Faster Crystallization Rate Research Articles

Effects of Poly(D-lactide acid) on the Properties of Crystallization and Thermal Behavior of Poly(L-lactide acid)

In order to improve the heat deflection temperature and ability of crystallization of poly(L-lactic acid) (PLLA) based materials, Poly (D-lactic acid) (PDLA) was used to complex with PLLA to form a stereocomplex. The effects of annealed time and different amounts of poly(D-lactic acid) (PDLA) on the crystallization, thermal behavior and mechanical properties of poly(L-lactic acid) (PLLA) were investigated by differential scanning calorimeter (DSC), X-ray diffraction (XRD), polarized optical microscopy (POM) ,et al. The results indicated that the PDLA was an effective nucleating agent for PLLA. A higher density of the stereocomplex crystallites was consistent with the increased content of the PDLA and annealed time. Samples that PLLA complexed with PDLA have the fastest crystallization rate at 393 K. The complexed materials have high heat deflection temperature (HDT), high Tm, high crystallinity and crystallization rate. With the annealed time and PDLA content in PLLA matrix increased, the tensile strength, modulus and elongation-at-break of the PLLA increased.

Rheological investigation of shear-enhanced crystallization for polylactide composites with different aspect ratio carbon nanotubes

Abstract Crystallization processes of polylactide (PLA)/acid oxidized multiwalled carbon nanotube (A-MWCNT) composites were detected by dynamic rheological measurements. The influences of pre-shear, crystallization temperature, concentration and aspect ratio of A-MWCNT, on the crystallization kinetics of PLA, were investigated. The addition of small amounts of A-MWCNT, can more or less accelerate the crystallization of the PLA matrix. Composites with smaller aspect ratio A-MWCNT (A-MWCNT–S) show faster crystallization rates than those with larger aspect ratio A-MWCNT (A-MWCNT–L), due to the more heterogeneous nucleation sites of the former. Pre-shear can significantly promote the crystallization process of PLA composites, and with the increase in shear rate, the crystallization rate becomes faster. Pre-shear results in an accelerated crystallization compared to PLA/A-MWCNT–S composites at the same concentration, without pre-shear, due to a more serious deformation from entanglement to orientation of A-MWCNT–L.

Preparation of Fully Biodegradable Poly(L-lactide) and Poly(ethylene succinate) Blend and its Crystallization Property

Both poly(L-lactide) (PLLA) and poly(ethylene succinate) (PES) are biodegradable synthetic aliphatic polyesters with excellent biocompatibility. They are semicrystalline thermoplastic polymers processed by most conventional processing methods. In view of their complementary properties, blending PLLA with PES becomes a promising approach to improve the properties of PLLA without compromising its biodegradability. In the present work, fully biodegradable PLLA/PES blend(the weight ratio 90/10) was melt-blended in stainless steel chamber. The crystallization morphology was investigated using a polarized optical microscope (POM), showing more clear spherulites with typical crosslink extinction patterns and faster crystallization rate of PLLA in blend at the lower heating rate. It clearly showed that the addition of PES largely increased the crystallization rate of PLLA and improved the crystallinity of PLLA in blends.

Effect of compounding procedure on morphology and crystallization behavior of isotactic polypropylene/high‐density polyethylene/carbon black ternary composites

The effect of compounding procedure on morphology and crystallization behavior of isotactic polypropylene/high‐density polyethylene/carbon black (iPP/HDPE/CB) composite was investigated. iPP/HDPE/CB composites were prepared by four compounding procedures (A: iPP + HDPE + CB; B: iPP/HDPE + CB; C: HDPE/CB + iPP; D: iPP/CB + HDPE). Scanning electron microscopy observation showed that CB particles are mainly distributed in HDPE in all composites, and the phase morphology of composites was obviously affected by a compounding procedure. The size of the HDPE/CB domains in the composites prepared by procedures A and D decreased with the increase in CB content, whereas that of HDPE/CB in the composites prepared by procedures B and C rarely changed with the increase in CB content. The crystallization behaviors of the composites were significantly affected by their phase morphology, which resulted from the variation of compounding procedure. The isothermal crystallization rate of iPP in the composites prepared by procedures A and D was obviously increased, which may originate from the small HDPE/CB droplets dispersed in the iPP phase. The non‐isothermal crystallization curves of composites prepared by procedure D represented two peaks because the iPP component in these composites had the fastest crystallization rate, whereas the curves of composites prepared by other compounding sequences only exhibited one peak. Moreover, the crystallinity of HDPE almost increased by one time with the incorporation of only 1 phr CB because the CB particles selectively located in the HDPE phase, and the crystallinity of HDPE decreased with the further increase of CB content because of the strong restriction of CB on the HDPE chains. Copyright © 2011 John Wiley & Sons, Ltd.

Effect of tetra-needle-shaped zinc oxide whisker (T-ZnOw) on mechanical properties and crystallization behavior of isotactic polypropylene

Isotactic polypropylene (iPP) composites with unmodified and modified tetra-needle-shaped zinc oxide whisker (T-ZnOw) were prepared via melt blending in a twin-screw extruder. The mechanical properties, thermal properties, morphology and crystallization behavior were investigated. The mechanical properties of iPP with T-ZnOw modified by silane based coupling agent were much better than that of iPP with unmodified T-ZnOw. The reason was that the interfacial interaction between T-ZnOw and iPP matrix became stronger when T-ZnOw was modified by coupling agent. Besides, the addition of T-ZnOw improved thermal properties of iPP, including heat deflection temperature and thermal degradation temperature. Furthermore, wide angle X-ray diffraction (WAXD) results showed that T-ZnOw can be used as a novel β-nucleating agent, and the relative content of β-crystal was 0.38 when T-ZnOw content was 5 wt.%. iPP/T-ZnOw composites were found to exhibit higher crystallization temperature, smaller spherulites, and faster crystallization rate than those of pure iPP, indicating that T-ZnOw played a role of nucleating agent on the crystallization of iPP and could induce accelerated crystallization.

Improvement of thermal properties of polyhydroxybutyrate by grafted chemicals

AbstractThe crystallization and thermal degradation behaviors of polyhydroxybutyrate (PHB) grafted with maleic acid (MAc) and exo-3,6-epoxy-1,2,3,6-tetrahydrophthalic anhydride (ETA) are analyzed with differential scanning calorimetry (DSC), dynamic thermogravimetric analysis (TGA), and gel permeation chromatography (GPC). It is obtained that the PHB grafted with MAc or ETA has a faster crystallization rate, larger crystallinity, and better thermal stability than the as-received PHB. The thermal stability of specimens are in the order of the as-received PHB < PHB grafted with ETA < PHB grafted with MAc. The grafting ratio of MAc is larger than that of ETA. The thermal stability of PHB can be greatly improved by grafting a small amount of MAc. The activation energy of the PHB degradation reaction are little affected by the grafted chemicals, nonetheless, the reaction order and frequency factor are significantly affected by the grafted MAc.

Effect of annealing on the crystallization and properties of electrospun polylatic acid and nylon 6 fibers

AbstractElectrospinning experiments for solutions of two polymers with different crystallization rates, polylatic acid (PLA), and nylon 6 solutions have been carried out. Hexafluoroisopropanol (HFIP) was used as a common solvent to dissolve both PLA and nylon 6. Using the fact that nylon 6 has the faster crystallization rate than PLA, the effect of thermal annealing conditions on the crystal structures of two electrospun fibers has mainly been studied via thermal and mechanical analysis. First, optimal conditions for the formation of uniform nanofibers have been determined by observing their morphology and rheological properties. Thermal analysis revealed that PLA as‐spun fiber exhibits cold crystallization due to insufficient crystal growth in spinline. Also, the effect of thermal annealing on the structural change of nylon 6 fibers including the degree of flow‐induced crystallization is totally different from that of PLA fibers. Finally, tensile measurements of annealed fibers show that the ultimate tensile strength and Young's modulus gradually increases with increasing annealing time for both PLA and nylon 6 mats, resulting from the further growth of crystal structures. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Non-isothermal crystallization kinetics and thermal stability of the in situ reinforcing composite films based on thermotropic liquid crystalline polymer and polypropylene

The non-isothermal crystallization kinetics and thermal stability of the in situ reinforcing composite thin film, comprising of 10 wt% of thermotropic liquid crystalline polymer (TLCP, Rodrun LC5000) and polypropylene (PP), prepared by a two-step method were investigated using differential scanning calorimetry (DSC) and thermogravimetry (TG), respectively. The DSC results revealed that Mo method was suitable for the crystallization behavior description of both neat PP and 10 wt%TLCP/PP films. The in situ formation of TLCP fibrils in the PP matrix led to reduction in both values of half-time of crystallization (t 1/2) and the kinetic parameter of Mo equation F(T), resulting in a significant increase in the crystallization rate of PP phase. The remarkable lower in crystallization activation energy of the composite films also confirmed that in situ formed TLCP fibrils could influence the molecular chain of PP easier to crystallize, and hence resulted in the faster crystallization rate. The nucleation activity value of composite indicated that TLCP fibrils acted as effective nucleating agents. From TG results and the higher decomposition activation energy, the thermal stability of the composite can be improved by the presence of in situ-formed TLCP fibrils.

A novel initiating system for ring‐opening polymerization of ε‐caprolactone: Synthesis of triarm star‐shaped poly(ε‐caprolactone)

AbstractThe rare earth compound, scandium trifluoromethanesulfonate [Sc(OTf)3], has been used as a water‐tolerant catalyst for the synthesis of star‐shaped poly(ε‐caprolactone)s (SPCLs) with trimethylol propane as trifunctional initiator in solvent at 40°C. Triarm SPCLs have been successfully prepared. The molar mass of SPCLs were determined by end‐group 1H NMR analyses, which could be well controlled by the molar ratio of the monomer to the initiator, and were independent of the amount of Sc(OTf)3 used. Differential scanning calorimetry analyses suggested that the maximal melting point, the cold crystallization temperature, and the degree of crystallinities of SPCLs increased with the increasing of the molar mass and were lower than the linear poly(ε‐caprolactone) (LPCL) with similar molar mass. Furthermore, polarized optical microscopy indicated that LPCL showed fast crystallization rate and good spherulitic morphology with apparent Maltese cross pattern, whereas SPCLs exhibit much lower crystallization rate and poor spherulitic morphology. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Studies on the isothermal crystallization kinetics and crystal morphology of PTT/SGF composites

AbstractThe kinetics of isothermal crystallization and crystal morphology of poly(trimethylene terephthalate)/short glass fibers (PTT/SGF) composites were investigated by using differential scanning calorimetry (DSC) and polarized optical microscopy (POM). DSC data was analyzed by the Avrami equation and Hoffman‐Lauritzen theory. The results show that SGF plays a role as the nucleating agent, which largely accelerates the crystallization rate of PTT. SGF below 20% will increase the crystallinity of the composites but 30% SGF will decrease the crystallinity, which is also verified by the crystallinity results calculated from the wide‐angle x‐ray diffraction (WAXD) experiments. The crystal morphology of the composites exhibits so much microcrystallites because of the fast crystallization rate and the strong interaction between SGF and polymers, which is consistent with the results analysized by the Avrami theory. The nuclei exponent Kg is increased sharply as the SGF added into polymer, and SGF makes PTT easier to crystallize during isothermal crystallization process. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers

Synthesis and comparative study of biodegradable poly(alkylene sebacate)s

AbstractNovel biodegradable polyesters, such as poly(ethylene sebacate) (PESeb), poly(propylene sebacate) (PPSeb), and poly(butylene sebacate) (PBSeb), were synthesized and studied with respect to melting behavior, crystallization kinetics, and enzymatic hydrolysis. PESeb and PPSeb showed multiple melting behavior. Wide angle X‐ray diffractometry measurements at various temperatures, standard, step‐scan, and high‐rate differential scanning calorimetry methods were applied to elucidate the appearance of multiple endotherms in heating scans, which was interpreted in the context of partial melting‐recrystallization and final melting. PBSeb did not show any multiple melting behavior but only a weak tendency for recrystallization on heating. The melting temperatures of PESeb, PPSeb, and PBSeb were measured equal to 78, 57, and 71 °C, respectively. The equilibrium melting points were estimated to be Tm° = 90.2, 69.9, and 77.4 °C for PESeb, PPSeb, and PBSeb, while the corresponding enthalpy of fusion values were found to be ΔHf = 170 ± 10, 140 ± 10, and 155 ± 10 J/g, respectively. The polyesters showed fast crystallization rates under both isothermal and nonisothermal conditions. Crystallization kinetics was thoroughly investigated using macrokinetic models and isoconversional analysis. Enzymatic hydrolysis rate in the presence of lipases Rhizopus delemar and Pseudomonas cepacia was found to be fast for PPSeb, whereas PESeb and PBSeb showed slow rates and comparable with those of PCL. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 672–686, 2010

Dispersion and crystallization studies of hyper-branched poly(urea-urethane)s-grafted carbon nanotubes filled polyamide-6 nanocomposites

Multiwalled carbon nanotubes (MWNTs) are functionalized by hyper-branched poly(urea-urethane)s (HPUs) through a polycondensation technique (MWNTs-HPUs). MWNTs-HPUs filled polyamide-6 (PA6) nanocomposites are prepared by melt compounding. MWNTs-HPUs is observed to disperse uniformly in the PA6 matrix, because the grafting of HPUs improves the compatibility between MWNTs-HPUs and PA6. Dynamic mechanical properties of PA6 are effectively modified after the addition of MWNTs-HPUs. The improved dispersion of MWNT-HPUs in PA6 results in the more efficient load-transfer from polymer to MWNTs. In addition, because of the hydrogen-bond between hydroxyl groups on the surface of MWNTs-HPUs and carbonyl groups on the PA6 chains, the interfacial adhesion of PA6 and MWNT is further improved. Compared with PA6/MWNTs nanocomposite, PA6/MWNTs-HPUs nanocomposite has lower crystallization temperature, faster crystallization rate and higher crystalline degree. This is because the grafted HPUs change the surface of MWNTs, which weakens the effect of MWNTs on the crystallization of PA6.

Nucleation effect of cyclodextrin inclusion complexes on the crystallization of isotactic poly(1‐butene)

AbstractThe β‐cyclodextrin (β‐CD) and γ‐cyclodextrin (γ‐CD) inclusion complexes (ICs) with four kinds of polyolefin were prepared. The crystallization behavior of isotactic poly(1‐butene) (iPB‐1) blended with these CDs and ICs was investigated by differential scanning calorimetry, polarized optical microscopy, and wide‐angle X‐ray diffraction. The iPB‐1 blended with the ICs was found to exhibit higher crystallization temperature (TC), smaller spherulites, and faster crystallization rate than neat iPB‐1. These results indicate that the ICs can act as nucleating agent on the crystallization of iPB‐1 and induce the accelerated crystallization. The guest molecules of ICs play an important role in the nucleation effect of ICs on the crystallization of iPB‐1. ICs with polyolefin having higher TC as guest molecules have higher nucleation effect than the one with polyolefin having lower TC as guest molecules. And, the CDs and ICs induce different crystal form of iPB‐1. The crystal of iPB‐1 blended with CDs is defective, whereas the crystal of iPB‐1 blended with ICs is more perfect. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 389–395, 2010

Relationship between free volume properties and structure of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) membranes via various crystallization conditions

The relationship between the crystalline structure and the free volume properties within the amorphous phase of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) membranes, which were prepared via isothermal crystallization processes at various crystallization temperatures, was investigated. From the data of positron annihilation lifetime (PAL) spectroscopy over a temperature range of 25–90°C, the temperature dependence of free volume size, amount, distribution, and relative fractional free volume and thermal expansion behaviors of free volume were discussed. The existence of a rigid-amorphous fraction (RAF) which constrained long-range motions in the semi-crystalline polymer was demonstrated by DSC trace and considered to interpret the temperature dependence of those free volume properties. Based on the crystallization conditions, the effect of the crystallization rate of PHBV polymer was proposed to explain the thermal expansion coefficients of free volume size. The membranes crystallized at faster crystallization rate would accompany higher thermal expansion coefficients of free volume size and the larger free volume size at higher temperatures. Larger distribution of the free volume size and higher thermal expansivity of o-Ps lifetime dispersion (dσ/dt) of isothermally melt-crystallized membranes were observed as a result of the bimodal distribution of the lamellar periodicity, which was obtained by small-angle X-ray scattering (SAXS), and less amount of RAF than that of a cold-crystallized membrane. Conclusively, free volume size and thermal expansion of the free volume in PHBV membranes were affected by the kinetics of crystallization; comparatively, the total amount of free volume and fractional free volume were determined by the final crystallinity. The size distribution of free volume was associated with the crystalline lamellar structure which was dominated by the crystallization conditions.

Nucleation effect of inclusion complexes with different polyolefin as guest molecules on the crystallization of polypropylene

AbstractThe γ‐cyclodextrin (γ‐CD) inclusion complexes (ICs) with four kinds of polyolefin (PO) as guest molecules were prepared. The crystallization behavior of isotactic polypropylene (iPP) blended with the γ‐CD and γ‐CD–PO ICs was investigated by differential scanning calorimetry, polarized optical microscopy, and light scattering. The iPP blended with the ICs was found to exhibit higher crystallization temperature (TC), smaller spherulites, and faster crystallization rate than those of neat iPP, indicating that the ICs play a role of nucleating agent on the crystallization of iPP and induce accelerated crystallization. The IC with PO having higher TC as guest molecules showed higher nucleation effect than the IC with PO having lower TC as guest molecules. The results suggest that the nucleation effect of these ICs was affected by the kinds of the guest molecules. The higher TC guest molecules could result in higher nucleation effect. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Crystallization Behavior of Poly(Lactic Acid) Filled with Modified Carbon Black

A novel method was employed to modify the surface of carbon black (CB) by an organic small molecule in a Haake Rheomix mixer. The modified carbon black (MCB) was dispersed uniformly in poly(lactic acid; PLA). The crystallization behaviors of PLA, PLA/CB and PLA/MCB composites were investigated by differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS) and polarizing optical microscopy. It is found that the addition of CB or MCB can influence the crystallization behavior of PLA. PLA/MCB has a faster crystallization rate and higher crystallization peak temperature than PLA/CB. For non-isothermal studies, Jeziorny and Mo equations were employed. The Mo equation can well describe the non-isothermal crystallization of the three samples. For PLA/CB and PLA/MCB composites containing 3wt% fillers, the nucleating activity for CB is about 0.32, and about 0.16 for MCB. All these results show that MCB is an effective nucleating agent. PLA/MCB has a higher nucleation rate than PLA/CB because of the finer dispersed particles size and improved interaction between MCB and PLA.

Fabrication of FePt Nanoparticle Langmuir-Blodgett Films

Polycrystalline silicon thin-film transistors fabricated by metal induced lateral crystallization (MILC) have been demonstrated as promising devices for realizing electronics on large area, inexpensive glass substrates. However, it takes a relatively long time to crystallize amorphous silicon thin films. In this work, we proposed a new crystallization method using copper and nickel metals together and proved that this method could be used to fabricate thin-film transistors. It turned out that a four times faster crystallization rate could be obtained and that device performance was similar to conventional nickel MILC TFTs. This means that the proposed crystallization method is quite acceptable for practical use.

Melt-crystallization behavior and isothermal crystallization kinetics of crystalline/crystalline blends of poly(ethylene terephthalate)/poly(trimethylene terephthalate)

The melt-crystallization and isothermal melt-crystallization kinetics of poly(ethylene terephthalate)/poly(trimethylene terephthalate) blends (PET/PTT) were investigated by differential scanning calorimetry (DSC) and polarized optical microscopy. Although PET and PTT in the binary blends are miscible at amorphous state, they will crystallize individually when cooled from the melt. In the DSC measurements, PET component with higher supercooling degree will crystallize first, and then the crystallite of PET will be the nucleating agent for PTT, which induce the crystallization of PTT at higher temperature. On the other hand, in both blends of PET80/PTT20 and PET60/PTT40, the PET component will crystallize at higher temperature with faster crystallization rate due to the dilute effect of PTT. So the commingled minor addition of one component to another helps to improve the crystallization of the blends. For blends of PET20/PTT80 and PET40/PTT60, isothermal crystallization kinetics evaluated in terms of the Avrami equation suggest different crystallization mechanisms occurred. The more PET content in blends, the fast crystallization rate is. The Avrami exponent, n = 3, suggests a three-dimensional growth of the crystals in both blends, which is further demonstrated by the spherulites formed in all blends. The crystalline blends show multiple-melting peaks during heating process.

Dimensional stability of single‐site ethylene copolymers in rotational molding

AbstractThe dimensional stability of ethylene copolymers in rotational molding was studied by comparing the warpage observed for a series of conventional and single‐site catalyzed ethylene copolymers. Bench‐scale molding trials were carried out under controlled molding conditions. The rapid cooling of the mold using a water spray resulted in greater warpage. Under such conditions, molded parts made using the single‐site resins showed less warpage compared to the Ziegler‐catalyzed copolymers with otherwise comparable densities. The Ziegler‐catalyzed copolymers were characterized by a faster crystallization rate, and were shown to generate larger crystallinity gradients through the part thickness during the cooling process. Second to temperature gradients, crystallinity gradients are a leading cause for the development of residual stresses and causing warpage. Differences in the crystallization rates between single‐site and Ziegler‐catalyzed copolymers are discussed based on their intermolecular comonomer distributions. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers

Concentration Dependence of Crystalline Poly(L-lactide) Prepared by Freeze-drying Solutions

Poly(L-lactide) was freeze-dried from p-xylene solutions with varying concentrations. The samples were characterized by differential scanning calorimetry. It was found that the sample produced from the dilute solution exhibited higher non-isothermal crystallization temperature and faster crystallization rate. There was a critical concentration corresponding with the critical overlap concentration proposed by De Gennes in the polymer solutions. In the solution below the critical concentration, the PLLA chains were isolated from each other, resulting in an acceleration of melt crystallization for the recovered samples. It appears that the chain entanglement is a barrier to the melt crystallization of polymer.