The size effect in the thermo-mechanical behavior of polymeric materials is a critically important
phenomenon and has been the subject of many researches in past decades. For example,
polystyrene (PS), a widely used polymeric material, is brittle at the bulk state. When the
dimensions decreases to the nanoscale, such as PS in nanofibers, their ductility becomes
orders higher than their bulk state. In recent years a number of diverse applications, such
as scaffolds in tissue engineering, drug delivery devices, as well as soft robotics, are designed
by utilizing the unique properties of polymers at nanoscale. However, the inside mechanism
of the size dependency in polymeric materials are still not clear yet. In this dissertation, systematic
computational and experimental studies are made in order to understand the origins
of the size effect for one- and two-dimensional polymeric materials. This framework is also
expanded to investigate the size-dependent multi-physics response of functional polymeric
materials (shape memory polymers) which are actuated by high-intensity focused ultrasound
(HIFU). Our computational studies are based on molecular dynamic (MD) simulations at
the atomistic scale, and experimentally-validated finite element models at the bulk level.
From bottom-up direction, molecular dynamics can reveal the mechanisms of the size effect
in polymers at molecular level, and help predict properties of the bulk materials. In this
research, MD simulations are performed to track the origins of the size-effect in the mechanical
properties of PE and PS nanofibers. In addition, the size-dependent thermal response
of functional polymeric films is also studied at the atomistic scale by utilizing molecular dynamics simulations to predict the thermal properties and actuation mechanisms in these
materials when subjected to HIFU fields. From top-down direction, experiments and finite
element analysis, are also conducted in this research. An experimentally-validated finite
element framework is built to study the mechanical response of shape memory polymers
(SMPs) triggered by HIFU. As an external trail towards application fields, a SMP composite
with enhanced shape memory ability and also a two-way SMP are synthesized. A smart
gripper and also a self-rolling structure are designed by using these SMPs, which approves
that these SMPs are good components in designing soft robotics. Finally, The influence of
evaporation during fiber forming process is investigated by molecular dynamics simulation.
It is found that the formation of the microstructure of polymeric fibers at nanoscale depends
on the balance of stretching force and evaporation rate when the fiber is forming. / Doctor of Philosophy / Thermomechanical properties of a thin fiber, a thin film and a cube made of a polymer are
significantly different. Although, based on the extensive research that has been performed in
recent years our understanding of this size-dependency is advanced to a great degree in the
past decades, there are still many unanswered basic questions that can only be addressed
by performing computational and experimental investigation at different length scales, from
atomistic up to bulk level in polymers. In this research we target exploring some unknown aspects
of the size dependency in the thermomechanical properties of polymers by investigating
their deformation mechanisms at different length scales. As the first step, we will investigate
the mechanical properties of polymeric fibers. For these fibers, the mechanical properties
are strongly connected to the fiber's diameter. The prevailing hypothesis is that this size
dependency is closely related to the thickness of the surface layer of the nanofibers. Our
results show some unknown origins behind the size dependency of the mechanical properties
in polyethylene (PE) and polystyrene (PS) nanofibers, which originate from the deformation
mechanisms at the atomistic scale. In addition, not just the mechanical properties, the
thermal properties and response of functional polymers subjected to an external stimulation
are also related to their size. For example, the thermal conductivity of a fiber, a sheet and a
cube may be significantly different. Our study shows the thermal responses of different polymers
triggered by ultrasound are also different. The size and the type of the polymers will
both have influence on the final temperature in the polymeric materials, when the polymeric materials are heated by same ultrasound source. We also have applied our computational
and experimental frameworks to investigate this phenomenon. In addition, we also used a
new shape memory polymer composite and a two-way shape memory polymer on designing
soft robotics-like structures. Overall this research indicates that both mechanical response
and thermal responses of polymers are highly related to their dimension. Taking advantage
of these unique size effects, and by tailoring this property, diverse devices can be made for
being used in a broad range of applications.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/111074 |
| Date | 06 January 2021 |
| Creators | Peng, Kaiyuan |
| Contributors | Mechanical Engineering, Mirzaeifar, Reza, Shahab, Shima, Safranski, David Lee, Acar, Pinar, Nain, Amrinder |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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