Sealing Method of Thermoplastics Mediated By Surface Chemical Modification
Authors: Lee Nae Yoon 
Published: February 2020

Thermoplastics are optically transparent, lightweight, mechanically hard, and highly cost-effective material, making them suitable for fabricating integrated physical, chemical, or biological sensors. Thermoplastics are easily moldable mainly via stamping or engraving method. Stamping utilizes hard material having positive structures as a stamp to print patterns repeatedly on a substrate one example of which is imprint molding, while engraving carves the pattern using sharp object one example of which is a computer numerical control (CNC) micromilling. After pattern formation, thermoplastics need to be sealed. Many thermoplastics adopt sealing method known as "thermal bonding" which is realized by applying high pressure and temperature, closer to the glass transition temperature of the thermoplastic used. However, due to high heat and pressure applied, patterns tend to deform or collapse, sacrificing surface morphology. Moreover, thermal bonding is restricted to sealing two homogeneous substrates. For this reason, alternative sealing methods have been reported such as solvent-assisted bonding, adhesive layer bonding, and microwave welding, to name a few, to obtain high fidelity in pattern morphology and realize sealing between heterogeneous substrates.
In this study, the innate properties of thermoplastics were employed to graft chemical functionalities to tune the wettability of original thermoplastics. The core idea is to make surface flexible to become highly wetting after surface oxidation of thermoplastics. In general, thermoplastics are not easily hydroxylated by means of plasma treatment, UV exposure, or UV/ozone treatment. However, it was reported that carbonate or carbonyl backbone can undergo aminolysis, which opens a possibility that chemical functionalities can be grafted onto thermoplastics without having to go through hydroxylation. In specific, addition of amine functionality causes scission of the carbonate or carbonyl backbone by the donation of the lone pair of electrons to the carbonyl group, opening the double bond between carbon and oxygen. This results in the formation of robust urethane linkage between amine-bearing chemicals and a thermoplastic which contains carbonate or carbonyl backbone as its component. Among many choices of chemicals, silane coupling agents were widely adopted. Silane coupling agents are synthetic hybrid inorganic-organic compounds widely used for bridging silicon with non-silicon compounds. In this study, silane coupling agent bearing amine functional group as its organic moiety is used to graft it onto thermoplastics bearing carbonate or carbonyl backbone. After the grafting, the dangling inorganic moiety at the terminal surface of the thermoplastic, in this case alkoxysilane, makes the substrate feasible for surface hydroxylation, which is not readily realized with plasma treatment, UV exposure, or UV/ozone treatment. Two hydroxylated surfaces then form robust siloxane bond (Si–O–Si) at the interface, facilitating thermoplastic sealing. After realizing the sealing, the chemically modified surfaces were further tuned freely into hydrophilic or hydrophobic by subsequent reaction with silane coupling agents bearing versatile organic moieties. In particular, after thermoplastic sealing, microchannel structure engraved on thermoplastic was selectively modified into hydrophobic to realize sequential injection of multiple reagents for solid-phase-based nucleic acid purification, without the need for precise valve control.
Besides realizing sealing between two hard thermoplastics, we extend the concept for sealing thermoplastics with another widely adopted material, poly(dimethylsiloxane) (PDMS). PDMS is a transparent elastomer, widely utilized for the fabrication of microfluidic devices owing to high transparency, simple replica molding, easy surface oxidation, low cost, and low material toxicity. Sealing between hard thermoplastic and soft PDMS poses numerous advantages particularly offering construction of microvalves or micropumps where flexible components become necessary. Besides, these heterogeneous assemblies can provide desirable environment for performing cell-based research because PDMS is gas permeable. It is experimentally proven that heating the contacted substrates after surface oxidation of both PDMS and thermoplastics does not realize sealing. In this study, simple and room temperature processes for realizing sealing between PDMS and thermoplastics were introduced employing variety of silane coupling agents. First, we introduce the formation of amine-epoxy bond employing two silane coupling agents. Later, we simplify the process by adopting only one silane coupling agent in two different ways. One method is mediated by the formation of urethane linkage followed by robust siloxane bond formation, and another method is mediated by instantaneous chemical reaction between nucleophiles and electrophiles followed by siloxane bond formation. Permanent sealing was readily realized at room temperature condition on a large area homogeneously, making these sealing method highly feasible particularly when thermally fragile microstructures or heat-sensitive biomolecules must be integrated onto or incorporated into the PDMS-thermoplastic microdevice prior to sealing.