The purpose of this section is to review various aspects of main chain TLCP rheology and mechanical properties that pertain to rotational molding. In fact, their unique performance and rheological behavior inspired their selection for rotational molding because they may extend the processing technique to higher performance applications where conventional rotational molding polymers are insufficient.
The section begins with the review of TLCP mechanical properties in section 2.2.1. As some mechanical properties are dependent on the processing technique, it should be stated that reported values might not be representative of what may be obtained from rotational molding. This is especially true in properties that are strongly dependent on the degree of molecular orientation. Section 2.2.2 focuses on various rheological aspects of TLCPs.
2.2.1 Mechanical Properties
Strength and Modulus
Mechanical properties, especially tensile strength and modulus, depend upon the degree of orientation achieved. This is limited by the fabrication method and geometry of the manufactured item. A compression molded unoriented LCP has mechanical properties similar to that of a conventional isotropic polymer [104]. Injection molding imposes higher deformation rates that increase the degree of orientation, especially in the skin, which makes the major contribution to stiffness [31]. Injection molded main chain TLCPs show superior tensile moduli to those molded from conventional glass fiber reinforced isotropic polymers. As the degree of orientation increases in the testing direction, the mechanical properties approach those of main chain TLCP fibers [21, 130].
Simplistically, the layered structure of injection molded TLCP can be viewed as a microcomposite composed of layers with varying directional orientation [104]. Each layer contributes in an integral manner to the mechanical properties of the entire molding.
Modulus is dependent upon layer thickness and the direction and degree of orientation of polymer chains in those layers. Each of these factors depends on the processing conditions and the fluid’s rheological response to the imposed flow conditions. Stiffness can also be varied through manipulation of the chemical composition. The flexural modulus of injection molded tensile bars with varying HBA content was studied. As HBA content increases and becomes great enough for the polymer to transition to a liquid crystalline phase, the modulus monotonically increases [14]. Orientation was measured as a function of depth through the moldings. Despite the obvious skin-core effects, it was
found that an increase in molecular orientation was mainly responsible for the increase in modulus [13]. From this, it was concluded that simply creating a liquid crystal mesophase is not sufficient to acquire a system with high, self-reinforcing modulus; the mesophase must become sufficiently oriented. However, if this is done effectively, TLCPs can exhibit exceptionally high strength and modulus. Moldings may retain these properties at elevated temperatures (>200°C), which make them suitable for high temperature applications.
TLCP articles possess some degree of anisotropy, a difference in properties when tested parallel and perpendicular to the flow direction, which is reported as the anisotropy ratio. The anisotropy ratio increases with the degree of orientation and is therefore greatest in fibers. The anisotropy ratio for injection molded TLCP has been shown to fall somewhere between 4:1 to 10:1, increasing with decreasing thickness as the proportion of skin and core increases [118, 62, 29]. Introducing fillers tends to increase the anisotropy ratio of conventional isotropic polymers, but disrupts molecular alignment and reduces the ratio for TLCPs [118]. Although not typically desirable, this is one way of improving the cross flow properties.
Dimensional Stability
TLCPs are used for high precision components that require close tolerances on dimensional quantities. There is very little difference in molecular configuration, and therefore a negligible density change between the melt and solid states for TLCPs. They also typically possess little elastic recovery. This means that when molded, they exhibit
very low mold shrinkage and warpage when compared with isotropic polymers. Once molded, they retain their molded dimensions well. One reason for this is that TLCPs absorb very small quantities of water (typically less than 0.2% when immersed in water), which makes the effects of swelling from moisture absorption negligible [104]. The coefficient of linear thermal expansion is much lower for TLCPs than for conventional polymers, even when conventional polymers have been reinforced with glass fibers [31].
Incidentally, they are quite similar to values for metals (TLCPs < 1, 1< Metals < 3 cm/cm/°Cx10-5). This similarity results in good integrity and minimal strain during thermal cycling of components including both materials [104].
Barrier Properties and Chemical Resistance
TLCPs act as excellent barriers to gases, with permeability coefficients for He, H2, Ar, N2, CO2 comparable to or smaller than those for polyacrylonitrile, one of the least permeable polymers known [26]. See Table 2.6 for a comparison between Vectra A900 and PAN. They also have exceptionally low oxygen and water vapor permeabilities. The low permeability seems to be more of a result of low gas solubility rather than low gas diffusion, though it has been shown that diffusion can be greatly reduced by thermal treatment in systems that are crystallizable [22]. It was calculated that crystalline content would have to be near 90% or more to explain the solubility. However, crystallinity is often below 20% for an unannealed sample so it seems that liquid crystalline order is responsible [104].
Table 2.6. Comparison of Gas Transport Properties at 35°C of Vectra A900 and PAN [162]
LCP PAN LCP PAN LCP PAN
He 17,700 71,000 6,600 2,700 2.0 200
N2 3.0 2.9 1.4 0.042 1.6 52
Ar 10 18 1.5 0.042 5.4 330
CO2 70 280 0.96 0.023 51 9200
P x 1015 D x 1010 S x 103
Gas ( )
⋅
⋅
⋅ Hg cm cm
cm STP cm
2 sec
3 (cm2 sec) [cm3(STP) cm3⋅atm]
A precise explanation for the low solubility observed in TLCPs has not been determined. Two explanations have been proposed. The liquid crystalline phase is the only viable region for gas sorption, but solubility is orders of magnitudes less than in amorphous polymers because of efficient chain packing. Another explanation is that the polydomain structure of the mesophase is responsible [162]. It was suggested that domain boundaries are separated by a material having gas solubility and diffusion characteristics similar to flexible chain polymers. If this boundary material accounts for only a few percent of the total volume, it would explain the behavior. This two phase depiction of permeability was used with reasonable success to estimate the values for Vectra type materials [162].
TLCPs have excellent resistance to a wide range of organic solvents and exhibit good hydrolysis resistance. Retention of properties in both acidic and basic environments is also very good [21]. As with gas barrier properties, it is likely that liquid crystalline
order is somewhat responsible; it is possible to decrease chemical resistance in TLCPs by increasing backbone flexibility [21].
Interfacial Strength
A major problem identified during the injection molding of TLCPs is poor weld- line strength. Adjoining flow fronts have difficulty reestablishing equilibrium molecular structure across the interface. This can dramatically reduce tensile strength as demonstrated by the injection molded tensile bar study, where tensile strength is reduced to roughly 10% of the continuous sample [104]. It has also been shown that fillers may improve strength, but not significantly. The current approach to handle this issue is to manipulate weld line position to areas where they will have the least effect on properties.
A detailed study of the adhesion mechanism, or lack thereof, in TLCPs has not been completed. Flexible chain polymers that are void of ionic interactions mend by molecular diffusion to establish equilibrium across the interface. The study of molecular diffusion in TLCPs has neglected interfacial or domain effects. It has revealed that the rigid nature of the LC molecule requires the combination of both rotation and translation.
Diffusion is anisotropic, dependent on molecular dimensions and the degree of orientational order [161]. Incidentally, it has been shown that orientation can be strongly influenced at fluid boundaries, even at free surfaces [148]. This may imply that boundary induced orientation inhibits diffusion across an interface.
Miscellaneous
Main chain TLCPs possess several other notable properties. They are rather tough materials that typically do not fail in a brittle or ductile fashion upon impact.
Instead, their failure is more benign, similar to that of long fiber reinforced polymers or natural wood [104]. Moldings also show extremely low flammability, without the need for flame retardant additives. They tend to have low dielectric constants (less than 3.0) and high dielectric strength (greater than 7000 V/ 25àm).
2.2.2 Rheology of Thermotropic Liquid Crystalline Polymers
TLCPs exhibit strong interdependence between morphology, rheology, and processing [119]. This makes attempting to characterize the fluid’s rheology difficult. In fact, the flow behavior of these materials is so sensitive to temperature and deformation history that sample preparation and loading procedures change the initial morphology (initial conditions) and alter rheological results, especially the transient response. One way to reduce response fluctuation is to impose reproducible initial conditions by heating the melt into the isotropic state to erase nematic texture. This eliminates any previously induced preferential orientation due to deformation or temperature, thus generating a randomly oriented (no bulk orientation) polydomain structure upon quenching back into the LC phase [157]. This is common practice for main chain TLCPs with flexible spacers, and it is partially responsible for why they are referred to as model nematic TLCP systems (the other reason is their lack of residual crystallinity in the melt state and subsequent aversion of biphasic influences) [51, 157]. However, the procedure does not apply to more rigid main chain TLCPs because the nematic-isotropic transition is usually above degradation temperatures, making it inaccessible. Also, in those systems,
recrystallization or transesterification may occur at melt temperatures, leading to time dependent material properties [99, 113]. It is also possible to obtain reproducible data by following identical sample loading protocols and applying a flow field to introduce repeatable initial conditions prior to the rheological measurement. Unfortunately, the initial morphological state is still unknown, making interpretation of start-up transients difficult and inhibits meaningful comparisons between studies [6, 157].
To familiarize the reader with the current state of TLCP rheological characterization, a review of TLCP rheology is contained in this section. This review is not meant to encompass all TLCP rheology. Instead, emphasis has been placed upon the responses important to the rotational molding process. This includes behavior common, and particular, to main chain thermotropic systems with nematic mesophases.
Observations of the LCP response to shear and shearfree flows are summarized in the two sections that compose this chaper, 2.2.2.1 and 2.2.2.2.
2.2.2.1 TLCP Response to Shearfree Flow
It was shown in the review on particle sintering that the deformation kinematics for coalescence are equibiaxial extension. For this reason the rheological response of TLCPs in extensional flow is reviewed.
Despite inherent difficulties involved with extensional rheometry, several experimental studies exist on the extensional behavior of TLCPs. In a lyotropic system of HPC in acetic acid, it has been shown that the transient uniaxial elongational viscosity,
ae+, was independent of extension rate over the range 0.02 to 10 sec-1 [117]. It was also demonstrated that the Trouton ratio was approximately 9, three times greater than the linear visoelastic limit observed in flexible chain polymers. The generality of these results are somewhat questionable. Experiments were performed with a gravity spinning apparatus, where the applied deformation rate is not constant and the results for flexible chain polymers do not agree with homogeneous extensional flows.
Experiments have been performed on HPC as a thermotropic system [25]. A single rotary clamp device was used to measure uniaxial shearfree stress growth with extension rates from 0.005 to 0.05 sec-1 under isothermal conditions (180°C). At short times, ae+ was independent of rate but showed rate thinning with increasing time. The curves also passed through a maximum at strains around 0.2 strain units, which is lower than what would be anticipated for a steady state linear viscoelastic fluid. The generality of the results from this study are also limited because yield stresses have been reported in HPC melts at 180°C, which could explain the rate thinning behavior [44, 152].
Another series of thermotropic extensional tests were performed with HPC and also included Vectra A 900 [164]. Extension was applied with a rotary clamp device that could be operated as either a single clamp or a twin clamp. In both materials, the transient uniaxial elongational viscosity was found to respond similarly to that of isotropic melts of linear polyolefins. At small strains linear viscoelastic behavior was observed, which progressed to strain hardening as strains increased, see Figure 2.7. The HPC was tested in both the isotropic and anisotropic states. It was concluded that the
differences in ae+ between the two states were related to differences in structure of the melt and may indicate the influence of the melt state on the flow behavior of LCPs. Once again, the conclusions drawn from these results may be limited due to existence of residual crystallinity.
Figure 2.7. Comparison of Transient Uniaxial and Shear Viscosities [163]
A similar study was performed on Vectra A 950 with extension rates between 0.005 and 1 sec-1 [49]. It was found that the material did not reach steady state before the samples broke, which were above 3 to 4 strain units. The transient response, for similar strains, was rate thinning with ae+ and positively deviated from Trouton ratio at moderate
strains. Also, samples that were injection molded consistently demonstrated greater values for viscosity than compression molded counterparts. It was determined that the results from injection molded samples were strongly influenced by skin-core effects. The samples did not melt uniformly and the skin often separated from the core as a result.
Experimental studies of TLCP extensional flow properties has not been limited to uniaxial flow. A series of thermotropic copolyesters (60 HBA/ 40 PET, 80 HBA/ 20 PET, and 73 HBA/ 23 HNA) were studied in equibiaxial extensional flow [36].
Lubricated squeezing flow was performed to measure the transient biaxial extensional viscosity. Results showed that ab+ was slightly lower at small strains than 6h+. At greater strains the transient biaxial elongational viscosity exceeded the Trouton ratio because of strain hardening. The results of the study were limited to relatively small strains because the equibiaxial response becomes contaminated with shear effects when proceeding through larger strains due to the loss of lubrication effectiveness.
2.2.2.2 TLCP Response to Shear Flow
The previous discussion of the rheological response of TLCPs in shearfree flows alluded to the use of the Trouton ratio to establish a correlation between shear and shearfree flows for linear viscoelastic materials. From the experimental results reviewed in the previous section, this relationship is expected to remain valid for TLCPs throughout the coalescence process because it is limited to low rates and small strains.
Therefore, the viscosity behavior at low shear rates is of particular interest because of its role in coalescence.
Steady Response
Onogi and Asada proposed an explanation for a well documented feature of the steady shear viscosity [122]. They suggested that a general, three-region flow curve could be used to describe the viscous behavior of LCPs in response to steady shear flow, as shown in Figure 2.8. In addition, by introducing the concept of domains and polydomain structure, with individual molecular orientation in each domain, they were able to relate each of the three-regions to different stages of destruction of polydomain texture to form a single nematic phase [10, 119]. Region I occurs at low rates and has shear-thinning behavior that suggests the existence of a yield stress. The mesophase is dominated by defects that result in polydomain texture [108]. Region II exists at moderate rates where shear-thinning transitions to a Newtonian plateau as a dynamic balance of shear induced texture [119]. The polydomain texture becomes more refined as the directors in the various domains begin to align [10]. Region III occurs at even higher rates and is another shear-thinning region. If rates are great enough, the polydomain texture is fully reduced to a monodomain [10, 49].