Molecular Design And Synthesis Of Amphiphilic Copolymers And Their Use For Surface Modification Of The Hydrophobic Polymer Membranes

PVC,PVDF,and PSF have been widely used to fabricate polymeric Microfiltration and Ultrafiltration membranes for decades.However,their hydrophobic character makes them highly susceptible to fouling.A second problem limiting the applications of PVC,PVDF,and PSF membranes is their poor flux,selectivity,and functions.An obvious approach to solving the problems of fouling and selectivity is the surface modification of these hydrophobic membranes to impart engineered surface chemistries.Compared to the conventional surface modification methods such as coating and grafting,blending is a facile and efficient method to prepared hydrophilic membranes.Over the past decades,many studies have been devoted toward the development of methods to modify the surfaces of polymeric membranes through the surface segregation of amphiphilic polymer additives.When an amphiphilic copolymer is blended with the hydrophobic membrane materials and prepared a blend membrane via immersion-precipitation process,the amphiphilic copolymer can preferentially segregated to the surfaces of blend membranes,and the location of amphiphilic copolymer at membrane surface results in the formation of a hydrophilic "brush" of the hydrophilic chains,while the hydrophobic segments intermixes with the membrane base component,serving as an anchor for the additives.In this method, the performances of blend membranes are depend to a large extent on the compatibility between the membrane base component and the hydrophobic segments of the amphiphilic copolymers,thus,the molecular design and synthesis of the amphiphilic copolymer additives is very important.With this in mind,in this thesis, we designed and synthesized a series of amphiphilic copolymers with various chemical composition and molecular structures,the using those copolymers as additives to prepared hydrophilic and anti-fouling PVC,PVDF,and PSF blend membranes were also studied.An amphiphilic graft copolymer which having PVC as the hydrophobic chains and P(PEGMA) as the hydrophilic chains,i.e.PVC-g-P(PEGMA),have been synthesized via ATRP.The molecular weight of PVC-g-P(PEGMA) which calculated based on ¹H-NMR was 5.5×10⁵ g/mol,the weight fraction of P(PEGMA) was 82.6wt%.PVC-g-P(PEGMA) was used as an additives to modified PVC membranes, the investigation results demonstrated that PVC-g-P(PEGMA) were mainly located on the membrane surfaces and impart the blend membranes with excellent hydrophilicity and anti-fouling properties.For instance,when the weight fraction of PVC-g-P(PEGMA) in blend membrane was 15wt%,the initial water contact angle of the blend membrane was 52.2°,and the water contact angle decreased to 0°within 15s, the pure water flux of 15wt%PVC-g-P(PEGMA)/PVC membrane was 584.4 L/m²Â·h, the BSA rejection ratio was higher than 92%,the water flux recovery was 84.7%.An ABA type amphiphilic copolymer with "comb-block-comb" structure,i.e. P(PEGMA)-PMMA-P(PEGMA),was synthesized via two steps RAFT.On the other hand,a amphiphilic graft copolymer which containing PVDF as hydrophobic backbone and having an sugar-carrying polymer as the hydrophilic side chains,i.e. PVDF-g-PMAG,was synthesized via ATRP using PVDF as macroinitiator.The GPC molecular weights of P(PEGMA)-PMMA-P(PEGMA) and PVDF-g-PMAG was 1.9×104 g/mol and 3.12×105 g/mol,respectively.When small amounts(less than 5wt%) of P(PEGMA)-PMMA-P(PEGMA) or PVDF-g-PMAG was blend with PVDF to prepare PVDF blend membranes,the water contact angle of PVDF membranes decreased from 110°to 50～56°.Beside this,the protein-resistance properties of PVDF blend membranes were also enhanced greatly.Using PSF-CH₂Cl as macroinitiator,an amphiphilic copolymer which having PSF as the hydrophobic backbone and with P(PEGMA) as the hydrophilic side chains, i.e.PSF-g-P(PEGMA),was synthesized via ATRP.The molecular weight of PSF-g-P(PEGMA) was calculated as 6.34×10⁴ g/mol based on IH-NMR results,the molar fraction and weight fraction of P(PEGMA) in the copolymer was 71.1%and 39.6 wt%,respectively.PSF-g-P(PEGMA) can be swollen but were not soluble in H20.A series of blend membranes containing various amounts of PSF-g-P(PEGMA) were prepared via phase inversion process.The investigation results showed that the introduction of PSF-g-P(PEGMA) can obviously improved the hydrophilicity and anti-fouling property of PSF blend.For example,the initial water contact angle of 20wt%PSF-g-P(PEGMA)/PSF membrane was 43.5°,after 50s,the blend membrane was completely wetted by water.The apparent BSA adsorption amount of the 20wt%PSF-g-P(PEGMA)/PSF membrane was extreme low compared to that of the pure PSF membrane.Two kind of ABA type amphiphilic copolymer having PSF segment as the hydrophobic composition,i.e.P(PEGMA)-PSF-P(PEGMA) and PMAG-PSF-PMAG, respectively,were synthesized via ATRP,using a difunctional PSF as macroinitiator. The molecular weights and molecular weights distributions of these copolymers were determined by GPC.P(PEGMA)-PSF-P(PEGMA) and PMAG-PSF-PMAG were not soluble in water but they can be swollen by water.The porous membranes prepared from the blends of P(PEGMA)-PSF-P(PEGMA) or PMAG-PSF-PMAG with PSF exhibited higher porosity and average pore diameters.Moreover,when the contents of the amphiphilic copolymer in the blend membranes was 10wt%,the apparent BSA adsorption of blend membrane were neared to zero.PSF-g-PtBA and PSF-g-PNIPAAm were synthesized by ATRP,using PSF-CHECI as macroinitiator.After acidolysis treatment,the tert-butyl groups on PSF-g-PtBA were completely removed,thus an amphiphilic copolymer,i.e.PSF-g-PAA,was obtained.The molecular weights of them were determined by GPC(5.04×10⁴ g/mol for PSF-g-PAA and 5.58×10⁴ g/mol for PSF-g-PNIPAAm,respectively).Three kinds of blend membranes were prepared from the blends of PSF-g-PAA or/and PSF-g-PNIPAAm,i.e.20wt%PSF-g-PAA/PSF(A),20wt%PSF-g-PNIPAAm/PSF(B), and 10wt%PSF-g-PAA/10 wt%PSF-g-PNIPAAm/PSF(C),respectively.Water contact angle measurement result demonstrated that A,B,and C membrane can be wetted by water within 35s,90s,and 120s,respectively.Moreover,the water flux of A was pH-dependent,the water flux of B was temperature-dependent,and the water flux of C was pH and temperature dual- dependent.