Main-chain Thermotropic Liquid Crystalline Polymers Research Articles

Degradation of a Wholly Aromatic Main-Chain Thermotropic Liquid-Crystalline Polymer Mediated by Superbases

Degradation of a Wholly Aromatic Main-Chain Thermotropic Liquid-Crystalline Polymer Mediated by Superbases

Friction-induced surface textures of liquid crystalline polymer evaluated by atomic force microscopy, spectroscopy and nanoindentation

The main-chain thermotropic liquid crystalline polymer (LCP), Vectra A950, has been reported to be able to simultaneously lower dynamic friction coefficient and wear rate when introduced into poly(vinylidene fluoride) (PVDF) as lubricant. In this investigation, we evaluate the friction surfaces of Vectra A950 through atomic force microscopy (AFM), spectroscopy and nanoindentation. By AFM, various frozen nanometer-level textures are observed at friction surfaces of LCP particles after cessation of friction. Besides, orientation and microscopic hardness of textures for Vectra A950 are unveiled via polarized Raman spectroscopy and nanoindentation tests, respectively. It is worth noting that as an important hint of orientation for LCPs under shear, observation of LCPs textures is conventionally conducted at molten state by polarized-light optical microscope and shear apparatus, however, which hardly detects textures morphology at solid state. Overall, this investigation not only proposes a new approach to evaluate textures of LCPs, but also provides guide for further exploring tribological enhancement mechanism of LCPs.

Friction-induced lubricating nanocoatings of main-chain thermotropic liquid crystalline polymer

The main-chain thermotropic liquid crystalline polymer (LCP) has been reported to be able to simultaneously lower friction coefficient and wear rate when introduced into poly (vinylidene fluoride) (PVDF) as the lubricant. In this investigation, we aim to obtain some insight into the lubricating mechanism of LCP. Through atomic force microscopy (AFM) and Raman spectroscopy, friction-induced in-situ nanocoatings of LCP are observed on friction PVDF domains, which arises from increasing surface temperatures and high shear thinning properties of LCP. Consequently, LCP nanocoatings obviously enhance microscopic hardness of friction PVDF domains. In other words, LCP nanocoatings could be treated as a type of protective lubricant to prevent the direct contact between counterface and PVDF domains, promote wear resistance and lower friction coefficient. To our best knowledge, such an interesting lubricating mechanism is rarely reported. Overall, this investigation not only proposes the novel lubricating mechanism of LCP, but also provides support for further tribological applications in the polymeric systems.

Tribological enhancement effect of main-chain thermotropic liquid crystalline polymer

Although main-chain thermotropic liquid crystalline polymer (LCP) has been extensively investigated as self-reinforcing composites, tribological application is rarely reported. This paper explores tribological enhancement effect of LCP on representative matrix poly (vinylidene fluoride) (PVDF). Moreover, structures of PVDF/LCP blends are assessed in detail. Due to immiscibility, LCP in blends is uniformly dispersed in the form of particles or microfibrils, dependent on LCP content. Besides, LCP improves storage modulus of PVDF. Most importantly, tribological performance of PVDF is obviously promoted. Compared with neat PVDF, the wear rate, coefficient of friction and oscillation amplitude of blend containing 20 wt% LCP simultaneously decrease by 97.7%, 80.7% and 80.2%, respectively. In particular, frequency-enhancement phenomenon is observed. By probing worn PVDF domains with nanoindentation test and atomic force microscopy (AFM), preliminary tribological enhancement mechanism of LCP is unveiled. Overall, this finding provides a new, but promising lubrication route of polymer.

In-depth analysis on thermal phase transition mechanism of main-chain phosphorus-containing thermotropic liquid crystal copolyester

ABSTRACTIn order to surmount drawbacks of the infrared spectroscopy (IR) itself during investigating the mesophase-transition behaviours and mechanism of the thermotropic liquid crystalline polymers (TLCPs), the elemental phosphorus as an internal marker was introduced into the main-chain TLCPs. The detail mechanism of the glass transition and mesophase phase transition of the phosphorus-containing aromatic liquid crystalline copolyester (poly(-hydroxybenzate-co-DOPO-benzenediol dihydrodiphenyl ether terephthalate) [PHDDT]) was revealed through tracing the internal marker with the perturbation correlation moving window 2-dimensional correlation and 2-dimensional correlation analysis (2DCOS) correlation IR spectra. The results showed that the phosphorus-containing unit did not participate in the glass transition of the PHDDT. The results of the 2DCOS showed that the PHDDT mesophase phase transition took place through adjustment of the phosphorus-containing units. Simultaneously, the adjustment of the phosphorus-containing unit also can induce the motion of the other groups, and the sequential orders of the spectral changes were Ar–O–Ar → ester C–O → C=O. However, the sequential orders of the spectral changes were converse during the PHDDT glass transition.

Phase transition of LCP fluids confined in nanochannels through MD simulation

Manipulation of molecular orientation alignment in MCTLCPs (main-chain thermotropic liquid crystalline polymers) by pure shear at nano scale has been investigated for the first time using molecular dynamics (MD) simulation. Results indicate that high planar shear induces long-range uniform orientation ordering (liquid crystalline phase) of initially randomly orientated molecules of MCTLCP fluid confined in a nanochannel, which is confirmed by analyzing the orientation order parameter and the snapshots of MCTLCP liquid in a nanochannel under different shear rates. Insights into the origin of the phase transition phenomena are given at molecular level through investigating the thermodynamic density distribution of MCTLCP molecules in the nanochannel, suggesting that the energy shift due to a radical jump of system density affects both the magnitude and the orientation of the molecular ordering. Simulation results also show that there is a critical shear rate for transforming isotropic phase into liquid crystalline phase. The critical shear rate is dependent on the temperature of the MCTLCP system. Findings in this paper may present useful information for processing TLCP molecules at nano scale and the understanding of nanoflow.

Bimodal Orientation Defects in Main-Chain Thermotropic Liquid Crystalline Polymer Fibers

The molecular orientation in as-spun fibers of main-chain thermotropic liquid crystalline polymers (TLCP) has been investigated using electron diffraction at high spatial resolution. Although a lar...

Molecular orientation in quenched channel flow of a flow aligning main chain thermotropic liquid crystalline polymer

We report measurements of molecular orientation in solid specimens of a main-chain thermotropic liquid crystalline polymer (LCP) that were quenched from mixed shear-extensional channel flows. The polymer under investigation is a random copolyether with mesogens separated by flexible hydrocarbon spacers. This polymer is known to exhibit ‘flow aligning’ dynamics under slow shear flow. Experiments were designed to preserve the molecular orientation state, representative of steady, isothermal channel flow in the solid samples, so that comparisons could be made against in situ channel flow measurements on other main chain thermotropes without flexible spacers, including a commercial fully aromatic copolyster. In the flow aligning material, little change in orientation was found in slit-contraction flows, and only modest drops in orientation were found in slit-expansion flow. This contrasts strongly with data on commercial LCPs, suggesting that these materials may be of the ‘tumbling’ type.

Phase Behavior, Rheology, and Morphology of Binary Blends of Semiflexible Main-Chain Thermotropic Liquid-Crystalline Polymers

The phase behavior, rheology, and morphology of binary blends of semiflexible main-chain thermotropic liquid-crystalline polymers (TLCPs) were investigated. Specifically, binary blends consisting of poly[(phenylsulfonyl)-p-phenylene alkylene-bis(4-oxybenzoate)]s (PSHQn) having five methylene groups (PSHQ5) and 11 methylene groups (PSHQ11) were prepared by solvent casting. It was found from differential scanning calorimetry (DSC) that PSHQ5, PSHQ11, and their blends are glassy thermotropic polymers, exhibiting only glass-to-nematic and nematic-to-isotropic (N−I) transitions. Approximate phase diagrams were constructed for PSHQ5/PSHQ11 blends based on DSC data. Using a cone-and-plate rheometer, transient shear flow experiments were conducted for the PSHQ5/PSHQ11 blends (i) at 160 °C in the biphasic region where PSHQ11 forms an isotropic phase and PSHQ5 forms a nematic phase and (ii) at 130 °C in the nematic region where both PSHQ5 and PSHQ11 formed the nematic phase. It was found for such PSHQ5/PSHQ11 blends that the steady-state shear viscosity at 130 °C (in the nematic region) is lower than that at 160 °C (in the biphasic region). However, the first normal stress difference at 130 °C exhibits a very large overshoot followed by an oscillatory decay until reaching a steady state, while it is virtually zero at 160 °C. The time evolution of morphology for the PSHQ5/PSHQ11 blends, upon shear startup and also upon cessation of shear flow, was investigated using a specially designed optical microrheometer equipped with a polarizing optical microscope. Contrasting observations are reported for the case of nematic PSHQ5 in isotropic PSHQ11 when compared to the nematic PSHQ5/nematic PSHQ11 blend. Shearing of a nematic/nematic blend induces a much larger birefringence change than does shearing a nematic/isotropic blend, and a shear-induced isotropic-to-nematic transition is observed from a mixture of isotropic phases containing two TLCPs.

Shear Aligning Properties of a Main-Chain Thermotropic Liquid Crystalline Polymer

We report the first direct, quantitative measurements of the shear aligning properties of a main-chain thermotropic liquid crystalline polymer (LCP). We find that a model thermotrope with alternating mesogen and spacer structure is of the shear aligning type throughout its nematic range. The director rotates uniformly in the shear flow toward the Leslie alignment angle as probed by in situ flow conoscopy. The Leslie alignment angle becomes progressively closer to the flow direction as temperature decreases, corresponding to a decrease of the tumbling parameter λ with increasing order parameter S. Our measurements of λ(S) enable direct comparison with predictions from molecular models, which predict that shear alignment prevails in the limit of flexible nematic chains. This is in direct contrast to rodlike lyotropic LCPs for which director tumbling is the rule.

Shear Stress Overshoots in Flow Inception of Semiflexible Thermotropic Liquid Crystalline Polymers: Experimental Test of a Parameter-Free Model Prediction

We explore the origins of shear stress overshoots upon flow inception of main-chain thermotropic liquid crystalline polymers from a random polydomain initial condition. A simple polydomain simulation strategy, based on Ericksen's transversely isotropic fluid model, is capable of predicting a shear stress overshoot arising from the reorientation dynamics of domains toward a steady state flow aligned state. When model parameters are estimated using elementary molecular theories for semiflexible main-chain thermotropes, both the relative magnitude of the shear stress overshoot and the steady state ratio of first normal stress difference to shear stress are determined solely by the “tumbling” parameter, λ. This implies a universal correlation between these two variables. We test this prediction against an extensive data set on semiflexible polyesters and find good qualitative agreement between model and experiment.

Detection of nematic-isotropic transition of a main-chain thermotropic liquid crystalline poly(aryl ether ketone) by dynamic rheological technique

Thermotropic liquid crystalline polymers (TLCPs) have attracted much research attention in the past two decades due to the attracting properties, especially melt processing characteristics [1]. Most of the main-chain TLCPs synthesized up to date were polysters with flexible spacers [2–4]. The commonly used techniques in investigating phase behavior and to identifying the mesophase textures of LCPs include differential scanning calorimetry (DSC), wide angle X-ray diffraction (XRD) as well as polarized optical microscopic (POM) [5, 6]. The application of POM in TLCP characterization is sometimes found to be problematic because of the high viscosity of polymeric materials. Rheological response is very sensitive to morphological variation of polymers and in turn can be utilized to investigating phase transition of TLCPs [7, 8]. Rheological studies on lyotropic liquid crystalline polymers have afforded fruitful achievements [9, 10]. Comparatively dear rheological results from TLCPs were available. The reason lies in the fact that most TLCPs which have accessible mesophase-to-isotropic phase transition temperatures and mesophase temperature region broad enough that is suitable for rheological studies have not been synthesized in large quantity and at the same time most commercial available TLCPs have inaccessible mesophase-to-isotropic transition temperatures because they will decompose before reaching the clearing point. Poly(aryl ether ketone)s are a very important class of special engineering plastics which have been widely used in applications because of their excellent comprehensive properties [11]. The major drawbacks relative to processing are the high melting temperature and the high melt viscosity. Bennett and Farris first reported in 1994 thermotropic liquid crystalline poly(aryl ether ketone) copolymers based on 4,4′biphenol, bulk substituted hydroquinone and 1,4difluorobenzophene [12]. Afterwards several series of thermotropic liquid crystalline poly(aryl ether ketone) copolymers have been prepared [13, 14]. The realization of thermotropic liquid crystalline properties in poly(aryl ether ketone)s is showing promise in solving the above-mentioned problem in processing because they have been shown to have melt viscosity significantly lower that the isotropic polymers [15]. Here in the present work we describe a dynamic rheological detection of the mesophase to isotropic transition of a novel main-chain thermotropic liquid crystalline poly(aryl ether ketone) which was synthesized via an aromatic nucleophilic substitution reaction of 4,4′-biphenol, tert-butylhydroquinone with 1,4-bis(4-fluorobenzoyl)benzene. The details leading to the copolymer synthesis was described in one of our previous papers [16]. The copolymer containing 30 mol% biphenol moiety with a 19F NMR-based number-averaged molecular weight of 11 000 was employed in rehological measurements. The polymer has a nominal melting temperature of 335 ◦C and a nematic to isotropic phase transition temperature of 402 ◦C as determined by DSC. This copolymer is high temperature resistant. Thermogravimetric analysis demonstrated that it had a decomposition temperature above 500 ◦C at a 5% weight loss in nitrogen. This polymer is referred to as BB3 throughout this paper, where the number 3 stands for the 30 mol% tert-butylhydroquinone moiety in the copolymer. The ARES (Advanced Rheological Expansion System, Rheometrics, USA) was used for rheological measurements. The 25 mm-parallel platen geometry was adopted. The sample was subjected to dynamic oscillatory stressing (small-amplitude oscillatory shear, SAOS) in which the range of frequencies covered was from 0.1 to 100 rad/s. Before applying a frequency sweep, strain sweep at each fixed frequency was performed to determine the range of linear viscoelastic response (LVR) of the sample. The strain amplitude was varied from 2% to 20%, over which LVR holds. This strain amplitude also ensured to achieve sufficient levels of transducer torque during temperature increasing period for all the experiments. The temperature precision of the rheometer was satisfactorily controlled to be within ±1 ◦C. Data acquisition was completed by using a computer that was interfaced with the rheometer. The unique feature of polymer BB3 is the presence of both the crystal-to-liquid crystal and liquid crystal-toisotropic state transitions on its DSC thermogram. The

Morphological and Rheological Responses to Shear Start-up and Flow Reversal of Thermotropic Liquid-Crystalline Polymers

Novel rheo-optical methods have been used to directly observe morphology evolution, during shear start-up and reversal flow, in semiflexible main-chain thermotropic liquid-crystalline polymers (TLCPs). Using a specially designed microrheometer allowing for simultaneous transient optical and mechanical observations, we observed band formation upon reversal of flow direction. It was seen that this band formation causes asymmetric light diffraction in HV small-angle light scattering mode, indicating a tilted arrangement of the regularly spaced bands. Also conducted were shear start-up and flow reversal experiments using a cone-and-plate rheometer under the same thermal and deformation histories as those in rheo-optical experiments for polymers of differing spacer lengths at equal temperature difference below the nematic-to-isotropic transition temperature (TNI). It was observed that, during both shear start- up and flow reversal, the first normal stress difference N1 + (A o , t) exhibits a large overshoot followed by an oscillatory decay, while shear stress U + (A o , t) exhibits a large overshoot followed by a monotonic decay. It was found that the higher the applied shear rate, the larger the overshoot of N1 + (A o , t) and U + (A o , t), and the longer the persistence of oscillations in N1 + (A o , t). Similarity was found between the ratio N1 + (A o , t)/ U + (A o , t) and flow birefringence ¢n + (A o , t) during shear start-up and flow reversal of the TLCPs investigated in this study. Further, we found that the ratio U + (t,A o )/U scales with A o t but the ratio N1 + (A o , t )/ N1 does not, where U denotes shear stress at steady state and N1 denotes first normal stress difference at steady state.

Optical and Mechanical Rheometry of Semiflexible Main-Chain Thermotropic Liquid-Crystalline Polymers with Varying Pendant Groups

A specially designed flow cell was used to investigate birefringence in transient shear flow, ¢n + (A o ,t), of three semiflexible main-chain thermotropic liquid-crystalline polymers (TLCPs), poly((x)-p- phenylene 1,10-decamethylenebis(4-oxybenzoate)) (PxHQ10) with varying pendant side groups (x ) oxyethyleneethoxy, tert-butyl, or phenylsulfonyl). Also, shear rheometry was used to investigate the growth of shear stress U+(A o ,t) and first normal stress difference N1 +(A o ,t) during transient shear flow of the same TLCPs. Upon startup of shear flow, ¢n + (A o ,t) initially increases rapidly, follows two minor overshoots, and then levels off to attain a steady-state value ¢n in all three TLCPs. The steady-state value of ¢n for PSHQ10 (PxHQ10 with x ) phenylsulfonyl pendant side group) was found to be greater than that for PEHQ10 (PxHQ10 with x ) ethoxy pendant side group) and PTHQ10 (PxHQ10 with x ) tert-butyl pendant side group). This behavior is strongly correlated to the large van der Waals volume of phenylsulfonyl pendant side groups in PSHQ10 compared to that of ethoxy pendant side groups in PEHQ10 and tert- butyl pendant side groups in PTHQ10. It is also observed that N1 + (A o ,t) exhibits a very large overshoot followed by an oscillatory decay to attain a steady-state value N1, while U + (A o ,t) shows a very large overshoot followed by a rapid monotonic decrease to attain a steady-state value, U. The peak values of N1 + (A o ,t) and U + (A o ,t) for PSHQ10 were found to be 3-4 times greater than those for both PEHQ10 and PTHQ10, which we again attribute to the bulky phenylsulfonyl pendant side groups in PSHQ10. Only positive values of N1 + (A o ,t) and N1 were observed over the entire range of shear rates investigated. Variations of the ratio N1 +(A o ,t)/U+(A o ,t) with shear strain (A o t) were found to be very similar to variations of ¢n+(A o ,t) with A o t for all three TLCPs investigated, except for that the ratio N1 + (A o ,t)/U + (A o ,t) reached a steady state sooner than ¢n+(A o ,t). In steady-state shear flow, it is observed that the ratio N1/U overlaps ¢n over the entire range of temperatures investigated; specifically, both the ratio N1/U and ¢n remain constant at temperatures far below the clearing temperature (TNI), decrease gradually as the temperature approaches TNI, and drop precipitously to zero value at and near TNI.

Molecular orientation in channel flows of main-chain thermotropic liquid crystalline polymers

In situ wide angle X-ray scattering is employed for quantitative measurements of flow-induced molecular orientation in channel flows of thermotropic hydroxypropyl-cellulose (HPC). An extrusion die, constructed to allow X-ray access, generates slit flow, slit flow with superimposed contractions and expansions in cross-section, and slit flow past an obstruction. In slit flow, weak molecular orientation develops slowly with downstream position. Superimposed extension associated with contraction flows leads to a strong enhancement in orientation, which persists with distance further downstream of the contraction. Conversely, transverse extension present in expansion flows generates a bimodal orientation state and substantial reductions in average molecular orientation. These results are compared to earlier measurements on a commercial fully aromatic thermotrope. HPC is found to respond more strongly to superimposed extension, and more weakly to the prevailing inhomogeneous shear flow than the commercial material.

Effect of Bulkiness of Pendent Side Groups on the Rheology of Semiflexible Main-Chain Thermotropic Liquid-Crystalline Polymers

The effect of bulkiness of pendent side groups on the rheology of semiflexible main-chain thermotropic liquid-crystalline polymers (TLCPs) was investigated. The TLCPs investigated have aromatic triads with 10 methylene groups as flexible spacer and varying pendent side groups: ethoxy group, tert-butyl group, and phenylsulfonyl group. The TLCP having ethoxy pendent side groups (PEHQ10) undergoes smectic−nematic transition at 148 °C and nematic−isotropic (N−I) transition at 239 °C, the TLCP having tert-butyl pendent side groups (PTHQ10) undergoes only N−I transition at 193 °C, and the TLCP having phenylsulfonyl pendent side groups (PSHQ10) undergoes only N−I transition at 179 °C. Comparison of the rheological behavior of the three TLCPs was made at 40 °C below the N−I transition temperature (TNI) and at 15 °C above the TNI. The following observations were made. Upon startup of shear flow all three TLCPs exhibited a large overshoot in shear stress (σ) followed by monotonic decay and a very large overshoot in first normal stress difference (N1) followed by oscillatory decay. The peak values of overshoot in both σ and N1 increased with increasing bulkiness of pendent side groups, and only positive N1 was observed over the entire range of shear rates and temperatures investigated. Upon cessation of steady-state shear flow the relaxation rate of both σ and N1 decreased with increasing bulkiness of pendent side groups, and during rest after cessation of steady-state shear flow the rate of increase in reduced dynamic loss modulus was fastest in PTHQ10 and slowest in PSHQ10. In the nematic state the differences in bulkiness of pendent side groups played an important role, while in the isotropic state the differences in molecular weight played an important role in determining the viscosities of the TLCPs.

Practical considerations in the study of main-chain thermotropic liquid-crystalline polymers by vibrational microscopy

Current approaches to the study of the mesomorphic proparties of main-chain thermotropic liquid crystalline polymers by vibrational microscopy are discussed, highlighting with some examples the information available, emphasising advantages and disadvantages of the techniques, and offering perspectives on how new technological developments may affect the field.

A rheo-optical and dynamic X-ray-scattering study of flow-induced textures in main-chain thermotropic liquid-crystalline polymers. The influemce of molecular weight

In situ optical microscopy, small–angle light–scattering and X–ray–scattering techniques have been applied to study flow–induced textures in main–chain thermotropic liquid–crystalline polymers (LCPs). In order to gain more insight into the underlying mechanism leading to the formation of microstructure, the effect of molecular weight has been investigated. The well–known ‘banded’ texture, commonly observed in fibres and sheared thin films of LCPs, is observed to develop after cessation of shear. Furthermore, the banded–texture formation is molecular–weight dependent, in that it occurs only above a certain ‘critical’ molecular weight, M c w . For polymers with M w > M w c the banded texture forms within a few seconds of cessation of shear, relaxing to a disordered ‘tight’ texture. On the other hand, for polymers with M w M w c , a ‘striped’ texture, with stripes parallel to the flow direction, develops after cessation of shear, relaxing to a ‘domain’ texture, with no evidence of banded–texture formation. The ‘striped’ texture corresponds simply to the elongation in the shear direction of the original microstructure of disclinations. The critical molecular weight for banded–texture formation corresponds approximately to the molecular weight threshold for the transition from log rolling to flow aligning previously observed for these materials by Romo–Uribe and Windle. Rheological measurements (stress relaxation and recoverable strain) along with microscopic observations suggest that a retraction process along the nematic director at the cessation of shear provides a mechanism for crumpling the microstructure and thus for banded–texture formation.

Effect of Flexible Spacers on the Rheological Behavior of Main-Chain Thermotropic Liquid-Crystalline Polymers Having Bulky Pendent Side Groups

The effect of flexible spacers on the rheological behavior of main-chain thermotropic liquid-crystalline polymers having bulky pendent side groups, poly[(phenylsulfonyl)-p-phenylenealkylenebis(4-oxybenzoate)]s (PSHQn), was investigated. For the study we synthesized a homologous series of PSHQn polymers containing methylene groups, varying from 3 to 12, as the flexible spacer and then investigated their transient, steady-state, and oscillatory shear flow behaviors using a cone-and-plate rheometer. We identified the factors which give rise to, during experiment, time-dependent rheological behavior by taking wide-angle X-ray diffraction (WAXD) patterns of the specimen right before and after each rheological measurement. The initial conditions (i.e., the initial morphology) for transient shear flow were controlled by first heating an as-cast specimen to the isotropic region, shearing there at a rate of 0.1 s-1 for ca. 10 min, and then cooling slowly to a preset temperature in the nematic region. Only positive values of steady-state first normal stress difference were observed in all the PSHQn polymers synthesized, except for PSHQ3 having 3 methylene groups of flexible spacer, at all temperatures and shear rates investigated. Interestingly enough, we found that PSHQ3 exhibits Newtonian behavior at low shear rates over the entire range of temperatures tested, showing no evidence of liquid crystallinity from a rheological point of view. This observation is consistent with our earlier findings based on differential scanning calorimetry and WAXD that PSHQ3 is a glassy, non-liquid-crystalline homopolyester. When steady-state shear viscosity (η) in the nematic region is plotted against T − TNI, where T is a measurement temperature and TNI is a clearing temperature, we found that PSHQn moieties with odd numbers of methylene groups have higher values of η in the nematic region compared to PSHQn moieties with even numbers of methylene groups of the flexible spacer.

Viscoelastic Properties of Low Temperature Thermotropic Liquid Crystalline Polymers

Abstract Main-chain type thermotropic liquid crystalline polymers HPC-P having lower liquid crystalline temperature were obtained by esterification of hydoxypropyl cellulose. Two different samples were obtained, the one is HPC-P(l) having DS = 2.4 and another one is HPC-P(2) having DS = 2.8. Logarithmic curves of dynamic modulus(G') and loss modulus(G') against frequency obtained at different temperatures within liquid crystalline temperature range can be superposed to obtain a single composite curve for both samples. The composite curves for G' show lower frequency side plateau(Ca. 300 dyne/cm2). The log G' against temperature curves for both samples decrease with increasing temperature upto transition temperature and then increase upto TC and then decrease again with increasing temperature above TC showing maxima at around Tc.