59th Annual Report on Research 2014

Our work has focused on understanding the interactions between confinement and interfaces on the T_g and structural relaxation in nanoparticles (NPs) of glassy polymers.^1-7 Undertaking these studies provided a means to examine the generality of size-effects on glassy properties aside from thin films. Furthermore, these studies mitigated issues related to the processing of thin films that distorts chain conformations and introduces stresses, which have been argued to be the origin of size-dependent effects on glassy properties. In our initial work, we measured T_g via calorimetry of aqueous suspended polystyrene (PS) NPs; see Fig. 1. Our work demonstrated a strong influence of diameter on the T_g of PS NPs. That is, with a decreasing diameter, a significant suppression in T_g was observed. We developed a phenomenological model with the premise that the effect originated from a reduction in T_g at the free surface, which, in this case, is the polymer/water interface, and T_g reductions at the free surface propagated tens of nanometers into the nanoparticle interior. The importance of the free surface was investigated by capping PS nanoparticles with a rigid silica shell, i.e., removing the free interface. We observed that PS nanoparticles confined within a silica shell did not exhibit a size dependent T_g as observed for bare PS nanoparticles. The T_g recovery could be significant for smaller size nanoparticles, e.g., T_g,PS - T_g,PS/silica = - 42 K for nanoparticles with a diameter of 130 nm. The absence of the T_g–confinement effect for PS/silica core-shell nanoparticles could be understood if the free surface is considered a major reason for the size dependence of T_g for polymer nanoparticles. It then follows that if the silica shell was effective in eliminating the polymer free surface, the T_g-confinement effect, which is a consequence of surface effects, is eliminated.

Rapid cooling, the typical route to a glass is accompanied by changes in temperature and volume, both of which contribute to slow dynamics. A potentially powerful study to understanding confined polymers would be to investigate the slow dynamics under isobaric and isochoric confinement conditions in an effort to separate the temperature and volume contributions to glass formation. We developed an approach to quantify the fragility in both isobaric and isochoric conditions; our ability to directly measure the isochoric fragility in either bulk or confinement represents a major achievement. Isochoric confinement was achieved via the capping of polymer nanoparticles with a rigid inorganic shell (Fig. 2a), and fragility was measured via variable-cooling rate DSC using the Moynihan/Angell approach. As illustrated in Figure 2b, both the isobaric and isochoric fragilities decrease with confinement, with a nearly identical onset value for poly(4-methyl styrene) (P4MS). Surprisingly, T_gdecreases and remains constant with isobaric and isochoric confinement, respectively.

Relating changes in glassy properties under confinement to a fundamental length-scale of glass formation remains a major challenge. A classical theory to explain the rapid increase in cooperative segmental relaxation time as T_g is approached from higher temperatures is the Adam-Gibbs (AG) theory. The central feature of the AG theory is that a growing length scale (x_a) is required for cooperative segmental motion or rearrangement, as T is decreased towards T_g. Within the framework of the AG theory, fragility should correlate with both the size of a CRR at T_g, i.e., the characteristic length of the glass transition (x_a), and the rate of change in x_a, with temperature at T_g. We examined the relationship between x_a and m_v for polymers under confinement. We found that a linear correlation between x_a and m_v was observed for silica-capped polymer nanoparticles of PS and P4MS as illustrated in Figure 3 for PS and P4MS. However, as T_g remained constant with confinement for silica-capped NPs, how our results satisfy the AG theory is intriguing; see ref. 3 for more discussion.

1. C. Zhang, Y. Guo and R.D. Priestley,"Glass transition temperature of polymer nanoparticles under soft and hard confinement." Macromolecules 44, 4001 (2011)

2. Y. Guo, C. Zhang, C. Lai, R.D. Priestley, M.D'Acuzni and G. Fytas,"Structural relaxation of poymer nanospheres under soft and hard confinement: Isobaric versus isochroic glass formation." ACS Nano 5, 5365 (2011)

3. C. Zhang, Y. Guo and R.D. Priestley,"Characteristic length of the glass transition in isochorically confined polymer glasses." ACS Macro Letters 3, 501 (2014)

4. C. Zhang and R.D. Priestley,"Fragility and glass transition temperature of polymer confined under isobaric and isochoric conditions." Soft Matter 9, 7076 (2013)

5. C. Zhang, Y. Guo, K. Shepard and R.D. Priestley,"Fragility of an isochorically confined polymer glass." Journal of Physical Chemistry Letters 4, 431 (2013)

6. C. Zhang, Y. Guo and R.D. Priestley,"Confined polymer properties of nanoparticles." Journal of Polymer Science Part B Polymer Physics 51, 574 (2013)

7. C. Zhang, V.M. Boucher, D. Cangialosi and R.D. Priestley,"Mobility and glass transition temperature of polymer nanospheres." Polymer 54, 230 (2013)