Font Size: a A A

The Synthesis, Structure And Properties Of The Organic-Inorganic Hybrid Poly (N-Isopropylamide) Polymers Containing Polyhedral Oligomeric Silsesquioxane (POSS)

Posted on:2013-03-01Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y C ZhengFull Text:PDF
GTID:1221330362967344Subject:Materials science
Abstract/Summary:PDF Full Text Request
A series of modified poly (N-isopropylamine)(PNIPAAm) hydrogel networks byheptaphenyl polyhedral oligomeric silsesquioxane (POSS), poly(ethylene oxide)(mPEO)and mPEO-b-P(MA POSS) were synthesized via combination of atom transfer radicalpolymerization (ATRP), reversible addition-fragment transfer polymerization (RAFT) and“click” chemistry. The chemical structures of model block copolymers were investigatedby means of nuclear magnetic resonance spectroscopy (NMR) and gel permeationchromatography (GPC). The morphology and self-assembly behaviors were characterizedusing differential scanning calorimetry (DSC), atomic force microscopy (AFM),small-angle X-ray scattering (SAXS), transition electron microscopy (TEM) and fouriertransform infrared spectroscopy (FTIR). The lower critical solution temperature (LCST)was performed on ultraviolet-visible spectroscopy (UV-Vis) and dynamic laser scattering(DLS). The deswelling and reswelling experiments were employed to characterize to thetemperature responsive properties. The main results are being summarized as follows:1. Synthesis and characterization of heptaphenyl polyhedral oligomericsilsesquioxane-capped poly(N-isopropylacrylamide)s.A novel initiator bearing heptaphenyl polyhedral oligomeric silsesquioxane (POSS)was synthesized via the copper-catalyzed Huisgen1,3-cycloaddition (i.e., click chemistry).With this initiator, the ATRP of N-isopropylacrylamide (NIPAAm) was carried out toafford the POSS-capped PNIPAAm. The organic–inorganic amphiphiles werecharacterized by means of NMR and GPC. AFM showed that the POSS-cappedPNIPAAm amphiphiles in bulk displayed microphase-separated morphologies. In aqueous solutions, the POSS-capped PNIPAAm amphiphiles were self-assembled into micelle-likeaggregates as evidenced by DLS and TEM. It was found that the sizes of theself-organized nanoobjects decreased with increasing the lengths of PNIPAAm chains. Bymeans of UV–Vis spectroscopy, the LCST behavior of the organic–inorganic amphiphilesin aqueous solution was investigated and the LCSTs of the organic–inorganic amphiphilesdecreased with increasing the percentage of POSS termini. It is noted that theself-assembly behavior of the POSS-capped PNIPAAm in aqueous solutions exerted thesignificant restriction on the macromolecular conformation alteration of PNIPAAm chainswhile the coil-to-globule collapse occurred.2. Hydrogels modified by POSS with RAFT polymerization are of rapidthermo-responsive characteristics confirmed by the deswelling and reswelling tests. Theimproved properties compared with that of pure PNIPAAm gels attribute to the POSSnanodomains formed by the self-assembly of POSS moiety in the PNIPAAm networkswhich were confirmed by the results of DSC and AFM measurements. Themicrophase-separated POSS segments lead to a part of PNIPAAm chains keepinghydrophilic property without phase transformation occurring during the temperatureacross the LCST. The part of PNIPAAm chains without phase transformation occurringacts as the water absorbing and released channels in the swollen hybrid hydrogelspromoting their temperature-responsive rate.3. Preparation, characterization and rapid deswelling and reswelling behavior ofpoly(ethylene oxide)-grafted poly(N-isopropylacrylamide) hydrogels via RAFT.PNIPAAm-g-mPEO networks were prepared via RAFT of N-isopropylacrylamide(NIPAAm) with trithiocarbonate-terminated poly(ethylene oxide) andN,N’-methylenebisacrylamide as the chain transfer agent and the crosslinking agent,respectively. It was found that the PNIPAAm-g-mPEO copolymer networks weremicrophase-separated and that mPEO microdomains were dispersed in the PNIPAAmmatrix. The hydrogel behavior of the PNIPAAm-g-mPEO networks was investigated usingswelling, deswelling and reswelling tests. The PNIPAAm-g-mPEO hydrogels displayedfaster responses to external temperature changes than did the control PNIPAAm hydrogel.4. The hybrid gels of poly(N-isopropylacrylamides) containing polyethylene glycolmonomethyl ether-block-poly (methacryloxypropylheptaphenyl polyhedral oligomericsilsesquioxane) copolymer (mPEO-b-POSS) were prepared by RAFT. Due to the immiscibility of PNIPAAm with mPEO and POSS segments, the cross-linkedPNIPAAm-g-mPEO-b-POSS networks displayed the microphase-separated morphology. Itis found that the hybrid hydrogels significantly exhibited faster volume phase transitionsbased on swelling, deswelling, and reswelling experiments than the conventionalPNIPAAm hydrogels. The improved thermo-responsive properties of hydrogels have beeninterpreted on the basis of the formation of the microphase-separated nanostructures in thehydrogels. The self-organized morphology in the hydrogels was further confirmed bydynamic laser scattering (DLS). The DLS result shows that the linear modelmPEO-b-PNIPAAm-b-POSS copolymers of networks were self-organized into micellestructures, i.e., the mPEO and POSS nanodomains simultaneously existed in the swollenhydronetworks and exerted influence on hydrogel behavior.5. Synthesis and self-assembly behavior of organic–inorganic poly(ethyleneoxide)-b-poly(MA POSS)-b-poly(N-isopropylacrylamide) triblock copolymers.The MA-POSS macromer is used to synthesize mPEO-b-P(MA-POSS)-b-PNIPAAmtriblock copolymers via sequential ATRP. The organic-inorganic, amphiphilic andthermoresponsive ABC triblock copolymers are characterized by means of NMR and GPC.DSC and AFM show that the hybrid ABC triblock copolymers are microphase-separatedin bulk. Cloud point measurements show that the effect of the hydrophiphilic block (i.e.,mPEO) on the LCSTs is more pronounced than the hydrophobic block. Both TEM andDLS show that all the triblock copolymers can be self-organized into micellar aggregatesin aqueous solutions. The sizes of the micellar aggregates can be modulated by changingthe temperature. The temperature-tunable self-assembly behavior is interpreted using acombination of the highly hydrophobicity of P(MA-POSS), the water-solubility of mPEOand the thermore-sponsive property of PNIPAAm in the triblock copolymers.
Keywords/Search Tags:Polyhedral oligomeric silsesquioxane nanocomposites (POSS), temperatureresponsive hydrogels, poly (N-isopropylacrylamide), block copolymer, self-assembly
PDF Full Text Request
Related items