Far-infrared spectra and γ-relaxation in glass-like polymers

PolymerScience U.S.S.R. Vol. 32, No. 1, pp. 90-95, 1990

0032-3950/90 $10.00 + .00 (~) 1991 Pergamon Press plc

Printed in Great Britain.

FAR-INFRARED SPECTRA AND 'v-RELAXATION IN GLASSLIKE POLYMERS* V. A. R v z H o v , V. A. BERSHTEIN and A. B. SINANI A. F. Ioffe Physicotechnicai Institute, Academy of Sciences of the U.S.S.R.

(Received 25 July 1988)

The parameters of the far-infrared spectra of several oligomers, linear and moderately cross-linked polymers are compared with those of mechanical y-relaxation. Absorption at ~120-170cm -1 in the materials studied can be used for elucidating the molecular mechanism of y-relaxation. Except for some special cases this transition in flexible chain glass-like polymers is due to restricted torsional vibrations in fragments of the backbone, which are significantly shorter than the statistical segment.

IN FLEXIBLE-CHAINglass-like polymers relaxational behaviour is often observed over the temperature range 80-170 K (1 Hz), intermediate in relation to the/3-transition ( T < To-relaxation) and the low temperature g-transition [1-4]. This 7-transition,t is manifested differently, i.e. in the form of a specific relaxation peak, a fl-peak with a diffuse low temperature leg, or an inflexion on this leg. There are different points of view on the origin of 'y-relaxation. In some cases the molecular assignment of the transition is indicated unambiguously. This applies firstly to a conformational transition of the "chair-chair" type in the cyclohexyl ring of polycyclohexyl methacrylate [6, 7], other cyclohexyl-containing polymers [2], and cyclohexane itself [6], which is manifested in the form of a narrow relaxation peak at 190 + 10 K (1 Hz). Secondly, this motion of long alkyl side groups at ~100-120 K in polyalkyl methacrylate and other polymers [2, 6] is independent of the main chain. The given transition can also appear as a consequence of the motion of polar impurities in the polymer [3, 7]. This case is a specific feature of 'y-relaxation. Tentative assignments of the -y-transition of a more general character also exist, i.e. assignment to the vibrations of the side groups of any structure or the ends of chains [2-4, 8], torsional vibrations in short sections of chains within the limits of potential pits, without overcoming internal rotation barriers [9]. The first of these explanations is not very likely, since the role of the end groups in high molecular polymers consists in preserving the 'y-transition negligible, and, as a rule, potential barriers significantly lower than the activation energy Qv [10] correspond to the torsional vibrations of the side groups. As noted in recent reviews [2, 3], so far there is no experimentally based explanation of the mechanism of 'y-relaxation in linear polymers as a general phenomenon. It was shown previously in Refs [11, 12], and then by the present authors in Ref. [13] that in many cases information on the molecular motion units taking part in relaxation transitions can be obtained by far-infrared (FIR) spectra of the polymers. It is known that absorption due to torsional vibrations of atomic groups on different scales can fall into this spectral range, apart from intra- and inter-molecular vibrational transitions. Thus, absorption due to small angle (15 °) libration (torsional * Vysokomol. soyed. A32: No. 1, 90-95, 1990. t In the case of polyethylene and other highly crystalline polymers this term occasionally indicates a transition close to the glass transition temperature (in polyethylene at 140-170 K), corresponding to a /3-transition in amorphous and low crystallinity polymers [5].

90

Glass-like polymers

91

vibrations) of atomic groups close in magnitude to the macromolecule monomer unit lies within the range 20-130cm-~; these determine the relaxational 8-transition at 20--70K (1Hz)[10, 13]. Absorption due to correlated torsional vibrations of the units occur at a somewhat higher frequency in the FIR spectra of the polymers. In particular, as shown by Bershtein and Ryzhov [14, 15], the region 200--260 cm -1 is associated with torsional-vibratory motion in sections of the main chain close in size to the statistical segment; this can be used for determining/3-relaxation [16] and for assessing the presence of conformational chain mobility (T-G-transitions) [17] corresponding to it. The object of this work was to study the nature of y-relaxation in polymers, also using FIR spectra. The initial assumption was as follows: apart from the above considered correlated torsional vibrations, determining the conformational mobility of the main chain, torsional vibrations are also possible involving more than one unit, but not resulting in surmounting of the internal rotation barrier. The role of these should increase when motion of the segments, for example because of cross-linking of the chains at distances between the cross-links of N~ ~
92

V . A . RYzHov et al. M,CM" I~ C/~-!

15 /5O i

2

1001 o

= 100

~ ZOO

FIG. 1. FIG. 1.

_ , v, c r n - 7

$0 I I

I-

gO0 v, crn--

I00

FIG. 2.

Far-IR spectra of PMS (1) and its oligomers with n = 4 (2), 7 (3), and 14 units (4) at 298 K. Spectra 2-4 displaced along the vertical axis.

F~G. 2. Far-IR spectra of P M M A (1) at 298 K and its oligomers with n = 50 (2), 9 (3), 7 (4), and 2 units (5) at 80 K and spectrum of P D M E G (cross-linked P M M A ) (6) at 298 K. Spectra 1-4 and 6 displaced along the vertical axis.

absorption determined by libration and absorption associated with correlated torsional-vibrational motion in the main chain. It can be seen that the absorption bands at intermediate frequencies are clearly marked in the spectra of the oligomers and cross-linked polymer. In the PMS series, with transition from a polymer to oligomers and shortening of the molecules of the latter an absorption band arises at ~120-150 cm -t (Fig. 1) and increases in intensity. Similar changes are observed in the intermediate frequency range in the PMS spectra of PMMA oligomers and in cross-linking of PMMA (Fig. 2). The FIR spectra of PMMA and its 50-mer are almost identical, but an increase in absorption at 140--180cm -t is characteristic of the 9-mer, and an absorption band at 140-160 cm-~ appears, corresponding to the 7-mer and the dimer; the spectrum of the dimer also has a band at 190 cm- ~. The new band at 150 + 20 cm- i in the spectrum of PMMA increases as the degree of cross-linking of its macromolecules increases. As an example Fig. 2 shows the spectrum of cross-linked PMMA with Nc ~ 1. It should be noted that the increase in absorption in the intei-mediate frequency range discussed was observed earlier in cross-linking of PS macromolecules [14]. A priori this absorption could explain not only the torsional vibrations of the chain fragments, but also the appearance of end group vibrations in the oligomers or network units in the case of cross-linked systems. However, analysis fails to confirm this assumption, since the appearance of intermediate absorption occurs when the chemical structures of the proposed oscillators are quite different. Thus, oligomethyl methacrylates were obtained by catalytic chain transfer of cobalt to porphyrin molecules, and contained vinyl groups at the ends of the molecules [20]. Molecules of a-methylstyrene oligomers, obtained by organometallic synthesis, had isopropyl-benzene groups at the ends, which almost corresponded to the repeating unit. The units in networks of type M M A - D M E G and based on copolymers of styrene and divinylbenzene differed fundamentally: apart from the different chemical structure, the first have fairly flexible (they contain "hinged" oxygen atoms) transverse bridges, and in the second the rigid benzene ring serves as a bridge. It is thus not very probable that such different atomic groups should absorb in one and the same spectral

Glass-like polymers

93

range. This is supported by the absence of correlations between the intensity of the new intermediate absorption band and the concentration of end groups or network cross-links. It also follows from analysis that the observed effect of the appearance of an intermediate absorption band is not associated with vibrations in the transverse bridges and in network defects [18]. Accordingly, only the original hypothesis as to the origin of the given absorption* remains for discussion. If this is true, then absorption at intermediate frequencies vi, in cross-linked polymers and oligomers is determined by motion in short (with Nin'~S units) chain fragments. We will evaluate the potential barriers of such motion ain and the value of Nin. According to the method proposed by Ryzhov and Bershtein [13, 16], the value of Qin can be determined from the ratio of the frequencies of the skeletal torsional vibrations Vsk and Vin: ain = Qsk(Vin/Vsk)2, where Qsk ~ Qa ~" QvS + Qo and includes librational barriers Q1, S units of the correlation section, and an internal rotation barrier Q0 for rotation around the C - - C bond, which is equal to 15 kJ/mole. Correspondingly Qi, = apNin + Q 0 , whence Ni. = ( Q i n - Q o ) / a p • The values of ain thus calculated for the oligomers and polymers under study are given in Table 1. It is shown by comparison with the activation energies of the y-transition Qv that in all cases, as has been suggested, in the case of absorption bands at/-'in ~--140+ 20 cm -1 Qin ~ Q./. Evaluation of Nin shows that in the act of torsional-vibrational motion two adjacent monomer units take part. Consequently, on disturbance (decrease) of segmental motion in the main chain restricted skeletal torsional vibrations begin to play a part, which also determine the y-relaxation. T A B L E 1.

COMPARISON OF THE C H A R A C T E R I S T I C S

OF THE HIGH FREQUENCY T O R S I O N A L - - V I B R A T I O N A L

MOTION OF THE MAIN

CHAIN IN ')/-RELAXATIONS IN GLASS-LIKE POLYMERS

Qi,

225 225-230

140-160 160-190

28-42 42-56

43 [27]

170

245 245

120-150 140

32+7 28+5

35 [2] 33 [28]

140 132

vsk, cm- l

Cross-linked Oligomer

14 14

Cross-linked

13 11

State

PMMA

PMS PS

vin, cm-

Ql, k J/mole

Polymer

Qv kJ/mole

Tv, K ( ~ 1 Hz)

Note. The values of Q~ and/~'sk are taken from previous papers [10, 16].

The symbatic character of the changes in the FIR spectra of oligomethacrylates and specimens of cross-linked PMMA in the internal friction spectra of the latter is another confirmation of this conclusion. With increase in the degree of cross-linking of the PMMA molecules a simultaneous increase in absoprtion at 1200150 cm -~ and a decrease in absorption at 225-230 cm -t are noted; a similar "transfer" is observed as the PMMA chains are shortened, and on transfer with oligomers with n ~ 2 and 7, absorption at 1200150cm -l appears in the form of an individual band (Fig. 2). Figure 3 shows some of the mechanical loss moduli as a function of temperature. It can be seen that with increase in cross-linking of the PMMA chains the height of the relaxational fl-peak is gradually decreased ( T o ~ 3 0 0 K ) , and the cooperative a-transition of the glass transition (Tg~400K) is associated with inter-molecular correlational motion of such segments [21, 22], and the intensity of y-relaxation (T:,--170 K) increases. The increase in absorption in the intermediate region of the FIR spectrum of PS on chemical cross-linking was noted above. In this case also the mechanical losses in PS at 1400160 K increase, * Apart from torsional skeletal vibrations, the vibrations of hydrogen bonds and specific heavy atoms, which are absent in the systems under investigation, also fall within the frequency range 100-200 cm ~.

94

V . A . RvzHov et al.

G" MPa ~ -,

G', _MPa_I

9O

50~

ao 227

- 1oo

o

FIG. 3. FIG. 3.

foo

~

°C

;

~1

1o~ =<,.

FIG, 4.

Mechanical loss modulus of PMMA (1) and M M A - D M E G copolymers as a function of temperature at DMEG concentrations in them of 5 (2), 10 (3), 50 (4), and 100 mol.% (5).

FIG. 4. Changes in intensity of FIR absorption in M M A - D M E G copolymers at 225 (1) and 140 + 20 cm- 1 (2), of PMMA oligomers at 225 (3) and 160-190cm -I (4) as a function of the mean distance between cross-links Nc or the mean length of the oligomers n; the change in intensity of mechanical relaxation (loss modulus) in the regions of the B- (5) and y-transition (6) in M M A - D M E G copolymers as a function of Nc is also shown.

i.e. in the 3,-relaxation region 23]; the same is observed in radiation cross-linking of PS [24]. Figure 4 shows the changes in intensity of FIR absorption and the relaxational characteristics as a function of the mean distance between the cross-links Nc or the mean length of the oligomer molecules n. The picture is qualitatively the same: i.e. when the values of Nc or n are decreased to a size close to the length of a statistical segment (in PMMA s ~6), a significant decrease in the intensity of /3-relaxation and absorption at 225-230cm -1 and a simultaneous increase in the intensity of 3,-relaxation and absorption at 140-160 cm -1 are observed. Finally, evaluation of the displacement activation volumes of transitions in PMMA supports the considered concept of 33-relaxation: in the case of a v-transition va ~ 900/~3, which corresponds to a volume of ~6 units [25], and at y-relaxation temperatures va ~ 400-500/~3, which is close to the volume of 2-3 units [26]. From the point of view under consideration the appearance of 1,-relaxation must obviously be associated with the presence of strong inter-molecular bridges in the polymer, established by hydrogen bonds. For example, the 33-transition in PA and PU at 120-160 K are associated with torsional-vibrational motion of polymethylene sequences in some CHz groups located between amide groups [2]. Hydrogen bond bridges can obviously be established by absorbed water, and can lead to an increase in 3,-relaxation in PMMA [27]. The given data support the original hypothesis as to the possibility of a fairly general mechanism of 3,-relaxation in flexible chain glass-like polymers as being torsional vibrations in short sections of the main chain. Accordingly, the FIR spectroscopy method provided a means of defining mechanisms of molecular motion responsible for ~5-[10]/3- [16], and 3,-transitions in polymers and of establishing their connection with the main molecular characteristics, i.e. structure of the monomer unit, cohesion energy, and thermodynamic rigidity of the macromolecules. Translated by N. STANDEN

Glass-like polymers

95

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