactions in determining the properties of the nanocomposites are not very clear. Here, we demonstrate the influence of the interfacial layer dynamics on the fragility and dynamical heterogeneity (DH) of athermal and glassy PNCs. The IL properties are altered by changing the grafted to matrix polymer size ratio, f, which in turn changes the extent of matrix chain penetration into the grafted layer, λ. The fragility of PNCs is found to increase monotonically with increasing entropic compatibility, characterised by increasing λ. Contrary to observations in most polymers and glass formers, we observe an anti-correlation between the dependence on IL dynamics of fragility and DH, quantified by the experimentally estimated Kohlrausch-Watts-Williams parameter and the non-Gaussian parameter obtained from simulations.

Correlation between grafted nanoparticle-matrix polymer interface wettability and slip in polymer nanocomposites

Controlling and understanding the flow properties of polymer nanocomposites (PNC) is very important in realising their potential for various applications. In this study we report molecular dynamics simulation studies of slip between a rotating polymer-grafted nanoparticle and the surrounding free linear matrix chains. By varying the interface wettability between the nanoparticle and matrix chains defined by the parameter f, the ratio of the graft to the matrix chain length, or the graft chain density, Σ, we were able to tune the interface slip, δ, significantly. Both f and Σ alter the interface wettability by changing the matrix chain penetration depth, λ, into the graft chain layer. We observed a large value of δ at smaller f or Σ which reduces with an increasing value of the respective parameters. Since interface slip is also likely to affect other properies of PNCs, like viscosity and the glass transition, we suggest that these parameters could become useful tools to control the flow and mechanical properties of PNCs made with grafted nanoparticles.

Stratification and two glass-like thermal transitions in aged polymer films

Using X-ray reflectivity, spectroscopic ellipsometry and Raman spectroscopy, we have studied the stratified structure and the two glass-like thermal transitions in sufficiently aged glassy polystyrene films. We find that favorable interaction between the solid substrate and the polymer film induces stratification within the film resulting in different densities across the film thickness. Existence of two glass-like thermal transitions (one at 70 °C and the other at 95 °C) is independently confirmed by temperature dependent spectroscopic ellipsometry and Raman spectroscopy measurements. Interestingly, the thermal coefficient of expansion of the polymer film displays anomalous behavior with temperature and is found to have the lowest value over the temperature range 70-95 °C, i.e. between the two observed glass-like thermal transition temperatures.

Size Dependent Mechanical Properties of Monolayer Densely Arranged Polystyrene Nanospheres

In contrast to macroscopic materials, the mechanical properties of polymer nanospheres show fascinating scientific and application values. However, the experimental measurements of individual nanospheres and quantitative analysis of theoretical mechanisms remain less well performed and understood. We provide a highly efficient and accurate method with monolayer densely arranged honeycomb polystyrene (PS) nanospheres for the quantitatively mechanical characterization of individual nanospheres on the basis of atomic force microscopy (AFM) nanoindentation. The efficiency is improved by 1-2 orders, and the accuracy is also enhanced almost by half-order. The elastic modulus measured in the experiments increases with decreasing radius to the smallest nanospheres (25-35 nm in radius). A core-shell model is introduced to predict the size dependent elasticity of PS nanospheres, and the theoretical prediction agrees reasonably well with the experimental results and also shows a peak modulus value.