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Thermally Stimulated Depolarization Currents in Poly (Ethyleneterephthalate-ran-p-Hydroxybenzoates.

Thermally stimulated polarization (TSPC) and depolarization currents (TSDC) are measured on polyethylene terephthalate (PET) copolymerized with p-hydroxy benzoic acid (PHB). These systems are Important since they comprise components of many commercial polymer liquid crystals (PLCs). There has been some speculation about the origins of the multiple transition peaks in TSDC of PLCs and secondary peaks have frequently been designated as resulting from Maxwell-Wagner interphase polarization. The applicability of this designation for main chain PLCs has been investigated. TSDC as a function of field strength and window polarization has been conducted and the transitions have been identified as glass transition peaks as opposed to space charge relaxations. A comparison of the TSDC current versus the polarization current (TSPC) reveals that TSPC is not generated for transitions associated with the PHB component.

1. INTRODUCTION

Polyethylene terephthalate (PET) copolymerized with p-hydroxy benzoic acid (PHB) is a common component of many commercial polymer liquid crystals [1]. Determining the glass transition of polymer liquid crystals is difficult using differential scanning calorimetry or dynamic mechanical methods, because of longer relaxation times and small change in specific heat at the glass transition of the rigid rod component. Dielectric techniques have shown potential for discerning the transitions related to individual components within the copolymers. To this end, thermally stimulated depolarization currents (TSDC) have been increasingly utilized to determine the glass transitions of polymer liquid crystals [2-6]. Much of this work has been conducted for side chain liquid crystals where two consecutive peaks are associated with the glass transition relaxation [7, 8]. The secondary peak has been assigned as due to a space charge relaxation. To examine the potential for TSDC in main chain (longitudinal) thermotropic polyme r liquid crystals, we utilize a known and extensively researched copolymer of polyethylene terephthalate: PET/xPHB where x denotes the mole fraction of PHB. The phase diagram of PET with PHB has been developed [9] using DSC, dynamic mechanical and dielectric spectroscopy from numerous sources in the literature. The wide range of potential transition temperatures was associated with instrumentation sensitivity and sample preparation differences. Zachmann et al. conducted a detailed examination on the effect of PHB fractions and concluded that liquid crystallinity was only possible for copolymers containing beyond 30 mol % PHB [10].

Thermally stimulated discharge (TSD) refers to the field induced discharge current upon buildup (Thermally Stimulated Polarization Current-TSPC) and/or release (Thermally Stimulated Depolarization Current-TSDC) of charges in a dielectric placed between two electrodes. The polarization phenomenon is a consequence of the rotation or migration of charges originating from and remaining within the dielectric. This leads to surface charges, which have the opposite polarity to those of the polarizing electrode [11]. Under simultaneous application of electric field during a temperature ramp, the orientation of the dipoles results in the formation of an electret. The first electrets were formed by Eguchi [12]. The effective frequency of operation varies between [10.sup.-3] and [10.sup.-5] Hz, making it especially suitable for studying systems having long relaxation times. The technique has a long history with nonpolymer glasses [13-17]. As Vanderschueren [18] points out, this technique was developed independently by several researchers. TSDC is also referred to as electret thermal analysis, thermal current spectra, thermally stimulated depolarization, thermally activated depolarization and the ionic thermocurrent technique [19]. Relating these relaxations to fundamental mechanisms of charge storage and release in nonmetallic systems was initiated by Bucci and Fiesehi [20]. One of the first detailed investigations on its potential in polymers was initiated by van Turnhout [19. 21]. There have been numerous applications of the technique to determine glass transition temperatures in amorphous and semicrystalline polymers [22], tacticity and chemical structure [23], water absorption [24-26], interfaces In composites [27].

The first application of the TSDC technique In PLCs was conducted by Simon [4] for side chain (comb) PLCs, in order to estimate the glass transition, degree of mesogenic alignment and degree of stored polarization, which is related to their use in optical storage media and non-linear optics. Recognizing its potential in resolving the complicated chain dynamics associated with PLCs due to the positional and orientational order, Brostow et al. conducted one of the first detailed investigations into the relaxational behavior of the PET/0.6PHB copolyrner (Unitika) [28]. More recently, Sauer et al. conducted tests on a variety of PLC systems and established the effects of matrix morphology in relationship to PLC architecture [3]. In using TSDC for main chain polymer liquid crystals, they recommend cautious use of "compensation plots" because of the different states of order. Compensation plots show the relationship between the logarithm of the pre-exponential factor of the Arrhenius equation and the apparent acti vation energy [29, 30]. Caution is required in interpretation since the linear relationship can be the result of an artificial statistical compensation pattern due to experimental errors [31], or it may reveal real physical behavior as shown by McCrum et al for glass transition relaxations [32]. Collins and Long [6] studied films of the commercial Vectra [Hoechst Celanese] material. He found a large degree of cooperativity above the [T.sub.g] (when defined as the temperature related to the peak in enthalpy-polarization temperature ([T.sub.p]) plot].

Characterization of the relaxational behavior of PLCs has traversed the spectrum of available techniques. TSDC and dielectric spectroscopic techniques are far more sensitive in resolving PLC transitions as compared to calorimetric, dilatometric and dynamic mechanical techniques. Spatial resolution of morphology through Wide Angle X-ray Scattering [WAXS] and polarizing microscopy are commonly conducted. Investigations on the molecular mobility of the liquid crystalline state using NMR have also been conducted [33]. Cao et al. studied PLCs using the DSC and reported on the width of the mesophase transition that extended to the melting point of the PLC [34]. Thermal cycling of rapid quenching from the melt state to above the glass transition followed by slow cooling within the glass transition regime was able to distinguish transitions from phases having similar [T.sub.g]'s. A distinct second step was found to be more pronounced with every subsequent thermal cycle. Regardless of the benefits of thermal cycling, it becomes apparent from Fig. 1 that the temperature related to the glass transition of the PHB rich component is hard to clearly discern due to its breadth and small change in specific heat at the glass transition. Sauer reports that this breadth Is related to the degree of order of the PLC [2, 3]. Dynamic Mechanical Analysis done on a modified torsion braid internal friction Instrument has shown two transitions for some PLCs [35]. However, in many copolymeric PLCs, an overlap of the transitions is often evident leading to broad transition peaks. Dielectric Spectroscopy conducted at [10.sup.-3] Hz has been effective in discerning individual peaks. The major difficulty using dielectric relaxation spectroscopy of PLCs is that the loss spectra are broad and highly asymmetric with coalescence of the different bands and without clearly resolvable features [18].

To further isolate individual relaxations related to the macroscopic relaxation, Lacabanne [36] and Chatain [37] proposed a modified TSDC method termed thermal sampling or windowing polarization. They attempted to experimentally deconvolute individual relaxation contributions from the global relaxation spectra. The relaxation time-temperature relationship associated with each window is used to isolate elementary Debye type relaxations of the molecules over the entire relaxation spectrum. Physically, the existence of multiple relaxations can be explained by several mechanisms, including dipole-dipole interactions, variations in size and shape of the rotating dipolar entities, anisotropy of the internal field in which the dipoles reorient (internal rotation, bending and twisting in polymers etc.). This technique is a further development of the attemp by Bucci et al. to isolate overlapping relaxations [38]. To isolate the transitions for a material having two peak temperatures [T.sub.m1] and [T.sub.m2], they po larized the material at [T.sub.p1] such that [T.sub.m1] [less than] [T.sub.p1] [less than] [T.sub.m2], to allow the dipoles associated with [T.sub.m1] to be polarized but those associated with [T.sub.m2] to be undisturbed. The TSDC curve would then show only the relaxation associated with [T.sub.m1]. The relaxation associated with [T.sub.m2], could be isolated by polarizing the material at [T.sub.p2] such that [T.sub.m2] [less than] [T.sub.p2] and removing the field at [T.sub.d] such that [T.sub.m1] [less than] [T.sub.d] [less than] [T.sub.m2] [18]. Window polarization has recently been utilized to probe space charge relaxations [39]. Space charge relaxations were demarcated from glass transition relaxation by a minimal shift in peak temperatures for different temperature windows.

Thermally stimulated polarization has also been used to probe the relaxations of dielectrics [40]. Vanderschueren points out that in the high temperature range, TSPC spectra are usually less well resolved because the relaxational current can be superimposed or masked by the conduction current [41]. Since TSPC does not involve a heating stage prior to data collection, differences between TSPC and TSDC are useful in determining the effects of physical aging behavior of thermally unstable structures.

In this paper, the results of a systematic study on a range of copolymers based on polyethylene terephthalate (PET) and poly (hydroxy benzoic acid) (PHB) are presented. A comparison between depolarization versus polarization currents for PLCs is demonstrated. Experimental parameters associated with determining the transitions are explored. The transition temperatures are determined for four copolymers and the PET homopolymer. The peak currents as a function of voltage for each material are investigated for linearity. TSPC and window polarization as a means to resolve transition temperatures are examined. The assignation of secondary peaks to Maxwell-Wagner inter-phase polarizations is examined for main chain copolymers.

2. EXPERIMENTAL

2.1 Samples

PET and copolymers with PHB (24, 50, 60 and 80 mol%) provided by W. Brostow, University of North Texas, Denton, and Michael Hess, University of Duisburg, were investigated.

2.1 Differential Scanning Calorimetry (DSC)

DSC heating runs were conducted on a Perkin Elmer DSC-7. Experiments were conducted by heating from 30[degrees]C to 200[degrees]C, annealing for 20 minutes, quenching to 30[degrees]C and heating to 200[degrees]C at 10[degrees]C/min.

2.2 Thermally Stimulated Polarization and Depolarization

Thermally stimulated polarization was conducted by heating the sample up at 5[degrees]C/min under the application of a 100 V electrical field. For the depolarization measurements, the sample was heated to 120[degrees]C, the voltage was applied for 10 minutes and the sample was then cooled at 50[degrees]C/min to -50[degrees]C, at which time the voltage was turned off. After 5 minutes, the depolarization currents were measured during a 2[degrees]C/min temperature ramp to 150[degrees]C.

2.3 TSDC Window Polarization

Window polarization was conducted between the temperatures obtained from the TSDC global measurements. The polarization temperature was increased in 5[degrees]C increments between 30[degrees]C and 120[degrees]C, with the discharge temperatures being 5[degrees] below the polarization temperature ([T.sub.p]). The samples were cooled to [T.sub.p] -40[degrees]C at 20[degrees]C/min and then discharge currents measured by heating it to [T.sub.p] + 40 at 50[degrees]C/min.

3. RESULTS

As shown in Figs. 2a and 2b, two peaks are evident from the global TSDC measurements on the homopolymer and copolymers. The pure PET homopolymer has a transition of 80[degrees]C. The peak temperatures as a function of PHB mol% are shown in Fig. 3. Two peak temperatures are evident for copolymers containing 24, 50 and 60 mol%. There are two different interpretations of this phenomenon in these systems. Zachmann et al. [10] assign the low temperature relaxation (around 60[degrees]C) to the LC phase and the higher temperature peak (around 104[degrees]C) to the isotropic phase. Brostow et al. [9] associate the lower temperature peak with a PET rich phase and the higher temperature peak with a PHB rich phase. The decrease in the transition temperature of the PET phase in the copolymer was associated with plasticization of the PET rich domains by fractions of the PHB. The reason for plasticization of the PET phase was attributed to disruption of order in the PET as a result of introduction of the PHB phase. The PE T homopolymer used here has a peak at 82[degrees]C while the copolymer containing 80 mol% PHB has a single [T.sub.g] at 98[degrees]C. The presence of a single [T.sub.g] for copolymers having PHB contents greater than 70 mol% PHB has been shown by Zachmann [10].

In validating results for TSDC it is important to consider other techniques. The higher sensitivity of TSDC over that of DSC concerning solid state relaxations (in contrast to melting transitions) is shown in Fig. 1. From the DSC results It can be seen that the PET system had a [T.sub.g] of 75[degrees]C, cold crystallization at 150[degrees]C. and a melting point of 148[degrees]C. On addition of PHB the cold crystallization and melting transition was suppressed for PHB = 0.6 and 0.8. The PET rich transition is clearly evident In the copolymers, but as can be seen, the PHB rich phase has a broad transition. The difference between the pure PET [T.sub.g] by DSC and TSDC is due to the difference in effective frequency of the two techniques. However, the wide temperature range of the glass transition and limited discernibility of transition temperatures highlight the limitations on using this method for [T.sub.g] determination. Complementary use of the DSC is important in determining the melting transitions of thes e systems. Processing parameters are aided by analysis of DSC results, while applications utilizing the anisotropic behavior of PLCs are aided by TMA. Thus, while the TSDC is valuable, the utility of other techniques for measurements should be considered.

The absence of two clear glass transition relaxations in DSC has led to some question about the validity of the higher temperature peak being assigned to a glass transition. However, ac dielectric measurements and internal friction measurements on a PET/0.6PHB and PET/0.5PHB copolymer show two relaxations [42, 43]. Further, the internal friction measurements done on two copolymers showed higher peak heights of the PET rich phase than the PHB rich phase with increasing PHB content. This result was attributed to the disruption in crystallinily of the PET phase, leading to greater amorphous characteristics with Increasing PHB content [42]. To determine whether the two peaks were glass transition relaxations or space charge relaxations, the samples were studied under different field strengths. A linear relationship between peak current and field strength is indicative of a true molecular relaxation. Space charge peaks are associated with stored charges between crystalline domains in semicrystalline material and due to water or other absorbed impurities in amorphous materials [18]. Space charges are characterized by a non-ohmic relationship between current and voltage. Mano et al. [7] studied thermotropic liquid crystalline polymers having sidechain mesogenic groups of acrylate, methacrylate and siloxane backbones. Mano [8] and Kohler [44] studied model side chain PLCs. They found that the peak current increased with increasing field strengths for low field strengths. For higher values of field strength, peak current decreased. This behavior was ascribed to the detrapping of ionic impurities and/or homocharges at higher voltages with the consequence that the motion of charges through the sample would make dipolar orientation a function of field owing to motion of space charges.

The increase in relaxation strength with varying voltage for the PET/0.5PHB system is shown in Fig. 4. Figure 5 shows the results of the peak current versus voltage for PET and the copolymers. The single linear relationship together with literature evidence for one copolymer [35] is indicative of the relaxation being a material relaxation as opposed to a space charge relaxation mechanism. Sauer and Avakian [45] note, however, that It is possible to have linear dependence of polarization current when Maxwell-Wagner charging is 100%. More recently, window polarization was utilized as a means to Investigate space charge relaxations in poly(methlyrnethacrylate] [39]. Space charge relaxations show a minimal shift in peak temperatures for different temperature windows. Therefore, complementary use of window polarization and evidence from other techniques was conducted.

Window polarization results for PET and PET/ 0.5PHB copolyiner are shown In Fig. 6a and b. As can be seen, the peak maxima center around the two global

relaxation peaks in the copolymer. More important for the higher temperature relaxation, a gradual shift in peak temperature is associated with increasing polarization temperatures. This is further confirmation that the two peaks recorded in the global TSDC spectra correspond to molecular relaxations and not to space charge effects, which would show no change in discharge temperature due to changes in the temperature window (39).

The potential for using TSPC for TSDC for PLCs was examined using PET and PET/0.5PHB (Fig. 7). If dipolar or ionic processes are involved, these show similar peaks as in global TSDC [18, 46]. A higher current in TSPC compared to TSDC was related to the formation of ionic space charge during polarization at high temperatures in TSDC [47]. As can be seen, only the transition corresponding to the PET phase is evident. However, the presence of PHB strongly affects the TSPC curve, as shown for PET/0.5PHB. The peak temperature of the polarization current occurs around 10[degrees]C prior to the TSDC peak. The difference between the peaks is Indication of changes in the copolymer induced as a result of the temperature ramp prior to TSDC measurement. Strong injection currents after the PET rich transition precluded data collection above the lower temperature peak. This has been found for many other systems [18, 48-50].

5. CONCLUSIONS

TSDC is a valuable technique In determining the glass transition temperatures of copolymer containing rigid rod sequences. The low effective frequency makes easy discernment of individual relaxations clear. The two peaks in longitudinal polymer liquid crystal copolymers are due to the relaxations of the copolymer components and not due to space charge relaxations. This was demonstrated through both the linear relationship between peak current and field strength, peak shifts in window polarization and literature evidence for one copolymer. TSPC is not as effective as TSDC in analyzing relaxations due to PHB sequences but was useful for pure PET homopolymers showing no shift in peak maxima in TSDC or TSPC.

ACKNOWLEDGMENTS

The author thanks Witold Brostow for kindly supplying these materials and introducing polymer liquid crystals to her. Deep gratitude is due to Michael Hess, University of Duisburg, for supplying some of the copolymers and making suggestions on the content of this manuscript. The author acknowledges with appreciation TherMold Partners, Inc., Stamford, Connecticut, for providing equipment and technical support.

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Author:D'SOUZA, NANDIKA ANNE
Publication:Polymer Engineering and Science
Date:Jun 1, 2001
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