Synthesis of thermo-magneto-responsive poly(n-isopropylacrylamide)-based composite hydrogels for adsorption-desorption of chromium (iii) ions / Chen Jian Jie
Hidrogel komposit yang bersifat perangsang-responsif telahpun menjadi perintis penyelidikan untuk aplikasi mereka dalam penjerapan dan pelepasan ion logam melalui perubahan bentuk. Penyediaan hidrogel komposit yang bersifat termo- dan magnet-responsif memerlukan penglitupan teras nanozarah magnet ferum oksida (MNPs) dengan petala berfungsi secara lapisan-demi-lapisan. Namun begitu, penyalutan berpetala MNPs yang berbilang peringkat menjadi penghalang utama dalam penghasilan hidrogel-hidrogel komposit yang mempunyai kestabilan koloid mantap dan sifat dwi-responsif yang berfungsi dengan baik. Dalam kajian ini, hidrogel-hidrogel komposit bertautsilang MNPs bersalutkan homo-polimer poli (N-isopropilakrilamid) (PNIPAM-MNPs) telah disintesiskan secara mudahnya melalui penglitupan lapisan-demi-lapisan dengan tanpa menggunakan pelopor silana, 3-(trimetoksisilil)propil metakrilat (MPS). Ini didapati bahawa PNIPAM boleh digelkan terus ke atas MNPs berfungsian polivinilpirolidon (PVP) yang telah dilitupi silika (silika-PVP-MNPs) melalui pempolimeran radikal bebas tanpa penggunaan MPS untuk menambahbaikkan kestabilan koloid dan sifat termo-magneto-responsif. Selain itu, hidrogel komposit MNPs bersalutkan ko-polimer poli(N-isopropilacrilamid-ko-asid akrilik) [(PNIPAM-ko-AA)-silika-PVP-MNPs] dihasilkan demi menunjukkan perbezaan dalam mekanisme penjerapan antara kumpulan-kumpulan pengkelat karboksilat (-COO-) terkandung dalam moieti AA dan amid-amid (-CONH) dalam moieti NIPAM. Dalam ujian penjerapan-nyahpenjerapan di bawah manipulasi suhu, nyahpenjerapan Cr3+ menjadi semakin ketara selaras dengan peningkatan suhu daripada 298 K sehingga 323 K bagi PNIPAM-silica-PVP-MNPs. Penjerapan semula Cr3+ berlaku semasa hidrogel-hidrogel komposit tersebut dilindap ke suhu 298 K untuk kepekatan permulaan Cr3+ yang lebih rendah (20 – 80 mg L-1) telah menunjukkan bahawa nyahpenjerapan dapat dikecapi untuk penjerapan permukaan. Sebelum pemanasan, data penjerapan keseimbangan Cr3+ dapat dipadankan dengan baik dalam model Flory-Huggins dan Frumkin, menunjukkan bahawa pengkelatan ion-ion Cr3+ berlaku melalui penggantian molekul-molekul air pada tapak pengikat. Tambahan pula, PNIPAM-silika-PVP-MNPs mempunyai kapasiti penjerapan maksimum, qm (434.78 mg g-1) yang lebih tinggi berbanding dengan (PNIPAM-ko-AA)-silika-PVP-MNPs (qm = 243.90 mg g-1) seperti yang diekstrapolasikan daripada model isoterma Langmuir bahawa kedua-dua hydrogel komposit telah menunjukkan pemadanan yang baik dalam model tersebut. Analisis kinetik penjerapan pula menunjukkan bahawa penjerapan Cr3+ pada PNIPAM-silika-PVP-MNPs tertakluk pada resapan intra-zarah serta penjerapan fizikal berbalik kerana data-data hydrogel itu mengikuti model pseudo-pertama, pseudo-kedua dan resapan intra-zarah. Di samping itu, penjerapan kimia permukaan adalah dominan untuk (PNIPAM-ko-AA)-silika-PVP-MNPs kerana hydrogel ini hanya mengikuti model pseudo-kedua.
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Stimuli-responsive composite hydrogels have been in the vanguard of researches for their application in metal ion adsorption and its release via conformational change. The preparation of composite hydrogels with both thermo- and magneto-responsiveness requires careful layer-by-layer coatings of functional shells onto the core of iron oxide magnetic nanoparticles (MNPs). However, multiple stages of shell encapsulation of MNPs remains a major setback on the production of composite hydrogels with adequate colloidal stability and well-functioned dual-responsiveness. In this study, homo-polymeric poly(N-isopropylacrylamide)-encapsulated magnetite nanoparticles (PNIPAM-MNPs) cross-linked composite hydrogels were facilely synthesized via layer-by-layer coatings with and without employing silanization precursor, 3-(trimethoxysilyl)propyl methacrylate (MPS). It was found that PNIPAM could be gelated directly onto the silica-coated, poly(vinylpyrrolidone) (PVP) functionalized MNPs (silica-PVP-MNPs) via free radical polymerization without MPS to improve its colloidal stability and both thermo-magneto-responsive. Besides, co-polymeric poly(N-isopropylacrylamide-co-acrylic acid)-encapsulated MNPs ((PNIPAM-co-AA)-silica-PVP-MNPs) composite hydrogels were prepared for elucidating the difference in adsorption mechanisms between chelating groups of carboyxlates (-COO-) contained by AA moiety and amides (-CONH) of NIPAM moiety. In the temperature manipulated adsorption-desorption tests, desorption of Cr3+ gradually predominated as temperature increased from 298 K to 323 K for PNIPAMsilica-PVP-MNPs. Re-adsorption of Cr3+ by the composite hydrogel took place as being quenched to 298 K for lower initial Cr3+ concentration (20 – 80 mg L-1) which showed that desorption can be realised for surface adsorption. Before heating, the equilibrium adsorption data of Cr3+ fitted well into Flory-Huggins and Frumkin models, that elucidated the chelation of Cr3+ ions occurred via replacement of water molecules on the binding sites. Moreover, PNIPAM-silica-PVP-MNPs had higher maximum adsorption capacity, qm (434.78 mg g-1) compared to (PNIPAM-co-AA)-silica-PVP-MNPs (qm = 243.90 mg g-1) as extrapolated by Langmuir isotherm model in which the data of both composite hydrogels also showed good fit to the model. The adsorption kinetic analysis indicated that Cr3+ adsorption on PNIPAM-silica-PVP-MNPs was governed by intra-particle diffusion and reversible surface physisorption as its data followed pseudo-first, pseudo-second- and intra-particle diffusion models. On the other hand, surface chemisorption predominated over (PNIPAM-co-AA)-silica-PVP-MNPs as it followed only pseudo-second model.
Synthesis of thermo-magneto-responsive poly(n-isopropylacrylamide)-based composite hydrogels for adsorption-desorption of chromium (iii) ions / Chen Jian Jie
Synthesis of thermo-magneto-responsive poly(n-isopropylacrylamide)-based composite hydrogels for adsorption-desorption of chromium (iii) ions_Chen Jian Jie_K4_2019_MYMY