Effects of Crystals, Nano-reinforcement, and Electrospinning on Confinement in Semicrystalline Polymers.
Abstract: 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. ... read moreIt 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
Thesis (Ph.D.)--Tufts University, 2012.
Submitted to the Dept. of Physics.
Advisor: Peggy Cebe.
Committee: Roger Tobin, William Oliver, Cristian Staii, and Daniel Schmidt.
Keyword: Physics.read less