Hermans Orientation Function Research Articles

Cooperative chain relaxation in miscible polymer blends

AbstractOrientation relaxation of dissimilar chains in the molten miscible blends, poly(methyl methacrylate)/poly(vinylidene fluoride) and poly(methyl methacrylate)/poly(vinylidene fluoride‐co‐trifluoroethylene), were investigated by measuring (1) the change of infrared dichroic ratio with time after the uniaxial stretching of film specimens, (2) the shear stress relaxation spectrum, and (3) birefringence relaxation in shear. The dissimilar polymers showed an identical time variation of the normalized Hermans orientation function. The blend showed a relaxation spectrum with a single characteristic relaxation time τc, depending on the blend composition. The birefringence relaxed monotonically, remaining positive. These results suggest that the dissimilar polymers do not relax independently but cooperatively. This behavior may be induced by a constraint due to the specific interactions between the dissimilar polymers, e.g., weak hydrogen bonding. For the cooperative chain relaxation, a third power relationship was found; τc/τe vprop; (M/Me),3 where τe and Me are the relaxation time and molecular weight of entanglement strand, respectively, and M is the number average molecular weight in the blend.

An Exact Method to Determine the Chain Axis Orientation Distribution in High Performance Fibers from a Single Inter-Chain Reflection

ABSTRACTAn exact pole inversion method is used to calculate the complete chain-axis orientation distribution from a single inter-chain reflection. A brief outline of the formulation and computation method is given. This method is demonstrated by comparing the measured chain-axis distribution from a (002) reflection of oriented polyethylene samples, to the calculated distribution from (110) and (200) reflections of the same sample. The error in the Hermans Orientation function is less than 0.5% for the two samples tested.

The Effect of Molecular Weight Distribution on the Physical Properties and Morphology of Polypropylene Stretched Tape

Stretched tape and film were made from isotactic polypropylene (iPP) homopoly mer produced with SHAC* catalyst (Shell High Activity Catalyst). The resins used were nominal 3.5 and 8.0 melt flow (MF) resins with narrow, medium, and broad molecular weight distributions (MWD). Measurements were made of tape and film physical properties as a function of MWD and process heat history. Small angle x-ray two-theta scans through the meridional direction were done to obtain Long Period Spacing (LPS) Long period spacing for an idealized semicrystalline model is the combined distance through both a crystalline lamella and the amorphous region between the lamellae. LPS was correlated to resin melt flow and MWD. Wide angle x-ray scattering (WAXS) techniques using pole figures and Hermans orientation func tions were used to determine molecular orientation. Density and physical properties (percent elongation at break, Young's modulus, and tenacity) did not change significantly as a function of MWD for the 3.5 and 8 MF tapes. The tenacities of the 8 MF tapes averaged 0.8 gm/denier less than those of the 3.5 MF tapes. The density of the 8 MF tapes was higher than that of the 3.5 MF tape density (0.9039 and 0.8998 gm/cm3, respectively) indicating a higher percent crystallinity for the 8 MF tapes. The lower molecular weights of the 8 MF resins apparently contribute to a more highly ordered structure; the latter keeps the physical properties of the tape from deteriorating as molecular weight decreases. The net result is that lower molecular weight resins (higher MF) can produce stretched tape with acceptable physical prop erties at high processing speeds.

Determination of the crystalline and noncrystalline molecular orientation in oriented polypropylene by infrared spectroscopy

AbstractThe application of orientation to enhance the properties of a wide range of macromolecular materials has stimulated an interest in the characterization of the effects of such orientation processes. In semicrystalline polymers, this requires the characterization of the orientation in both the crystalline and noncrystalline phases. An infrared dichroism method was developed to simultaneously measure the orientation in the crystalline and noncrystalline phases of polypropylene. The method was applied to a series of melt‐crystallized and gel‐crystallized polypropylene specimens drawn up to a maximum of about 50 ×. The Hermans orientation functions of each phase were determined. The crystalline orientation was found to increase rapidly and approach perfect orientation at relatively low draw ratio (i.e., > 10 × ). However, the noncrystalline orientation was found to increase much more slowly up to about 50 × draw ratio. The Youngs modulus was found to increase nearly linearly up to about 50 × and to correlate with the increase in the noncrystalline‐orientation function and not with the crystalline‐orientation function.

Infra-red characterization of oriented poly(phenylene vinylene)

We present i.r. spectra of oriented films of poly(phenylene vinylene), PPV. Stretch-alignment of the amorphous precursor polymer, poly( p-xylene- α-(dimethyl sulphonium chloride)) during its thermal conversion to PPV enables highly oriented films of PPV to be produced that exhibit considerable crystallinity. From an analysis of the dichroic ratios of the main infra-red-active modes for a film of stretch ratio 5, we obtain an orientation parameter, s = 0.04, corresponding to a value of 0.94 for the Hermans orientation function (〈 p 2(cos γ)〉 = 0.94). This is an unexpectedly high degree of orientation for these initially amorphous films. Using the orientation parameter we derive values for the orientation of the transition moments of the main infra-red modes with respect to the molecular chain axis. We find in particular that the vinylene CH stretch mode at 3024 cm −1 lies at 30° to the chain axis and that this is very different from the value of 74° expected from the molecular geometry. This deviation may be accounted for if the vibration induces a large charge flux along the chain. Such behaviour is consistent with a well delocalized π electron system.

Molecular orientation of polydimethylsiloxane induced by elongational and shearing strains

AbstractA study examining the molecular orientation of poly(dimethylsiloxane) for different combinations of elongational and shear strains is presented. Three different cases were studied: (1) pure elongational strain; (2) increasing shear and decreasing elongational strains; (3) increasing shear and increasing elongational strains. The experiments were performed in a converging flow cell (at room temperature), where elongational and shearing strain rates achieved values of 370 s−1 and 640 s−1 respectively. Values of the Hermans orientation function were obtained from measurements of birefringence and polarization angles while strain rates were estimated from laser Doppler anemometry velocity measurements. Prospects for predicting molecular orientation from the stress‐optical laws and rheological flow models are outlined and commented on.

Quantitative determination of the monoclinic crystalline phase content in polyethylene by 13C n.m.r.

Solid-state 13C n.m.r. spectroscopy involving the techniques of cross-polarization (CP), magic angle spinning (MAS), and high power proton decoupling, has been used to determine quantitatively the ratio of monoclinic to orthorhombic crystalline phases in compression moalded ultra-high molecular weight polyethylene (UHMWPE) sheet which had been stretched uniaxially. Criteria for expecting quantitative relative intensities in 13C CP-MAS spectra are discussed. Attenuation of the non-crystalline (NC) signals relative to crystalline signals was observed. Experiments were therefore carried out to ascertain whether measurable relative intensity distortions exist between the monoclinic crystalline phase (MCP) and the orthorhombic crystalline phase (OCP) resonances due to possible differences in proton ‘spin diffusion’ between the NC and the two crystalline phases during cross-polarization. No relative intensity distortions were detected. This result, coupled with experiments in which spin diffusion was monitored at times longer than those used for cross-polarization, suggests that the average distance from the protons in a given crystalline phase to the nearest protons in the NC regions is the same for the MCP and the OCP. Finally, non-spinning 13C spectra of the deformed polyethylene were recorded to determine the orientation of the chains in the crystalline and NC regions. The Hermans orientation function, F c, was determined independently for the crystalline (combined OCP and MCP) and NC regions, and found to be 0.66 + 0.06 and 0.23 + 0.04 respectively. The occurrence of orientation in the NC regions may be evidence for internal stresses, which, it is suggested, also stabilize the metastable MCP in the stretched sample.