In this thesis, the effect of confinement on the spherulite morphology, crystallinity (x_c) and mechanical properties of semi-crystalline polymer is investigated in detail. Free-standing poly(ε-caprolactone) (PCL) polymer films with thicknesses (h) from 90 to 1000 nm were used as the model system. Our result shows that these films have a critical thickness of h* ~500 nm about which the spherulites morphology transitions from three-dimensional (3D) to two-dimensional (2D). Specifically, upon reducing h below 500 nm, the spherulite radius changes from being independent of h to increases with decreasing h according to ~1/h. Quantitative analysis with infrared spectroscopy shows that x_c of the PCL films increases with decreasing h below h*. Micro-Raman suggests that the center of the spherul...[ Read more ]

In this thesis, the effect of confinement on the spherulite morphology, crystallinity (x_c) and mechanical properties of semi-crystalline polymer is investigated in detail. Free-standing poly(ε-caprolactone) (PCL) polymer films with thicknesses (h) from 90 to 1000 nm were used as the model system. Our result shows that these films have a critical thickness of h* ~500 nm about which the spherulites morphology transitions from three-dimensional (3D) to two-dimensional (2D). Specifically, upon reducing h below 500 nm, the spherulite radius changes from being independent of h to increases with decreasing h according to ~1/h. Quantitative analysis with infrared spectroscopy shows that x_c of the PCL films increases with decreasing h below h*. Micro-Raman suggests that the center of the spherulites have the lowest x_c. As such, the larger spherulite radii found of thinner PCL films below h* naturally explains the increases in x_c observed. For mechanical properties, the Young’s modulus (E₀), mechanical relaxation time (τ) and the yield stress (σ_yield) were measured for PCL films with different h. We found that E₀, τ, and σ_yield all approach a constant value for h >> 500 nm, but become decreasing with decreasing h for h < 500nm. Typically, E₀ increases with x_c in polymers, which is, however, contradictory to the result found here. We think that the crossover of the spherulite morphology into 2D is the reason for the noted reversal in the E₀-x_c relation. Specifically, in 2D the spherulite centers are connected in series with the rest of the films but in 3D they are more likely to be connected in parallel with an off-center, high-x_c region of another spherulite. We believe that this may also be the reason why τ and σ_yield decrease with decreasing h below h*.