Double Glass Transition Temperatures of Poly(methyl methacrylate) Confined in Alumina Nanotube Templates

Recently, confinement of polymers with different geometries has become a research hotspot. Here, we report the dramatic deviation of glass transition behaviors of poly­(methyl methacrylate) (PMMA) confined in cylindrical nanopores with diameter significantly larger than chain’s radius of gyration (<i>R</i>_<i>g</i>). Fast cooling a PMMA melt in the nanopores results in a glass with one single glass transition temperature (<i>T</i>_g). But two distinct <i>T</i>_gs are detected after slow cooling the melt. The deviation in <i>T</i>_g could be as large as 45 K. This phenomenon is interpreted by a two-layer model. During vitrification under slow cooling two distinct layers are formed: a strongly constrained interfacial layer showing an increased <i>T</i>_g as compared to that of the bulk polymer and a core with a decreased <i>T</i>_g. By thermal annealing experiments, we find that these two <i>T</i>_gs are inherently correlated. In addition, the deviation of <i>T</i>_g for PMMA confined in nanopores reveals a dependence on molecular weight

Effect of Molecular Chain Architecture on Dynamics of Polymer Thin Films Measured by the Ac-Chip Calorimeter

It was reported that glass transition temperature (<i>T</i>_g) measured by differential alternating current (ac) chip calorimetry showed little thickness dependence for polymer films. Here we demonstrate the detection of <i>T</i>_g in thin films by ac-chip calorimeter and show that <i>T</i>_g is decreased as the thickness is reduced for oligomers and star-shaped polymers, as compared with their long linear analogues. The deviation range is a few to more than ten Kelvin. Such a depression in <i>T</i>_g is quite pronounced for ac-chip calorimetric measurement at a high frequency of 10 Hz. We argue that the perturbation in the increased interfacial free volume for spin-cast oligomers and dendrimers is the major reason for increasing segmental dynamics for ultrathin films

Acceleration of Crystal Growth of Amorphous Griseofulvin by Low-Concentration Poly(ethylene oxide): Aspects of Crystallization Kinetics and Molecular Mobility

This study aims to investigate the crystallization behavior and molecular dynamics of amorphous griseofulvin (GSF) in the presence of low-concentration poly­(ethylene oxide) (PEO). We observe that the addition of 3% w/w PEO remarkably increases the crystal growth rate of GSF by two orders of magnitude in both the supercooled liquid and glassy states. The liquid dynamics of amorphous GSF in the presence and absence of PEO are characterized by dielectric spectroscopy. With an increase of the PEO content, the α-relaxation times of the systems decrease, indicating the increase of global molecular mobility. The couplings between molecular mobility and crystallization kinetics of GSF systems show strong time-dependences below <i>T</i>_g. The overlapping of α-relaxation times of GSF in presence and absence of PEO as a function of <i>T</i>_g/<i>T</i> suggest the “plasticization” effect of PEO additives. However, the crystallization kinetics of amorphous GSF containing low-concentration PEO do not overlap with those of pure GSF on a <i>T</i>_g/<i>T</i> scale. The remarkable accelerating effect of crystal growth of amorphous GSF by low-concentration PEO can be partially attributed to the increase of global mobility. The high segmental mobility of PEO is expected to strongly affect the crystal growth rates of GSF. These findings are relevant for understanding and predicting the physical stability of amorphous pharmaceutical solid dispersions

Thickness Dependence of Glass Transitions Measured by AC-Chip Calorimetry in Films with Controlled Interface

When most prior studies on thin polymer films have shown that glass transition temperature (<i>T</i>_g) decreases under nanoconfinement, the differential alternating current chip (ac-chip) calorimetric method shows little dependence of <i>T</i>_g on thickness for supported film. To reveal this contradiction, we have manipulated a controlled interface by spin-coating polystyrene (PS) with immiscible surfactants such as tetraoctylammonium bromide or citric acid. Since the immiscible surfactants did not show plasticizing effect for PS, there was no observable reduction of <i>T</i>_g from the bulk value ether in powdered blends or in thick films. However, the ultrathin film with thickness <i>h</i> ∼ 25 nm, consisting of 95 wt % PS and 5 wt % surfactants, showed a reduction of <i>T</i>_g by 6–7 °C, as compared to thick film with the same composition. We propose that the surfactant molecules assembled on the interface between thin film and substrate due to phase separation. The molecular mobility of molecules at the interface was dramatically increased, which was detected by ¹NMR with dipolar filter sequence. It appeared that the deviation range was not so large as that measured by other methods. But considering that we were measuring <i>T</i>_g at a high frequence (10 Hz), this amount of deviation was quite significant for ac-chip calorimetry. As a result, ac-chip calorimetry measured <i>T</i>_g data unambiguously demonstrate that thickness dependence of <i>T</i>_g is a real property of confined thin film

Diffusion Behavior of Polystyrene/Poly(2,6-dimethyl-1,4-phenylene oxide) (PS/PPO) Nanoparticles Mixture: Diffusion Mechanism for Liquid PS and Glassy PPO

We investigated the diffusion behavior of polystyrene/poly­(2,6-dimethyl-1,4-phenylene oxide) (PS/PPO) nanoparticles mixture prepared by the nanoprecipitation method. The diffusion experiments of liquid PS into the glassy PPO matrix (<i>l</i>-PS/<i>g</i>-PPO) were conducted by annealing the PS/PPO mixture at temperatures between the glass transition temperatures (<i>T</i>_gs) of the PS and PPO components. By tracing the <i>T</i>_g evolution of the PS-rich domain behind the diffusion front, we obtained the master curve of PS volume fraction during diffusion by time–temperature superposition (TTS) and studied the diffusion mechanism of the <i>l</i>-PS/<i>g</i>-PPO system based on the core–shell model. As there is ongoing debate on the diffusion mechanism for the liquid/glassy polymers interdiffusion, herein we confirm that the diffusion behavior of PS/PPO nanoparticles mixture follows the characteristics of the Fickean mechanism rather than the case II mechanism. Both of the shift factors (<i>a</i>_T) and the diffusion coefficients in the initial (<i>D</i>_initial) obey the Arrhenius equation, which yield almost the same apparent activation energy (<i>E</i>_df) (about 153.6 kJ/mol). As the PS/PPO nanoparticles mixture is a limited liquid supply system, both of the calorimetric and rheological measurements reveal the departure in the time scaling laws, which corresponds to the change of PS chain dynamics from the reptation type to the Rouse type during the diffusion process

Growth of Polymer Nanorods with Different Core–Shell Dynamics via Capillary Force in Nanopores

The dynamics of poly­(<i>n</i>-butyl methacrylate) confined in porous anodic aluminum oxide (AAO) templates are investigated using differential scanning calorimetry (DSC) and fluorescence nonradiative energy transfer (NRET). Two glass transition temperatures (<i>T</i>_g,low and <i>T</i>_g,high) are obtained at higher infiltration temperatures via capillary force followed by slow cooling. <i>T</i>_g,low resembles the <i>T</i>_g of the bulk phase and represents the transition of the core layer. <i>T</i>_g,high represents the transition of the adsorbed layer in the confined polymer glass. The temperature threshold to form one or two glass transitions is determined by adjusting the infiltration temperatures and the pore diameters. It is shown that the adsorbed layer has increased interchain proximity relative to the bulk. In addition, the glass transition behavior is hypothesized to be mediated by the counterbalance of the size and interfacial effects in the confined space. The easily synthesized core–shell nanofibers with one glassy and one rubbery component without the need for block polymers have promising potential for use in several processing strategies

Detecting Surface Hydration of Poly(2-hydroxyethyl methacrylate) in Solution <i>in situ</i>

Understanding the interfacial molecular structures of antifouling polymers in solutions is extremely important in research and applications related to chemistry, biology, and medicine. However, it is generally challenging to probe such buried solid/liquid interfaces <i>in situ</i>. We herein report a molecular-level study on detecting the interfacial molecular structures of an antifouling hydrogel material, poly­(2-hydroxyethyl methacrylate) (PHEMA), in contact with water and bovine serum albumin (BSA) solution <i>in situ</i> using sum frequency generation (SFG) vibrational spectroscopy. To compare to and validate our <i>in situ</i> experiments, molecular-level structures of the substrate/PHEMA interface before and after water exposure were also detected. The detected strong O–H vibrational signals from water and hydroxyethyl and carbonyl vibrational signals from PHEMA prove that the PHEMA surface hydration was attributed to the interaction between water and PHEMA side hydrophilic groups. SFG experimental results verify that the adsorbed BSA molecules at the PHEMA/solution interface were disorderly arranged, supported by data from the laser scanning confocal microscopic (LSCM) experiment. This indicates the weak interaction between the BSA molecules and PHEMA surface. This direct detection of the surface hydrated structures of PHEMA sheds light on understanding the interfacial properties of antifouling materials in aqueous environments. The capability reported here to probe the PHEMA/solution interface and the hidden substrate/PHEMA interface after water exposure can be applied to investigate a broad range of interfaces of antifouling materials

Contribution of the Polarity of Mussel-Inspired Adhesives in the Realization of Strong Underwater Bonding

Although the role of 3,4-dihydroxyphenyl-<i>L</i>-alanine­(DOPA)­in mussel foot proteins (mfps) in the realization of underwater bonding has been widely recognized, the role of the polarity of the polymer was largely overlooked. Here, by systematically comparing the underwater bonding properties of four mussel-inspired adhesives with different amide/lactam contents but similar catechol contents and molecular weights, we came to the conclusion that the polarity of the polymers also contributes to the strong underwater bonding. With the increase in the amide/lactam contents, the polarity of the polymeric adhesive increases, which correlates to the improved underwater bonding strength. A dielectric constant is introduced to evaluate the polarity of the polymer, which may be used as a guidance for the design of mussel-inspired adhesives with even better underwater bonding properties

Glass Transitions of Poly(methyl methacrylate) Confined in Nanopores: Conversion of Three- and Two-Layer Models

The glass transitions of poly­(methyl methacrylate) (PMMA) oligomer confined in alumina nanopores with diameters much larger than the polymer chain dimension were investigated. Compared with the case of 80 nm nanopores, PMMA oligomer confined in 300 nm nanopores shows three glass transition temperatures (from from low to high, denoted as <i>T</i>_g,lo, <i>T</i>_g,inter, and <i>T</i>_g,hi). Such phenomenon can be interpreted by a three-layer model: there exists an interphase between the adsorbed layer and core volume called the interlayer, which has an intermediate <i>T</i>_g. The behavior of multi-<i>T</i>_g parameters is ascribed to the propagation of the interfacial interaction during vitrifaction process. Besides, because of the nonequilibrium effect in the adsorbed layer, the cooling rate plays an important role in the glass transitions: the fast cooling rate generates a single <i>T</i>_g; the intermediate cooling rate induces three <i>T</i>_g values, while the ultraslow cooling rate results in two <i>T</i>_g values. With decreasing the cooling rate, the thickness of interlayer would continually decrease, while those of the adsorbed layer and core volume gradually increase; meanwhile, the <i>T</i>_g,lo gradually increases, <i>T</i>_g,inter almost stays constant, and the <i>T</i>_g,hi value keeps decreasing. In such a process, the dynamic exchanges between the interlayer and adsorbed layer, core volume should be dominant

Sensitive Characterization of the Influence of Substrate Interfaces on Supported Thin Films

The perspective by Ediger and Forrest stated that, while we know that the dynamics of polymers in ultrathin films can be significantly altered by substrate interfaces, our understanding of how this depends on the polymer structure and the particular interfaces is rudimentary. Here, we show that fluorescence nonradiative energy transfer (NRET) is an extremely sensitive method for characterizing the interfacial adsorption of polystyrene onto silicon dioxide, even though their interaction is often suggested to be weak. We observed that tensile stress was generated in the supported film by substrate adsorption, which imposes constraints on molecular motion and prevents a reduction of the glass transition temperature (<i>T</i>_g). Furthermore, our investigation suggests that modifying the surface chemistry of the substrate can change the film conformation and dynamics when the film is thinner than 40 nm