Tuning Morphology and Melting Temperature in Polyethylene Films by MAPLE

Abstract

The control of structure and thermal stability in semicrystalline polymer films remains an important challenge in applications ranging from solar energy devices to packaging films. Here, we demonstrate the ability to dramatically alter the morphology and melting temperature (T m ) of low-molecularweight linear polyethylene (PE) by employing an innovative vapor-assisted deposition process termed matrix assisted pulsed laser evaporation (MAPLE). We report the ability to tune T m of PE films by 20 °C by simply adjusting the deposition temperature during MAPLE processing. This unique capability stems from the ability of MAPLE to exploit confined crystallization during thin film growth. In addition, we demonstrate the ability to exploit MAPLE to design PE films that exhibit the same T m as their melt-crystallized analogues but have an ∼25% higher degree of crystallinity. Our investigation offers new insights into how confinement effects in polymer crystallization can be utilized in the emerging field of polymer film fabrication by MAPLE to control structure and key material properties of semicrystalline polymer films.

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

“…Jeong and co‐authors also discovered increasing crystallinity of vapor deposited PE crystals with increasing temperatures in the range from −2°C to 100°C . Such trend has been related to higher chain mobility enabled at elevated substrate temperatures for chain disentanglement and reorganization …”

“…By studying the x‐ray diffraction (XRD) pattern, they reported the crystalline fraction scaled with substrate temperature from 200°C to 350°C, while the film remained amorphous below 200°C . Jeong and co‐authors also discovered increasing crystallinity of vapor deposited PE crystals with increasing temperatures in the range from −2°C to 100°C . Such trend has been related to higher chain mobility enabled at elevated substrate temperatures for chain disentanglement and reorganization …”

“…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.…”

“…Compared to the more commonly used solution processing techniques, that is, spin casting and solution casting, substrate temperature and deposition rate can be precisely manipulated during PVD to drive the semi‐crystalline morphology of thin films . It allows for simultaneous film growth and crystallization, as recently advocated . Furthermore, PVD approaches are amenable for co‐depositing polymers that are insoluble in a common solvent or designing multilayered films composing chemically similar or dissimilar polymers …”