Transcript Characterization

Characterization
X-ray diffraction and crystal structure
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X-rays have a wave length, l0.1-10Å.
This is on the size scale of the
structures we wish to study
X-rays interfere
constructively when the
interplanar spacing is
related to an integer
number of wavelengths in
accordance with Bragg’s
law:
nl  2d sin 
Because of the numbering system,
atomic planes are perpendicular to
their corresponding vector,
e.g., (111) is perpendicular to [111]
The interplanar spacing for a cubic
crystal is:
d hkl 
a
h2  k 2  l 2
Because the intensity of the
diffracted beam varies depending
upon the diffraction angle, knowing
the angle and using Bragg’s law
we can obtain the crystal structure
and lattice parameter
Bragg’s law only describes the size and shape of the unit cell
If there are parallel planes inside the unit cell, their reflections can
interfere constructively and result in zero intensity of the reflected beam
hence, different crystal structures will only allow reflections of particular
planes according to the following rules:
Spectroscopy
Electromagnetic energy, E = hn
can excite electrons to higher
molecular orbitals. The amount of
energy varies with chemical structure.
Wavelengths may cover the range
from ultraviolet-visible (UV-VIS) to
infrared (IR)
UV-VIS Spectrometers
The source beam (at a frequency determined by the filter) is split
and projected through the sample and a reference. The amount of
transmitted energy is compared. This is repeated at different
frequencies. If the sample absorbs energy at a particular wavelength
it shows up as a difference in the amount of energy transmitted.
Standard curves are generated by measuring a known material in
varying concentrations at a fixed wavelength (above).
Concentrations of unknown samples can be determined by
comparing absorbance to that of the standard curve.
The amount of energy absorbed may be calculated
using the Beer-Lambert law:
A = elC
Where
A = absorbance
e = molar absorption coefficient
(material dependent)
l = sample thickness
C = molar concentration
Infrared (FTIR) Spectroscopy
Infrared radiation excites molecules that contain a permanent dipole
to vibrate strongly at resonant frequencies that are material
dependent.
Each material has a unique “molecular fingerprint”, allowing
identification of through spectra comparison, and provides
information on polymer structure and composition
Common types of 3-dimensional
vibrations between atoms. “+”
indicates movement out of the plane
of the page.
IR spectrum of poly(styrene).
Locations of high absorbance
(e.g.3010-3100, 1500-1600,
690-900 above) are compared
to reference data to determine
structure.
Nuclear Magnetic Resonance (NMR)
NMR excites changes in the nucleus of molecules in the radio-frequency range
The nuclei behave as small
magnets, whose orientation can be
flipped from low to high energy
states at particular field strengths
by applying an external magnetic
field. Biomaterials of interest are
molecules containing H and C
NMR spectrum of poly(DL-lactic
acid-b-ethylene glycol)monomethyl ether diblock
copolymer. By comparing relative
shifts in resonant frequencies to a
known standards (often
tetramethylsilane ((CH3)4Si) the
environment of the individual H
atoms produces alterations in
localized magnetic fields, resulting
in different shifts that allow
chemical structure to be
distinguished.
Mass Spectrometry
Mass spectroscopy measures the
atomic or molecular masses of various
species in a material.
The sample is bombarded with high energy particles (typically
electrons) then the irons are forced through a magnetic field, which
deflects them from a linear path. Light elements are deflected more
than heavy elements, allowing them to be distinguished from one
another.
High Performance Liquid Chromatography (HPLC) and
Size-Exclusion Chromatography
•Provides information based on size and charge
•SEC based on filtration by size
•Determine molecular weight of synthetic and natural polymers
System contains mobile and
stationary phases
Sample is dissolved into a
liquid solvent (mobile phase)
Porous silica or polymer beads
are the stationary phase
Analyte is removed from the
mobile phase and retained by
the porous structure
Retention time is affected by
the size of the analyte (i.e.
smaller = longer)
Residence time leads to
separation of compounds by
molecular weight
Results from SEC analysis of poly(ethylene glycol). (A)
Size-exclusion chromatographs of eight PEG
standards of known molecular weight, which is used to
calibrate curve (B), which is used to determine the
molecular weight, Mw or molecular number, Mn