Dependence of the Surface Structure of Polystyrene on Chain

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Dependence of the Surface Structure of Polystyrene on Chain Molecular Weight Investigated by Sum Frequency Generation Spectroscopy Wasef Bzeih, Amara Gheribi, Paula M Wood-Adams, and Patrick L. Hayes J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08574 • Publication Date (Web): 09 Jan 2018 Downloaded from http://pubs.acs.org on January 14, 2018

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The Journal of Physical Chemistry

Dependence of the Surface Structure of Polystyrene on Chain Molecular Weight Investigated by Sum Frequency Generation Spectroscopy

Wasef Bzeih1, Amara Gheribi2, Paula M. Wood-Adams1*, Patrick L. Hayes2*

(1) Department of Mechanical and Industrial Engineering, Concordia University, Montréal, Québec, Canada (2) Department of Chemistry, Université de Montréal, Montréal, Québec, Canada

Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT Sum frequency generation spectroscopy is used to probe the surface structure of monodisperse polystyrene (PS). The amplitudes of the ν20a and ν2 resonances in SSP polarization are found to depend on polystyrene molecular weight between 6 and 102 kDa whereas the amplitude of the ν20b resonance is invariant, which all together indicates a reorientation of the phenyl ring with different chain lengths. The measured resonant amplitudes are consistent with the C2 axis of the phenyl ring lying flat in the surface plane at the lowest molecular weights and tilting towards the surface normal at the highest molecular weights. Such reorientation is supported by the observation that the SFG intensity increases with polymer molecular weight in SSP polarization, but the spectrum exhibits no significant difference when PPP polarization is used. It is clear from these results that molecular weight can influence the surface structure of polystyrene in ways that are important to surface tension and surface segregation of smaller chains. Such understanding is key to providing a fundamental picture of the relationship between polymer molecular weight distribution, local surface structure and macroscopic properties.


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1. Introduction Interfaces, between polymers and their environment and between different constituents in a multiphase polymeric material, play a significant role in macroscopic end-use properties and processing behaviors such as: the crystalline content and morphology, the strength of a composite material, tribological properties, surface appearance properties, slip under flow and rate-limiting production problems such as surface fracture or die drool. While much is known about the surface and interfacial properties of polymers, there is a lack of molecular-level understanding of the relationship between the molecular structure of polymers, especially the molecular weight distribution, and the local structure and composition at interfaces. In liquid mixtures, components with the lowest surface tension segregate at interfaces. For polymers, this is regulated by enthalpic and entropic forces.1 Interfacial segregation in polymers has been observed directly with mass spectrometry2 and spectroscopy3-5 as well as indirectly via surface tension.6 For many polymers, surface tension increases with molecular weight,7 decreases as frequency of pendant groups increases8 and decreases with long chain branching6 although chain end functionalization will also influence the observed surface tension. The direct experimental results showing surface enrichment in the literature are for polymer blends, but there are theoretical results showing that simple polydisperse polymers can exhibit similar segregation behaviour.9-10 This previous work predicts that for a linear polydisperse homopolymer, there is an excess of low molecular weight chains at the polymer melt-air interface. This surface segregation is predicted to increase as the width of the molecular weight distribution increases and is especially dependent on the higher moments of the distribution.10 Macroscopic interfacial tension studies are consistent with such surface segregation occurring in bidispserse polystyrene.6


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In this context, the objective of this study is to utilize sum frequency generation (SFG) spectroscopy to analyze and understand how changes in molecular weight and surface tension influence the structure of the polymer surface. An advantage of SFG spectroscopy is that the technique does not rely on chemical labelling or other approaches that alter surface chemistry of the molecules in the polymer sample. Furthermore, this nonlinear vibrational spectroscopy provides very fine interfacial specificity because only molecules in an environment that breaks the inversion symmetry of the bulk contribute to the SFG spectrum.11-13 For the polystyrene surface, inversion symmetry is only broken at the surface because both air as well as atactic polystyrene have no net alignment and are centrosymmetric for the optical coherence length of the SFG technique.14 SFG provides thus excellent surface specificity and has been used extensively to study both polystyrene as well as other polymer surfaces and interfaces.12-13,15 However, previous SFG-based studies of the connection between chain length and surface structure are very limited, have not explored changes systematically using monodisperse films,16 and have not specifically studied the industrially-important polymer polystyrene. To address this gap in our knowledge of polymers surfaces, we present SFG spectra of the polymer surface for a series of monodisperse polystyrene films, and identify molecular weight dependent characteristics of the spectra. After assigning the resonances to specific vibrational modes, the ratios of the mode amplitudes are used to determine the tilt angle of the C2 axis of the phenyl ring in polystyrene, which then permits evaluation of the reorientation of the polymer surface groups as a function of molecular weight. This work thereby provides a more complete understanding of the polystyrene surface at the molecular-level, and it may serve as a basis for future efforts to develop an experimental method for probing the molecular weight of polymer chains at an interface and surface segregation in bidisperse and polydisperse polymers.


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2. Experimental 2.1 Materials and polystyrene sample preparation. Solutions (2 wt%) of eight monodisperse polystyrenes (Scientific Polymer Products Inc) were prepared in toluene. The molecular weight and polydispersity index of the polystyrenes studied are summarized in Table 1 and were reported by the supplier. Thin films were deposited on glass slides (Bio Nuclear Diagnostics Inc., catalogue number LAB-033, thickness of 1.0 mm) using a spin coater in a clean room. The slides were cleaned with deionized water before film deposition. (Furthermore, the SFG spectra of polystyrene reported below are fully consistent with other studies,15,17-18 and show no indication of contamination.) Spin coating was done at 2000 RPM for 1 minute for every slide. Following spin coating, the samples were carefully transferred to a vacuum oven, and annealed under vacuum for a period of 4 to 5 hours at a temperature of 110ᴼC, which is above the glass transition temperature of all the monodisperse polystyrene samples (105ᴼC or slightly less). The cooling rate after annealing was 2°C min-1. The homogeneity and thickness of a typical film was verified and measured to be approximately 130 nm by analyzing a subset of samples with atomic force microscopy (AFM). The samples for AFM analysis were prepared following the same procedure used for the SFG samples and were then gently scratched with a sharp blade in order to provide a contrasting region without polymer. The similarity of the SFG spectra to those measured in previous studies of polystyrene films, including studies of polystyrene films on 2 nm SiO2 terminated Si substrates by Briggman et al.19, confirms that the spectra measured in our work indeed arise from the polystyrene/air interface. A full set of eight samples with different molecular weights were prepared and processed at the same time, including spin coating and annealing, and thus were handled under identical conditions. After their preparation and annealing, the samples were immediately analyzed by


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SFG spectroscopy in order to minimize sample aging effects. Furthermore, collection of SFG spectra was completed for all samples on the same day.

Table 1. Properties of polystyrene resins used in this study. Nominal molecular weight

MW (kg/mol)

MW/MN

6 kDa

6.3

1.05

13 kDa

13.7

1.06

18 kDa

18.0

1.01

29 kDa

29.3

1.09

48 kDa

48.9

1.01

59 kDa

59.5

1.07

76 kDa

76.2

1.17

102 kDa

102.7

1.04

2.2 Sum frequency generation spectroscopy. In the SFG experiment, visible and infrared lasers with frequencies ωvis and ωIR are overlapped on a sample surface producing a third beam with a frequency that is the sum of the incident frequencies. The vibrational spectrum is then measured by tuning the IR frequency and measuring the intensity at ωvis+ωIR, which increases when the incident IR laser frequency approaches the frequency of a vibrational resonance of a molecule at the surface.20-21 The emitted SFG field, ESFG, is proportional to the second-order nonlinear polarization of the interface, PSFG, as shown in Equation 1 (using SI units). () E ∝ = χ E

(1)

Above, Evis and EIR are the visible and IR electric fields, χ(2) is the second-order nonlinear susceptibility tensor, and εo is the vacuum permittivity. Within the electric dipole approximation,

χ(2) is nonzero only in non-centrosymmetric and ordered environments,22 from which the ACS Paragon Plus Environment

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surface-selectivity of SFG arises. Furthermore, the second-order susceptibility may be written as follows in Equation 2,11,23 where N is the number of molecules per unit area and β is the secondorder hyperpolarizability for which the indices l, m and n correspond respectively to the different possible projections of the SFG, visible and IR fields onto the molecular coordinate system.

χ () = ∑#%&〈R # (θ, ψ, φ)R% (θ, ψ, φ)R & (θ, ψ, φ)β#%& 〉

(2)

In addition, Ril(θ,ψ,φ)Rjm(θ,ψ,φ)Rkn(θ,ψ,φ) is the product of the necessary rotation matrices using all three Euler angles to convert from the molecular to surface coordinates. The indices i. j and k denote the different 27 elements of the susceptibility tensor. The brackets in the equation above indicate that the second-order hyperpolarizability is averaged over the molecular orientations at the surface. Quantum mechanical expressions for β have been derived for SFG spectroscopy, and a simplified version of the general equation specific to the SFG experiment is shown here where ωIR is near a vibrational resonance and ωvis is far from an electronic transition.11,23 (

β#%& = ħ (ω

*+, -.

/ 0ω12 0 Γν )

(3)

In Equation 3, ων is the frequency of the resonant vibrational mode, and Γν is a damping constant. Furthermore, M and A are the Raman and IR transition moments, respectively. Thus the second-order hyperpolarizability can be calculated if the Raman and IR transition moments are known. To analyze SFG spectra empirically, it is often more practical to express the product of the transition moments as a single value, the amplitude, as shown in Equation 4. The SFG intensity is then taken to be the combination of a constant non-resonant background contribution


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and a series of Lorentzian functions representing the frequency-dependent resonant contributions.11 I (ω ) = 6A8 9 φ + ∑ν

;ν

20°) are not consistent with the experimental results, indicating the polystyrene surface structure is relatively well-ordered. For the lowest molecular weight polystyrene sample, the tilt angle is closer to 90° from the surface normal or the distribution of tilt angles is narrower or both differences exist at the same time. These two changes in surface structure, which are inferred from the ratio of the ν2 and ν20b mode amplitudes, are also consistent with the decreasing (increasing) intensity in the SSP spectra with decreasing (increasing) molecular weight. From the SFG results, it is clear that the molecular weight can influence the surface structure of polystyrene films in way that has important implications for surface tensions as well as surface segregation of smaller chains. Furthermore, the differences in orientation are consistent with previously hypothesized increases in excess free volume at polystyrene surfaces with decreasing molecular weight, which may allow an orientation of the phenyl ring C2 axis along the surface plane without unfavorable steric interactions with other functional groups in the lateral direction. While this current work is focused on monodisperse samples, it may be possible in future studies to use SFG spectroscopy and tilt angle analysis to evaluate surface


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segregation due to molecular weight differences in bidisperse or polydisperse polymers. While molecular weight driven surface segregation has been predicted in theoretical studies,10 experimental confirmation has been limited to studies of mixtures of distinct chains that were probed by tagging chains with deuterium atoms or fluorine atoms.3 The use of tagged polymer molecules, which necessarily perturbs the polymer surface chemistry, influences the segregation of the chains and is not suitable for the ultimate goal of isolating and observing the influence of the molecular weight distribution on polymer surface segregation. Finally, it would be interesting in the future to expand the SFG studies described here to other interfaces such as the sapphire/polymer and fused silica/polymer interfaces, where the molecular-weight dependent differences in molecular orientation could be evaluated and corroborated using both SFG as well as other interface specific spectroscopic techniques such as total internal reflection Raman (TIRRaman) spectroscopy.34-35

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Full set of SFG spectra collected for all eight monodisperse polystyrene samples using SSP and PPP polarization. Parameters obtained after the SSP and PPP spectra were fitted using Equation 4.


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AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS. This work was partially supported by the Natural Science and Engineering Research Council of Canada Discovery Grant Program (RGPIN/05002-2014 and RGPIN/218056-2012), the Canada Foundation for Innovation (Project 32277), as well as Concordia University and the Université de Montréal.

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TOC Graphic Phenyl ring orientation changes with MW

ϕ

ψ

θ


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Dependence of the Surface Structure of Polystyrene on Chain

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