Dynamical properties in glass forming polymers

Journal of

MOLECULAR STRUCTURE

ELSEVIER

Journal of Molecular Structure 448 (1998) 261-268

Dynamical properties in glass forming polymers V. Crupi, D. Majolino*, P. Migliardo, V. V e n u t i Dipartimento di Fisica dell'Universith di Messina e INFM, P.O. Box 55, 98166 S. Agata (Messina), haly Received 31 October 1997: accepted 1 December 1997

Abstract Raman low-frequency depolarized light scattering measurements were performed on polymers, namely isotactic polypropylene (iPP), polyethylene (PE) and their blends with hydrogenated oligo cyclo pentadiene (HOCP) at the melting point. In the solid state these blends have a lamellar morphology, crystalline iPP layers alternating to amorphous iPP + HOCP layers. Detailed study of the experimental data showed the main role played by the effective vibrational density of states in comparison with the reorientational diffusion contribution. On the other hand the existence of a boson peak, characteristic of glass forming systems, whose centre-frequency shifts towards higher values, increasing the percentage of HOCP, denotes the disorder effect connected with the presence of this component in the polymeric blends, the occurrence of which is also shown by the evolution of the dynamical correlation length, R~. Furthermore, in the very low-frequency range a crossover (~o,.,,-0.1 THz) from a spectral phonon-like contribution to a fracton-like contribution is detected. © 1998 Elsevier Science B.V. All rights reserved Kevwords: Raman depolarized light scattering; Glass forming polymers; Vibrational dynamic properties

1. Introduction Polymeric blends have been investigated for several decades due to the possibility of enhancing the properties of the single constituents. In general both the amorphous and the crystalline phases contribute to the final properties of the blends. Isotactic polypropylene (iPP) together with polyethylene (PE) have excellent chemical, mechanical, physical and electrical properties. However, their properties can be improved by blending with proper plasticizers, additives, modifiers, etc. For example films of iPP blended with hydrogenated oligo cyclo pentadiene have been introduced into the packaging industry because the blends possess a reduced permeability * Corresponding inl'm.it

author.

E-mail:

[email protected].

to oxygen and aromas compared to plain iPP films. After their introduction into the packaging industry, these blends have been carefully studied from morphological and mechanical points of view [1,2]. iPP and PE manifest a great tendency to crystallize. At low temperature these polymers are in the solid state without a high periodicity as in the case of organic solids, also due to the high viscosity that does not allow the chains to uncoil. Hence in the case of polymers there is a sort of disorder that persists even when the temperature is so low as to make the crystalline structure stable. In this work we report Raman depolarized light scattering measurements at low frequency on iPP, PE and their blends with hydrogenated oligo cyclo pentadiene (HOCP) at the melting point. In particular we studied the following compositions (w/w) of the blends PE/HOCP: 100/0 and 40/60 (w/w) at T= 140°C

0022-2860/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PII S 0 0 2 2 - 2 8 6 0 ( 9 8 ) 0 0 3 5 8 - 5

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V. Crupi et al./Journal of Molecular Structure 448 (1998) 261-268

and of the blends iPP/HOCP: 100/0 at T = 180°C, 80/ 20 at T = 250°C, 60/40 (w/w) at T = 218°C. It is well known [3,4] that there are two contributions to low-frequency excitation Raman spectra of glass formers: relaxations and vibrations. It could be shown from the analysis of low-frequency depolarized Raman spectra that the weight of vibrational over relaxational excitations is larger for less fragile glass formers (Angell's classification) [3]. This vibrational dynamics in glasses or liquids depends strongly on the static or transient organization, respectively, or on the disordered structure. In the frequency domain from about 1 to 100 cm-L universal features appear in the different responses, which are functions of the density of vibrational states (DVS). In particular the inelastic neutron scattering manifests a DVS excess. The low-frequency broad band in the Raman scattering spectrum, boson peak, is another characteristic of glass forming systems, present in several liquids. However up to now it is not clear if the boson peak is related to an excess in the DVS.

2. Experimental set-up We performed experimental low-frequency Raman depolarized scattering measurements, at the melting point, on iPP, Moplen T305 Montedison with molecular weight Mw = 3.0 x 105, PE, Eltex A1050P purchased by Exxon Co. with Mw = 3.15 × 105 , crystallinity 65% and their blends with HOCP, Escorez of ESSO Chemical Co. with Mw = 630 and density d = 1.07 g/cm 3. The blends were prepared by extruding the two components with a twin-extruder at about 280°C. After the extrusion the blends were cooled at room temperature and granulated [5]. The experimental data were recorded by a highresolution fully computerized Spex-Ramalog 5 triple monochromator, with a good signal to noise ratio thanks to the good optical quality of the entire system (sample and sample holder). It has been used in a 90 ° scattering geometry coupled with the 5145 ,~ vertically (with respect to the scattering plane) polarized line of a unimode Ar + laser (Spectra Physics mod. 165) working with a mean power of 300 mW. The scattered photons in horizontal polarization were automatically normalized for the incoming beam

intensity. A Glan-Thompson polarizer was used with an extinction ratio better than 5 x 105 as tested experimentally. The intensity of the obtained spectra were subsequently corrected for the density O, for the refractive index n and for local-field effects by means of the factor np-I(n 2 +2) -4, and the Stokes and anti-Stokes sites of the spectra were properly normalized by means of the detailed balance law. Following a well established procedure, we used a spectral resolution (0.10 +- 0.05)cm ~half width at half maximum (HWHM) in the region ( - 5 to + 5) cm-', 0.25 in the region ( - 30 to + 30) cm l, 1.5 in the region (-100 to + 200) cm ~.

3. Results and discussion The polymer iPP is able to order itself much more than others because of the chemical structure of the chains, in fact it has the side-groups located on the same side of the chain. In such a case if the portion of the polymeric chain is close to another, they arrange so as to minimize the repulsions and to enhance the attractions. The kind of order can be different, in particular if the lamellar structure is considered, we can define a characteristic length, It, depending on the polymer's nature and external parameters such as temperature, pressure and density. The crystalline lamellae interact with each other giving layers of lamellae among which a portion of non-crystalline polymer is present. The thickness of the amorphous part between the two crystalline lamellae, l~, is a function of external parameters too. The quantity L = lc +/a is defined as the long period. It has been reported that the Raman spectrum of solid n-paraffins contain a peak in the low-frequency region, whose fundamental frequency is inversely proportional to the chain length, over the frequency range 26-150 cm ~ [6,7]. These observations have been simply accounted for by treating a chain as a continuum and using elementary vibrations theory. It has been found that these results can be described by a longitudinal vibration, in which the chain is replaced by a solid rod, that is called LAM (longitudinal acoustic mode). The results obtained in the case of n-paraffins [6] are really useful in order to he able to explain what happens in the polymers. The well established observation of a LAM in PE [7] can

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V. Crupi et al./Journal of Molecular Structure 448 (1998) 261-268

furnish a direct measure of the lamellar crystalline thickness between the two amorphous phases, since PE also has a crystalline structure similar to iPP. As far as iPP is concerned, we observed in the experimental low-frequency Raman spectrum (see Fig. 1) the LAM peak related to the lamellar components of the sample. In fact: (a) its frequency is inversely proportioned to the chain length; (b) the band intensity is very high as for n-paraffins and PE; (c) the band intensity is comparable to the maximum of SAXS (small angle X-ray scattering) spectrum indicating that it is associated with the lamellar component of the sample and in particular to its crystalline thickness [8]. It is important to point out that the crystalline iPP, quenched from the liquid state using solid CO2 ( - 8 0 ° C ) , cannot be obtained in the amorphous phase but, as for most polymers, in a mixed phase crystyalline + amorphous. As it can be observed in Fig. 1 the LAM-1, corresponding to the first vibrational mode along the chain, is asymmetrical and is deconvolved in two gaussian bands centred at 11.8 and 1 5 c m - L By applying a unidimensional acoustic oscillation model [6] we obtained the propagation lengths of the ophonon in phase without dumping that are l] = 38 A and 12 = 30 A, respectively. Therefore iPP, unlike other previous measurements [9], shows two coherent lengths

corresponding probably to two ordered lengths of a linear chain present in the system. Because the amorphous phase has to have a DLAM, which is characteristic of a long range conformational disorder in the chain, with an enlarged peak [10], the two sub-bands could correspond to the chain length in the crystalline phase lc and the average chain length in the amorphous phase la, giving a long period L = la + It -- 68 A. Our Raman results in quenched crystalline iPP do not agree with previous SAXS measurements [5], that give I, = 7 0 - 8 0 A and lc = 40 and hence L -- 1, + l~ -- 110-120 ,~. This discrepancy can be explained in terms of the enlargement and asymmetry of the length distribution of chain segments. Fig. 2 shows, as an example, the reduced Raman intensity of liquid iPP at T = 180°C Ir=IVH/(n(w, T ) + 1)

in which we observed a boson peak centred at about 20 cm -~, due to excess density of vibrational states in viscoelastic systems. It is well known that the real physical origin of this peak is not yet clear. One of the most used theoretical models to explain the presence of this peak at low frequency is the Mode Coupling Theory (MCT) [11,12] that takes into account the vibrational and

1000

IPP Crystal 800 t-

6oo

e-0 t-

400

¢-

E r~

200

10

15

2O

A~o(cm -1) Fig. 1. Longitudinal acoustic mode (LAM) in crystal iPP with theoretical best fit (continuous line) and the single components (dashed lines).

264

V. Crupi et al./Journal qf Molecular Structure 448 (1998) 261-268

300

|

250

IPP T=180°C

f

=c 8oo

•

|

!

..O 200

v

.,_>,

I J

600

4oo C t-

I,,.,.

E

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+ I-

o

i

150

10

20

15

,~o (cm"1)

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0

i 0

I 40

i

I

,

80

I 120

J

I 160

i

ml

I 200

Aco(cm -1 ) Fig. 2. Raman reduced intensity of isotactic polypropylene at T = 180°C. The inset shows the longitudinal acoustic mode in the crystal phase.

relaxational contributions describing the Raman response of viscoelastic samples in terms of o~ and/3 relaxations. But since our systems in the liquid phase are composed by more or less entangled chains with partial order related to a resulting sliding of the chain with a large length distribution, we apply a modified version of the disorder induced scattering mechanism. So the observed low-frequency peak for all the systems could have a vibrational nature instead of relaxational. It is possible to quantify the vibrational contribution in comparison with the relaxational one, introducing [3] a new parameter R ], defined as the ratio of the scattered intensity in the first minimum observed in the depolarized spectrum IVH to the maximum of the boson peak: RI=

(lO,,,i,, (lr)max"

(1)

Fig. 3 shows the typical behaviour of Ir in a log-log plot used to calculate the R ] values for both the pure

and blends systems. A high value of this parameter corresponds to a considerable relaxational contribution in comparison with the vibrational one. As reported in Table 1, we obtained R 1 ~ 0.1 in agreement with viscoelastic systems characterized by a negligible relaxational contribution. Moreover by analysing the boson peak in pure iPP and PE and in their blends it appears clear that C0maxdepends on the sample. Furthermore, taking into account the experimental data in the crystalline phase, we are able to hypothesize that the boson peak in these liquid polymers is directly related to the propagation of transversal and longitudinal acoustic waves, namely D-LAM [10], similar to LAM-1 present in the crystalline phase. In other words it is generally ascribed to quasi localized collective atomic vibrations involving a great number of atoms. The frequency of the boson peak maximum, C0ma,, is usually related to a so-called dynamic correlation radius Rc = V/Wma~where v is the sound velocity.

V. Crupi et al./Journal (~['Molecular Structure 448 (1998) 261-268

265

1000

IPP T=180°C

t-

~401 ,.,°,,°o

"%.,

°•~ ,",° ...II-

,•

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....

I

. . . . . . . .

I

0.01

. . . . . . . .

0.1

I

i

i

i

i I i 1

10

1

o(THz) Fig. 3. L o g - l o g plot of R a m a n reduced intensity of isotactic polypropylene at T = 180°C.

The values of Rc are reported in Table 1. From an inspection of the table, a diminishing of Rc can be noticed when HOCP increases, indicating that the 'patches' dynamically correlated reduce, confirming a disordering effect of the additive previously shown by SAXS experiment [1]. Another interesting result from our study regards the spectral region between 0.01 THz (co--0.3 cm -~) - 1 THz (co--33.3 cm -I) in which we observe a discontinuity on the vibrational dynamics• It is well known that in a glass forming system and in a viscoelastic liquid the effective density of vibrational states, g~r(co), can be measured being connected with the IvH by the relation lvH(co)= (co()- co)4[n(co,T) + 1]gR-f(CO)

(2)

where n(co, T)=

and

g~-,-(co) = C(co)g(w) where C(co) is the usual vibron-boson coupling factor. R Figs. 4 - 7 show gert(co) vs. co in a log-log plot• The experimental data point out a clear discontinuity in the corresponding crossover frequency co~,, strictly dependent on the system. In fact, co~L,shifts towards higher frequencies when HOCP increases in the blends, indicating that the correlation length of fractonic modes [13,14] is diminishing, confirming our results on Re.

4. C o n c l u s i o n s

The analysis of anisotropic spectral contributions in high molecular weight polymers (iPP, PE, blends with HOCP) shows the disappearance of reorientational

1

e h~'/k"T- 1

Table 1 Numerical values of the relevant parameters calculated from the experimental data T (°C)

Samples

R i = I,,i,/I .....

R~ = v/c0...... ( 4 )

c0c,, (THz)

Slopes

180 252 218 140 140

iPP i P P / H O C P 80% + 2 0 % iPP/HOCP 60% + 40% PE PE/HOCP 40% + 60%

0.10 0.15 0.26 0.18 0.10

9.3 8.8 7.1 I 0.0 8.8

0.068 0.077 0.560 0.059 0.097

3 1 = 2 , 3 ~ ' = 1.3 31 = 1.8, 3_~ = 1.3 ~3j = 1.5, 32 = 1.l 3, = 1.8, 3", = 1.3 3t = 2,~3~= 1.4

V. Crupi et al./Journal of Molecular Structure 448 (1998) 261-268

266

1000

IPP 100

C

g 10 $

~0co 0.1

,

.

,

,

,

,,I

0.01

,

,

,

,

,

,

,

0.1

c0(THz) Fig. 4. Effective vibrational density of states and crossover frequency of isotactic polypropylene at T = 180°C.

induced from a Debye elastic continuum to a localized spectral contribution with lower dimensionality than the eulerian one, characterized by a crossover frequency O~cobetween the phononic and fractonic regime. This observed behaviour is common in many viscoelastic systems and it has been explained on the basis of the recent theories on the excess of spectral density in glass forming systems.

diffusion effects and the presence of a density of vibrational states. We observed: a)

b)

A boson peak whose maximum shifts in frequency when the additive HOCP increases, causing a diminishing of the dynamic correlation length R c. A clear evolution from a spectral contribution

IPP/HOCP 60/40

100

fl

T=218oC lO

3 a~

co 0.1

, 0.01

,

,

,

,

'

'

''

'

'

'

0.1

co(THz) Fig. 5. Effective vibrational density of states and crossover frequency of isotactic polypropylene/hydrogenated oligo cyclo pentadiene 60/40 blend at T = 218°C.

V. Crupi et al./Journal of Molecular Structure 448 (1998) 261-268

267

100 •

e

o

PE

T=140°C lO r-

$

/ 01

i

i

i

(0co i

i

0.01

i

i

I

,

,

,

i

,

,

,

0.1

0~(l-Hz) Fig. 6. Effective vibrational density of states and crossover frequency of polyethylene at T = 140°C. 1000

PE/HOCP 40/60 lOO

¢-

~

lO

o.1 O.Ol

.

.

.

.

.

.

~,c° :L o.1

. . . . . . .

e(THz) Fig. 7. Effective vibrational density of states and crossover frequency of polyethylene/hydrogenated oligo cyclo pentadiene 40•60 blend at T= 140°C.

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[5] E. Caponetti, D. Chillura Martino, S. Cimmino, M.A. Floriano, E. Martuscelli, C. Silvestre, R. Triolo, J. Mol. Struct. 383 (1996) 75. [6] S.I. Mizushima, T. Simanouchi, J. Am. Chem. Soc. 71 (1949) 1320. 171 R.F. Schaufele, T. Simanouchi, J. Chem. Fis. 47 (1967) 3605. 18] S.L. Hsu, S. Krimm, S. Krause, G.S.Y. Yeh, Polymer Letters Edition 14 (1976) 195. [9] W. Steffen, B. Zimmer, A. Patkowski, G. Meier, E.W. Fischer, J. Non-cryst. Sol. 37-42 (1994) 172.

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V. Crupi et aL/Journal o['Molecular Structure 448 (1998) 261-268

[10] R.G. Snyder, N.E. Schlotter, R. Alamo, L. Mandelkem, Macromolecu[es 19 (1986) 621. [I I] H.Z. Cummins, W.M. Du, M. Fuchs, W. G6tze, S. Hildebrand, A. Latz, G. El, N.J. Tao, Phys. Rev. E 47 (1993) 4223. [12] A.P. Sokolov, E. R6ssler, A. Kisliuk, D. Quitmann, Phys. Rev. Lett. 71 (1993) 2062.

[13] F. Rocca, A. Fontana, Phil. Mao. B 59 (1989) 57. [14] D. Majolino, F. Mallamace, P. Migliardo, F. Aliotta, N. Micali, C. Vasi, Phys. Rev. E 47 (1993) 2669.