Functionalization of silica synthesized by sol-gel process with PDLLA via "grafting to" method

Due to its properties of biodegradability and biocompatibility, polylactide has been first designed for the medical field, in particular for tissue reconstruction by tissue engineering. However, for this kind of application, its mechanical properties are too low. The addition of silica has been proposed to reinforce the mechanical strength of this biopolymer. The main challenging aspect to realize this nanocomposite is to achieve a good dispersion of nanofillers in the polymer matrix. An effective method to improve compatibility between inorganic particles and polymer matrix relies upon the preparation of PLA grafted silica nanoparticles. To form a network structure with PLA matrix, molecular weight of these grafted chains has to be superior to the critical entanglement molecular weight (CEMW) of 8.5 kDa [1].
This work is dedicated to the functionalization of sol-gel silica by the “grafted to” method. A recurrent problem with this method is the great dependence of the grafting density on the molecular weight of the PDLLA chains. When weight is above the CEMW, grafting densities greater than 0.001 chains/nm2 of silica are difficult to achieve [1]. To increase grafting density of PDLLA chains, these chains were firstly synthesized via ring opening polymerization with Sn(oct)2 as catalyst and with a organosilane (alkoxyde XSi(OR)3), the (3-Aminopropyl)triethoxysilane (APTES) as initiator, instead of a classic protic compound. This process led to the formation of PDLLA capped with three terminal functions Si-O-CH2-CH3. The resulting polyester was then dissolved in tetrahydrofuran under inert atmosphere to prevent hydrolysis of Si-OR functions. After dissolution, silica was synthesized by sol-gel reaction by mixing prehydrolysed tetramethyl orthosilicate (TMOS), dissolved polymer, water and APTES followed by three days of gelation at 65°C. Finally, these nanoparticles were washed several time with methylene chloride and cycle of centrifugation to remove unbound PDLLA and APTES. GPC and 1H NMR have confirmed the targeted molecular weight of PDLLA of 11 kDa with a monomer conversion closed to 100%. FTIR measurement and solid 29Si NMR have confirmed the covalent attachment of PDLLA chains on silica surface. BET analysis, performed after calcination of silica at 400°C, gave a specific surface of 350 m2/g. Finally, TGA allowed us to calculate a grafting yield of 25 % and a grafting density of 0.035 chains/nm2 in a reproducible and manner.
As a conclusion, our chemical strategy has been demonstrated successful to increase significantly the grafting density of polyester chains on silica according to a reproducible methodology easier to apply compared to former publications [1][2].