PhD Thesis
Polymeric Nanoparticles for the Modification of Polyurethane Coatings
Sandrine Tea (11/2006-06/2011)
Support: Axel H. E. Müller
Rubber-based nanomodifiers were successfully synthesized following two different strategies and were used as impact modifiers in polyurethane (PU) automotive clearcoats to improve chip resistance.
Various narrowly distributed polybutadiene-b-poly(methyl methacrylate) (B-M) block copolymers differing in composition and molecular weights were synthesized and studied with respect to their self-assembly in organic selective solvents. Dynamic light scattering and transmission electron microscopy measurements revealed that spherical micelles were obtained in acetonitrile for all block copolymers, independently of the polymer concentration. Their radii varied from 11 to 69 nm depending on the molecular weight of the initial linear block copolymer and their aggregation behavior in acetonitrile followed the model established by Förster and Antonietti for strongly segregated block copolymers. In DMF and acetone, block copolymers with 85 %wt PMMA were dissolved as unimers. For lower methacrylate contents, the sizes of the obtained spherical micelles were decreasing from DMF to acetone independently of the polymer concentration. The calculated interaction parameters confirmed acetonitrile as the best solvent for PMMA followed by DMF and acetone as the poorest one. The size of the spherical aggregates could be tuned by the molecular weight and/or by the nature of the selective solvent. Polybutadiene-b-poly(n-butly acrylate) (B-nBA), polybutadiene-b-poly(n-butyl methacrylate) (B-nBMA) and polybutadiene-b-poly(t-butyl methacrylate) (B-tBMA) did not show such a large choice in selective solvents and spherical micelles were obtained in DMF, DMAc and acetone respectively.
Cross-linking of the polybutadiene core of the obtained micelles was performed in solution using two different methods: cold vulcanization and radical reaction upon the decomposition of a photo-initiator under UV radiation. Both methods allow retaining the spherical shape of the micelles leading to narrowly distributed non fusible nanospheres. In the case of B-M nanoparticles, the degree of cross-linking seemed independent of the amount of cross-linker used. Unlikely, B-nBMA and B-nBA nanoparticles exhibited increasing degrees of cross-linking with the amount of photo-initiator introduced. Their degrees of cross-linking were particularly lower than those of B-M nanoparticles.
The hydrolysis of the t-BMA corona of the nanoparticles obtained from B-tBMA linear block copolymers self-assembly in selective solvent resulted in water soluble nanoparticles carrying acid functions and thus potentially exhibiting pH-responsive behavior.
Various hyperstars consisting of a hyperbranched PB core and (meth)acrylate arms were synthesized by anionic self-condensing vinyl copolymerization (SCVCP) of divinylbenzene and butadiene followed by the anionic polymerization of the linear (meth)acrylate arms. The amount of hyperbranched products resulting from SCVCP could be enhanced by introducing additional DVB to the reaction while polymerizing. The topology of the hyperbranched PB cores was confirmed by viscosity measurements. All Mark-Houwink-Sakurada exponents were significantly below the value for linear PB. The initiation of (meth)acrylate arms was confirmed by NMR spectroscopy. Upon the arm-growth reaction, the branched topology was retained as witnessed by further viscosity measurements.
The introduction of cross-linked nanoparticles based on linear block copolymers did not disturb the transparency of PU coatings. Even after curing reaction, the nanoparticles were well-dispersed into the coating. TEM observations confirmed this last result where neither aggregation nor flocculation of the cross-linked nanoparticles was observed.
Hyperstar polymers were found to undergo self-assembly upon the curing reaction leading to onion-like structured aggregates, in the case of PMMA hyperstars, with sizes as large as 200 nm. Aggregates of the same size order were observed for the other hyperstars but no defined structures were found. For all hyperstar modified coatings, the transparency of the films was altered.
In both cases, cross-linked nanoparticles and hyperstar modified coatings, improvements of chip resistance were observed. The improvements were even better with increasing amount of cross-linked nanoparticles but no effect was noticed on the hardness of the coatings. Similar trends were observed for the hyperstar modified coatings.