Fourier transform infrared spectroscopy of the polymorphic forms of syndiotactic polystyrene

Makmmol. Chem. 191,2111-2119 (1990) 2111

Fourier transform infrared spectroscopy of the polymorphic forms of syndiotactic polystyrene

Gaetano Guerraa), Pellegrino Must0 b), Frank E. Karasz? William .l MacKnight

Department of Polymer Science & Engineering, University of Massachusetts, Amherst, Massachusetts 01003, USA

(Date of receipt: October 23, 1989)

SUMMARY: Infrared spectra of syndiotactic polystyrene exhibiting various crystalline forms and

modifications, and in the amorphous state, are reported in this contribution. In addition to strong conformational effects, the spectra demonstrate striking effects of chain packing. Several infrared spectral differences also exist between the two crystalline forms which present a trans-planar conformation of the chains. Spectral changes are also reported for solutioncrystallized samples, which depend on the solvent present in the clathrate compound formed with the polymer.

Introduction

The synthesis of fully syndiotactic polystyrene (s-PS) has been reported recently ‘p2) .

Fourier transform infrared (FTIR) studies of syndiotactic polystyrene have been described in which spectra for solution-cast sample^^^^) as well as for melt-crystallized samples’) are discussed. It has been concluded that the spectra of cast samples are consistent with results of previous studies which suggest a helical conformation for s-PS and which indicate that heat treatments can cause a transition to a trans-planar form 4).

Recent structural studies of s-PS have shown a very complex polymorphic behaviour 5-9). Using the nomenclature proposed in ref. ’), this can be described in terms of two crystalline forms, a and B, containing planar zig-zag chains while two others, the y and 6, contain (2/1)2 helical chains corresponding to a sequence of dihedral angles along the backbone of the type (TTG+G+). The 6 forms always incorporate solvent molecules; their exact structures are therefore a function of the specific solvent employed@. The general pattern is further complicated by the fact that both the a and p forms can exist in different modifications characterized by differing degrees of structural order, which are intermediate to those of the two limiting disordered modifications (the a’ and p’), and the two limiting ordered modifications (u” and p”). In these descriptions, structural disorder refers to the positioning of the polymer backbone; the order in the positioning of the substituent phenyl rings remains unaltered ’**).

*) Permanent address: Dipartimento di Chimica, Universith di Napoli, via Mezzocannone 4,

b, Istituto di lknologia dei Polimeri e Reologia del C.N.R., via Toiano 6, 80072 Arc0 Felice 80134 Napoli, Italy.

(Napoli), Italy.

0 1990, Htkthig & Wepf Verlag, Base1 CCC 0025-1 16X/90/$03.00

2112 G. Guerra, P. Musto, F. E. Karasz, W. J. MacKnight

In this contribution FTIR spectra of amorphous s-PS, as well as of samples in the above mentioned polymorphic forms and modifications of s-PS are reported. In addition to the anticipated large conformational effects, the FTIR spectra also show clear packing effects.

Experimental part

The fully syndiotactic polystyrene (s-PS) with weight-average molecular weight aw = 6,6 * lo5 as determined using gel permeation chromatography (GPC) was supplied by Zstituto Guido Donegani of Montedison, Novara. The polymer fraction insoluble in ethyl methyl ketone amounted to 93% which implies a very high syndiotactic Entent'). The atactic and isotactic samples were from Dow (number-average molecular weight M,, = I,@ * lo5 and aw = 2,3 * lo') and Polymer Laboratories, respectively.

The amorphous isotactic polystyrene (i-PS) and s-PS samples were obtained by quenching melted samples in liquid nitrogen. The crystallization conditions for the different crystalline forms of s-PS are described briefly in the next section. A more detailed discussion on the crystallization processes of s-PS is reported in ref.8). The respective crystalline forms of the samples used for FTIR measurements were identified by wide angle X-ray diffraction analysis').

Infrared spectra were obtained at a resolution of 1 cm-' with a Bruker IBM FTIR spectrophotometer. The wavenumber range scanned was 4800 - 400 cm-'. Qpically 50 spectra were signal-averaged in the conventional manner to reduce spectral noise. The results reported were obtained with films with thicknesses in the range 30-50 bm. The sample thickness was chosen so that the spectral region between 1400 and 800 cm-' would lie in the linear absorbance range; this region is the most sensitive to conformation. Measurements were also performed on powders in the form of KBr pellets with similar results. AU spectra are reported in absorbance units and are normalized when necessary with respect to the band at ') 1601 cm-'.

To gain a more accurate evaluation of the spectral changes induced by temperature, the annealing experiments were performed in a Spectra-lkh HT32 high temperature cell mounted directly in the spectrophotometer. The latter unit was controlled by an Omega CN-2010 programmable heat controller with an accuracy of f 1 OC. After the various annealing procedures the temperature was allowed to stabilize at 60 "C before starting the spectral data acquisition. This procedure removed any temperature effect on the frequency and intensity of the infrared absorption bands under investigation. The comparison of all the spectra at the same temperature was possible since the investigated transitions are irreversible '* ').

Results and discussion

The FTIR spectra of fully amorphous polystyrene samples of different tacticity are shown in Fig. I. The three spectra differ slightly in the 500- 1400 cm-I region where a number of conformation-sensitive vibrations are found.

In particular: i) the spectrum of amorphous s-PS (curve C in Fig. 1B) the 1250- 1400 cm-l region is markedly different from those of atactic polystyrene (a-PS) and amorphous isotactic polystyrene (i-PS); ii) the band at 1070 cm-' of a-PS, which gives rise to a more complex pattern for i-PS1o*ll), becomes sharper in amorphous s-PS (Fig. IB); iii) the very intense absorption at 758 cm-I which is present in both the atactic and isotactic samples splits for the syndiotactic sample into two bands located at 750 and 765 cm-' (Fig. 1A); iv) particularly informative is the 500-580 cm-* region (Fig. I C), since studies of polystyrene model compounds are available 12) which show that a band close to 540 cm-' is directly associated with the proportion of trans-trans local conformations present. The a-PS presents a broad

Fourier transform infrared spectroscopy of the polymorphic forms of . . . 2113

Fig. 1. Infrared spectra of (a) atactic, @) amorphous isotactic,

diotactic polystyrene, in the 400- 1700 cm-' (A), in the 1050- 1400 cm-' (B), and in the 400-650 cm-' regions (C)

a d (c) ~ O ~ h O u S SW-

1600 lL00 1200 1000 800 600 LOO Wavenumber in cm-'

Wavenumber in cm-'

a

b

C c

600 550 500 150 100 Wavenumber in cm-'

21 14 G. Guerra, P. Musto, F. E. Karasz, W. J. MacKnight

asymmetrical band with a well defined maximum at 540 cm-l suggesting a broad distribution of conformations with a prevalence of trans-trans local conformations; the amorphous i-PS also presents a broad band but the maximum is at 555 cm-' which most probably corresponds to a prevalence of conformations close to the minimum energy TG + conformation 12); finally the amorphous s-PS presents a sharper band at 538 cm-l which is, of course, related to the trans-planar conformation and which corresponds to the conformational energy minimum 13), but also shows two further weak bands at 515 and 571 cm-'; it is important to note that the band at 515 cm-' is absent in both atactic and isotactic amorphous samples.

It is worth noting that the sharpness of the bands at 1029 and 1070 cm-l in the fully amorphous s-PS sample, indicates that these bands are not of crystalline origin, a result in disagreement with previous interpretations').

The FTIR spectrum of an s-PS sample in the 6 form obtained by CHC1,-induced crystallization of amorphous s-PS7) is shown in Fig. 2. The spectrum is essentially identical to spectra of samples cast from solution at low temperatures 3 9 4 ) , which are in the same crystal forms). This spectrum is overall similar to that of the amorphous s-PS sample (curve c in Fig. I A); however, significant differences are present in some regions as, for instance, in the 900- lo00 cm-l region (Fig. 3). In particular, a well defined doublet at 943 and 934 cm-l, assigned to the helical conformation'), is present in the spectrum of the semicrystalline 6-form sample, while only a broad shoulder is present in the amorphous sample.

Fig. 2. Infrared spectrum of a fully syndiotactic polystyrene (s-PS) sample in the 6 form obtained by CHCI3-induced crystallization of amorphous s-PS. The spectra shown in the insets detail spectral changes caused by annealing at 170 "C which results in the conversion of the sample to the y-form. The heavy lines show spectra of samples before annealing; light lines are spectra of samples after annealing. (The band at 1219 cm-' which disappears on annealing, is due to residual solvent)

The FTIR spectra of samples in the 6 form prepared from different solvents are all generally similar. However, noticeable differences are present in the 950-980 cm-l region, as shown for instance in Fig. 3, in the PS samples derived from the three


Fig. 3. Infrared spectra of syndiotadc polystyrene (s-PS) samples in the (a) amorphous state, (b) 6-form from CHCl, , (c) &form from 1 f-dibromo- ethane, (d) &form from 1,2-dichloro- ethane, (e) y-form s

1000 950 Wavenumber in cm-'

different solvents. The solvents used have no absorption bands in this region; moreover after solvent adsorption, the respective spectra remain unchanged regardless of the extent of solvent removal (performed at 60 "C under vacuum). It was also observed that the adsorption of the same solvents in s-PS samples originally in the 8-form, which adsorbs solvent molecules only into the amorphous phase9), has no significant influence on the bands in the 950-980 cm-' region. Thus it is reasonable to conclude that these spectral differences for &form samples originating from different solvents are due to interactions of the chains with the solvent molecules in the crystalline phase and/or to the different kinds of chain packing6V8). Tb our knowledge, Figs. 2 and 3 present the first reported spectra of different clathrate structures.

The different &form samples are all transformed into identical y-form samples after annealing at 17OoC8). The FTIR spectrum of s-PS in the y form (Figs. 2 and 3) is similar to that of the S form spectrum. In fact, other than an increase in the intensities of a number of bands (eg., those at 538 cm-', 571 cm-l, I 155 cm-', 1278 cm-', 1321 cm-', 1354 an-'), which may correspond only to an increased crystallinity'), only minor changes in the 950-980 cm-l region (Fig. 3) are observed.

The &form sample obtained by crystallization of amorphous s-PS from CHCl, , the spectrum of which is shown in Fig. 2, was annealed for 5 min at 170°C and at 200 "C. The spectra of the annealed samples are compared in Fig. 4. Striking spectral changes are observed which correspond to the transition of a crystal with helical chains to a crystal with trans-planar structure'). No further significant spectral changes were observed after annealing at higher temperatures. This result is in agreement with differ- ential scanning calorimetry (DSC) and wide-angle X-ray diffraction measurements which indicate that the y to a' transition is already complete at 200 "C').

crystalline modifications are shown in Fig. 5 (curves a, b, c and d, respectively). The a' sample was prepared by annealing an amorphous sample at 200°C while the a" sample was prepared by compression molding s-PS at 280 "C*). The spectra of the two modifications of the a- form are essentially identical (curves a and b) and are also identical to those o f the

The FTIR spectra of samples in the a', a'', f3' and

21 16 G. Guerra, P. Musto, F. E. Karasz, W. J. MacKnight

,

Fig. 4. Infrared spectra of the sample the spectra of which are shown in Fig. 2, after annealing at 170T (y- form; heavy line) and at 200 "C (a'-form; light line, cf. inset spectra in Fig. 2)

1500 1000 500 Wavenumber in cm-'

solvent-crystallized samples annealed at high temperatures (Fig. 4 and ref.4)) which are also in the a'-crystalline modification8). The fl' sample was prepared by compression-molding at 330 "C while the f3" sample was prepared by casting at 150 "C from an o-dichlorobenzene solution8). For the f3 form, as for the a form, the FTIR spectra of the two different modifications, shown here for the first time, are essentially indistinguishable.

However, well defined differences exist between the spectra of the a and fj crystalline forms, although both contain a plain-type planar conformation of the chains. In our opinion this observation is particularly relevant. Differences which appear to be independent of the particular modification obtained and of the preparative route are present in the 1300- 1400 cm-l and 900 cm-I regions. Expanded FTIR spectra in three different regions for a- and &form samples as well as for an amorphous (s-PS) and a &form sample, are shown in Fig. 6. The 1222 cm-I band, associated with long trans sequences", which is absent in the amorphous and in the 6 (and y form) is present in the spectra of both the a and 0 forms (Fig. 6A). However, the spectra of the two forms differ in the 1300- 1400 cm-l range in terms of peak locations and relative intensities (Fig. 6A). Particularly interesting is the behaviour of the broad band present at 906 cm-' both in the amorphous and &form (and y-form) samples (Fig. 6B). For the a- and f3-form samples this band presents a sharper maximum at 902 cm-I and 911 cm-I, respectively, together with a shoulder close to 906 cm-' (Fig. 6B), due to the presence of the amorphous phase in these samples (the degree of crystallinity, evaluated by X-ray diffraction is approximately 50%).

Also informative is the comparison of the spectra in the 500-580 cm-l region (Fig. 6C). Narrow bands appear at 540 cm-I which confirm the assignment of this band to a trans-planar conformation12). For the 6-form samples, in addition to the 538 cm-I band, bands at 571 cm-', 548 cm-', and 505 cm-' are present, and may be associated


1600 1100 1200 1000 800 600 LOO

Wavenumber in cm-'

1 I

3 9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . s

1100 1200 1000 800 600 LOO

3 A 1600

Wavenumber in cm-'

Fig. 5. Infrared spectra of syndiotactic polystyrene (s-PS) samples: (a) amorphous state, after annealing at 200T (a' form); (b) compression-moulded at 280°C (a" form); (c) compression- moulded at 330 "C (p' form); (d) cast from o-dichlorobenzene solution at 150 "C (p" form), see text

2118 G. Guerra, P. Musto, F. E. Karasz, W. J. MacKnight

$400 1350 1300 1250 1200 1150 Wovenumber in cm-'

c . , 9LO 920 900 880 860

Wovenumber in cm-'

C

0

b Fig. 6. Expanded infrared spectra of syndiotactic polystyrene (s-PS) samples in three different regions (A) 1 100- 1400 cm-',

E (B) 860-940 cm-', 0 (C) 40-650 cm-'; Ef. (a) amorphous; (b) 6-

form; (c) a-form; (d) 2 < . . _ . , . . . . , . . . . , . . . _ I _

+I .I

r,, , B-form LOO 600 550 500 L50

Wavenumber in cm"

Fourier transform infrared spectroscopy of the polymorphic forms of . . . 21 19

with the TTG+G+ helical conformation, which is present in the crystalline phase6gt3). The similarity in this region of the spectra of the 6 form and the amorphous samples suggests that the shoulders present at 571 cm-I and 515 cm-' for the amorphous s-PS sample (curve c in Fig. 1 C and curve a in Fig. 6C) could be related to a substantial presence in the amorphous form of local conformations of the type TTG+G+, together with the prevalent trans-planar local structure. This interpretation is also supported by energy calculations showing nearly isoenergetic minima for trans-planar and (2/1)2 helical conformations for an isolated chain 13).

The application of methods such as spectral subtraction analysis to the determination of the crystallinity content by FTIR is currently in progress and will be discussed in a future article.

Dr. E. Albizzati of the Istituto Guido Donegani (Himont) is gratefully acknowledged for useful discussions. FEK acknowledges support from AFOSR, grant # 88-001.

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Fourier transform infrared spectroscopy of the polymorphic forms of syndiotactic polystyrene