Investigation of the Detection Limit for the Determination,
by Pyrolysis-Capillary GC, of the Thickness of Ultra Thin
PMMA Layers Coated on to PET Film
Xigao Jin*Polymer Physical Laboratory, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China
Naoki Oguri, Akira Onishi, and Sigeki Uchino
Japan Analytical tridustry &o. Ltd, 208 Musashi, Mizuho, Nishitama, Tokyo 1 90-12, Japan
Key words:
Pyrolysis-capillary GC
Quantitative analysis
Detection limit
Ultra-thin layer poly(methyl methacrylate)Poly(ethylene terephthalate)
Summary
The determination of nanometer thick layers of poly(methylmethacrylate) coated on to the surface of poly(ethytene terephthalate) film has been Investigated by high resolution pyrolysis
gas chromatography without sample pretreatment or modification of the instrumentation used. A good linear relationship wasobserved between the quantity of the characteristic pyrolysate
and the thickness of the potpotyy (methyl methacrylate) layer ; thedetection limit was sufficient to enable the quarrtttatton of
poty
poty (methyl methacrylate)-to-poly(ethylene terephthalate) film
thickness ratios of 1 :20000 in composite materials.

1 Introduction
The characterization of polymer surfaces has, in general, been
performed by spectroscopic means such as X-ray photoelectron
spectroscopy (XPS). secondary ion mass spectrometry (SIMS),
and Fourier transform infrared attenuated total reflection spectroscopy (FTIR-ATR); these techniques are useful for analyzing
surface layers as thin as 1-10 nm [1. 2].
Pyrolysis - gas chromatography (Py-GC) has many advantages
which make it suitable for application to polymer analysis.
Minimum sample preparation is required and the technique canbe applied even to complex mixtures of formulated and treatedproducts such as composite polymers. The small amount ofsample required, the high resolution separation, and the qualitative and quantitative information available from this techniquehave made Py-GC an essential tool in polymer analysis [3]; evenso, the technique has been applied relatively rarely to studies
involving micro quantitative analysis. Hida etal. recently reported
that the presence of metal powder and inorganic salt additives iseffective in improving the sensitivity and reproducibility ofpyrolysis [4]. The trace determination of poly(vinyl pyrrolidone) incomplex mixtures [5] and in the end groups of macromonomers[6, 7] has been performed by Py-GC with the highly sensitive andselective detectors. TSD, FPD, and NPD.
In the present work, we have applied high resolution Py-GC to thedetermination of ultra thin (nanometer thick) poly(methyl methacrylate) layers coated on to the surface of poly(ethylene terephthalate) (PET) films, an analysis previously performed only byXPS, SIMS, and special FTIR-ATR. We have investigated thedetection limit which may be achieved without sample pretieatment or modification of the instrumentation.
2 Experimental
2.1. Sample Preparation
Dilute solutions of different concentrations of PMMA in chloroform were deposited on PET film(thickness 120 μ m) by means of a
microsyringe; the solvent was then slowly evaporated, at a
relatively low temperature, in an environment nearly saturated
with chloroform. The thickness of the PMMA layer was estimatedfrom the weight increment and the bulk density of 1.19 g/cm3 for
PMMA; samples with layers 10, 24, and 36 nm thick were selectedas calibration standards.
2.2. Apparatus
Pyrolysis GC was performed with a Model JHP-3 Curie-point
pyrolyzer (Japan Analytical Industry Co.) combined with anon-line Mode! HP 5890A gas chromatograph (Hewlett-Packard)
and a Model D-2520 integrator (Hitachi). The chromatograph wasequipped with flame iomzation detection and a 30 m x 0.25 mm
i.d. fused silica capillary, column coated with a 0.25 μ m film ofDB-1701 (immobilized poly(phenyl cyanopropyl) polydimethylsiioxane; J & W). Identification of the pyrolysates was performed
with a quadrupole mass spectrometer (Model QP 2000A, Shimadzu) equipped with an El ionization source and interfaced directlywith the Py-GC.
2.3 Conditions for Py-GC
A piece of the sample film (0.4-0.5 mg) was wrapped in
Pyrofoil-500ºC (8 x 23 x 0.05 mm) in a manner ensuring good
contact between both. The sample and foil were loaded into aquartz pyrolysis chamber (40 x 4 mm i.d.) maintained at 200ºC,
and heated inductively for 5 s. The column temperature wasmaintained at 30ºC for 10 min following pyrolysis and thenprogrammed to 300ºC at 8ｺ/min. Helium was used ascarrier gas;
the total flow of 50 ml/min was spiit at a ratio of 50:1. The El
source of the mass spectrometer was operated at 70 eV and250ºC.

3 Results and Discussion
3.1 Pyrogram
It is well known that the major component formed upon pyrolysisof PMMA is the monomer methyl methacrylate (MMA): theamount of the compound in the total pyrolysate sometimes
approaches 100 % [6].
The features of the thermal degradation of PET have also been
extensively studied. The origins of the main pyrolysates, such asoligomers. monomer (divinyl terephthalate, DVT), benzoic acid,vinyl benzoate. benzene (B) and styrene (S), etc.. appear to bereasonably understood: their formation is based mainly on chain

scission, ester exchange, p-CH transfer, in addition to secondary
pyrotysis [8. 9].
Relative to the amount of PET. the quantities of PMMA in thesamples studied in this work were extremely small (PMMA : PET
film thickness ratio of 1 : 3.000 to 1 : 10.000 for standard films). All
experimental parameters, especially sample size, pyrolysis temperature, and GC separation conditions, would, therefore, have asignificant influence on the quantitative measurement of thePMMA fraction against the PET background. When the samples
were pyrolyzed at the lowest temperatures Investigated (3 1 5 and423º C) they were not completely degraded, even after extended
pyrolysis times. Pyrolysis at the highest temperatures tested (e.g.
590ºC), on the other hand, resulted in the formation of largeamounts of the lower boiling point pyrolysates, the peaks fromwhich overlapped the very weak peak from MMA. As a result of
these observations, a pyrolysis temperature of 500ºC was chosen
for the analysis.
Rather large sample sizes were necessary for the determination of
the trace amounts of PMMA in the layers; because heat istransferred more slowly in large samples, however, this has beenfound to influence reproducibility, [8]. The data obtained in thisstudy showed that the smaller the sample size, the morehomogeneous was the sample temperature and the wider thelinear dynamic range of the calibration graph: a reasonablesample size range was found to be 0.4-0.5 mg (incorporating
0.05-0.15 ug PMMA). The GC conditions presented in Section 2.3
were found suitable for separating the MMA peak from otherpeaks within a relatively short analysis time; the pyrograms
obtained from standard calibration samples under these conditions, following Pyrolysis at 500ºC, are illustrated In Figure 1 ;nwhich the small but remarkably distinct MMA peak is clearly
apparent.
3.2 Calibration
Those structurally characteristic pyrolysates from PET, such as
benzene (B), styrene (S), and divinyl terephthaiate (DVT), etc.,
which are completely separated in the pyrograms may be used as
Internal standards for the quantitative analysis. The peak areas
observed for MMA, B, S and DVT, and the peak area ratios
MMA/B, MMA/S, and MMA/PET obtained from samples of PET
coated with various thicknesses of PMMA are listed in Table 11.
The plots of the peak area ratios MMA/B, MMA/S and MMA/DET
against the thickness of the PMMA film are shown in Figure 22,excellent linear relationships may be observed. Figure 2 also
shows the dependance of the peak area ratio on the identity of theinternal standard chosen: we advocate the use of peaks, such as
those of benzene and styrene, with intensities and retention times
comparable with those of the MMA peak, as being the mostsuitable for use as internal standards in this work. The relative
standard deviation resulting from 5 replicate measurements waswithin 4 %.

3.3 Determination of Unknown Samples
Figure 2 can be simply used for quantitative determination of ultra
Thin layers of PMMA coated on to PET films within a PMMA PET
film thickness ratio of 10-4. Two unknown samples, coded X andY, were analyzed on the. Basis of these calibration curves, the data
are summarized inTable2. The correlation of the values obtained
from the different calibration curves indicates the suitability of themethod for the quantitative trace analysis of ultra thin layers of
PMMA coated on to PET film. In addition, the thickness measuredfor the PMMA film of sample Y implies that the detection limit is
low enough for application of the method to the detection of
PMMA films when the PMMA/PET film thickness ratio of thecomposite material is as low as 1/20,000.