A successful alternative for the development of new polymeric materials is blending of the already existing polymers, for attaining a balance among the desired prop­erties exhibited by the individual components. Development of characterization techniques has improved understanding of the mechanisms involved in polymers mixing, of their fundamental interactions, and of the manner in which these interac­tions affect their final properties.66 The relation between molecular interactions and the physical and engineering properties continues to be an important challenge from both scientific and industrial perspectives, due to the increasing economical impact of polymer blends and alloys in many domains, directly affecting our everyday life. The rheology, as well as the interface properties or morphology of polymer blends involving cellulose materials have been the subject of numerous researches.26’65’6770 As a consequence, a good knowledge on the rheological properties of these solu­tions is important for their handling and formulation, and also for better understand­ing and controlling the processes involved in cellulose acetate phthalate/hydroxy — propyl cellulose blends for different applications.

In this context, literature shows that the microstructure and CAP/HPC-solvent interactions are observed by rheological studies involving shear experiments, which can evidence the lyotropic properties in different solvents at higher concentrations.26 Figures 3.10 and 3.11 plot the modification of dynamic viscosity, П, versus shear rate, і, and concentration, c, respectively, for different compositions of the polymer mixtures, including pure polymers, in 2-methoxyethanol. The concentration domain is lower than that corresponding to the occurrence of crystal liquid properties — where, at high shear rates, viscosity at high concentration is lower that viscosities obtained for smaller concentrations.26

FIGURE 3.10 Log-log plots between dynamic viscosity and shear rate for CAP/HPC blends at different concentrations in 2-methoxyethanol.69

FIGURE 3.11 Log-log plots between dynamic viscosity and different concentrations in 2-methoxyethanol for CAP/HPC blends: (a) 0/100 wt./wt., (b) 25/75 wt./wt., (c) 50/50 wt./ wt. (d) 75/25 wt./wt., (e) 100/0 wt./wt.69

In addition, a Newtonian behavior for CAP was observed for all concentrations, as well as a thinning behavior for HPC and CAP/HPC solutions at higher HPC compositions. Figure 3.11 shows that dynamic viscosity increases with increasing concentration, the maximum slope occurring at 50/50 wt./wt. CAP/HPC, in the ab­sence of any liquid crystal phenomenon.69 The dependence of viscosity on concen­tration at a shear rate of 1.6 s-1 is described by a power law cx, with exponents of 2.27, 3.14, 3.69, 2.64 and 2.53 for 100/0, 75/25, 50/50, 25/75 and 0/100 wt./wt. CAP/HPC, respectively. These dependencies coincide with the theoretical predic­tion for semidiluted polymer solutions close to the entanglement domain, especially for 50/50 wt./wt. CAP/HPC. As already mentioned, dynamic viscosity is propor­tional with c1.25 and c3.7 in the semidilute unentangled regime for nonassociating polymers, as well as in the semidilute entangled regime, respectively.60

The interactions between chain segments, which reflect the existence of polymer unentanglements or entanglements and hydrogen bonding, are described by the acti-

E

vation energy, a (Eq. (9)):71-72

E

ln n = ln n +—— (9)

0 RT

where n0 “ e-AS/R represents a preexponential constant,73 as is the flow activation entropy, “R” is the universal gas constant and “T” is absolute temperature.

Generally, this property is directly related to the disengagement ( E dis, with posi­tive contribution) of the associated chain formation, (Eass, with negative contribu­tion), according to Eq. (10):

Ea = Edis + Eass (10)

Thus, for presented blends,69 the activation energy Ea > 0, Edls > Eass, de­creases with increasing the CAP content (see Fig. 3.12). The association phenomena generated by hydrogen bonding appear preponderantly in the HPC/2-Me system (as due to the presence of a higher number of hydroxyl groups and 2-Me, which is a poorer solvent for HPC than for CAP, as shown by viscometric data). The anhy — droglucose units of HPC reduce chain flexibility in 2-methoxyethanol, so that the entropy of the systems is lower, resulting in the highest mean activation energy values Ea = 26.81 + 29.28 kJ/mol, comparatively with the CAP pure components, for which Ea = 6.48 + 20.42 kJ/mol. A lower value of Ea implies a lower energy barrier for the movement of an element in the fluid. In the here discussed case, this barrier is related to the interaction between chain segments, and is determined by polymer entanglements or by specific interactions, such as hydrogen bonding. A pseudoplastic behavior and consequently, a decrease in viscosity with increasing shear rate are observed both in HPC solutions and in their corresponding blends with CAP. The dependence of shear stress, °, on shear rate partially obeys the power law relationship described by Eq. (11):

1.1 3.2

1000/T, К

In n vs. 1000/T for ▲- 0/100 CAP/HPC, V — 25/75 CAP/HPC, ▼ — 50/50 CAP/HPC, A — 75/25 CAP/HPC and ■ — 100/0 CAP/HPC blends (wt./wt.) in 2-methoxyethanol at different concentrations69

S= K — f (11)

where n and K are the flow behavior and consistency index, respectively.

It is estimated that n = 1 for a Newtonian behavior of fluids, n < 1 for a thinning behavior, and n > 1 for a thickening behavior.7475 Table 3.4 confirms the thinning behavior introduced by HPC in polymer blends; the flow behavior indices are sub­unitary for HPC and at a higher HPC content, and around the unit for CAP and at a higher CAP content in polymer blends, according to the shear thinning rheological behavior evidenced in Fig. 3.10. The solution consistency index, K, induced by pure HPC takes higher values than those induced by pure CAP, and decreases with decreasing the HPC content and solution concentrations in polymer blends. These results, influenced by both composition and solution concentrations of the polymer blends, are a consequence of the modification of polymer interactions in the system,

such as hydrogen bonding interactions. Likewise, HPC helps to the specific molecu­lar rearrangement in solution, being influenced by temperature and concentration.

TABLE 3.4 Flow Behavior Index, n, and the Consistency Index, K, (® ‘ s ), for CAP/ HPC Casting Solutions in 2-Me at Different Concentrations, c (g/dL), and Mixing Ratio (wt./ wt.)69 CAP/HPC blends c n K c N K c n K 0/100 3 0.66 3.55 4 0.49 20.89 5 0.47 25.12 25/75 3 0.67 2.29 4 0.51 10.23 5 0.55 13.80 50/50 3 1.04 0.15 4 0.66 0.43 5 0.66 5.01 75/25 3 0.95 0.05 4 0.92 0.20 5 1.01 0.20 100/0 3 0.98 0.01 4 0.99 0.02 5 1.03 0.02

Viscoelastic measurements significantly contribute to the knowledge and differ­entiation of polymer systems, completing the rheological studies developed in shear regime. Prior to the measurements of these CAP/HPC blends, suitable strain am­plitude tests must be performed, to establish the domain of shear stress over which the storage and loss moduli are constant. Figure 3.13 exemplifies the variation of storage and loss modulus versus shear stress at a constant frequency of 1 Hz, for a 4 g/dL concentrated CAP/HPC blend.69 Initially, G’ and G" are constant, with G’ < G”, after which both moduli decrease at higher deformation, showing viscoelastic fluid properties.

At the same time, the constant zone is seen as dependent on the intermolecular association from the polymer blends. The aspect of the curves for both moduli is related to polymer topological entanglements and association phenomena.76 There­fore, knowledge on the morphological and rheological properties as a function of shear stress provides insight into molecular interactions and surface organization. In addition, Fig. 3.13 permits the selection of shear stress on 2 Pa (from the linear vis­coelastic domain versus shear stress) in viscoelastic measurements versus frequen­cy. The tendency towards aggregation phenomena in casting solutions of CAP, HPC and their blends is reflected in Fig. 3.14, where moduli G’ and G» are presented as a function of frequency.69 Over the low frequencies domain of 0.3-0.9 Hz, the storage and loss moduli are proportional to frequency — where the exponents for G’ and G» are between 2.1-0.9 and 1.1-0.8, respectively, maintaining the behavior characteris­tic to a viscoelastic fluid, where G’ < G».77 These exponents decrease at lower values of polymer concentrations and at lower 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 2-Me, which increase with increasing the CAP content in polymer blends.

All these aspects reflect the specific molecular rearrangements in the system, through modification of the mixing ratio of polymers, under the influence of hydro­gen bonding interactions between the polymer components and solvent.

The rheological, as well as the morphological properties, are influenced by the hydrophobic characteristics of both CAP and HPC components (Fig. 3.15),78,79 in which the disperse components take values of 38.8 mN/m and 34.3 mN/m, re­spectively, while the electron-acceptor interactions (of Y+ = 0 59 mN/m and Y+ = 0.18 mN/m, respectively) are lower than the electron-donor ones (of

Y-v = 18.86 mN/m and = 1623 mN/m, respectively) (Table 3.5). For 50/50 wt./wt. CAP/HPC films, the apolar component achieves maximum value, while the polar components, with electron donor and electron acceptor parameters, and total surface tensions have minimum values.

FIGURE 3.15 Surface free energy (AGW) vs. water contact angle of CAP/HPC blends (wt./ wt.) in 2-Me for 4 g/dL concentration. AGw >-113 mJ/m2 correspond to more hydrophobic materials.78

TABLE 3.5 Surface Tension Parameters (mN/m) of CAP/HPC (wt./wt.) Blends in 2-Me for 4 g/dL Concentration, According to the Geometric and Acid/Base Method78 CAP/HPC Y sv Yr sv y+ sv Y- Ysv 100/0 38.80 6.69 0.59 18.86 45.49 75/25 28.20 4.22 0.23 19.97 32.42 50/50 35.32 3.03 0.12 19.16 38.35 25/75 31.50 3.28 0.14 19.36 34.78 0/100 34.34 2.87 0.18 16.23 37.21

According to Table 3.6, which summarizes the data provided by AFM images (Figs. 3.16-3.20), variation in polymer blend compositions and concentration of the casting solutions in 2-Me favors a different surface morphology.69

FIGURE 3.16 2D-AFM images at 10*10 pm2 scan area, including the small images corresponding to 3D-AFM images, and cross-section profiles corresponding to the insert line in small 2D images, of CAP films obtained from solutions in 2-Me at different concentrations, 3 g/dL, 4 g/dL, and 5 g/dL.69

FIGURE 3.17 2D-AFM images at 10*10 pm2 scan area, including the small images corresponding to 3D-AFM images, and cross-section profiles corresponding to the insert line in small 2D images, of HPC films obtained from solutions in 2-Me at different concentrations, 3 g/dL, 4 g/dL, and 5 g/dL.69

FIGURE 3.18 2D-AFM images at 10*10 pm2 scan area, including the small images corresponding to 3D-AFM images, and cross-section profiles corresponding to the insert line in small 2D images, of 75/25 wt./wt. CAP/HPC films obtained from solutions at different concentrations, 3 g/dL, 4 g/dL, and 5 g/dL.69

FIGURE 3.19 2D-AFM images at 10×10 pm1 scan area, including the small images corresponding to 3D-AFM images, and cross-section profiles corresponding to the insert line in small 2D images, of 50/50 wt./wt. CAP/HPC films obtained from solutions in 2-Me at different concentrations, 3 g/dL, 4 g/dL, and 5 g/dL.69

FIGURE 3.20 2D-AFM images at 10×10 pm2 scan area, including the small images corresponding to 3D-AFM images, and cross-section profiles corresponding to the insert line in small 2D images, of 25/75 wt./wt. CAP/HPC films obtained from solutions in 2-Me at different concentrations, 3 g/dL, 4 g/dL, and 5 g/dL.69

TABLE 3.6 Pore Characteristics (Area, Volume, Depth and Diameter) and Surface Roughness Parameters (Average Roughness, Sa, and Root Mean Square Roughness, Sq) for CAP/HPC Films Obtained from Casting Solutions in 2-Me at Different Concentrations, c, Corresponding to AFM Images69 CAP/HPC (wt./wt.) c, (g/dL) Pore characteristics Surface roughness Area (pm2) Volume (pm2 .m) Depth (nm) Diameter (pm) Sa (nm) Sq (nm) 100/0 3 0.68 27.49 77.41 0.85 11.58 15.76 75/25 3 0.44 164.72 474.31 0.74 99.25 134.15 50/50 3 1.30 514.37 — 1.26 70.57 88.64 25/75 3 1.45 663.35 — 1.35 66.55 84.43 0/100 3 — — — — 12.72 16.22 100/0 4 1.13 41.24 89.23 1.14 7.43 11.67 75/25 4 1.09 162.39 411.07 1.17 53.74 77.14

TABLE 3.6 (Continued) CAP/HPC (wt./wt.) c, (g/dL) Pore characteristics Surface roughness Area (mm2) Volume (pm2 .m) Depth (nm) Diameter (mm) Sa (nm) Sq (nm) 50/50 4 4.55 4.23 — 2.39 153.25 188.58 25/75 4 0.38 95.79 — 0.69 41.62 51.65 0/100 4 — — — — 11.74 16.13 100/0 5 1.77 62.16 57.87 1.33 4.27 5.59 75/25 5 1.74 129.13 206.93 1.47 50.50 66.21 50/50 5 7.66 5.26 — 3.09 151.71 184.49 25/75 5 5.52 2351.66 — 2.62 87.21 110.01 0/100 5 — — — — 17.57 21.83

Different morphological aspects of the CAP film are the effect of chain con­formation modification generated by the hydrogen bonding between the acetyl and hydroxyl groups, and also by the specific interactions, including hydrogen bonding, with 2-Me. Increase of casting solutions concentration determines modification of pores number and of their characteristics; thus, surface roughness decreases, both area and volume of pores increase, while pores depth decreases with increasing concentration.

The structure, formed after the slow evaporation of 2-Me at room temperature, evidences the importance of evaporation kinetics and concentration of the cast­ing solution in making the different morphological aspects of the HPC films. The changes of molecular distribution in the corresponding films depend on solutions concentration. At a concentration of 3 g/dL, a large number of HPC molecules ag­gregate, showing their potential to form self-assembled structures generated by side-chain hydrogen bonding interactions. The dimensions of aggregates are in the 1.5 — 2.7 mm range, increasing with the concentrations of the casting solution over the 3-5 g/dL domain. Under some specific conditions, at higher concentrations, HPC films morphology gets modified, exhibiting a crystal liquid behavior and/or texture characteristics.8081

Polymer-polymer miscibility is generally considered as a result of the specific interactions between polymer segments, which include donor-acceptor, dipole-di­pole, hydrogen-bonding, ion-ion, acid-base, and ion-dipole interactions.82 In CAP/ HPC blends, with hydrophobic and electron-donor characteristics, miscibility is due especially to hydrogen bonding. 2-Me is known as a solvent with electron-donor properties, so that the morphological aspects of polymer blends films are a conse­quence of hydrogen bondings presence in the casting solution system, along with other kinds of apolar interactions; therefore, polymers dissolution is assured by the interfacial free energy of the polymers, on taking into account the polar as well as the apolar surface tension parameters.83 A small content of HPC in the CAP/ HPC blends generates pores with higher volumes and depths, and also an increased surface roughness, comparatively with the values corresponding to pure CAP; at a higher content of HPC, domains with large areas and lower average roughness ap­pear. Both components are stabilized by hydrogen bonds interactions, which lead to areas whose diameter increases with increasing the concentration and composition of HPC. The occurrence of different areas, expressed as supernodular aggregates,84 coincides with the thinning behavior evidenced by rheological data — caused by nu­merous hydrogen bonding interactions at higher HPC content. The next subchapter evidences that in some solvents, lyotropic mesophases usually have a characteristic critical concentration, where the molecules first begin to orient themselves into the anisotropic phase (which coexists with the isotropic one); the anisotropic or ordered phase increases with solution concentration in some domain of the biphasic region. At higher concentration, the solution becomes anisotropic.85