Glass transition and alpha-relaxation dynamics of thin films of labeled polystyrene

The glass transition temperature and relaxation dynamics of the segmental motions of thin films of polystyrene labeled with a dye, 4-[N-ethyl-N-(hydroxyethyl)]amino-4-nitraozobenzene (Disperse Red 1, DR1) are investigated using dielectric measurements. The dielectric relaxation strength of the DR1-labeled polystyrene is approximately 65 times larger than that of the unlabeled polystyrene above the glass transition, while there is almost no difference between them below the glass transition. The glass transition temperature of the DR1-labeled polystyrene can be determined as a crossover temperature at which the temperature coefficient of the electric capacitance changes from the value of the glassy state to that of the liquid state. The glass transition temperature of the DR1-labeled polystyrene decreases with decreasing film thickness in a reasonably similar manner to that of the unlabeled polystyrene thin films. The dielectric relaxation spectrum of the DR1-labeled polystyrene is also investigated. As thickness decreases, the $\alpha$-relaxation time becomes smaller and the distribution of the $\alpha$-relaxation times becomes broader. These results show that thin films of DR1-labeled polystyrene are a suitable system for investigating confinement effects of the glass transition dynamics using dielectric relaxation spectroscopy.Comment: 10 pages, 11 figures, 2 Table

On the equivalence between the thermodynamic and dynamic measurements of the glass transition in confined polymers

7th IDMRCS: Relaxation in Complex SystemsUnderstanding why the glass transition temperature (Tg) of polymers deviates substantially from the bulk with nanoscale confinement has been a 20-year mystery. Ever since the observation in the mid-1990s that the Tg values of amorphous polymer thin films are different from their bulk values, efforts to understand this behavior have intensified, and the topic remains the subject of intense research and debate. This is due to the combined scientific and technological implications of size-dependent glassy properties. Here, we discuss an intriguing aspect of the glassy behavior of confined amorphous polymers. As experimentally assessed, the glass transition is a dynamic event mediated by segmental dynamics. Thus, it seems intuitive to expect that a change in Tg due to confinement necessitates a corresponding change in molecular dynamics, and that such change in dynamics may be predicted based on our understanding of the glass transition. The aim of this perspectives article is to examine whether or not segmental dynamics change in accordance with the value of Tg for confined polymers based on bulk rules. We highlight past and recent findings that have examined the relationship between Tg and segmental dynamics of confined polymers. Within this context, the decoupling between these two aspects of the glass transition in confinement is emphasized. We discuss these results within the framework of our current understanding of the glass transition as well as efforts to resolve this decoupling. Finally, the anomalous decoupling between translational (diffusion) and rotational (segmental) motion taking place in the proximity of attractive interfaces in polymer thin films is discussed.RDP acknowledge support from the National Science Foundation (NSF) Materials Research Science and Engineering Center program through the Princeton Center for Complex Materials (DMR-0819860), the NSF through a CAREER Award (DMR-1053144) and the AFOSR through a YIP Award (FA9550-12-1-0223). SN acknowledges financial support from the Defay Foundation and the funds FER of the Université Libre de Bruxelles. DC acknowledges the University of the Basque Country and Basque Country Government (Ref. No. IT-654-13 (GV)), Depto Educacion, Universidades e investigacion and Spanish Government (Grant No. MAT2012-31088) for their financial support.Peer Reviewe

Decoupling of glassy dynamics from viscosity in thin supported poly(n-butyl methacrylate) films

We utilized fast scanning calorimetry to characterize the glass transition temperature (Tg) and intrinsic molecular mobility of low-molecular-weight poly(n-butyl methacrylate) thin films of varying thicknesses. We found that the Tg and intrinsic molecular mobility were coupled, showing no film thickness-dependent variation. We further employed a unique noncontact capillary nanoshearing technique to directly probe layer-resolved gradients in the rheological response of these films. We found that layer-resolved shear mobility was enhanced with a reduction in film thickness, whereas the effective viscosity decreased. Our results highlight the importance of polymer–substrate attractive interactions and free surface-promoted enhanced mobility, establishing a competitive nanoconfinement effect in poly(n-butyl methacrylate) thin films. Moreover, the findings indicate a decoupling in the thickness-dependent variation of Tg and intrinsic molecular mobility with the mechanical responses (shear mobility and effective viscosity).M.C. acknowledges support from the Science and Engineering Research Board through the Ramanujan Fellowship (Grant No. SB/S2/RJN-084/2018) and the Early Career Research Award (ECR/2018/001740) of the Department of Science and Technology, India. Additionally, support from IIT Bombay through a SEED grant (RD/0519-IRCCSH0-033) is acknowledged. R.D.P. acknowledges support from the National Science Foundation (NSF) Materials Research Science and Engineering Center Program of the Princeton Center for Complex Materials (grant numbers DMR-1420541 and DMR-2011750) and the NSF through grant number CBET-1706012. D.C. acknowledges MICINN-Spain and FEDER-UE (grant PGC2018-094548-BI00) and the Basque Government (grant IT-1175-19).Peer reviewe

Physical aging of hydroxypropyl methylcellulose acetate succinate via enthalpy recovery

Amorphous solid dispersions (ASDs) utilize the kinetic stability of the amorphous state to stabilize drug molecules within a glassy polymer matrix. Therefore, understanding the glassy-state stability of the polymer excipient is critical to ASD design and performance. Here, we investigated the physical aging of hydroxypropyl methylcellulose acetate succinate (HPMCAS), a commonly used polymer in ASD formulations. We found that HPMCAS exhibited conventional physical aging behavior when annealed near the glass transition temperature (Tg). In this scenario, structural recovery was facilitated by α-relaxation dynamics. However, when annealed well below Tg, a sub-α-relaxation process facilitated low-temperature physical aging in HPMCAS. Nevertheless, the physical aging rate exhibited no significant change up to 40 K below Tg, below which it exhibited a near monotonic decrease with decreasing temperature. Finally, infrared spectroscopy was employed to assess any effect of physical aging on the chemical structure of HPMCAS, which is known to be susceptible to degradation at temperatures 30 K above its Tg. Our results provide critical insights necessary to understand better the link between the stability of ASDs and physical aging of the glassy polymer matrix.This work was supported by the National Science Foundation (NSF) Materials Research Science and Engineering Center Program through the Princeton Center for Complex Materials (PCCM) (DMR-2011750). The authors also thank Lonza Pharma (Bend, Oregon) for donating the three grades of HPMCAS used for this study. The authors acknowledge the use of Princeton's Imaging and Analysis Center, which is partially supported through the Princeton Center for Complex Materials (PCCM), a National Science Foundation (NSF)-MRSEC program (DMR-2011750). D. Cangialosi acknowledges the grant PID2021-123438NB-100 funded by MCIU/AEI/FEDER, UE and financial support of the Basque Government (Eusko Jaurlaritza), code IT1566-22. B. Zuo acknowledges the grant number 22011530456 funded by the National Natural Science Foundation of China.Peer reviewe

Mobility and glass transition temperature of polymer nanospheres

Recent studies have illustrated a decoupling between cooperative segmental mobility and the glass transition temperature (Tg) of thin polymer films and nanocomposites. Here, we use dielectric spectroscopy to probe the cooperative segmental mobility and capacitive dilatometry to determine the Tg of films of polystyrene nanospheres with diameters (d) less than 400 nm. We find that both capacitive dilatometry and calorimetry revealed nearly identical suppressions in Tg as the size of the nanospheres was reduced. While Tg was impacted by confinement, in the range 130 nm ≤ d ≤ 400 nm, in stark contrast, the cooperative segmental mobility, i.e., the peak position of the α-relaxation process was not. Furthermore, when d ≤ 200 nm, an additional molecular relaxation process, not observed in bulk, was present. We interpret these findings as evidence of a decoupling between Tg and cooperative segmental mobility in nanospheres. That is, the latter may be impacted by confinement under conditions in which the former is not.We acknowledge usage of the PRISM Imaging and Analysis Center, which is supported in part by the NSF MRSEC program through the Princeton Center for Complex Materials (DMR-0819860). C.Z. acknowledges support by the Department of Defense (DoD) through the National Defense Science & Engineering Graduate Fellowship (NDSEG). R.D.P. acknowledges the donors of the American Chemical Society Petroleum Research Fund (PRF 49903-DNI10) and the 3M-nontenured faculty grant program for partial support of this work. V.M.B and D.C. acknowledge the University of the Basque Country and Basque Country Government (Ref. No. IT-436-07, Depto. Educación, Universidades e investigación) and Spanish Minister of Education (Grant No. MAT 2007-63681) for their financial support. V.M.B. acknowledges CSIC for the JAE-Doc contract, co-financed by the European Social Fund (ESF).Peer Reviewe

Direct Measurement of the Local Glass Transition in Self-Assembled Copolymers with Nanometer Resolution

Nanoscale compositional heterogeneity in block copolymers can impart synergistic property combinations, such as stiffness and toughness. However, until now, there has been no experimental method to locally probe the dynamics at a specific location within these structured materials. Here, this was achieved by incorporating pyrene-bearing monomers at specific locations along the polymer chain, allowing the labeled monomers’ local environment to be interrogated via fluorescence. In lamellar-forming poly­(butyl methacrylate-<i>b</i>-methyl methacrylate) diblock copolymers, a strong gradient in glass transition temperature, <i>T</i>_g, of the higher-<i>T</i>_g block, 42 K over 4 nm, was mapped with nanometer resolution. These measurements also revealed a strongly asymmetric influence of the domain interface on <i>T</i>_g, with a much smaller dynamic gradient being observed for the lower-<i>T</i>_g block

Characteristic Length of the Glass Transition in Isochorically Confined Polymer Glasses

We report the effect of isochoric confinement on the characteristic length of the glass transition (ξ_α) for polystyrene (PS) and poly­(4-methylstyrene) (P4MS). Utilizing silica-capped PS and P4MS nanoparticles as model systems, ξ_α values are determined from the thermal fluctuation model and calorimetric data. With decreasing nanoparticle diameter, ξ_α decreases, suggesting a reduction in the number of segmental units required for cooperative motion at the glass transition under confinement. Furthermore, a direct correlation is observed between ξ_α and the isochoric fragility (<i>m</i>_v) in confined polymers. Due to a nearly constant ratio of the isochoric to isobaric fragility in confined polymer nanoparticles, a correlation between ξ_α and <i>m</i>_v also implies a correlation between ξ_α and the volume contribution to the temperature dependence of structural relaxation. Lastly, we observe that when the fragility and characteristic length are varied in the same system the relationship between the two properties appears to be more correlated than that of across different bulk glass-formers

A One-Step and Scalable Continuous-Flow Nanoprecipitation for Catalytic Reduction of Organic Pollutants in Water

Efficient treatment of organic pollutants in water by a facile and green technique is a great challenge for environmental remediation. In this study, we report a simple and low-energy strategy for catalytic reduction of organic pollutants in water by continuous-flow flash nanoprecipitation. The one-step processing technique integrates rapid metal@polymer nanoparticle production and catalytic reaction in a continuous-flow fashion. Such a concept is successfully demonstrated for simultaneous formation of Au@polymer nanospheres and catalytic reduction of organic pollutants (e.g., methylene blue and 4-nitrophenol) in water. Furthermore, the catalytic reaction rate could be easily tuned by varying the processing parameters (e.g., feeding concentration). The activity of the nanocatalyst was demonstrated in five recycles without any detectable loss. The characteristics of continuous-flow mode make the one-step process scalable, promising processing methodology for wastewater treatment

A One-Step and Scalable Continuous-Flow Nanoprecipitation for Catalytic Reduction of Organic Pollutants in Water

Efficient treatment of organic pollutants in water by a facile and green technique is a great challenge for environmental remediation. In this study, we report a simple and low-energy strategy for catalytic reduction of organic pollutants in water by continuous-flow flash nanoprecipitation. The one-step processing technique integrates rapid metal@polymer nanoparticle production and catalytic reaction in a continuous-flow fashion. Such a concept is successfully demonstrated for simultaneous formation of Au@polymer nanospheres and catalytic reduction of organic pollutants (e.g., methylene blue and 4-nitrophenol) in water. Furthermore, the catalytic reaction rate could be easily tuned by varying the processing parameters (e.g., feeding concentration). The activity of the nanocatalyst was demonstrated in five recycles without any detectable loss. The characteristics of continuous-flow mode make the one-step process scalable, promising processing methodology for wastewater treatment