Structure-Property Relations in Polymers - American Chemical Society

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25 Investigation of Polycarbonate Degradation by Fluorescence Spectroscopy and Its Impact on Final Performance Makarand H. Chipalkatti and Joseph J. Laski G T E Laboratories Incorporated, 40 Sylvan Road, Waltham, M A 02254

A novel use of fluorescence spectroscopy as a sensitive, rapid, and nondestructive technique for the study of degradation in solid-state polycarbonate is described. The technique is especially suited for the study of degradation in aromatic polymers such as polycarbonate, but in principle it should have wider application. A comparison is made of emission and excitation spectra of bisphenol A polycarbonate with spectra of model compounds selected to represent previously established degradation products. The reported thermal and UV degradation products for polycarbonate in the presence and absence of air, respectively, are verified by a comparison of the emission and excitation peaks of the model compounds and the degraded polymer. The progress of thermal degradation as a function of processing temperature has been tracked by measuring the emission peak intensities of the degradation products and then correlating them with bulk performance measures of the polymer. The technique described is intended to serve as a rapid method of identifying the presence of known or proposed degradative pathways rather than as a substitute for more rigorous techniques of chemical and physical analysis. Potential applications of this technique as an aid in polymer processing as well as its extension to UV degradation studies and relationship with optical performance are also discussed.

THE

INTERACTION

O F T H E STRUCTURE,

MORPHOLOGY,

A N D PROCESSING

conditions o n the final properties o f polymers is very critical i n d e t e r m i n i n g 0065-2393/93/0236-0623$06.00/0 © 1993 American Chemical Society

In Structure-Property Relations in Polymers; Urban, M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1993.

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STRUCTURE-PROPERTY RELATIONS IN POLYMERS


the performance a n d reliability o f plastic parts over extended periods o f t i m e . T h i s interaction is especially important i n products for high-precision applica tions w h e r e very n a r r o w performance tolerances are specified. F o r instance, bonates, especially polycarbonate o f b i s p h e n o l A , are i n great d e m a n d as w i n d o w or lens materials. I n such applications, even small amounts o f degradation i n d u c e d d u r i n g processing affect the mechanical performance o f thin-walled m o l d e d parts as w e l l as clarity a n d visual appeal. I n this chapter, the use o f fluorescence spectroscopy to study the extent o f U V a n d thermal degradation o f polycarbonate is discussed. L i k e many other aromatic polymers, polycarbonate a n d its major degradation products have chromophores that absorb a n d emit electromagnetic radiation i n a range conveniently addressed b y c o m m e r c i a l spectrophotometers. O u r approach relies o n the vast b o d y o f p r i o r w o r k that elucidates the degradation pathways a n d products o f b i s p h e n o l A polycarbonate ( 1 - 3 ) b y independent means. T h e emission a n d excitation spectra due to m o d e l compounds o f k n o w n polycarbonate degradation products have b e e n used as "signatures" o f the emission spectra o f key degradation products i n the solid-state polymer. T h e intensity o f emission is recorded as a function o f exposure a n d is correlated w i t h other macroscopic s t r u c t u r e - p r o p e r t y attributes, such as the dynamic modulus and viscosity a n d molecular weight distributions. O w i n g to its remarkable sensitivity to small traces o f compounds a n d the influence o f the molecular environment, U V fluorescence spectroscopy is ideal for m o n i t o r i n g subtle degradation reactions. A l t h o u g h aromatic polymers are most suitable for this analytical approach, other polymers w i t h degradation products dis playing significant fluorescence c o u l d also be investigated b y similar means. T h e value o f the technique described here lies i n its ability to m o n i t o r k n o w n degradation products. L i k e other methods of spectroscopy that deal w i t h electronic transitions o f a material, it is not as w e l l suited to the elucidation o f u n k n o w n c h e m i c a l reactions as is vibrational spectroscopy. H o w e v e r , i f the key degradation products are k n o w n a n d can b e represented b y m o d e l compounds, o u r technique can be a r a p i d a n d sensitive m e t h o d o f tracking the degradation. F o r example, i n situations where a material has b e e n exposed to u n d e f i n e d U V - t h e r m a l conditions, the technique can be used to determine i f degradation is the result o f U V or t h e r m a l degradation, feasible i n such a straightforward manner w i t h traditional optical transmission measurements or mechanical analyses. F u r t h e r m o r e , the sensitivity o f this technique a n d its ability to detect trace materials have a significant advantage over conventional methods. I n a process control loop, the response t i m e for corrective action to m i n i m i z e degradation is dramatically r e d u c e d . A b r i e f review o f the approach a n d experimental techniques o f previous workers a n d representative examples are c i t e d here to differentiate o u r approach. T h e study o f degradation products and pathways for organic


25.

CHIPALKATTI AND LASKI

Fluorescence Spectroscopy Degradation

625

molecules i n general a n d polymers i n particular is often based o n the use o f infrared spectroscopy of samples exposed to U V radiation or t h e r m a l degrada tion. T h e classical approach relies o n the complete alkaline hydrolysis o f the degraded p o l y m e r a n d the chromatographic separation o f the various p r o d ucts, f o l l o w e d b y infrared spectroscopy (4, 5). O t h e r techniques that involve luminescence p h e n o m e n a i n polymers for the purpose o f analysis or monitor ing c h e m i c a l change (e.g., chemiluminescence due to photooxidation) have also b e e n discussed i n the literature (6). M o r e recently, solution electronic Downloaded by NORTH CAROLINA STATE UNIV on May 3, 2015 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/ba-1993-0236.ch025

spectra of polycarbonate before a n d after U V degradation were r e p o r t e d i n an effort to elucidate the mechanisms and products o f degradation (7).

In

this article G u p t a et al. used gel permeation chromatography to follow chain-scission reactions, a n d exposure to U V was m o n i t o r e d v i a actinometric measurements o f o-nitrobenzaldehyde rearrangement. Fluorescence data o f various concentrations o f polycarbonate solution as w e l l as t h i n films were reported to demonstrate the effect o f q u e n c h i n g b y degradation products. M o d e l compounds have b e e n u s e d previously for comparison o f solution U V absorption spectra w i t h those o f the degradation products ( 8 ) o f flash photolysis o f polycarbonate solution i n 1, 2-dichloroethane. M o l e c u l a r weight distributions were m o n i t o r e d b y gel permeation chromatography a n d intrinsic viscosity measurements. T h u s , the ratio o f rearrangement reactions to chain scission were c o m p a r e d . I n yet another study (9), light-scattering of solutions o f polycarbonate i n c h l o r o f o r m was p e r f o r m e d before a n d after t h e r m a l degradation to determine the weight average molecular weight. P h o t o d e c o m position was b o t h i n d u c e d a n d studied b y irradiation o f h i g h - a n d low-density polyethylenes, polyvinyl chloride, polypropylene, a n d polycarbonate, w i t h a 351.3-nm excimer laser (10).

L a s e r - i n d u c e d emission spectra as w e l l as

time-dependent q u e n c h i n g were used to assess the progress o f degradation i n the materials just n a m e d . A l t h o u g h a variety o f techniques, i n c l u d i n g U V absorption of polymers, polycarbonates i n specific, o u r approach is relatively n o v e l i n that it is based on the measurement o f U V

fluorescence

o f the degradation products. V e r y

little p r i o r w o r k presents (as w e do) polycarbonate emission spectra a n d the evolution o f its

fluorescence

spectra

as a f u n c t i o n o f thermal a n d U V

degradation. O u r approach, therefore, provides a n o v e l a n d sensitive m e t h o d for the analysis o f solid-state samples where a r a p i d nondestructive approach is desirable. T h i s technique is particularly useful because it involves the analysis o f degradation products that r e m a i n e m b e d d e d i n the matrix p o l y m e r without any n e e d for the separation techniques discussed earlier. F u r t h e r more, emission spectroscopy is c o n d u c t e d off the surface of the

sample

(hence m i n i m a l sample preparation is required) a n d is essentially nondestruc tive. C o n v e n i e n t correlation w i t h the effect o f processing history a n d performance allows this approach to be used as a process c o n t r o l tool.


final


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Processing o f polymers often results i n some amount o f thermal- or shear-induced degradation (11). S u c h degradation may o c c u r i n the c o m p o u n d i n g stage or i n injection m o l d i n g , w h e r e the action o f the extruder screw or the effect o f pressure may result i n mechanochemistry- or pressurei n d u c e d " d e p o l y m e r i z a t i o n " . T h e processing temperature significantly affects b o t h the properties a n d structure o f the polymers (12, 13) a n d results i n t h e r m a l degradation v i a r a n d o m chain scission, cross-finking, or oxidative degradation. O p t i c a l methods o f processing, such as U V cure o f adhesives a n d lithography, increasingly are b e i n g used. These treatments may also result i n exposure o f the substrate resin to U V a n d heat simultaneously or separately, and consequently, may affect b o t h performance a n d reliability o f p o l y m e r parts. T h e degree o f degradation often may be too subtle to detect b y conventional means such as i n f r a r e d spectroscopy a n d yet be sufficient to affect long-term properties. I n such cases fluorescence spectroscopy is ideally a r a p i d a n d extremely sensitive alternative once a thorough audit and labeling o f spectral features o f the p o l y m e r a n d its degradation products has b e e n made.

Experimental Notes A l l the instrumentation and compounds used are o f the standard c o m m e r c i a l variety, so only a f e w b r i e f comments are made here about the experimental aspects o f this w o r k . F o r the yellowness index measurements, the samples were injectionm o l d e d plaques o f B P A - P C ( M o b a y M a k r o l o n 2 6 0 0 - 1 0 0 0 ) . W i t h the excep tion o f d i p h e n y l carbonate o f b i s p h e n o l A , w h i c h was synthesized a n d p u r i f i e d at G T E Laboratories as described b y P o c h a n et al. (14), all the m o d e l compounds a n d the polymers were u s e d as received f r o m A l d r i c h , A l l reported solution spectra were generated f r o m as-prepared solutions without any attempt to exclude oxygen, a n d all solvents are o f spectrophotometric grade used as received f r o m A l d r i c h . Because o u r objective was not the identification o f u n k n o w n compounds, b u t rather the tracking o f emis sions f r o m k n o w n compounds, d e - a i r i n g was not d e e m e d essential. A l l the reported spectra were corrected for instrument response (e.g., different wavelengths c o u l d be c o m p a r e d relative to each other. A l t h o u g h this renders the data suitable for quantitative measurements, no quantifica tion was attempted here. S o l i d samples used for spectroscopic analysis w e r e generally w i p e d w i t h isopropyl alcohol once p r i o r to analysis. T h e emission a n d excitation spectroscopy were p e r f o r m e d o n a spectrofluorometer ( S L M 8 0 0 0 C ) w i t h a 4 5 0 - W xenon light source (wavelength range 2 0 0 - 9 0 0 n m ) a n d a c o m p u t e r controller ( I B M P C X T ) .


25.

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Fluorescence Spectroscopy Degradation

627

T h e gel permeation chromatography was carried out o n a chromatogra phy instrument ( M i l l i p o r e M a x i m a 820) w i t h T H F as the carrier solvent. T h e creep a n d complex viscosity measurements w e r e made o n a dynamic mechan ical analyzer ( D M A ; D u p o n t 983). Yellowness index calculations were based o n visible transmission spec troscopy carried out o n a spectrophotometer ( C a r y 1 7 D ) . T h e transmission data were t h e n converted to yellowness index values i n accordance

with


A S T M D 1925-70.

Results and Discussion Effect of Processing History.

O u r study is based o n comparisons

of the emission a n d excitation spectra of pristine and degraded p o l y m e r w i t h the spectra o f suitable m o d e l compounds. A m o d e l c o m p o u n d is considered suitable i f its emission a n d excitation spectra differ only shghtly f r o m what w o u l d be expected i f it were incorporated into the polymer. T h i s c o n d i t i o n occurs i f the only additional bonds r e q u i r e d for incorporation into

the

p o l y m e r m a i n chain are σ bonds, w h i c h only shghtly p e r t u r b the I T electron configuration i n the relevant c h r o m o p h o r i c species.

I n such a case, the

spectra of the m o d e l c o m p o u n d are quahtatively identical to those o f the degradation products e m b e d d e d i n the polymer, w i t h peaks

sometimes

shifted b y a f e w nanometers. In the case o f b i s p h e n o l A polycarbonate ( B P A - P C ) , the m o d e l c o m p o u n d , d i p h e n y l carbonate o f B P A ( D P C ) , was f o u n d to be a suitable m o d e l o f B P A - P C electronic a n d molecular structure. T h e D P C as synthesized h a d to b e substantially p u r i f i e d b y several washings p r i o r to preparation o f a solution w i t h no trace impurities that w o u l d be evident i n the absorption or emission spectra. T h e validity o f this m o d e l selection is clearly demonstrated i n the solution emission a n d absorption spectra o f F i g u r e 1. T h e spectra for the p o l y m e r a n d m o d e l c o m p o u n d are very similar a n d they b o t h share the same degree o f symmetry b e t w e e n their respective emission a n d absorption spectra, w h i c h demonstrates a h i g h degree o f correspondence. T h e identical p r i n c i p l e also has b e e n a p p l i e d i n the selection o f m o d e l compounds for the various t h e r m a l a n d U V degradation by-products that are specifically discussed w h e n appropriate d u r i n g the course o f this report. In the transition f r o m a solution sample to the solid state, a comparison is made between the m o d e l c o m p o u n d a n d the c o m m e r c i a l B P A - P C sample. T h e two emission spectra (excitation at 280 n m ) are c o m p a r e d i n F i g u r e 2. T h e similarities o f the spectra o f F i g u r e 2 are quite remarkable; specifically, the m o d e l c o m p o u n d . T h e additional peaks superimposed o n the p o l y m e r emission at 330, 347, a n d 365 n m , respectively, are also notable.



628


(b) • Excitation^'

Emission

-o-c-o-

Structure-Property Relations in Polymers - American Chemical Society

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