Adhesion Engineering in Polymer-Metal Comolded Joints with Biomimetic Polydopamine.

Joints that connect thermoplastic polymer matrices (TPMs) and metals, which are obtained by comolding, are of growing importance in numerous applications. The overall performance of these constructs is strongly impacted by the TPM-metal interfacial strength, which can be tuned by tailoring the surface chemistry of the metal prior to the comolding process. In the present work, a model TPM-metal system consisting of poly(methyl methacrylate) (PMMA) and titanium is used to prepare comolded joints. The interfacial adhesion is quantified by wire pullout experiments. Pullout tests prior to and following surface modification are performed and analyzed. Unmodified wires show poor interfacial strength, with a work of adhesion (Ga) value of 3.8 J m-2. To enhance interfacial adhesion, a biomimetic polydopamine (PDA) layer is first deposited on titanium followed by a second layer of a poly(methyl methacrylate-co-methacrylic acid) (P(MMA-co-MAA)) copolymer prior to comolding. During processing, the MAA moieties of the copolymer thermally react with PDA, forming amide bonds, while MMA promotes the formation of secondary bonds and molecular interdigitation with the PMMA matrix. Control testing reveals that neither PDA nor the copolymer provides a substantial increase in adhesion. However, when used in combination, a significant increase in adhesion is detected. This observation indicates a pronounced synergistic effect between the two layers that strengthens the PMMA-titanium bonding. Enhanced adhesion is optimized by tuning the MMA-to-MAA ratio of the copolymer, which shows a maximum at a 24% MAA content and a greatly increased Ga value of 155 J m-2; this value corresponds to a 40-fold increase. Further growth in the Ga values at higher MAA contents is hindered by the thermal cross-linking of MAA; MAA contents above 24% restrict the formation of secondary bonds and molecular interdigitation with the PMMA chains. Our results provide new design principles to produce thermoplastic-metal comolded joints with strong interfaces.

Atomic force microscopy based thermal lithography of poly(tert-butyl acrylate) block copolymer films for bioconjugation.

In this paper, we report on the local thermal activation of thin polymer films for area-selective surface chemical modification on micrometer and nanometer length scales. The thermally induced activation of tert-butyl ester moieties in polystyrene- block-poly(tert-butyl acrylate) (PS- b-PtBA) block copolymer films leads to the formation of pending carboxylic acid groups, which are among the versatile functionalities for subsequent bioconjugation. From Fourier transform infrared (FTIR) spectroscopic analyses, the apparent activation energy (Ea) for the tert-butyl ester deprotection in thin films was calculated to be 93 +/- 12 kJ/mol, which is in good agreement with values reported for the bulk. The availability of the deprotected carboxylic acid groups in subsequent wet chemical grafting reactions on neat thermolyzed films was confirmed by covalently immobilizing fluoresceinamine and amino end-functionalized poly(ethylene glycol) (PEG-NH2) using established 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC)/N-hydroxysuccinimide (NHS) chemistry. Local thermal deprotection on micrometer and sub-micrometer length scales was achieved by scanning thermal microscopy using an atomic force microscope with heatable probe tips. Passivating PEG and fluoresceinamine layers were selectively covalently coupled to locally deprotected areas as small as 370 nm x 580 nm.