Effects of Crystals, Nano-reinforcement, and Electrospinning on Confinement in Semicrystalline Polymers

This thesis focuses on new insights into the characterization of the confinement that exists in semicrystalline polymers, polymer nanocomposites and electrospun fibers using the combination of thermal analysis techniques and other experimental approaches.;The concept of rigid amorphous fraction, RAF, was introduced to explain the confinement exerted by crystals over four decades ago. It can be quantified using two modes of differential scanning calorimetry (DSC), standard DSC and temperature modulated DSC (TMDSC). In this thesis, another advanced thermal analysis technique quasi-isothermal (QI) TMDSC is adopted to obtain the temperature dependent RAF and further investigate its vitrification and devitrification behaviors. For Polytrimethylene terephthalate, PTT, most of the RAF vitrifies between 451 K and Tg step by step during QI cooling after the crystals have formed. The constraints imposed by the crystal surfaces reduce the mobility of the highly entangled polymer chains. It is suggested that the vitrification of RAF proceeds outward away from the lamellar surfaces in a step by step manner during QI cooling. Upon reheating, devitrification of RAF occurs at a temperature above its previous vitrification temperature, due to the effects of densification brought by physical aging during the long period of quasi-isothermal treatment.;Then, the concept of RAF was broadened and we investigated the RAF in polymer nanocomposites and electrospun nanofibers.;In Polyethylene terephthalate (PET)/silica nanocomposite fiber, we found evidence of the existence of RAF in the absence of crystals. The RAF induced by the interaction between the nanofillers and the polymeric matrix, was analyzed based on changes in the heat capacity step in the glass transition region. The effect of the silica particles on the phase structure (fraction of crystals, mobile amorphous phase, and rigid amorphous phase) of electrospun PET fibers as a function of the amount of silica was also investigated. We found with the increase of silica fillers, a large increase in RAF was observed and RAF reaches a value of 0.22 for untreated, as-spun PET fibers with 2.0% silane-modified silica. Using quasi-isothermal TMDSC, it is found that no matter what the origin of the RAF, whether formed by interaction with silica particles or with lamellar crystals, the RAF in the ES PET fibers devitrifies during heating before the temperature reaches the start of the crystal melting endotherm.;The existence of another origin of RAF was confirmed in highly-aligned poly(d-lactide) PLA nanofibers by means of thermal analysis and wide angle X-ray scattering WAXS. This RAF is present as a result of molecular orientation induced by the electrical and mechanical force during the electrospinning. The extension of polymer chains initiates a level of orientation in the as-spun fiber, and these portions of polymer chains acts as an intermediate state (mesophase) between the crystalline and amorphous phases, and cannot undergo the glass transition of the bulk due to the constrained mobility of polymer chains. Like RAF, the mesophase does not contribute to the heat of fusion either. A new phase structure model was proposed to interpret the unique confinement in highly-aligned ES PLA nanofibers.