Local Structure Contributions to Surface Tension of a StereoregularPolymerKshitij C. Jha, Ali Dhinojwala, and Mesfin Tsige*

Department of Polymer Science, The University of Akron, Akron, Ohio 44325, United States

*S Supporting Information

ABSTRACT: We have used all-atom molecular dynamics (MD) simulations to calculate the surface tension of melt poly(methylmethacrylate) (PMMA) as a function of tacticity. Computation of surface tension using the Kirkwood-Buff approach requiredhundreds of nanoseconds of equilibration. The computed slopes of surface tension versus temperature are in very goodagreement with reported experimental values. Using a rigorous treatment of the true interface, which takes into account themolecular roughness, we find that isotactic PMMA, in comparison to syndiotactic and atactic PMMA, shows a larger surfaceconcentration of polar ester-methyl and carbonyl groups on the surface versus nonpolar α-methyl groups. A mechanistichypothesis based on the helical nature of the isotactic PMMA chains, their relative flexibility, and their reported conformationalenergies is proposed to explain the trends in composition near the surface. We highlight here how surface composition andsurface tension are controlled by both polarity and steric constraints imposed by tacticity.

The conformations and configurations of poly(methylmethacrylate) (PMMA) stereoisomers have been topicsof substantial research interest in order to gain insight into theeffect of tacticity on the physical properties of this commonpolymer.1−5 Energetically unfavorable overlaps between theoxygens of the carbonyl and ester-methyl groups for differentstereoregular forms provides a considerable barrier betweentrans−trans and trans−gauche conformers of the backbonechain.1,2,6 In addition, the cooperative motion of the sidegroups7 contributes to chain mobility and flexibility, whichleads to comparatively large differences in glass transitiontemperature,3 dipole moments,8 and characteristic ratios1

between isotactic and syndiotactic forms. Here, we have usedan all-atom molecular dynamics (MD) approach to study thedifferences in surface tension of free-standing thin films ofPMMA as a function of tacticity. Ultrathin stereoregularPMMA films find applications in lithographic coatings andresists, geodesic lenses, nanoreactors, micro RNA detectors,and LEDs (where they enhance the LED quantumefficiency).9−11 Chemically modified forms of acrylates arealso used as biomaterials12−14 and advanced filtrationmembranes.15−17 Knowledge of near-surface concentration offunctional groups as a function of tacticity is required, as the

surface plays a much more critical role compared to the bulkproperties as thickness decreases.In this paper, we have implemented several important

improvements for MD simulations of thin PMMA films. First,we have used force fields and parameters that have beenvalidated by comparing with the surface structures determinedusing infrared-visible sum frequency generation spectrosco-py.18,19 Second, we ran simulations for hundreds of nano-seconds to calculate equilibrated surface tension values as afunction of temperature using the Kirkwood-Buff approach(augmented by a tail correction to account for long-rangeinteractions). Third, we calculated the surface excess by using atrue interface that takes into account the roughness of thesimulated PMMA surface. The effect of surface properties ofpolymer surfaces on toxicity20 and biocompatibility21 arecontrolled by the orientation and surface concentrations ofpolar and nonpolar groups.22,23 This new approach provides uswith a key insight into how chain conformations control thesegregation of chemical groups, and how this leads to subtle

Received: August 27, 2015Accepted: October 22, 2015Published: October 23, 2015

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changes in surface tension. The simulation provides quantifi-cation that is often challenging to obtain through surface-sensitive spectroscopy and contact angle measurements. Inparticular, the contact angle measurements limit interpretationson near-surface composition in vacuum due to surfacereconstruction in contact with liquids.All-atom molecular dynamics (MD) were performed on

PMMA systems, of all three tacticities, comprising 40-monomerchains with an initial surface area of about 74 × 74 Å2 and a filmthickness of about 90 Å. All structures were generated initiallythrough the Discover module of Materials Studio 6.0 fromAccelrys Inc. The chain length and box dimensions werechosen for optimal computational efficiency, allowing enoughsurface area for an individual chain to fully adsorb on thesurface, sufficient thickness to prevent chains from spanning thevertical dimension of the systems, and allow for atacticconformer to have random placement of side groups, with anoverall dyad replication probability of 0.5. To check theappropriateness of box dimensions, MD simulations werecarried out for systems of overall greater dimensions (100 ×100 × 100 Å3), as well as one with double the thickness (74 ×74 × 180 Å3). The results obtained were similar to those for thesmaller systems used in the current study. Total number ofchains for our box size was 64 for each of the systems.Energetically stabilized melt structures were obtained throughdynamics at 600 K with equilibrated bulk densities. After filmformation from bulk the structures were cooled to 300 K with acooling rate of 20 K/2 ns in the NVT ensemble. Selecttemperatures (500, 520, 540, 560 K) were run for additionalhundreds of nanoseconds. Further details on equilibration andfilm formation can be found in our previous work.19 Particle−particle particle-mesh (PPPM) Ewald summation with 10−5

accuracy for electrostatic interactions was used. A 12 Å cutofffor van der Waals summation using the 12−6 potential wasimplemented. Integration time-step of 1 fs was used for allsimulations. Periodic boundary conditions were maintained inall three directions, and removed in the z-direction afterequilibration to obtain a film. The OPLS force field24 has beenshown repeatedly to accurately reproduce the dynamics,structure and relaxation behavior of single-component systemsof PMMA as well as blends.25−28 Modifications to the OPLS-AA force field with a more accurate torsion potential for estergroups29 yield structure and dynamics properties that are closeto those obtained through neutron scattering.30 The modifiedversion has been used in the current study.The surface tension is computed following the Kirkwood-

Buff theory:31

γp =L2

z[⟨pz⟩ −⟨ ⟩ + ⟨ ⟩p p

2x y ], where Lz is the length of the

simulation box in the z-direction, divided by 2 to account for

symmetry of the two surfaces and ⟨pz⟩ and⟨ ⟩ + ⟨ ⟩p p

2x y represent

the time averaged normal and tangential pressure components,respectively.The plot of surface tension as a function of time is shown in

Figure 1. To reach a well-equilibrated value of surface tensionrequires hundreds of nanoseconds of simulation time, and itcan be observed from Figure 1 that at least the first 100 ns arenot useful for statistical analyses. Previous MD simulations ofbulk PMMA have pointed out the need for large equilibrationtimes.32 This is one of the reasons why a number of simulationstudies, that compare the effect of PMMA tacticity onenergetics and dynamics, have not attempted to compute

surface tension. Most of the prior work in the field utilizessimulated trajectories over tens of nanoseconds or less.32−36

Our work utilizes trajectories for a cumulative simulation time,for all systems and temperatures, of more than 4 μs (Figures 1and 2). Tail corrections to the surface tension values in Figure 1

are required to account for long-range interactions, and werecomputed from the mass density profile (ρ(z)),6 with ρ(z)being fit to an error function, the details of which can be foundin our previous publication.37 The final, tail-corrected values ofsurface tension for PMMA at various temperatures are shownin Figure 2.The surface tension of isotactic (iso-) PMMA is higher than

both syndiotactic (syn-) and atactic (atac-) PMMA. Forinstance, at 500 K, the surface tension values for iso-, syn-,and atac-PMMA are 26.5, 25.6, and 24.9 dyn/cm. Thedifferences in surface tension lie outside a maximum error of±0.15 dyn/cm. Using a linear fit (shown by solid lines in Figure1), the values of the slope of surface tension versus temperatureis found to be −0.076 (±0.007) for atactic, −0.081 (±0.004)for syndiotactic, and −0.085 (±0.003) for isotactic PMMA,respectively. Wu et al.,38 using a pendant drop method,measured the slope of surface tension versus temperature to be−0.076, which is in very good agreement with the MD results.

Figure 1. Accumulated time averaged surface tension vs time for iso-(black), syn- (green), and atac- (blue) PMMA at 540 K.

Figure 2. Surface tension vs temperature for iso- (squares), syn-(circles), and atac- (triangles) PMMA after tail corrections. Maximumerror values are within the symbol size (±0.15 dyn/cm). Broken linesare linear fits to the data.

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The experiments were carried out in a window of 373−453 Kand did not analyze the effect of tacticity on surface tension.Temperatures exceeding 500 K in MD simulations aresufficiently above the glass transition for all three tacticities tohave the requisite chain mobility to reach equilibrium states. Adecrease in temperature of a few degrees would extend thesimulation time required for equilibration to the point whereall-atom MD would be infeasible for computing surface tension.The linear slope of surface tension versus temperature is veryuseful in extrapolating to the surface tension values at lowertemperatures.38

Slopes of surface tension versus temperature, which representthe difference in entropy between the surface and the bulk, aresimilar for all three tacticities. The magnitude of the slopesshows a slight increase in value for isotactic (iso-) PMMAcompared to syndiotactic (syn-) and atactic (atac-) PMMA andshow the following general trend iso- > syn- > atac-. However,the slopes are not significantly different up to a 0.1 significancelevel (Supporting Information (SI), section S1). The nature ofthe helical chain, discussed in our mechanistic explanation,would allow isotactic PMMA to rearrange itself better at theinterface, leading to higher changes in packing from the bulk.Small differences in surface tension can be attributed toexperimental observations of differences in chain conformation,surface roughness, and near surface chemical composition withtacticity.39−42

To understand the relationship between conformation andsurface composition, we first need to define the interface. In ourprevious work, we have reported the surface composition ofPMMA surface by defining the first 1 nm as the surfaceregion.19 However, the surface is not flat in these simulationsand experimental results using both X-ray reflectometry andatomic force microscopy report a surface roughness ofapproximately 1 nm.11 In our previous work, we determinedroughness to be approximately 5 Å for PMMA surfaces.19 Theroughness in our context is due to capillary waves and theinterface is not smooth as shown in our table of contents figure.To correct for this roughness, we traverse the contour of thepolymer by using a 2 × 2 Å2 grid parallel to the interface.Different mesh sizes were used and 2 × 2 Å2 was the minimumgrid size to capture molecular details. Any other smaller gridsize shows voids.The computed ratios of chemical groups (α-methyl, ester-

methyl, and carbonyl) for iso-/syn-, iso-/atac-, and syn-/atac-PMMA at 560 K as a function of depth are shown in Figure 3.Relative ratios with tacticity provide an explanation for

differences in observed surface tension outlined above. Thepolymer film is probed from the local maximum of the grid.The value at a given depth is the cumulative sum up to thedepth. The choice of “increment in probe depth” proved verysensitive in terms of characterizing the surface behavior (see SI,Figure S1, for a probe increment of 5 Å that misses the crestsand troughs shown by choosing a probe increment of 1 Å).This carefully calibrated true interface approach provides acomprehensive picture in terms of relative chemical groupratios that is representative of the ultrathin PMMA filmbehavior at the vacuum interface.In Figure 3, we see ratios tend to converge around 10−15 Å

with overlap occurring at approximately 20−25 Å for cases ofboth iso-/syn- and iso-/atac-PMMA. This point of convergencecan be used to define the interface. All ratios remain above 1 foriso-/syn- and iso-/atac- as the presence of all chemical groupsstudied is enhanced at the surface of isotactic PMMA, in partdue to reported higher density and better packing, and possiblyin part due to better surface arrangement of chains. Oncealmost all chemical groups have been counted, the ratios equal1, as expected (see SI, Figure S2). Note that as we move awayfrom the bulk toward the extremities of the interface, the(absolute) density of the chemical groups decreases sharply vis-a-vis that of the bulk. Even so, we see higher presence of ester-methyl chemical groups at the interface up to 1.5×, asrepresented in the iso-/atac- values.There are some distinct differences in the relative ratios of

polar and nonpolar groups at the surface of iso-PMMA whencompared with syn- and atac-PMMA. The α-methyl groups arerelatively depleted and ester-methyl and carbonyl groups are inrelative excess at the surface for iso-PMMA in comparison tosyn-PMMA. This makes the iso-PMMA surface relatively morehydrophilic, which is consistent with the higher surface tensionof iso- PMMA. The presence of ester-methyl groupsdominating surface composition, in relation to α-methyl groupsis also consistent with the surface composition measured usinginfrared-visible sum frequency generation spectroscopy(SFG).18,19

In comparison to iso-/syn-, the ratios for iso-/atac- arehigher, implying much larger fraction of ester-methyl forisotactic PMMA (vs atactic PMMA). This contributes to atacticPMMA being the most hydrophobic of the three tacticities asrepresented by surface tension values. For a direct comparison,we analyze ratios of syn-/atac- shown in Figure 3C. We observethat syn-PMMA has a larger number for all chemical groups.There is a pronounced peak for α-methyl groups, which is

Figure 3. Chemical group ratios for (A) iso-/syn-, (B) iso-/atac-, and (C) syn-/atac-PMMA at 560 K in vacuum as a function of distance from thetop surface, as measured through the local maxima.

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consistent with our previous observations on syn-PMMAhaving dominant α-methyl groups at the interface.19 It alsodemonstrates the competing effect of chemical group ratios: alarger relative concentration of the carbonyl and ester-methylgroups at the interface would be counterbalanced by increase inrelative α-methyl group concentration. The increase in α-methyl groups at the syn-PMMA surface appears to increasewith increasing temperature (SI, section S5).Previous studies using Lateral Force Microscopy and ATR-

FTIR spectroscopy have shown that the polar ester-methylgroups are on the same side for isotactic PMMA consequentlymaking it relatively easier for the helical chains to reorient inthe presence of a polar environment.6,41−43 The relative ease oforientation in polar environments is also attributed to theflexibility of the helical isotactic PMMA.6,44 Based on the aboveobservations, the syn- stereomer is locked, with the hydro-phobic groups pointing up toward the air surface, while the iso-PMMA with its greater flexibility is able to reorient even if theenergies are slightly unfavorable in order to pack itself better atthe surface, indicating higher hydrophobicity for syn-PMMA.The atac-PMMA is free of these steric constraints and is able torearrange to present the most favorable conformation withlesser penalties in comparison to the stereoregular forms. Thisimplies that steric restrictions of chemical groups could be usedto control surface energies of functional stereoregular acrylates.Measurement of contact angles for stereoregular PMMA

(hence, their corresponding phobicity) remains an experimen-tal challenge. So far, the results for contact angle measurementsfor stereoregular PMMA have been inconclusive.45−47 It hasbeen hypothesized, however, that iso-PMMA is the mosthydrophilic of the three tacticities and exhibits amphiphilicbehavior.48 This is borne out in the most recent reportedvalues,47 as well as in the corresponding utilization ofdifferences in hydrophilicity, by the same research group,21

for application in biocompatible electrospun fibers. Computedsurface tension values (Figure 2) follow the same trend, withiso- having the highest surface tension, implying a relativelyhigher presence of polar groups at the vacuum surface. It is alsoimportant to note that measurement of contact angle is notsufficient to make inferences on the surface energy of PMMAbecause of possible restructuring of the surfaces in contact withliquids and the fact that the measured angles are affected bysurface roughness. Also, all measurements for contact anglehave been done below glass transition and melting point, whilethe current study analyzes near-surface composition and surfacetensions at much higher temperatures to allow for sufficientchain relaxation.In conclusion, we have shown that computation of surface

tension for stereoregular PMMA requires equilibration overhundreds of nanoseconds. Furthermore, our values for slopes ofsurface tension versus temperature are in very good agreementwith experiment. We have correlated the differences in surfacetension of iso-, syn-, and atac-PMMA to the relative surfaceexcess of polar ester-methyl and carbonyl groups and proposeda mechanistic hypothesis to explain the trends therein. Througha grid-based approach that takes into account the roughness ofthe polymer surface, we have shown that the trends in relativeratios for iso-/syn-, iso-/atac-, and syn-/atac- are qualitativelyconsistent with the expected interfacial behavior. We emphasizethe importance of surface composition of polar and nonpolargroups in modulating surface behavior of polymer films.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsmacro-lett.5b00612.

Values of surface tension, analysis of significant differencefor slopes of surface tension with temperature, anddetails on computation of surface tension. Chemicalgroup ratios for probe depth of 5 Å showing sensitivity toprobe depth. Chemical group ratios showing decay to 1as all groups are counted. Chemical group ratios for 500,520, and 540 K. Details on viability of temperature rangeselected for analysis (PDF).

■ AUTHOR INFORMATIONCorresponding Author*E-mail: mtsige@uakron.edu.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was made possible by funding from ACS PetroleumResearch Fund (ACS PRF 54801-ND5) and Procter & Gamble(K.C.J. and M.T.). A.D. acknowledges support from NationalScience Foundation (NSF DMR-1105370). The authors thankDr. Selemon Bekele for help with visualization of theinstantaneous surface, He Zhu, Emmanuel Anim-Danso,Nishad Dhopatkar, and Pushkar Sathe for fruitful discussionson approaches to spectroscopic characterization at polymerinterfaces, and Dr. Gary Leuty, Dona Foster, Iskinder Arsano,and Jacob Hill for general comments on the paper.

■ REFERENCES(1) Sundararajan, P.; Flory, P. J. Am. Chem. Soc. 1974, 96, 5025−5031.(2) Vacatello, M.; Flory, P. Macromolecules 1986, 19, 405−415.(3) O’Reilly, J. M.; Bair, H. E.; Karasz, F. E. Macromolecules 1982, 15,1083−1088.(4) Yoon, D.; Flory, P., II Macromolecules 1976, 9, 299−303.(5) Grohens, Y.; Brogly, M.; Labbe, C.; Schultz, J. Polymer 1997, 38,5913−5920.(6) Blokhuis, E.; Bedeaux, D.; Holcomb, C.; Zollweg, J. Mol. Phys.1995, 85, 665−669.(7) Grohens, Y.; Prud’homme, R. E.; Schultz, J. Macromolecules 1998,31, 2545−2548.(8) Shima, M.; Sato, M.; Atsumi, M.; Hatada, K. Polym. J. 1994, 26,579−585.(9) Kim, J.-J.; Jung, S.-D.; Hwang, W.-Y. ETRI J. 1996, 18, 195−206.(10) Le, N. C. H.; Gubala, V.; Clancy, E.; Barry, T.; Smith, T. J.;Williams, D. E. Biosens. Bioelectron. 2012, 36, 250−256.(11) Pandit, P.; Banerjee, M.; Gupta, A. Colloids Surf., A 2014, 454,189−195.(12) Rydholm, A. E.; Bowman, C. N.; Anseth, K. S. Biomaterials2005, 26, 4495−4506.(13) Grande, D.; Baskaran, S.; Chaikof, E. L. Macromolecules 2001,34, 1640−1646.(14) Pignatello, R.; Bucolo, C.; Spedalieri, G.; Maltese, A.; Puglisi, G.Biomaterials 2002, 23, 3247−3255.(15) Hua, H.; Li, N.; Wu, L.; Zhong, H.; Wu, G.; Yuan, Z.; Lin, X.;Tang, L. J. Environ. Sci. 2008, 20, 565−570.(16) Cheng, Q.; Zheng, Y.; Yu, S.; Zhu, H.; Peng, X.; Liu, J.; Liu, J.;Liu, M.; Gao, C. J. Membr. Sci. 2013, 447, 236−245.(17) Nagarale, R.; Shahi, V. K.; Schubert, R.; Rangarajan, R.;Mehnert, R. J. Colloid Interface Sci. 2004, 270, 446−454.

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(18) Zhu, H.; Jha, K. C.; Bhatta, R. S.; Tsige, M.; Dhinojwala, A.Langmuir 2014, 30, 11609−11618.(19) Jha, K. C.; Zhu, H.; Dhinojwala, A.; Tsige, M. Langmuir 2014,30, 12775−12785.(20) Izzo, L.; Griffiths, P. C.; Nilmini, R.; King, S. M.; Wallom, K.-L.;Ferguson, E. L.; Duncan, R. Int. J. Pharm. 2011, 408, 213−222.(21) Sharma, J.; Zhang, X.; Sarker, T.; Yan, X.; Washburn, L.; Qu, H.;Guo, Z.; Kucknoor, A.; Wei, S. Polymer 2014, 55, 3261−3269.(22) Ma, Y.; Cao, X.; Feng, X.; Ma, Y.; Zou, H. Polymer 2007, 48,7455−7460.(23) Lu, X.; Jin, Y.; Tan, S.; Zhang, L.; Liu, Y.; Zhang, X.; Xu, J. J.Adhes. Sci. Technol. 2008, 22, 1841−1852.(24) Jorgensen, W. L.; Maxwell, D. S.; Tirado-Rives, J. J. Am. Chem.Soc. 1996, 118, 11225−11236.(25) Chen, C.; Maranas, J. K.; García-Sakai, V. Macromolecules 2006,39, 9630−9640.(26) Wu, C. J. Mol. Model. 2014, 20, 1−11.(27) Rissanou, A. N.; Harmandaris, V. J. Nanopart. Res. 2013, 15, 1−14.(28) Wu, C. Mol. Simul. 2015, 41, 547−554.(29) Kahn, K.; Bruice, T. C. J. Comput. Chem. 2002, 23, 977−996.(30) Chen, C.; Depa, P.; Maranas, J. K.; Sakai, V. G. J. Chem. Phys.2008, 128, 124906.(31) Kirkwood, J. G.; Buff, F. P. J. Chem. Phys. 1949, 17, 338−343.(32) Soldera, A. Macromol. Symp. 1998, 133, 21−32.(33) Soldera, A. Polymer 2002, 43, 4269−4275.(34) Soldera, A.; Grohens, Y. Polymer 2004, 45, 1307−1311.(35) Soldera, A.; Metatla, N.; Beaudoin, A.; Said, S.; Grohens, Y.Polymer 2010, 51, 2106−2111.(36) Soldera, A. Polym.-Plast. Technol. Eng. 2000, 39, 457−467.(37) Yimer, Y. Y.; Dhinojwala, A.; Tsige, M. J. Chem. Phys. 2012, 137,044703.(38) Wu, S. J. Phys. Chem. 1970, 74, 632−638.(39) Wu, S.; Beckerbauer, R. Polym. J. 1992, 24, 1437−1442.(40) Tretinnikov, O. N. Langmuir 1997, 13, 2988−2992.(41) Tretinnikov, O. N.; Ohta, K. Langmuir 1998, 14, 915−920.(42) Tretinnikov, O. N. J. Adhes. Sci. Technol. 1999, 13, 1085−1102.(43) Fujii, Y.; Akabori, K.-i.; Tanaka, K.; Nagamura, T. Polym. J.2007, 39, 928−934.(44) Horinouchi, A.; Tanaka, K. RSC Adv. 2013, 3, 9446−9452.(45) Park, J. H.; Hwang, D.; Lee, J.; Im, S.; Kim, E. Thin Solid Films2007, 515, 4041−4044.(46) Sharma, V. K.; Pethrick, R. A.; Affrossman, S. Polymer 1982, 23,1732−1736.(47) Lester, C.; Abdulkadir, A.; Washburn, L.; Kucknoor, A.; Su-Ying,W. Fluorescent Biocompatible Tactic PMMA-QDs Thin Films.International Conference on Material Science and Application(ICMSA 2015),Suzhou, China, June 13−14, 2015, Atlantis Press:Amsterdam, Netherlands, 2015.(48) Brinkhuis, R.; Schouten, A. Langmuir 1992, 8, 2247−2254.

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