(660c) Effect of 3-D Nanoscale Confinement on the Glass Transition Temperature: Comparison of Fluorescence Measurements of Polystyrene Films and Particles

The seminal work in the field of polymer glass transition temperature (T_g)-confinement effects came in the mid 1990’s when Keddie et al. showed via ellipsometry that a 10-nm-thick polystyrene (PS) film supported on silica exhibited a reduction of up to 29 K in T_g relative to T_g,_bulk PS.¹  They postulated that a “liquid-like” layer existed at the free surface which increasingly influenced the overall chain dynamics of the film when the ratio of surface area to volume is increased with geometric confinement. Many other (pseudo-)thermodynamic measurements of T_g in supported, thin PS films have shown confinement effects similar to those of Keddie et al.  Free-standing PS films show considerably larger T_g-confinement effects than supported PS films.^2,3  For PS with M_n< 350 kg/mol, the free-standing films exhibit the same T_g-confinement effect as supported films when T_g is plotted as a function of characteristic length, which is volume divided by free surface area.²Such a plot accounts for free-standing films having twice the number of free surfaces as supported films.  

Historically, T_g-confinement studies have focused on the one-dimensional film geometry; far fewer confinement studies have been reported on three-dimensional nanospheres possibly because of the increased complexity in preparation and measurement of samples.  Nanoparticles represent an alternative geometric system to investigate the effects of free surfaces and confinement on T_g.  Nanosphere confinement studies found in literature vary greatly in methods of sample preparation, strength of the reported T_g-confinement effect, and conclusions reached.^4–6 Sasaki et al.⁴ reported that PS spheres prepared via emulsion polymerization having diameters of 42 - 548 nm exhibited an “unambiguous glass transition very near the T_g of bulk PS” while both Zhang⁵ and Feng⁶ reported an apparent T_g-confinement effect as a function of particle diameter.  Using surfactant-free emulsion polymerization, Zhang⁵ reported that particles ~150 nm in diameter exhibited a reduction in T_g of ~45 K from bulk PS T_g while Feng⁶ showed that ~150 nm diameter “surfactant-free” particles (PS synthesized via conventional emulsion polymerization followed by removal of surfactant via dialysis) exhibited a more modest T_g reduction of ~10 K from T_g,bulk. 

We have prepared PS nanoparticles and evaluated the T_g-confinement effects. To ensure that our particles are free of surfactant and unreacted monomer, we employ a facile nanoprecipitation method.  Stable nanoparticles possessing diameters ranging from 40 - 950 nm were produced by precipitating in water various concentrations of polystyrene (M_n = 91 kg/mol) dissolved in tetrahydrofuran.  The effect of nanoscale confinement on T_g was determined using the temperature dependence of a fluorescence intensity ratio associated with trace levels of covalently labeled pyrene dye.  We find that a strong effect of confinement on T_g is obtained for particles with diameters below 170 nm; bulk T_g was measured for particles with larger diameter.  A quantitative comparison between the T_g-confinement effect seen in particles and films was made possible by defining a characteristic length equal to the ratio of volume to surface area.  The size-dependent T_g values of our PS particles are in good agreement with those of supported films (of all molecular weights) and free-standing films (M_n< 350 kg/mol) when the T_g data are compared as a function of characteristic length (volume/area). This indicates that there is a common physical origin of the T_g-confinement effect in the different geometries which is associated with how the free surface perturbs T_gsome tens of nanometers in the film or particle interior.

(1)      Keddie, J. L.; Jones, R. A. L.; Cory, R. A. Europhys. Lett. 1994, 27, 59–64.

(2)      Mattsson, J.; Forrest, J. A.; Borjesson, L. Phys. Rev. E 2000, 62, 5187–5200.

(3)      Kim, S.; Torkelson, J. M. Macromolecules 2011, 44, 4546–4553.

(4)      Sasaki, T.; Shimizu, A.; Mourey, T. H.; Thurau, C. T.; Ediger, M. D. J. Chem. Phys. 2003, 119, 8730.

(5)      Zhang, C.; Guo, Y.; Priestley, R. D. Macromolecules 2011, 44, 4001–4006.

(6)      Feng, S.; Li, Z.; Liu, R.; Mai, B.; Wu, Q.; Liang, G.; Gao, H.; Zhu, F. Soft Matter 2013, 9, 4614.