Additive Growth and Crystallization of Polymer Films

Abstract

We demonstrated a polymeric thin film fabrication process in which molecular-scale crystallization proceeds with additive film growth, by employing an innovative vapor-assisted deposition process termed matrix-assisted pulsed laser evaporation (MAPLE). In comparison to solution-casting commonly adopted for the deposition of polymer thin films, this physical vapor deposition (PVD) methodology can prolong the time scale of film formation and allow for the manipulation of temperature during deposition. For the deposition of molecular and atomic systems, such a PVD manner has been demonstrated to facilitate molecular ordering and delicately manipulate crystalline morphology during film growth. Here, using MAPLE, we deposited thin films of a model polymer, poly­(ethylene oxide) (PEO), atop a temperature-controlled substrate with an average growth rate of less than 10 nm/h. The mechanism of deposition is sequential addition of nanoscale liquid droplets. We discovered that the deposition process leads to the formation of two-dimensional (2D) PEO crystals, composed of monolamellar crystals laterally grown from larger nucleus droplets. The 2D crystalline coverage and crystal thickness of the films can be manipulated with two processing parameters, deposition time, and temperature.

Exploiting physical vapor deposition for morphological control in semi‐crystalline polymer films

“…Developed nearly two decades ago, MAPLE is a PVD technique that can effectively minimize polymer damage during deposition . While it utilizes a laser sources similar to PLD, such as UV excimer lasers, IR lasers, and FEL lasers, MAPLE uniquely adapts a sacrificial solvent in the target composition to absorb the majority of the laser energy. Such design preserves the target polymers and produces films with identical chemical structure and molecular weight to the starting material .…”

“…Film formation proceeds by the addition of polymer molecules, as the volatile solvent gets pumped away . The polymer‐friendly technique has been applied to crystalline polymers such as PEO and PE for systematic crystallization studies . Furthermore, it allows for fabricating electronic and photonic devices with polymers .…”

“…Our investigation on crystallization of polymers thin films by MAPLE has been focused on the governing rules of crystallization kinetics occurring during extremely slow film growth . While MAPLE achieves concurrent film growth and crystallization similar to other PVD technologies, it is distinguished in that (1) the polymer is ejected from the target in the form of a polymer‐matrix clusters rather than a gas phase, and (2) the chemical nature and molecular weight of the target polymer can be preserved.…”

“…During laser ablation of a target matrix, polymer chains trapped in the matrix are entrained together in the ablation plume . Subsequently, the polymers deposit as confined droplets atop a substrate, as schematically shown in Figure A . The diameter of the droplets typically ranges from tens of nanometers to a few micrometers.…”

“…By short time MAPLE deposition with slow film growth rates, we could examine the early stage film morphology composed of confined polymer droplets. Figure B shows an atomic force microscopy (AFM) height image of the morphology of a PEO ( M n = 4600 g/mol, M w / M n = 1.1) film deposited onto a silicon (Si) substrate by MAPLE (MAPLE‐PEO) . The substrate temperature during deposition ( T dep ) was 25°C, which is much lower than the melting temperature of the target PEO ( T m ~ 60°C).…”