NIR-FT Raman image of solid-state polymerization of PTS diacetylene

SPECTROCHIMICA ACTA PART A

Spectrochimica Acta Part A 53 (1997) 67-79

ELSEVIER

NIR-FT

Raman image of solid-state polymerization diacetylene Alexander

N. Shchegolikhin*,

of PTS

Olga L. Lazareva

Department of Electronics of Organic Materials, Joint Institute of Chemical Physics, 4 Kossygina St., Moscow, 117334, Russia Received

28 April

1996; accepted

9 May

1996

Abstract The paper compares the visible wavelengths excited resonance Raman and the near-infrared Fourier transform (NIR-FTR) Raman spectroscopy methods in terms of their convenience and informativeness for studies of diacetylenes (DA) and polydiacetylenes (PDA). It is shown that the combination of excellent precision with the reliability of measurements on the samples which are essentially transparent to both the exciting laser wavelength, 1064 nm, and the NIR-Raman photons makes NIR-FTR spectroscopy an ideal tool for quantitative measurements of the solid-state structural transformations in PDAs. In particular, thermally induced polymerization of a model DA, PTS has been reinvestigated in situ by the NIR-FTR method. The data so obtained suggest occurrence of a sharp but continuous and concerted structural transition during the polymerization of PTS. Other peculiarities, namely the apparent lack of both the intensity and frequency dispersions with the polymer rc-conjugated length in the NIR-FTR spectra, as well as a non-trivial reasonant enhancement of the latter are noted. Keywords:

NIR-FT

Raman

images;

Solid-state

polymerisation

1. Introduction 1.1. General consideration Traditionally, Raman spectroscopic investigations of conjugated polymers have been performed by using mainly laser excitations, falling either in strong or in preresonance with electronic, vibronic or excitonic transitions associated with the conjugated z-electron system of the polymer backbone. Although in part it is explainable by purely technical reasons [l], much more funda* Corresponding 0584-8539/97/$17.00

author. Copyright

PII SO584-8539(96)01757-6

0 1997 Elsevier

mental grounds for using namely resonance Raman (RR) spectroscopy when dealing with conjugated polymers have been put forward [2]. The late 1980s saw rapid advancements in Raman instrumentation which, specifically, lead to development of the near-infrared Fourier transform (NIR-FT) Raman method [3]. Certainly, the latter has been promptly adopted for studies of doped conjugated polymers [4]. Being doped with electron donor or acceptors, these usually show additional absorption maxima in the NIR. This allows the molecular structure of the polymer to be probed using resonance-enhanced FT-Raman spectroscopy excited at 1064 nm.

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A.N. Shchegolikhin, O.L. Lazareva 1 Spectrochimica Acta Part A 53 (1997) 67-79

However, although it may be less obvious, NIR-Raman spectroscopy seems to be of much value also for studies of undoped conjugated polymers, when the 1064 nm excitation is expected to be the off-resonance one. Thus, for instance, the study of phase transitions in two PDAs (dispersive NIR-excited Raman) [5] as well as NIR-FTR studies of DAs polymerization kinetics [6] and of C,, and C,, fullerenes [7] have implicitly proved the potential of the NIR-excited Raman spectroscopy for advanced structural analysis of undoped conjugated systems. 1.2. Motivation for the study Dealing with acetylenes, DAs and PDAs, we have used the NIR-FTR in order to study, e.g. the anomalous intensity distribution in different wavelengths-excited Raman spectra of photoconductive copper phenylacetylides [8], the solid-state polymerizability of nitroxyl radicals-substituted DA monomers [9], phase segregation phenomena in spin-labelled block-co-poly(diacetylene)ether urethanes [lo], the kinetics of the solid-state topochemical polymerization of DAs and kinetics of thermo- and mechanochromic phase transitions in PDAs [l 11. The results and experience we have gained during those studies generally prove most of the claimed or expected advantages of the technique [l]. Moreover, we have to suggest that the NIR-FTR is able to bring plenty of additional information unobtainable (or obscured) by the visible wavelengths excited RR measurements on the same samples. There are, however, a number of questions (or topics of debate) usually arising during interpretation of NIR-FTR spectra of conjugated polymers, which can be exemplified by the following ones. (i) Does NIR-FTR see conjugation length of an undoped conjugated polymer, i.e. do NIR-Raman

lines show frequency dispersion with the conjugation length? (ii) Can we detect and/or should we take into account any possible variations in Raman line intensity as induced by changes in the conjugation length, that is, do NIR-FTR lines show intensity dispersion with conjugation length? (iii) Can we use the intensities of NIR-FT Raman bands for quantitative (or ‘semi-quantitative’) estimates? (iv) What does the shape of NIR-FTR band usually reveal and what does it hide? (v) Considering that most of the conjugated polymers have some degree of second or third order nonlinear optical activity, should we take into account possible contribution of the higher order processes?; etc. Keeping in mind that, in principle, different Raman spectra should be observed with excitation in resonance versus not in resonance and being faced the above questions in our current work, we have felt the need to compare both types of Raman spectra for a well charac-terized sample of an undoped conjugated polymer. For a number of reasons, polydiacetylenes have been considered to be quite appropriate for such investigation. The feasibility of quantitative NIR-FTR measurements on this class of materials has been addressed in this work primarily. 1.3. The chosen material Among conjugated polymers, polydiacetylenes are unique in that they are obtainable as true single crystals of macroscopic size through topochemical polymerization of the corresponding disubstituted diacetylene monomer crystals [12]. Polymerization of DA monomer crystals is induced by thermal annealing, UV radiation or high-energy radiation, and proceeds in the solid state by virtue of 1,Caddition reaction according to the following general scheme:

A.N. Shchegolikhin,

O.L. Lazareva

/ Spectrochimica

The majority of experimental studies of mechanisms of PDA formation have been dealt with bis(p-toluene sulfonate) of 2,4-hexadiyne-1,6-diol, hereafter referred to as PTS, for which R = -CH,OSO,C,H,CH,. Polymerization of this compound was first described by Wegner [13], and was then all-embracingly studied, for example, by optical spectroscopy [14], RR spectroscopy [ 1517] and electron spin resonance [ 18,191. Thermally induced polymerization has been compared with UV- and y-ray initiated ones [20]. Plenty of calorimetric studies have been published [21-241. The variation in the PTS monomer crystal lattice parameters during polymerization has been studied by X-ray diffraction [25], etc. In fact, this compound has a status of a model diacetylene and our understanding of the mechanisms of polymerization of DAs is mainly based on studies made on PTS. Summing all the above considerations, we have chosen namely PTS diacetylene for the present study.

2. Experimental The NIR-FTR spectra were measured on a Perkin-Elmer 1725-X FTIR spectrometer equipped with a Perkin-Elmer NIR-FT Raman attachment, comprising a Spectron c/w polarised Nd:YAG laser operating in TEM,, mode. The 1.064 urn line (9398 cm ~ ‘) from this laser was used for excitation. The detector was a 2 mm diameter semiconductor photo-diode made from InGaAs (IGA). It operates at ambient (3500-200 cm-‘) as well as at liquid nitrogen (LN,) temperatures (3000-200 cm - ’ shifts). A 180” back scattering geometry, 4 cm - ’ resolution, normal apodisation function, scan speed 1, gain 1 and LN,-cooling of the IGA detector were normally used throughout this work. For heating of the samples we used a Spectra-Tech temperature controller and heater, the latter being detached from the respective ‘Collector’ DRIFT accessory. This controller/heater set was equipped with a chromel-alumel thermocouple and gave + 0.5”C precision in temperature readings. The NIR-FTR spectra were recorded using both single crystals and microcrystalline samples

Acta Part A 53 (1997) 67-79

69

of PTS DA monomer. In the first case high-purity monomer PTS crystals, synthesized and grown as reported elsewhere [26], were cleaved parallel to (100) surface to obtain the platelets with about 0.5- 1.0 mm thickness and about 5 mm length in the expected polymer chains direction. The single crystals were fixed in good thermal contact directly in the cylindrical recess on the brass end of the Spectra-Tech heater by means of a thin quartz cover plate and a miniature wire spring clip. For analysis of the particulate PTS samples, a copper cylindrical block (about 9 mm ext. dia. and 6 mm height) with a hole drilled in the centre (about 1 mm dia. and 5 mm deep) has been used as a sample holder. Alternatively, spectra were recorded also for microcrystalline samples prepared by impregnation of pre-cut pieces of a filter paper with a concentrated solution of PTS monomer and allowing it to evaporate rapidly. In any case, before the polymerization run, the position of the PTS monomer sample was optimized, while in the instrument monitoring mode, to achieve a maximum intensity of the monomer v(C=C) Raman band at 2272 cm- ‘. After that, the power of the incident laser beam was fixedly set at a lowest possible level (usually, 5-10 mW), and was not varied until the end of the experiment. To rule out any possibility of an inhomogeneous polymerization across the sample owing to local extra heating of the sample under the laser beam, the intermittent regime (sample compartment door is opened and the laser beam is blocked for the time between recording of two successive spectra) has been used in this work. Thermal polymerization of the PTS samples was then accomplished in situ, being started by setting a desirable temperature (usually from 70-85°C interval) on the temperature controller/heater. NIR-FTR spectra of the PTS samples in the course of polymerization were taken successively over the measured time intervals. Usually two to ten individual scans were coadded (0.1 - 1.O min). All NIR-FTR spectra are reported uncorrected for the instrumental response function. IR spectra were measured on a Perkin-Elmer 1725-X FTIR with the aid of a Perkin-Elmer PEDR diffuse reflectance accessory by using a modified diffuse reflection-absorption (DRAFT) technique [27].

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A.N. Shchegolikhin, O.L. Lazareva / Spectrochimica Acta Part A 53 (1997) 67-79

Differential scanning calorimetry (DSC) measurements were performed on a Perkin-Elmer DSC-7 analyser.

3. Results and discussion Typical NIR-FTR spectrum of a PTS monomer single crystal is shown in Fig. 1 (lower trace). Expectedly, owing to high polarizability of the -C&-C-Cfragment of the monomer PTS molecule, v(C=C) stretching mode centered at 2272.7 cm - ’ is the most prominent feature of the spectrum. Contrary to the conventional dispersive RR spectra of PTS monomer, the NIR-FTR spectrum contains plenty of additional information, which, being incorporated into a complete vibrational spectrum, may be of help for detailed analysis of structural transformations in materials of this class. In particular, such complete vibrational spectra should be of value during, e.g. the dynamic vibrational spectroscopy studies [28,29] of thermoor mechanisms of molecular mechanochromic phase transitions in many PDAs [I 11, that is, in those where transformations of molecular structure of the conjugated backbone are accompanied by significant changes in molecular structure of the side chains (substituents) or vice versa. While the optical spectra of the polymerizing diacetylene crystals may be readily interpreted as being owing to polymer chains alone [30], they

Fig. 1. A complete vibrational spectrum of PTS monomer. IR-DRAFT [27] spectrum of a microcrystalline sample (upper trace) and NIR-FT Raman spectrum of a single crystal (lower trace).

Fig. 2. Comparison of NIR-FT Raman spectra of slightly polymerized PTS monomer (lower trace) and 100% PTS polymer (upper trace) single crystals. NIR-FT Raman bands corresponding to PTS polymer backbone vibrational modes are marked by asterisks.

cannot provide sufficient evidence for detailed molecular structure of the growing PDA chains. Infrared (IR) absorption measurements on PDAs usually reveal only rather weak and broad absorptions in the backbone C-C stretching vibration region, owing to the symmetric nature of the functional moieties along the PDA backbone; on the contrary, strong bands in v(C=C and v(C=C) regions have been expected and observed in earlier Raman spectral studies of PDAs [31]. Typical NIR-FTR spectra of PTS monomer and polymer single crystals are shown in Fig. 2. It should be noted that the ordinate readings in Fig. 2 are valid for both spectra. However, despite considerably lower (about five times) excitation laser power used, the NIR-FTR spectrum of PTS polymer crystal had approximately 10 times higher overall intensity compared with the corresponding spectrum of the monomer. Therefore, to illustrate weak features on both spectra, only a lower part of the polymer spectrum is shown shifted upward for clarity. Importantly, the NIRFTR spectrum of PTS monomer simultaneously contains Raman bands, having principally distinguished origin. Thus, for example, bands centered at 1601, 1383 and 1174 cm-l have been tentatively assigned to breathing phenyl, v(O-SOJ and v(O=S=O) vibrational modes, respectively [ll], and hence belong to vibrations within the substituents of the PTS monomer molecule. On the contrary, the lines marked by asterisks origi-

A.N. Shchegolikhin, O.L. Lazareva / Spectrochimica Acta Part A 53 (1997) 67-79

nate from trace amounts of PTS polymer inevitably present in the monomer single crystal. These five peaks are usually seen in the NIR-FTR spectrum of the monomer near 2010 cm - ‘, 1475 + 1455 cm-’ (doublet), 1190 cm-’ and 955 cm-’ (shoulder) and are characteristic for the PTS polymer conjugated backbone vibrational modes, which have been assigned previously to v(C=C), v(C=C) + scissors 6(-CH,-), S(C=C-C) and S(C=CC) vibrations, respectively [32,33]. Measurements in situ with the aid of NIR-FT Raman spectroscopy reveal that on thermally induced PTS polymerization the asterisk marked lines in the monomer spectrum (see Fig. 2) exhibit a dramatic increase in intensity, as represented in Fig. 3 for initial stages of the solid-state reaction. Notably, intensity of the PTS monomer v(C=C) stretching mode at 2272 cm-’ exhibits accordingly a slight, but measurable, decrease during the same time interval. Beyond doubt, the PTS monomer v(CrC) stretching mode has the highest polarizability among other chromophores in the pristine PTS monomer molecule, and its polarizability value as well as symmetry should not experience any dramatic changes during initial slow stages of the polymerization reaction, since the last still takes place in the essentially monomer matrix. Being a building block for the growing PDA chains, the -C=C-C=Cfragment of the monomer is slowly consumed in the early stages of the polymeriza-

20

10

150 00

n

$ _*

Fig. 3. A 3-D image of the polymer backbone vibrational bands evolution vs. time during initial stages of thermally induced (80fOS”C) PTS single crystal polymerization as measured in situ by NIR-FT Raman

71

2ioo

Fig. 4. A 3-D NIR-FT Raman image of complete process thermally induced (80 k OS‘C) solid-state polymerization PTS single crystal.

of a of

tion reaction, so the observed intensity decrease for the PTS monomer v(C=C) stretching mode is believed to be a purely concentrational one. A typical complete set of NIR-FT Raman spectra obtainable in the course of thermally induced polymerization of PTS monomer single crystal is represented in Fig. 4. It is evident that actually all parameters of the main PDA backbone bands, i.e. their intensities, positions and shapes, are changing during the polymerization. Moreover, a systematic intensity growth of all polymer backbone Raman bands with polymerization time, following an S-shape function, is apparent. It is worth noting that, contrary to the corresponding visible wavelengths excited RR spectra, the NIR-FTR ones show neither overtones nor combinations for the whole PTS polymerization process. Several weaker bands are indeed seen at lower Raman shifts in the NIR-FTR spectra (as well as in the RR ones). The dynamic NIR-FT Raman experiment provides evidence, however, that these lowfrequency features grow synchronously with the bands originating from the conjugated backbone vibrational modes and, hence, are seem to be the crystal lattice or phonon modes coupled with the latter [ll]. The evolution of the lower frequency bands is illustrated by Fig. 5. A general comparison of PTS polymer Raman spectra obtainable by

A.N. Shchegolikhin, O.L. Lazareva / Spectrochimica Acta Part A 53 (1997) 67-79

12

experimental or instrumental variables were kept as far as possible constant. Principally, in order to test the applicability of the NIR-FTR spectroscopy for kinetic measurements of the solid state PTS polymerization reaction one needs to compare a curve based on the FT Raman data with the corresponding ‘standard’ polymerization kinetics curves. The spectra represented in Fig. 4 show that, owing to their high intensity and good overall SNR, actually any vibrational band seems to be appropriate for quantitative evaluations. It may be guessed that, as a first approximation, peak intensity values of the NIR-FTR spectral bands could be used for obtaining a kinetic curve. Indeed, an interesting approach to kinetic analysis of DAs polymerization based on use of NIR-FT Raman lines intensities has been reported recently [6]. More close examination reveals, however, that actually all backbone bands in the NIR-FTR spectra of PTS change their shape and/or exhibit some degree of asymmetry in the course of polymerization reaction, representing, in fact, an evolution of a distristructurally similar bution of particular chromophores during the polymerization. Generally, such distribution may change in a complex manner [l 11.In this respect, we have guessed that an integrated NIR-FTR band area value taken at a given time ought to be a more representative measure of an instant monomer to polymer conversion. An exemplary NIR-FT Raman kinetics curve which has been produced by using integral band area values for the polymer v(C=C) stretching mode is represented in Fig. 6. The well studied polymerization curve of PTS has a sigmoidal shape showing a slow initial polymerization rate up to about 10% (by weight)

Fig. 5. Evolution of NIR-FT Raman low-frequency vibrational bands in the course of isothermal (80 $- 0.5”C) solidstate polymerization of PTS single crystal.

RR and NIR-FT Raman methods is illustrated by data in Table 1. A very important point is that during conventional visible wavelengths excited RR spectroscopy of PDAs the Raman photons usually have frequencies falling into the region of intense optical absorption of the sample. In the case of NIR-FTR experiment, on the contrary, the Raman photons have energies corresponding to 9200-6000 cm - ’ and PDAs are essentially clear in this spectral region. Keeping this in mind and taking into account that the whole set of successive spectra (see, for example, Fig. 4) has been recorded in situ from the same strictly geometrically defined domain of the polymerizing PTS single crystal, we have guessed that the systematic variations in the vibrational bands parameters, which have been observed during the isothermal heating experiment, could more or less quantitatively reflect a dynamics of molecular structure transformations of the sample, provided other Table I Comparison

T 09

300 300

of main Raman Ref

121 This work

bands

peak positions Raman

band

for RR and NIR-FT wavenumber/cm

Raman

spectra

of single crystal

PTS polymer

samples

’

v(C=c)

v(C=c)

v(CpC)

S(C-C=c)

s(c~c=c)

2088 2087

1487+ 1464 1486f 1466

1342” 1272b

1205 1205

955 953.8

a Positively absent in the NIR-FT Raman spectra. The nearest observable negligible intensities in the FT Raman. b Tentative assignment [1 11.

bands

at 1372, 1358, 1344 and

1334 cm-’

all have

A.N. Shchegolikhin, O.L. Lazareva / Spectrochimica Acta Part A 53 (1997) 67-79 1.1,

I

Fig. 6. Brutto-kinetics curves for isothermal solid-state polymerization of PTS based on: (0) NIR-FT Raman data for 79 f 0.5”C, (-) DSC data for 80°C and (-) theoretical calculations for 80°C according to Ref. [35].

conversion to polymer followed by an extremely rapid polymerization to about 90% conversion and a final slow approach to completion [12](b). Usually, the plots of polymer conversion versus time are built basing on the results of a gravimetric analysis of the extracted partially polymerized samples. Other possible ways of obtaining the polymerization kinetics curves for DAs rely on dilatometry [34] and DSC [22,23] techniques. The latter method is much more convenient than the first ones, and, since PTS is known to polymerize quantitatively, we considered that the corresponding DSC curve can be used as a testing standard in our case. This curve is represented by a full bold line in Fig. 6, while empty circles represent the integrated Raman band area values taken from the corresponding set of NIR-FT Raman spectra, for the polymer v(C&) vibrational mode in this case. The full thin line, representing a theoretical curve for isothermal PTS, polymerization at 8O”C, has been borrowed from Ref. [35]. Note that the time scales of the theoretical and the two experimental curves have been made superimposable by normalizing them to the 50% monomer-to-polymer conversion times in each case. The apparent correspondence of the three polymerization kinetics curves may probably be classified as ‘acceptable’. Note that the identical conversion versus time plots are obtainable based

13

on any FT Raman spectral band belonging to vibrations of chromophores embedded in the conjugated backbone. The dynamic NIR-FTR spectroscopy turned out to be equally suitable for kinetics analysis of other solid-state polymerizations of PTS. As Fig. 7 shows, kinetics of either UV- or thermally-induced polymerizations of PTS can be measured with the aid of NIR-FTR by using microcrystalline samples prepared by impregnation of pieces of a filter paper with a concentrated solution of PTS monomer. The conversion versus polymerization time plots presented in Fig. 7 were built analogously to that shown in Fig. 6, using a normalization constants, t,,,, which in each case were the times required to reach 50% conversion from monomer to polymer. For recording the UV-initiated polymerization, the sample of microcrystalline PTS on a filter paper was exposed periodically to a laboratory low intensity UV source. The filter paper impregnated with PTS is partly transparent to the 1064 nm laser beam (i.e. the last is able to ‘see’ through the composite sample). Accordingly, to achieve more even UVirradiation efficiency, the sample fixed in an ordinary IR slide film holder was irradiated for equal times from both sides before each successive NIRFTR run. The observed disappearance of the monomer v(C=C) band as well as the overall intensity saturation of the NIR-FTR spectra are

x ._

B 8

Fig. 7. Comparison of NIR-FTR measured kinetic curves for (0) UV-initiated polymerization of microcrystalline PTS on filter paper; (e) in situ thermal (80 _+0.5”C) polymerization of microcrystalline PTS on filter paper; and (0) in situ thermal (79 f 0.5”C) polymerization of PTS single crystal.

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A.N. Shchegolikhin, O.L. Lazareva / Spectrochimica Acta Part A 53 (1997) 67-79

indicative that, indeed, eventually 100% conversion to polymer may be achieved by this route. The corresponding conversion versus time plot (see Fig. 7) evidences that the brutto-kinetics of the UV-initiated process is heavily differs from that of the thermally induced ones. Delaying a detailed analysis of the photo polymerization [l 11, note that the above example is reported here only to illustrate the NIR-FTR afforded possibility to obtain quantitative data by using this convenient sample geometry. Despite the slide with the sample has been removed from and repositioned on the NIR-FTR sample stage many times, quite reasonable kinetic data have been easily obtained

WI. Obviously, the same technique should be of more value during studies of y-induced polymerizations. It should be noted also that the other two sets of NIR-FTR data in Fig. 7, both being relevant to the thermally induced polymerization, provide strong evidence that, as has been noted earlier [23], significant differences in the kinetics are observed when a single crystal is substituted by a polycrystalline sample of small crystallite size, z lo-100 urn, and these differences should be taken into account properly [l 11. What additional spectral or structural information can be safely derived from the above NIRFTR spectra? Firstly, the NIR-FTR spectra (similarly to the RR ones) do not provide any clear frequency dispersion of the Raman bands with a-conjugation length for polymerizing PTS samples. In fact, continuous gradual frequency shifts have been observed for all the backbone vibrational bands in the course of PTS polymerization (see Fig. 4) but, in very general terms, these shifts are totally justifiable by the continuously relaxing elastic strain experienced by the polymer chains during the topotactic ‘monomerpolymer crystal’ structural transformation [36,37]. Typical ‘raw’ NIR-FTR bands peak frequency shifts measurable during PTS polymerization are illustrated by plots in Fig. 8. Superficial comparison of these plots with the kinetic curves presented in Fig. 6 reveals that these two types of plots seem to correlate in general. Both have a region of slow changes, followed by a region of sharp changes, and a

region of slow approach to completion. The beginning of sharp variability in peak frequencies for all the PTS backbone modes in Fig. 8 coincides with the start of the autocatalytic period of the polymerization reaction in Fig. 6. More interestingly,.however, that a slowing down of the fast frequency changes takes place at the time (about 0.9-0.97 abscissa readings in Figs. 8 and 6), corresponding to 40-45% conversion to polymer (see Fig. 6). Of utmost knportance, perhaps, is the fact that such behaviour of the NIR-FTR peak frequencies strongly correlates with the reported [25] changes of the monoclinic parameter b of the average lattice of PTS as a function of polymerization time. This parameter reduces from 5.165 8, in PTS monomer down to 4.905 8, in fully polymerized PTS and, reflecting the contraction of the single crystal in the polymer chain direction during polymerization, also exhibits a characteristic sharp decrease during the same period of the autocatalytic stage of the reaction. Thus, the second important result is that the curves of the three types, namely: (1) monomer to polymer conversion versus polymerization time; (2) parameter b value versus polymerization time [25] and (3) NIR-FT Raman vibrational mode frequency versus polymerization time turned out to be cross-correlated one with another. Further, recall that the plots in Fig. 8 were built on the basis of NIR-FTR data all gathered in situ from a geometrically fixed domain within the same PTS single crystal. Note also that the corresponding PTS polymerization kinetics curve in Fig. 6 (empty circles) has been built by using the same set of NIR-FTR data gathered in the same experiment. Owing to this, for any v(C=C) mode frequency value (see Fig. 8) the corresponding degree of conversion from monomer to polymer is simply defined by the respective point (see Fig. 6). By this route the conversion dependencies for any vibrational mode of PTS polymer backbone are obtainable without recourse to the solvent extraction procedures. Probably the weakest point in this approach is the default assumption that the last spectra in a set correspond to 100% degree of polymerization. For single crystal PTS, however, such assumption seems to be quite reasonable. Moreover, we hope that the NIR-FT Raman

A.N. Shchegolikhin, O.L. Lazareva / Spectrochimica Acta Part A 53 (1997) 67-79

75

Fig. 8. NIR-FT Raman v(C=C), v(C=C) 6(-CH,-) scissors, S&X-C) and b(W-C) vibrational modes peak frequencies vs. time plots obtainable during thermally induced (79 * 0.5”C) PTS single crystal polymerization in situ. The to.5 normalization constant is the time required to reach 50% conversion from monomer to polymer.

technique may be useful for quantitative polymerization. kinetics studies of other DAs, provided the limiting degree of conversion to polymer for a given sample would be confidently determined once. Alternatively, we guess that after a slight modification of the experimental set-up [39], the limiting degree of conversion of a DA monomer as well as polymerization kinetics can be NIRFTR measurable in terms of relative decay of the monomer v(C=C) band integral area during the polymerization.

Nonetheless, the shape of the plots in Fig. 8 seems to deserve a closer examination. All these plots reveal a region of fast frequency changes which begin simultaneously with the start of transition of the polymerization reaction from the induction stage to the autocatalytic one. The origin of this effect is rather peculiar and will be discussed in detail separately [l 11, so only the most important points will be attempted to elucidate here. Accordingly, keeping in mind that (i) the rigid rod-like PDA chain is considered to be a

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direct and an excellent Raman probe of internal stresses existing in the crystalline matrix of polymerizing DAs [32] and that (ii) partly polymerized DA crystal may be considered as a solid solution of polymer chains all extended along a common crystallographic axis (the monoclinic axis b for PTS) in a monomer matrix [12], the overall PTS polymerization process may be interpreted in terms of the above NIR-FTR data as follows. The first polymer chains should have the monomer periodicity (5.18 A) in the lattice, i.e. they should experience an elongation of about 5% with respect to the polymer natural periodicity (4.91 A). As the polymerization proceeds, the monomer matrix should become more and more stressed by the growing amount of the polymer in lattice. Owing to high rigidity of the polymer chains, the parent lattice should, obviously, respond to the increasing stress by self-adjusting the current parameters toward the stress relaxation, i.e. by gradual lowering the lattice periodicity along the b direction. The tension on the polymer at the same time is gradually released. While the polymer concentration is low, this co-operative process proceeds rather slowly. In terms of Raman frequencies of the polymer backbone vibrations this state is illustrated by the left-side portions of the plots in Fig. 8, showing a slow frequency increase for all modes except the ECC deformational one. However, by giving way before the increasing stress excerted by the slowly rising polymer content, the parent lattice eventually drives itself into a critical state. This state is reached at about 7-8% conversion to polymer, when the parent (still essentially monomer) lattice attains a critical periodicity along the b direction. From this time on, the distance between neighbouring monomer units in the stack rapidly becomes more and more favourable for enhanced polymer growth. It means that the same amount of chain initiation events can now produce more polymer by virtue of longer polymer chains formation. Soon after the start of this transient stage the parent lattice finds itself in an autocatalytitally (and, perhaps, catastrophically!?) changing situation, when its further attempts to release stress by lowering the parameter b lead to an avalanche-like polymer growth and to corre-

spondingly rising stress. To escape a phase segregation into polymer and monomer lattices (both would find a relaxed state in this case), the parent lattice should respond adequately. The proper response of the parent (still enough pliable) lattice comes, obviously, to the rapid and substantial decrease of the parameter b. It automatically means that during this period the new rapidly growing polymer chains will grow in a rapidly changing (and more and more comfortable) environment. In terms of NIR-FTR spectroscopy, this highly co-operative process is illustrated by the central portions of the plots in Fig. 8, showing the accelerated frequency changes for all Raman bands during about 0.8-0.97 normalized time interval. Namely during this period the parent essentially monomer lattice is converted into the essentially polymer one. Further growth of the polymer by virtue of involving the remaining monomer into the polymerization reaction (from about 40-45% conversion on) is accomplished in the optimized lattice but with progressively slower rate owing to markedly decreased concentration of the monomer in the crystal. As the plots in Fig. 8 show (right-side portions), the lattice and/or the polymer chains continue to relax slowly during this period up to the completion of the reaction. Despite the fact that plots in Fig. 8 exhibit sharp but not discontinuous changes, we admit that the region of fast frequency changes may be confused with the discontinuous jump in fre500

1

n

Fig. 9. Sets of NIR-FT Raman spectra showing evolution of the polymer v(CEC) vibrational bands for PTS thermal polymerization reaction proceeding inhomogeneously, with uneven degrees of conversion in different layers of the crystalline sample (left), and proceeding more or less “homogeneously” (right).

A.N. Shchegolikhin, O.L. Lazareva / Spectrochimica Acta Part A 53 (1997) 67-79

quency indicative for appearance of an inhomogeneity in the polymerizing sample. As it has been noted [16], such inhomogeneity may be connected, for example, either with segregation of the polymer into its own phase from the parent lattice or with uneven degrees of monomer to polymer conversion through the sample depth. To elucidate the situation, let us address to the NIR-FTR spectra presented in Fig. 9. The situation depicted by the left 3-D plot in Fig. 9 has been artificially modelled by using excessively high laser power (more than 100 mW) and the uninterrupted regime of the spectra recording (see Experimental section). The suddenly appeared peak, centered at about 2077 cm-‘, originates from the polymer generated in the front surface layers of the crystal owing to extra heating excerted by the laser beam. Note that, indeed, this peak appears at frequencies close to those characteristic for fully polymerized PTS. It may be seen that initially prevailing (‘normal’) band at about 2032 cm - ’ and the suddenly emerged band at about 2077 cm - ’ grow then essentially independently from each other. The fact that they grow with different rates clearly indicates that the band at 2032 cm - l belongs to a material still residing at the induction stage, while the high-frequency band represents the crystal domains polymerizing already in the autocatalytical regime. The right 3-D plot in Fig. 9 illustrates a situation normally encountered during the in situ NIRFTR recording of PTS single crystals polymerization. Note that, in this case, the initial ‘normally’ growing band reveals rapid but rather smooth shifts of its peak to higher frequencies with the polymerization time. Most importantly, the intensity of the NIR-FTR signal at the frequencies, which just a few moments ago were the peak ones, turns out to be simultaneously decreased, revealing, in fact, a downfall on the subsequent spectra. Such behaviour of the Raman band indicates that the older polymer species represented by this particular band (being accumulated within the crystal during the induction period of the polymerization reaction and, hence, rather stressed) have simultaneously transformed into the more relaxed ones and now are repre-

17

2120 2110 2100 2090 2080 2070 2060 2050 2040 2030 Wavenumkr(cm-1)

Fig. 10. A typical structure of the v(CK) band as measured by NIR-FT Raman for fully polymerized FTS samples. Two Lorentzian components are shown by dotted lines.

sented by vibrations shifted to higher frequencies. Thus, this set of NIR-FTR spectra reveals a superposition of at least two processes, namely: (a) the process of enhanced growth of new polymer species in a modified-lattice and (b) the process of relaxation of the older (and more stressed) polymer species. The very important point, however, is that these two processes proceed more or less simultaneously and co-operatively. In other words, the NIR-FTR spectra suggest the occurrence of a rather sharp but continuous and concerted structural transition during the solid-state thermally induced polymerization of PTS. Similar analysis of the dynamic behaviour of the other NIR-FTR bands, originating, e.g. from v(C=C) or S(C-C=C) PTS backbone vibrations, would lead to the same conclusion. As to the obvious asymmetry and the multicomponent structure of the NIR-FTR bands at the beginning of the transition, we argue [l l] that these are quite natural for the case of in situ experimental probing of the thermally induced polymerization of PTS single crystal. Both the asymmetry and the composite structure of the bands are explainable in part by the non-ideality of the heating regime used. We guess, however, that owing to the poor heat conductivity of PTS, any heating regime employed for preparation of PTS polymer samples (including isothermal DSC conditions) in real life is, strictly speaking, not ideal. It is of importance also that selected from different batches samples of fully polymerized PTS prepared by using different heating conditions and regimes (isothermal and temperature

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A.N. Shchegolikhin, O.L. Lazareva 1 Spectrochimica Acta Part A 53 (1997) 67-79

programmed DSC, heating oven, thermostated oil bath etc.) all gave actually identical NIR-FTR spectra of the final polymer. As Fig. 10 shows, thermally polymerized single crystals of PTS contain at least two types of polymer species represented by the corresponding Lorentzian peaks. In the respective NIR-FTR spectra recorded at higher instrument resolution (not shown) the two component structure of the band is even more obvious (cf. also dispersive NIR-Raman spectra of PTS [5]). The minor Lorentzian peak centered at about 2070 cm ~ l usually represents about 8- 11% of the total polymer material. The origin of this fraction is not clear yet but, supposedly, it well could be the material which had been formed during the induction period. Moreover, it has been noted above that the asymmetric shape of the PTS polymer backbone lines in the NIR-FTR spectra, changing in ‘the course of polymerization reaction, may reflect an evolution of a distribution of structurally similar polymer species during the polymerization. Thus, it seems reasonable to assume that even those polymer PTS single crystals which usually are classified as being defect free or ‘perfect’ may be characteristically ‘inhomogeneous’ in terms of simultaneous presence of the spectrally resolvable populations of differently stressed polymer species within the same lattice. Obviously, the reported study could not answer all the above cited questions (see Section 1.2) and, moreover, actually it has formulated a number of new ones. Thus, for example, while the apparent absence of the polymer backbone modes frequency dispersion with the polymer n-conjugation length may be justified by the obscuring effects of the strong mechanical stress in the PTS crystal, the lack of the NIR-FTR bands intensity dispersion with rr-conjugation length in the course of the polymerization seems to be rather surprising. The apparent insensitivity of the skeletal modes intensities to changes in the refractive index of the material during the polymerization is also suspicious. Further, the intensities of the polymer backbone NIR-FTR bands seem to be unproportionally high relative to those, belonging to the side group chromophores vibrations or to the monomer v(C=C) mode. Specifically, average in-

tensity growth of the polymer backbone modes during the polymerization is estimated to be in excess of 300, while intensities of the bands belonging to vibrations of the side chains seem to remain practically unchanged [ 111. This strongly suggests that, despite the use of expectedly nonresonant excitation wavelength, the backbone vibrations turned out to be clearly resonantly enhanced. Although the linear polarizability per segment along the PTS polymer conjugated chain is more than double that in the monomer PTS molecule [38], it is hardly able to be totally responsible for the observed backbone modes intensity. Based on preliminary results of the studies which are currently in progress in our laboratory, we have to assume that, rather, nontrivial vibrational resonances involving lattice phonons as well as non-linear optical effects are operative in this case [39].

4. Conclusions Using PTS DA as a model compound, we have shown that the application of the NIR-FT Raman method for in situ monitoring of the solid-state topochemical polymerization of diacetylenes permits one to obtain reliable, reproducible and quantitative results. The useful kinetic information can be elucidated from the corresponding dynamically recorded sets of NIR-FTR spectra. Owing to the reliability of the NIR-FTR measurable spectral band parameters, such as position, width, intensity etc., obtaining of new pieces of structural information relevant to PDAs becomes feasible. Whilst the method is for sure widely applicable to DA polymerization studies, the laser powers used in the experiments should be chosen judgeously, to simplify subsequent interpretation of the NIR-FTR spectra. The main problem we have encountered during the prolonged isothermal heating experiments is one connected with imperfection of the conventional heating cell used, which is able to produce sometimes (in case of carefree arrangement of the sample in it) erratic or non-reproducible data. Minor cell modification, however, should permit to overcome this problem. Notably, the reported results have been

A.N. Shchegolikhin, O.L. Lazareva 1 Spectrochimica Acta Part A 53 (1997) 67-79

obtained on the machine equipped with a wideband KBr/Ge beamsplitter and by using a watercooled Nd:YAG laser. Hopefully, the spectra obtainable on the new generation instruments optimized for the work in the NIR region and equipped with more stable photo-diode lasers should be more precise. Undoubtedly, the higher resolution aforded by the new NIR-FTR instruments will permit to get much more deeper in sights into the mechanisms of structural transformations in this class of materials.

Acknowledgements

The authors thank Dr Ray H. Baughman (Allied Signal Corp.), Dr Valerio Grassi (PerkinElmer Corp.) and Dr Isao Noda (Procter and Gamble Co.) for encouragement during planning this study. Partial financial support from the International Science and Technology Center under Grant ISTC 015-94 is gratefully acknowledged.

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