Ceramics are known for their good biocompatibility, corrosion resistance, and high compression resistance. Drawbacks of ceramics include, brittleness, low fracture strength, difficult to fabricate, low mechanical reliability, lack of resilience, and high density. Polymer composite materials provide alternative choice to overcome many shortcomings of homogenous materials mentioned above. A lot of polymers are used in various biomedical applications; such as polyethylene (PE), polyurethane (PU), polytetrafluoroethylene (PTFE), polyacetal (PA), polymethylmethacrylate (PMMA), polyethylene terepthalate (PET), silicone rubber (SR), polysulfone (PS), poly(lactic acid) (PLA), and poly(glycolic acid) (PGA). Each type of material has its own posi­tive aspects that are particularly suitable for specific application. One of these appli­cations is an orthopedic implant. One of the major problems in orthopedic surgery is the mismatch of stiffness between the bone and metallic or ceramic implants. In the load sharing between the bone and implant, the amount of stress carried by each of them is directly related to their stiffness. In this respect, the use of low-modulus ma­terials such as polymers appears interesting; however, low strength associated with low modulus usually impairs their potential use. Since the fiber reinforced polymers, that is, polymer composite materials exhibit simultaneously low elastic modulus and high strength, they are proposed for several orthopedic applications 39. The mechani­cal properties of various polymers are shown in Table 2.5.39

When the diameters of polymer fiber materials are decreased from micrometers to submicrons or nanometers, there appear several amazing characteristics such as very large surface area to volume ratio, flexibility in surface functionalities and superior mechanical properties (stiffness and tensile strength) compared with any other known form of the material. A number of processing techniques such as draw­ing 40, template synthesis 41, phase separation 42, self-assembly 43, electrospinning 44, etc. have been used to prepare polymer nanofibers in recent years.

Cellulose, (C6H10O5)n, is the major component in the rigid cell walls in plants, a linear polysaccharide polymer with many glucose monosaccharide units. As an ac­etate ester of cellulose, cellulose acetate (CA) is a biodegradable polymer. CA scaf­folds have been used for growing “structurally mature” and “functionally compe­tent” cardiac cell networks 45. For ceramic-polymer hybrid systems, poor dispersion of ceramic powders in the polymer matrix and agglomeration of ceramic component has been a grave issue to the manufacturing of artificial scaffolds.

The use of nanotechnology has helped overcome these limitations. The synthe­sis of hybrid ceramic polymer nanofibers by means of electrospinning is a major breakthrough in biotechnology-related nanomanufacturing. Nanosized HAp pre­pared from eggshells and CA were combined to form novel hybrid 3D scaffolds mimicking the extracellular matrix (ECM) architecture.

The absorption of sound mainly results from the dissipation of acoustic energy due to viscosity and heat conductivity of the medium. A number of dissipation mecha­nisms have been proposed by some authors.10,13,28,29 Attenborough and Ver17 cite the friction between the solid body (fiber) of the absorber and the fluid moving in it (air) as the main cause of sound attenuation. Similarly, Cox and D’antonio14 refer to the friction due to viscosity of air as the primary cause and thermal conduction as the secondary cause of the sound energy loss. Fahy13 explains the sound absorption phe­nomena on a molecular level as a combination of viscosity, thermal diffusion and re­laxation processes which take place in the boundary layers next to “pore” surfaces. The next section gives information about sound absorption measuring techniques.

The smoke density for a sample having dimension of 120 mm x 100 mm * 4 mm in size was measured by using a smoke density chamber as per ASTM D 2843-0414. The smoke generated (flaming mode) in the process of burning of sample was mea­sured by the change of the light intensity. This test was useful for measuring and observing the relative amounts of smoke obscuration produced by burning or de­composition of the material. Smoke density rating which represents the total amount of smoke present in the chamber for 4 min was measured by Eq. (7).

A

Smoke density rating = j x 100 (7)

where, A and T are the area under the light absorption versus time curve and total area of the curve, respectively.

Table 6.7 shows that the smoke density rating of the natural rubber latex based jute composite is less than that of the traditionally used noise control material, fiber glass.

Biaxial tension tests were carried out to study the tensile behavior of the flax fabric. The results showing the biaxial behavior of the fabric in the weft direction are pre­sented in Fig. 7.25 for different values of the parameter k. The parameter kt= ef є is defined as the ratio between the strain in the weft direction over the one in the

warp

warp direction. The results show that an increasing value of kt leads to higher strain in the weft tows. This means that the crimp effect decreases in this case. The crimp effect is the lowest in the case of the unidirectional test (k =e /e =0).

c warp weft

Figure 7.25 indicates that the maximum failure strain is obtained in the case of the uniaxial test. This phenomenon is normal and is due to the crimp effect. The maximum strain to failure is observed for the case of the uniaxial test (kC=0). For all the considered cases, the failure strain is lower than 4.5%. These values are much lower than the ones measured on the tetrahedron face (Figs. 20, and 21). It is there­fore probable that failure occurs in the tightest tows of the face during the process.

In a general way, the biaxial tensile behavior of the fabric is very similar to the ones observed on woven fabrics made from glass or carbon fibers.6773 Even if the tows are not constituted with continuous fibers, the entanglement between the fibers provides a sufficient continuity to the tow so that this one behaves like a homoge­neous entity.

Polystyrene is an aromatic polymer made from an aromatic monomer (styrene) which is commercially manufactured from petroleum.107 High Impact Polystyrene (HIPS) is a type of polystyrene that is versatile, economical and easy to fabricate. HIPS is mostly used for low strength structural applications where impact resis­tance, machinability and low cost are needed. Polybutadiene is added during the polymerization of HIPS. This polybutadiene is generally known as rubber, which provides HIPS with the required toughness and impact resistance.57

9.4.1 MECHANICAL PROPERTIES

9.4.7.1 EFFECT OF FIBER LOADING

A study on the effect of SPF loading on the tensile properties of SPF/HIPS compos­ite was reported by Sapuan and Bachtiar57. Fiber contents of 0, 10, 20, 30, 40 and 50% by weight were incorporated in HIPS matrices. The tensile strength of short SPF/HIPS composites slightly decreased compared to the neat HIPS (0% loading) as the fiber content in them increased from 10 to 30%. The average tensile strength of the neat HIPS was 29.92 Mpa, which was approximately 12.4, 22.5 and 35.5% higher than SPF/HIPS composites with 10, 20 and 30% fiber contents, respectively (see Fig. 9.15). The decrease of tensile strength of the composites with fiber load­ings from 10 to 30% by weight emanated from the weak fiber-matrix interface as a result of differing polarities of the hydrophilic SPF and hydrophobic HIPS matrix. On the other hand, considerable increment in the tensile strength of the composites was realized as the fiber loadings increased from 40 to 50%. This may be attributed to the better interaction and improved dispersion of SPF in the HIPS matrix, which enhanced the interfacial bonding between fibers and matrix. Generally, good tensile strength depends more on effective and uniform stress distribution.96 The high ten­sile strength of composites at fiber content of 40 to 50% indicates the fibers were effectively capable of transfer the load to one another. This illustrates that the fibers (40 to 50% weight fraction) effectively participated in the stress transfer causing the effect of crack inhibition more dominant over the effect of crack initiators.96

The tensile moduli of short SPF/HIPS composites increased from 1516 to 1706 MPa with increase fiber loading from 10 to 30% by weight (see Fig. 9.16). The maximum tensile modulus value obtained at fiber content of 30% in HIPS matrix was 25.6% higher than neat HIPS (0% fiber loading). However, the addition of short SPF by 40 to 50% inflicted slight decrease on the tensile modulus of the compos­ites. The decrease of tensile modulus beyond SPF loading of 30% was due to the decrease of bond quality between the fibers and HIPS matrix. At high fiber contents, the extent of fiber wetting reduces because it is difficult to achieve good consolida­tion of the composites during fabrication process.107

The Type I tensile bars injection molded for each composite were dried in an oven for 24 h at 100 ± 2°C and weighed. Tests were conducted in an incubator at 25 ± 2°C under a photosynthetic photon flux density of 180 pmolm2s-1 using a photoperiod of 12 h light/12 h dark. Tensile bars were placed in distilled water at room tempera­ture for 872 h. At predetermined time intervals the specimens were removed from the distilled water, the surface water was blotted off with paper towels, and their wet masses were determined. Water absorption, measured as weight gain percentage, was computed using the following formula,

Weight gain (%) = (mt — mo)/mo x100 (3)

where mo denotes the oven-dried weight and mt denotes the weight after soak time t. Thickness swelling (TS) was calculated using the following formula,

where TS is the thickness swelling (%) at time t, To is the initial thickness of the specimen, and Tf is the thickness at end of soak time t (872 h).

15.1.5.1.1 AB INITIO

The term “Ab Initio” is Latin for “from the beginning.” This name is given to com­putations, which are derived directly from theoretical principles, with no inclusion of experimental data. Most of the time this is referring to an approximate quan­tum mechanical calculation. The approximations made are usually mathematical approximations, such as using a simpler functional form for a function or getting an approximate solution to a differential equation.

The most common type of Ab initio calculation is called a Hartree Fock calcu­lation (abbreviated HF), in which the primary approximation is called the central field approximation. This means that the Coulombic electron-electron repulsion is not specifically taken into account. However, it’s net effect is included in the cal­culation. This is a variational calculation, meaning that the approximate energies calculated are all equal to or greater than the exact energy. The energies calculated are usually in units called Hartrees (1 H = 27.2114 eV). Because of the central field approximation, the energies from HF calculations are always greater than the exact energy and tend to a limiting value called the Hartree Fock limit.

PLA is a renewably derived thermoplastic polyester and is completely biodegrad­able and bioabsorbable.96 PLA, one of the oldest and most promising biodegradable polymers (aliphatic polyester) which is obtained from agricultural products such as corn, sugarcane, etc., is at the forefront of emerging biodegradable polymer used in industries through improved manufacturing practices that lower its production cost.97 Poly(lactic acid) and polylactide are the same chemical products and both are abbreviated as PLA. The only difference between them is how they are produced. Lactic acid is a chiral molecule existing as two stereoisomers, l — and d-lactic acid which can be produced in different ways, that is, biologically or chemically syn — thesized.98 In the first case, lactic acid is obtained by fermentation of carbohydrates from lactic bacteria, belonging mainly to the genus Lactobacillus, or fungi.99 This fermentative process requires a bacterial strain and is a sources of carbon (carbo­hydrates), nitrogen (yeast extract, peptides, etc.) and mineral elements to allow the growth of bacteria and the production of lactic acid. The lactic acid as-formed ex­ists almost exclusively as l-lactic acid and leads to poly(l-lactic acid) (PLLA) with low molecular weight by polycondensation reaction. However, Moon et al.100 have proposed an alternative solution to obtain higher molecular weight PLLA by the polycondensation route. In contrast, the chemical process could lead to various ratio of l — and d-lactic acid. Indeed, the chemical reactions leading to the formation of the cyclic dimer, the lactide, as an intermediate step to the production of PLA, could form macromolecular chains with l — and d-lactic acid monomers. This mechanism of ring-opening polymerization ROP from the lactide explains the formation of two enantiomers. This ROP route has the advantage of reaching high molecular weight polymers101 and allows control of the final properties of PLA by adjusting the pro­portions and the sequencing of l- and d-lactic acid units. At present, due to its avail­ability on the market, PLA has one of the highest potentials among biopolyesters, particularly for packaging and medical applications.102

Our understanding of the extrusion process of spider silks from the MA gland has advanced over the past decade, but much still remains a mystery. A large empha­sis has been focused on the ampulla and the spinning duct. The ampulla acts as a protein repository, whereas the spinning duct mediates much of the chemical and physical processes that result in fiber assembly. Based upon histological studies, the spinning duct has been divided into three limbs. The duct is comprised of a thin cuticle that functions as a dialysis membrane responsible for selective movement of water, surfactants and lubricants and the ions sodium, potassium, chloride and phosphate. A careful analysis of the elemental constituents in the MA gland has revealed that sodium and chloride ion concentrations are higher in the ampulla but decrease as the dope moves down the spinning duct.13 It has been hypothesized that the elevated levels of the sodium and chloride ions in the ampulla facilitate fibroin storage, resulting in a highly concentrated liquid spinning dope. Quite impressively, the concentration of the protein mixture, often referred to as the spinning dope, has been reported to represent 30-50% (w/v) or 300-500 g/L.20 During storage in the ampulla, the secondary structure of the fibroin mixture represents a random coil and alpha helical state.21 This high storage concentration results in the formation of a lyotropic liquid crystalline phase in the duct, which represents a unique phase that flows as a liquid but maintains the molecular orientational order that is characteristic of a crystal.22 Levels of phosphate and potassium ions have also been reported to in­crease moving down the spinning duct. Based upon the chemical properties of these elements, it has been suggested that these ions promote precipitation of the fibroins. Intriguingly, elemental sulfur levels have been shown to increase down the spinning duct; however, no theories have been proposed to explain the role of elevated levels of sulfur down the spinning duct during the assembly process.16 One possibility is that changes in the redox state, in some manner, help trigger fiber formation. Con­sistent with this assertion is the decrease in mass of protein complexes stored in the MA gland after treatment with reducing agents.16 It has also been suggested that dehydration occurs during the late stages of the extrusion process, a procedure that uses specialized epithelial cells that recover water prior to extrusion near the third limb of the spinning duct. For air-spinning spiders, which represent the predominant focus in this chapter, the process of evaporation also helps remove water after extru­sion. In addition to these chemical changes, physical forces influence the conver­sion of the liquid-crystalline phase to a solid transition. Microscopic analysis of the MA gland reveals the spinning duct narrows as the tubing approaches the spigot or exit point on the spinneret. This geometry increases the flow rate and shear forces as the silk is extruded through the duct. Collectively, the chemical and physical forces facilitate the alignment of the fibroin molecules with the direction of the flow, drive beta-sheet structure formation in the poly A blocks, and dehydrates the fiber to produce a solid material. Further enhancement of the mechanical properties of the extruded threads is accomplished by pulling or tugging on the threads, a process the spider executes with its legs, generating what is referred to as a postspin draw.

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

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.

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.

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

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