We investigate bimodal mixtures of polypropylenes (PP) in which we vary the molecular character (concentration, molecular weight, regularity) of the high molecular weight mode. We apply a number of in situ characterization tools (rheo-optics, rheo-WAXD) to the development of transient structure and interpret our findings in light of ex situ investigation (polarized light microscopy, TEM) of the final morphology.
Introduction................................................................................................... I-1
- Morphology and nucleation in quiescent polymer crystallization
- Flow-induced morphology and nucleation in polymer crystallization4
- Motivation
- Thesis organization
- Bibliography
McHugh, "A continuum model for the dynamics of current-induced crystallization." Journal of Polymer Science Part B-Polymer Physics, 1996. Maffettone, "Relation between Kinetics and Rheological Parameters in Current-Induced Crystallization of Thermoplastic Polymers." Macromolecular Symposia, 2002.
Experimental Instrumentation and Methods............................................... II-1
- Shear-induced crystallization apparatus
- Experimental protocol
- Sample preparation
- Characterization methods
- In situ characterization
- Ex situ characterization
- Bibliography
The geometry of the flow cell (length/depth ~ 100) limits the total experimental wall shear stress (γw) to ~ 100 stress units to ensure that any fluid element reaching the observation point was already in the channel at t = 0 (hence subject to the specified thermal and flow history). This effect is consistent with a simple mechanistic model of the role of long chains in the propagation of filament-like precursors.
Effect of Long Chain Concentration in Bidisperse Blends .......................III-1
Materials
The narrow chain length distribution for L-PP (Figure 3.1) was achieved using solvent gradient fractionation, in which the solvent composition of o-dichlorobenzene and diethylene glycol monomethyl ether in the polymer solution was gradually changed.24 The methods used used for molecular characterization are shown below in section 3.2.7.25. The difference in stereoregularity and Mw led to a difference of 16.3 °C in the apparent melting point of the two samples. Bidisperse mixtures were prepared with different concentrations of L-PP (c), ranging from 0 to twice the critical concentration of L-PP (c*) at which molecular coils of L-PP begin to overlap (Table 3.1).
In situ rheo-optical measurements
In situ rheo-WAXD
The resulting patterns are analyzed in terms of a circular average of the scattered intensity versus crystalline poles of perfect uniaxial orientation at an angle φ from the axis of the orientation sphere and plot a cone on the orientation sphere (Figure 3.4).
Ex situ microscopy (OM and TEM)
On our flat plate image, the flow direction and axis of the orientation sphere are along the equator (horizontal direction) of the 2-D image. Therefore, the scattered intensity detected on the meridian (vertical direction) of the 2-D image of crystalline poles oriented at φ = 90° appears less intense than an equivalently oriented group of crystalline poles oriented along the equator φ = 0 °.
DSC measurements
Measurement of molecular weight
The MALLS model DAWN DSP from Wyatt Technology Corporation, equipped with a He-Ne laser (λ = 632.8 nm), was connected to the GPC instrument. The ASTRA software package from Wyatt was used for the analysis of GPC data and scattering data, which were collected simultaneously at 17 scattering angles ranging from 18° to 155° (details25 are given elsewhere).
Results
- Quiescent crystallization kinetics
- In situ rheo-optical
- In situ rheo-WAXD
- Ex situ microscopy (OM and TEM)
The difference can be attributed to the amount of oriented crystallites as measured by the amplitude of the 110 parent peaks (Figure 3.13B). TEM images of the skin layer of the same samples examined by OM reveal shish-kebab structures (Figure 3.20).
Discussion
- Isolating the effect of long chain content
- Sequence of events in shear-enhanced crystallization
- Molecular perspective on shear-enhanced crystallization
Second, the relaxation time of long chains increases with increasing long-chain–long-chain interaction, especially with the onset of long-chain–long-chain entanglement. Interestingly, adding long chains beyond c* to 2c* produces little further improvement in thread formation.
Conclusion
Bibliography
In addition to the dramatic effects of long chains on the formation of oriented coatings, WAXD patterns also show an enhancement of unoriented crystallization in bimodal blends relative to B0 (Figure 3.14C). Mandelkern, "Structural and Kinetic Factors Governing the Formation of the Gamma Polymorph of Isotactic Polypropylene." Macromolecules, 1999.
Effect of Long Chain Molecular Weight in Bidisperse Blends ............... IV-1
Materials
Bidisperse mixtures, prepared via the solvent mixing procedure described in the chapter, are referred to according to the Mw of the long chains, eg, B507 indicates the mixture of f507-PP with base-PP. Due to the extremely high molecular weight of 3500-PP, even this low concentration caused the viscosity of the solution to be significantly higher during solution mixing and required a longer time to obtain a homogeneous mixture. Using the extrusion rates (wex/ts) of the blends at a low enough stress that crystallization does not affect the extrusion rate, we estimate the ratio of the blends' viscosity to the viscosity of the base polymer to be ~1.28:1 (B B3500) .
In situ rheo-optical measurements
The maximum ts was determined as that at which ≈ 100 mg of supercooled polymer was extruded. When the optical delay is less than half a wave (λ/2), the birefringence can be calculated from the ratio of the perpendicular component of transmitted light between crossed polarizers to the total intensity (I⊥/Itot) according to Equation 2.1. The time at which the delay reaches a half wave is marked by a maximum in I⊥/Itot and a full wave is marked by a minimum.
In situ rheo-WAXD
The orientation of the crystal microstructure is determined by the development of birefringence in the sample. Consecutive minima correspond to increments, odd multiples of λ/2, and successive maxima correspond to even multiples of λ/2. However, spectrographic studies of the birefringence of liquid crystalline polymers by Burghardt and co-workers9 showed that the use of extrema to calculate birefringence is robust even in the presence of depolarization.
Optical microscopy
DSC measurements
Measurement of molecular weight
Results
- Quiescent crystallization kinetics
- In situ rheo-optical
- In situ rheo-WAXD
- Ex situ optical microscopy
This signature of the creation of oriented precursors under flow is absent if the long chains are ~500 kg/mol. The time scale for crystallization as measured by sample turbidity is also unchanged by the addition of long chains with ML ~ 500 kg/mol and only mildly altered by the addition of long chains with ML kg/mol (Figure 4.2B). Examination of the final crystalline morphology by polarized light microscopy readily reveals the reinforcement with long chains with ML ≥ 862 kg/mol (Figure 4.6).
Discussion
- Importance of relaxation-time separation on shish-kebab formation . 7
Examples of the resulting WAXD difference patterns at 130 s after shift (dashed line on lower graph) for regions shown in the schematic list (core not shown) (middle). The FWHM (Figure 6.10B) of the fit to the parent peaks (φ = 90° and 270°) indicates that the orientation distribution is established early in the crystallization process—. Chain adsorption at the core surface depends on the molecular regularity of the adsorbing chains.
Conclusion
Bibliography
Meijer, "Development and validation of a reproducible strain-based model for flow-induced polymer crystallization.". Kornfield, "Shear-Mediated Crystallization of Isotactic Polypropylene: The Role of Long Chain and Long Chain Overlap." Macromolecules, 2002. 34; Visualization of molecular fluctuations near the coil-stretch critical transition point in polymer elongation." Macromolecules, 2003.
Toward the Determination of Propagation Velocity Using Real-Time
Materials
In situ rheo-optical measurements
In situ rheo-WAXD
Real-time “depth sectioning”
Fortunately, the stress-depth relationship also enables a meaningful analysis of the depth dependence of real-time data. The main feature is the examination of the incremental contribution to the data in real time from one shear stress to another (with all other conditions fixed), allowing us to attribute the change to a small spatial region of the sample (Figure 5.1). . Repeating this procedure with progressively lower wall shear stress yields a depth-dependent profile of X-ray patterns of the sample subjected to σw,1 = 0.064 MPa.
Results
- In situ rheo-optical
- In situ rheo-WAXD
- Real-time “depth sectioning”
The growth rate of the parent peak area 110 increases gradually with ts up to 4 s, then strongly with further increases in ts (Figure 5.9B). As the distance from the wall increases, the orientation distribution of the parent crystallites broadens significantly and the parent-daughter ratio decreases (Figures 5.11B and 5.11C). Identical analysis of a series of cutting conditions for ts = 7 s provides a glimpse of the depth-dependent effects of cutting time (Figure 5.12).
Discussion
- Propagation of threadlike precursors
- A depth dependent sequence of events
- Toward the determination of thread propagation velocity
The collision time manifested by the decrease in growth rate of the parents provides an indirect estimate of the distance between shish: (impingement time) * (growth rate) ~ (distance at which the growth front moved). The small change in the collision time in the outer 35 µm portion when comparing the 7 s and 12 s shear times suggests a negligible increase in the wire length/vol near the wall after 7 s of shear. If we were to take 400 nm as an estimate of the shish separation in the saturated case, we would conclude that the shish length per unit volume is roughly 4 µm-2.
Conclusion
However, translating the real-time data into a linear propagation velocity for shish awaits future ex situ images of the final morphology for at least one shear time. The dotted line indicates an average of the fit values and the error bars indicate the range. The dotted or solid lines indicate an average of the fit values and the error bars indicate the range.
Bibliography
JaneschitzKriegl, "On the evolution of elongated particles as precursors to polymer crystallization from shear flow: The origin of the so-called fine-grained layers." Rheologica Acta, 1996. JaneschitzKriegl, "The role of long molecules and nucleating agents in shear-induced crystallization of isotactic polypropylenes." International Polymer Processing, 1997. Srinivas, "A simultaneous small- and wide-angle X-ray scattering study of the early stages of melt crystallization in polyethylene." Polymers, 2000.
Effects of Long Chain Regularity in Bidisperse Blends .......................... VI-1
Materials
The samples used in the study to demonstrate the effects of monomer-level defects distributed on the high molecular weight portion of MWD include both model polymer systems available in gram quantities and systems designed to be produced commercially. A set of large-scale bimodal materials was designed to include pilot-scale nonwoven fabric production at trial scale. In order to make a sufficiently large quantity of each mixture, we chose to use melt extrusion mixture and selected the individual components from commercial resins that were available in large quantities.
Molar mass
To produce blends that have similar rheological properties and overall comonomer content, we chose “long” pairs and.
Melt flow rate
Quiescent crystallization
Comonomer content
Flow-induced crystallization
For the large-scale blends, Tcs were chosen so that the quiescent crystallization times of the two blends matched. The ratios of the long and short components that gave the best agreement of MFRs and C2 content of LEP/S and L/SEP. We will demonstrate that the difference in stationary crystallization kinetics is undeniable in light of the overwhelming effects seen in flow-induced crystallization.
Flow-induced solid-state morphology
To compensate for the discrepancy in kinetics, the choice of crystallization temperatures (142 °C for LEP/S and 145 °C for L/SEP) was guided by the DSC and turbidity results (Figure 6.3), which differed from each other. Raw WAXD samples were collected and analyzed according to the methods outlined in Chapter 3 to extract the crystallinity index (xc) and directional crystallization metrics based on 110 azimuthal scans: parent peak area, FWHM, amplitude and ratio of parent to daughter crystallites . The number of spherulite superstructures in the sample core can be used to determine the nucleation density.
Fiber morphology and properties
Results
- Shear-induced crystallization of model blends
- Shear-induced crystallization of large-scale blends
- Trends with stress
- Transient morphology development
- Ex situ WAXD on spun fibers
- Fiber mechanical properties
However, the degree of orientation and intensity of reflections were much weaker for LEP/S than L/SEP. We analyze the transient evolution of oriented crystallization by fitting the azimuthal scan peaks of reflection 110. A comparison of the parent-daughter relationship for L/SEP at moderate and high stress reveals.
Discussion
- Shear-induced morphology and kinetics
- Interpretation of transient morphology development in the skin
- Effect of long chain regularity on spun fiber structure and properties18
Based on a conceptual model of the strong effect of long chains in mixtures of uniform molecular order (Chapter 3), we predict (Figure 6.13) the effect of molecular defects on the participation of long chains in events leading to oriented crystallization under flow . They appreciated the fact that aPP would be repelled by crystallinity, but argued that the extended orientation of long chains would be so. In particular, the threshold stress to induce oriented growth can be decreased and the rate of strand propagation can be increased as the perfection of long chains increases (all other parameters are kept fixed).
Conclusion
Ex situ optical micrographs of L/SEP and LEP/S viewed through crossed polarizers after being subjected to shear pulses (σw .. Note that ts for L/SEP has been reduced to match total strain between rheo-optical and rheo-WAXD experiments to maintain ). Transient morphology development in the skin layer results in distinct slope changes in the real-time transmission that correlate with the onset of shish-kebab collision indicated by change in parent peak area growth rate.. resolved WAXD analysis of A) crystallinity index (xc), B ) full width at half-maximum (FWHM) of Lorentzian fits to parent 110 peaks, and C) parent:daughter ratio (see text) for 110 peaks for L/SEP (145. For L/SEP the effect is minimal since long chains dominates this process, but for LEP/S there is a large effect as defective long chains are difficult to attach.
Bibliography
Richeson, "The influence of isotactic, ethylene comonomer content and nucleating agent additives on the structure and properties of melt-spun isotactic polypropylene filaments." Journal of Applied Polymer Science, 1996. Spruiell, "The Effects of Atacticity, Comonomer Content, and Configurational Defects on the Equilibrium Melting Temperature of Monoclinic Isotactic Polypropylene." Journal of Applied Polymer Science, 2001. Temperature of Polychlorotrifluoroethylene." Journal of Research of the National Bureau of Standards Division a-Physics and Chemistry, 1962.
