Michael Lo

Spatial Differentiation of Sub-Micrometer Domains in a Poly(hydroxyalkanoate) Copolymer Using Instrumentation that Combines Atomic Force Microscopy (AFM) and Infrared (IR) Spectroscopy

Atomic force microscopy (AFM) and infrared (IR) spectroscopy have been combined in a single instrument (AFM-IR) capable of producing sub-micrometer spatial resolution IR spectra and absorption images. This new capability enables the spectroscopic characterization of micro-domain-forming polymers at levels not previously possible. Films of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) were solution cast on ZnSe prisms, followed by melting and annealing to generate crystalline microdomains of different sizes. A tunable IR laser generating pulses of the order of 10 ns was used for excitation of the sample films. Short duration thermomechanical waves, due to infrared absorption and resulting thermal expansion, were studied by monitoring the resulting excitation of the contact resonance modes of the AFM cantilever. Dramatic differences in the room-temperature IR spectra are observed in the 1200–1300 cm−1 range as a function of position on a spatial scale of less than one micrometer. This spectral region is particularly sensitive to the polymer backbone conformation. Such dramatic spectral differences have also been observed previously in bulk IR measurements, but only by comparing room-temperature spectra with ones collected at higher temperatures. Less dramatic, but significant, AFM-IR spectral differences are observed in the carbonyl stretching region around 1720 cm−1 as a function of location on the sample. Two overlapping, but relatively sharp, carbonyl bands are observed near 1720 cm−1 in more crystalline regions of the polymer, while a broader carbonyl stretching band appears centered at 1740 cm−1 in the more amorphous regions. Using this spectral region, it is possible to monitor the development of polymer crystalline structures at varying distances from a nucleation site, where the site was generated by bringing a heated AFM tip close to a specific location to locally anneal the sample.

Nanoscale mid-infrared imaging of phase separation in a drug–polymer blend

The applicability of nanoscale mid-infrared (mid-IR) spectroscopy for the study of the micro- and nanostructure of pharmaceutical drug-polymer systems was explored. Felodipine-poly(acrylic acid) (PAA) blends were used as model systems. Standard atomic force microscopy evaluation as a function of drug-polymer composition suggested limited miscibility, in line with previous findings. Localized spectra on a 50:50 (w/w) felodipine-PAA dispersion revealed that the discrete submicrometer domains formed corresponded to an amorphous felodipine-rich phase while the continuous phase tended to be rich in PAA. Further, spectroscopic imaging at selected wavenumbers, enabling discrimination between both constituents, confirmed this finding and made it possible to chemically image differences in composition between each phase with submicrometer resolution.

Nanoscale Mid-Infrared Evaluation of the Miscibility Behavior of Blends of Dextran or Maltodextrin with Poly(vinylpyrrolidone)

Determining the extent of miscibility of amorphous components is of great importance for certain pharmaceutical systems, in particular for polymer-polymer and polymer-small molecule blends. In this study, the application of standard atomic force microscopy (AFM) measurements combined with nanoscale mid-infrared (mid-IR) spectroscopy was explored to evaluate miscibility in binary polymer blends. The miscibility characteristics of a set of 50/50 (w/w) polymer blends comprising of poly(vinylpyrrolidone) (PVP) with dextran or maltodextrin (DEX) of varying molecular weights (MWs) were investigated. Standard AFM characterization results show good agreement with inferences drawn from differential scanning calorimetry (DSC) analysis in terms of forming either single or two phase systems. AFM analysis also provided insight into the microstructure of the two phase systems and how domain sizes varied as a function of polymer MWs. Nanoscale mid-IR evaluation of the blends, performed by collecting local mid-IR spectra or spectral maps, provided an extra dimension of information about the dependence of polymer MWs on chemical composition of the different phases. AFM, combined with nanoscale mid-infrared analysis, thus appears to be a promising technique for the evaluation of miscibility in certain pharmaceutical blends.

Nanoscale infrared (IR) spectroscopy and imaging of structural lipids in human stratum corneum using an atomic force microscope to directly detect absorbed light from a tunable IR laser source

An atomic force microscope (AFM) and a tunable infrared (IR) laser source have been combined in a single instrument (AFM‐IR) capable of producing ~200‐nm spatial resolution IR spectra and absorption images. This new capability enables IR spectroscopic characterization of human stratum corneum at unprecendented levels. Samples of normal and delipidized stratum corneum were embedded, cross‐sectioned and mounted on ZnSe prisms. A pulsed tunable IR laser source produces thermomechanical expansion upon absorption, which is detected through excitation of contact resonance modes in the AFM cantilever. In addition to reducing the total lipid content, the delipidization process damages the stratum corneum morphological structure. The delipidized stratum corneum shows substantially less long‐chain CH2‐stretching IR absorption band intensity than normal skin. AFM‐IR images that compare absorbances at 2930/cm (lipid) and 3290/cm (keratin) suggest that regions of higher lipid concentration are located at the perimeter of corneocytes in the normal stratum corneum.

Localization of Human Hair Structural Lipids Using Nanoscale Infrared Spectroscopy and Imaging

Atomic force microscopy (AFM) and infrared (IR) spectroscopy have been combined in a single instrument (AFM-IR) capable of producing IR spectra and absorption images at a sub-micrometer spatial resolution. This new device enables human hair to be spectroscopically characterized at levels not previously possible. In particular, it was possible to determine the location of structural lipids in the cuticle and cortex of hair. Samples of human hair were embedded, cross-sectioned, and mounted on ZnSe prisms. A tunable IR laser generating pulses of the order of 10 ns was used to excite sample films. Short duration thermomechanical waves, due to infrared absorption and resulting thermal expansion, were studied by monitoring the resulting excitation of the contact resonance modes of the AFM cantilever. Differences are observed in the IR absorbance intensity of long-chain methylene-containing functional groups between the outer cuticle, middle cortex, and inner medulla of the hair. An accumulation of structural lipids is clearly observed at the individual cuticle layer boundaries. This method should prove useful in the future for understanding the penetration mechanism of substances into hair as well as elucidating the chemical nature of alteration or possible damage according to depth and hair morphology.

Applications of AFM-IR—Diverse Chemical Composition Analyses at Nanoscale Spatial Resolution

The combination of infrared (IR) spectroscopy and atomic force microscopy (AFM) has produced a technique, called AFM-IR, which is becoming one of the most important recent developments in the field of IR spectroscopy and chemical imaging. Conventional Fourier transform infrared (FT-IR) microspectroscopy is well established as a technique for chemical characterization of small samples down to the 3–10 mm size range. This diffraction-imposed size limit has prevented the application of FT-IR microspectroscopy to smaller analysis regions that are relevant to analysis problems in polymer materials and the life sciences. The nanoIR™ instrument (Anasys Instruments, Santa Barbara, CA) described here uses an AFM probe as the IR absorbance sensor and hence breaks through the diffraction limit to attain spatial resolution improvements of between one and two orders of magnitude beyond previous techniques. Thus, the AFM-IR concept provides chemical information from nanoscale regions of polymers and other organic materials. This article describes the physics behind the technique, followed by results from several applications.

Interface Analysis of Composites Using AFM-Based Nanoscale IR and Mechanical Spectroscopy

Introduction Composite materials are becoming increasingly important in today’s world, where lighter materials with enhanced properties are in high demand. Carbon fibers, carbon black, graphite, graphine, carbon nanotubes, quartz particles, nanocrystalline cellulose, and clays are among the materials being added to bulk polymers in an effort to achieve better properties and performance. It is important not only to determine the size and locations of nanoparticle inclusions in bulk polymers, but also to characterize the important interphase region where the components interact. This article describes how an atomic force microscope (AFM) combined with infrared (IR) spectroscopy and mechanical spectroscopy can be used to not only locate and determine the size of inclusions, but also to characterize them chemically and mechanically. After introducing AFM-IR spectroscopy and Lorentz contact resonance (LCR) methodology for obtaining nanoscale mechanical spectra and images, results from three specific applications will be discussed. These applications include an isotactic poly(propylene) film with added SiO2 particles, a polymer with carbon black particles incorporated under different processing conditions, and a carbon-fiber/epoxy composite material. The first example uses AFM-IR spectroscopy and IR absorbance imaging. The second example employs LCR mechanical property spectroscopy and imaging. The final example includes a combination of AFM-IR and LCR to obtain corroborating information about the important interphase region between carbon fiber and epoxy domains.

Nanoscale chemical composition mapping of polymers at 100nm spatial resolution with AFM-based IR spectroscopy

Atomic Force Microscopy (AFM) and infrared (IR) spectroscopy have been combined in a single instrument capable of producing sub-micron spatial resolution IR spectra and images. This new capability enables the sprectroscopic characterization of microdomain-forming polymers at levels not previously possible. Films of poly(3-hydroxybutyrate-co-3-hydroxyheanoate) were solution cast on ZnSe prisms. Dramitic differences in the IR spectra are observed in the 1200-1300 cm-1 range as a funstion of position on a spatial scale of less than one micron. This spectral region is particularly sensitive to the polymer crystallinity, enabling the identification of crystalline and amorphous domains within a single spherulite of this polymer.

Nanoscale Chemical Imaging via AFM coupled IR Spectroscopy

Conventional infrared spectroscopy is one of the most widely used tools in science and industry to identify materials via vibrational resonances of chemical bonds, but optical diffraction limits its spatial resolution to the scale of many microns. Atomic force microscopy (AFM) enjoys excellent spatial resolution and can measure mechanical, electrical, magnetic and thermal properties of materials, but has historically lacked the ability to perform robust chemical analysis. Two techniques, (1) AFM-based infrared spectroscopy (AFM-IR) and (2) scattering scanning near field optical microscopy (s-SNOM) have been developed which couple AFM with an IR source allowing the chemical identification capabilities of IR spectroscopy to extend to the nanoscale. As complementary techniques, AFM-IR and s-SNOM together provide an unrivaled capability to perform nanoscale chemical analysis on a diverse range of organic, inorganic, photonic and electronic materials. The AFM-IR technique achieves nanoscale spatial resolution by using the AFM probe as a local sensor of the IR absorption [1]. A sample is irradiated with light from a pulsed, tunable infrared laser. When the IR laser is tuned to a wavelength where the sample has an absorption peak, a portion of the incident IR light is absorbed and converted into heat resulting in a rapid thermal expansion of the absorbing region. This generates an impulse force on the AFM tip, inducing resonant oscillations of the AFM cantilever. IR absorption spectra can then be obtained by measuring the cantilever oscillation amplitude as a function of laser wavenumber. Because the amplitude of induced cantilever oscillation is directly proportional to the IR absorption of the sample [2], absorption spectra obtained by AFM-IR compare very well to conventional FTIR without the use of modelling or reference samples. More recent extensions of this technique have allowed chemical measurements to be performed on samples as thin as individual monolayers [3]. Figure 1 shows an example application of the AFM-IR measurement, nanoscale chemical measurements of the environmental degradation of a biomedical material. One challenge to implantable devices is the possible degradation of the material due to exposure to the in vivo environment. Polyurethane was exposed to this simulated environment for sufficient time to allow the initiation of surface degradation. The chemical mechanism of this degradation was then analyzed and mapped using the AFM-IR technique. The AFM-IR spectra clearly show the chain scission which occurs in the ether bonds of the polyurethane by the reduction in the peak at 1108 cm -1 . It also shows the formation of alcohol groups by the increase in the 1176 cm -1 absorption band and carboxylate groups (COO-) shown by the increase in the broad bands at 1652 and around 1400 cm -1 . These absorption bands can be used to map the degradation and determine how the degradation initiates and extends into the sample, information that was previously not achievable at this length scale. The s-SNOM technique has been used in the SPM field for more than two decades [4]. It uses a metallized AFM tip to enhance and scatter radiation from a nanometer scale region of the sample. The scattered radiation is detected in the far field and carries information about the complex optical properties of the nanoscale region of the sample under the metallized tip. Both the optical amplitude