Article pubs.acs.org/JPCC
Effects of Relative Humidity on the Surface and Bulk Structures of Linear Polyethylenimine Thin Films Geoffrey A. Lott, Matthew D. King, Michael W. Hill, and Lawrence F. Scatena* Boise Technology, Inc., 5465 East Terra Linda Way, Nampa, Idaho 83687, United States S Supporting Information *
ABSTRACT: Polyethylenimine (PEI) polymers have become increasingly utilized for a myriad of applications including self-decontaminating materials and nonviral gene transfection. While the bulk properties of PEIs have been studied in detail, their surfacespecific structure/behavior remain unexplored. Here, we report the effects of relative humidity on the surface structure of linear polyethylenimine (LPEI), as investigated by vibrational sum frequency generation (VSFG) spectroscopy. Results show that the surface structure of (as prepared) anhydrate LPEI is highly dependent on relative humidity. As the relative humidity is varied from 0% to 75%, surface spectra of LPEI in the C−H and N−H stretching regions reveal multiple crystalline and amorphous states, including the gel-phase amorphous state that has previously only been observed in appreciable quantities above LPEI’s upper critical solution temperature (64 °C). Utilizing DFT calculations, we have assigned large characteristic frequency shifts (∼50 cm−1) of LPEI anhydrate crystalline methylene modes to the Bohlmann effect, which is the delocalization of the nitrogen lone electron pair causing a weakening of the C−H bonds of methylene moieties adjacent to the amine functionality. Similar frequency shifts (∼20 cm−1) observed in the hydrated crystalline forms are likely due to intermolecular interactions mediated by hydrogen bonding within the LPEI/water matrix.
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INTRODUCTION Polyethylenimines (PEIs) have found widespread utility in numerous fields including chemical and biological sensing,1 carbon dioxide capture,2 and electrical energy storage.3 Recently, some of the more interesting and impactful applications are based around PEI’s biocompatibility, where it is being investigated for its use as a nonviral vector for gene transfection,4−8 enzyme or protein stabilization,9,10 and cell culture.11 Additionally, due to the generally high concentration of reactive amine moieties present in PEI materials, they are also of interest for reactive/self-decontaminating technologies for protection against nerve-type chemical warfare agents (CWAs), wherein amine groups hydrolyze CWAs to produce less lethal or nonlethal byproducts. Although surface reorganization of various polymeric materials due to environmental adsorbates (water and organics) has been established,12−16 the characterization of self-decontaminating materials’ environmentally mediated surface reorganization and its effects on surface functionality and reactivity is largely absent. To these ends, developing a surface-specific, molecular level understanding of the behavior of polyethylenimine films in varied environmental conditions will provide valuable insights into the molecular-level details that mediate the functionality of these materials for numerous applications. In the present study, we employ linear polyethylenimine (LPEI) as a model system for understanding the effects of relative humidity, and therefore hydrogen bonding (amine− amine, water−amine, and water−water hydrogen bonding), on the surface structure of amine-bearing polymers at the solid/ © 2014 American Chemical Society
vapor interface. In addition, LPEI provides a more simplistic molecular structure to characterize the spectroscopic signatures of secondary amine-bearing molecules at an interface and serves as a foundation for investigating more complex PEI systems that contain primary, secondary, and tertiary amine functionalities such as branched polyethylenimine. Although the molecular structure of linear polyethylenimine (LPEI) is simple, being comprised of repeating −(CH2−CH2−NH)− ethylenimine monomer units, LPEI exhibits complex crystalline structures and phase behavior as a function of water content.17−25 In its anhydrate form, approximately 40% of bulk LPEI exists as a parallel array of double-stranded helices stabilized by hydrogen bonds between the secondary amine groups of neighboring PEI chains. The remaining 60% consists of rigid amorphous domains that arise from the inevitable entanglement of the polymer chains.23 Similar to the crystalline anhydrate, the rigid amorphous domains are devoid of absorbed water molecules, but disordered, with their mobility restricted by the anhydrate crystalline structure and secondary bonding (hydrogen bonds) of the amine groups.26−30 As anhydrate LPEI absorbs water, the anhydrate crystalline and rigid amorphous domains undergo transitions to a series of three distinct crystalline hydrates. The crystalline hydrates consist of LPEI strands exhibiting all-trans (planar-zigzag) conformations with hydrogen bonds now formed between the intercalatedReceived: May 2, 2014 Revised: July 2, 2014 Published: July 21, 2014 17686
dx.doi.org/10.1021/jp504321r | J. Phys. Chem. C 2014, 118, 17686−17698
The Journal of Physical Chemistry C
Article
Figure 1. (a) Distinct crystal structures of linear polyethylenimine and (b) chain conformations of the double-helical anhydrate and planar-zigzag hydrated forms. Reprinted with permission from ref 21. Copyright 2002 American Chemical Society.
below (above) ∼64 °C. Therefore, in water at room temperature, LPEI exists as phase-separated dihydrate crystalline fibrous aggregates.35 These experiments also revealed an exceptionally small IR spectral contribution in the C−H stretching region from hydrated amorphous “gel-phase” LPEI in room-temperature (unsolvated) LPEI solutions.31,35 Conversely, this spectral feature is the predominant C−H stretching mode observed in IR spectra of LPEI solutions above the UCST. To date, studies have nearly exclusively focused on elucidating the bulk properties of LPEI. As such, the surfacemediated behavior of LPEI as a function of hydration has not previously been characterized. To characterize the surface structure of LPEI thin films and its humidity-dependent behavior, a technique is needed that is surface specific, capable of probing solid/vapor interfaces in situ, and able to provide molecular specificity. Vibrational sum frequency generation (VSFG) spectroscopy is an established technique that is capable of elucidating surface-specific molecular structure information such as the presence, orientation, and conformation of functional groups at a surface or interface. We therefore utilize broadband vibrational sum frequency generation (BBVSFG) spectroscopy in these investigations to characterize the surface vibrational signatures of thin-film LPEI samples and provide a detailed look into the surface behavior and the surface molecular conformation of the LPEI films. VSFG is a second-order nonlinear effect that requires a break in centrosymmetry to generate a signal because it is dipoleforbidden in centrosymmetric bulk media.36,37 This leads to the sum frequency “selection rule” that signal is only derived from surfaces and interfaces such as at solid/vapor,14,36−38 solid/ liquid,14,15,38−40 and liquid/liquid41−43 interfaces. This selection rule breaks down if the molecules residing at the interface possess a molecular structure that preserves local inversion symmetry. We will show that as the molecular structure of LPEI changes with hydration, the well-established hydrated crystalline states, which possess local inversion symmetry, are largely absent from VSFG surface spectra (in agreement with VSFG theory). IR spectroscopy is used to complement the surfaceselective VSFG studies described herein, and resulting spectra are in agreement with established literature.
water and secondary amine groups in the LPEI chains, instead of directly between the secondary amine groups of LPEI strands (Figure 1). These bulk hydrated phases can reach up to 90% crystallinity.23 Although the all-trans molecular conformation is conserved across all three of the crystalline hydrates, the packing structures of both LPEI and water differ in each and are highly dependent on the ratio of absorbed water molecules to ethylenimine monomer units. The three hydrated crystalline states have ratios of 1:2, 1.5:1, and 2:1 for the hemihydrate, sesquihydrate, and dihydrate forms, respectively. The monohydrate of LPEI is unstable under all known conditions.17,31 Seminal research on bulk LPEI was performed by Chatani et al. with X-ray crystallography to determine the crystalline structures of the individual anhydrate and hydrated crystalline forms.17−19 This research concluded that, for anhydrate crystalline LPEI, hydrogen bonding between each secondary amine on one polymer strand and two secondary amines on a second LPEI strand facilitates the formation of the double-helix structure characteristic of the anhydrate. Following the introduction of water, the solid LPEI sample transitions to the extended all-trans (planar-zigzag) crystalline hydrates via O−H···N and N−H···O bonds with absorbed water molecules. Variations in crystalline structure between the different hydrates are due to differences in the hydrogen-bonding network and the amount of water absorbed into the matrix. As a consequence, each of the three hydrate crystalline states exhibits a distinct water structure that is defined by the number of amine/water molecules that each absorbed water molecule coordinates with. The characterization of these unique properties led to significant interest in the behavior of, and the transitions between, bulk crystalline forms of LPEI as a function of temperature and water content. Investigations into these areas have been performed by utilizing techniques including IR,21−23,31,32 Raman spectroscopy,20,24 wide-angle X-ray diffraction,24,25 small-angle X-ray scattering,24 differential scanning calorimetry,25,33 multiple-angle incidence resolution spectroscopy,34 and infrared external reflection spectroscopy.34 Solution-phase behavior of LPEI has also been studied. As a function of temperature, aqueous solutions of LPEI reveal upper critical solution temperature (UCST) behavior,31,35 in which LPEI is predominantly insoluble (soluble) in water 17687
dx.doi.org/10.1021/jp504321r | J. Phys. Chem. C 2014, 118, 17686−17698
The Journal of Physical Chemistry C
Article
assignments, and for direct comparison to VSFG spectra generated from samples exposed to similar conditions. Spectra were acquired as averages of 4 scans at 2 cm−1 resolution in standard transmission mode. VSFG Spectroscopy. Vibrational sum frequency generation spectroscopy is a second-order nonlinear optical technique that is inherently surface/interface specific due to its selection rules, which in the electric dipole approximation allow for signal generation only where there is a break in centrosymmetry. For this reason, functional groups in centrosymmetric bulk materials generally do not generate VSFG signal. At a surface or interface between two materials, however, inversion symmetry is broken and VSFG signal generation is allowed. As there are many excellent reviews about the theory and application of VSFG spectroscopy,15,44−46 most of the intimate details will not be repeated here. In the electric dipole approximation, the VSFG signal intensity is proportional to the square of a material’s secondorder nonlinear susceptibility, χ(2). The second-order nonlinear susceptibility is comprised of nonresonant and resonant (2) contributions: χ(2) = χ(2) NR + χR . The VSFG signal intensity can therefore be described by
The results of our investigations show that the behavior of the LPEI surface at low relative humidity (
