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MPhil Thesis Presentation 
Confinement effect on crystallinity and mechanical properties of poly(E-caprolactone) thin films
Speaker Mr Tao ZHANG
Department of Physics, The Hong Kong University of Science and Technology
Date 23 July 2021 (Friday)
Time 14:30
Venue Online (Zoom)

In this thesis, the effect of confinement on the spherulite morphology, crystallinity (xc) and mechanical properties of semi-crystalline polymer is investigated in detail. Free-standing poly(E-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 xc of the PCL films increases with decreasing h below h*. Micro-Raman suggests that the center of the spherulites have the lowest xc. As such, the larger spherulite radii found of thinner PCL films below h* naturally explains the increases in xc observed. For mechanical properties, the Young’s modulus (E0), mechanical relaxation time (τ) and the yield stress (yield) were measured for PCL films with different h. We found that E0, τ, and yield all approach a constant value for h >> 500 nm, but become decreasing with decreasing h for h < 500nm. Typically, E0 increases with xc 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-xc 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-xc region of another spherulite. We believe that this may also be the reason why  and yield decrease with decreasing h below h*.


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