Page 116 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Molecular Weight of Polymers 79

TABLE 3.4
Mass Spectrometry Approaches Used in the Determination of Molecular Weights
of Oligomeric and Polymeric Materials
(Typical) Upper Molecular
MS Type Weight Range (Da)
(Usual) Electron impact (EI) To 2,000
Fast atom bombardment (FAB) To 2,000
Direct laser desorption (Direct LD) To 10 4
Matrix-assisted laser desorption/ionization (MALDI) To 10 7

3.8.2 MASS SPECTROMETRY
Certain MS procedures allow the determination of the molecular weight or molecular mass of olig-
omeric to polymeric materials (Table 3.4).
In matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS), the polymer
is dissolved, along with a “matrix chemical,” and the solution deposited onto a sample probe. The
solution is dried. MALDI MS depends on the sample having a strong UV absorption at the wave-
length of the laser used. This helps minimize fragmentation since it is the matrix UV-absorbing
material that absorbs most of the laser energy. Often employed UV-matrix materials are 2,5-dihy-
droxybenzoic acid, sinnapinic acid, picplinic acids, and alpha-cyano-4-hydroxy cinnamic acid. The
high energy of the laser allows both the matrix material and the test sample to be volatilized. Such
techniques are referred to as “soft” since the test sample is not subjected to (much) ionizing radia-
tion and hence little fragmentation occurs.
Mass accuracy on the order of a few parts per million are obtained. Thus, chain content can be
determined for copolymers and other chains with unlike repeat units. Polymer MWD can also be
determined using MALDI MS and related MS techniques. More about MALDI MS and other MS
techniques is described in Section 13.4.
MALDI MS was developed for the analysis of nonvolatile samples and was heralded as an
exciting new MS technique for the identification of materials with special use in the identifi ca-

tion of polymers. It has fulfi lled this promise to only a limited extent. While it has become a well
used and essential tool for biochemists in exploring mainly nucleic acids and proteins, it has been
only sparsely employed by synthetic polymer chemists. This is because of the lack of congruency
between the requirements of MALDI MS and most synthetic polymers. MALDI MS requires that
the test material, polymer, be soluble in a relatively volatile solvent. Proteins, nucleic acids, and
poly(ethylene glycols) are water soluble so allow the use of MALDI MS for analysis of their chain
length. Most polymers are not readily soluble in such solvents so MALDI MS is of little use in the
analysis of these materials. Carraher and coworkers have developed an approach that is applicable
to most materials that focuses on analyzing the fragmentation of the polymers. While this technique
is not able to give molecular weights and MWDs, it allows the identification of ion fragments and

ion fragment clusters up to several thousand daltons.
Recently, MS combinations have been available, including the TG-MS combination developed
by Carraher that allows the continuous characterization of evolved materials as a polymer under-
goes controlled thermal degradation.

3.9 VISCOMETRY

Viscosity is a measure of the resistance to flow of a material, mixture, or solution. Here we will
consider the viscosity of solutions containing small, generally 1 g/100 cc (called 1% solutions) and
less, amounts of polymer. The study of such dilute polymer solutions allows a determination of a

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