R E V I EW

Exploiting physical vapor deposition for morphological controlin semi-crystalline polymer films

Yucheng Wang1 | Hyuncheol Jeong1 | Mithun Chowdhury1 | Craig B. Arnold2,3 |

Rodney D. Priestley1,3

1Department of Chemical and Biological

Engineering, Princeton University, Princeton,

New Jersey

2Department of Mechanical and Aerospace

Engineering, Princeton University, Princeton,

New Jersey

3Princeton Institute for the Science and

Technology of Materials, Princeton University,

Princeton, New Jersey

Correspondence

Rodney D. Priestley, Princeton Institute for the

Science and Technology of Materials,

Princeton University, Princeton, NJ 08544.

Email: rpriestl@princeton.edu

Funding information

Kwanjeong Educational Foundation in South

Korea; Division of Materials Research (DMR),

Materials Research Science and Engineering

Center, Grant/Award Number: 1420541;

National Science Foundation (NSF)

Research in semi-crystalline polymer thin films has seen significant growth due to their fascinat-

ing thermal, mechanical, and electronic properties. In all applications, acquiring precise control

over the film morphology atop various substrates or in the free-standing film geometry is key to

advancing product performance. This article reviews the crystallization of polymer thin films

processed via physical vapor deposition (PVD). Classical PVD techniques are briefly reviewed,

highlighting their working principles as well as successes and challenges to achieving morpho-

logical control of polymer films. Subsequently, the recent development of a unique PVD tech-

nique termed Matrix Assisted Pulsed Laser Evaporation (MAPLE) is highlighted. The non-

destructive technology overcomes the major drawback of polymer degradation by classical

PVD. Recent advances highlighting how MAPLE can be exploited to control polymer film mor-

phology in ways not achievable by other methods are presented. Challenges and future scope

of PVD for polymer film deposition concludes the review.

KEYWORDS

confinement, interfacial interactions, matrix assisted pulsed laser evaporation (MAPLE),

physical vapor deposition (PVD), polymer crystallization, thin films

1 | INTRODUCTION

Polymer crystallization, especially in thin film geometry, is an impor-

tant topic attracting intensive research efforts to unveil its complex

nature. The studies are of tremendous importance for functional

polymers, which enable a broad range of applications such as

solar cells,[1–4] transistors,[5–8] non-volatile memories,[9,10] and

sensors.[11–13] Among all the applications, film morphology, arising

from the complex semi-crystalline nature of polymers, critically deter-

mines performance.[8,14–17] For example, it is generally accepted that

a higher extent of crystallinity in the active layer of semiconducting

polymer thin films can enhance device efficiency[8,14,15,18] and a lon-

ger lateral dimension of defect-free crystalline lamellae can reduce gas

permeability through polymer thin films.[11] Despite its significance,

ways to drive polymer thin film crystallization into a desired morphol-

ogy has not been fully understood or mastered, especially since the

process is known to deviate from bulk crystallization. During the

process of melt-crystallization, the mutually entangled polymer chains

are driven by both thermodynamic and kinetic factors into different

metastable states comprising both crystalline and amorphous

regions.[19] It is strongly influenced by crystallization temperature,[20]

molar mass,[21] and additives in the bulk composition.[22] In contrast,

in thin films, additional processing parameters such as geometric

confinement,[23–25] polymer/substrate interactions,[26–28] and sample

history of non-equilibrated films[29] can further influence crystalliza-

tion, thus impacting the resulting film morphology. Furthermore, the

method in which a thin film is processed, including solution proces-

sing, physical or chemical vapor deposition (CVD), and melt-

crystallization can profoundly impact the film morphology. In this

review article, we highlight recent advances in controlling the film

morphology of semi-crystalline polymers produced by physical vapor

deposition (PVD). We focus on elucidating general processing-struc-

ture-property relationships of films formed by this approach. Com-

pared to the more commonly used solution processing techniques,

that is, spin casting and solution casting, substrate temperature andYucheng Wang and Hyuncheol Jeong contributed equally to this study.

Received: 30 April 2018 Accepted: 12 July 2018

DOI: 10.1002/pcr2.10021

Polymer Crystallization. 2018;e10021. wileyonlinelibrary.com/journal/pcr2 © 2018 Wiley Periodicals, Inc. 1 of 13https://doi.org/10.1002/pcr2.10021

deposition rate can be precisely manipulated during PVD to drive the

semi-crystalline morphology of thin films.[30–33] It allows for simulta-

neous film growth and crystallization, as recently advocated.[34] Fur-

thermore, 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.[35,36]

The critical roles of geometric confinement, polymer/substrate

interactions, and crystallization temperature have been reported for

polymer thin film crystallization.[30,37,38] To set the context for the

continued need of advancements in top-down PVD, Box 1 provides a

brief summary of selected studies exploiting solution-based proces-

sing methods to manipulate morphology in semi-crystalline polymer

films. Challenges still remain to fully understand and control the crys-

tallization mechanism of polymer thin films. For example, many stud-

ies have discovered that the orientation of crystalline lamellae atop

attractive substrates would preferentially transition to flat-on as film

thickness was decreased.[39–41] Researchers have proposed different

models to explain such experimental observations.[41,42] Nevertheless,

understanding how parameters such as polymer/substrate interfacial

energy affect the crystallization process, for example, crystal orienta-

tion, still remains a subject of intense debate.[38] Part of the reason

arise from the difficulty to tune processing parameters in a systematic

manner during the fabrication of polymer thin films. In other words, to

better understand how processing affect morphology, it would be

useful to exploit a film fabrication technique that has the ability to

precisely tune the crystallization conditions at a slow film growth rate.

For instance, one potential approach is to characterize the nanoscale

crystalline morphology of thin films in a layer-by-layer manner, which

would be accessible as a result of top-down film deposition. Such a

distinctive approach can be challenging for most processing

techniques.

It is, therefore, critical to explore various approaches to proces-

sing semi-crystalline polymer thin films. To date, the most fre-

quently reported film production techniques are solution processes

such as drop casting, spin casting,[43] and Langmuir-Blodgett deposi-

tion.[44] In contrast, dry processes such as vapor deposition, includ-

ing PVD[30] and CVD,[45,46] can provide new perspectives by

altering the processing route to which a thin film is achieved. With-

out incorporating chemical reactions, PVD is a viable way of explor-

ing the processing-structure-property relationships of crystalline

thin films by precisely tuning growth rate, crystallization tempera-

ture, and interfacial interactions at distinct stages of film formation.

The major drawback of PVD that limits the ability to undertake sys-

tematic studies of polymer thin film has been the prospects of

polymer degradation during processing due to the high energy

input into the target material.[47] Despite this challenge, there are

many examples in which PVD has been successfully applied to con-

trol the semi-crystalline morphology of polymer films. Furthermore,

a recently developed PVD technique termed matrix assisted pulsed

laser evaporation (MAPLE) has offered a substantial opportunity to

bring deeper insights into the field by diminishing material

degradation.[31]

The scope of the present article will be specific to the crystalliza-

tion of polymer thin films via PVD. We start with a description of clas-

sical PVD techniques, including a discussion on the uniqueness and

challenges for each technique in the context of polymer thin film fab-

rication. Next, we summarize previous studies related to vapor depos-

ited semi-crystalline films, highlighting how PVD was applied to

manipulate film morphology by varying key parameters such as sub-

strate temperature and substrate type. We then highlight recent

advances in the field by the utilization of MAPLE, with particular

emphasis on work from our group demonstrating, for instance, the

ability to control crystal lamellar thickness and extent of crystallinity.

Moreover, it has immense potential in the application of polymeric

semiconducting devices.[48–51] Future opportunities are discussed at

the end of the review.

2 | PVD TECHNIQUES

In this section, we briefly discuss several classical PVD techniques:

sputtering, thermal evaporation, and pulsed laser deposition (PLD).

We describe the mechanism of film formation and the uniqueness/

challenges of each process. Finally, we briefly highlight how each

approach has been exploited to deposit semi-crystalline polymer thin

films.

2.1 | Sputtering

A typical polymer sputtering process requires a vacuum chamber

coupled to a radio frequency generator.[56,57] A target polymer and a

substrate are placed on the cathode and the anode, respectively. In

most cases, deposition is initiated by the creation of energetic parti-

cles in a chemically inert gas via an ionization process. These particles

subsequently bombard the solid target with high kinetic energy,

thereby ejecting molecules from the target which lead to film forma-

tion atop the adjacent substrate.[57,58] Sputtering is, therefore, a dry

process that circumvents the use of solvent. It also supports the fine

control of deposition rate.[59] Nonetheless, the transfer of kinetic

energy from the particles to the target molecules can result in chemi-

cal degradation by random polymer chain cleavage.[60,61] This imposes

a big constraint to morphological control and characterization of poly-

mer thin film properties.[62] So far, the employment of sputtering has

frequently been used to acquire thin film coatings of fluoropolymers,

such as poly(tetrafluoroethylene) (PTFE), for enhancing water-repel-

lent[63,64] or gas separation properties[65] where polymer degradation

can be tolerated.

2.2 | Thermal evaporation

Thermal evaporation deviates from sputtering in that the energy input

into the target polymer is in the form of heat. The thermal energy

evaporates the molecules in vacuum. Subsequently, film formation

proceeds by the transfer of evaporated molecules that condense atop

the substrate. In a typical thermal evaporation process, the substrate

temperature is held below the polymer melting temperature to control

the degree of supercooling during deposition. Analogous to sputter-

ing, thermal evaporation is a dry process so that the dissolution of

polymer is not required. With the control of film thickness spanning

from a monolayer to several microns by varying deposition time, this

2 of 13 WANG ET AL.

technique has been applied to deposit crystalline polymers such as

polyethylene (PE),[66] poly (vinylidene fluoride) (PVDF),[30] and

π-conjugated polymers.[67–69] The flexibility of substrate type has

enabled the decoration of thin films atop structured substrates such

as graphite, polymers, and carbon nanotube (CNT).[70,71] These advan-

tages render thermal evaporation as one of the most widely used pro-

cessing methods for making small molecular weight thin films,

including organic molecules that were found to exhibit exceptional

properties.[72,73] However, material degradation is commonly reported

when polymers are used as the target due to the required high energy

input.[30,66,74,75] The scission of polymer chains may occur by random

degradation or a depolymerization mechanism, producing low-

molecular weight fragments that are undesirable for producing a uni-

form film structure.[47] Despite these challenges, new literature is

emerging in which in situ polymerization is incorporated into thermal

evaporation processes to make polymer films[76,77] or the creation of

low-energy polymer glasses.[78]

2.3 | Pulsed laser deposition

Pulsed laser deposition is a dry process that utilizes laser pulses at suf-

ficiently high energy density to ablate a solid polymer target. Upon

absorption of photons, macromolecules at the target surface rapidly

vaporize, creating a plume composed of polymer that is ejected

toward the substrate.[79,80] The most frequently used laser sources

include ultraviolet (UV),[81] infrared (IR),[82] F2,[83] and CO2.

[84] By

sequentially ablating chemically similar materials, PLD can be readily

implemented to design multilayered film structures that are challeng-

ing for solution processing techniques.[35] Unfortunately, material deg-

radation by thermal dissociation and photochemical reactions are

commonly observed with direct laser ablation,[47,85] making PLD

unsuitable for applications that require preservation of bulk chemis-

try.[86] Consequently, polymers should be kept at low molecular

weight for PLD.[31,87] In order to minimize the destruction of poly-

mers, careful selection of the laser source and the corresponding

wavelength should also be considered. For instance, when an ArF

laser (λ = 193 nm) was used, the deposition of low-molecular weight

(Mw = 1400) polyethylene glycol (PEG) revealed structure modifica-

tion, as identified by Fourier transform infrared spectroscopy.[86]

However, no clear observation of chemical modification was found

when the authors switched to a free-electron laser (FEL) with λ = 2.9

or 3.4 μm, wavelengths that are in resonance with specific bond vibra-

tions of PEG.[88,89]

BOX 1

CONFINED CYSTALLIZATION OF SOLUTION PROCESSED

POLYMER FILMS

Polymer thin film crystallization atop substrates can be markedly

different from the bulk process. In thin films, geometric confine-

ment and polymer/substrate interactions can play critical roles.

This Box briefly highlights the basic concepts of how confinement/

interfaces can influence polymer thin film crystallization. Confine-

ment can be utilized to suppress heterogeneous nucleation of poly-

mer crystals to allow for the attainment of homogeneous

nucleation. Massa and co-authors created impurity-free

poly(ethylene oxide) (PEO) droplets atop a thin layer of polystyrene

by dewetting.[23] They successfully achieved and characterized

homogeneous nucleation at large supercooling of micron-sized

droplets, as shown in Figure B1A. Homogeneous nucleation was

observed to occur randomly for equal-sized droplets because the

energy barrier to nucleation is only an intrinsic function of PEO

itself.[23] The nucleation kinetics was determined by the volume of

the droplet, as a faster ratewas found for larger droplets.[24]

Interactions at the polymer/substrate interface can influence

the film morphology by changing the crystal orientation.[27] Hu

and co-workers applied two substrates, that is, highly oriented

pyrolytic graphite (HOPG) and amorphous carbon-coated mica,

to spin coat poly(di-n-hexylsilane) (PDHS) films.[53,54] Ultrathin

films of PDHS revealed edge-on lamellae atop HOPG as a result

of epitaxial crystal growth at the interface of highly ordered sub-

strates, while flat-on lamellae were found atop substrates coated

by amorphous carbon.[27,53] Mareau and Prud'homme also

observed flat-on lamellae for ultrathin poly(ε-caprolactone) (PCL)

films atop silicon wafers and carbon-coated substrates, as illus-

trated in Figure B1B.[39] Additionally, they found a nonlinear

reduction of the crystal growth rate as film thickness decreased,

as the reduction was more pronounced for films thinner than the

lamellar thickness. They attributed this suppression of crystalliza-

tion kinetics to a slower diffusion of polymer chains to the growth

front, imposed by both size confinement and polymer/substrate

interactions.[39] Napolitano and Wübbenhorst applied dielectric

spectroscopy for spin coated poly(3-hydroxybutyrate) atop alu-

minum coated glass slides.[28] They claimed that the slowing

down of the crystallization kinetics was related to a reduced

mobility layer, that is, irreversibly adsorbed layer, at the interface

between polymer and substrate.[28]

Besides the effect of geometric confinement and polymer/

substrate interactions, crystallization temperature also influ-

ences the crystallization processes and morphology as it deter-

mines chain dynamics. Grozev et al. examined the

crystallization of mono-lamellar PEO crystals in spin coated

poly-2-vinylpyridine-poly(ethylene oxide) films atop silicon

wafers. After performing melt-crystallization for the films in a

temperature range between 45�C and 57�C, they identified

that the crystal structure of PEO changed with temperature, as

can be compared in Figure B1C. Both lamellar thickness and

width of the side branches increased with increasing

crystallization temperature.[52] Similarly, Jeon and co-author

observed a morphological transition for low-density polyethyl-

ene thin films atop silicon wafers when the crystallization tem-

perature changed from 90�C to 115�C.[55] As a summary,

confinement, polymer/substrate interactions, and crystalliza-

tion temperature are critical factors in determining the final

film morphology of semi-crystalline polymers.

WANG ET AL. 3 of 13

3 | CRYSTALLIZATION OF POLYMER THINFILMS BY PVD

Here, we discuss priori work that has employed PVD approaches to

probe or control the crystallization of polymer films deposited atop

various substrates. By controlling processing parameters such as sub-

strate temperature and substrate type, the change in crystal form,

extent of crystallinity, or chain orientation has been achieved for

model polymers such as PTFE, PVDF, and PE.

Owing to the slow film growth rate of PVD processes, the crystal-

lization of molecules take place at the substrate.[30,90] Therefore, one

major advantage of PVD is that it can effectively tune the degree of

supercooling by simply altering the substrate temperature during film

growth. This key parameter enables tunability toward desired poly-

morphism and crystallinity. For example, the ferroelectric polymer,

PVDF is known to adopt distinct crystalline forms based on the

method of processing.[91,92] PVDF crystalline forms are not perma-

nent, as they can be interconverted by applying external heating, elec-

tric field, or pressure.[93] Takeno and co-workers produced PVDF

films exhibiting α and β forms atop silicon wafers by thermal evapora-

tion at 25�C and −190�C, respectively.[94] Without the need of extra

treatment, the β form showed piezoelectricity because of the in situ

supercooled crystallization condition provided.[95] The γ form of PVDF

was also made using a substrate temperature of 125�C and a much

slower deposition rate compared to the previous conditions. The

authors further proposed a phase diagram where they claimed the

crystal form to be a function of deposition rate and temperature, as

illustrated in Figure 1.[96] To enhance the control of dipole orientation

for exhibiting piezoelectricity, thermal evaporation can also be

equipped with an electric field or an ionization process.[97,98] The pro-

longed time scale of film growth by PVD facilitates the control of

chain reorientation under certain conditions, thus allowing for the

achievement of strong piezoelectricity in a single step that would

commonly require a multi-step process by other means.

Substrate temperature is also known to strongly influence the

extent of crystallinity of vapor deposited polymer films. Norton

et al. investigated PTFE films atop glass substrates deposited at different

temperatures by PLDwith a KrF excimer laser. By studying the x-ray dif-

fraction (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.[99,100] 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.[34] Such trend has been

related to higher chain mobility enabled at elevated substrate tempera-

tures for chain disentanglement and reorganization.[34,100]

FIGURE B1 A, Optical microscopy image of a small section of the sample (1000 μm wide) obtained at a crystallization temperature of -2.6�C.Amorphous droplets appear dark and semi-crystalline droplets appear white under nearly crossed polarizers. The plot shows the fraction ofcrystallized droplets as a function of temperature upon cooling (0.4�C/min) for homogeneous nucleation. Adapted with permission.[24] Copyright

2004, the American Physical Society. B, Tapping mode atomic force microscopy (AFM) height images of PCL films crystallized at roomtemperature on carbon-coated substrates at film thicknesses of (1) 15 and (2) 6 nm. Adapted with permission.[39] Copyright 2005, AmericanChemical Society. C, Typical optical micrographs of crystals in a 40 nm thick film grown at (1) 48�C and (2) 52�C. The average growth rate of thediagonals from center to tip is about 2.9 μm/min for (1) and 0.9 μm/min for (2), respectively. The size of the images is 65 by 65 μm2. Adapted withpermission.[54] Copyright 2008, Springer Nature

FIGURE 1 Phase diagram and molecular orientation in poly

(vinylidene fluoride) (PVDF) thin films as a function of the depositionrate and the substrate temperature. α, β, and γ denote the crystalforms in PVDF. The symbols ? and k refer to the molecularorientation perpendicular and parallel to the substrate, respectively.Adapted with permission from Ref.[30]. Copyright 1994, Elsevier

4 of 13 WANG ET AL.

Another key parameter that can tune the crystalline film morphol-

ogy is the interaction between polymer and substrate. The ease of

depositing polymers by PVD atop various substrates has been

exploited by many researchers to manipulate the molecular orienta-

tion by epitaxial crystallization. The concept of epitaxial growth of a

polymer crystal is often considered as the matching of crystallographic

orientation between the guest polymer and the crystalline sub-

strate.[26,101] Typical substrates used for this purpose are alkali halide

or crystalline polymers.[30,70,74] In 1975, Hattori and co-workers

reported the epitaxial growth of low-molecular weight PE by perform-

ing thermal evaporation atop KCl substrates.[74] At room temperature,

rod-like crystals of PE were formed on the cleaved face of a KCl single

crystal, with the chains preferentially aligned parallel to the substrate.

The crystal structure developed as ellipse-like and disk-like patterns at

substrates temperature of 100�C and 150�C, respectively. At approxi-

mately the same time, the epitaxial growth of thermally evaporated

polymers was also applied to reveal the surface topography of the

crystal underlayers, and further be exploited as a means to tune sur-

face functionality.[70,102,103] Wittmann and Lotz deposited a decorat-

ing layer of PE crystallized atop a solution processed PE single crystal;

the resulting film structure is shown in Figure 2A.[104] The vapor

deposited PE aligned as edge-on lamellae atop the crystal substrate,

with chains parallel to the direction of the growth face, as illustrated in

Figure 2B.[30,104] The crystallization features of the top layer revealed

sectorization of the underlying solution processed PE during crystalli-

zation. It is worth noting that this type of multiple layered film com-

prising the same polymeric material can only be achieved by dry

processes such as vapor deposition. Applying the same concept but

using CNTs as the substrate, Li and co-authors created unique two-

dimensional (2D) patterns of PE crystals termed nanohybrid shish

kebab (NHSK). Figure 3A is a representative transmission electron

microscopy (TEM) micrograph of a PE-coated single-walled carbon

nanotube (SWNT).[71] With a schematic illustration in Figure 3B, the

formation of the 2D structure, where the rod-like PE crystals stack

with their long axes normal to the nanotube, was attributed to a soft

epitaxial mechanism due to the extremely small diameter of the nano-

tubes limiting the strict lattice matching of PE crystal chains.[71,105]

Film morphology after vapor deposition can also be affected by

other parameters including target shape and laser source in the case

of PLD. For instance, Li et al. deposited PTFE films atop silicon wafers

by PLD from two different targets: polished pellets and pressed pow-

der. After ablation by a KrF (λ = 248 nm) excimer laser at a substrate

temperature of 300�C, the film made from the pressed powder was

FIGURE 2 A, Decorated single crystal of a polyethylene fraction. Note that sectorization in four quadrants revealed by the orientation of the

decorating rods. Scale bar: 1 μm. Adapted with permission from Ref. [104]. Copyright 1985, John Wiley and Sons. B, Illustration of epitaxial crystalgrowth of polyethylene on a polyethylene single crystal. Adapted with permission from Ref. [30]. Copyright 1994, Elsevier

FIGURE 3 A, TEM image of PE-decorated SWNTs. Arrows indicate

“nanocentipede-like” 2D nanostructures, which are named 2Dnanohybrid shish kebabs. SWNTs form the central shish and PE rodsform the kebabs. The inset shows a “nanonecklace” formed by PEdecorating on a SWNT loop. Note that PE rods are locallyperpendicular to the SWNT axis. (b) Schematic representation of the2D NHSK. PE chain axes are perpendicular to the PE rod crystals, whilethey are also parallel to the SWNT axis. 2, 3, and 4 show the PE chainorientation on CNTs with different chirality. No matter whether theCNT possesses an armchair (2), a zigzag (3), or a chiral configuration (4),PE chains are always parallel to the CNT axis, suggesting a soft epitaxygrowth mechanism. Both images adapted with permission from Ref.[71]. Copyright 2006, American Chemical Society

FIGURE 4 Schematic of matrix assisted pulsed laser evaporation

(MAPLE)

WANG ET AL. 5 of 13

smoother than the one from polished pellets due to rapid depolymeri-

zation of the polished pellets.[106] Jiang and co-authors utilized XRD

and TEM to characterize the film morphology of PTFE atop glass

slides or carbon-coated copper grids by PLD using a KrF (λ = 248 nm)

laser. At a substrate temperature between 200�C and 450�C, the films

comprised both amorphous and crystalline regions. The crystalline

structure was highly ordered with the long helical molecular chains

oriented parallel to the interface plane with the substrate.[100] A simi-

lar film structure for PTFE atop silicon wafers at a substrate tempera-

ture of 300�C was also observed using a femtosecond pulsed Ti:

Sapphire (λ = 800 nm) laser.[107] Nonetheless, when PTFE films atop

silicon wafers were made by a synchrotron beam (critical wavelength

of 1.5 nm), the chain orientation became perpendicular to the sub-

strate surface at room temperature deposition, compared to parallel

chain orientation of the same films made using a Yttrium Aluminium

Garnet (YAG) laser (λ = 266 nm).[108] Zhang et al. reasoned the differ-

ence to be the result of a different deposition mechanism. Employing

a quadrupole mass spectroscopy, they found saturated fluorocarbons

were ejected from the target by a photochemical reaction mechanism

when a synchrotron beam was used, as opposed to a normal laser

ablation yielding radicals of monomers by thermal unzipping of poly-

mer chains.[108] The larger oligomers from the synchrotron irradiation

were more susceptible to form lamellar structures with the molecular

axes normal to the interface.[108–110]

The current section has discussed the ability of PVD to control

the crystal form, extent of crystallinity, and crystal chain orientation

by exploiting the effects of substrate temperature, substrate type and

other parameters such as target shape and laser source on film mor-

phology. However, many of these approaches are limited toward fully

understanding the processing-structure-property relationships due to

polymer degradation. One way of overcoming such challenge is to

employ newly developed techniques. Recently, a PVD technique

termed MAPLE has been proven to minimize chemical modification of

polymers. The gentler deposition approach sets the stage for studying

polymer thin film crystallization in greater details. In the next section,

we discuss the unique film formation mechanism of MAPLE, followed

by highlighting our work in advancing the field by employing MAPLE

to enhance the understanding of structure-property relationships of

polymer films formed by top-down deposition.

4 | MATRIX ASSISTED PULSED LASEREVAPORATION

Developed nearly two decades ago, MAPLE is a PVD technique that

can effectively minimize polymer damage during deposition.[111] While

it utilizes a laser sources similar to PLD, such as UV excimer

lasers,[86,112] IR lasers,[48,113,114] and FEL lasers,[115] 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.[86,116] A schematic of MAPLE is illus-

trated in Figure 4. Deposition is initiated by laser ablation on a cryogen-

ically frozen target comprising a dilute polymer solution. Subsequently,

phase explosion occurs at the target surface, releasing a vaporized

plume containing both polymer and solvent into the vacuum. Film for-

mation proceeds by the addition of polymer molecules, as the volatile

solvent gets pumped away.[31] The polymer-friendly technique has

been applied to crystalline polymers such as PEO and PE for systematic

crystallization studies.[34,112,117] Furthermore, it allows for fabricating

electronic and photonic devices with polymers.[48–50] The size of these

polymers range from 3 to 70 kDa. For example, Ge et al. recently

reported a solar-cell performance of over 3% by MAPLE-deposited

poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester using

an Er:YAG laser (λ = 2.94 μm).[51] Yet application wise, the major limita-

tion of MAPLE is the unfavorable surface feature. In many cases,

MAPLE-deposited films have notable surface roughness with a large

size distribution of polymer globules.[114] Molecular dynamics

FIGURE 5 A, Schematic showing the transport of polymer-matrix clusters during MAPLE process. Film fabrication is achieved by the deposition

of concentrated polymer droplets. Adapted with permission from Ref. [34]. Copyright 2018, American Chemical Society. B, AFM height image ofMAPLE-PEO film formed on Si substrate with Tdep = 25�C, featuring the growth of 2-dimensional mono-lamellar crystals (MLC) from nucleatingdroplets marked with N1-N3. The inset schematically shows the structure of PEO crystalline islands. Adapted with permission from Ref. [112].Copyright 2016, American Chemical Society. C, AFM height image of MAPLE-PE formed on Si with Tdep = 75�C, showing the morphology of PEnanocrystals consisting of a single lamella or a few stacked lamellae. The inset schematic shows the structure of the PE nanocrystals. Adaptedwith permission from Ref. [34]. Copyright 2018, American Chemical Society

6 of 13 WANG ET AL.

simulation by Leveugle et al. explained the surface roughness to be a

result of the deposition mechanism itself, which involved cluster forma-

tion consisting of polymer chains.[118] In order to improve the film qual-

ity, researchers have managed different ways such as creating

emulsions within the target composition[119] and selecting solvents

which have high absorption of the laser source.[120,121]

5 | CRYSTALLIZATION OF POLYMER THINFILMS BY MAPLE

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.[34,112,117,122] While MAPLE achieves

concurrent film growth and crystallization similar to other PVD technolo-

gies, 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. This results in some unique features in crystal nucleation-

growth mechanism atop the substrate, and more importantly, allows the

manifestation of long-chain effects of the molecules in crystallization,

that is, chain-folding and adsorption to substrate surface. In the following

section we will address our recent work in advancing the understanding

of polymer crystallization in thin film geometry byMAPLE.

5.1 | Crystal nucleation and growth mechanism inMAPLE

In MAPLE processing, film fabrication is achieved by the assembly of

nano-sized to micron-sized polymer droplets rather than individual mole-

cules. During laser ablation of a target matrix, polymer chains trapped in

the matrix are entrained together in the ablation plume.[123] Subsequently,

the polymers deposit as confined droplets atop a substrate, as schemati-

cally shown in Figure 5A.[112,124,125] The diameter of the droplets typically

ranges from tens of nanometers to a few micrometers. Depending on the

substrate temperature and droplet size, the droplets either self-nucleate,

crystallize from a crystal growth front, or remain in the liquid phase.

The distribution of size in deposited polymer droplets plays a critical

role in crystal nucleation due to finite size effects on the rate of nucle-

ation.[24] The nucleation probability is greater in larger liquid droplets.[24]

When polymer droplets are larger than a critical size, they self-nucleate

during deposition, while smaller droplets remain in the liquid state.[19,126]

By short time MAPLE deposition with slow film growth rates, we could

examine the early stage film morphology composed of confined polymer

droplets. Figure 5B shows an atomic force microscopy (AFM) height

image of the morphology of a PEO (Mn = 4600 g/mol, Mw/Mn = 1.1)

film deposited onto a silicon (Si) substrate byMAPLE (MAPLE-PEO).[112]

The substrate temperature during deposition (Tdep) was 25�C, which is

much lower than the melting temperature of the target PEO (Tm ~ 60�C).

The film comprised three distinct structures: nucleating droplets, 2D

dendritic patterns consisting of mono-lamellar crystals, and the sur-

rounding amorphous phase. Figure 5B illustrates three large droplets

denoted N1, N2, and N3, which are nucleating droplets formed by self-

nucleation. Once nucleating droplets form, they render the crystallization

of small droplets into 2D patterns on the surrounding. The resulting

structure is schematically described in the inset of Figure 5B. The liquid

layer is formed by small droplets isolated from any crystalline islands and

is incapable of self-nucleation and epitaxial nucleation from growth

fronts of the crystalline islands.

For strong crystal formers, however, such size-effect on nucleation

may not be evident. Figure 5C shows an AFM height image of nanoscale

polymer crystals formed by the deposition of low-molecular weight PE

(Mn = 3000, Mw/Mn = 1.10) atop Si at Tdep = 75�C (MAPLE-PE), which

FIGURE 6 A, AFM height image magnifying the PE nanocrystals consisting of a single lamella formed by MAPLE deposition at Tdep = 75�C.Adapted with permission from Ref. [34]. Copyright 2018, American Chemical Society. B, Representative height profiles of PE MLCs formed atvarious Tdep. The MLC formed at Tdep = 116�C had a uniform thickness of ~27 nm, which corresponds to the fully extended chain length of thetarget PE (Mn = 3000). Adapted with permission from Ref. [34]. Copyright 2018, American Chemical Society. C, AFM height image showing thePEO film morphology formed on Si with two sequential MAPLE depositions (1st and 2nd MAPLE) at different Tdep; the substrate temperaturewas maintained at 25�C during 1st MAPLE and maintained at 35�C during the following 2nd MAPLE, as schematically described in the inset. D,AFM height profiles of the four PEO MLCs marked with P1-P4 in panel (c). The dashed circle highlights ~3 nm step change in the thickness of thefour MLCs, which was caused by the sudden change in Tdep from 25 to 35�C during deposition

WANG ET AL. 7 of 13

is more than 50�C below the typical melting temperature of the target

PE (Tm ~ 127�C).[34] Most of the isolated PE nuclei were comprised of

~10-nm-thick PE mono-lamellae, as schematically shown in the inset of

Figure 5C, which implied that the nucleation capability of PE was suffi-

cient to form single-lamellar nuclei. As a result, the structural formation

of PE films formed byMAPLE was highly driven by nucleation of individ-

ual droplets, which was in stark contrast to the case of PEO whose self-

nucleation event was only limited tomicron-sized droplets.

5.2 | Control of lamellar thickness and meltingtemperature

The folded-chain structure of polymer crystals has a critical influence on

the metastability as well as the mechanical,[127–130] and electrical prop-

erties of semi-crystalline polymers.[131–134] Therefore, controlling the

lamellar thickness is of significant interest in polymer processing. Since

chain-folding is strongly governed by crystallization temperature,[135,136]

precise tuning of the crystallization temperature (Tc) over a wide range

is desirable to engineer the lamellar thickness in semi-crystalline poly-

mer films. PVD has a significant advantage in that it enables a facile con-

trol of the substrate temperature at which the target material

condensates; this prevents the occurrence of crystallization at untar-

geted temperature on slow cooling from a liquid state. Conventional

PVD techniques, however, were not suitable for systematically studying

the chain-folding behavior due to the fracture of macromolecules during

vaporization. In this sense, the use of MAPLE processing was necessary.

Via MAPLE, a strong correlation between lamellar thickness of

the deposited polymers and Tdep has been identified with two model

polymers, low-molecular weight PE (Mn = 3000, Mw/Mn = 1.10) and

PEO (Mn = 4600 g/mol, Mw/Mn = 1.1). Figure 6A exhibits the AFM

morphology of PE single-lamellar crystals atop silicon wafers, whose

height corresponds to the lamellar thickness. The height profiles of PE

single crystals formed at four different Tdep, that is, 75, 80, 100, and

116�C are compared in Figure 6B. While the crystals formed with

Tdep = 116�C exhibited a fully extended chain length of ~27 nm, the

lamellar thickness clearly decreased as Tdep was lowered.[34]

In addition, by manipulating Tdep during MAPLE, crystals with dif-

ferent lamellar thickness can be formed in one film. Figure 6C shows

an AFM height image of PEO lamellar crystals, grown using the tem-

perature protocol described in the inset of Figure 6C. The deposition

started with Tdep = 25�C (MAPLE 1st), temporarily ceased to raise the

Tdep to 35�C and continued again with Tdep = 35�C (MAPLE 2nd). The

rise of Tdep from 25�C to 35�C led to a ~3 nm step increase in lamellar

thickness of the growing PEO crystals, as shown in Figure 6D that

measures the height profiles along P1-P4 in Figure 6C. Such a sudden

change of Tc while growing different layers of crystals by top-down

approach would not be possible in the case of solution processing and

melt-crystallization, where crystals rapidly grow from a consolidated

liquid phase. By MAPLE, a delicate control of polymer crystallization

could be realizable by extremely slow film growth combined with the

capability of tuning Tc by Tdep.

The control over the film morphology with Tdep during MAPLE

leads to different thermal properties of the deposited polymer films.

Due to the metastability of lamellar crystallites bounded by amor-

phous surfaces, their Tm strongly varies with lamellar thickness, fol-

lowing the Gibbs-Thomson relationship.[41] The strong correlation

between Tdep and Tm in MAPLE-deposited PE films was successfully

verified using flash-calorimetry (flash-DSC) technology.[34] Flash-DSC

can measure heat flow from an infinitesimal amount of a sample

(<1 μg) placed on a flash-DSC chip with ultra-high rates of cooling and

heating.[137–139] By MAPLE, PE was directly deposited onto a flash-

DSC chip as shown in Figure 7A. The temperature of the chip during

deposition (=Tdep) was controlled by a MAPLE substrate heater.

Figure 7B plots the flash-DSC first heating thermograms of

MAPLE-PE samples obtained with various Tdep, where melting peaks

were clearly captured. The manipulation of Tdep over 118�C (−2-116�C)

resulted in a dramatic change in Tm over 20.6�C (105.7-126.3�C). For

reference, Figure 7B also illustrates the melting peaks of the same PE

obtained by melt-crystallization in bulk (~mg) with two cooling rates;

one by 0.5�C/min (dashed line in Figure 7B) and the other by quench-

ing into liquid nitrogen (LN2, dotted line in Figure 7B). Those two

FIGURE 7 A, Optical image that shows the sample stage of a flash-DSC chip after MAPLE PE deposition. The white dashed line marks the region

of the deposited film. B, Plots comparing normalized flash-DSC first heating thermograms of MAPLE-PE formed with various Tdep, ranging from−2 to 116�C. The dotted curve and dashed curve are the normalized heating thermograms of LN2–quenched bulk PE (5.1 mg) and slowly cooledbulk PE (12.7 mg) with a cooling rate of 0.5�C/min, respectively, which were obtained using a conventional DSC (bulk-DSC). C, Plots showing theTm-Tdep relationship of MAPLE-PE (MAPLE, green circles) and the Tm-Tc relationship of melt-crystallized PE. For melt-crystallization, two methodswere used: Isothermal crystallization using flash-DSC (ISO, blue triangles) and non-isothermal crystallization by cooling with various rates usingbulk-DSC (NIC, red squares). The target crystallization temperature in ISO and the peak temperature of the exotherm in NIC were defined as Tc.line 1 was obtained from all MAPLE, NIC, and ISO data ranging from Tdep (or Tc) = 75 to 116�C, while line 2 was obtained from MAPLE-PE dataranging from Tdep = −2 to 75�C. All images are adapted with permission from Ref. [34]. Copyright 2018, American Chemical Society

8 of 13 WANG ET AL.

extreme cooling rates only resulted in 1.8�C difference in Tm, that is,

Tm = 128.2�C with 0.5�C/min and Tm = 126.4�C with LN2 quenching.

This is mainly because a typical melt-crystallization process is limited to

achieve nucleation at a large supercooling. This comparison clearly

highlights the exceptional tunability of Tm by MAPLE.

How Tdep governs the crystallization of PE during MAPLE can be

further clarified by comparing the correlation between Tdep and the

actual crystallization temperature of PE during film formation.

Figure 7C plots the Tdep-Tm relationship of MAPLE-deposited PE and

Tc-Tm relationship of the same PE by melt-crystallization. Traditional

DSC (bulk-DSC) and flash-DSC were used to obtain the melt-

crystallization data. Above 75�C of Tdep or Tc, all three data sets were

reasonably fit to one straight line (line 1 in Figure 7C), indicating that

the crystallization of PE in MAPLE occurred at Tdep (Tdep = Tc). Such lin-

ear relationship between Tc and the corresponding Tm was expected

for polymer crystals formed at a relatively low undercooling (ΔT = Tm� -

Tc), where Tm� is the equilibrium melting temperature.[140–142] In the

Tdep range below 75�C, however, the Tdep-Tm data followed a different

line (line 2 in Figure 7C), at which Tm depression rate was about 3.9

times smaller than the rate on line 1. In this case the PE prematurely

crystallized prior to reaching Tdep (Tc > Tdep), suggesting that the charac-

teristic nucleation induction time in PE droplets (τnucl) is smaller than

the time scale of heat equilibration at the substrate (τheat) in such a low

temperature range. Considering the dominant droplet size of PE in

MAPLE as 1 μm or below in radius, the target PE material is expected

to nucleate prior to thermal equilibration (τnucl < τheat) at a temperature

below ~63�C, which reasonably supports the deviation of Tdep-Tm

below Tdep ~ 75�C.[34] While the crystallization temperature in MAPLE

is also eventually bounded by the nucleation of PE, the non-zero slope

of line 2 suggests that the depositing PE chains are still effectively

cooled prior to nucleation at temperatures where τnucl < τheat.

5.3 | Adsorption of polymers during MAPLE

At the moment of deposition atop a substrate, polymer droplets may

physically adsorb to the surface, thereby impeding crystallization. As

hinted previously, such adsorption would be evident when the

nucleation capability of the deposited polymer droplets is low, that is,

when the droplet is smaller than the critical size for nucleation or Tdep

is higher than the temperature allowed for self-nucleation. While

MAPLE of PE does not show a noticeable evidence of adsorption due

to the strong nature of PE to crystallize, the effect of droplet size and

Tdep on adsorption can be clearly observed in MAPLE-deposited PEO.

PEO films grown atop Si by MAPLE exhibit an adsorbed layer formed

by the deposition of small PEO droplets that failed to nucleate.[112,117]

MAPLE of PEO at a temperature slightly below the bulk melting tem-

perature (~60�C) can form a film that remains in liquid phase across

the deposited area.[112,117]

Adsorption during MAPLE can lead to the loss of the crystallizability

of deposited polymer films. In the case of metals and molecular com-

pounds deposited via PVD, any deposited species would eventually crys-

tallize by forming isolated nuclei or incorporating into any nearby

growing crystals.[143,144] In MAPLE of PEO, however, adsorbed molecules

can stay adsorbed persistently even upon contact with crystal growth

fronts.[112,117] Figure 8A presents the evolution of 2D PEO crystals during

the total 6 hours of MAPLE at Tdep = 25�C atop Si. The deposition was

divided into three distinct 2 hours depositions to capture optical images

of the same region in the intervals. While the 2D crystals formed in the

first MAPLE showed clear evidence of growth during the second MAPLE

interval, no detectable growth of the crystals was observed during the

third deposition. Figure 8B captures the AFM morphology of a PEO film

formed after 6 hours of deposition and the corresponding height profile.

Clearly, the growth front of 2D crystals was surrounded by an adsorbed

layer with nanometer thickness (nanolayer) without a depletion zone. This

suggests that the amorphous PEO in direct contact with the crystalline

face is kinetically stable against crystallization.

The kinetic stability of the adsorbed PEO nanolayer against crys-

tallization can depend highly on deposition temperature. In the above

study, while adsorbed PEO formed at Tdep = 25�C was stable against

crystallization, MAPLE at Tdep = 50�C rendered strong crystal growth

from the nanolayer when the substrate temperature was cooled to

25�C after the deposition.[117] The effect of Tdep on the stability of

nanolayer was investigated by post-deposition aging of the two

MAPLE-PEO samples (Tdep = 25, 50�C) at the same aging temperature

FIGURE 8 A, Optical microscopy images captured with 405 nm-laser showing the growth of 2D crystals after additive PEO deposition atop Si. The same

region of the sample was captured after 1st, 2nd, and 3rd MAPLE deposition, each of which was performed at Tdep = 25�C for 2 hours. B, AFM heightimage showing the morphology of MAPLE PEO film formed after 6 hours of deposition (upper panel) and the corresponding AFM height profile (lowerpanel). Dendritic 2D crystals, a surrounding nanolayer, and an area scraped with a razor blade (Si surface) are depicted. No depletion zone exists between2D crystal growth front and the contacting nanolayer. Both images are adapted with permission from Ref. [117]. Copyright 2016, American Chemical Society

WANG ET AL. 9 of 13

(Tag = 25�C) under N2 environment. In both samples the morphology

of a ~5 nm thick nanolayer, formed by 3 hours of MAPLE deposition,

was monitored during aging. Figure 9A compares the AFM images

when Tdep = 25�C captured before (left panel) and after 27 days of

aging (right panel), which showed no evidence of the growth of 2D

crystals into the nanolayer region. In contrast, when Tdep = 50�C crys-

tallization in the nanolayer during aging at 25�C was observed; see

Figure 9B. Figure 9C exhibits the AFM image of film when Tdep = 50�C and after crystallization during aging. The ~10 nm height of the 2D

crystals (see lower panel of Figure 9C) provides evidence that the

crystallization occurred during aging at 25�C.

The stability of adsorbed PEO against crystallization was correlated

to the degree of attractions between the PEO and the underlying sub-

strate. The degree of adsorption was conjectured by measuring the resid-

ual nanolayer thickness of MAPLE-deposited PEO samples after washing

in toluene. Washing the polymer film in a solvent can uncover loosely

adsorbed chains, therefore a positive correlation is expected between the

degree of adsorption and residual film thickness.[145–147] Before solvent

washing, both Tdep = 25�C and 50�C samples had an ~5-nm thick uniform

nanolayer as a result of MAPLE deposition for 3 hours. Figure 9D shows

a representative AFM profile of a residual nanolayer deposited at Tdep =

25�C after solvent washing. As compared in Figure 9E, there was no

meaningful decrease in the thickness of the nanolayer after washing,

which proved the strong adsorption of PEO to the substrate. In

comparison, the AFM profile of a residual nanolayer deposited at Tdep =

50�C shown in Figure 9F clearly exhibited a noticeable loss of PEO after

washing. Figure 9G compares the average nanolayer thickness before and

after solvent washing, which showed a noticeable decrease from ~5 to

~1 nm. While the origin of how deposition temperature affects the

degree of adsorption is still unclear, the results here imply that the deposi-

tion temperature in PVD can critically affect the conformation (or energy

state) of the deposited polymer chains. Another intriguing fact from the

above results was that the crystallization of long-chain molecules in PVD

can exhibit anomalies compared to other crystalline solids due to their

ability to strongly interact with underlying substrates.

In this focused section, we have discussed two model polymers

deposited by MAPLE and their differences in film morphology and the

resulting thermal properties. We first showed that substrate tempera-

ture during MAPLE has a critical impact on crystal lamellar thickness

and melting temperature. Deposition temperature-lamellar thickness-

melting temperature relationships observed in MAPLE of PE agree

well with our conventional understanding of polymer crystallization

from solution or liquid phase. Due to the rapid cooling effect during

the deposition of confined polymer droplets, tuning of lamellar thick-

ness and melting temperature could be achieved over a wide tempera-

ture range. In addition, the extremely slow deposition rate of

polymers allows for the manipulation of substrate temperature during

FIGURE 9 A, AFM height images comparing the film morphology of MAPLE-PEO film made with Tdep = 25�C right after deposition (left panel)

and after 27 days of post-deposition aging at 25�C under N2 environment (right panel). The yellow dashed lines indicate the position of crystalgrowth fronts in as-deposited sample, highlighting that there was no growth of the 2D crystals into the nanolayer region during the 27 days ofaging. B, Optical microscopy images captured with 405 nm-laser showing 2D crystal growth during post-deposition aging at 25�C in a MAPLE-PEO film made at Tdep = 50�C. The film was quickly transferred onto a 25�C temperature stage after MAPLE for aging and optical microscopyobservation. The upper panel was captured right after crystallization in droplet N1 and the lower panel was captured after 1 hour of aging. C,AFM height image (upper panel) showing 2D crystals grown from a nanolayer during post-deposition aging. Left side of the film was scraped witha razor blade. The measured height of the 2D crystals was ~10 nm as depicted in the lower panel. D, AFM height profile of a residual nanolayerafter 20 minutes of toluene washing in MAPLE-PEO film made at Tdep = 25�C. E, Graph comparing nanolayer thickness in MAPLE-PEO film madeat Tdep = 25�C before and after toluene washing. F, AFM height profile of a residual nanolayer in MAPLE-PEO film after 20 min of toluenewashing. The original film was made at Tdep = 50�C. G, Graph comparing nanolayer thickness in MAPLE-PEO film made at Tdep = 50�C beforeand after toluene washing. All images are adapted with permission from Ref. [117]. Copyright 2016, American Chemical Society

10 of 13 WANG ET AL.

deposition, resulting in a non-uniform lamellar thickness of crystals in

the same film. Next, we demonstrated that the deposition tempera-

ture could affect the stability of the adsorbed polymers formed at the

substrate surface. In MAPLE deposition of PEO, the adsorption

strongly affects the nucleation of deposited PEO droplets as well as

the crystal growth kinetics. Interestingly, the stability of adsorbed

layers against crystallization showed a dependence on deposition

temperature. We anticipate structure-property relationships observed

for these two model polymers can be generally applied to other semi-

crystalline polymer systems processed via PVD.

6 | FUTURE WORK

Future directions of using PVD to study polymer crystallization in thin

film geometry are vast. The development of the technique itself

should focus on addressing the limitation of material degradation,

unfavorable surface features, and technical concerns in commercial

scale-up.[148,149] Overcoming these challenges will attract more

research efforts aiming to understand the crystallization of polymer

thin films in greater details, and to expand the use of PVD application

wise. Furthermore, the fundamental studies of polymer film crystalli-

zation should exploit the film formation mechanism of PVD to design

film structures such as multilayered films comprising chemically similar

or dissimilar polymers, films made by co-polymers or polymer blends,

and polymers atop various substrates. By creating different hard or

soft interfaces, the effect of heterointerfaces or confinement on the

crystallization kinetics, and the role of irreversible adsorption on the

film properties can be further explored. This will inspire ways of engi-

neering film morphologies and material properties to meet various

application needs. Last but not least, more parameters can be investi-

gated by utilizing the advantage of additive growth mechanism from

PVD techniques such as MAPLE to monitor how they influence the

film growth at different stages. For example, the film growth rate,

molecular weight of polymers, and side branching.

7 | CONCLUSIONS

In this review, we have discussed the advances in the field of polymer

thin film crystallization by PVD. As a top-down deposition method,

PVD offers a new route aiming to unveil the processing-structure-

property relationships and to control film morphology for various

application needs. The key parameters such as substrate temperature

and substrate type have been addressed by studies using classical

PVD techniques such as thermal evaporation and PLD, as they can

control the crystal form, extent of crystallinity, and polymer chain ori-

entation. Furthermore, we have shown that a more recent PVD tech-

nique known as MAPLE can expand the research topics by

overcoming the limitations caused by polymer degradation. In MAPLE,

polymers are confined within separate droplets. The crystallization of

these droplets atop a substrate has revealed a dependence of crystal

lamellar thickness and melting temperature on the deposition temper-

ature, thereby proving the tunability of film morphology by selection

of MAPLE deposition protocols. By changing the substrate

temperature, or effectively the crystallization temperature, we

showed the tunability of melting temperature by 20.6�C for MAPLE-

deposited PE. Additionally, we proposed different kinetic stability of

MAPLE-deposited PEO films at various deposition temperatures due

to the development of irreversibly adsorbed layers during the deposi-

tion. Despite the distinct film formation mechanism of each PVD tech-

nique described above, we believe ways to control film morphology

and thermal properties by tunable parameters such as substrate tem-

perature and substrate type are generally applicable to other top-

down vapor deposition techniques when polymer degradation is

prevented.

ACKNOWLEDGMENTS

H.J. acknowledges support from Kwanjeong Educational Foundation

in South Korea. C.B.A. and R.D.P. acknowledge the support of the

National Science Foundation (NSF) Materials Research Science and

Engineering Center Program through the Princeton Center for Com-

plex Materials (DMR-1420541). R.D.P. acknowledges the support of

the AFOSR through a PECASE Award (FA9550-15-1-0017).

ORCID

Rodney D. Priestley http://orcid.org/0000-0001-6765-2933

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AUTHORS' BIOGRAPHIES

YUCHENG WANG received his B.S. in Chemical

Engineering from Purdue University in 2015.

He is currently a Ph.D. candidate in the

Department of Chemical and Biological Engi-

neering at Princeton University. His research

interests include MAPLE deposition of poly-

mer thin films to achieve control over film

morphology and properties.

HYUNCHEOL JEONG received his B.S. in Chemical

Engineering from Pohang University of Sci-

ence and Technology (POSTECH) in South

Korea in 2012 and obtained his Ph.D. in

Chemical and Biological Engineering from

Princeton University in 2017. During his Ph.

D., he worked on crystallization of polymer

films by MAPLE, with interest on the phase transition of polymers

under nanoscale confinement and laser processing. Currently, he

works in Intel Corporation.

MITHUN CHOWDHURY is an associate research

scholar in the Department of Chemical and

Biological Engineering at Princeton Univer-

sity. He obtained his Ph.D. in Physics

(Polymer) from the University of Freiburg,

Germany in 2012. He completed brief post-

doctoral stays in Trinity College Dublin, Ire-

land and University of St Andrews, UK. His research interests

include glassy polymers, polymer crystallization, mechano-rheo-

logical, optoelectronic, and responsive properties of polymers, all

with an emphasis on the effect of interfaces.

CRAIG B. ARNOLD is a Professor of Mechanical

and Aerospace Engineering at Princeton Univer-

sity and is the Director of the Princeton Institute

for the Science and Technology of Materials

(PRISM). He earned his Ph.D. from Harvard Uni-

versity, and was an NRC postdoctoral fellow at

the US Naval Research Laboratory. His interests

include laser deposition, printing, and manufacturing methods, mate-

rials processing, optical design, and energy storage/conversion tech-

nologies. He is a fellow of OSA and SPIE.

RODNEY D. PRIESTLEY is an Associate Professor

in the Department of Chemical and Biological

Engineering at Princeton University. He

obtained his Ph.D. in Chemical Engineering

from Northwestern University in 2008. He

completed an NSF/Chateaubriand postdoc-

toral fellowship at Ecole Superieure de Phy-

sique et Chimie Industrielles de la Ville de Paris. His research

interests include polymer glasses, nanoconfined polymer dynam-

ics, polymer thin film and nanoparticle processing, MAPLE,

responsive and sustainable polymers.

How to cite this article: Wang Y, Jeong H, Chowdhury M,

Arnold CB, Priestley RD. Exploiting physical vapor deposition

for morphological control in semi-crystalline polymer films.

Polymer Crystallization. 2018;e10021. https://doi.org/10.

1002/pcr2.10021

WANG ET AL. 13 of 13