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Title: Stress-memory of polymeric materials and its application in smart compression stockings
Authors: Narayana, Harishkumar
Degree: Ph.D.
Issue Date: 2018
Abstract: Shape memory polymers (SMPs) have been considered as an important class of stimuli-responsive smart polymers over past few decades due to their compelling behaviors. SMPs are not only limited to shape memory functions (i.e. fixity or recovery), instead other physical parameters such as stress, temperature, chrome can also be memorized and retrieved with an external stimulus such as temperature. Thus, these polymers could be regarded as memory polymers (MPs). MPs in the current context have both shape and stress memories. Similar to shape memory, stress-memory represents a newly discovered phenomenon where the stress in a polymeric material can be programmed, stored and retrieved reversibly upon an external heat stimulus. It can serve different smart functions in stress control. This innovative research project aims to exploit the memory potentials (stress/strain) in polymeric film, filaments/fibers, and textile fabric structures towards designing and investigating the smart medical compression stocking to manage the phlebological and lymphatic related chronic venous disorders such as varicose veins, venous ulcers, and venous stasis. Most of the reported studies pertaining to stress in the MPs are basically related to shape recovery, namely, recovery force/stress to recover the original shape from the deformed shape caused mainly by the elasticity or viscous strain. But it is still a misconception. The response of the recovery stress is not stable over time due to presence of impure memory stress, viscous stress, and elastic stress components. Several examples can be found where MPs are used in sensors and actuators, however the presence of impeditive stress-strain components restrain the full potential of MPs. The present study aims to distinguish, quantify, and characterize the total stress-strain components in the polymer network and proposes unique programming methods to selectively eliminate the impeditive parts and utilize the pure memory stress from the MP network, hence will expand the application potential of MPs. To further extend the potential domains of MP, the present research exploits the stress memory potential of MPs for designing smart stocking for functional compression management. So far, there exists several shortcomings in the current compression products such as stockings/bandages; 1) once applied on the limb with predetermined elastic force; there is no any further means to change the pressure externally, 2) lack of massage function for old or hospitalized immobile patients having limited calf muscle function, 3) selection of the stockings with improper size, 4) different classes of stockings to provide different levels of compression, 5) It is difficult to achieve the targeted pressure level and pressure drop over time is also a major concern due to the time dependence of the system behaviors. Hence, there is an imperious need of any smart material and textile structures with novel functions to address these problems with profound scientific approach.
To overcome the unsolved research gap, an impending approach in this research project has been drawn the attention to find a most suitable smart material i.e. stress-memory polymer. A novel stress-memory behavior has been discovered and optimized right from the polymeric film, filaments/fibers, and textile structures for designing multi-functional smart compression stockings. Herein, semi-crystalline polyurethanes based on poly(ε­caprolactone) diol (PCL-4000 g.mol -1) and poly(1,6-hexamethylene adipate) diol (PHA­3000 g.mol -1) as a soft segment, 4,4'-methylenediphenyl diisocyanate (MDI), and chain extender 1,4-butanediol (BDO) as hard segment have been synthesized to prepare the stress-memory polymers. A thermomechanical smart tensile stress-memory programming is newly proposed to eliminate the negative components (elastic and viscous stress/strain) to obtain the pure "memory stress" which is stable, cyclically repeatable/controllable without loss over time. This unique property is termed as "stress-memory" behavior and the concept was enlightened with a novel "switch-spring-frame" model to elucidate the evolution of memory stress. The stress analysis in the MP film is investigated to unveil and quantify the total stress-strain components. A constitutive model based on phase transition approach has been used to predict and analyze the individual stress components during thermomechanical process. In contrast to earlier models, a new approach of using relaxed or memory modulus has been proposed to precisely predict stress components. The predicted results have significant agreement with experimental data. The further approach is to unveil and comprehend the stress-memory behavior especially at filament/fiber level which is prerequisite to design the smart stocking for the first time. MP is melt spun and produced filaments to investigate stress-memory behavior. The results have shown maximum memory stress compared to film and the underlying reason is unveiled by mechanical and structural characterizations. Stress-memory filaments were further integrated as a main load bearing element into optimized fleecy or mock inlay textile knitted structure to control the stress/interfacial pressure via external heat stimulus. Based on the interfacial pressure analysis, it is shown that the level of stress and massage effect in the stocking structure can be controlled via parameters such as textile structures and physical parameters including leg radius, deformation level, and temperature range. In addition, the massage effect was confirmed by Doppler ultrasound test to measure the change in venous blood flow. This is the summary of entire research work to advent the multi-functional compression stocking which unifies pressure gradient, massage effect, single size stocking, and selective pressure control for advanced level of compression therapy to overcome existing major shortcomings. Scientific findings in semi-crystalline stress-memory polymeric materials may inspire people to re-examine existing materials such as elastomers/rubbers and develop novel structures and smart functions in soft materials, particularly polymers. Mechanisms revealed in stress-memory behavior of polymers, their fibers and fabrics right through compression stockings would bridge the gap between fundamental study and practical applications. This study helps to shed insights for broadening applications of MPs to interdisciplinary areas such as artificial muscles, soft skins, massage devices and electronic sensors/actuators.
Subjects: Hong Kong Polytechnic University -- Dissertations
Pressure suits -- Therapeutic use
Socks -- Therapeutic use
Shape memory polymers
Pages: xxxi, 212 pages : color illustrations
Appears in Collections:Thesis

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