Jan 30, 2012 - 1 J. J. Lin, F. S. Bates, D. A. Hammer and J. A. Silas, Phys. Rev. Lett., ... 24 O. B. Wright, B. Perrin, O. Matsuda and V. E. Gusev, Phys. Rev. B:.
View Online / Journal Homepage / Table of Contents for this issue
C
Soft Matter
Dynamic Article Links
80 nm. Morevover, it has been shown for atactic polystyrene (PS), a model hydrophobic polymer, that the glass transition temperature, related to the relaxation processes, does not significantly decrease for film thicknesses >100 nm.17 This comparison supports our finding that the motion of the polymer chains is not confined in the PLGA shells we study. The lifetime of the Brillouin oscillations, related to the relaxation processes, is also matched by adjusting the attenuation in the shell at Gs ¼ 1.8 mm1 for both capsules. By analogy with a Kelvin–Voigt viscoelastic model, since the phonon wavelength lpb/2ns Gs1, the longitudinal storage and loss modulus of the shell are defined as M0 s ¼ rsvs2 ¼ 8 GPa and M00 s ¼ 2vs3Gsrs/2pfs ¼ 0.6 GPa, respectively, where rs ¼ 1.35 g cm3 is the mass density.36 The longitudinal kinematic viscosity, defined as hLs ¼ M00 s/2pfsrs ¼ 4 mm2 s1, lies within the range observed in glassy polymers at GHz frequencies.18 Since there is no confinement in the shells we probe, our data can be compared with lower frequency bulk PLGA values. From the longitudinal sound velocity and attenuation measured by Parker et al.33 at 10 MHz, we calculate similarly M0 s z 8 GPa and M00 s z 0.1 GPa. This weak fs-dependence of M00 s is the signature of thermally activated processes in glasses. Such behavior has been successfully described by the Gilroy–Phillips (GP) model in PS in the GHz range.37 The GP model describes the relaxation processes as thermally activated transitions within asymmetric double-well potentials with an exponential distribution of barrier heights.38 At low frequencies (typically 100 GHz in polymer glasses), it yields Gs f fs(1 + a), where a is related to the mean activation barrier. In PLGA we find a ¼ 0.2, a typical value for polymer glasses,37,38 further suggesting the applicability of the GP model.{ Measurements at various temperatures should give support for this conclusion.37 We simultaneously determine the attenuation in the PFOB core at Gc ¼ 1.6 mm1, giving a longitudinal kinematic viscosity hLc ¼ 1.2 mm2 s1 associated with the Kelvin–Voigt analogy. This value is in agreement with that given in the literature at 10 MHz and 25 C,31 demonstrating that the frequency dispersion is negligible for such a low viscosity liquid. The phonon lifetime in the PFOB core of the 80 nm shell capsule presented in Fig. 3(b) seems to be matched only up to 700 ps, corresponding to a critical propagation distance zc 600 nm inside the capsule. For such small capsules, the contact adhesion force is FA ¼ 3pWAR/2 2 mN,41 where WA 0.1 J m2 is the typical Dupre’s energy for a metal–glass system.42 FA dominates the counteracting gravitational and Archimedes’ forces, and maintains the capsule in contact with Ti6Al4V. Considering the Johnson–Kendall– Roberts theory applied to an homogeneous compliant elastic PLGA sphere in adhesive contact with a rigid flat surface41 gives a contact area of radius Rc ¼ (3FAR/Ec)1/3 ¼ 0.3 mm, where Ec z 0.7 GPa is the 2588 | Soft Matter, 2012, 8, 2586–2589
Young’s modulus of PLGA.43 This situation is drawn to scale in the inset of Fig. 2. Considering Rc as the width of a piston-like acoustic source acting at the Ti6Al4V–capsule interface, the near-field limit is Rc2fc/vc ¼ 0.6 mm. This value, comparable to zc, indicates that the phonons may transit from the near to the far field of the acoustic source within the probed area of the core, resulting in a non-exponential attenuation profile, as observed after 700 ps. This feature suggests that the GHz phonon propagation in soft micro-objects can be extremely sensitive to the adhesion conditions. To conclude, we have probed the longitudinal moduli of the shell and of the core of single polymer microcapsules at GHz frequencies in a liquid environment. Analysis of the GHz phonon relaxation has revealed thermally activated processes in the PLGA shell, while the PFOB core behaves as a low-viscosity fluid. We have also determined the shell thickness and demonstrated that no confinement occurs in the shell for thicknesses >80 nm. Probing of GHz phonon relaxation in each compartment of such confined multiple-phase micro-objects, at various temperatures and frequencies, should provide further understanding of the intricate dynamics of the submicrometer polymer shell, and of the influence of chemical interactions between the soft shell and the liquid core. PU investigation of polymers with longer chain lengths should also help analyzing confinement in the capsule. In the future, our method should allow the probing of the influence of external stimuli on the GHz relaxation mechanisms of polyelectrolyte microcapsules or of biological micro-objects such as cells.
Acknowledgements Authors thank V. Nicolas (IPSIT, Univ Paris-Sud) for the confocal microscopy measurements and M. F. Trichet (ICMPE, CNRS) for access to the SEM facility.
Notes and references ‡ Solving the 3D heat diffusion equation29 in a bulk Ti half-space25 provides an estimate of the laser-induced steady-state temperature rise of 0.7 K at the capsule–Ti6Al4V interface for a 3 pJ pulse focused to a 2 mm-radius spot. x Thermal properties of PLGA and PFOB, taken equal to those of silica and of water, respectively, have negligible influence since the velocity of thermal expansion at the Brillouin frequencies is slower than the acoustic velocity.35 { The longitudinal modulus M is a linear function of the bulk and shear moduli K and G. The distributions of relaxation times for K, G, and consequently for M, are taken equal, as observed for low-MW polymer glasses.39,40 1 J. J. Lin, F. S. Bates, D. A. Hammer and J. A. Silas, Phys. Rev. Lett., 2005, 95, 026101. 2 G. B. Sukhorukov, E. Donath, S. Moya, A. S. Susha, A. Voigt, J. Hartmann and H. M€ ohwald, J. Microencapsulation, 2000, 17, 177–185. 3 M. Delcea, H. M€ ohwald and A. G. Skirtach, Adv. Drug Delivery Rev., 2011, 63, 730–747. 4 C. G omez-Gaete, E. Fattal, L. Silva, M. Besnard and N. Tsapis, J. Controlled Release, 2008, 128, 41–49. 5 E. Pisani, et al., Adv. Funct. Mater., 2008, 18, 2963–2971. 6 E. Pisani, N. Tsapis, J. Paris, V. Nicolas, L. Cattel and E. Fattal, Langmuir, 2006, 22, 4397–4402. 7 R. Dıaz-L opez, N. Tsapis, D. Libong, P. Chaminade, C. Connan, M. M. Chehimi, R. Berti, N. Taulier, W. Urbach, V. Nicolas and E. Fattal, Biomaterials, 2009, 30, 1462–1472.
This journal is ª The Royal Society of Chemistry 2012
Downloaded by UNIV BORDEAUX 1-SCD-BIBLIO UNIVERSITAIRE on 10 February 2012 Published on 30 January 2012 on http://pubs.rsc.org | doi:10.1039/C2SM07146K
View Online 8 S. R. White, N. R. Sottos, P. H. Geubelle, J. S. Moore, M. R. Kessler, S. R. Sriram, E. N. Brown and S. Viswanathan, Nature, 2001, 409, 794–797. 9 X. Liu, C. Gao, J. Shen and H. M€ ohwald, Macromol. Biosci., 2005, 5, 1209–1219. 10 Y. Jing, Y. Zhu, X. Yang, J. Shen and C. Li, Langmuir, 2011, 27, 1175–1180. 11 P. A. L. Fernandes, M. Delcea, A. G. Skirtach, H. M€ ohwald and A. Fery, Soft Matter, 2010, 6, 1879–1883. 12 J. Ara ujo, E. Vega, C. Lopes, M. A. Egea, M. L. Garcia and E. B. Souto, Colloids Surf., B, 2009, 72, 48–56. 13 H. Richardson, I. L opez-Garcıa, M. Sferrazza and J. L. Keddie, Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys., 2004, 70, 051805. 14 R. D. Priestley, C. J. Ellison, L. J. Broadbelt and J. M. Torkelson, Science, 2005, 309, 456–459. 15 J. K. Black, L. E. Tracy, C. P. Roche, P. J. Henry, J. B. Pesavento and T. Adalsteinsson, J. Phys. Chem. B, 2010, 114, 4130–4137. 16 G. Ha€ıat, R. Berti, B. Galaz, N. Taulier, J.-J. Amman and W. Urbach, J. Acoust. Soc. Am., 2011, 129, 1642–1652. 17 K. Fukao and Y. Miyamoto, Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top., 2000, 61, 1743–1754. 18 D. Fioretto and F. Scarponi, Mater. Sci. Eng., A, 2009, 521–522, 243– 246. 19 H. Liem, J. Cabanillas-Gonzalez, P. Etchegoin and D. Bradley, J. Phys., 2004, 16, 721–728. 20 C. Thomsen, H. T. Grahn, H. J. Maris and J. Tauc, Phys. Rev. B, 1986, 34, 4129–4138. 21 C. J. Morath and H. J. Maris, Phys. Rev. B: Condens.Matter, 1996, 54, 203–213. 22 L. Shelton, F. Yang, W. Ford and H. Maris, Phys. Status Solidi B, 2005, 242, 1379–1382. 23 T. Pezeril, C. Klieber, S. Andrieu and K. A. Nelson, Phys. Rev. Lett., 2009, 102, 107402. 24 O. B. Wright, B. Perrin, O. Matsuda and V. E. Gusev, Phys. Rev. B: Condens. Matter Mater. Phys., 2008, 78, 024303.
This journal is ª The Royal Society of Chemistry 2012
25 B. Audoin, C. Rossignol, N. Chigarev, M. Ducousso, G. Forget, F. Guillemot and M. C. Durrieu, Ultrasonics, 2010, 50, 202–207. 26 T. Sasaki, M. Misu, T. Shimada and M. Teramoto, J. Polym. Sci., Part B: Polym. Phys., 2008, 46, 2116–2125. 27 H. S. Lim, M. H. Kuok, S. C. Ng and Z. K. Wang, Appl. Phys. Lett., 2004, 84, 4182–4184. 28 T. Dehoux, et al., Opt. Lett., 2009, 34, 3740–3742. 29 S. Kashiwada, O. Matsuda, J. J. Baumberg, R. Li Voti and O. B. Wright, J. Appl. Phys., 2006, 100, 073506. 30 T. Dehoux, M. Perton, N. Chigarev, C. Rossignol, J. M. Rampnoux and B. Audoin, J. Appl. Phys., 2006, 100, 064318. 31 J. G. Riess, Chem. Rev., 2001, 101, 2797–2920. 32 J. N. Marsh, C. S. Hall, S. A. Wickline and G. M. Lanza, J. Acoust. Soc. Am., 2002, 112, 2858–2862. 33 N. G. Parker, M. L. Mather, S. P. Morgan and M. J. W. Povey, Biomed. Mater., 2010, 5, 055004. 34 T. Dehoux, N. Chigarev, C. Rossignol and B. Audoin, Phys. Rev. B: Condens. Matter Mater. Phys., 2007, 76, 024311. 35 T. Dehoux, N. Chigarev, C. Rossignol and B. Audoin, Phys. Rev. B: Condens. Matter Mater. Phys., 2008, 77, 214307. 36 T. A. Litovitz and C. M. Davis, in Physical Acoustics, ed. W. P. Mason, Academic, New York, 1965, vol. 2A. 37 N. V. Surovtsev, J. A. H. Wiedersich, V. N. Novikov, E. R€ ossler and A. P. Sokolov, Phys. Rev. B: Condens. Matter, 1998, 58, 14888– 14891. 38 K. S. Gilroy and W. A. Phillips, Philos. Mag. B, 1981, 43, 735–746. 39 L. Grassia, A. D’Amore and S. L. Simon, J. Rheol., 2010, 54, 1009– 1022. 40 U. Buchenau, Phys. Rev. B: Condens. Matter, 2001, 63, 104203. 41 K. L. Johnson, K. Kendall and A. D. Roberts, Proc. R. Soc. London, Ser. A, 1971, 324, 301–313. 42 S. Y. Kim, K. Kim, K. Hong and J.-L. Lee, J. Electrochem. Soc., 2009, 156, J253–J257. 43 S. Choi and T. Park, J. Biomater. Sci., Polym. Ed., 2002, 13, 1163– 1173.
Soft Matter, 2012, 8, 2586–2589 | 2589
