Structures And Properties Of Functional Layered Double Hydroxide/Polymer Nanocomposites

Layered double hydroxide (LDHs) is a class of anionic clay with layered structure, represented by the general formula [MⅡ1-xMⅢx(OH)2]x+[An-x/n·yH2O]x-. LDHs can be synthesized with tunable morphology, composition and dispersibility by simple control over the conditions of preparation. Therefore, LDHs with designed morphology and function can be used as functional nanofillers in LDHs/polymer nanocomposites. There are various methods that can be applied to fabricate polymer based functional nanocomposites. Basically it can be categorized into three groups: functional polymer based, functional nanofiller based and structure based functional nanocomposites. Using functional polymer as matix of nanocomposites is the most direct way to obtain thermo-and photo-responsive, electro-conductive nanocomposites. However, functional polymers are limited in a few kinds of polymers and application fileds. Thus functional polymer based functional nanocomposites are rarely studied. On the contrary, functional nanocomposites fabricated by homogeneous dispersion of functional nanofiller into common polymer matrices will significantly enhance properties of nanocomposites, endowing common polymers with funtions. The other methods of fabricating functional nanocomposites are to deliberately design the structure of nanocomposites, e.g. electrospun nanofibers, oriented thin films, nanocomposites with micro/nano hybrid structures, etc. As a result, nanocomposites with superhydrophobic surfaces, superbarrier effect, and selective adorption, and so on, can be formed.This thesis is focused on the studies of relationship between structure and properties of nanocomposites. Inorganic funtional LDHs are selected as nanofillers to fabricate exfoliated LDH/polymer nanocomposites and highly oriented LDH/polymer hybrid films by melt compounding and controlled orientation method, respectively. Besides, in situ growth of LDHs on mica and electrospun SiO2 fiber gave vertically oriented LDHs thin films with 3D hybrid structure, which show versatile functions and properties. The functionality of LDHs, dispersion morphology of LDHs in matrices, structure and properties of nanocomposites were studied and discussed.In the first part, microsized LDH crystals were syntheized via urea hydrolysis method and used as green carrier for copper ions to fabricate functional MgCuAl-LDH which will be a new class of environmental friendly thermal stabilizers with efficient smoke suppression effect and mechanical enhancement for poly(vinyl chloride) (PVC). MgCuAl-LDH/PVC nanocomposites were fabricated by melt compounding. The thermal stablility, degradation behavior and smoke properties of nanocomposites were studied and discussed. It is found that MgCuAl-LDH not only retains the smoke suppression effect of copper ions, but also keeps the properties of LDHs. After anion exchange, MgCuAl-DS-LDH can be delaminated in PVC, forming exfoliated LDH/PVC nanocomposites. The relationship between dispersion morphology and performance of nanocomposits were consequently discussed.The second part is mainly the fabrication and characterization of highly oriented LDH/polymer hybrid films and related studies of coassemly of LDH nanosheets with anions or polymers in formamide. LDHs with different lateral size, CoAl-LDH (6μm) and NiAl-LDH (500 nm), were syntheized, anion exchanged and delaminated in formamide to form exfoliated LDH nanosheets/formamide colloidal dispersion. The thickness of LDH monolayer was characterized to be about 0.8 nm by AFM. After vacuum filtration of LDH/formamide colloidal dispersion, highly oriented LDH thin films on filter membrane can be transfer-printed onto different substrates, forming transparent, strong and flexible LDH thin films. The morphology and structure studies of LDH films reveal different stacking modes of CoAl-LDH and NiAl-LDH. Utilizing coassembly of positively charged LDH nanosheets with polymeric anions, negatively charged LDH nanohybrids can be obtained. By adding various types of anions into colloidal dispersion of LDHs and subsequent filtration, LDH thin films with tunable composition and nanostructure can be prepared easily. This facile filtration method will facilitate the fabrication of functional LDH thin films.The third part is the studies of in situ growth of LDHs on surfaces of mica and electrospun SiO2 micro-fiber. LDHs crystals vertically oriented on surfaces of substrates and can be easily controlled by tuning the hydro-thermo conditions. The in situ growth of LDHs on electrospun SiO2 micro-fiber built functional oriented LDHs thin films with 3D hybrid structure which have numerous potential applications.