Orientation of Phenyl Rings and Methylene Bisectors at the Free Surface of Atactic Polystyrene

Clancy, Thomas C.; Jang, Jae Hoon; Dhinojwala, and Ali; Mattice, Wayne L.

doi:10.1021/jp011588e

Cited by 55 publications

(78 citation statements)

References 24 publications

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“…10 The typical uncertainties in the surface energies in our method are (10 erg/cm 2 . 6 The plateau density, F 0 , is also close to the bulk density expected by considering the composition of the chain, and the correlation length, , is comparable to those obtained previously for other amorphous polymeric hydrocarbons. 44 Later on we will also compare our results for the aPP surface with those obtained by Mansfield and Theodorou.…”

Section: Resultssupporting

confidence: 87%

“…[1][2][3][4][5] In general, it is reported that, for all systems investigated so far, side groups are found to orient outward and normal to the free surface. Simulation results 6,7 on selected homopolymers are in agreement with the experimental results by SFG. Recently, 5 SFG has also been used to study surfaces of random poly(ethylene-co-propylene) with a focus on understanding the effect of changing ethylene fraction on the surface structure.…”

Section: Introductionsupporting

confidence: 82%

“…When applied to atactic polystyrene, this approach gave excellent agreement with experimental results. 6 In the present simulation of the ethylene-propylene copolymers, the film is first equilibrated on a high coordination lattice for 40 million MCS using the standard Metropolis criterion for acceptance of moves. The length of the simulation is chosen well-exceed the equilibration time, which is monitored in two ways: first by making sure that the mean square displacement of both kinds of beads in the chain is of the order of the thickness of the film and second the autocorrelation function of the endto-end vector, 〈r(t + t 0 )‚r(t 0 )〉, goes to zero.…”

Section: Methods and Simulation Detailsmentioning

confidence: 99%

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Atomistic Simulation of Orientation of Methyl Groups and Methylene Bisectors, and Surface Segregation, in Freely Standing Thin Films of Atactic Poly(ethylene-co-propylene)

2004

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Atomistically detailed models of free-standing thin films of random poly(ethylene-co-propylene) were produced using a method recently applied to atactic polystyrene (Clancy, et al. J. Phys. Chem. B 2001, 105, 11493). Monte Carlo simulations of the copolymer were carried out at ethylene fractions that cover the entire range of composition investigated in a recent experimental work (Opdahl, et al. J. Phys. Chem. B 2002, 106, 5212) based on sum-frequency generation (SFG) spectroscopy. We find that there is a minor enrichment of methyl groups on the surface, as compared to the bulk composition. Interestingly, the orientation distribution of methyl and methylene units is not unimodal. For methyl units, there is about 50% probability of orientation perpendicular to the surface. The other 50% are randomly oriented. For the methylene bisector, the simulation shows that the distribution has two broad peaks at opposite angles (at 180°apart) to the surface normal. More conclusive results may be obtained by using larger sample sizes and analysis of a larger number of snapshots, both of which increase the computation time significantly. The opposite angle pairs nearly cancel each other in the computation of an average order parameter and give us a misleading net result that the bisectors are randomly oriented. With increasing ethylene fraction, the orientation distribution of methyl and methylene units tends to become more uniform; however, this change is small. The strong average orientation of methyl groups and weak average orientation of methylene groups is in agreement with experiments. However, the simulation does not show the significant relative increase in the surface methyl groups contributing to the SFG signal as compared to the bulk concentration or an increase in average orientation of methylene units, with an increase in the ethylene fraction, as inferred from the experimental results. We find that the raw experimental data obtained with an increase in ethylene fraction can be explained by the changes in the number of methyl and methylene groups on the surface rather than the increase in surface excess of the methyl groups contributing to the SFG signal, trans cancellation, and increase in orientation of methylene bisectors as deduced from the experiments.

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Section: Resultssupporting

confidence: 87%

Section: Introductionsupporting

confidence: 82%

Section: Methods and Simulation Detailsmentioning

confidence: 99%

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Atomistic Simulation of Orientation of Methyl Groups and Methylene Bisectors, and Surface Segregation, in Freely Standing Thin Films of Atactic Poly(ethylene-co-propylene)

2004

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“…Their finding is consistent with molecular dynamics simulations [45], and with earlier results from SFG spectroscopy [37,38]. All of these previous studies attribute the observed anisotropy entirely to the phenyl rings because of their highly anisotropic bonding.…”

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confidence: 90%

Molecular anisotropy effects in carbonK-edge scattering: Depolarized diffuse scattering and optical anisotropy

Stone

Kortright

2014

Phys. Rev. B

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Some polymer properties, such as conductivity, are very sensitive to short-and intermediaterange orientational and positional ordering of anisotropic molecular functional groups, and yet means to characterize orientational order in disordered systems are very limited. We demonstrate that resonant scattering at the carbon K-edge is uniquely sensitive to short-range orientation correlations in polymers through depolarized scattering at high momentum transfers, using atactic polystyrene as a well-characterized test system. Depolarized scattering is found to coexist with unpolarized fluorescence, and to exhibit pronounced anisotropy. We also quantify the spatially averaged optical anisotropy from low-angle reflectivity measurements, finding anisotropy consistent with prior visible, x-ray absorption, and theoretical studies. The average anisotropy is much smaller than that in the depolarized scattering and the two have different character. Both measurements exhibit clear spectral signatures from the phenyl rings and the polyethylene-like backbone. Discussion focuses on analysis considerations and prospects for using this depolarized scattering for studies of disorder in soft condensed matter.

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“…Previous MD study on atactic polystyrene surface revealed that the methylene groups were weakly ordered and was able to explain why the dominant signal in the SFG spectra was from the side chain phenyl groups. 27 MD analysis in conjunction with SFG measurements helped in explaining the strong SFG signal associated with the ordering of poly(dimethylsiloxane) (PDMS) methyl groups in contact with the methyl head groups of the self-assembled monolayer. 28 In addition to polymer surfaces, MD has also been used to explain the SFG spectra of interfacial water 29 and aqueous solutions.…”

Section: ■ Introductionmentioning

confidence: 99%

Molecular Structure of Poly(methyl methacrylate) Surface II: Effect of Stereoregularity Examined through All-Atom Molecular Dynamics

et al. 2014

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The chemical composition and molecular structure of polymeric surfaces are important in understanding wetting, adhesion, and friction. Here, we combine interfacesensitive sum frequency generation spectroscopy (SFG), allatom molecular dynamics (MD) simulations, and ab initio calculations to understand the composition and the orientation of chemical groups on poly(methyl methacrylate) (PMMA) surface as a function of tacticity and temperature. The SFG spectral features for isotactic and syndiotactic PMMA surfaces are similar, and the dominant peak in the spectra corresponds to the ester-methyl groups. The SFG spectra for solid and melt states are very similar for both syndiotactic and isotactic PMMA. In comparison, the MD simulation results show that both the ester-methyl and the α-methyl groups of syndiotactic-PMMA are ordered and tilted toward the surface normal. For the isotactic-PMMA, the α-methyl groups are less ordered compared to their ester-methyl groups. The backbone methylene groups have a broad angular distribution and are disordered, independent of tacticity and temperature. We have compared the SFG results with theoretical spectra calculated using MD simulations and ab initio calculations. Our analysis shows that the weaker intensity of α-methyl groups in SFG spectra is due to a combination of smaller molecular hyperpolarizability, lower ordering, and lower surface number density. This work highlights the importance of combining SFG spectroscopy with MD simulations and ab initio calculations in understanding polymer surfaces. ■ INTRODUCTIONOne of the simplest and still commonly used technique to characterize surfaces of polymers is the measurement of water contact angle. Although this is a very simple technique to judge the wettability of surfaces, it lacks molecular level sensitivity to identify the chemical composition and molecular structure of surfaces. In addition, the rearrangement of molecules at the surface after exposure to water complicates the interpretation of contact angle results. 1 X-ray photoelectron spectroscopy (XPS), a vacuum-based technique, provides excellent information regarding the elemental composition present at the surfaces. However, to identify different hydrocarbon groups (or bondings) is not trivial and relies on subtle shifts of carbon binding energies in the high-resolution XPS analysis. 2 Attenuated-total-reflectance infrared can be used to pick up molecular fingerprints, but this technique samples a much thicker layer than the surface region that affects wetting, surface energy, and friction. 3 In the past two decades, surface-sensitive infrared−visible sum frequency generation spectroscopy (SFG) has been used to study polymer surfaces, 4−7 polymer/liquid interfaces, 7−11 polymer/metal or metal oxide interfaces, 5,12−15 and polymer/polymer interfaces. 12,16−19 SFG is a second-order nonlinear optical technique, and under electric dipole approximation, the SFG signal is only generated by the ordered molecules at surfaces or interfaces and the contribution from ...

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Orientation of Phenyl Rings and Methylene Bisectors at the Free Surface of Atactic Polystyrene

Cited by 55 publications

References 24 publications

Atomistic Simulation of Orientation of Methyl Groups and Methylene Bisectors, and Surface Segregation, in Freely Standing Thin Films of Atactic Poly(ethylene-co-propylene)

Atomistic Simulation of Orientation of Methyl Groups and Methylene Bisectors, and Surface Segregation, in Freely Standing Thin Films of Atactic Poly(ethylene-co-propylene)

Molecular anisotropy effects in carbonK-edge scattering: Depolarized diffuse scattering and optical anisotropy

Molecular Structure of Poly(methyl methacrylate) Surface II: Effect of Stereoregularity Examined through All-Atom Molecular Dynamics

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