Cold Crystallization Of Poly Research Articles

Nanograin Evolution in Cold Crystallization of Syndiotactic Polystyrene As Illustrated via in-Situ Small/Wide-Angle X-ray Scattering and Differential Scanning Calorimetry

Structural evolution of syndiotactic polystyrene (sPS) in a cold crystallization process was quantitatively examined with in-situ small/wide-angle X-ray scattering and differential scanning calorimetry (SAXS/WAXS/DSC). After removal of background scattering from fractal-like matrix structure, SAXS profiles obtained during programmed heating of an amorphous sPS specimen from 30 to 240 at 10 °C/min can be interpreted with a similar sequence of events previously observed in cold crystallization of poly(9,9-di-n-octyl-2,7-fluorene) (PFO). Specifically, the nanograin evolution of sPS involves four stages: (1) the frozen-in stage below 90 °C, (2) the nucleation of oblate-like nanograins with a constant radius of gyration Rg ≈ 2.6 nm between 90 and 130 °C, (3) the growth of nanograin size to Rg ≈ 3.2 nm concomitant with the emergence and development of WAXS-determined crystallinity (Xc,WAXS) from 130 to 180 °C, and finally (4) the coalescence (and thickening) of the nanocrsytals into a greater size of Rg ≈ 4.6 n...

The role of the amorphous phase in the re-crystallization process of cold-crystallized poly(ethylene terephthalate)

The process of re-crystallization in poly(ethylene terephthalate) is studied by means of X-ray diffraction (SAXS and WAXS) and dynamical mechanical thermal analysis. Samples cold-crystallized for 9h at the temperatures T(c) = 100°C and T (c) = 160°C, i.e. in the middle of the alpha relaxation region and close to its upper bound, respectively, are analyzed. During heating from room temperature, a structural rearrangement of the stacks is always found at T (r) approximately T (c) + 20°C. This process is characterized by a decrease of the linear crystallinity, irrespective of T(c); on the other hand, the WAXS crystallinity never increases with T below T(c+30)°C. The lamellar thickness in the low-T(c) sample decreases significantly after the structural transition, whereas in the high-T(c) sample the lamellar thickness remains almost unchanged. In both, high- and low-T(c), the interlamellar thickness increases above T(r). Moreover, the high-T(c) sample shows a lower rate of decrease of the mechanical performance with increasing T as the threshold T(r) is crossed. This result is interpreted in terms of the formation of rigid amorphous domains where the chains are partially oriented. The presence of these domains would determine i) the stabilization of the crystalline lamellae from the thermodynamic point of view and ii) the increase of the elastic modulus of the amorphous interlamellar regions. This idea is discussed by resorting to a phase diagram. An estimation of the chemical-potential increase of the interlamellar amorphous regions, due to the enhancement of the structural constraints hindering segmental mobility, is offered. Finally, previous calculations developed within the framework of the Gaussian chain model (F.J. Baltá Calleja et al., Phys. Rev. B 75, 224201 (2007)) are used here to estimate the degree of chain orientation induced by the structural transition of the stacks.

Structural Evolution of Nanograins during Cold Crystallization of Poly(9,9-di-n-octyl-2,7-fluorene) As Revealed via in Situ Small-Angle X-ray Scattering/Wide-Angle X-ray Scattering/Differential Scanning Calorimetry

By means of in situ small-angle X-ray scattering/wide-angle X-ray scattering/differential scanning calorimetry (SAXS/WAXS/DSC), structural evolution of poly(9,9-di-n-octyl-2,7-fluorene) (PFO) in a cold crystallization process was quantitatively examined. After removal of background scattering from the fractal-like structure, SAXS profiles obtained during programmed heating of an amorphous PFO specimen unveil a sequence of stages of structural evolution. These include (1) a frozen-in stage below the glass transition temperature (Tg ≈ 62 °C), (2) the nucleation of prolate nanograins with radius of gyration Rg ≈ 3.3 nm from 65 to 85 °C, (3) growth of the prolate ellipsoids up to Rg ≈ 5.0 nm between 95 and 105 °C, and (4) coalescence (and thickening) of the nanocrsytals into oblates (of Rg ≈ 10 nm) upon further heating to 145 °C. There were no further changes in morphological features in the subsequent isothermal annealing at 145 °C for up to 1 h, as the size of the coalesced nanograins quickly reached a threshold value where the orientation and attachment of neighboring nanograins via thermally activated Brownian rotation became seriously hindered. Developments in the DSC- determined crystallinity (Xc,DSC) and the degree of heterogeneity (Qinv from SAXS) coincided with the nucleation and growth stages, respectively; in contrast, buildup of the WAXS-determined crystallinity (Xc,WAXS) proceeded mainly in the subsequent coalescence stage where large nanocrystallites were developed.

Cold-Crystallization of Poly(trimethylene terephthalate) Studied by Photoluminescence of Its Amorphous Portion

Dynamic processes of isothermally cold-crystallized poly(trimethylene terephthalate) (PTT) was investigated by fluorescence spectroscopy of its amorphous region. The key issue lay in the fact that the emission at 390 nm originating from the main chain phenylene ring in the amorphous phase was correlated to the cold-crystallization. A reduction in this emission peak intensity corresponded to a gradual transition of the phenylene ring in the amorphous to crystal region. Accordingly, molecular chain movement and structure evolution of PTT in the course of cold-crystallization were carefully revealed. The experimental results indicated that the kinetics parameters measured by the fluorescence spectroscopy method are in good agreement with those by differential scanning calorimetry (DSC). What is more, the former was capable of providing detailed information about structural variations during cold-crystallization, for example, molecule arrangement in induction phase.

Recrystallization studies on isotropic cold-crystallized PET: Influence of heating rate

The nanostructural changes associated to the multiple melting behaviour of isotropic cold-crystallized poly(ethylene terephthalate) (PET) have been investigated by means of simultaneous wide- and small-angle X-ray scattering, using a synchrotron radiation source. Variations in the degree of crystallinity, coherent lateral crystal size and long period values, as a function of temperature, for two different heating rates are reported for cold-crystallized samples in the 100–190°C range. The Interface Distribution Function analysis is also employed to provide the crystalline and amorphous layer thickness values at various temperatures of interest. Results suggest that samples crystallized at both low (Ta=100–120°C) and high (Ta=160–190°C) temperatures are subjected to a nearly continuous nanostructural reorganization process upon heating, starting immediately above Tg (≈80°C) and giving rise to complete melting at ≈260°C. For all the Ta investigated, a melting–recrystallization mechanism seems to take place once Ta is exceeded, concurrently to the low-temperature endotherm observed in the DSC scans. For low-Ta and slow heating rates (2°C/min), a conspicuous recrystallization process is predominant within Ta+30°C≤T≤200°C. In contrast, for high-Ta, an increasingly strong melting process is observed. For both, high- and low-Ta, an extensive structural reorganization takes place above 200°C, involving the appearance of new lamellar stacks simultaneously to the final melting process. The two mechanisms should contribute to the high-temperature endotherm in the DSC scan. Finally, the use of a high heating rate is found to hinder the material's overall recrystallization process during the heating run and suggests that the high-temperature endotherm is ascribed to the melting of lamellae generated or thickened during the heating run.

Phenomenological model for the confined dynamics in semicrystalline polymers: the multiple α relaxation in cold-crystallized poly(ethylene terephthalate)

The segmental relaxation in poly(ethylene terephthalate), crystallized from either an isotropic or a cold-drawn glass, is investigated by means of dielectric spectroscopy. It is shown that there exist two distinct alpha relaxation modes: a slow one, characterized by a rather wide frequency interval, and a faster, much narrower one. A simple phenomenological model is developed in order to analyze the polarization autocorrelation functions phi(t)'s associated with these relaxation modes. The model is based on the idea that the growth of crystalline domains causes a progressive confinement of the amorphous regions where, eventually, the observed alpha processes take place. The mechanism of confinement is accounted for by applying to the case of constrained density fluctuations, well known concepts introduced by Adam and Gibbs [J. Chem. Phys. 43, 139 (1965)] concerning the relaxation dynamics in liquids close to the glass transition. Randomness on confining conditions is then introduced, leading to the derivation of analytical expressions which are used afterwards to fit the asymptotic behavior of the phi's for long-time tails. It is found that the slow, broad alpha process takes place in regions where the confining effect of crystals is strong, whereas the amorphous domains relaxing via the fast mode are those where the confinement effect of crystals is weak. The analysis of the phi's by means of this model allows us to relate the fitting exponents to the dispersion in the free energy associated with structural rearrangement.

The link between rigid amorphous fraction and crystal perfection in cold-crystallized poly(ethylene terephthalate)

The rigid-amorphous fraction (RAF) in cold-crystallized and subsequently annealed poly(ethylene terephthalate) (PET) was investigated as a function of crystallinity and crystal perfection. During cold-crystallization, the amount of RAF increases non-linearly with crystallinity to a maximum of 44% at a crystallinity of 24%. Vitrification of the RAF is almost complete at the cold-crystallization temperature. Increasing the crystallinity from 24% after cold-crystallization to 44% by subsequent annealing at higher temperatures decreases the RAF. The specific RAF, i.e. the ratio of RAF to crystallinity at the glass transition temperature, T g, of the mobile-amorphous fraction decreases from almost 2.0 after cold-crystallization to 0.75 after the subsequent annealing. The decrease in the specific RAF is attributed to crystal perfection which decreases the strain transmitted to the amorphous phase. Analysis of the reversing specific heat capacity of the annealed samples on cooling leads to the conclusion that the remaining RAF at the temperature of annealing is still glassy up to at least 490 K. An observed excess heat capacity above 490 K is due to reversible melting and is discussed within the frame of the concept of the specific reversibility of melting.

Structural formation and gelation behavior of cold-crystallized poly(trimethylene terephthalate)

Experimentally observed a +-type Hv and a circular-type Vv scattering patterns from small-angle light scattering (SALS) measurement indicated a random assembly of sheaf-like or rod-like superstructure for poly(trimethylene terephthalate) (PTT) under cold-crystallization. The long period and crystallite thickness of cold-crystallized PTT have been determined by small-angle X-ray scattering (SAXS). The crystallite thickness is very small, ca 2nm, which is close to the c-axis (chain axis) of unit cell in PTT crystal, 18.64Å. The intermolecular crystallization may be viewed as a behavior of physical gelation. The liquid–solid transition in PTT crystallization from glassy state through the gel point has been investigated by dynamic mechanical experiments. The power law relaxation modulus, E(t)=St−Δ identifies the gel point, and small frequency window is sufficient to determine the relaxation exponent, Δ. The gel point occurs at the very early stage of crystallization, in which the relative degree of crystallinity is found to be about 2–3%. The Δ value is ca 0.52, which is independent of crystallization temperature.

Positron Annihilation Lifetime Spectroscopy of Poly(ethylene terephthalate): Contributions from Rigid and Mobile Amorphous Fractions

Systematic divergences in the orthopositronium (o-Ps) annihilation lifetimes, τ3, and intensities, I3, are observed, when comparing melt-crystallized and cold-crystallized poly(ethylene terephthalate) (PET) as a function of crystallinity. Following a previous analysis of corresponding deviations in oxygen permeability, the divergences in I3 and τ3 are traced to distinct characteristic values for the probability of o-Ps formation and o-Ps lifetime in the rigid amorphous phase (RAF) associated with the crystalline lamellae and the mobile amorphous regions (MAF) which are unperturbed by the presence of the crystal phase. Utilizing independent information on the volume fractions of RAF and MAF, a quantitative analysis of the o-Ps annihilation parameters is possible.

Effect of aging on the mechanical properties of cold-crystallized poly(trimethylene terephthalate)

The changes in the mechanical and thermal properties of cold-crystallized poly(trimethylene terephthalate) during aging at 60 and 80 °C were investigated. A significant increase in the tensile modulus and stress at yield and a decrease in strain at yield were observed for both aging temperatures. Differential scanning calorimetry (DSC) scans of the aged sample showed an endothermic annealing peak 10–20 °C above the previous aging temperature, the maximum temperature and enthalpic content of these peaks increased with aging time. Dynamic mechanical measurements indicated a relaxation process starting at about 20 °C above the aging temperature and correlate with the annealing peak detected by DSC. Density measurements and wide-angle X-ray scattering investigation revealed that neither the crystallinity increased significantly nor did the crystal structure changed. These results were explained by the existence of a third phase besides the crystalline and the ‘classical amorphous’ which involves oriented and constrained ‘non-crystalline’ polymer chain sequences close to the crystalline lamellae.

Role of thermal history on quiescent cold crystallization of PET

The cold crystallization of poly(ethylene terephthalate) (PET) has been studied as a function of the initial structure of the glass using density, microhardness, wide angle X-ray scattering, small angle X-ray scattering and DSC measurements. Glassy PET samples varying from slightly crystalline to completely amorphous phase were investigated. Results reveal that differences in the inner structure of the starting glassy material induce different crystallization rates from the glassy amorphous state. Thus, it is observed that crystallization rate decreases with the increasing cooling rate used to quench the samples. Results have been analyzed using the Kolmogroff–Avrami–Evans theory. A good agreement between theoretical and experimental data is obtained providing accurate values for kinetic constants. The different crystallization rates obtained are explained in terms of differences in nucleation density.

The Semicrystalline Morphology of Poly(ether−ether−ketone) Blends with Poly(ether−imide)

The microstructure of cold-crystallized poly(ether−ether−ketone)/poly(ether−imide) (PEEK/PEI) blends has been investigated by a combination of wide- and small- angle X-ray scattering (SAXS), dielectric spectroscopy, and transmission electron microscopy. Both SAXS and dielectric spectroscopy indicate that most PEI is excluded from interlamellar regions and segregates essentially into interfibrillar regions upon crystallization. Interlamellar inclusion was found to be limited to low PEI levels. Transmission electron microscopy (TEM) observations on stained thin sections from bulk samples confirm these conclusions. The restricted character of the interlamellar inclusion of PEI is probably due to a more ordered structure of noncrystalline regions near PEEK crystal surfaces. In addition, our results on PEEK/PEI blends corroborate the model of cold-crystallized pure PEEK which consists of thin lamellar crystals separated by thicker noncrystalline interlayers that are grouped into stacks of limited coherence and uniformly fill the entire sample volume without any separate amorphous pockets or gaps.

COLD CRYSTALLIZATION OF POLY (ETHYLENE TEREPHTHALATE)

In order to clarify polymer conformations prior to crystallization, crystallization behavior of poly (ethylene terephthalate) (PET) from a quenched, amorphous state has been examined. The crystallinity of the samples annealed in the low temperature range 130°C to 180°C was nearly constant at ca. 40%, supporting a model similar to Yeh's folded chain fringed micellar grain model but without local orientation of the segments.IR measurements (CH2 rocking and CO stretching) showed that the content of trans-CH2-CH2-bond was as low as ca. 10% in the original sample, indicating the existence of quite local bends in the PET chains in the amorphous state. Tg, as determined by dilatometery as a function of the annealing temperature, showed an abrupt drop at 170°C, indicating the destruction of the “fringed micellar network”. Weight loss vs time curves during treating the samples by monoethylamine also reflected the destruction of the “network” and the formation of new lamellar crystals at higher temperatures than 170°C. Finally, DSC measurements of samples before and after treating by the amine, suggested that the small peak appeared at ca. 160°C for the sample annealed at low temperatures (120°C_??_160°C) may be due to the melting of the crystals formed in the grain boundary proposed by Yeh.