Surface Dynamics in Rubbed Polymer Thin Films Probed with Optical Birefringence Measurements

Schwab, Alexander D.; Agra, D. M. G.; Kim, Jae-Hoon; Kumar, Satyendra; Dhinojwala, Ali

doi:10.1021/ma9919514

Cited by 87 publications

(101 citation statements)

References 34 publications

(139 reference statements)

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“…9 In particular, good quantitative agreement between the change in T g 19 and the change in T 0 as the PS film thickness is decreased from 50 to 10 nm supports the viewpoint that T 0 is directly related to T g . 8,9 This result, consistent with the previous finding that T 0 has the same M w dependence as T g , 12 suggests that the ␣ relaxation may dominate the observed relaxations of the rubbed PS films near T g . Moreover, at thicknesses greater than 30 nm, the T 0 's of PS films on HOSi are lower than those on SiO 2 surfaces.…”

Section: Resultsmentioning

confidence: 62%

“…Similar behavior has been observed before. 8,9 For PS films on SiO 2 , T 0 becomes smaller for thinner films. For PS films on HOSi, however, T 0 remains the same for all thicknesses.…”

Section: Resultsmentioning

confidence: 99%

“…9,10 It has been observed 8 -11 that substantial relaxation of the mechanically induced molecular alignment starts to occur at temperatures tens of degrees below T g , where most of the ␣-relaxation activities should be quenched. Although the ␤ and ␥ modes are still active, the timescale of the relaxation in rubbed films is on the order of minutes, too long to be accountable by these two modes.…”

Section: Introductionmentioning

confidence: 99%

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Rubbing‐induced molecular alignment and its relaxation in polystyrene thin films

Tsang

Xie

Tsui

et al. 2001

J Polym Sci B Polym Phys

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Rubbing-induced molecular alignment and its relaxation in polystyrene (PS) thin films are studied with optical birefringence. A novel relaxation of the alignment is observed that is distinctly different from the known relaxation processes of PS. First, it is not the Kohlrausch-Williams-Watts type but instead is characterized by two single exponentials plus a temperature-dependent constant. At temperatures several degrees or more below the glass-transition temperature (T g ), the relaxation time falls between that of the ␣ and ␤ relaxations. Second, the decay time constants are the same within 40% for PS with weight-average molecular weights (M w 's) of 13,700 -550,000 Da at temperatures well below the sample T g 's, indicating that the molecular relaxations involved are mostly local within the entanglement distance. Nonetheless, the temperature at which the rubbing-induced molecular alignment disappears (T 0 ) exhibits a strong M w dependence and closely approximates the T g of the sample. Furthermore, T 0 depends notably on the thickness of the polymer in much the same way as previously found for the T g of supported PS films. This suggests that the ␣ process becomes dominant near T g . Preliminary spectroscopic studies in the mid-infrared range show a significant degree of bending of the phenyl ring toward the sample surface, with the COC bond connecting the phenyl ring and the main chain tends to lie along the rubbing direction, which indicates that the relaxation is connected with the reorientation of this COC bond. We exclude the observed relaxation, as predominantly a near-surface one, because detailed studies on the effects of rubbing conditions on the degree of molecular alignment indicate that the alignment is not local to the polymer-air surface.

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Section: Resultsmentioning

confidence: 62%

“…Similar behavior has been observed before. 8,9 For PS films on SiO 2 , T 0 becomes smaller for thinner films. For PS films on HOSi, however, T 0 remains the same for all thicknesses.…”

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Rubbing‐induced molecular alignment and its relaxation in polystyrene thin films

Tsang

Xie

Tsui

et al. 2001

J Polym Sci B Polym Phys

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“…With all the above observations, it is puzzling that hours of annealing at temperature much higher than the T g is needed for PMMA surface to equilibrate, as it is expected that molecules on the surface relax much faster than the ones in the bulk. 25 In addition, there are limited surface studies of PMMA as a function of tacticity, in spite of the impact tacticity may have on the surface properties. 26 Recent SFG study from Tanaka et al 7 concluded that syndiotactic PMMA (syn-PMMA) had both backbone methylene and estermethyl groups ordered at the surface.…”

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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“…24 The mechanical rubbing of polystyrene (PS) surfaces can also produce homogeneous planar LC alignment layers, where the LC molecules are aligned perpendicular to the rubbing direction. [25][26][27][28][29][30][31][32][33] However, this perpendicular LC alignment produces a very low pretilt angle of about 0 o26,31 as well as a weak azimuthal anchoring energy of less than 10 -7 -10 -8 J/m 2 , 31-33 which is too low to produce a stable LC cell for practical applications in LCD devices. The homogeneity of an LC cell fabricated with the rubbed PS film is lost after 1 day.…”

Section: Introductionmentioning

confidence: 99%

The effect of phenoxymethyl side groups on the liquid crystal alignment behavior of polystyrene derivatives

2009

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Abstract:We synthesized a series of polystyrene derivatives containing various side chain terminal moieties, such as phenoxymethyl, 4-methoxyphenoxymethyl, 4-fluorophenoxymethyl, 4-methylphenoxymethyl, and 4-trifluoromethoxyphenoxymethyl groups, using polymer analogous reactions, in order to investigate the effect of the side group on their liquid crystal (LC) alignment behaviors. The polymers containing 4-fluorophenoxymethyl, 4-methylphenoxymethyl, or 4-trifluoromethoxyphenoxymethyl side groups had lower surface energy values and the LC cells fabricated using the unrubbed films of these polymers showed homeotropic LC alignment behavior. The LC cells fabricated using the rubbed films of the polymers containing phenoxymethyl or 4-fluorophenoxymethyl groups showed homogeneous planar LC alignment behavior in which the LCs were aligned perpendicular to the rubbing direction. This homogeneous planar and perpendicular alignment behavior was ascribed to the favorable anisotropic interactions between the LC molecules and the side groups preferentially oriented perpendicular to the rubbing direction.

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Surface Dynamics in Rubbed Polymer Thin Films Probed with Optical Birefringence Measurements

Cited by 87 publications

References 34 publications

Rubbing‐induced molecular alignment and its relaxation in polystyrene thin films

Rubbing‐induced molecular alignment and its relaxation in polystyrene thin films

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

The effect of phenoxymethyl side groups on the liquid crystal alignment behavior of polystyrene derivatives

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