Chemical Vapor Jet Deposition of Parylene Polymer Films in Air

Parylene films are commonly used as transparent, flexible coatings in electronic devices and biomedical applications, exhibiting barrier properties against corrosion, low dielectric constant, and moisture resistance. Reactive vapor deposition of parylene results in conformal coverage of features at room temperature, which is advantageous for passivating, for example, organic optoelectronic devices. Conventional parylene deposition methods, however, coat surfaces virtually indiscriminately and utilize separate chambers for vaporization, pyrolysis, and polymerization, resulting in a large footprint and limited processing integration ability, especially at a laboratory scale. Here, we demonstrate the vaporization and pyrolysis of the di-<i>p</i>-xylylene (parylene dimer) in a single compact nozzle, producing a jet of monomer that polymerizes into a film upon contact with the substrate at room temperature. A guard flow jet is employed to shield the reactive monomer molecules <i>en route</i> to the substrate, thereby enabling polymer deposition and patterning in ambient atmosphere. We present an analytical model predicting film growth rate as a function of process parameters (e.g., gas flow rate and source, pyrolysis & substrate temperatures). The effect of jet flow dynamics on film morphology is also discussed. A 100% increase in the lifetime of air-sensitive OLEDs is demonstrated upon encapsulation of the devices with parylene-N film deposited by this technique. Potential advantages of this approach include increased material utilization efficiency, localized conformal coating capabilities, and an apparatus that is compact, inexpensive, and does not require vacuum

Thermal Conductance in Cross-linked Polymers: Effects of Non-Bonding Interactions

Weak interchain interactions have been considered to be a bottleneck for heat transfer in polymers, while covalent bonds are believed to give a high thermal conductivity to polymer chains. For this reason, cross-linkers have been explored as a means to enhance polymer thermal conductivity; however, results have been inconsistent. Some studies show an enhancement in the thermal conductivity for polymers upon cross-linking, while others show the opposite trend. In this work we study the mechanisms of heat transfer in cross-linked polymers in order to understand the reasons for these discrepancies, in particular examining the relative contributions of covalent (referred to here as “bonding”) and nonbonding (e.g., van der Waals and electrostatic) interactions. Our results indicate cross-linkers enhance thermal conductivity primarily when they are short in length and thereby bring polymer chains closer to each other, leading to increased interchain heat transfer by enhanced nonbonding interactions between the chains (nonbonding interactions being highly dependent on interchain distance). This suggests that enhanced nonbonding interactions, rather than thermal pathways through cross-linker covalent bonds, are the primary transport mechanism by which thermal conductivity is increased. We further illustrate this by showing that energy from THz acoustic waves travels significantly faster in polymers when nonbonding interactions are included versus when only covalent interactions are present. These results help to explain prior studies that measure differing trends in thermal conductivity for polymers upon cross-linking with various species