On macroscopic scale, liquid crystalline phase of fluids polymers possesses long — range order. Generally, their molecules have asymmetric shapes to form liquid crys­talline phases and mesogenic groups, linked either on the main chain with flexible segments, or on the side chain attached to the flexible main chain. Some polymers without mesogenic groups are also capable of forming liquid crystalline phases. These polymers have a rigid or semirigid backbone, arising from the steric effects or intramolecular hydrogen bonding, which restricts chain flexibility.

Based on a lattice polymer, Flory was the first to predict that linear rod-like polymer would form ordered phases in concentrated solutions.86 According to this theory, at a given concentration, where the critical volume fraction of the polymer depends on the axial ratio, expressed as a length — diameter ratio, the rigid polymer solution would separate into two phases — isotropic and anisotropic.

Both entropy and enthalpy contribute to the stability of nematic mesophases, although the dominant factor is the entropy term, which is controlled by the size and shape of the molecule.8788 The original theory, which assumed that the polymer chains were completely rigid and rod-like, does not reflect the real situation. Be­sides, polydispersity, distribution of the substituted side groups as well as polymer- solvent interactions affect the flexibility of the polymer, so that even the most rigid polymers will have some degree of flexibility.

Formation of the liquid crystalline phase is highly dependent on specific poly­mer-solvent and polymer-polymer interactions. Transition from the isotropic to the ordered phase of lyotropic systems of semirigid polymers represents a balance of polymer-polymer and polymer-solvent interactions.8990 Not all cellulose derivatives form liquid crystalline phases at high concentrations; it is only those with appropri­ate solubility in a particular solvent that can form liquid crystalline solutions. In some cellulosic/solvent systems, increasing polymer concentration from the semi­dilute state, where already microgels appear, leads directly to the gel state, with no evidence of liquid crystallinity, even if the backbone of these derivatives has simi­lar stiffness with that of the derivatives producing mesophases. Due to the strong polymer-polymer interactions cellulose derivatives are not sufficiently soluble in a particular solvent to achieve the concentrations necessary for mesophase formation. Literature shows that the lyotropic liquids crystalline phase formed by cellulose de­rivatives occurs in highly concentrated solutions, ranging from 20-70% by weight, higher than those of the rigid rods predicted by Flory’s theory. The semiflexible nature of chains is due to substituents nature, substitution degree, substituents dis­tribution for partially substituted cellulose derivatives, temperature and solvents. Thus, the stiffness of cellulose derivatives in solutions is a function of the steric in­teractions occurring between adjacent units, intrachain hydrogen bonding, polymer — polymer and polymer-solvent intermolecular interactions.91

On the other hand, liquid crystal polymer blends have been intensively investi­gated as to their unique electrical, optical, mechanical properties.92 The self-aligning nature of the liquid crystal (LC) is used for obtaining organic polymer thin-film transistors in liquid crystal display devices.93 Recently, a comprehensive study has been devoted to the cellulose triacetate-nitromethane system, to explore its phase separation for different concentration and temperature values.94 The physical state of the polymer is identified within the coexistence phase limits on the phase diagram, which included three types of phase separation: amorphous, crystal, and liquid crys­tal. The limits of the regions determining the coexistence of the liquid crystal and of the partially crystal phase are found to be inside the region of amorphous liquid — liquid phase separation. For cellulose ester-solvent systems, this state diagram is the first experimental evidence for the possible coexistence of several phases with amorphous, liquid crystal, and crystal polymer ordering. The phase state of the sys­tem develops under the influence of temperature and concentration. At temperatures above the binodal and liquidus, all mixtures are homogeneous transparent solutions. If temperature is lowered and the configurative point moves to the metastable range, the system of various concentrations may have various morphologies, depending on polymer concentration and distance from the stability range. For low-concentrated solutions, under conditions of a rather high kinetic mobility of the molecules, amor­phous phase separation is observed with lowering temperature. The solutions are visually characterized by opalescence and occurrence of precipitation. For medium­concentrated solutions with low viscosities, in time the system splits into two phas­es. A dense white precipitate is formed in the polymer-concentrated phase, the other phase appearing as an opalescent solution with suspended particles. Literature9598 shows that transition to LC state is possible under certain thermodynamic condi­tions, when the selective solvation of the different polymeric groups determines the extended helical conformations of macromolecules9499102 and the appearance of liquid crystal ordering. In the same context, the structure, intra and intermolecular interactions, and conformations of macromolecules in cellulose derivative films — under conditions of the liquid crystal state formation during vapor sorption of some solvents — show that the number of intramolecular hydrogen bonds stabilizes the rigid helical conformation of the macromolecules.103 The anisotropic structure is also preserved after desorption of the vapor sorbed solvent. The capability of cel­lulose derivative films to form a liquid crystal state in vapor solvent and to preserve the anisotropic structure after solvent desorption, is especially important for prepar­ing new functional materials.

In crystalline state, the structure of polymers is influenced by solvent and con­centration.6970 Fundamental research on the formation of banded textures in thin — film samples from lyotropic solutions subjected to shear is important, due to the large number of physical interactions here involved.104 105 Surface anisotropy and the mechanical and optical properties of the polymer films,106 together with their potential use as alignment layers for liquid crystal displays, make these systems par­ticularly interesting and promising for new applications. Flow behavior is the most thoroughly studied rheological property. Some studies hypothesized the universal existence of three shear flow regimes to describe the viscosity of polymer liquid crystals namely: a shear thinning regime at low shear rates (Region I), a Newtonian plateau at intermediate shear rates (Region II), and another shear thinning regime at high shear rates (Region III). Region I, observed at low shear rates, shows shear thinning, exhibiting yield stress, as in some plastic materials. This region is charac­terized by distortional elasticity, associated with spatial variation in the director field (average local molecular orientation). Region II is a Newtonian plateau, reflecting a “dispersed polydomain” structure and Region III is a shear-thinning zone, showing viscoelastic behavior. In Regions I and II, the flow is not strong enough to affect molecular orientation while, in Region III, the flow field is very strong, so that the shear induces molecular orientation. Cellulose derivatives do not always cover the entire domain from Region I to Region III, because not every regime lies within the accessible shear rate range.107

In the preceding section, the specific interactions of CAP/HPC blends in 2-me — thoxyethanol are presented, such as hydrogen bonding, ion-ion pairing, and elec­tron-donor and electron-acceptor complexation, which generates miscibility be­tween components, have been discussed. In this context, the ATR-FTIR spectra of CAP, HPC and their blends (100/0, 75/25, 50/50 25/75 and 0/100 wt./wt. CAP/ HPC), plotted in Fig. 3.21, show that equilibrium in polymer blends is assured by the hydrogen bonds.69 A remarkably similar aspect of the spectra is observed for both polymers. Broad transmission bands are distinguished at 3421 cm-1 for HPC and 3431 cm-1, respectively, for CAP, produced by stretching of the — OH groups, at 1723 cm-1 for HPC and 1719 cm-1 for CAP, produced by stretching of the C=O groups from the ester, carboxylic acid, and at 1252 cm-1, respectively, for HPC and 1329 cm-1 for CAP, produced by stretching of the C-O-C ester bond. The pres­ence of hydrogen bond structures in blends are evidenced from peaks shape and intensity of the absorption band of the hydrogen stretching vibration.108 The differ­ences observed among the shape, broadening and shifting of the mentioned peaks for polymer blends suggest the existence of hydrogen bonding generated by — OH, C=O and C-O-C groups. However, the mentioned free and associated groups assure the equilibrium in these polymer blends via hydrogen bonds.

FIGURE 3.21 ATR-FTIR spectra of CAP, HPC and CAP/HPC wt./wt. blends.69

Quantitative measurements of weight loss are shown as TG/DTG plots (Fig. 3.22) for the same blends.69 The highest mass loss was of 100/0 wt./wt. CAP/HPC, followed by 75/25, 50/50 25/75 and 0/100 wt./wt. CAP/HPC. It has been found out

that the components of the blends decompose over different temperature ranges, the films loose water, and the curve for pure HPC shows an one-stage degradation step within the 300-400 °C range, with a maximum at 373 °C. The CAP and CAP/HPC blends showed a two-stage degradation step within the 210-298 °C and 280-560 °C ranges, respectively. The first weight loss is caused by the phthalic anhydride from CAP; with increasing the HPC content, this process is less evident, while the second weight loss is caused by the remaining cellulose. In addition, Table 3.7 shows that a percentage of 10 wt.% from the CAP and HPC mass is lost at 194 °C and 340 °C, respectively. For CAP/HPC blends, the temperatures at which 10 wt.% of their mass is lost represent intermediary values of the above-mentioned ones, increasing with increasing the HPC content.

T, °С

FIGURE 3.22 Experimental TG/DTG curves of CAP, HPC and CAP/HPC wt./wt. blends.69

TABLE 3.7 Degradation Stage, Temperature at which the Thermal Decomposition Begins (T.), Temperature at which the Degradation rate is Maximum (T. ,), Temperature at v onset77 r ° v main peak77 r End of the Process (T^.), Weight Losses (W), and Temperatures Corresponding to 10 and 50 wt.% Weight Losse“ (T10, T50) from TG/DTG Curves for CAP, HPC and CAP/HPC wt./wt. Blends69 CAP/HPC blends Degradation Stage, °C T onset’ °C T ■ k main peak, °C T endset, °C W, % T A10’ °C T A50’ °C 0/100 30-560 337 373 403 91.70 340 372 residue 8.30 25/75 30-140(a) — — — 2.70 140-225(b) — — — 3.28 270 369 225-290(c) — — — 6.10 290-560(d) 349 375 403 81.75 residue 6.17 50/50 30-120(a) — — — 3.20 120-224(b) — — — 6.54 226 359 224-280(c) — — — 8.50 280-560(d) 325 374 405 72.86 residue 8.90 75/25 30-120(a) — — — 2.70 120-212(b) — — — 7.10 213 333 212-280(c) 215 246 271 18.23 280-560(d) 301 342 411 62.44 residue 9.53 100/0 30-117(a) — — — 5.63 117-185(b) — — — 3.40 194 308 185-298(c) 202 274 293 37.38 298-560(d) 312 340 366 43.05 residue 10.54

(a) Degradation stage corresponding to the water loss.

(b) Degradation stage corresponding to the phthalic anhydride and acetic acid losses from CAP.

(c) Degradation stage corresponding to the phthalic anhydride losses from CAP.

(d) Degradation stage corresponding to the thermal decomposition of the remaining cellulose.

Thermogravimetric measurements show that CAP is less thermally stable than the HPC and CAP/HPC blends, as the anhydroglucose units increase the rigidity of the HPC chain.69 These interactions impart specific properties to the polymer blends. In N, N-dimethylacetamide (DMAc), the anisotropic behavior appears under specific conditions of concentration and/or blend composition. Some studies have reported that the polymer structures, their mixing ratio and the used solvent influ­ence the interactions from the systems and, consequently, the ordered domains in rheological behavior.109,110 In this respect, Figure 3.23 presents the modification of dynamic viscosity, П, versus shear rate, Y, for c = 20, 40, 60 wt. % at 25 °C, for
pure CAP and pure HPC in DMAc. In addition, Fig. 3.24 shows the same modi­fication of dynamic viscosity versus shear rate at different temperatures over the 25-45 °C domain for HPC in DMAc.

FIGURE 3.23 Log-log plots ofdynamic viscosity versus shear rate at different concentrations and also versus concentrations at different shear rates for CAP and HPC samples in DMAc Logarithmic viscosity values of HPC in log n versus log І are shifted upwards to 1 and 2 for 40 and 60 wt. %, respectively, for a better visualization70

-10123

log у (S-1)

As one can see, for CAP solutions with lower concentrations, a Newtonian be­havior appears over the entire shear rate domain while, with increasing concentration to 60 wt. %, the thinning effect reduces dynamic viscosity. The shear experiments

performed on lyotropic HPC solutions in DMAc reveal that the viscosity-shear rate dependence at different concentrations appears only in Regions II and III. As concentra­tion increases, the Newtonian plateau becomes smaller, being shifted to lower shear rates. At low shear rate, dynamic viscosity increases with concentration for both cellu­lose derivatives, showing a maximum at intermediary concentration, in the 20-60 wt. % range, which means a transition from the isotropic to the anisotropic phase.

Ordering of macromolecules in both polymers at higher shear rate is associated with a viscosity lower than that of the isotropic solutions, revealing liquid crystal be­havior. Below a certain critical shear rate and at a lower content of HPC, the isotro­pic solutions are Newtonian. Literature data show that the viscosity peak, observed for all lyotropic polymer solutions, is a decreasing function of shear rate, ascribed to several mechanisms.70111 A competition between the ordering induced by shear and that thermodynamically produced was suggested by Hermans,111 while a correlation between maximum viscosity and the anisotropic phase appearance — valid only at lower shear rate — was suggested by Zugenmaier.91

For CAP/HPC blends in DMAc,70 a distinct change in the rheological properties occurs at the critical concentration of 40 wt. %. Therefore, viscosity at higher shear rates takes lower values than at lower concentrations, at equivalent shear rates. Upon formation of the anisotropic phase, viscosity begins to decrease. Considering the de­pendence of viscosity on concentration for concentrations above 40 wt. %, the solu­tion is in a fully liquid-crystalline mesophase. Below this concentration, the solution is biphasic. Similar results were obtained for HPC aqueous solutions89112 or CAP/HPC blends — in which Regions II and III have different extensions, the Newtonian behav­ior decreasing with increasing concentration and HPC content (Fig. 3.25).

CAP/HPC: 100/0 wt/wt n = 0.94; К = 147.23 ‘ ____ , • _ * 4 n = (fs8; К = 14:94 "

Deviation of the CAP solution from the Newtonian behavior is also analyzed by the power law relationship between shear stress and shear rate (equation 11). Figures 3.26 and 3.27 show the values of the flow and consistency indices, obtained at different concentrations and blend compositions, from the slope and intercept of log shear stress versus log shear rate plots, over the domain of lower shear rates. It was observed that, as concentration increases, the resistance of the fluid to the ap­plied rate of shear or force (called shear stress) decreases, causing a decrease of the flow index to values < 1, and an increase in pseudoplasticity. Decrease of the flow index occurs when the HPC content increases and temperature decreases. At the same time, the consistency index increases with increasing concentration and HPC content, and decreases with the increase of temperature.

0.0 0.8 log y, 1/s

FIGURE 3.26 Log-log plots of shear stress versus shear rate for 100/0, 25/75, 0/100 wt./ wt. CAP/HPC blends in DMAc at different concentrations: (■) 20 wt. %; (▲) 40 wt. %; (▼) 60 wt. %.70

FIGURE 3.27 Log-log plots of shear stress versus shear rate for 50/50 wt./wt. CAP/HPC blends in DMAc at different concentrations and temperatures: (■) 25 °C; (▲) 35 °C; (▼) 45 °C.70

Figure 3.28 shows that the flow activation energies for CAP and HPC in DMAc, determined in the region of shear rate with Newtonian behavior, increase with con­centration and are higher for HPC.

6	CAP/HPC: 100/0 wt/wt	^3.2	CAP/HPC: 50/50 wt/wt		CAP/HPC: 0/100 wt/wt ▼
°?4 я	Ea = 36.33 KJ/mol	ш 24	Ea = 50.98 KJ/moJ^-»»^	«8	Ea = 39.10 KJ/mol
CL 2	_ A.——————————————————— "	1.6		CL	_—
So	Ea = 33.28 KJ/mol	°0 d ‘U)	Ea = 4§264iCJ/mole	•E7	*Ea = 38.91 KJ/mol
c _	_______________ a	C _ _		c6
—-2	л__________ _ ___________________	— 0.0	■-—■
-4	Ea = 31.72 KJ/mol	-0.8	Ea = 38.50 KJ/mol	5	‘ Ea = 36.50 KJ/mol

3.04 3.12 3.20 3.28 3.36 3.04 3.12 3.20 3.28 3.36 3.04 3.12 3.20 3.28 3.36 1000/T, К 1000/T, К 1000/T, к FIGURE 3.28 Arrhenius plots for CAP, HPC and 50/50 wt./wt. CAP/HPC blends in DMAc at (■) 20 wt. %, (▲) 40 wt. %, and (▼) 60 wt. % concentrations.70

Similarly with Fig. 3.13, Fig. 3.29 illustrates the variation of storage and loss moduli versus shear stress at a constant frequency of 1 Hz, for CAP, HPC and CAP/ HPC blend solutions at 60 wt. % concentrations in DMAc. G’ is constant and lower than G», showing viscoelastic fluid properties for 100/0 and 75/25 wt./wt. CAP/ HPC blends over a larger deformation region. Increase of the HPC content deter­mines higher values of G’ than of G», generated by transition from the isotropic to the anisotropic phase.

FIGURE 3.29 Log-log plots of storage and loss moduli versus shear stress for different compositions of CAP/HPC blends in DMAc at a concentration of 60 wt. %.70

The ordering tendency in casting solutions of CAP, HPC and their blends in DMAc is illustrated in Fig. 3.30, where moduli G’ and G» are presented as a func­tion of frequency, in the same manner as in Fig. 3.14 (at lower concentrations, in 2-Me).

FIGURE 3.30 Log-log plots of storage (•) and loss (■) moduli as a function of frequency for different compositions of CAP/HPC blends in DMAc at a concentration of (a) — 20 wt. %; (b) — 40 wt. %; 60 wt. %.70

For all concentrations of CAP and CAP/HPC blends at higher content of CAP, over the low frequencies domain of 0.2-0.9 Hz, the storage and loss moduli are proportional to frequency — when the exponents for G’ and G» take values between 1.6-0.6 and 0.9-0.4, respectively, maintaining the characteristic of a viscoelastic fluid, where G'< G» .76-77-113 These exponents decrease both with increasing polymer concentrations and at higher HPC compositions in the polymer blend. In addition, the frequencies corresponding to the crossover point, which delimits the viscous flow from the elastic one, and for which G’ = G», exhibit lower values for HPC in DMAc, and become higher with increasing the CAP content in polymer blends. A gel characteristic proves the presence of a liquid crystal phase, which appears for HPC, and a higher content of HPC at 40 wt. % and 60 wt. % concentrations. In these cases, the storage modulus, G’, is always higher than the loss modulus, G», over the entire frequency range, being characterized by G’ ~ f0267-0475 and G» ~ f0 249-0399 ; these dependencies are observed when the lyotropic phase becomes predominant.

Literature shows that the liquid crystalline order in cellulose derivative solutions observed by rheological investigations is preserved in solid films by slow evapo­ration of the solvent.114 Consequently, the microstructure of films depends on the drying conditions; also, films prepared under ambient conditions show polydomain

structures with the helical axes of different chiral nematic domains pointing in dif­ferent directions. Moreover, applying a magnetic field during drying increases the size of the chiral nematic domains and affect the orientation of the helical axes with respect to the film plane.115 Studies on the microstructure of such chiral nematic solid films reveal parabolic focal conic defects, a symmetrical form of focal conic defects in which the line defects produce a pair of perpendicular, antiparallel, and confocal parabolas.114 Depending on the substance and manner of sample prepara­tion, the parabolas lie horizontally or vertically in the sample plane.116 In vertical position, the parabolas intersect the top and bottom sample surfaces with their ends. The focal regions are located in midplane. The fact that these structures are captured in a solid film permits to study this highly symmetrical defect structure by other means than optical microscopy, for example, by atomic force microscopy (AFM). In addition, a chiral nematic system with a helical pitch large enough to be resolved by an optical microscope allows visualization of the individual structural layers.

Thus, AFM images from Figs. 3.31-3.34, for cellulose derivative films prepared under ambient conditions, show the polydomain structures with the helical axes of different chiral nematic domains pointing in different directions. Moreover, films obtained from CAP and HPC liquid crystalline solutions in both pure state and in mixture evidence some lights areas whose sizes depend on the HPC content and increase with increasing concentration. Intensity of the polarization colors varies cy­clically, from zero up to a maximum brightness at 45 degrees, for HPC (Fig. 3.31 (b, b,’ b”)). A rotating stage and centration of samples in a polarized light microscope is a critical element for determining the quantitative aspects of the HPC liquid crystal. Centration of the objective make the center of the stage rotation coincides with the center of the field, for maintaining the specimen exactly in the center, when rotated.

In the case of CAP/HPC blends, the number of methylene units (i. e., side chain length) inserted by increasing the HPC ratio in blends has a significant influence on the selective reflection characteristics of cholesteric liquid crystals.117 Some forma­tions of different sizes and intensities, namely droplets, appear. At a c = 20 wt. % concentration for all mixing ratios, the presence of the HPC liquid crystal is visible (white areas), without forming the droplets, such formations tending to appear start­ing from a 40 wt. % concentration (Figs. 3.32-3.34).

FIGURE 3.32 Optical microscopy images of films obtained from solutions of25/75 wt./wt. CAP/HPC blends in DMAc at 40 wt. % and 60 wt. % concentrations.70

FIGURE 3.33 Optical microscopy images of films obtained from solutions of 50/50 wt./wt. CAP/HPC blends in DMAc at 40 wt. % and 60 wt. % concentrations.70

Atomic force microscopy studies, according to Figs. 3.33-37, 70 also evidence the occurrence of liquid crystal phases including these formations. Under particu­lar conditions, HPC liquid crystalline films exhibit a characteristic structure, called “band texture,”81,91 consisting of alternating bright and dark areas.

FIGURE 3.35 2D and 3D — AFM images at 40×40 /nm2 scan area, and histograms — in which surface profiles were introduced as small plots, — of CAP films obtained from solutions in DMAc at different concentrations: 20 wt. % — (a, a,’ a”); 40 wt. % — (b, b,’ b”); 60 wt. % — (c, c,’ c”).70

FIGURE 3.36 2D and 3D — AFM images at 40×40 /nm2 scan area, and histograms — in which surface profiles were introduced as small plots, — of HPC films obtained from solutions in DMAc at different concentrations: 20 wt. % — (a, a,’ a”); 40 wt. % — (b, b,’ b”); 60 wt. % — (c, c,’ c”).70

As the percent of the liquid crystalline counterpart decreases, the surface pattern is maintained, while its dimensions become higher and the “small” bands disappear. In the case of CAP/HPC blends in DMAc, hydrogen bonds appear only between HPC and DMAc, therefore band texture is larger and increases with decreasing the amount of HPC. A natural tendency is noticed for the semirigid segments of HPC, namely to self-align into ordered domains, thus lending high performance properties to these materials.118 Also is observed that the LC phases are strongly dependent on the number of methylene units from the side chains of polymer blends.

On the other hand, although in optical microscopy images droplets cannot be seen at c = 20 wt. % concentration, in AFM images they are visible for all films, with the exception of the pure sample. They start to appear from c = 20 wt. %, 50/50 CAP/ HPC wt./wt. blends, their dimensions being around 3.69mm. These values agree with those from literature,119 obtained for hydroxypropylcellulose/polydimethylsi — loxane blends, where the size of droplets varies between 7-15mm. With increasing concentration, domains without droplets appear, as well as areas with droplets in training. They become more visible for 25/75 CAP/HPC wt./wt. blends, increase of the HPC content and, implicitly, of the methylene units leading to a more compact network, caused by HPC/DMAc bonding. Thus, the size of droplets decreases with increasing concentration (Table 3.1). In addition, as shown by Table 3.1 and by the AFM images of films made from pure samples, the surfaces present low roughness, pores appearing only at 75/25 CAP/HPC wt./wt., their dimension increasing with increasing concentration. Both components are present on the surface after solvent evaporation, being stabilized by hydrogen bonds interactions, which leads to the formation of droplets/pores of different sizes and intensities. The periodicity of the

average peak-to-valley height for these images is evident from the profile line plot­ted in Figs. 3.35-3.39.

In the same context, some data on the rheological properties of epiclon — based polyimide/HPC blend solutions have been reported.26 2D topography AFM images (40×40 /um2) of the corresponding films, prepared by mixing a
60 wt. % HPC solution with a 49 wt. % polyimide solution, shows that the hydrogen bonds between HPC and DMAc influence band texture, which increases with decreasing the amount of HPC (Fig. 3.40 (a) and (b)).

FIGURE 3.40 2D-AFM images for 30/70 wt./wt. polyimide/HPC (a) and 70/30 wt./wt. polyimide/HPC (b).26

As a result of the symmetry properties of the HPC liquid crystal solution, large domains of well-oriented polymer chains are formed during shear flow, while the defects are squeezed into small regions. Under specific shear flow conditions, the cholesteric liquid crystalline cellulose derivatives exhibit unwinding of the cho­lesteric helix and a cholesteric-to-nematic transition.120 When shear is stopped, the system will first relax at a characteristic time to a transient state. In this state, the distortion energy is minimized and the orientational order is maintained, resulting a banded structure. The shear-induced anisotropy is affected by the inevitable relax­ation of the chains, when the external field is removed. By introducing polyimide into the HPC solution, relaxation will take place collectively, due to the fact that the highly concentrated and aligned polymers cannot individually relax and, therefore, the inner stress induces a periodical contraction in the whole liquid crystalline poly­mer and different packing modes are observed. Structural relaxation after cessation of shear depends on the shear history of the mixture and on the dominant mechanism of stress relaxation. The band morphology of the blend is influenced by precursor and polyimide solution composition, solvent evaporation rate, film thickness, rate and duration of shear.121 The effect of the chemical structure and composition on the viscoelastic properties is reflected on the orientation or mobility of segments in the shear field. The inherent long-range ordering tendencies of LC itself — and specifically its pattern-forming properties — open new perspectives to produce or­dered polymer microstructures. The morphology of the ordered domains provides a means of “imaging,”, respectively potentially novel aspects of the pattern-forming LC states. Pure and applied researches related to the shear-induced morphology and structural relaxation after cessation of shear in polymer/LCP blends will become more and more important for developing high performance alignment layers used in display devices.

Consequently, the occurrence of droplets coincides with the thinning behavior evidenced by rheological data, both being caused by the more numerous hydrogen bonding interactions at higher HPC content. In this context, the results are in agree­ment with the literature ones, which show that cellulose derivatives form lyotropic mesophases.

All these results reflect the specific molecular rearrangements produced in the system through modification of the mixing ratio of polymers in certain solvents. As some biological polymers are insoluble in organic solvents or water, ionic liquids have attracted the attention of industry, especially because of the current need for creating a green chemistry environment. Thus, literature recommends 1-butyl-3- methylimidazolium acetate as a solvent for the study of sol/gel and liquid crystal transition of hydroxypropylcellulose (HPC).25 According to the experimental data obtained from the parameters: relaxation time, hysteresis ratio, loss modulus, and also observing the LC textures via a polarization optical microscope, the liquid crys­tal transition concentration of HPC is slightly higher than sol/gel transition concen­tration, and increases with temperature which is also the case of common solvents.

Furthermore, the structural and molecular heterogeneity of cellulose derivatives, their high molecular masses, low diffusion coefficient values, high viscosities of even moderately concentrated solutions, and specific intermolecular and molecule — solvent interactions, as well as their behavior in different conditions raise numer­ous experimental problems. The investigations developed in this chapter describe the character of the structural transformations observed, starting from rheological and morphological data. Knowledge of phase separation kinetics mechanism which modifies the morphology of cellulosic systems allows designing of materials with desired properties for specific applications.