Bryan B. Sauer

The nature of secondary crystallization in poly(ethylene terephthalate)

Abstract The nature of secondary crystallization in poly(ethylene terephthalate) (PET) was examined during isothermal crystallization and subsequent melting by time-resolved synchrotron small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC) and temperature modulated DSC (MDSC) techniques. In one experiment, the process of isothermal crystallization was sustained over 72xa0h to induce a relatively large crystallinity (46%, by weight). The purpose of this experiment was to resolve the issue of controversial assignment for the crystal lamellar thickness (lc) by the correlation function analysis of the SAXS data. Results suggest that a two-stage decrease mechanism exists in both long period (L) and lc during isothermal crystallization: (1) a significant decrease in the initial stage (primary crystallization dominant), and (2) a much slower decrease in the later stage (secondary crystallization dominant) that is nearly linear with log time. We attribute this behavior to the formation of thinner separate stacks of lamellae between the primary stacks by secondary crystallization. Both secondary and primary stacks can undergo a great deal of crystal perfection and rearrangement with time. From DSC measurements, a triple-melting behavior was observed in the samples crystallized at 205 and 215°C for 1xa0h, and a double-melting behavior at higher temperatures of 225 and 231°C for 2xa0h. Temperature scanning SAXS and MDSC directly characterize aspects of crystal perfection and melting. Consistent with some of the literature, we confirm that for short annealing (∼hour) at 200–220°C, the first (low) endotherm is related to melting of secondary crystals, the middle endotherm is due to melting of primary crystals, and the third endotherm is due to melting of crystals reorganized during heating. With prolonged crystallization at 231°C for 24 and 72xa0h, a single higher melting endotherm was observed even though SAXS experiments indicate a slight decrease in average lamellar thickness. In PET, ester exchange reactions contribute to unusual high mobility, allowing chains to avoid topological constraints such as entanglements and tie chains. The results suggest that the change in population of tie molecules in the non-crystalline phase reduces the entropy of melting causing an increase in Tm, and that this overwhelms the contribution of the decrease in lc.

Temperature modulated DSC studies of melting and recrystallization in polymers exhibiting multiple endotherms

Abstract Temperature-modulated DSC (TMDSC) is used to characterize melting and recrystallization in polymers exhibiting multiple melting endotherms. Poly(ethylene-2,6-naphthalenedicarboxylate)(PEN) and poly(oxy-1,4-phenyleneoxy-1,4-phenylenecarbonyl-1,4-phenylene)(PEEK) are chosen, and the data show the detailed contributions of thermal and processing histories to properties. The results are supplemented by standard DSC at different heating rates. By independent very rapid heating rate methods, the temperature at which the polymer first completely flows is used as a measure of the end of melting of crystals originally present in the sample, and is shown to be well below the final DSC melting point because of recrystallization during the DSC heating scan. This is true even for long annealing times at moderately high temperatures. The TMDSC signal detects endothermic peaks or shoulders corresponding to the melting of crystals originally present in the sample, and such information are not available from standard DSC because of offsetting exothermic and endothermic signals. The TMDSC data prove that the “low endotherm”—routinely detected by standard DSC a few degrees above isothermal annealing temperatures—is not a true “low endotherm”, but is a superposition of early melting of secondary crystals with almost simultaneous exothermic recrystallization. It is not a distinct endotherm because the degree of recrystallization measured in the non-reversing signal of TMDSC increases continuously up to and sometimes through the final melting region. This description of the thermal scan considers both primary crystals, secondary crystals, and recrystallization during the heating scan.

Correct determination of crystal lamellar thickness in semicrystalline poly(ethylene terephthalate) by small-angle X-ray scattering

Abstract For the purpose of resolving an uncertainty over the correct determination of the crystalline lamellar thickness in semicrystalline poly(ethylene terephthalate), PET, via small-angle X-ray scattering (SAXS) analysis, a gel-crystallization method from oligomeric poly- (ethylene glycol) solution was used to prepare samples with high crystallinity (57%). By using simultaneous synchrotron SAXS and wide-angle X-ray diffraction (WAXD) measurements, the heating and cooling processes of the gel-crystallized PET sample were monitored. Results support the assignment of the larger thickness value from the SAXS correlation function analysis as the lamellar crystal thickness. Analysis of WAXD 0 1 1 reflection line broadening gives the minimum lamellar thickness (in the chain axis) and verifies the thickness assignment for gel and melt crystallized samples. This assignment is critical as it affects the correct interpretation of the crystallization behavior in semicrystalline polymers of relatively low crystallinity.

Temperature modulated DSC studies of melting and recrystallization in poly(ethylene-2,6-naphthalene dicarboxylate) (PEN) and blends with poly(ethylene terephthalate) (PET)

The thermal properties of amorphous and melt crystallized poly(ethylene-2,6-naphthalene dicarboxylate) (PEN) and its blends with poly(ethylene terephthalate) (PET) were investigated. Temperature modulated DSC (TMDSC) was used over a broad range of annealing times and temperatures. PEN under all moderate temperature crystallization conditions was found to exhibit secondary crystal melting in the low endotherm region, followed by melting of primary crystals superimposed with a large endotherm due to melting of DSC scan-induced recrystallized species. The exothermic signal due to recrystallization was separated into the TMDSC non-reversing signal. Annealing time had a small effect on thermal properties at moderate annealing temperatures, and varying the annealing temperature had a larger effect on the recrystallization properties above the annealing temperature. The results show that TMDSC provides excellent resolution of recrystallization and related events compared to standard DSC. Blends of PEN and PET were evaluated as a function of melt blending time, and the phase properties were studied at 260 and 285°C and related to transesterification reactions. The evolution from a phase separated mixture to a single phase but still crystallizable copolymer, to finally a non-crystallizable copolymer with a higher degree of randomness was studied. The crystallization properties measured using TMDSC were contrasted with those of PEN.

Nylon 66/poly(vinyl pyrrolidone) reinforced composites:: 1. Interphase microstructure and evaluation of fiber–matrix adhesion

Abstract Nylon 66, an aliphatic semicrystalline polyamide, was reinforced with E-glass fibers or high-modulus (AS4) carbon fibers. As in many reinforced semicrystalline thermoplastics, an interphase composed of transcrystallinity developed owing to the high nucleation density of the polymer on the fiber surface. The influence of this region on the fiber–matrix adhesion was studied with a modified microdebond test. E-glass fibers were freshly prepared in our laboratory by traditional glass-forming techniques and embedded in a film of Nylon 66 or a Nylon 66/poly(vinyl pyrrolidone) (PVP) blend. Previous work has shown that PVP, an amorphous polar polyamide, has a dramatic influence on the morphology of Nylon 66. This phenomenon was utilized to manipulate the interphase formation in the Nylon 66 composite from one having a complete transcrystalline interphase to a composite with the absence of an interphase. PVP was introduced to the matrix by solution blending with Nylon 66 and/or to the fibers as a sizing prior to embedment. The resulting morphologies were studied by polarized hot-stage optical microscopy. From the microdebond and morphology results, it was shown that the fiber–matrix adhesion in this composite system is dependent upon interphase microstructure. Composites containing transcrystallinity have higher interfacial shear strength values than those that do not contain this interphase. This has profound implications for the bulk mechanical properties of the composite, which are addressed in Part 2 of this paper.

Thermal and morphological properties of main chain liquid crystalline polymers

Abstract Temperature modulated differential scanning calorimetry (TMDSC), variable heating rate DSC, and tapping atomic force microscopy (AFM) were used to study semi-crystalline liquid crystalline polymers (LCPs). Main chain LCPs included a random copolyester (Vectra® A950) and an azomethine alternating copolymer. For the azomethine LCP the TMDSC non-reversing signal detected broad exothermic transitions associated with melting and recrystallization as the slow DSC heating scan induced surprisingly large morphological changes. Non-isothermally crystallized Vectra® and some isothermally crystallized samples at lower temperatures exhibited different levels of DSC scan induced crystal reorganization. Such crystal metastability was also studied by variable heating rate DSC and an independent technique for estimating the melting point at very rapid heating rates. The TMDSC characterization of the scan induced crystal perfection in Vectra® was substantially different than for the other polymers studied. In most cases even though crystal perfection was occurring, no clear exotherm was detected in the non-reversing signal. High temperature annealing for long times resulted in degrees of crystal perfection which could be studied by DSC with minimal scan induced reorganization. High resolution tapping AFM was used to elucidate details of crystal morphology for mechanically oriented and non-oriented Vectra® before and after annealing. Structures resembling lamellae were found to be oriented perpendicular to the chain direction in the oriented Vectra®. In the non-oriented film broad and sometimes curved ‘lamellae’ were detected. They were about 1000xa0nm long and between 20 and 35xa0nm wide, with the width increasing slightly as a function of increased annealing time at 260xa0°C melt crystallization conditions. Substructure of the lamellae in both oriented and non-oriented Vectra® consisted of smaller stacked crystallites which are detected by AFM studies of these surfaces.

Nanometer resolution of crystalline morphology using scanning probe microscopy

A semicrystalline segmented polyamide has been used as model system to investigate the morphology of thick and thin solvent cast films. Tapping atomic force microscopy (AFM) with height and phase detection was used to resolve the ribbon-like crystals near the surface. Methods are described for obtaining nanometer resolution of domains. Much thinner and smaller crystals were imaged in films approximately 30u2009nm thick due to crystallization under constrained environments. The results illustrate the danger of casting ultrathin films where the thickness is found to have a large influence on the final morphology. The results also show that details of crystallite organization in spherulites can be characterized by lower resolution phase data, and illustrate the advantages of the phase data for polymer films with long-range roughness. n n n n© 2000 Society of Chemical Industry

TMDSC and Atomic Force Microscopy Studies of Morphology and Recrystallization in Polyesters Including Oriented Films

The thermal and crystal morphological properties of poly[ethylene teraphthalate] (PET) and poly(ethylene-2,6-naphthalenedicarboxylate) (PEN) biaxially oriented films were compared to amorphous and other isotropic semi-crystalline samples. Crystal melting as a function of temperature was characterized by temperature modulated DSC (TMDSC) and found to begin just above the glass transition for both oriented films. About 75°C above the glass transitions, substantial exothermic recrystallization begins and continues through the final melting region in oriented films. The maximum in the non-reversing TMDSC signal for the oriented films signifies the maximum recrystallization exothermic activity with peaks at 248°C and 258°C for PET and PEN, respectively. The final melting endotherm detected was 260°C and 270°C for PET and PEN, and is shown by the TMDSC data and by independent rapid heating rate melting point determinations to be due to the melting of species recrystallized during the heating scan. The results are compared with TMDSC data for initially amorphous and melt crystallized samples. The volume fraction of rigid species (Frigid=total crystal fraction plus ‘rigid amorphous or non-crystalline species’) were measured by TMDSC glass transition data, and contrasted with the area fraction of rigid species at the oriented film surface characterized with very high resolution atomic force microscopy (AFM) phase data. The data suggest that the 11 nm wide hard domains in PET, and 21 nm wide domains in PEN film detected by AFM consist of both crystal and high stiffness interphase species.

Dynamic mechanical and dielectric relaxation in a series of main chain thermotropic liquid crystalline polyesters

Abstract The effects of structural changes on the frequency dependent dynamic mechanical (d.m.a.) and dielectric relaxation behaviour in a series of wholly aromatic thermotropic main chain liquid crystalline (LC) polyesters were investigated. The polymers were side-group substituted poly(hydroquinone-terephthalates), modified with biphenols and 4-hydroxybenzoic acid. The side groups, R, were varied as R = methyl, phenyl, or t-butyl. Data were also obtained for Vectra A950. These LC polymers exhibit three prominent relaxation processes. Some of the LC polymers in this series are ‘non-crystalline’ with strong and narrow glass transitions which are shown to vary as much as 80°C depending on the choice of substitution of the hydroquinone group. Tertiary butyl substitution on the hydroquinone units was found to lead to the highest glass transition ( ca 185°C). Another common feature of most of these LCPs is a sharp but weak second ‘glass transition’ observed at lower temperatures than the main glass transition. This is attributed to motions of non-substituted aromatic ester species including 4-hydroxybenzoic acid (HBA) due to their relatively low barriers to rotation. Comparisons are made with the wholly aromatic, HBA-rich, LC polymer, Vectra®. Three of the LC polymers exhibit essentially the same subglass γ relaxation at ca 60°C (1Hz) which is attributed to local motion involving the non-substituted aromatic units such as HBA. Adding methyl side group substituents to the ortho position of the biphenol group restricts the subglass γ process in a kinetic sense, shifting the process to higher temperatures. The effect of substitution of the hydroquinone group on the γ relaxation was also systematically investigated.

Crystallization study of poly(ether ether ketone)/poly(ether imide) blends by real-time small-angle x-ray scattering

Abstract Morphological changes during isothermal melt crystallization in poly(aryl ether ether ketone) (PEEK)/poly(ether imide) (PEI, Ultem® 1000) blends were monitored via real-time small-angle x-ray scattering (SAXS) using synchrotron radiation. SAXS data were analyzed using a novel combination of correlation and interface distribution functions to determine the lamellar crystal thicknesses l c and interlamellar noncrystalline or “amorphous” layer thicknesses l a. The higher glass transition noncrystalline PEI component slows the PEEK crystallization substantially, but l a (about 40 A) and l c (about 85 A) are independent of crystallization time and blend composition. This is consistent with the known independence of melting temperature with blend composition. These results indicate that PEEK crystallizes in densely crystalline lamellar stacks through all stages of primary crystallization, and that the noncrystalline PEI is almost entirely excluded from the stacks at all times during spherulitic growth....