Variation in glass transition temperature of polymer nanocomposite films driven by morphological transitions Sivasurender Chandran, J. K. Basu, and M. K. Mukhopadhyay Citation: J. Chem. Phys. 138, 014902 (2013); doi: 10.1063/1.4773442 View online: http://dx.doi.org/10.1063/1.4773442 View Table of Contents: http://jcp.aip.org/resource/1/JCPSA6/v138/i1 Published by the American Institute of Physics. Additional information on J. Chem. Phys. Journal Homepage: http://jcp.aip.org/ Journal Information: http://jcp.aip.org/about/about_the_journal Top downloads: http://jcp.aip.org/features/most_downloaded Information for Authors: http://jcp.aip.org/authors Downloaded 17 Jan 2013 to 203.200.35.11. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissions THE JOURNAL OF CHEMICAL PHYSICS 138, 014902 (2013) Variation in glass transition temperature of polymer nanocomposite films driven by morphological transitions Sivasurender Chandran,1 J. K. Basu,1,a) and M. K. Mukhopadhyay2 1Department of Physics, Indian Institute of Science, Bangalore, 560 012, India 2Applied Materials Science Division, Saha Institute of Nuclear Physics, Kolkata, 700 064, India (Received 2 September 2012; accepted 12 December 2012; published online 7 January 2013) We report the variation of glass transition temperature in supported thin films of polymer nanocom- posites, consisting of polymer grafted nanoparticles embedded in a homopolymer matrix. We observe a systematic variation of the estimated glass transition temperature Tg, with the volume fraction of added polymer grafted nanoparticles. We have correlated the observed Tg variation with the under- lying morphological transitions of the nanoparticle dispersion in the films. Our data also suggest the possibility of formation of a low-mobility glass or gel-like layer of nanoparticles at the inter- face, which could play a significant role in determining Tg of the films provided. © 2013 American Institute of Physics.[http://dx.doi.org/10.1063/1.4773442] I. INTRODUCTION properties especially on their glass transition.14, 15 Needless to say that, for various practical applications such compos- Polymer thin films have been extensively studied in the ites will eventually have to be prepared in the form of a film last two decades to explore the possibility of finding a length or a coating. Thus, studying the properties of thin films of scale underlying glass formation in polymers.1–8 Several re- these materials is of vital importance. However, it turns out view articles summarize our current understanding of the that the interface plays a crucial role in such thin films,14, 15 subject.9, 10 Despite, a large body of work suggesting the exis- to the extent that the nature of dispersion in the bulk could tence of a finite size effect on glass transition temperature, T , g be significantly modified. The film and substrate processing of polymers, several examples contradicting the results exist conditions, which are crucial for thin polymer films, turn out in literature2 so that the outcome of an enormous amount of to be much more critical in PNC thin films.29 These in turn research in the last two decades is inconclusive on this as- could lead to large changes in thermo-mechanical properties pect. It is clear that part of this discrepancy stems from the of thin films of PNCs as well. Therefore, to explore the in- significant contribution of surface/interface effects along with terplay of strong confinement of polymer segments by em- possible finite size effects. An alternative method to explore bedded nanoparticles at high volume fractions and the sur- possible finite size effects as well as the role of the inter- face effects, especially the role of particle dispersion, we have face on polymer thermo-mechanical properties is to impreg- studied the glass transition of PGNP embedded PNC thin nate it with nanoparticles.7, 11–13 Equivalence of the perturba- films. tion of the bulk glass transition of polymers when confined Here, we report a comprehensive measurement, follow- in the form of thin films or impregnation with nanoparticles ing up on our earlier work15 of T variation in polystyrene has been demonstrated.11–13 A large body of work in the area g films of thickness ∼70 nm embedded with thiol termi- of polymer nanocomposites (PNC) has emerged in the last nated polystyrene (PST) capped gold nanoparticles (Au NP) decade,11–17 driven not only to explore the finite size and in- of fixed size and various properties as shown in Table I. terface effects on polymer physical properties, but also to cre- The volume fraction of the embedded gold in the polymer ate new materials with novel physical properties. A crucial matrix, φ has been varied from 0.1−10, as indicated in aspect in determining the ultimate success of this strategy, p Table II. We have estimated the Tg variation from the tem- and hence to maximize the benefits of the anticipated elec- perature dependence of film thickness using spectroscopic trical, optical, and magnetic properties,18 is the ability to tune ellipsometry and correlated this variation with the detailed the dispersion of the particles in the embedded polymer ma- three-dimensional morphology of the film using atomic force trices, and to prevent the thermal degradation of the polymer microscopy (AFM), field emission scanning electron micro- matrix. Although research11–13 seems to indicate T variation g scope (FESEM), and X-ray reflectivity (XRR). We observe with increase in volume fraction of added particles, some re- step-wise decrease in T of the polymer films as a function cent work seems to indicate no T variation in PNCs.19 A suc- g g of added PGNP. We provide a model for this T variation cessful dispersion strategy has been to use polymer grafted g in terms of the underlying morphological phase transition in nanoparticles (PGNP) in the identical polymer matrix to cre- the dispersion of PGNPs and also allude to the existence of ate an athermal blend.5, 11–13, 20 The morphological phase di- possible viscosity gradient along the film thickness similar agrams of such blends are beginning to be elucidated.20–28 to the recent observations.3, 17 The overall impact of parti- However, very few studies have been made on the physical cle loading, morphological transitions, processing conditions, and possible viscosity gradients on Tg variation of PNC thin a) E-mail: [email protected]. films are discussed. 0021-9606/2013/138(1)/014902/7/$30.00138, 014902-1 © 2013 American Institute of Physics Downloaded 17 Jan 2013 to 203.200.35.11. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissions 014902-2 Chandran, Basu, and Mukhopadhyay J. Chem. Phys. 138, 014902 (2013) TABLE I. Properties of PGNP. ∼3×10−2 mbar. The chamber, especially the quartz windows used in the beam path, were tested for appearance of the pos- R σR c e sible spurious polarization changes from residual stress due Sample nm nm Chains/nm2 nm R /R e g to temperature variation. PST-Au 2.1 ± 0.2 1.5 1.98 ± 0.2 3.6 0.45 X-ray reflectivity measurements on the samples, de- scribed in Table II, were performed at BL 18B in Photon Fac- tory synchrotron, Tsukuba, Japan, at an incident x-ray energy II. EXPERIMENTAL DETAILS of 10 KeV as well as with a D8 Discover lab based reflectome- ter (Bruker, Germany) at 8 KeV. The electron density profiles Polymer grafted nanoparticles consisting of a core of (EDP) ρ(z) of the various PNC films were extracted from gold nanoparticles (Au NP) and corona of thiol terminated the measured reflectivity, R, as a function of the perpendic- polystyrene (PST of molecular weight 3 Kg/mol, degree of ular wave vector transfer, qz (=4πsinθ/λ, where θ and λ are polymerization N∼27), grafted to the Au NP core, were syn- the angle of incidence and wavelength of the incident x-rays 15, 20, 22, 30, 31 thesized by a method described earlier. Trans- on the samples). Atomic force microscopy (NT-MDT, NTE- mission electron microscopy(TEM, Technai, T20) and ther- GRA) measurements were performed, in contact mode using mogravimetric analysis (TGA, METTLER) were used to NT-MDT cantilevers to find the surface morphology. The lat- estimate the grafting density σ of the PST chains on PGNP eral dispersions of particles were seen using FESEM (Ultra core. The average thickness of the PST shell, on the Au NPs Zeiss, Germany), operated at 8 keV with a working distance has been estimated to be ∼1.5 nm from inter-particle spacings of 3 mm. obtained from TEM images. The total radius of the PGNP Re(=+Rc, Rc is the radius of the PGNP core) is therefore estimated to be ∼3.6 nm. PGNP solutions were mixed with III. RESULTS AND DISCUSSIONS the polystyrene (PS) (molecular weight 97.4 Kg/mol, degree A. Glass transition of polymerization P∼936; radius of gyration, Rg = 8nm) solutions in appropriate ratios as indicated in Table II.The For estimating the Tg of PNC films, they were heated to ◦ − mixtures were stirred for ∼24 h to ensure the formation of a 150 C (at a vacuum better than ∼3×10 2 mbar) and held homogeneous dispersion. Thin films of these PGNP-polymer at this temperature for 2 h to maintain equilibrium condi- suspension were then prepared (using the solutions mentioned tions in the sample. Ellipsometric angles and were mea- ◦ above) on polished silicon wafers (Vin Karola Inc, USA) sured, in situ, continuously from 150 C to room tempera- cleaned using standard methods described earlier.15 The films ture, over a wavelength range of 300−600 nm, at a cool- ◦ ∼ ◦ were annealed at ∼150 C (well above the Tg of bulk PS ing rate of 0.8 C/min.