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Preferential Organization of Methacrylate Monomers and Polymer Thin Films at the Air Interface Using Femtosecond Sum Frequency Generation Spectroscopy Katherine Leslee Asetre Cimatu, Stephanie Chong Chan, Joon Hee Jang, and Kasey Hafer J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b07688 • Publication Date (Web): 15 Oct 2015 Downloaded from http://pubs.acs.org on October 16, 2015

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The Journal of Physical Chemistry

Preferential Organization of Methacrylate Monomers and Polymer Thin Films at the Air Interface Using Femtosecond Sum Frequency Generation Spectroscopy Katherine A. Cimatu,*,1 Stephanie C. Chan,1 Joon Hee Jang,2 Kasey Hafer1 1

Department of Chemistry and Biochemistry, 100 University Terrace, 136 Clippinger

Laboratories, Ohio University, Athens, Ohio, 45701-2979, USA. 2

Department of Nanomedicine, Houston Methodist Research Institute, 6670 Bertner Ave.,

Houston, Texas, 77030, USA. Corresponding Author * Katherine A. Cimatu, Ohio University, Department of Chemistry and Biochemistry, 295 Clippinger Laboratories, Athens, Ohio 45701; telephone (740) 593-2308; fax (740) 593-0148; email [email protected]

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Abstract Monitoring and understanding monomer conformational changes as substituents are varied is an important step in investigating in-situ polymerization and controlling the polymerization process as well as regulating the functionality of polymer surfaces to provide efficient polymer coatings. In this study, the preliminary stage involves characterization of the interfacial structures of methacrylate-based functional monomers and their polymer thin films at the air-liquid interface using femtosecond sum frequency generation spectroscopy (FSFGS). By varying the substituted ethyl group of the methacrylate monomers with hydroxy (-OH), chloro (-Cl) and phenoxy (OPh), the alpha-methyl (α-CH3), alkene-methylene (alkene-CH2), and methylene (CH2) groups are observed to have preferential surface ordering toward the air interface. A peak positioned at ~3000 cm-1 was observed in the spectrum of the 2-hydroxyethyl methacrylate (HEMA) monomer and assigned to alkene-CH2 group. This functional group signifies that the pure monomer has not undergone polymerization. However, the SFG spectra of the polymer versions of these monomers revealed that for the poly(2-hydroxyethyl methacrylate), the α-CH3 group dominated the surface; in both poly(2-chloroethyl methacrylate) and poly(2-phenoxyethyl methacrylate), the α-CH3 and CH2 groups were both segregated at the interface. These observations on the conformational changes of the monomer units and polymer thin films indicated that substitution of the ethyl group of the methacrylates affected the behavior of the interfacial molecules.

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Introduction Polymers are of interest because of their applications: functioning as adhesives and lubricants to coatings for biomedical devices and petroleum industry. The chemical structures of polymers are routinely modified to enhance and improve chemical, physical, and mechanical properties of polymers at the bulk. However, methods such as polymer blending and grafting can also affect the interfacial properties of polymer surfaces. Numerous studies have investigated the molecular structures of polymer surfaces and the response of these molecular interfacial interactions at their air, liquid, and solid interfaces.1-4 Despite these studies, there is still a gap in the understanding of the mechanism of monomer units transitioning to oligomers and polymers. Monomers are the building blocks of polymers, thus it is important to investigate monomer units of varied substituents as a step to understand how their conformation at the interface affects polymerization and polymer surfaces. For this study, a preferential organization of monomer units is observed as a function of varied functional groups. In addition, the polymer thin films of the substituted monomers are also investigated to observe any differences in the composition or ordering at the interface, in comparison, with the results obtained from monomers. Specifically, it becomes necessary to study the methacrylate-based functional monomers and understand their organization at air interface when the substituted ethyl terminal group is varied while α-methyl (α-CH3) and unreacted alkene-methylene (alkene-CH2) groups in the backbone structure remain unchanged. Among other surface-sensitive techniques used to study polymer surfaces,3,

5-11

sum

frequency generation (SFG) spectroscopy has interface-specificity, chemical sensitivity, nondestructive and labelling-free characteristics. This makes SFG a suitable characterization tool to study the interfacial molecules of the selected methacrylate monomers and their molecular 3



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conformation. SFG studies have been performed on specific polymer surfaces to compare the interaction

of

polymers

interact

with

different

interfaces,

e.g.,

air/polymer

and

polymer/substrate1, 4, 8, 12-13. For example, the phenyl ring of polystyrene was found to orient parallel with respect to the surface normal at the polystyrene/air interface, while for a polystyrene/sapphire interface, the phenyl ring was oriented perpendicularly.14 In addition to polymer/substrate interfaces, surface ordering of polymers with liquids have also been investigated.15-16 One experiment reported the interaction of poly(butyl methacrylate)(PBMA) with air and water and observed that methyl group of the ester side chain of PBMA surface is more ordered when in contact with water because of surface restructuring.17 In this study, neat monomers at the air interface are characterized by using substituted monomers that can affect the conformation of each monomer unit at the interface. SFG technique is used as a spectroscopic tool to help identify the molecules at the interface. The polymer thin films of these substituted monomers were also prepared and characterized to investigate any differences on surface ordering of the interfacial molecules. Another motivation is to observe whether hydrogen bonding, and/or dipole-dipole molecular interactions contribute to the molecular conformation of the interfacial molecules by halide- and bulky substitution. In the experiment, the substituted ethyl methacrylate (EMA) with –OH group will be investigated as a control for the orientation of the monomer; most studies were performed using poly(2hydroxyethyl methacrylate) (polyHEMA), which suggested that the –OH group has preferential orientation towards the bulk.8, 18 The other two substituted ethyl methacrylate molecules will also be characterized containing –Cl and –OPh groups for 2-chloroethyl methacrylate (CIEMA) and 2-phenoxyethyl methacrylate (PhEMA) monomers, respectively, and are chosen to affect molecular conformation changes at the interface structurally by dipole-dipole intermolecular 4


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interactions. As shown in Figure 1A, the substitution is positioned at the ethyl group of the methacrylate monomer while the methyl group and methylene groups are assigned as the α-CH3 and alkene-CH2 group of the methacrylate monomer, respectively. Figure 1B shows a substituted polymer; again, X is the position of substitution. In the future experiments, this study will be expanded to understand whether electronic and/or steric effects will have a contribution in the behavior of the interfacial molecules by varying the substituents, as described in Figure 1. It is worth noting that available literatures are also very limited in studying the characterization of ClEMA19-25 and PhEMA26-27 as monomers and even as polymers, so it will be interesting to characterize and investigate the behavior of these substituted methacrylate monomers and polymers. In order to confirm monomer structure and vibrational peak assignments, IR, Raman, and nuclear magnetic resonance (NMR) spectra of HEMA, ClEMA, and PhEMA monomers are also presented.

A

B

Figure 1. A. ChemDraw representation of methacrylate-based functional monomer where a. α-CH3 refers to methyl group at the methacrylate side, b. alkene-CH2 refers to methylene group at the methacrylate side, and c. is the substituted ethyl group where X = OH, Cl, and OPh. B. A ChemDraw representation of a methacrylate-based functional polymer.

Theoretical Background SFG spectroscopy, a second-order nonlinear vibrational technique, probes the chemical structure and degree of conformational changes of the interfacial molecules. This powerful technique has the ability to distinguish oriented molecules at the surface from those of the bulk, wherein the SFG signal can only be generated from an environment that lacks inversion symmetry. The 5



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molecules tend to preferentially segregate at the interface where the forces on the molecules are asymmetric. Thus, this surface technique is ideal for investigating molecular species at different interfaces such as air, liquid, and solid in the presence of a bulk phase.28-31 To generate a broadband SFG signal, a fixed narrow bandwidth 795 nm beam is spatially and temporally overlapped at the sample surface with a broad bandwidth mid-infrared beam.32-34 The sum of both frequencies allows the generation of SFG photons. The intensity of the SFG signal is directly proportional to the square of second-order induced polarizability, P(2). P(2) can be further described by the relationship of the second-order nonlinear effective susceptibilities with the incident fixed 795 nm and mid-IR beams, as shown in Equation 1: 2

(2) (2) 𝐼(𝜔𝑆𝐹 ) ∝ |∑𝑖 𝑃𝑖,𝑗,𝑘 = ∑𝑖 𝜒𝑒𝑓𝑓 ∑𝑗,𝑘 𝐸𝑗 𝐸𝑘 |

(1) (2)

where i, j, and k coordinates are the representations of x, y, and z axes. 𝜒𝑒𝑓𝑓 is a third-rank tensor (2)

that contains the Fresnel factors and macroscopic second-order nonlinear susceptibilities, 𝜒𝑖𝑗𝑘 , at the interface. Each of the 27 elements of macroscopic susceptibility can be defined in Equation 2: (2)

(2)

𝜒𝑒𝑓𝑓,𝑖𝑗𝑘 = 𝜒𝑖𝑗𝑘 [𝐿𝑖 . 𝑒𝑖 ][𝐿𝑗 . 𝑒𝑗 ][𝐿𝑘 . 𝑒𝑘 ]

(2)

where L is the Fresnel factor and e is the unit optical field vector. There are 27 elements involved in macroscopic susceptibility, but due to the symmetry of the surface, some of the elements tend to vanish. (2)

𝜒𝑒𝑓𝑓 also contains the measurable vibrational and electronic information as a result of the (2)

response of the molecules to the electric fields. Thus, as shown in Equation 3, 𝜒𝑒𝑓𝑓 is composed (2)

of 1) the resonant term, 𝜒𝑅 , that is due to the vibrational transitions of the molecules at the 6


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(2)

(2)

interface, and 2) the nonresonant term, 𝜒𝑁𝑅 , originating from the substrate. 𝜒𝑁𝑅 is independent of the mid-IR frequency and its value is almost negligible for liquid and dielectric surfaces. (2)

(2)

𝜒𝑒𝑓𝑓 (2) = 𝜒𝑅 + 𝜒𝑁𝑅

(3) (2)

On the other hand, the resonant susceptibility, 𝜒𝑅 , can be further described by Equation 4 to incorporate the Lorentzian line profile for spectral fittings. This is represented by N as the number density of modes and the orientational averaged hyperpolarizability, 𝛽 (2) . Hyperpolarizability can also be described as the product of IR and Raman transition dipole moments. (2)

𝜒𝑅 =

𝑁〈𝛽 (2) 〉 𝜔𝐼𝑅 − 𝜔𝑞 +𝑖Γ𝑞

(4)

ωIR, ωq,, and the Γq correspond to the IR frequency, the frequency of the vibrational normal mode, and the damping constant of the qth vibrational mode, respectively. The denominator of the equation can also be explained this way: If the IR frequency matches any of the vibrational transitional frequency/ vibrational mode stretches of the interfacial molecules, the difference in the denominator approaches zero, which in effect resonantly enhances the SFG signal. The SFG intensity signal is therefore plotted as a function of IR frequency generating an SFG spectrum.28, 30, 35-38

Gaining information from SFG spectroscopy has been very successful in characterizing

the chemical identification and orientation of the molecules present at the surface.39-40 Experimental Methods Sample Preparation Gold, Octadecanethiol on Gold Thin Film and Eicosanoic Acid on Water. The detailed procedure for gold (Au) thin film, octadecanethiol (ODT) on Au and eicosanoic acid on water is

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available in the Experimental Section of the Supporting Information. These experiments were performed prior to the characterization of monomers and polymer thin films. Methacrylate Monomers. 2-hydroxyethyl methacrylate (≥99% contains ≤50 ppm mono methyl ether hydroquinone, MEHQ), 2- chloroethyl methacrylate (97% contains 500 ppm hydroquinone, HQ), and 2-phenoxyethyl methacrylate (97% contains 200 ppm HQ and 200 ppm MEHQ) were purchased from Sigma Aldrich, Alfa Aesar, and Polysciences, Inc., respectively, and used as received. The MEHQ and HQ are commonly used inhibitors to prevent polymerization of the monomers. 1H-NMR experiments were performed to ensure structure and purity of the monomers. Figure S1 in the Supporting Information shows the NMR spectra and peak assignments of the methacrylate monomers. A clean petri dish was used as a reservoir to hold the sample. The petri dish was boiled with micro-90 cleaning solution, then washed copiously and boiled with Milli-Q 18 MΩ water twice before being placed in the oven to dry. The petri dish was removed from the oven only when it was about to be used for the SFG experiment. Approximately 50-mL of the methacrylate monomers was placed in an open system of petri dish to make certain that the alignment at the detection side of the SFG spectroscopy setup was not affected by the formation of a liquid meniscus along the walls of the sample dish. The SFG area of the spectroscopic setup was semi-enclosed with black hardboard and blackout fabric with rubberized coating to reduce air current. The room temperature was 23 °C and the relative humidity was in the average of 20% when the spectra were collected. Polymerization Process. The monomer 2-chloroethyl methacrylate (97% with 500 ppm HQ) was obtained from Alfa Aesar and 2-phenoxyethyl methacrylate (200 ppm HQ and 200 ppm MEHQ) was obtained from Polysciences. The radical initiator, azobisisobutyronitrile (98% purity), was obtained from Sigma-Aldrich. The solvents used were 2-butanone (97% purity) and 8


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methanol, which were purchased from Alfa Aesar and Fisher Scientific, respectively. The monomers were polymerized using 0.25% by weight of azobisisobutyronitrile (AIBN) as the initiator in the presence of 2-butanone at 70 ºC. Afterward, the polymers were acquired through precipitation by adding 10 ml of methanol. The polymers were then vacuum-dried.41-43 Thin Film Polymerization. PolyHEMA (MW=20,000) was purchased from Sigma-Aldrich. The solvent used for to prepare a 1% polyHEMA was ethanol, which was obtained from PharmcoAAPER. On the other hand, the other two polymers were dissolved inchloroform to prepare 1% poly(2-chloroethyl methacrylate) (polyClEMA), and 0.5% poly(2-phenoxyethyl methacrylate) (polyPhEMA. The polymer solutions were then spin-coated (MTI Corporation, VTC-100 Vacuum Spin Coater) at 3000 rpm for 30 seconds [1] on a clean fused silica (25.4 mm in diameter and 3.18 mm thickness) bought from ESCO Products. All the polymer films were annealed at 80 ºC for 12 hours.13, 44-45 IR and Raman Spectroscopic Techniques. The detailed procedure in performing both IR and Raman spectroscopic techniques is available in the Experimental Section of the Supporting Information. Femtosecond SFG. Instead of utilizing a picosecond laser to gain a narrow-bandwidth fixed visible beam32, the custom-built broadband femtosecond SFG setup used a Fabry-Perot etalon to convert the broadband 795 nm (100 fs) to a picosecond narrow-bandwidth beam. The etalon provided a spectral resolution of ~7 cm-1 for the SFG beam. The time-asymmetric pulse that allowed the typically broad nonresonant background can also be suppressed by introducing a time delay (τ) between the visible and infrared pulses. 33-34, 46-50 In our laboratory, the sum frequency generation spectra are acquired using a broadband SFG scheme34, 47-49, 51 in which the femtosecond laser is a one-box ultrafast amplifier system 9



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(Solstice) from Spectra-Physics consisting of a regenerative amplifier (RGA) seeded by a Mai Tai oscillator with a tunable wavelength centered at 795 nm at

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