Segmental Dynamics in Poly(Methyl Acrylate) on Silica: Molecular-mass Effects

The effect of molecular mass on the segmental dynamics of poly(methyl acrylate) (PMA) adsorbed on silica was studied using deuterium quadrupole-echo nuclear magnetic resonance (NMR) and modulated differential scanning calorimetry. Samples adsorbed on silica (all about 1.5 mg PMA/m2 silica) were shown to have more restricted segmental mobility, and higher Tg\u27s, than the corresponding bulk PMA samples. Around the glass-transition region, adsorbed samples exhibited segmental mobility, which could be classified as heterogeneous due to a superposition of more-mobile and less-mobile components present in the deuterium NMR spectra. This heterogeneity was consistent with a motional gradient with more-mobile segments near the polymer-air interface and the less-mobile species near the polymer-silica interface. The mobility of the adsorbed 77 kDa PMA sample was the lowest among the four different molecular-mass samples studied. Samples studied with masses both larger and smaller than 77 kDa had larger mobile-component fractions in the adsorbed polymer. The additional mobility was attributed to the presence of either longer tail and loop conformations in the higher molecular-mass samples or the inherent mobility of the tails in the lower molecular-mass samples on the surface

Molecular Mass and Dynamics of Poly(Methyl Acrylate) in the Glass Transition Region

The segmental dynamics of bulk poly(methyl acrylate) (PMA) were studied as a function of molecular mass in the glass-transition region using 2H NMR and modulated differential scanning calorimetry (MDSC). Quadrupole-echo 2H NMR spectra were obtained for four samples of methyl-deuterated PMA-d3 with different molecular masses. The resulting spectra were fit using superpositions of simulated spectra generated from the MXQET simulation program, based on a model incorporating nearest-neighbor jumps from positions on the vertices of a truncated icosahedron (soccer-ball shape). The lower molecular-mass samples, influenced by the presence of more chain ends, showed more heterogeneity (broader distribution) and lower glass transitions than the higher molecular-mass samples. The MDSC experiments on both protonated and deuterated samples showed behavior consistent with the NMR results, but temperature shifted due to the different frequency range of the measurements in terms of both the position and breadth of the glass transition as a function of molecular mass

Thermogravimetric Studies of PMMA on Silica

The thermal degradation of poly(methyl methacrylate) (PMMA) has been studied extensively. It is well accepted that the degradation process is a radical chain reaction involving initiation, depropagation, transfer, and termination reactions. In the degradation process, the factors having observable effects on the decomposition behavior include molecular mass, polydispersity, tacticity, and sample dimensions. The effects of tacticity on the degradation behavior of PMMA were investigated by Kashiwagi et al., Jellinek et al. And Chiamtore et al. The results indicated that both isotactic and syndiotactic PMMA have similar decomposition pathways and activation energies. Isotactic PMMA will decompose at a slightly lower temperature and over a broader range, compared to syndiotactic PMMA with the similar chain ends and molecular masses. It has also been reported that isotactic PMMA is more sensitive to electron-beam radiation, namely, it degrades more easily, than syndiotactic PMMA. The PMMA-SiO2 system has been investigated by a few groups. The results indicated that the interaction between the carbonyl groups and silica surfaces can increase the decomposition temperature of PMMA. However, how different variables affect the decomposition behavior of PMMA is still far from clear. In the present work, we report studies of the degradation of ultrathin adsorbed PMMA on silica with a specific focus on the effects of adsorbed amount and tacticity on decomposition

Dynamics of PIPA-d₇ on Silica Surface

Molecular motion of polymer chains is an important determinant in understanding the physical properties of polymeric materials. Glass transition temperature (Tg) is a physical property of polymers, which is of primary interest. The study of the dynamics of polymer segments assists in understanding the dependence of Tg on polymer structure.1 For decades, studies have addressed the molecular motion in various polymers. Some of them have probed the dynamics of polymer backbones.2,3 the properties of a polymer at an interface may change because of the type of polymer, the substrate, or other variables. The side chain of a polymer can also play an important role in terms of the interaction between a polymer and a substrate at an interface.4 the strength of the surface-segment interaction affects the mobility of polymer-chain segments. Several techniques have been used to investigate the effects, including modulated differential scanning calorimetry (MDSC)5,6 and nuclear magnetic resonance (NMR).2,3,7,8 in this work, relatively narrow polydispersity poly(isopropyl acrylate)-d7 (PIPA-d7) has been selected for study. The large and bulky group on the PIPA side-chain, would provide a different probe for segmental mobility than that of previously studied poly(methyl acrylate)-d3 (PMA-d3).9-11 the PIPA sidechain contains two methyl groups, branched at a methane carbon atom. Additionally, when different amounts of polymers are deposited on a surface, individual unique behaviors become evident that are different from the behavior of bulk polymers. Deuterium solid-state NMR was used to characterize the polymer segmental motions in both bulk deuterium-labeled PIPA and polymer thin films on silica. The 2H quadrupole-echo NMR spectra were collected as a function of temperature. The interpretation of those spectra can provide valuable information on the molecular motion and the physical properties of the polymer including glass transition temperatures. Calorimetry, the most widely accepted technique for measuring the glass transition temperature (Tg), was also performed for comparison. This work is an update of that previously presented.1

Segmental Mobility of Chain Ends in Poly(Methyl Acrylate)-d3

Better control of polymeric materials can be achieved with a thorough understanding of the dynamics of their constituents. In the present study, we consider polymer chains as composed of chain middles and chain ends. Even though chain ends do not comprise much of the sample by mass, they may play a crucial role in the ultimate properties of the polymers. Although chain ends have been assigned a higher mobility, as compared to chain middles, there have not been a large number of experimental studies that directly probe their mobility. Among those, the studies of Kitahara et al.1 and Miwa et al.2 demonstrated the higher mobility of chain ends using ESR for polyethylene and polystyrene (PS), respectively. They observed that the transition temperatures for the onset of rapid molecular motion at the chain ends were 5 K lower than those inside the chains. A molecular mass dependence was also observed for the transition for the chain ends. A specular neutron reflectivity (SNR) study3 indicated that segments in the center sections of PS have a lag in mobility across a welded interface, as compared to chain ends. Previous deuterium NMR studies,4,5 by our group, have shown the a molecular-mass dependence on segmental dynamics through the glasstransition region. More heterogeneous segmental dynamics were observed in the NMR spectra of lower molecular mass poly(methyl acrylate) (PMA) compared to those of higher molecular mass. This heterogeneity was attributed to the presence of a higher concentration of more-mobile chain ends. In this study, the segmental dynamics of the PMA samples, with methyl-groupdeuterated chain ends, was studied using the 2H quadrupole-echo NMR technique. These results provided significant insight into the role of chain ends in the glass transition region of polymers

Segmental Dynamics of Poly(Isopropyl Acrylate)-d7 on Silica

For a polymer film deposited on a surface, the strength of the surfacesegment interaction affects the mobility of polymer-chain segments. The selfconsistent field lattice model of Scheutjens and Fleer,1 based on mean-field lattice models of polymer at interfaces,2 has been used to describe the distribution of conformations of polymers on surfaces. Adsorbed-polymer segments may be classified as belonging to loops, trains or tails. There are different techniques used to study the molecular motion of the polymer including modulated differential scanning calorimetry (MDSC)3 and nuclear magnetic resonance (NMR).4,5 in this work, solid-state deuterium (2H) NMR was used to characterize the polymer segmental motions. Solid-state 2H NMR is an excellent tool for studying segmental dynamics. The interpretation of solid-state 2H NMR spectra of a deuterium-labeled polymer can provide valuable information on the molecular motion and the physical properties of the polymer.6,7 Relatively narrow polydispersity poly(isopropyl acrylate)-d7 (PIPA-d7) has been studied using deuterium NMR. PIPA has two methyl groups, branched at a methine carbon atom. Substitution of deuterons onto these methyl groups provides a different probe for the segment mobility than that used in previous studies.7-9 the structure of the side chains of PIPA-d7 is different than that of poly(methyl acrylate) previously studied. Bulk and surface-adsorbed PIPA-d7 polymers were investigated using the 2H quadrupole-echo NMR technique as a function of temperature

Thermal Analysis of Ultrathin PS-r-PMMA Copolymer Films on Silica

Composite materials represent a major part of man-made materials that are used in many applications. The interaction of polymers with surfaces plays a crucial role in the final properties of these materials and, by understanding the surface processes and adsorption mechanisms, better systems can be designed. Therefore, the behavior of polymer molecules at interfaces has been the topic of many studies in recent years and a rough picture has been obtained. Keddie et al. Obtained experimental results showing the dependence of the glass transition temperature (Tg) on the thickness of supported polystyrene films using ellipsometry. Later, they also investigated supported poly(methyl methacrylate)(PMMA) films. Their results indicated that a strongly attractive interaction between the polymer film and substrate (e.g. H-bonding between PMMA and the silicon native oxide) was responsible for an increase in Tg with decreasing film thickness. On the other hand, a weak interaction, as in PS with silicon oxide, resulted in a decrease in Tg with decreasing film thickness. It was suggested that the reduction in the Tg value was caused by the presence of a liquid-like layer at the polymer-air interface. Estimates based on the observed thermal expansivities suggest that the characteristic length scale for this layer is ~80-130 Å. Similar results were obtained by other techniques, such as Brillouin light scattering, X-ray reflectivity, positron annihilation lifetime spectroscopy, fluorescence recovery after patterned photobleaching, atomic force microscopy and magnetic resonance (NMR and ESR)3-7. Although many new techniques have been used and much useful information has been obtained, the understanding of the properties of polymers adsorbed on solid surfaces is still far from complete. Porter and Blum used modulated differential scanning calorimetry (MDSC) to investigate the thermal behavior of PMMA thin films adsorbed on silica. They found that the Tg of the adsorbed PMMA layer, at maximum adsorbed amount from toluene, was raised to 136o C and 158o C for half of that amount. Song et al. used MDSC to quantify the interfacial fractions in polymer blends. These studies suggest that MDSC may be a useful tool for investigating very small amounts of species on surfaces. We report use of MDSC to investigate silica adsorbed PS-r-PMMA copolymers as a function of the adsorbed amount and copolymer composition. The derivative of the heat capacity signal, dCp/dT, from MDSC was used to elucidate the fractions of different mobility. In this way we were able to see the changes of fractions with different mobilities directly from the DSC curves

Segmental Dynamics in Poly(Methyl Acrylate)-d3 on Strongly and Weakly Adsorbing Silica Surfaces

The segmental dynamics of a polymer on a solid surface appears to be significantly different than that in bulk. The strength of the interaction between polymer segments and a solid surface is likely to be an important factor affecting the segmental mobility of the polymers. Studies of polymers on a weakly adsorbing surface would provide additional understanding of their motional characteristics on surfaces. In a recent NMR study by Rivillon et al., chain dynamics was seen to be different for samples, even when there was no specific interaction with the surface. Peanasky et al. have found a restricted segmental dynamics for the polyphenylmethylsiloxane samples at weakly adsorbing surfaces. Zheng et al. Observed a significant difference from bulk dynamics on both strongly and weakly interacting interfaces for poly(styrene) (PS) and poly(2-vinylpyridine). Mechanical properties of weakly interacting poly(methyl acrylate) (PMA)- silica composites were seen to become poorer than the ones with stronger interactions. Contrasting results for PS on silicon surfaces with X-ray reflectivity also highlighted the importance of the surface-polymer interaction for thin films.5,6 Wallace et al. explained that the contrasting results were due to a difference in the surface characteristics. A stronger surface-polymer interaction on a hydrogen-terminated silicon resulted in an increase in glass transition temperature (Tg) for thin films of PS, while Keddie et al.â s work on a silicon native-oxide surface with the same polymer indicated a decrease in Tg for thin films due to a weaker interaction. The segmental dynamics of PMA-d3 on a strongly adsorbing surface was previously studied using the deuterium (2H) quadrupole-echo NMR and modulated differential scanning calorimetry. In this study, the segmental dynamics of PMA on a weakly adsorbing surface was investigated with 2H quadrupole-echo NMR and compared to that in bulk and on a strongly adsorbing surface

Molecular Mass and Dynamics in PMA-d₃ in the Glass Transition Region

The segmental dynamics through the glass transition region, by which the glassy polymer becomes rubbery, are not well understood. Free volume, thermodynamic and kinetic theories have been used to describe the phenomenon behind the glass transition and some of those theories were tested with computer simulations. Different experimental techniques, such as NMR, thermo-mechanical analysis (TMA), differential scanning calorimetry (DSC), dilatometry and dynamic mechanical analysis (DMA) have all been also used to probe this important phenomena. Deuterium (2H) NMR has been a valuable tool for the investigation of the dynamics of macromolecules. Deuteration of macromolecules at specific locations on the chains does not significantly affect the properties of polymers. Spiess et al. Investigated motions in the glass transition region using 1D and 2D exchange NMR experiments on deuterated polystyrene. Rössler et al. studied the molecular dynamics in deuterated binary liquids close to the glass transition temperature (Tg) using 2D Exchange NMR. Blum et al. studied the effect of molecular mass on dynamics through glass transition for poly(methyl acrylate) (PMA). They found a “homogeneity” of the dynamics in the glass transition region for high molecular mass sample, but heterogeneity for the low molecular mass sample. In addition, the polydispersities of the samples were large. In this paper, we report studies of the dynamics of more monodisperse poly(methyl acrylate)-d3 (PMA)-d3 samples around the glass transition region using 2H quadrupole echo NMR and modulated DSC (MDSC)

NMR and Modulated Differential Scanning Calorimetry of Adsorbed Poly(Methyl Acrylate) on Silica

The interaction between polymers and solid substrates can be studied by a variety of techniques. The results from different experiments can be affected by the nature of the polymer, surface, polymer-surface interactions and, also, the experiment. A great deal has been learned about the behavior of polymers at interfaces, but there is also much to be learned about the interactions and how they affect the physical properties of adsorbed polymers. In the present study, we report both deuterium nuclear magnetic resonance (NMR) and modulated differential scanning calorimetry (MDSC) of poly(methyl acrylate) (PMA) adsorbed on Cab-O-Sil silica. The focus of this study is on the dynamics of polymer segments and the interfacial glass transition temperature (Tg). In our case, very small amounts of polymer were adsorbed, and we can view behavior of polymer segments in close proximity to the silica surface. We found that both NMR and MDSC experiments provide consistent insight into the dynamics of adsorbed polymers. Both experiments show the heterogeneous nature of the adsorbed polymers, with the polymer segments closer to the solid surface being the most motionally restricted resulting in a higher Tg for those segments