Microscopic transport properties in liquid crystalline polymeric matrices: dependence on the thermal history

IOURNA L OF

ELSEVIER

Journal of Non-Crystalline Solids 172- 174 (1994) 943-949

Microscopic transport properties in liquid crystalline polymeric matrices: dependence on the thermal history L. Andreozzi a, M.P. Fontana h, F. Francia, M. Giordano a, D. Leporini a'*, M.

Rateo b

a Dipartimento di Fisica dell'Universita, 1-56100 Pisa, ltaly b Dipartimento di Fisica dell'Universith, 1-43100 Parma, Italy Abstract

The dependence on the thermal history of the dynamical and structural properties of a polymer liquid crystal has been evidenced by differential scanning calorimetry, infrared dichroism and non-linear electron spin resonance (ESR) spectroscopy in a wide range of temperatures. The behaviour of the microscopic viscosity as a function of both the isothermal annealing and the temperature is studied and its salient features are interpreted in terms of cooperativity. The multiexponential decay of the orientation correlation functions, relevant in the relaxation processes of linear and non-linear ESR spectroscopies, occurs in the supercooled and glassy phases, whereas the single exponential decay recovers above the isotropization temperature.

1. I n t r o d u c t i o n

Up to now, the study of the dynamics of structural properties of polymeric materials is one of the most stimulating interdisciplinary fields of investigation. Polymers open new prospects to materials science [1]; they are also very suitable for investigating the dynamics of complex systems, since they develop supercooled and glassy phases very easily. The results on the disordered systems such as the non-Arrhenius divergence of the transport properties and the extreme broad distribution of the correlation times of any relaxation phenomena have been particularly interesting [2,3], and attempts have been made to find theoretical unified explanations [-3] and a quantitative definition of the cooperativity and its role [4].

* Corresponding author. Tel: + 39-50 91 50 48 277.

1284. Telefax: +

39-

In this general context, liquid crystalline polymers play a prominent role in that they combine the richness of structures and phases of anisotropic fluids with the complexity of structure and dynamics of polymeric systems. In particular, side chain liquid crystalline polymers [5] allow the control of the mesogenic order and, at the same time, they can maintain this ordered arrangement down to below the glass transition temperature. Both from a fundamental and an applied point of view, the understanding of the molecular dynamics and the laws governing transport properties is of paramount importance in these materials. More specifically, besides the investigation about the relationships existing, if any, between macroscopic and microscopic transport laws, microscopic transport properties of polymeric liquid crystalline matrices are basic in the study of the maintenance of the mesogenic order near the glass transition temperature. This work reports on structural and dynamic studies carried out on a side chain liquid crystalline

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polymer [6] of the family of the polyacrylates using different spectroscopic techniques. Reorientational relaxation has been followed from the glassy up to the isotropic phase of the polymeric mesophase and monitored as a function of the isothermal aging. Structural results, obtained by IR dichroic ratio measurements and differential scanning calorimetry, support dynamical information provided by the use of a non-conventional electron spin resonance (ESR) spectroscopy: the longitudinally detected ESR (LODESR). 2. LODESR spectroscopy

Differently from the usual ESR spectroscopy, the observable of interest in LODESR spectroscopy is the longitudinal component of the dynamical magnetization (i.e. the component parallel to the static magnetic field) whose relaxation time is given by TI [7]. This feature allows the extension of the observation time to the millisecond region and makes this spectroscopy useful to study slow dynamics in polymeric systems. LODESR spectroscopy consists in irradiating a paramagnetic sample in resonance conditions, with two transverse waves at frequencies 0)1 ~ 0)2 ~'~ (Do ((DO being the Larmor frequency), and in detecting the first harmonic signal of the oscillating longitudinal magnetization at the frequency difference A = [o91- 0)2[ [8], The dependence of the LODESR signal, 6S~zIJ (0)1,0)2), on the frequency difference of the two irradiating waves is given by [9-11] ~S~ 1) (0)1, 0)2) = C(Ho, '~1, /~2) [T1/x/(AT1) 2 + 1], (1) where C(Ho, 21,22) is a constant depending on the chosen value of the static magnetic field Ho and on the intensities 21 and 22 of the two microwave transversal fields. The non-linear response of the spin system to multiple irradiation is discussed elsewhere [11]. A general expression of T1 is given elsewhere [10]. Eq. (1) assumes that the longitudinal magnetization decays with a single relaxation time 7'1 which is appropriate for the present case [9-11]. In this paper, we turn our attention to the case of a spin S = 1/2 interacting with a nuclear spin I = 1, as for nitroxide radicals (widely used as

spin probes in diamagnetic systems) [12]. If the relevant microscopic correlation times of the reorientation of the probe are shorter than the longitudinal relaxation time, a proper coarse-graining procedure of the time scale, reminiscent of the Redfield approach, leads to express the longitudinal relaxation time T1 as [10,13] r i -1 = A 0)b~-~ + A

0)o' <<~H, *±,

(2)

whereJ~l a n d f i are known functions depending on the values of the magnetic anisotropies of the paramagnetic probe and the order parameters of the polymeric mesophase. Having focused on the case of nitroxide spin probe with cylindrical symmetry, two correlation times Zll and zi, are introduced to characterize the reorientational stochastic process. Zll refers to the rotation around the symmetry axis (spinning motion), whereas 3± refers to the rotation of the symmetry axis (tumbling motion). The explicit expression of J~l and f± can be derived according to the general theory reported in Ref. [9] and will be discussed elsewhere [14]; the simple case of S = 1/2 is presented in Ref. [15]. 3. Infrared spectroscopy

Vibrational bandshapes contain considerable information on molecular motions and ordering in condensed phases. Fluctuation Raman and infrared spectroscopies have been extensively applied to the study of simple molecular liquids [16]. For more complex systems, such as low molecular weight (200-300 Da) liquid crystalline materials, serious difficulties arise due to the problem of separating the reorientational relaxation contibutions to the bandshape in the presence of the much stronger vibrational relaxation. However, a systematic study of molecular orientational order and dynamics in aligned liquid crystalline mesophases has shown that IR spectroscopy can be quite useful, provided that suitable vibrational lines can be found [17]. In a macroscopically aligned anisotropic fluid the IR spectrum will be dichroic, i.e. the vibrational lines will be polarized according to the specific symmetry of the vibration and the degree of molecular alignment. A totally symmetric

L. Andreozzi et al. / Journal Of Non-Crystalline Solids 172-174 (1994) 943-949

vibration, related to an induced dipole parallel to the main symmetry axis of the rod-shaped mesogenic molecule, will be a good probe of the orientation of the molecules with respect to the laboratory frame. In fact, if we denote by Ax, Az the integrated area of the absorption band of such a vibration for light propagating in the y direction and polarized either perpendicular (x) or parallel (z) to the alignment axis (f.i. the nematic axis), the microscopic orientational order parameter (P2) is obtained (P2) = ( R - 1 ) / ( R - 2), where R = A~/Ax is the dichronic ratio and a complete macroscopic alignment is assumed. Typical strongly polarized vibrations are the benzene ring C = C stretching, in the 1600 cm-1 range, and the C-H in-plane deformation in the 1100 cm i range. Thus their dichroism can be used to determine the orientational order. Dynamical information can be obtained by analysing the IR bandshape. In the simplest approach, the reorientational relaxation times for spinning and tumbling molecular fluctuations are obtained from the difference, F in the half-widths (in cm -1) of the Azt~o) and Ax(~o) bandshapes, respectively. For tumbling motions, the vibrational mode should be strongly longitudinally polarized, whereas for spinning fluctuation the induced dipole should be directed away from the main molecular axis (the best sensitivity being achieved at the 'magic angle' of ~54°). For low molecular weight liquid crystals, it was shown [18], in the framework of the small-step rotational diffusion model, that F ~ (P2) D~I, where D~I is the spinning diffusion coefficient (to which we restrict our analysis). Thus, if two appropriate vibrational lines are available, it is possible to determine simultaneously the order parameter (P2) and the spinning correlation time r~l ~ 1/D[I. Recently, some efforts have been made to extend these techniques to polymeric liquid crystals [19]; in this case the situation is complicated not only by the much larger complexity of the molecules, but also by the very high viscosity, long structural relaxation times and tendency to microcrystallization which these systems exhibit.

4. Experimental The investigated polymer is the thermotropic nematic side chain (acronym PA) poly[[4-pen-

945

tiloxy-Y-methyl-4'-(6-acryloxyexyloxy)]azobenzene] [6]. Isotropization, melting and glass transition are 368, 353 and 293 K, respectively. To perform LODESR experiments, we dissolved the nitroxide spin probe cholestane [12] in the diamagnetic polymeric matrix, the molar ratio spin probe/monomer being 10 -2. Sample preparation included a thermal treatment: the sample was heated above the isotropization temperature for 10h and then quenched to below the glass transition temperature. To investigate low recrystallization kinetics we annealed samples for different times at the temperature T = Tg + 5 K. This procedure was used for both LODESR and DSC experiments. The LODESR apparatus together with its recent developments have been detailed elsewhere [20]. The DSC instrument was a standard Mettler TA4000. For the IR measurements, the sample was placed in a silicon cell. The plates (0.5 ~tm thickness) were separated by 15 tam spacers, and the inner surfaces treated to yield homogeneous alignment. Furthermore, an electric field was applied. The cell was positioned in a small optical oven in which temperature could be controlled to +0.1 K. The whole assembly was then placed in the sample compartment of a double beam prism spectrophotometer for the absorption measurements. In order to produce a homogeneous and well-aligned sample, the following procedure was pursued: the sample was first heated up to 393 K, i.e. well in the isotropic phase, and kept here under an ac (v = 10 KHz) electric field for about 6 h. Thereafter, it was slowly (rate ~ 0.1 K/min) cooled to about Ti - 1 K and kept here for about 30 min. Then, the dichroism measurements were taken as a function of temperature.

5. Results The thermogram of Fig. l(a) reveals features ascribed to the glass transition and the isotropization of the liquid crystalline phase (clearing point). The traces in Fig. l(b)-(f) are characterized by an increase in enthalpy at Tm as the annealing time goes by. They evidence that low kinetic reorganization occurs at room temperature.

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L. Andreozzi et al. / Journal of Non-Crystalline Solids 172 174 (1994) 943-949

The overall IR spectrum of this polymer has already been reported [19], together with the relevant band assignment. In the present work, we investigate the 1109 line assigned to an in-plane CH deformation mode; it is single and well-shaped and highly polarized, and thus very suitable to study orientational order. As a check, we have also studied the 1602 cm- ~ line, due to C = C aromatic ring stretching, which in low molecular weight liquid crystals is found to be strongly polarized. In Fig. 2, the behaviour of the dichroic ratios R as a function of the temperature (cooling model) is shown. We found that the values of R were reproduced upon heating from the room temperature glassy phase. The behaviours of 1/T~ as a function of the temperature for samples subjected to different aging times are shown in Fig. 3. T~ values are determined by fitting the experimental frequency swept

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Temperature (K) Fig. 1. DSC curves for PA on heating at 20 K/min. Quenched samples: (a) I00 K/rain; (b) 1 K/rain. Samples isothermically annealed at T 8 + 5 K for (c) 72 h; (d) 120 h; (e) 168 h; (f) 240 h.

LODESR curves by Eq. (1). The good agreement between the prediction and the experimental data allowed us to estimate the parameter. T1 with an error of 5%. Note that the validity of Eq. (1) states: (i) the timescale separation between the microscopic correlation times and 7"i; (ii) the homogeneity of the microscopic orientational sites within the timescale imposed by T~. Moreover, owing to the cigar-like shape of the cholestane spin probe, the values of the diffusion coefficients are highly anisotr o p i c ('c±/2711 /> 10) [21,22], SO that Eq. (2) reduces to

1 Ti -1 = f l 0902ZlI

(3)

and a linear proportionality between 7"1 and 2711is established. 6. Discussion

In the thermograms of Figs. l(b)-(f) the endothermic processes signal the occurred formation of crystalline nuclei inside the polymeric matrix. The area under the exothermic peak is smaller than the area of the endothermic process, suggesting that the endo peak at Tm is due both to the melting of the crystalline portion and the enthalpic contribution coming from the portion of the material reorganizing itself during the DSC temperature scan. An ever increasing portion of the sample is interested in these slow readjustments as the annealing time increases. In Fig. 2, the experimental results about dichroic ratio are compared with superimposed solid lines derived assuming a Maier-Saupe behaviour of the microscopic order parameter (P2), and the angle ~b of about 38 ° between the microscopic induced dipole moment and the main molecular axis of the side chain. Note the freezing of the value of (P2) as the temperature is lowered. This deviation from the ordinary Maier-Saupe behaviour is expected in polymeric glass forming systems [21]. The dichroic ratio gaurantees well-aligned samples, even though the overall value of R was not as high as usual for a strongly longitudinally polarized vibration. This, in turn, confirms that ff must be fairly large. This being the case, the l l90cm -I vibration, which should in principle be sensitive to tumbling

L. Andreozzi et al. / Journal of Non-Crystalline Solids 172-174 (1994) 943 949

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103/T(K) Fig. 3. Behaviour of the longitudinal relaxation time, T], as a function of the temperature. The annealing time is reported• (a), (b) Fit with Vogel-Fulcher law 1"1 oc e x p [ A / ( T - To)]; To = Tg - 55 K and A = 49 K, 43.1 K, respectively; (c) fit with Arrhenius law: activation energy 11 kJ/mol; (d)-(f) fit with doubly activated Arrhenius law: activation energies 16-3 kJ/mol; 20-2.5 kJ/mol; 25 2.6 kJ/mol, respectively.

fluctuations, becomes a probe of spinning reorientation, which, being much faster, will hide the tumbling contributions to the bandshape. A Lorentzian-Voigt fit of the polarized and depolarized

bandshapes of the 1109 cm-1 peak yield a halfwidth difference F ,~ 4 cm-~, which would imply a very fast spinning relaxation time of about 1 ps. However, it should be recalled that the sample is

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L. Andreozzi et al. / Journal of Non-Crystalline Solids 172-174 (1994) 943-949

under the action of an electric field. It is known from the literature that vibrational bandwidth can be broadened by the ordering effect of electric fields [23]. Thus the observed half-width difference can be due to a complex interplay of the intrinsic molecular reorientation and electric field effects. In Fig. 3 three regions can be identified in the temperature dependence of T1, as determined by LODESR spectroscopy. By increasing the temperature above the melting temperature Tin, a smooth decrease of T~ is observed ascribed to the smooth decrease of both the order parameters and the correlation times. Note that, as expected, the high temperature behaviour is independent of the thermal history of the material. It is also worth noting that the values of the spinning correlation time zlf as obtained from LODESR spectroscopy via Eq. (3) are in good agreement with the ones derived by the ESR profile, simulated according to the general stochastic approach detailed elsewhere [24]. For example, at T = 393 K from LODESR spectroscopy "ell= 2.3 x 10 -1° s, whereas from ESR spectroscopy we found a value of "ell ranging between 2.4 x 10-1o and 3.9 x 10- los. The uncertainty depends on the weak sensitivity of the ESR spectrum to the variations of the spinning diffusion component due to the high anisotropy of the diffusion tensor. In the fit procedure of both LODESR and ESR profiles, the reorientational process is assumed to be characterized by exponential correlation functions. The ESR spectroscopy provides information on the zero frequency spectral density J(0), i.e. on the area of the correlation function, whereas the LODESR spectroscopy is sensitive to J(too) and then to the decay of the correlation function close to the origin of the timescale (t ~ ~Oo1). The double test on the correlation function substantiates the hypothesis of a simple exponential decay of the orientational correlations. In the middle temperature region, an abrupt jump of the regular behaviour of 1/TI is observed at T ~ 326 K (marked by the dotted line in Fig. 3). The phenomenon occurs in the undercooled phase and is roughly independent of the thermal history of the sample. Around T ~ 326 K DSC thermograms exhibit a complex structure (Fig. 1) and the infrared spectroscopy signals that the local orientational order is frozen (Fig. 2). Owing to these evi-

dences and being the probe in the amorphous part of the sample [25], the discontinuity in the T1/temperature plot is ascribed to changes of the dynamical regime rather than of the amplitude of the random fields affecting the spin relaxation. The sensitivity of the LODESR spectroscopy to the environmental dynamic changes is even more impressive in the low temperature region. Differently from the previous findings, the dynamics of the paramagnetic probe depends markedly on the aging time and then on the crystallization degree. This is worth noting in that the probe is sited in the amorphous part of the polymeric host and it signals a strong interaction occurring between the amorphous and the crystalline part. It is remarkable that the same law, followed by the terminal diffusion coefficient in many glass forming systems [26], holds for the microscopic diffusion processes, Figs. 3(a)-(b). The observation of a Vogel-Fulcher law for zll (Figs. 3(a)-(b)) is consistent with a reorientation process occurring across a series of energy barriers [4,273 . The reciprocal of the overall diffusion coefficient 1/D s is simply expressed by the sum of the reciprocal of the diffusion coefficients related to the different barriers. Each term of the sum is expected to follow an Arrhenius law. The observed crossover to different regime (Figs. 3(c)-(e) seems to indicate that the reorientation occurs across parallel diffusion channels. In this case the overall diffusion coefficient Dp is expressed by a weighted sum over the different channels. Remarkably, in the low temperature region, ESR and LODESR yield different values of "rll (see Table 1). This can be explained as follows: they are sensitive to the spectral density of the reorientational process at frequencies zero and 09o, respectively. For non-exponential decay the effective "ell measured by ESR (LODESR) corresponds to the one derived by the slowest (fastest) component of the overall decay. As noted above, the coincidence between the two values signals the single exponential character of the decay. 7. Conclusions

Structural and dynamic properties of a liquid crystalline polymer have been investigated by using

L. Andreozzi et al. / Journal ~[" Non-Crystalline Solids 172 174 (1994) 943-949

Table 1 Comparison between 91 (ESR), Zll (LODESR) for different temperature values

T (K)

zll (ESR) (10 8s)

zlj (LODESR) (10 l°s)

268 283 293 303 318

42 _+ 10 19 _+ 3 13 _+ 1 8.9 +_ 0.4 5.9 +_0.3

15.0 _+ 0.7 8.8 _+0.4 5.8 + 0.3 5.1 _+ 0.3 4.4 _+ 0.2

Zll (ESR) is obtained by ESR lineshape simulations [24]; rll (LODESR) is calculated from Eq. (3). different s p e c t r o s c o p i c t e c h n i q u e s . F r e e z i n g of the o r d e r p a r a m e t e r of the l i q u i d c r y s t a l l i n e m e s o p h a s e a n d slow d y n a m i c processes o c c u r r i n g at r o o m t e m p e r a t u r e h a v e b e e n o b s e r v e d b y I R spectros c o p y a n d differential s c a n n i n g c a l o r i m e t r y , respectively. T h e c r o s s o v e r of the m i c r o s c o p i c viscosity from a V o g e l - F u l c h e r b e h a v i o u r to a d o u b l e act i v a t e d o n e has b e e n m o n i t o r e d in the low t e m p e r a ture r e g i o n b y n o n - l i n e a r E S R s p e c t r o s c o p y a n d i n t e r p r e t e d in t e r m s of serial a n d parallel diffusive processes. L i n e a r a n d n o n - l i n e a r E S R e v i d e n c e different c o r r e l a t i o n decays of the r e o r i e n t a t i o n a l corr e l a t i o n loss of the spin p r o b e at s h o r t (10 10 s) a n d l o n g times (10 7 s).

Note added in proof A n extensive i n v e s t i g a t i o n of the n o n - e x p o n e n t i a l c h a r a c t e r of the o r i e n t a t i o n c o r r e l a t i o n loss in s u p e r c o o l e d p o l y m e r s is discussed in Ref. [28].

References [I] L.H. Sperling, Introduction to Physical Polymer Science (Wiley, New York, 1992). [2] D. Richter and T. Springer, eds., Polymer Motion in Dense Systems (Springer, Berlin, 1988). [3] J.P. Hansen, D. Levesque and ]. Zinn-Justin, eds., Liquids, Freezing and Glass Transition (North-Holland, Amsterdam, 1991). [4] T.A. Vilgis, in: Polymer Motion in Dense Systems, ed. D. Richter and T. Springer (Springer, Berlin, 1988). I-5] C.B. McArdle, Side Chain Liquid Crystal Polymers (Blackie, Glasgow, 1989). [6] A.S. Angeloni, D. Caretti, M. Laus, E. Chiellini and G. Galli, J. Polym. Sci. 29 (1991) 1865.

949

[7] A. Abragam, The Principles of Nuclear Magnetism (Oxford, London, 1961). [8] M. Giordano, D. Leporini, M. Martinelli, L. Pardi, S. Santucci and C. Umeton, J. Chem. Phys. 88 (1988) 607, and references quoted therein. [9] L. Andreozzi, C. Donati, M. Giordano and D. Leporini, Phys. Rev. A46 (1992) 6222. [10] L. Andreozzi, C. Donati, M. Giordano and D. Leporini, Phys. Rev. E49 (1994) in press. [1 l] D. Leporini, Phys. Rev. A49 (1994) 992. [12] L.J. Berliner, Spin Labeling: Theory and Applications qAcademic Press, New York, 1976). [13] A.G. Redfield, IBM J. Res. Develop. 1 (1957) 19. [14] L. Andreozzi, M. Giordano and D. Leporini, in preparation. [15] L. Andreozzi, C. Donatiand D. Leporini, Colloids Surf. A72 (1993) 237. [16] J.H.R. Clarke, in: Advances in IR and Raman Spectroscopy, ed. J.H.R. Clarke and R.E. Ester (Heyden, London, 19771 p. 109; W. Rothschild, Dynamics of Molecular Liquids (Wiley, New York, 1984). [17] For a review, see M.P. Fontana, in: Phase Transitions in Liquid Crystals, ed. S. Martellucci and A.N. Chester (Plenum, New York, 1991) p. 259. [18] [. Dozov, N. Kirov and M.P. Fontana, J. Chem. Phys. 80 (1984) 2585. [19] M.P. Fontana, E. Anachkova, O. Monda, M. Rateo, A.S. Angeloni and M. Laus, Molec. Cryst. Liq. Cryst. (1993) in press. [20] A. Colligiani, M. Giordano, D. Leporini, M. Lucchesi, M. Martinelli, L. Pardi and S. Santucci, Appl. Magn. Reson 3 (1992) 107. [21] K.H. Wassmer, E. Homes, M. Portugal, H. Ringsdorf and G. Kothe, J. Am. Chem. Soc. 107(1985) 1511. [22] Y.K. Shin, J.K. Moscicki and J.H. Freed, Biophys. J. (1990) 445. [23] N. Kirov and P. Simova, Vibrational Spectroscopy of Liquid Crystals (B.A.S., Sofia, 1984). [24] M. Giordano, P. Grigolini, D. Leporini and P. Marin, Phys. Rev. A28 (1983) 2474; L. Andreozzi, M. Giordano and D. Leporini, in: Structure and Transport Properties in Organized Polymeric Materials. ed. M. Giordano, D. Leporini and E. Chiellini (World Scientific, New York) in preparation. [25] The probe is expected to be in the amorphous phase for several causes. The probe hampers the crystallization process. The observed homogeneous character of 7"1 excludes the presence of the probe in different environments. The values of 7"1 are not consistent with a strongly hindered motion. Finally, no anomalies in the LODESR signal and particularly in T1 are observed at the melting point (Fig. 3). [26] J.D. Ferry, Viscoelastic Properties of Polymers (Wiley, New York, 1980). [27] L. Andreozzi, F. Francia, M. Giordano, D. Leporini and M. Laus, Mater. Eng. 4 (1993) 345. [28] L. Andreozzi, M. Giordano and D. Leporini, J. Phys.: Condens. Matter 6 (1994) 1.