Probing polymer/polymer interfaces

Infrared-visible sum frequency generation spectroscopy (SFG) has been used to study the interface between poly(vinyl-N-octadecylcarbamate-co-vinyl acetate) (Comb) and deuterated or hydrogenated polystyrene (dPS or hPS) films. Strong methyl symmetric and Fermi resonance bands associated with the alkyl side chains of the Comb polymer are observed in the SFG spectra. In addition, for Comb/hPS spectra, symmetric and asymmetric vibration modes of phenyl groups are observed. The presence of asymmetric modes indicates the phenyl rings are tilted with respect to the interface normal.     

Changes in thermodynamic interactions at highly immiscible polymer/polymer interfaces due to deuterium labeling

Deuterium labeling has been shown previously to affect thermodynamic interactions at polymer surfaces, polymer/polymer heterogeneous interfaces, and in bulk (away from a surface or interface). However, the changes in polymer-polymer interactions due to deuterium labeling have not been thoroughly investigated for highly immiscible systems. It is shown here that deuterium labeling can influence polymer-polymer interactions at heterogeneous interfaces with highly immiscible systems, namely, polystyrene/poly(2-vinylpyridine) (PS/P2VP), polystyrene/poly(4-vinylpyridine) (PS/P4VP), and polystyrene/poly(methyl methacrylate) (PS/PMMA). Using secondary ion mass spectrometry, segregation of deuterium labeled polystyrene (dPS) in a dPS + unlabeled PS (dPS:hPS) blend layer was observed at the dPS:hPS/hP2VP, dPS:hPS/hP4VP, and dPS:hPS/hPMMA heterogeneous interfaces. However, a reference system involving PS on a PS brush shows no segregation of dPS to the interface.     

Sum frequency generation studies at poly(ethylene terephthalate)/silane interfaces: hydrogen bond formation and molecular conformation determination

To better understand the effects of interfacial molecular orientation on adhesion to plastics, the interfaces between poly(ethylene terephthalate) (PET) and different silane coupling agents were probed using sum frequency generation (SFG) vibrational spectroscopy. The polymer/air interface was dominated by the ester carbonyl, methylene, and phenyl groups. Upon contacting the PET film with the amino-functional silane 3-aminopropyltrimethoxysilane (ATMS), the ester carbonyl stretch shifted to a lower energy indicating the formation of hydrogen bonds between the polymer surface and the silane molecules. This shift was not observed when silanes that contained no hydrogen bond donors, such as (3-glycidoxypropyl)-trimethoxysilane and n-butyltrimethoxysilane, were placed into contact with the PET surface. Further evidence of silane ordering at the interface was observed as vibrational peaks attributed to the C-H stretching of the silane methoxy headgroups dominated the PET/silane spectra. It was determined that the conformation of the ATMS molecules at the interface was such that the amino endgroups were oriented toward the interface while the methoxy headgroups were directed toward the silane bulk.     

Diffusion of one or more components of a silane adhesion-promoting mixture into poly(methyl methacrylate)

The surface-sensitive technique of sum frequency generation (SFG) vibrational spectroscopy has been applied to study the buried interfaces between different polymers including deuterated polystyrene (d-PS) and deuterated poly(methyl methacrylate) (d-PMMA) and a two-component silane adhesion-promoting mixture (SAPM) composed of (3-glycidoxypropyl)trimethoxysilane (gamma-GPS) and a methylvinylsiloxanol (MVS). Because of the dissolution of d-PS, no SFG CH stretching signals could be collected from the d-PS/gamma-GPS interface, and SFG signals collected from the d-PS/SAPM interface gradually disappeared over time. SFG results also showed that gamma-GPS can diffuse through the d-PMMA film. The diffusion of gamma-GPS through the d-PMMA film was confirmed by SFG studies on the interface between gamma-GPS and a d-PMMA/PS two-polymer layer system. Initially the SFG signal from the PS layer was detected. However, after gamma-GPS diffused through the d-PMMA film, the PS film was dissolved by the silane, and thus the SFG signal from PS was lost. Similar experiments have been carried out at the interface between the SAPM and the d-PMMA/PS two-polymer layer system and it was found that the diffusion time of the gamma-GPS in the SAPM through the d-PMMA film was significantly longer. These results were much different to those from previous SFG studies on the analogous PET interfaces and appear consistent with differences in solubility parameters calculated for these systems.     

Probing alpha-helical and beta-sheet structures of peptides at solid/liquid interfaces with SFG

We demonstrated that sum frequency generation (SFG) vibrational spectroscopy can distinguish different secondary structures of proteins or peptides adsorbed at solid/liquid interfaces. The SFG spectrum for tachyplesin I at the polystyrene (PS)/solution interface has a fingerprint peak corresponding to the B1/B3 mode of the antiparallel beta-sheet. This peak disappeared upon the addition of dithiothreitol, which can disrupt the beta-sheet structure. The SFG spectrum indicative of the MSI594 alpha-helical structure was observed at the PS/MSI594 solution interface. This research validates SFG as a powerful technique for revealing detailed secondary structures of interfacial proteins and peptides.     

Entropically mediated polyolefin blend segregation at buried sapphire and air interfaces investigated by infrared-visible sum frequency generation vibrational spectroscopy

The segregation behavior of binary polymer blends at hydrophilic solid sapphire and air interfaces was investigated by infrared-visible sum frequency generation (SFG) vibrational spectroscopy. SFG spectra were collected from a bulk miscible blend consisting of identical molecular weight (approximately 54,000) and similar surface free energy (29-35 dyn/cm) components of atactic polypropylene (aPP) and aspecific poly(ethylene-co-propylene) rubber (aEPR). Characteristic CH resonances of the blend were contrasted with those of the individual components at both buried (sapphire/polymer) and free (air/polymer) interfaces. Preferential segregation of the aPP component was observed after annealing at both air/polymer and sapphire/polymer interfaces. SFG spectra revealed ordering of the polymer backbone segments with the methylene (CH2) groups perpendicular to the surface at the sapphire interface and the methyl (CH3) groups upright at the air interface. The SFG results indicate that the surface composition can be determined from the peak intensities that are characteristic of each component and that conformational entropy played a likely role in surface segregation. aPP occupied a smaller free volume at the surface because of a statistically smaller segment length (aPP is more flexible and has a shorter length). In addition, the high density of the ordered CH3 side branches enhanced the surface activity by allowing the long-chain backbone segments of aPP to order at the surface.     

Surface structures and properties of polystyrene/poly(methyl methacrylate) blends and copolymers

Sum frequency generation (SFG) vibrational spectroscopy has been applied to study the molecular surface structures of polystyrene (PS)/poly(methyl methacrylate) (PMMA) blends and the copolymer between PS and PMMA (PS-co-PMMA) in air, supplemented by atomic force microscopy (AFM) and contact angle goniometer. Both the blend and the copolymer have equal weight amounts of the two components. SFG results show that both components, PS and PMMA, can segregate to the surface of the blend and the copolymer before annealing, although PMMA has a slightly higher surface tension. Upon annealing both SFG results and contact angle measurements indicate that the PS segregates to the surface of the PS/PMMA blend more but no change occurs on the PS-co-PMMA surface. AFM images show that the copolymer surface is flat but the 1:1 PS/PMMA blend has a rougher surface with island like domains present. The annealing effect on the blend surface morphology has also been investigated. We collected amide SFG signals from interfacial fibrinogen molecules at the copolymer or blend/protein solution interfaces as a function of time. Different time-dependent SFG signal changes have been observed, showing that different surfaces of the blend and the copolymer mediate fibrinogen adsorption behavior differently.     

Segmental dynamics of interfacial poly(methyl acrylate)-d(3) in composites by deuterium NMR spectroscopy.

The interface in composite materials containing an ultrathin layer of poly(methyl acrylate)-d(3) (PMA-d(3)) on silica was studied using deuterium NMR. PMA-d(3) was deposited from solution at saturation coverage from toluene onto silica. The samples were dried and composite samples made by hot pressing the PMA-d(3)/silica samples with hydrogenated polystyrene (PS) and high (HMW) and low (LMW) molecular weight hydrogenated poly(methyl acrylate) (PMA) as the overlayer. The interfacial layers of PMA-d(3) were studied at the air-polymer-silica and polymer-polymer-silica interfaces using deuterium solid-state quadrupole-echo NMR and the results compared to those for the bulk polymer. It was found that for samples at the air-polymer-silica interface, some of the polymer segments in the surface sample had segmental mobility higher than that of the corresponding bulk PMA-d(3) sample at the same temperature. When overcoated with unlabeled polymer, the interfacial polymer at the polymer-polymer-silica interface showed reduced mobility due to the presence of the overlayer. The adsorbed PMA-d(3), in the composite samples, decreased in mobility in the order of LMW-PMA > HMW-PMA > PS. The PS sample caused the greatest reduction in the PMA-d(3) interfacial mobility. The order was consistent with the segmental mobilities of the polymers used for the overlayers. The lower the mobility of the polymer used for the overlayer, the more restricted were the polymer segments in the adsorbed PMA-d(3) layer.     

Structure of poly(methyl methacrylate) chains adsorbed on sapphire probed using infrared-visible sum frequency generation spectroscopy

We have studied the orientation of the train segments of a poly(methyl methacrylate) (PMMA) adsorbed layer at the CCl4-sapphire interface using surface-sensitive IR-visible sum frequency generation (SFG) spectroscopy. The SFG spectra of PMMA chains adsorbed on sapphire indicate ordered ester methyl groups. In comparison, we did not observe any significant contributions from the backbone methylene and alpha methyl groups, suggesting that these groups are disordered. No change in the structure of the adsorbed layer is observed upon cooling the solvent below the theta temperature; this is consistent with the picture of flat adsorbed chains on the surface. Interestingly, the orientation of the ester methyl groups of a spin-coated PMMA film at the PMMA-sapphire interface is similar to that of the same groups in the chains adsorbed from solution.     

Quantifying the ordering of adsorbed proteins in situ

We have investigated the orientation and conformation of protein molecules at the polystyrene (PS)/protein solution interface using sum frequency generation (SFG) vibrational spectroscopy, supplemented by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). In this research, we studied fibrinogen as a model protein. SFG studies indicate that fibrinogen adopts a bent structure after adsorbing to the PS surface. A broad orientation distribution of fibrinogen coiled-coils at the interface has been quantified by combining SFG and ATR-FTIR measurements. Error analysis for such a deduced distribution was carried out. This research demonstrates that quantitative structural information such as orientational and conformational ordering of proteins at interfaces can be studied using SFG supplemented by other spectroscopic techniques.     

Molecular level structures of poly(n-alkyl methacrylate)s with different side chain lengths at the polymer/air and polymer/water interfaces

Sum frequency generation (SFG) vibrational spectroscopy has been successfully applied to study molecular structures of several poly(n-alkyl methacrylate)s (PAMAs) with different side chain lengths at the PAMA/air and PAMA/water interfaces. We have observed that the ester side chains from all PAMAs always dominate the interface, but the orientation information of the methyl end group on the side chains varies, depending on the length of the side chain. The contributions from methylene groups on the side chains have been evaluated, and the surface structures have been related to the surface tension of these polymers. Different water restructuring behaviors have been observed for different PAMAs. This phenomenon and its reversibility are strongly dependent on the glass transition temperature of each polymer, which is influenced by the side chain length. Detailed data fitting and analysis has been discussed.     

Surface structural characterization of protein- and polymer-modified polystyrene microspheres by infrared-visible sum frequency generation vibrational spectroscopy and scanning force microscopy

Structural investigations of bare and surface-modified polystyrene microspheres (beads) have been carried out by infrared-visible sum frequency generation (SFG) vibrational spectroscopy and scanning force microscopy (SFM). Bead surfaces have been modified by either the covalent linking of immunoglobulin G (IgG) and bovine serum albumin (BSA) or the nonspecific adsorption of a Pluronic surfactant. After surface modification with protein, SFG signals in the aliphatic CH-stretch region are detected at both the buffer/bead and air/bead interfaces, indicating that some amino acid residues in proteins adopt preferred orientations. SFG results indicate that the hydrophobic poly(propylene glycol) moieties in the Pluronic order when adsorbed onto the bead, at both the buffer/bead and air/bead interfaces, whereas hydrophilic poly(ethylene glycol) groups align to a lesser extent. SFG spectra also show that the phenyl rings of bare polystyrene beads in contact with air or buffer are ordered, with a dipole component directed along the surface normal, but become less ordered after the adsorption of either proteins or the polymer. Molecular orientation and ordering at the bead surface affect its hydrophobicity and aggregation behavior. SFM results reveal the formation of nonuniform islands when bare beads with more hydrophobic character are spun-cast onto a silica substrate. In the presence of adsorbed protein, a hexagonal packing of beads, with some defects, is observed, depending on the bulk pH and the type of attached protein. Adsorbed Pluronic causes the beads to aggregate in a disordered fashion, as compared to the behavior of bare and protein-modified beads.     

Poly(alkyl α-chloroacrylates). IV. Tacticity analysis from 300 MHz proton magnetic resonance spectra

300 MHz proton magnetic resonance spectroscopy was applied to the determination of triad structures of three poly(alkyl α-chloroacrylates) of widely varying stereoregularities. Assignments and quantitative analyses were made for all of the tetrad peaks in the backbone methylene resonance of poly(methyl α-chloroacrylate), poly(ethyl α-chloroacrylate), and poly(isopropyl α-chloroacrylate). In addition the methyl singlet peaks of poly(methyl α-chloroacrylate), the methyl triplet peaks of poly(ethyl α-chloroacrylate) which had not been amenable previously to resolution at 220 MHz, and the methyl doublet peaks of poly(isopropyl α-chloroacrylate) were resolved, and the triad values which were obtained from these pendant ester group resonances compared favorably with triad values calculated from experimental tetrad values. As a demonstration of the internal agreement of the triad and tetrad peaks assignments, statistical calculations of the stereochemical structures of the actic and syndiotactic polymers were generally described well or to a first approximation by random selection statistics (Bernoullian) while the stereochemical statistics of the moderately to highly isotactic polymers were consistent with a nonrandom statistical process (first-order Markovian), as expected.     

Molecular restructuring at poly(n-butyl methacrylate) and poly(methyl methacrylate) surfaces due to compression by a sapphire prism studied by infrared-visible sum frequency generation vibrational spectroscopy

Infrared-visible sum frequency generation (SFG) vibrational spectroscopy, performed in visible wavelength total internal reflection (TIR) geometry, was used to determine the molecular structures of poly(n-butyl methacrylate) (PBMA) and poly(methyl methacrylate) (PMMA) surfaces in air and in contact with a smooth sapphire surface with and without the application of pressure. C-H vibrational resonances were probed optically to nondestructively examine the buried polymer/sapphire interfaces and obtain information about the molecular orientation in situ. These findings are contrasted with those of the same polymers cast from a toluene solution directly on the sapphire prism surface and annealed. Compared to polymer surface conformation in air, the SFG spectra of the deformed (compressed) PBMA at the sapphire interface illustrate that the ester butyl side chain restructures and tilts away from the surface normal. However, the molecular conformation in the similarly deformed PMMA at the sapphire interface is identical to that obtained in air, which is dominated by the upright-oriented ester methyl side chains. For PBMA and PMMA spin cast on sapphire and annealed, the surface structure of the undeformed PBMA at the sapphire interface is identical to that of the deformed PBMA at the sapphire interface, while the PMMA conformation is different and shows alpha-methyl group ordering. Since the glass transition temperature of PBMA is below room temperature, the rubbery state of PBMA demonstrates a melt-like behavior, evidenced by the fact that PBMA is in conformation chemical equilibrium at the sapphire surface even under compression. Due to the high glass transition temperature of PMMA, compression freezes PMMA in a metastable state, revealed by the restructured molecular conformation when annealed against the sapphire surface. The results of this study demonstrate that structural changes at buried polymer surfaces due to the application of contact pressure can be detected in situ by TIR-SFG vibrational spectroscopy.     

Molecular structure of an alkyl-side-chain polymer-water interface: origins of contact angle hysteresis

A new and direct approach to verify surface heterogeneity as the microscopic origin of contact-angle hysteresis is demonstrated. IR-visible sum-frequency-generation spectroscopy (SFG) was used to selectively probe the molecules at the interface of an alkyl-side-chain polymer [poly(vinyl n-octadecyl carbamate-co-vinyl acetate)] with water. The spectra indicate that in contact with water, the polymer surface is heterogeneous (having areas of differing surface energies). This evidence of surface heterogeneity supports the hysteresis observed in the advancing and receding contact angles of the polymer surface with water. The same measurements made for the chemically and structurally similar surface of an octadecyltrichlorosilane self-assembled monolayer indicates a homogeneous surface at the water interface. In this case, contact-angle hysteresis measurements implicate surface roughness as the cause of hysteresis. Atomic force microscopy measurements of roughness for these surfaces further support our conclusions. The polymer-water interface was probed using SFG at above-ambient temperatures, and an order-to-disorder transition (ODT) of alkyl side chains at the interface was observed, which closely follows the melting of crystalline side chains in the bulk. This transition explains the increased wettability of the polymer, by water, when the temperature is raised above the bulk melting temperature. Furthermore, the irreversibility of this ODT suggests that the disordered polymer-water interface is the thermodynamic equilibrium state, whereas the before-heating structure of this interface is a kinetically hindered metastable state.     

聚苯乙烯/蒙脱土纳米复合材料的自组装行为

聚合物／层状硅酸盐（PLS）纳米复合材料由于具有常规复合材料所没有的结构、形态以有较常规聚合物基复合材料更优异的物理力学性能等而引起人们的关注^[1],但以往文献^[1-3]主要报道PLS纳米复合材料的制备与性能表征,对于熔融加工过程中粘土粒子吸高分子的取向和结构研究很少。作者等^[4-6]发现了剥离型聚苯乙烯（PS）／蒙脱土纳米复合材料中的剪切诱导有序结构,并采用广角X射线衍射法（WAXD）、透射电镜法（TEM）和红外二向色性法对其形成机理进行了研究。结果表明,该有序结构的主要来源是分散在PS基体中的蒙脱土初级粒子（Primary particles）内部片层的规整排列以及沿平行于样品表面方面的平面取向,PS的苯环平面平行剪切流动方向取向,而烷项链未见明显取向。本文报道该纳米复合材料的剪切诱导有序结构在升温过程中出现的自组装行为,并用原位升温X射线衍射法和红外二向色性法对蒙脱土初级粒子的规整度以及PS的苯环和烷基链在升温过程中的取向行为进行了研究,在此基础上提出了可能的形成机理。     

Interfacial properties of free-standing poly(3-hexylthiophene) films

Using full atomistic classical molecular dynamics simulations, the interfacial properties of free-standing poly(3-hexylthiophene) (P3HT) films have been investigated. The orientations of different parts of the P3HT chain and the surface tensions of the films were calculated in a temperature range of 540 K-600 K. At the liquid/vacuum interface, the P3HT chain shows ordering by exposing hexyl groups at the interface, while the chain backbone lays flat with the thiophene ring preferentially tilt toward the surface. At the interface, the terminal methyl groups of hexyl side chains are in excess compared to the methylene groups or thiophene rings. The surface tension of P3HT in its melt state shows similar temperature dependence to that of polymers that have long alkyl side chains. The surface tension values are comparable to those polymers that expose methyl or methylene groups on the surface. The surface tension values determined for the melt state are lower than the experimental reported values for crystalline P3HT films, as expected.     

Surface structure relaxation of poly(methyl methacrylate)

Surface structure relaxations caused by temperature changes at the free surface of poly(methyl methacrylate) were studied using IR-visible sum-frequency generation (SFG). A polarization-rotating technique was introduced to enhance the sensitivity of SFG for monitoring the surface structure relaxations during a cooling process. A new surface structure relaxation was observed at 67 degrees C. This temperature does not match any known structure relaxation temperatures for the bulk and is 40 degrees C below the bulk glass transition temperature. As expected for a free-surface phenomenon, the surface relaxation temperature was found to be independent of film thickness in the range of 0.1-0.5 microm.     

A low temperature phase transition in Langmuir-Blodgett films

The influences of temperature on the SFG spectra of Langmuir-Blodgett films of cadmium stearate, ferric stearate, stearic acid and octadecanamide are reported. Upon cooling, all films display reversible discontinuous shifts of approximately 8 cm (-1) in the r+, r- and rfermi modes of the terminal methyl groups at approximately 150 K. Reversible changes in the relative intensities of these methyl group peaks, most pronounced in the PPP spectra, are also observed and attributed to a change in the environment of the methyl group that accompanies a discontinuous transition in the ordering of their alkyl chains. The onset of new spectral features at higher frequency is attributed to the observation of ordered water molecules contained within the films. The correlation between the onset of the water features and the onset of the reversible, discontinuous, spectroscopic changes of the amphiphiles argues for a causal connection between the two. In addition to the discontinuous behavior upon cooling, monolayer films of stearic acid and octadecanamide display activity of methylene modes upon exposure to vacuum. Films displaying SFG-active methylene groups at room temperature had them gradually become completely SFG-inactive by 100 K. Heating the films to room temperature revealed that the methylene group activity was reversible. Monolayer films of cadmium stearate and ferric stearate do not display this methylene activity upon exposure to vacuum, suggesting that this behavior may be linked to solvation of the amphiphile's headgroup. These observations suggest that water plays a key role in the stability and structure of LB supported monolayers, and have important implications to those interested in low temperature (cryogenic) effects of biological systems.     

Surface activity of myristic acid in the poly(methyl methacrylate)/supercritical carbon dioxide system

To confirm the surface activity of myristic acid in the dispersion polymerization of vinyl monomers in scCO2, the interfacial tension (IFT) at the polymer/supercritical carbon dioxide (scCO2) interface has been measured. For the IFT measurements, a high-pressure pendant drop apparatus was constructed. The IFT data was obtained by the axisymmetric drop shape analysis of melt polymer droplets formed at the tip of a capillary. The reliability of the apparatus was confirmed by measuring the IFT of polystyrene (PS)/scCO2 and polypropylene (PP)/CO2 systems. The IFT of the poly(methyl methacrylate) (PMMA)/scCO2 system with and without myristic acid was also measured. The IFT decreased on addition of myristic acid. The magnitude of the IFT depression due to the myristic acid was comparable to that of PS/scCO2 systems with the block copolymer surfactant, PS-b-poly(fluorooctyl acrylate). The surface activity of the myristic acid was confirmed by the decrease of IFT.