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Keywords: Natural Sciences
Chemistry & allied sciences
Analytical chemistry
Organic chemistry
Issue Date: 2009
Publisher: Quaid-i-Azam University Islamabad, Pakistan
Abstract: Block copolymers having polyamide with a trichlorogermyl pendant, (-CO- R'-CO–NH-Ar-NH-CO-R'-CO-) x as hard segment and aminopropyl-terminated polydimethylsiloxane H 2 N(CH 2 ) 3 SiO(CH 3 ) 2 (CH 3 ) 2 SiO) y Si(CH 3 ) 2 (CH 2 ) 3 NH 2 ], (PDMS) as the soft segment of general formula [(-CO-R'-CO-HN-Ar-NH) x -CO-R′- CO-NH(CH 2 ) 3 SiO(CH 3 ) 2 (Me 2 SiO) y Si(CH 3 ) 2 (CH 2 ) 3 NH 2 ] n , [n = 4.37 to 1175.0; x = 5 to 8; y = 360] where R′ = CH 2 CH(CH 2 GeCl 3 ); CH 2 CHGeCl 3 CH 2 ; CH 2 CH(GeCl 3 ); CH(CH 3 )CH(GeCl 3 ); CH(CH 3 )CH(GeCl 3 ) and Ar = C 6 H 4 ; (-C 6 H 3 -CH 3 ) 2 ; (-C 6 H 3 - OCH 3 ) 2 ; 2,5-(CH 3 ) 2 -C 6 H 2 and C 6 H 4 -O-C 6 H 4 were prepared by polycondensation reaction. A series of trichlorogermyl-substituted dicarboxylic acids of general formula HOOC-R-COOH where R = CH(CH 2 GeCl 3 )CH 2 - (2), -CH(GeCl 3 )CH 2 - (3), -CH 2 CH(GeCl 3 )CH 2 - (1), - –CH(CH 3 )CH(GeCl 3 )- (4) – CH(GeCl 3 )CH(CH 3 ) (5) were synthesized by the hydrogermylation reaction of unsaturated acids such as itaconic, trans-glutaconic, fumaric, mesaconic and citraconic acid with HGeCl 3 which was produced in situ by the reaction of GeO 2 with 37% HCl in presence of NaH 2 PO 2 .H 2 O. The synthesized trichlorogermyl substituted dicarboxylic acids were characterized by melting point, elemental analysis, FTIR, 1 H NMR and 13 C NMR. X-ray crystal structures of the trichlorogermyl substituted itaconic acids (1) and trans-glutaconic (2) were analyzed to show supramolecular structures in which central Ge atom in each of these structures is four-coordinated with a slightly distorted tetrahedral geometry. The trichlorogermyl substituted dicarboxylic acids were then converted into their respective acid chloride using dry, distilled SOCl 2 and condensed with slightly less than the required stoichiometric amounts of various para-substituted aromatic diamines dissolved in dry THF in the presence of an organic base Et 3 N under strict inert conditions to yield respective chloro terminated polyamide which were copolymerized in situ with aminopropyl-terminated polydimethylsiloxane (PDMS) dissolved in dry THF to obtain various series of seventeen new block copolymers PA 1 to PA 17 These block polymers were structurally characterized by elemental analysis, FT-IR, 1 H NMR, solid state 13 C NMR and molecular weight determination. Their molecular weights as determined by Laser Light Scattering technique were found in range 1.71×10 5 to 331×10 5 g/mol . The thermal stability of these copolymers was xivinvestigated by using various thermal analyses techniques such as TGA and DSC. These block copolymers show two Tgs, at ca -120 o C and the other between 350 o C to 400 o C indicating presence of soft component of PDMS and hard component polyamide with average decomposition temperature at 500 o C. TG-FTIR studies indicate that initial decomposition of these block copolymers starts with the evolution of CO. The pyrolysis of these polymers under inert conditions was done and the GCMS studies of the gases thus evolved revealed the presence of the oligomeric cyclic products of general formula [(CH 3 ) 2 SiO] n where n 3-7 and the polyamide fragments. Scanning electron microscopy (SEM) was used to study the surface morphology. Thermodynamic and kinetic parameters such as Ea, ∆H, ∆S, and n were calculated from Tg curves using Horowitz/Metzger methods. The values of activation energy so obtained fall in the range of 38.30 kJ/mol to 76.12 kJ/mol. Hydrogen bonding and inter-chain linkage give them increased activation energy and high stability. Due to the presence of polar site of polyamide block the copolymers absorb 5-17% moisture when soaked in water, using standard procedures at room temperature. The block copolymers were used for the covalent assembly of Layer by Layer (LbL) multilayers adopting a dipping technique for the deposition onto pre- activated Silicon or quartz substrates. Primarily, by using PDMS with a molar mass of about 2,500 g/mol and about 27,000 g/mol, the conditions (such as concentration of polymers, dipping time, rinsing etc.) for covalent layer-by-layer assembly were optimized to the point of identifying reaction conditions for this surface reaction that led to the construction of multilayers with a linear growth increment with respect to the number of layers chemisorbed and embedding of macromolecules with only 2 functional end-groups. As LbL multilayer assemblies are formed by the alternate deposition of polymers, therefore in the present study poly(ethylene-alt-maleic anhydride (PEMA) was used along with PDMS. Polyethylenimine (PEI) layer was deposited onto the substrate as the precursor layer. Thus the LbL film architecture was PEI(PEMA/PDMS)n where n = number of layer pairs deposited. The thickness of each layer pair was measured using an ellipsometer, while AFM, SAXR and UV Spectroscopy were also employed for the characterization of LbL nanofabricated multilayers. Aminopropyl terminated PDMS having only two functional groups have been successfully adsorbed onto PEMA utilizing the LbL technique resulting into multilayer buildup.
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