Fold Free Energy Research Articles

Influence of isotactic polypropylene grafted with styryl-group on the polymer crystallization behavior

4-Furfuryloxymethylstyrene (Sf) grafted isotactic polypropylene (iPP-g-Sf) was prepared by melt grafting reaction. The crystallization temperature of iPP after grafting increased to ∼130 °C. The isothermal and non-isothermal crystallization kinetics of iPP-g-Sf containing up to 1.50 mol% grafted Sf groups were compared with that of virgin iPP, and the isothermal and non-isothermal crystallization kinetics parameters of the polymers, including the fold surface free energy σe, the chain folding work q, and the crystallization activation energy ΔE, were calculated by Lauritzen-Hoffman theory and Kissinger method respectively. The results showed that iPP-g-Sf exhibited a unique self-nucleation phenomenon, which accelerating the crystallization rate of iPP and produced tiny crystal morphology than that for iPP. The introduction of only 0.23–1.50 mol% grafted Sf groups reduced the σe of the crystals by 44–57%. This might provide guidance for the design of polymer crystallization rate by controlling the molecular structures.

Poly(maleic acid-co-acrylic acid) ionomer as nucleating agent on the crystallization behavior and properties of poly(ethylene terephthalate)

Polyethylene terephthalate (PET) generally suffers from low crystallization rate and long molding cycle, which limits its application in injection molding of engineering plastics. To overcome the specific shortcoming, polymaleic acid-co-acrylic acid (PMA-AA) ionomer used as a nucleating agent was added to PET by melt blending, and its effect on the crystallization behavior of PET was investigated in this paper. It is indicated that the modified PET samples (MPET) with the addition of nucleating agents exhibit much higher molecular order and faster crystallization rate. The nucleation mechanism of MPET induced by PMA-AA ionomer was analyzed by means of Avrami equation and Hoffman–Lauritzen theory, revealing that the addition of PMA-AA ionomer provides more nucleation sites and reduces the fold surface free energy of PET. The nucleation and dispersion behavior of PMA-AA ionomer were further observed by SEM and POM morphologies. The results demonstrated that ionomer was dispersed homogeneously in the PET matrix, thus leading to faster crystallization rate of MPET than neat PET. In addition, the thermal stability, mechanical properties, and gas barrier of PET and MPET were characterized.

Influence of hydrogen bonding on the crystallization behavior of poly(ethylene oxide)/ionic liquids mixtures

Two types of imidazolium-based ionic liquids (ILs) [BMIM][BF4] and [BMIM][NTf2] and their mixtures were blended with poly(ethylene oxide) (PEO) matrix to tune the strength of intermolecular hydrogen bonding in the system. With the addition of ILs, particularly increasing [BMIM][NTf2] ratio, significantly reduced overall crystallization kinetics in the order of pure PEO > P80B20N0 > P80B16N4 > P80B6N14 > P80B0N20 (P80BxNy represents the mixture containing 80 wt% PEO, x wt% [BMIM][BF4] and y wt% [BMIM][NTf2]) were observed, which is mainly dominated by the enhanced hydrogen bonding. The early stage crystallization is accompanied by the breaking of hydrogen bonds as evidenced by in situ FT-IR, and much slower wavenumber shift of C-O-C groups on PEO backbone in P80B0N20 compared to that of P80B20N0 further demonstrates that the stronger hydrogen bonding between PEO and [BMIM][NTf2] could significantly inhibit the nucleation process. The fold surface free energy estimated from Lauritzen-Hoffman model is about 1.4 times in P80B0N20 than that in P80B20N0 indicating the stronger hydrogen bonding could increase the energy penalty associating to the nucleation and crystal growth. From SAXS, it is seen that hydrogen bonding only functions to the amorphous layer La rather than the lamellar thickness, that is, stronger hydrogen bonding results in larger La and more imperfect lamellar structure.

Crystallization and properties of poly(ethylene terephthalate)/layered double hydroxide nanocomposites

Poly(ethylene terephthalate) (PET) generally suffers from low crystallization rate and long molding duration, which as a result limit its application as engineering plastics. To overcome these drawbacks, series of PET/layered double hydroxide (LDH) nanocomposites were prepared by a solution blending process. The effect of metal composition (MgAl and CaAl) and organo-modification (stearic acid intercalated) for LDH fillers on the crystallization behavior of the nanocomposites was investigated. It was revealed that, compared with PET/CaAl-LDH, the PET/MgAl-LDH nanocomposite exhibits a higher crystallization temperature and faster crystallization rate, which is associated with the superior nucleation ability of MgAl-LDH. The nucleation mechanism of PET induced by LDHs was explored by means of Avrami equation and theory of Hoffman-Lauritzen, pointing out that the incorporation of LDHs reduce the free energy of nucleation and the fold surface free energy of PET. In order to improve the compatibility between LDH and PET, stearic acid (SA) intercalated MgAl-LDH was prepared and filled into PET matrix. The resultant PET/MgAl-LDH-SA shows a further enhanced crystallization temperature and accelerated crystallization rate, in comparison with PET/MgAl-LDH nanocomposites. In addition, the thermal stability, gas barrier and mechanical properties of PET/LDH composites were improved upon incorporation of LDH fillers.

Effect of WS₂ Inorganic Nanotubes on Isothermal Crystallization Behavior and Kinetics of Poly(3-Hydroxybutyrate-co-3-hydroxyvalerate).

Nanocomposites of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and tungsten disulfide inorganic nanotubes (INT-WS2) were prepared by blending in solution, and the effects of INT-WS2 on the isothermal crystallization behavior and kinetics of PHBV were investigated for the first time. The isothermal crystallization process was studied in detail using various techniques, with emphasis on the role of INT-WS2 concentration. Differential scanning calorimetry (DSC) and polarized optical microscopy (POM) showed that, in the nucleation-controlled regime, crystallization rates of PHBV in the nanocomposites are influenced by the INT-WS2 loading. Our results demonstrated that low loadings of INT-WS2 (0.1–1.0 wt %) increased the crystallization rates of PHBV, reducing the fold surface free energy by up to 24%. This is ascribed to the high nucleation efficiency of INT-WS2 on the crystallization of PHBV. These observations facilitate a deeper understanding of the structure-property relationships in PHBV biopolymer nanocomposites and are useful for their practical applications.

Nucleation barrier, growth kinetics in ternary polymer blend filled with preferentially distributed carbon nanotubes

Structural properties, evolution of morphology and crystallization kinetics in immiscible polymer blends filled with multiwall carbon nanotubes (MWNTs) in the droplet phase were systematically investigated in this study. By grafting suitable macromolecules (here SAN, styrene acrylonitrile), that can drive the MWNTs to the droplet phase, allowed the understanding of the rate of nucleation and growth of the semicrystalline matrix in droplet-filled blends in contrast to matrix-filled blends. By blending 90 wt% of PVDF with 10 wt% ABS, matrix-droplet morphologies were generated and by grafting SAN onto MWNTs, the localization of the nanotubes was tuned to fill the droplet phase which otherwise prefers the matrix phase (here PVDF); driven by thermodynamics. The evolution of morphology under quiescent annealing conditions was assessed by SEM. The blends with SAN-g-MWNTs also coarsened as a function of time similar to neat blends however, to a lesser extent. Although, droplet laden MWNTs did not suppress coarsening in the blends but it still improved the tensile properties when compared with the neat blends. The fold surface free energy (as evaluated from isothermal crystallization kinetics) was estimated to be less in case of blends with only MWNTs in contrast to blends with SAN-g-MWNTs. This was attributed to the fact that matrix (here PVDF) filled with MWNTs experiences faster crystallization rate due to heterogeneous dispersion of nucleating agents (here MWNTs) in the matrix. However, when the SAN-g-MWNTs were mostly localized in the amorphous droplet phase (here ABS), PVDF experienced similar crystallization behavior like the neat PVDF/ABS blends. Taken together, our study demonstrates that lower amount of energy is required for the arrangement of PVDF chains into the crystal lattice upon cooling from the melt state and accelerate the crystallization process when the heteronucleating agents are localized in the matrix phase than when they are localized in the amorphous droplet phase.

Relationship between melting behavior and morphological changes of semicrystalline polymers

The present study on the case of poly(hexamethylene succinate) is to provide a basis for a better understanding of the subtle relationship between melting behavior and morphological changes of semicrystalline polymers. The melting behavior and morphological changes of poly(hexamethylene succinate) during both isothermal secondary crystallization and annealing processes were investigated by DSC and SAXS. DSC results showed that, with increasing crystallization time or annealing time, the melting endotherm continuously shifted to higher temperature, which suggested that some minor structural or morphological changes must occur. However, almost no changes at all on the crystal thickness were observed from SAXS measurements. The observed evidence confirmed that the increase in the melting temperature is not attributed to crystal thickening but crystal perfection. More exactly, the rearrangement and smoothing of tie molecules at the folding surface result in the reduction of the fold surface free energy, which dominantly contributes to the increase in the melting peak temperature. The origin of the new endothermic peak observed after annealing at elevated temperature was also discussed. TMDSC results indicated that the annealing peak resulted from the enthalpy relaxation and devitrification transition of rigid amorphous fraction formed by the driving force of thermodynamic nonequilibrium, rather than usually regarded as the melting of thin lamellae or imperfect crystals formed by annealing secondary crystallization.

The Three-Phase Structure of Random Butene-1/Ethylene Copolymers

Abstract The three-phase arrangement of random copolymers of butene-1 with ethylene was investigated and compared with isotactic poly(butene-1) homopolymer (iPB-1). In all the analyzed compositions, isothermal crystallization leads to a three-phase structure, made of one crystal phase and two amorphous fractions that differ in mobility: the mobile amorphous fraction (MAF), made of the polymer chains that relax at the glass transition, and a rigid amorphous fraction (RAF) made of the amorphous segments coupled with the crystal phase. Copolymerization with ethylene leads to a drop in crystal fraction and to a sizable increase of both the RAF, and of the specific RAF, i.e. of the RAF normalized to crystallinity. Analysis of crystal growth rate allowed quantifying the fold surface free energy, which increases of about 50 to 100% in the copolymers, compared to iPB-1 homopolymer. In the butene-1/ethylene random copolymers, ethylene units are mostly excluded from the crystals and accumulate at the crystal/amorphous interphase, thus affecting the rigid amorphous area. The varied composition and higher mobility of the rigid amorphous fraction of the copolymers affects also the Form II to Form I transformation of poly(butene-1) crystals, which occurs with enhanced kinetics in the copolymers, compared to iPB-1 homopolymer.

Crystallization behaviour of syndiotactic polystyrene and benzoylated syndiotactic polystyrene

The crystallization behaviour of syndiotactic polystyrene (sPS) and benzoylated sPS (BzsPS) was investigated by POM, DSC and XRD. The results suggest β form of sPS has higher equilibrium melting temperature Tm0 and the fold surface free energy σe than α form. With increasing benzoylation degree the growth rates and Tm0 of two forms decrease, but their Tm0 decrement is very close; the σe of β form more increases than that of α form, the formation of α is more favourable. Upon loading trace amount of α nucleating agent PTBBNa in samples the growth rate of two forms can be strictly compared in the same melting and crystallization temperature condition, the results reveal that the growth rate of β form is faster than that of α form in sPS and BzsPS-2%. Using supercooling degree ΔT and σe two parameters has explained the formation competition mechanism of α and β forms.

Enhanced crystallization behaviour and impact toughness of poly(ethylene terephthalate) with a phenyl phosphonic acid salts compound

Low crystallization rate and inherent brittleness characteristics limit the wide application of PET. In this paper, it was found that a low molecular weight Phenyl phosphonic acid salts compound (TMC-210) is a very effective nucleator and can enhance the impact strength very much. So, the effect of TMC-210 on the crystallization behaviour and mechanical properties of poly(ethylene terephthalate) were systematically evaluated by differential scanning calorimetry (DSC), polarized optical microscopy (POM), wide angle X-ray diffraction (WAXD), scanning electron microscope (SEM) and mechanical properties test. The results show that TMC-210 obviously improves the crystallization temperature and accelerates the crystallization rate of PET and reflects a significant heterogeneous nucleating effect with a nucleation efficiency of 99.8 % when introducing a low content of 0.6 wt% TMC-210. The spherulites size and number of blended PET are greater than pure PET. The crystal structure of PET does not change but the blends with high TMC-210 content appears new diffraction peaks in x-ray diffraction spectrogram and it may attribute to the agglomeration of TMC-210 particles, which is verified by SEM observation. The impact fracture surface of PET develops a brittle ductile transition and thus the impact strength of PET improves significantly. Additionally, Lauritzen–Hoffman equation was used to discuss the effect of TMC-210 on the fold surface free energy (σ e) of PET in the crystallization process and found that the σ e values of PET/TMC-210 blends is smaller than that of pure PET.

Crystallization mechanism, microstructure and mechanical property of poly (L-lactic acid) modified by introduction of hydrogen bonding

In this work, 4,4′-Thiobis Phenol (TDP) was incorporated into PLLA using melt blending method. Overall crystallization mechanism and chain folding parameters in crystallization process were analyzed based on Avrami eq. and Lauritzen-Hoffman kinetic theory. Avrami exponent n for all samples were between 2.87 and 3.23, which indicated three dimensional crystal growth with heterogeneous nucleation. Meanwhile, overall crystallization rate of PLLA was significantly reduced by incorporation of TDP. The fold surface free energy (σ e) and work of chain folding (q) increased with TDP addition, implying that crystallization of PLLA was retarded. Small angle X-ray scattering (SAXS) analysis demonstrated that long period of PLLA was increased by TDP introduction, meaning more amorphous component between adjacent lamellae. As a result, the tensile strength decreased with TDP addition, while the elongation at break of PLLA was improved by more than 40 %.

Isothermal crystallization kinetics and melting behavior of poly(l-lactic acid)/WS2 inorganic nanotube nanocomposites

Environmentally friendly and biocompatible tungsten disulphide inorganic nanotubes (INT-WS2) were introduced into a poly(l-lactic acid) biopolymer matrix to generate novel nanocomposite materials through an advantageous melt-processing route. The effects of INT-WS2 on isothermal crystallization and melting behaviour of PLLA have been investigated. INT-WS2 has excellent acceleration effectiveness on the melt-crystallization of PLLA better than the promising nano-sized fillers reported in the literature (e.g. carbon nanotubes, graphene oxide, cellulose nanocrystals). In particular, the addition of INT-WS2 remarkably influences the energetics and kinetics of nucleation and growth of PLLA, reducing the fold surface free energy by up to 18 %. In the same way, the final crystallinity and subsequent melting behaviour of PLLA were controlled by both the incorporation INT-WS2 and variation of the crystallization temperature. These observations will enable the development of novel melt-processable PLLA/INT-WS2 nanocomposites with improved crystallization behaviour for many eco-friendly and biomedical applications.

Graphene-Induced Oriented Interfacial Microstructures in Single Fiber Polymer Composites.

Interfacial interactions between the polymer and graphene are pivotal in determining the reinforcement efficiency in the graphene-enhanced polymer nanocomposites. Here, we report on the dynamic process of graphene-induced oriented interfacial crystals of isotactic polypropylene (iPP) in the single fiber polymer composites by means of polarized optical microscopy (POM) and scanning electron microscopy (SEM). The graphene fibers are obtained by chemical reduction of graphene oxide fibers, and the latter is produced from the liquid crystalline dispersion of graphene oxide via a wet coagulation route. The lamellar crystals of iPP grow perpendicular to the fiber axis, forming an oriented transcrystalline (TC) interphase surrounding the graphene fiber. Various factors including the diameter of graphene fibers, crystallization temperature, and time are investigated. The dynamic process of polymer transcrystallization surrounding the graphene fiber is studied in the temperature range 124-132 °C. The Lauritzen-Hoffman theory of heterogeneous nucleation is applied to analyze the transcrystallization process, and the fold surface free energy is determined. Study into microstructures demonstrates a cross-hatched lamellar morphology of the TC interphase and the strong interfacial adhesion between the iPP and graphene. Under appropriate conditions, the β-form transcrystals occur whereas the α-form transcrystals are predominant surrounding the graphene fibers.

Morphology and crystallization behavior of poly(l‐lactide)/poly(butylene oxalate) blends

ABSTRACTThe isothermal crystallization of poly(l‐lactide) (PLLA) in blends with poly(butylene oxalate) (PBOX) is investigated by time‐resolved small‐angle X‐ray scattering, differential scanning calorimetry, and optical microscopy. We focus on the temperatures at which only PLLA crystallizes while PBOX is amorphous. It is obtained that the addition of PBOX causes a reduction of the melting temperature of PLLA. The lamellar thickness of PLLA crystals decreases whereas the amorphous layer thickness increases with blend composition, suggesting the occurrence of the interlamellar incorporation upon the addition of PBOX. The crystal growth rate and morphology of PLLA/PBOX blends are analyzed by polarized optical microscopy. The spherulite growth rate of PLLA is found to increase with the addition of PBOX. Analysis of the isothermal crystallization in terms of the Lauritzen and Hoffman equation give the reduction of the fold surface free energy upon the addition of PBOX in PLLA, indicating that the mobility of the PLLA chains is significantly improved due to the presence of PBOX. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 192–202

Poly(styrene‐co‐maleic anhydride) ionomers as nucleating agent on the crystallization behavior of poly(ethylene terephthalate)

ABSTRACTPoly(styrene‐co‐maleic anhydride) (SMA) ionomers were synthesized and designed as a new kind of nucleation agent according to the crystallization theory for improving the crystallization of poly(ethylene terephthalate) (PET). The crystallization behavior of PET with the addition of nucleation agents was investigated by differential scanning calorimetry, polarized‐light microscope, and X‐ray diffraction (XRD). Avrami equation and Hoffman–Lauritzen theory are adopted for analyzing isothermal and non‐isothermal crystallization kinetics, respectively. The results show that the addition of 1 wt % SMA ionomers effectively accelerates the crystallization rate and reduces the fold surface free energy of PET at high temperature regions. PLM results also indicated that the crystals impinge on each other, thus decreasing the spherulite size for PET/SMA ionomers samples compared with PET. XRD measurement revealed that the introduction of SMA ionomers does not change the crystal structure but indeed accelerates the crystallinity of PET. The results clearly demonstrate that our synthesized SMA ionomers are an efficient nucleating agent for PET. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41240.

Isothermal Cold Crystallization and Melting Behaviors of Poly(lactic acid)/Epoxy Vinyl Polyhedral Oligomeric Silsesquioxanes Nanocomposites

Poly(l-lactic acid) (PLA)/epoxy vinyl polyhedral oligomeric silsesquioxane (EP-POSS) nanocomposites were prepared via melt blending. The isothermal cold crystallization and melting behavors of pure PLA and its nanocomposites were investigated using differential scanning calorimetry (DSC). The crystal growth model of PLA was a mixture of two-dimensional discotic growth and three-dimensional spherulite growth, as evidenced by the values of the Avrami exponent ranging from 2.15 to 2.66. The crystallization rate displayed a great dependence on the EP-POSS loading level and crystallization temperature, and a transition from regime II to regime III was observed around 115°C. The values of fold surface free energy were calculated from the nucleation parameter which was determined by the Hoffman-Lauritzen equation. Polarizing microscope images (POM) indicated that EP-POSS promoted the nucleation of PLA.

Dynamic interactions between poly(3-hexylthiophene) and single-walled carbon nanotubes in marginal solvent.

Interfacial interactions between conjugated polymers and carbon nanotubes are pivotal in determining the device performance of nanotube-based polymer electronic devices. Here, we report on interfacial structures and crystallization kinetics of poly(3-hexylthiophene) (P3HT) in the presence of single-walled carbon nanotubes (SWNTs) in anisole by means of transmission electron microscope (TEM) and ultraviolet-visible (UV-vis) absorption spectroscopy. Confined on SWNT surfaces, the P3HT forms nanofibril crystals perpendicular to the long axis of SWNTs. The equilibrium dissolution temperature of the P3HT crystals in anisole is determined to be 381 ± 10 K according to the Hoffman-Weeks extrapolation approach. Upon cooling, the polymer solution spontaneously undergoes a time-dependent chromism. Various kinetics factors such as crystallization temperature, concentration, and SWNT loading have been investigated. It is found that the growth rate (G) of the crystals scales with concentration (C) as G ∝ C(1.70±0.16). The Avrami model is utilized to analyze the nucleation mechanism and the Avrami exponents vary between 1.0 and 1.3. The Lauritzen-Hoffman theory is applied to study the chain-folding process. The fold surface free energy is calculated to be (5.28-11.9) × 10(-2) J m(-2). It is evident that the addition of 0.30 wt % SWNTs reduces the fold surface free energy by 55.6%.

Novel poly(3-hydroxybutyrate) nanocomposites containing WS2 inorganic nanotubes with improved thermal, mechanical and tribological properties

Poly(3-hydroxybutyrate) (PHB) nanocomposites containing environmentally-friendly tungsten disulphide inorganic nanotubes (INT–WS2) have been successfully prepared by a simple solution blending method. The dynamic and isothermal crystallization studies by differential scanning calorimetry (DSC) demonstrated that the INT–WS2 exhibits much more prominent nucleation activity on the crystallization of PHB than specific nucleating agents or other nanoscale fillers. Both crystallization rate and crystallinity significantly increase in the nanocomposites compared to neat PHB. These changes occur without modifying the crystalline structure of PHB in the nanocomposites, as shown by wide-angle X-ray diffraction (WAXS) and infrared/Raman spectroscopy. Other parameters such as the Avrami exponent, the equilibrium melting temperature, global rate constant and the fold surface free energy of PHB chains in the nanocomposites were obtained from the calorimetric data in order to determine the influence of the INT–WS2 filler. The addition of INT–WS2 remarkably influences the energetics and kinetics of nucleation and growth of PHB, reducing the fold surface free energy by up to 20%. Furthermore, these nanocomposites also show an improvement in both tribological and mechanical (hardness and modulus) properties with respect to pure PHB evidenced by friction and nanoindentation tests, which is of important potential interest for industrial and medical applications.

Spherulite growth rate and fold surface free energy of the form II mesophase in isotactic polybutene-1 and random butene-1/ethylene copolymers

The spherulite growth rate of isotactic polybutene-1 and random butene-1/ethylene copolymers has been measured in a wide range of temperatures between the glass transition and the melting temperature. The presence of ethylene co-units in the butene-1 chain leads to a distinct decrease of both the maximum spherulite growth rate and the temperature of fastest growth. The data were analyzed within the frame of the Hoffman–Lauritzen theory of crystallization to obtain form II mesophase surface free energies. The robust performed analysis revealed that the form II mesophase fold surface free energy in random copolymers of butene-1 with less than 5 mol% ethylene is 50–100 % higher than in the homopolymer. It is suggested that the increase of the fold surface free energy is related to the exclusion of ethylene chain defects from crystallization and their accumulation at the basal planes of the form II mesophase.

Effect of surface-modified zinc oxide nanowires on solution crystallization kinetics of poly(3-hexylthiophene)

Interfacial interactions between donor and acceptor molecules are determinative to the device performance of hybrid photovoltaics. However, the dynamic process of such interactions remains largely obscure. In this work, we report the kinetic behavior of solution crystallization of poly(3-hexylthiophene) (P3HT) in anisole in the presence of ZnO nanowires by means of ultraviolet-visible absorption spectroscopy. ZnO nanowires are surface-modified by covalently attaching aliphatic and aromatic ligands to enhance the miscibility and interfacial interactions between P3HT and ZnO nanowires. Upon cooling the hot solution to room temperature, a significant time-dependent chromism occurs spontaneously. Analysis of the kinetics of isothermal solution crystallization across a range of crystallization temperature displays that the growth rate of the crystals scales with polymer concentration as R ∝ C1.6 for both the control P3HT and P3HT with ZnO nanowires. The Lauritzen–Hoffman theory of secondary nucleation is utilized to analyze the kinetic behavior of crystallization, and the fold surface free energies of the crystals of P3HT in anisole are calculated to be 6.6–10.3 × 10−2 J m−2. It is found that the addition of surface-modified ZnO nanowires decreases the fold surface free energy by 21.5% and 43.8%, respectively, for aliphatic and aromatic ligands.