Poly(ethylene oxide) crystallite growth during solvent vapor annealing in block polymer thin films

In recent years, numerous efforts have focused on leveraging block polymer (BP) nanostructure formation to enable next-generation nanotechnology and lithography applications [1], [2]. The appeal of BPs largely stems from the molecules’ inherent ability to undergo predictable self-assembly. This nanostructure formation process occurs when two or more chemically distinct and immiscible blocks microphase separate, and the resulting nanoscale morphology and feature sizes are readily tuned by manipulating key parameters such as block volume fractions, overall molecular weight, conformational asymmetry, and block order (for multi-block materials) [3]. This self-assembly of BPs can be harnessed in thin film geometries to create periodic arrays of nano-features over large areas ideal for use as membrane active layers, nanotemplates, sensors, etc. [4], [5]. Additionally, these thin films have the potential to replace or significantly streamline conventional lithographic techniques because of their ability to unlock new, low cost, energy efficient, and straightforward fabrication methods [6], [7], [8]. Unfortunately, the self-assembly process alone generally leads to short-range alignment and a relatively high number of film defects [9]. Thus, significant effort has been focused on the rapid and efficient generation of nanostructured BP thin films with long-range order and controlled orientation across large areas.

Various techniques, ranging from chemical and topological patterning to applied external fields, have been employed in an attempt to realize BP thin films with tunable long-range order and controlled orientation in a cost-effective and time-efficient manner [2]. Though there are some extremely promising avenues, the desired control over macroscopic arrays of nanoscale BP features is particularly challenging for amorphous-crystalline BP systems, as the final ordered morphology is impacted not only by BP self-assembly, but also by crystallization of the semi-crystalline block [10]. In these hierarchically assembling polymers, the crystallization behavior can be categorized as either confined or breakout [11]. Confinement occurs when the crystallization is restricted within the respective domain(s), as a result of strong segregation from the corresponding amorphous components. Alternatively, breakout arises when conditions favor a ‘more mobile’ amorphous block during crystallization and large, rogue crystallites are formed that span beyond the confining domains [12]. For the case of aligned features in nanostructured thin films, it is important to employ alignment methods that balance crystallization and facile alignment [10], [11].

One alignment method, which has shown recent promise for manipulating BP thin films, is flow-solvent vapor annealing (F-SVA), in which the effective χ can be tuned to modulate ordering, along with confined vs breakout crystallization [13]. This method uses an inert gas bubbling through solvent reservoirs to introduce a solvent-rich gas stream into an F-SVA chamber containing the BP film. Appropriate solvent selection allows preferential swelling of each polymer domain, and numerous factors contribute to assembly behavior including solvent choice, swollen film thickness, rate of deswelling, vapor exposure time, and vapor pressure [14], [15]. Thus, F-SVA is a versatile, multi-faceted method to achieve aligned, defect-free thin films in various BP systems with the potential for directional drying fronts to result in some orientation control [16]. If F-SVA is further coupled with an applied force, e.g. SVA with soft shear, long-range, controlled directionality is achievable [17].

This study used an amphiphilic diblock copolymer with one amorphous block and one semi-crystalline block, poly(styrene-block-ethylene oxide) (PS-PEO, M_n?=?33?kg?mol⁻¹, PEO volume fraction of 0.36). Thin films cast from this BP displayed a perpendicularly oriented, hexagonally-packed, cylinder morphology consisting of PEO cylinders inside a PS matrix. The flow-coated film was placed in an F-SVA chamber with two solvent vapor streams, water and toluene, modulated by separate bubblers joined at a T-junction [18], [19]. For the specific image presented herein, the film was swelled by ∼50% of its original thickness as monitored by spectral reflectance and held at that condition for 24?h. Following rapid deswelling, the modified film was imaged with an optical microscope revealing the large finger-like PEO crystallites.

In this film, breakout crystallization occurred as a result of a sufficiently high toluene vapor flow during F-SVA that likely reduced the PS glass transition temperature to below room temperature [20], eliminating the ability of PS to confine PEO crystallization. The water vapor, which preferentially swelled the PEO domain, increased chain mobility, and macroscopic crystallites formed that were readily visible by optical microscopy. For separate experiments in which the toluene vapor content was lowered, by reducing the inert gas flowrate into the toluene reservoir, these large crystallites were not detected and only confined crystallization behavior and longer-range nanoscale ordering were noted. Thus, the balance between crystal formation and alignment of amorphous-crystalline BP thin films comes from the delicate interplay between chain mobility, block segregation, and confinement, which in our case was modulated by the toluene:water vapor ratio. This issue’s cover image shows ∼100–150?μm long PEO crystals that form as a result of breakout crystallization due to enhanced mobility of the amorphous PS block during PEO crystallization.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors thank the Army Research Office (W911NF1820167) for financial support of the film fabrication and characterization efforts. T.H.E. also acknowledges financial support from the Thomas & Kipp Gutshall Professorship.

Poly(ethylene oxide) crystallite growth during solvent vapor annealing in block polymer thin films

Declaration of Competing Interest

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