Studies of functional shape memory fibers

Pao Yue-kong Library Electronic Theses Database

Studies of functional shape memory fibers


Author: Meng, Qinghao
Title: Studies of functional shape memory fibers
Degree: Ph.D.
Year: 2011
Subject: Shape memory effect
Textile fabrics -- Technological innovations
Smart materials
Hong Kong Polytechnic University -- Dissertations
Department: Institute of Textiles and Clothing
Pages: xxv, 204 leaves : ill. ; 30 cm.
InnoPac Record:
Abstract: Shape memory polymers (SMPs) can rapidly change their shapes from a temporary shape to the original (or permanent) shape under appropriate thermal stimulation. They have been widely used in the forms of solution, emulsion, film, bulk and foam. Recently, shape memory fibers (SMFs) prepared by different spinning methods were developed and studied. This project is intended to study the SMFs with novel functions, i.e., SMFs with thermal-responsive inner diameters, SMFs with temperature-regulating effect, SMFs with electro-active effect and SMFs with potential biological applications. In this project, first, a Tm type shape memory polyurethane (SMPU) was synthesized and corresponding SMFs were fabricated. The switching transition temperature of the SMF was the soft segment phase melting temperature at 47°C. The mechanical properties, shape memory effect, and thermal properties of the SMF were studied. The partially crystallized soft segment phase provided the SMF with partial elasticity at ambient temperature and the ability to fix the temporary shape once the fiber was cooled to ambient temperature. The SMF could recover its original length by reheating the fiber to a temperature above 47°C. The SMF had a tenacity of about 1.0 cN/dtex, and strain at break 562 ~ 660%. The shape fixity ratio reached 84% and the recovery ratio was up to 95%. The influences of heat treatments on the SMF properties were studied. Low temperature heat treatment increased breaking elongation and shape fixity ratios, while decreased boiling water shrinkage, tenacity and shape recovery ratios. High temperature heat treatment benefited the improvement of hard segment phase stability. High temperature heat treatment increased both the shape recovery ratios and fixity ratios. The SMF was expected to be treated at a high temperature. Unfortunately, the heat treatment at a very temperature could not be conducted because the SMF became too tacky and soft due to the melting of the soft segment phase. Shape memory hollow fibers could have special properties in comparison with cylindrical SMFs and could have many special applications. A shape memory hollow fiber with the thermal-responsive inner diameter was fabricated and the properties of the hollow fiber were studied. The hollow fiber had a switching temperature 41°C, a tenacity of about 1.14 cN/dtex, and strain at break 682%. The shape fixity ratio of the shape memory hollow fiber was above 80% and the recovery ratio above 90%. The inner diameter of the hollow fiber could be noticeably changed; and after being heated above the soft segment phase melting temperature, the hollow fiber inner cavity could recover its original diameter.
By employing poly(ethylene glycol) (PEG) as the soft segment of the SMF, a temperature-regulating fiber with shape memory effect was prepared. The PEG-based polyurethane fibers showed temperature-regulating effect and shape memory effect simultaneously. The fiber's phase change behaviors and crystalline morphology were investigated using polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). The prepared fiber had a tenacity of 0.7 cN/dtex, breaking elongation 488%, latent heat storage about 100 J/g, shape fixity ratio more than 85.8%, and shape fixity ratio above 95.4%. Presently, the degree of supercooling of the PEG-based SMF was still too high and the mechanical properties of the fiber were not satisfied. Furthermore, due to the significant melt transition at the low temperature, the fiber did not have good spinnability. Usually the shape memory effect is induced by directly heating the SMP to a temperature above the switching temperature. Electro-responsive shape memory effect of the SMFs may be achieved by Joule heating if the SMF is filled with conductive CNTs. To obtain electro-active shape memory effect by Joule heating, multiwalled carbon nanotubes (MWCNTs) were incorporated into SMFs by in-situ polymerization. Electro-responsive shape memory effect was observed on the SMFs. It was also found that the MWCNTs improved the recovery stress of SMFs markedly because of the interaction between the MWCNTs and SMPU, especially with the hard segments. The medical applications of the SMF for responsive medical devices are of great interests due to its combination of tailor-able transition temperatures, large deformation, high recoverability, and elastic properties. A Tg type SMF with the switching transition at around body temperature was fabricated. The Tg type SMF was used because of the easy adjustment of glass transition temperature (switching temperature) by varying the soft segment length and content. The biological properties of the SMF were evaluated in terms of cytotoxicity, haemolysis, sensitization and dermal irritant. Based on the test results, the shape memory fiber/fabric was not considered cytotoxic, haemolytic, sensitive and irritant.

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