Preparation, Structure And Properties Of Clay/polymer Nanocomposite Hydrogels

Polymeric hydrogels have become a very important part of our life, which have beenapplied in many fields, like molecular filters, superabsorbents, contact lenses etc. Smarthydrogels have been fully investigated for potential uses, like drug delivery system,substance seperation, microchannel components, shape memory material, andstimuli-responsive devices, because their unique sensitivity to outside stimulus. However,there are some shortcomings for smart hydrogels, like slow responsive rate, weakmechanical properties etc. Therefore, there are three trends in scientific research aboutsmart hydrogels: micro-, nano-or multifunctional gels, enhancing the responsive rate ofbulk smart hydrogels, and improving the mechanical properties of smart hydrogels.In this thesis, we have successfully prepared several kinds of clay/polymernancomposite hydrogels by in-situ polymerization, choosing a kind of modified hectorite(Laponite XLS, Clay-S) as the crosslinker. These nanocomposite hydrogels show excellentmechanical properties and some special stimuli-responsive behavior. Their preparation,structure and properties have been systematically investigated. The results and conclusionsare as follows:1. A series of Clay-S/poly(N-isopropylacrylamide)(PNIPAAm) nanocomposite hydrogels(S-N gels) have been prepared by in-situ polymerization, whose mechanical propertieshave been systematically inverstigated. These gels show very high tensile strength andhigh elongation at break, fast stress relaxation, high hysteresis and poor elasticrecovery. The results of the deswelling behavior, thermosensitive transparency, andDSC curves of S-N gels indicated that the macroscopic phase transition disappears athigh clay content(＞10wt%), for the clay platelets make PNIPAAm chains morehydrophilic. The results of XRD, SEM, AFM, and TEM indicate that clay platelets areuniformly dispersed in S-N gels. Moreover, the freezing dried S-N gels is porous, andmay be used as potential porous materials. 2. A series of Clay-S/polyacrylamide(PAAm) nanocomposite hydrogels (S-M gels) havebeen prepared by in-situ polymerization, which show excellent mechanical properties,like high tensile strength and ultrahigh elongation at break. In addition, compared withS-N gels, S-M gels show slow stress relaxation, small hysteresis and high elasticrecovery. The results of the transparency changes during polymerization of S-M gelsindicate that the hydrophilicity of polymer chains determine whether the transparencychanges occur or not. The swelling ratio of S-M gels decreases with increasing claycontent or increasing polymer content. XRD profile of S-M gels indicates that the clayplatelets are fully exfoliated in wet S-M gels, and partially intercalated in dried S-Mgels due to the volume shrinking. SEM photos of S-M gels indicate that clay plateletsform a good dispersion in freezing dried S-M gels at low clay content, and partiallyaggregated at high clay content.3. A series of Clay-S/Poly(NIPAAm-co-AAm) nanocomposite hydrogels (S-N-M gels)have been prepared by in-situ polymerization, which show good mechanical properties,like high tensile strength and high elongation at break. Their elongation at break ishigher than S-N gels, and their tensile strength is better than S-M gels. Furthermore,the mechanical properties and swelling bahavior of S-N-M gels can be modulated bychanging their composition. The mechanical properties of nanocomposite hydrogelsare determined by the hydrophilicity and flexibility of polymer chains. And it isreconfirmed that the transparency changes during polymerization of nanocompositehydogels are affected by the hydrophilicity of polymer chains; the higher thehydrophilicity, the smaller the changes.4. A series of Clay-S/PAAm nanocomposite hydrogels showing ultrahigh swelling ratio(S-M-T gels) have been prepared by post heat-treatment on S-M gels. The swellingratio of S-M-T gels is up to several thousands times. The reason of the improvement isthe increasing molecular weight among crosslink points, which come from many AAmrepeat units of the polymer chains detaching from clay platelets due to the highermobility of PAAm chains at high temperature. The swelling behavior of the S-N-Mgels show that their swelling ratio is sensitive to ion concentration in outside solution,which is similar to the swelling behavior of traditional ionic superabsorbents. Thismay be ascribed to the charges on the clay platelets. The mechanical properties ofS-M-T gels are not remarkably affected by post heat-treatment, and the S-M-T gelsstill show much better mechanical properties than traditional superabsorbents. 5. A series of Clay-S/PNIPAAm/PEG nanocomposite hydrogels (S-N-P gels) have beenprepared by in-situ polymerization. The introduction of PEG makes the LCST ofnanocomposite hydrogels reoccur at high clay content. It is estimated that PEGmolecules occupy some crosslinking sites on clay platelets, endowing PNIPAAmmolecules higher mobility. A kind of nanocomposite hydrogels with two criticalsolution temperature (UCST and LCST) has been successfully prepared by increasingPEG6000 content, and their UCST and LCST decrease with increasing PEG6000content. Moreover, these S-N-P gels with two critical solution temperature show hightransparency only in Very narrow temperature region.6. A series of thermosensitive nanocomposite hydrogel microcomponents have beensuccessfully fabricated by photomask microchannel in-situ polymerization. Thesemicrocomponents show good mechanical properties, and good thermosensitivity,which may be used as thermosensitive microvalve with high mechanical properties.Clay content is the most important factor affecting the properties of microcomponents.It is hard to get hydrogel microcomponents with good shape stability at low claycontent, and the thermosensitivity of the microcomponents prepared at high claycontent is bad. Thus, the microcomponents with good shape stability andthermosensitivity can only be fabricated at proper clay contents (about 10wt%). Otherfactors during preparation have also been. investigated, like polymerization time,photoinitiator content etc., but these factors affect the properties of microcomponentsmuch less than clay content.