Spherulite Growth Rate Research Articles

Accelerated Crystallization of Poly(lactic acid): Synergistic Effect of Poly(ethylene glycol), Dibenzylidene Sorbitol, and Long-Chain Branching

The crystallization of poly(lactic acid) (PLA) is usually slow and related to the content of d-lactide. A new approach is suggested in this work to accelerate the crystallization of PLA via the synergistic effect of nanofibril nucleating agent (dibenzylidene sorbitol, DBS), plasticizer (poly(ethylene glycol), PEG) and long-chain branching in PLA. It is found that premade DBS/PEG gel can act as an active nucleating agent of PLA, which makes the crystallization peak appear during cooling. The preparation of DBS/PEG gel before mixing with PLA is important because self-assembly of DBS directly in PLA melt is difficult even in the presence of PEG. The mixing temperature is also found to be critical, which determines the amount of residual nanofibrils after melt mixing. Tuning the chain structure into long-chain branching via multifunctional monomer pentaerythritol triacrylate (PETA) will further speed up the crystallization of PLA because of the additional interaction between DBS nanofibrils and the grafted monomers. It is proven that the acceleration of crystallization is not ascribed to the change of crystal form but is due to the dominating increase in the nucleation density as well as the faster growth rate of spherulites in the presence of the plasticizer. Therefore, the problems of low melt strength and slow crystallization of PLA can be solved simultaneously via the present approach.

Strong enhancements of nucleation and spherulitic growth rates through amplified interfacial effects for immiscible linear polymer/comb-like copolymer double-layer films

Cellulose-graft-poly[poly(ethylene glycol) methyl ether acrylate] (Cell-g-PPEGA) and poly[poly(ethylene glycol) methyl ether acrylate] (PPEGA) comb-like copolymers were synthesized by electron transfer atom transfer radical polymerization (ARGET ATRP). PPEGA/PLA and Cell-g-PPEGA/PLA pairs are both immiscible as indicated by slightly decreasing glass transition temperatures of PLA in the blends measured by differential scanning calorimetry (DSC) and phase separation morphologies observed by a phase contrast optical microscope (PCOM). Cell-g-PPEGA/PLA and PPEGA/PLA double-layer films and neat PLA film were prepared to measure the nucleation and spherulitic growth rates during isothermal crystallization at various temperatures above the melting points of Cell-g-PPEGA and PPEGA layers by using a polarized optical microscope (POM). In contrast to our previous results on the miscible polymer pairs, covering the immiscible molten PPEGA layer can greatly accelerate the spherulitic growth rates for PLA and the Cell-g-PPEGA layer shows a similar but less significant effect. Nevertheless, nucleation density is much higher for the Cell-g-PPEGA/PLA film than for the PPEGA/PLA and neat PLA films. A significant finding is that although the whole double-layer film systems represent phase separated ones, the densely grafted PEGA chains in the comb-like copolymers can simultaneously take action to amplify the segmental mobility of PLA chains through local contacts at interfacial layers between phase separated domains, which significantly enhances the formation of chain folding lamellae of the PLA, resulting in obvious enhancements of the crystallization kinetics.

Studies on crystallization kinetics of bimodal long chain branched polylactides

To investigate the effects of long chain branching (LCB) on the crystallization kinetics of polylactides (PLA), a series of long chain branched PLA samples have been prepared, which have been proven to be bimodal of linear PLA and long chain branched PLA with different branching degrees. The crystallization kinetics of these LCB PLA samples have been investigated using polarized optical microscopy (POM) and differential scanning calorimetry (DSC). The POM results show that the spherulitic growth rates of the LCB PLA samples are lower than that of the linear PLA precursor and the PLA spherulitic growth rate decreases with increasing branching degree at each isothermal crystallization temperature. In contrast, the nucleation density increases with increasing branching degree. The crystallization kinetics from the POM observation are analyzed using the Hoffman–Lauritzen theory. In the studied temperature range, both linear PLA and LCB PLA samples crystallize according to the Regime II mechanism. The nucleation constant (Kg) and fold surface free energy (σe) decrease with increasing branching degree, suggesting that the LCB PLA samples have lower free energy barriers for nucleation than the linear PLA precursor. Analysis of the DSC data by the Avrami equation indicates that the crystallization of both the linear PLA and LCB PLA samples follows a three-dimensional crystal growth. The half crystallization time reduces with increasing branching degree and the overall crystallization rates for the LCB PLA samples are higher than that of linear PLA.

Partially bio-based thermoplastic elastomers by physical blending of poly(hydroxyalkanoate)s and poly(ethylene-co-vinyl acetate)

Partially bio-based thermoplastic elastomers (bio-TPE) were designed and prepared by physical blending a com- mercial grade poly(hydroxyalkanoate)s (PHBM, Metabolix) and poly(ethylene-co-vinyl acetate) (EVA). The PHBM is mis- cible with EVA90 which has a vinyl acetate (VA) content of 90 wt% while it is not miscible with EVA at low VA content (!70 wt%). The PHBM/EVA90 blends exhibit high tensile strength and typical thermoplastic elastomeric characteristics e.g. high elongation at break (>800%), good strain-recovery (>60%) and melt processability. The spherulite growth rate of PHBM decreases with increasing EVA90 content. Consequently, a large number of fine PHBM spherulites were formed in the blends. The spherulites act as physical crosslink-points leading to a thermoreversable network in the blends. Such network and elastic EVA90 molecules result in the thermoplastic and elastomeric characteristics of the PHBM/EVA90 blends.

Influence of Thermally Reduced Graphene Low-Loadings on the Crystallization Behavior and Morphology of Biodegradable Poly(ethylene succinate)

Through a solution and coagulation method, biodegradable poly(ethylene succinate) (PES) and thermally reduced graphene (TRG) nanocomposites were prepared at low TRG loadings. The nonisothermal melt crystallization peak temperature values and overall isothermal melt crystallization rates are greater in the nanocomposites than in neat PES, indicative of a nucleating agent effect of TRG; however, the crystallization mechanism of PES remains unchanged in the nanocomposites, regardless of TRG loading and crystallization temperature. The nonisothermal and melt crystallization processes of the nanocomposites are found to vary with the TRG loading, exhibiting a maximum at 0.25 wt % TRG loading. The incorporation of a small amount of TRG shows little influence on the spherulitic growth rates but obviously affects the spherulites nucleation density values of the nanocomposites. TRG does not modify the crystal structure of PES in the nanocomposites.

Crystallization of Poly(vinylidene fluoride) in Blends with Poly(methyl methacrylate‐co‐methacrylic acid) Copolymers

A series of poly(methyl methacrylate‐co‐methacrylic acid) (PMMA‐co‐MAA) random copolymers ranging in MAA content from 0–15 mol% is synthesized and blended with poly(vinylidene fluoride) (PVDF). Using infrared spectroscopy, it is observed that the absorption bands attributed to hydrogen‐bonded carbonyl groups increase in intensity as the amount of MAA in the copolymer increases. In DSC analysis, the crystallization temperature of the PVDF in the blend initially decreases by ca. 12 °C with MAA contents ranging from 0 to 5.5 mol%; however, a PVDF blend with a 15 mol% MAA copolymer has a crystallization temperature that is only ca. 3 °C below that of pure PVDF. Similarly, spherulitic growth rate analysis initially shows a decrease in radial growth rate for PVDF in blends with PMMA‐co‐MAA copolymers containing less than 5.5 mol% MAA. At higher MAA copolymer contents, the spherulitic growth rate approaches that of pure PVDF. It is concluded that the presence of the MAA comono­mer in the PMMA‐co‐MAA copolymer initially (<5.5 mol% MAA) increases the intermolecular interactions between the copoly­mer and the PVDF. However, as the MAA content of the copolymer rises above 5.5 mol%, intramolecular hydrogen bonding interactions within the PMMA‐co‐MAA copolymer cause the copoly­mer to be less compatible with PVDF. image

Transition of spherulite morphology in a crystalline/crystalline binary blend of biodegradable microbial polyesters

Crystalline/crystalline binary blend films of microbial polyesters composed of poly[(R)-3-hydroxybutyrate-co-(R)-3hydroxyhexanoate] (P(3HB-co-3HH)) and poly[(R)-3-hydroxybutyrate] (P(3HB)) that exhibit a morphological change are prepared by solvent casting. Differential scanning calorimetry measurements indicate that P(3HB-co-3HH) and P(3HB) are miscible for all blend ratios because a single glass transition temperature is observed. Polarization optical microscopy is used to investigate the transition of spherulite morphology and measure the radial growth rate of spherulites in the blend films. P(3HB-co-3HH) and P(3HB) contain positive spherulites, whereas in the binary blends, spherulite morphology changes from positive to negative. This change is related to the different growth rates of P(3HB-co-3HH) and P(3HB) lamellar crystals. Partial enzymatic degradation of the film surfaces reveals that the lamellar crystals of negative spherulites are oriented both perpendicular and parallel to the radial direction of spherulites. A new growth mechanism for spherulites in crystalline/crystalline blends is constructed from the results obtained for the blend films.

Miscibility and crystallization behavior of biodegradable poly(butylene succinate-co-butylene carbonate) and tannic acid blends

Biodegradable poly(butylene succinate-co-butylene carbonate) (PBSC) and tannic acid (TA) blends were prepared via a solution and casting method in this work. Miscibility and crystallization behavior of PBSC/TA blends were investigated with various techniques. PBSC is miscible with TA as evidenced by both a single composition dependent glass transition temperature and the depression of equilibrium melting point of PBSC in the blends. Relative to neat PBSC, blending with TA does not modify the crystal structure of PBSC in the blends. The nonisothermal melt crystallization of PBSC was retarded by TA in the blends. Both the overall isothermal melt crystallization rates and spherulitic growth rates of PBSC/TA blends show the similar variation trends, which become slower with increasing crystallization temperature and the TA composition. The crystallization mechanism of PBSC does not change, regardless of crystallization temperature and the TA composition in the blends.

Regulation of Phase Separation in PP/EPR In-Reactor Alloy and Its Effect on Crystallization Kinetics

The low molecular weight ethylene–propylene random copolymer (LEPR) in a polypropylene (PP)/EPR in-reactor alloy is replaced by high molecular weight EPR (HEPR). It is found that the phase separation rate of PP/HEPR alloy is much slower than that of PP/LEPR alloy. The effect of phase separation on the crystallization kinetics of PP/HEPR alloy is studied and compared with that of PP/LEPR alloy. The pre-phase-separated PP/HEPR alloy exhibits a faster spherulitic growth rate and overall crystallization rate, as compared with the directly quenched one. This is contrary to the PP/LEPR alloy and is interpreted in terms of the relative rate of crystallization and phase separation based on phase diagram. It is also observed that directly quenched PP/HEPR alloy has a larger long period after crystallization. This can be attributed to more inclusion of EPR-rich phase into inter-lamellae, which is advantageous to the mechanical properties.

Non‐Radial Growth of Helical Homopolymer Crystals: Breaking the Paradigm of the Polymer Spherulite Microstructure

Radial symmetry is essential for the conventional view of the polymer spherulite microstructure. Typically it is assumed that, in the course of the spherulite morphogenesis, the lamellar crystals grow radially. Using submicron X-ray diffraction, it is shown that in banded spherulites of poly(propylene adipate) the crystals have the shape of a helix with flat-on crystals winding around a virtual cylinder of about 6 µm in diameter. The helix angle of 30° implies that the crystal growth direction is tilted away from the spherulite radius by this angle. The implications of the helical crystal shape contradict the paradigm of the spherulitic microstructure. The radial growth rate of such spherulites does not correspond to the crystal growth rate, but to the propagation rate of the virtual cylinder the ribbons wind around.

Hydrogen-bonding induced change of crystallization behavior of poly(butylene succinate) in its mixtures with bisphenol A

In the present work, the blend of poly(butylene succinate) (PBS) and bisphenol A (BPA) was prepared by solution mixing, and the intermolecular interactions between the two components were characterized by a combination of nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR). The results showed that intermolecular hydrogen-bonding forms between the carbonyl group of PBS and phenol hydroxyl of BPA. With the increase of BPA content, more hydrogen bonds were formed. The effect of hydrogen bonding on the crystallization behavior of PBS was investigated by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The results showed that the overall isothermal crystallization kinetics and the spherulite growth rate of PBS decrease with the increase of BPA content, while the PBS spherulite size increases with BPA content.

Complex phase behavior and state of miscibility in Poly(ethylene glycol)/Poly(l‐lactide‐co‐ε‐caprolactone) Blends

ABSTRACTA miscibility and phase behavior study was conducted on poly(ethylene glycol) (PEG)/poly(l‐lactide‐ε‐caprolactone) (PLA‐co‐CL) blends. A single glass transition evolution was determined by differential scanning calorimetry initially suggesting a miscible system; however, the unusual Tg bias and subsequent morphological study conducted by polarized light optical microscopy (PLOM) and atomic force microscopy (AFM) evidenced a phase separated system for the whole range of blend compositions. PEG spherulites were found in all blends except for the PEG/PLA‐co‐CL 20/80 composition, with no interference of the comonomer in the melting point of PEG (Tm = 64 °C) and only a small one in crystallinity fraction (Xc = 80% vs. 70%). However, a clear continuous decrease in PEG spherulites growth rate (G) with increasing PLA‐co‐CL content was determined in the blends isothermally crystallized at 37 °C, G being 37 µm/min for the neat PEG and 12 µm/min for the 20 wt % PLA‐co‐CL blend. The kinetics interference in crystal growth rate of PEG suggests a diluting effect of the PLA‐co‐CL in the blends; further, PLOM and AFM provided unequivocal evidence of the interfering effect of PLA‐co‐CL on PEG crystal morphology, demonstrating imperfect crystallization in blends with interfibrillar location of the diluting amorphous component. Significantly, AFM images provided also evidence of amorphous phase separation between PEG and PLA‐co‐CL. A true Tg vs. composition diagram is proposed on the basis of the AFM analysis for phase separated PEG/PLA‐co‐CL blends revealing the existence of a second PLA‐co‐CL rich phase. According to the partial miscibility established by AFM analysis, PEG and PLA‐co‐CL rich phases, depending on blend composition, contain respectively an amount of the minority component leading to a system presenting, for every composition, two Tg's that are different of those of pure components. © 2013 Wiley Periodicals, Inc. J. Polym. Sci. Part B: Polym. Phys. 2014, 52, 111–121

Influence of chain structure on crystal polymorphism of poly(lactic acid). Part 1: effect of optical purity of the monomer

The influence of chain structure on crystal polymorphism of poly(lactic acid) (PLA) with high l-lactic acid content (97.8–100 %) is detailed in this contribution. Upon usual processing conditions of PLA, only α and α′ crystals grow, which makes these two polymorphs of major interest for research. The two crystal modifications have similar chain packing, which complicates their quantitative analysis by diffraction methods. The two crystal modifications are instead easily identified by analysis of the crystallization kinetics, which varies not only with temperature, but also with crystal polymorphism. The dependence of the rate of ordering on temperature shows two distinct maxima around 105–110 and 120–125 °C, which are related to growth of α′ and α crystals, respectively. Addition of d-lactic acid co-units leads to a decrease of the overall crystallization rate of PLA, as well as of the rate of spherulite growth (G) of both the crystal modifications. The relative crystallization rates of α and α′ forms are highly affected by stereoregularity, especially in the PLA grades that have a high crystallization rate. A high d-lactic acid content results not only in an overall slower crystal growth, but also in a varied temperature range where each of the two crystal modifications prevail, with a shift to lower temperatures of both the maxima of the G vs. temperature plots, indicating that inclusion of d-lactic acid units in the PLA chain affects crystallization rate of both α and α′ crystal modifications.

Crystallization Regimes and Spherulitic Morphology of Poly(trimethylene terephthalate/isophthalate) Random Copolyesters

The crystallization regimes and spherulitic morphology of poly (trimethylene terephthalate/isophthalate) (TI) random copolyesters were studied in this work. A hot stage polarizing microscope was used to observe morphology and measure growth rate of the spherulite of TI copolyesters. Lauritzen-Hoffman secondary nucleation theory was applied to investigate crystallization kinetics and regimes of TI copolyesters. Results show that the spherulitic growth rate decreased with an increase in isophthalic acid content of the copolyesters. The analysis of Lauritzen-Hoffman secondary nucleation theory shows that regime I, II, and III were observed in TI11 and TI21. Regime III was observed in TI29. The spherulitic morphology of TI11, TI21, and TI29 changed from elliptical-shaped spherulite to irregular spherulite and then to normal spherulite at the regime transitions.

Crystallization kinetics of montmorillonite/poly(vinylidene fluoride) composites and its correlation with the crystalline polymer phase formation

Isothermal crystallization of poly(vinylidene fluoride), PVDF, was studied in the presence of montmorillonite, MMT, fillers by polarization optical microscopy, POM. The spherulite growth was monitored as a function of time for different crystallization temperatures and the crystallization kinetics was evaluated with the help of the Avrami theory. The incorporation of MMT fillers is responsible for a reduction of the crystallization time of the polymer based on an increase of the nucleation and a reduction of the spherulite growth rate, the nucleation effect dominating therefore the overall crystallization kinetics. The slowing of the spherulitic growth rate by the presence of the clays due to the clay–polymer interactions is responsible for development of the γ crystalline phase of PVDF in the composites.

Evolution of concentric spherulites in crystalline-crystalline poly(3-hydroxybutyrate-co-3-hydroxyvalerate)-b-poly(ethylene glycol) copolymers

Crystalline-crystalline poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and poly (ethylene glycol) (PEG) block copolymers (PHEGs) were synthesized by telechelic hydroxylated PHBV2000 and PEG with different number-average molecular weights. The synthesized PHEGs were analyzed using gel permeation chromatography and proton nuclear magnetic resonance. The cooling curves of the differential scanning calorimetry showed that the range of the melt-crystallization temperature of the PEG and PHBV blocks in the PHEGs partially overlapped. The spherulite of each PEG block and each PHBV block in the PHEGs crystallizes individually, but nucleated in the same site to form a concentric spherulite. The observations of the hot-stage polarized microscope (HSPM) showed that the first spherulite growth acted as a template for the later spherulite growth in the PHBV-b-PEG concentric spherulite. The spherulite growth rate of individual spherulite from the PEG block and PHBV block in PHEGs depends on the crystallization environment. The evolution of concentric spherulites in PHEGs at different crystallization conditions was studied in this study.

Increased spherulitic growth rates of semicrystalline polymer due to enhanced limited local chain segmental mobility at the interface of double-layer thin films

In this work the nucleation and growth of spherulites for the below polylactide (PLA) layer in poly(ɛ-caprolactone)/polylactide (PCL/PLA) double-layer films during isothermal crystallization at various temperatures above the melting point of PCL have been investigated by using polarized optical microscopy (POM). It is revealed that two types of spherulitic morphologies are observed in PCL/PLA double-layer films. One is the well defined highly birefringent spherulites, and the other one is the coarse spherulites. It is interesting to find that the spherulitic growth rate of the coarse spherulites is higher than that of the well defined spherulites. It is thought that the coarse spherulites nucleate and grow with the assistance of the interfaces between the PCL and PLA layers, and the well defined highly birefringent spherulites only nucleate and grow in the PLA layer.

Crystallization Kinetics and Morphology of Novel Miscible Crystalline/Amorphous Polymer Blends of Biodegradable Poly(butylene succinate-co-butylene carbonate) and Poly(vinyl phenol)

Biodegradable poly(butylene succinate-co-butylene carbonate) (PBSC) and poly(vinyl phenol) (PVPh) may form novel miscible crystalline/amorphous polymer blends that exhibit a single composition-dependent glass transition temperature and a depression of equilibrium melting point of PBSC in the blends. Relative to neat PBSC, blending with PVPh has not only suppressed the nonisothermal melt crystallization behavior of PBSC at the same cooling rate but also reduced both the overall isothermal melt crystallization rates and spherulitic growth rates of PBSC in the blends at the same crystallization temperature; however, the crystallization mechanism does not change for either neat PBSC or the PBSC/PVPh blends. Neat PBSC and its blends show the same crystal structure. The apparent increase in both long period and amorphous layer thickness values indicates that amorphous PVPh resides in the interlamellar region of PBSC spherulites in the blends.

Morphology and properties of renewable poly(3-hydroxybutyrate-co -3-hydroxyvalerate) blends with thermoplastic polyurethane

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)/thermoplastic polyurethane (PHBV/TPU) composites were prepared by solution blending combined with melt processing in an effort to improve the toughness of PHBV and overcome the coalescence phenomena of TPU at the same time. The compatibility, cross-section morphology, crystal structure, thermal behaviour, and mechanical properties of composites were investigated. Field emission scanning electron microscopy results showed that no distinct interface was found in composites, uniformly dispersed PHBV/TPU composites were obtained. The incorporation of TPU did not change the basic crystalline structure of PHBV, but decreased its crystallinity. What’s more, the integrity of PHBV spherulites structure was destroyed and the radial growth rate of spherulites was inhibited. Differential scanning calorimetry results analyzed by Jeziorny method indicated that the nucleation mode of composites did not change. Besides, when TPU content was up to 40 wt%, the initial degradation temperature of PHBV/TPU composite was increased by 5 � C, and the elongation-at-break was increased by 225% compared with those of PHBV. POLYM. ENG. SCI., 54:1113–1119, 2014. V C 2013 Society of Plastics Engineers

Poly(vinylidene fluoride)-acrylic rubber partially miscible blends: Crystallization within conjugated phases induce dual lamellar crystalline structure

The miscibility of melt mixed poly(vinylidene fluoride)(PVDF)/acrylic rubber(ACM) blends and the crystal morphology of PVDF in the blends over the whole composition range were investigated by dynamic mechanical analysis (DMA), optical microscopy(OM), Fourier transform infrared spectroscopy (FTIR), small angle X-ray scattering (SAXS), differential scanning calorimeter (DSC), and scanning electron microscopy (SEM). DMA analysis revealed that PVDF is partially miscible within the PVDF rich blends whereas it is fully miscible in the ACM rich blends. Phase diagram was obtained for α-PVDF/ACM blends using OM and DSC measurements. FTIR study showed that PVDF crystallizes in α form in the blends. Dendritic spherulites were observed for the miscible blends as well as for the ACM rich phase in partially miscible blends. Spherulite growth rate in miscible blends was analyzed by Lauritzen Hoffman equation. Blending with ACM increased free energy of folding for PVDF lamella. Lamellar thickness calculated for two conjugated phases in the partially miscible blends showed different values in agreement with the two peaks observed in SAXS profiles of partially miscible blends, suggesting dual lamellar crystal structure.