Rheological control in foaming polymeric materials: II. Semi-crystalline polymers

The influence of rheological properties and crystallization on foam structures, such as cell diameter, cell density and cell size distribution, of semi-crystalline polymer was investigated. The rheological properties of polypropylene (PP) were controlled by long chain branching (LCB) modification with free radical reaction and its crystallinity. The foaming behavior could be well correlated with the crystal structure and the rheological properties of polymers. The results showed that the long chain branching modification changed the crystallization speed, the diameter and the number of crystal and the rheological behavior as well. The interplay between the crystallization and the rheology of polymers with different chain structures can cause different nucleation mechanism in foaming. Both the cell size of linear PP and LCB PP decrease with crystallization time, and the cell density increases with crystallization time. The crystals in PPs acted as heterogeneous nucleation cites for bubbles, but the cell density of LCB PP is much higher than that of linear PP because of it higher spherulites density. The higher viscosity of branched PP further made its cell diameter smaller than that of linear one. Therefore, the foam structure can be well controlled by tuning the chain structure and crystal structures.     

Insight into how molecular structures of thiophene-based conjugated polymers affect crystallization behaviors

Combining the structural characterization of solution crystals fabricated from thiophene-based conjugated polymers with different molecular structures and a theoretical investigation of the polymer conformational transformability leads to an interesting discovery of the relationship between the molecular structures and their crystallization behaviors. The chain folding or nonfolding behavior of thiophene-based conjugated polymers in crystallization, an important factor to shape polymer crystals, is determined by their molecular structures, and can be estimated by the inter-ring rotation energy barriers of the polymer backbones. A quantitative theoretical calculation is proposed to evaluate the inter-ring rotation energy barriers, and the values are correlated with the experimentally observed chain folding or nonfolding behavior. The higher percentage of type I inter-ring σ bond (CH₃ and H are at 3 and 3′ position of adjacent aromatic rings, respectively) or the lower average rotation barrier in polymer backbones creates higher capability of polymer conformational transformation and higher tendency of chain folding. Our study provides a valid prediction of the crystallization behavior of thiophene-based conjugated polymers through a theoretical evaluation of conjugated polymer molecular structures, and offers an essential understand of the structure-property relationship of conjugated polymers.     

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 %.     

Roughness of growth faces of polymer crystals: Evidence from morphology and implications for growth mechanisms and types of folding

It is proposed that the growth faces of lamellar polymer crystals can have an equilibrium roughness (or crenellation). This can explain why some polymer crystals show no evidence of faceting. Support for this idea comes from the extensive theoretical developments on the nature of crystal surfaces. The characteristic habits of polyethylene are analysed in terms of a roughness which, on the {110} faces, increases progressively over a temperature range of about 100°C. At a temperature near 110°C the roughness becomes sufficient for there to be no free energy penalty for arbitrary crystal shapes (e.g. rounded) compared with one bounded by {110} faces. Above this temperature of crystallization most of the habits which are observed are leaf-shaped, with an apex along 〈010〉. Below 110°C{110} facets (or microfacets) are normally seen. There is no positive evidence that faces approximately parallel to (100) planes, observed for crystallization temperatures in the range 80°–110°C, are ever other than rough. The relative rates of growth on the {110} compared with the (100) increase with temperature, since {110} faces predominate at low but not high temperatures of crystallization. These changes are attributed to the increase in roughness with temperature on the {110} faces. The existence of surface disorder (roughness) requires that the binding energy between units in the crystal is comparable with KT. Hence this unit is probably several monomer units of polyethylene (rather than, for example, a complete stem which contains a hundred or more monomers). There is therefore a surface lattice on the growth faces with twenty or more units in the direction perpendicular to the lamellae. Monte Carlo calculations are cited for lattices of 20 by 50 units. These show that a cooperative increase in surface roughness with temperature and a transition between faceted and non-faceted growth can be expected for lattices of such limited extents. No explicit allowance has been made as yet for the consequences of the units being linked into chains and, for that reason, not being able to arrive or leave the surface independently. It is noted that changes in the alkane lattice with temperature indicate a possible evolution in binding energy, and in mobility, and hence may influence surface roughness. Theories of crystallization in polymers have normally assumed a growth surface which is molecularly smooth in equilibrium, and have emphasised nucleation events. Since this paper shows that the equilibrium structure may often be rough, it may be necessary to re-examine the basis of these theories. A brief review is included of the experimental evidence for surface nucleation events: nucleation may be a more important barrier at low temperatures than at high. The type of folding will be influenced by equilibrium roughness just as it will be by kinetic roughness, and some comparisons are made with neutron scattering results on this topic. The degree of adjacent folding is higher in the faceted regime as expected. Brief comments are made on the applicability of this idea to polymers other than polyethylene.     

限域空间内的结晶研究进展

在经典成核理论基础之上,综述了无机、有机化合物和大分子聚合物等几类物质在不同的限域空间内的结晶行为,并与普通溶液结晶行为进行了比较。分析了不同限域空间对晶体的成核、生长以及相转化产生的影响,如成核速率、生长取向、晶型种类等。研究表明,在高度分散的纳米孔基质中,孔径尺寸、形貌对晶体成核与生长的控制机理不同,从而影响了最终的晶体结构。     

Studies on nonisothermal crystallization of HDPE/POSS nanocomposites

The nonisothermal crystallization of HDPE/POSS nanocomposites (POSS content varying from 1 to 10 wt%) was studied using differential scanning calorimetry (DSC) technique. The Ozawa approach failed to describe the crystallization behaviour of nanocomposites, whereas the modified Avrami analysis could explain the behaviour of HDPE/POSS (90:10) nanocomposite only. The value of Avrami exponent n for HDPE/POSS (90:10) nanocomposite ranged from 2.5 to 2.9 and decreased with increasing cooling rate. It is postulated that the values of n close to 3 are caused by spherulitic crystal growth with heterogeneous nucleation while simultaneous occurrence of spherulitic and lamellar crystal growth with heterogeneous nucleation account for lower values of n at higher cooling rates. A novel kinetic model by Liu et al. was able to satisfactorily describe the crystallization behaviour of HDPE/POSS nanocomposites. Presence of POSS did not cause significant change in the activation energy for the transport of polymer segments to the growing crystal surface. POSS molecules exhibit nucleation activity only at 10 wt% loading in HDPE and are not effective nuclei at lower loadings.     

Solvent‐ and thermal‐induced crystallization of poly‐L‐lactic acid in supercritical CO2 medium

The effect of different annealing treatments with supercritical carbon dioxide (SCCO₂) on the structural and mechanical properties of semicrystalline poly‐L ‐lactic acid (L ‐PLA) was investigated. 2000, 27,000, 100,000, and 350,000 g mol^?1 molecular weight L ‐PLA polymers were used in the study. The solid‐state processing of L ‐PLA at temperatures lower than the effective melting point led to solvent‐ and thermal‐induced crystallization. Solvent‐induced and isothermal crystallization mechanisms could be considered similar regarding the increase of polymer chain mobility and mass‐transfer in the amorphous region; however, quite different microstructures were obtained. SCCO₂ solvent‐induced crystallization led to polymers with high crystallinity and melting point. On the contrary, SCCO₂ thermal‐induced crystallization led to polymers with high crystallinity and low melting point. For these polymers, the hardness increased and the elasticity decreased. Finally, the effect of dissolving SCCO₂ in the molten polymer (cooling from the melt) was analyzed. Cooling from the melt led to polymers with high crystallinity, low melting point, low hardness, and low elasticity. Distinctive crystal growth and nucleation episodes were identified. This work also addressed the interaction of SCCO₂‐drug (triflusal) solution with semicrystalline L ‐PLA. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Kinetic analysis on spherulite growth rate of polypropylene catalloys

The kinetic analysis on melt-crystallization of polypropylene catalloys (PP-cats) was conducted through measuring their spherulite growth rates. A multiple melting behavior of PP-cats was found through differential scanning calorimetry (DSC) and the corresponding crystalline microstructures of PP-cats were studied by wide-angle X-ray diffraction (WAXD). The calculated T_m^o value of propylene homopolymer (PP) suggests an obvious melting point depression of PP-cats. Moreover, it is found that the existence of ethylene-propylene copolymer could result in the changes of crystalline microstructure of PP and the PP crystal is in favor of growing along (040) lattice plane of α-monoclinic crystal. The crystal growth rate of PP-cats decreases with the increase of ethylene-propylene copolymer content in PP-cats. A comparison of crystallization kinetics between PP-cats and virgin iPP through a modified Lauritzen-Hoffman model indicates that there appears a transition from regimes II to III in iPP and PP-cats containing low ethylene-propylene copolymer content. However, for the PP-cats containing high ethylene-propylene copolymer content, crystallization always processes in regime II. In addition, both calculated nucleation parameter (K_g) and the fold surface free energy (σ_e) for PP-cats increase with the increase of ethylene-propylene copolymer content, implying that the existence of ethylene-propylene copolymer is unfavorable for the surface nucleation of PP and regular folding of the molecule chain. It is believed that an increase in viscosity of the melts induced by different compositions could remarkably slow crystallization growth down, because under this condition surface nucleation dominates as compared with crystal growth.     

Understanding crystal nucleation in solution-segregated polymers

We report dynamic Monte Carlo simulations of crystal nucleation in polymer bulk phase segregated from solutions. We found that poorer solvent enhances crystal nucleation in the concentrated phase of polymers. In addition, when the solvent becomes poor enough, crystal nucleation prefers to occur at the diffuse interfaces. The results are consistent with the predictions from theoretical phase diagrams, but something different from immiscible polymer blends. The surface-enhanced crystallization may explain the bowl-shaped crystal aggregates observed experimentally in poor solvent.     

Temperature and concentration dependent effect of partial glycerides on milk fat crystallization

Two diglycerides (distearin and diolein) and two monoglycerides (monostearin and monoolein) were added to milk fat in a concentration of 0.5% and 1%. The isothermal crystallization behavior was evaluated at 22 °C, 23.5 °C, 25 °C and 26.5 °C by DSC and pNMR. The crystallization kinetic was quantified by means of two models. It was noticed that the effect of the minor components on the crystallization behavior depends on temperature and concentration. The type of esterified fatty acids and the polar head of the amphiphilic molecule determine to what extent partial glycerides influence the nucleation and crystal growth of triglycerides. Moreover the degree of insolubility of partial glycerides in the melt determines which effect (on growth or on nucleation) predominates. Stearic acid based partial glycerides enhance nucleation at low temperatures, while at higher temperatures an interaction with the crystal growth predominates. Oleic acid based partial glycerides have an effect on the nucleation process while no interaction with the crystal growth was observed.     

Effects of short‐chain branches on strain‐induced polymer crystallization

We performed dynamic Monte Carlo simulations of a lattice polymer model to investigate strain‐induced crystallization behaviors of short‐chain branched polymers with variable sequence distributions, branch numbers and lengths, and their blending compositions with linear polymers. The results revealed that the branch density and distribution bring dominant effects. In the blends, there is no phase separation prior to crystallization, except that strain‐enhanced mixing is temporarily hindered by chain folding upon crystallization. Our observations shed lights onto the strain‐induced crystallization of short‐chain branched polymers. © 2018 Society of Chemical Industry     

The effect of grafting density on the crystallization behavior of one-dimensional confined polymers

Grafting density and confinement scale of the nano-area are significant elements affecting the crystallization behavior of polymers. In this work, the crystallization processes of confined and unconfined polymer systems with different grafting density were systematically studied by MC simulation. The results show that when the grafting density of confined systems is low, the crystallization rate is faster and the final crystallinity is higher. However, the crystallization ability is reduced for higher grafting density. We found that for this confined system, the segmental density of interfacial region is larger, and the movement of chain segment is lower. Due to the higher grafting density, the crowding effect of polymer chains is strong, which leads to the intermolecular nucleation. The critical nucleation free energy barrier is higher. Moreover, only homogeneous nucleation occurs in confined polymer systems. With the increasing grafting density, the crystals change from lying on the substrate surface to being perpendicular to the substrate surface in unconfined polymer systems. The crystals are mainly lying on the substrate surface in confined polymer systems. These simulation results are helpful to understand the microscopic mechanism of crystallization behaviors of polymer nanocomposites and provide a theoretical basis for the design of nanocomposites with excellent physical properties.     

Crystallization kinetics and morphology of poly(trimethylene terephthalate)

In this work, the isothermal crystallization kinetics of polytrimethylene terephthalate (PTT) was first investigated from two temperature limits of melt and glass states. For the isothermal melt crystallization, the values of Avrami exponent varied between 2 and 3 with changing crystallization temperature, indicating the mixed growth and nucleation mechanisms. Meanwhile, the cold crystallization with an Avrami exponent of 5 indicated a character of three-dimensional solid sheaf growth with athermal nucleation. Through the analysis of secondary nucleation theory, the classical regime I→II and regime II→III transitions occurred at the temperatures of 488 and 468 K, respectively. The average work of chain folding for nucleation was ca. 6.5 kcal mol⁻¹, and the maximum crystallization rate was found to be located at ca. 415 K. The crystallite morphologies of PTT from melt and cold crystallization exhibited typical negative spherulite and sheaf-like crystallite, respectively. Moreover, the regime I→II→III transition was accompanied by a morphological transition from axialite-like or elliptical-shaped structure to banded spherulite and then non-banded spherulite, indicating that the formation of banded spherulite is very sensitive to regime behavior of nucleation.     

Isothermal crystallization kinetics and melting behavior of modified high‐density polyethylene/barium sulfate nanocomposites

The melting/crystallization behavior and isothermal crystallization kinetics of high‐density polyethylene (HDPE)/barium sulfate (BaSO₄) nanocomposites were studied with differential scanning calorimetry (DSC). The isothermal crystallization kinetics of the neat HDPE and nanocomposites was described with the Avrami equation. For neat HDPE and HDPE/BaSO₄ nanocomposites, the values of n ranges from 1.7 to 2.0. Values of the Avrami exponent indicated that crystallization nucleation of the nanocomposites is two‐dimensional diffusion‐controlled crystal growth. The multiple melting behaviors were found on DSC scan after isothermal crystallization process. The multiple endotherms could be attributed to melting of the recrystallized materials or the secondary lamellae caused during different crystallization processes. Adding the BaSO₄ nanoparticles increased the equilibrium melting temperature of HDPE in the nanocomposites. Surface free energy of HDPE chain folding for crystallization of HDPE/BaSO₄ nanocomposites was lower than that of neat HDPE, confirming the heterogeneous nucleation effect of BaSO₄. POLYM. COMPOS., 2011. © 2010 Society of Plastics Engineers     

Poly(ethylene oxide) crystallization in single walled carbon nanotube based nanocomposites: Kinetics and structural consequences

The overall isothermal crystallization behavior of poly(ethylene oxide) (PEO) in single walled carbon nanotube (SWNT) based nanocomposites is studied with a focus on growth kinetics and morphological evolution of PEO using differential scanning calorimetry and in-situ small angle x-ray scattering measurements respectively. The characteristic time for crystallization of PEO increases due to the presence of lithium dodecyl sulfate (LDS) stabilized carbon nanotubes. Further, analysis of crystallization data using the Lauritzen-Hoffman regime theory of crystal growth shows the PEO chains stiffen in presence of LDS with an increased energy barrier associated with the nucleation and crystal growth, and the nanotubes further act as a barrier to chain transport or enhance the efficacy of the LDS action. The energy penalty and diffusional barrier to chain transport in the nanocomposites disrupt the crystalline PEO helical conformation. This destabilization leads to preferential growth of local nuclei resulting in formation of thinner crystal lamellae and suggests that the crystallization kinetics is strongly affected by the nucleation and crystal growth events. This study is particularly interesting considering the suppression of the PEO crystallinity in presence of small fraction of Lithium ion based surfactant and carbon nanotubes.     

Influence of Dopants on Nucleation and Growth of High-Quartz Solid Solution in Lithium Aluminosilicate Glass

The kinetic parameters of nucleation and crystal growth of high-quartz solid solution in multicomponent lithium aluminosilicate glasses doped with various transition-metalions were studied by nonisothermal DTA. The crystallization of glasses nucleated at different temperatures was carried out, and plots of the DTA peak versus the nucleation temperatures were used to determine the maximum nucleation rate temperature. Peak temperature data of nucleated samples at varying heating rates (5–20 K/min) were used to determine the activation energy for crystallization via the JMA equation. The temperature of maximum nucleation rate depends greatly on the doped transition- metal ions present. The activation energy for crystallization obtained for undoped glass or glasses doped with Fe₂O₃ is of the same order as that already published, and the Avrami exponent is consistent with predominantly three-dimensional crystal growth. The much higher activation energy values for glasses doped with CoO could be a consequence of two crystallization processes proceeding simultaneously.     

Characterization of a maleic anhydride-modified polypropylene as an adhesion promoter for glass fiber composites

Isothermal and nonisothermal crystallizations of isotactic polypropylene (iPP), maleic anhydride (MAH)-grafted PP, and MAH-modified iPP were studied by differential scanning calorimetry (DSC), to evaluate the influence of a small amount of MAH-grafted PP in iPP on its crystallization behavior. Isothermal crystallization was followed in the temperature range from 391 K to 403 K, and the rate constant and Avrami exponents were determined. Nonisothermal crystallization was carried out at different cooling rates (1-20 K/min). It was found that the crystallization kinetics of iPP was significantly altered by modification with the MAH-grafted polymer. A decreased equilibrium melting temperature, as well as decreased surface energy of folding and critical dimensions of a growing nucleus, was determined for the MAH-modified iPP, indicating faster growth of lamellae and a higher rate of crystallization. The improved nucleation ability of the modified polymer was shown to cause a shift in the crystallization peak temperature towards higher values (from 393.7 K to 399.6 K, at a cooling rate of 1 K/min), resulting in crystal structures less disposed to recrystallization. Model composites of iPP and MAH-modified iPP with glass fibers were also analysed. The apparent shear strength of single-fiber model composites with MAH-modified iPP was drastically increased compared with homo-iPP.     

The effect of lanthanum oxide (La2O3) on the structure and crystallization of poly(vinylidene fluoride)

Rare earth polymers, due to their excellent luminescence, fluorescence, laser protective, optical and magnetic properties, have attracted much research attention in recent years. However, little attention has been paid to the effect of rare earths on the structure and crystallization of polymers, which is of important significance in the development of functional polymers. X‐ray diffraction and differential scanning calorimetry were used to investigate the structure and crystallization behavior of a poly(vinylidene fluoride) (PVDF)/lanthanum oxide (La₂O₃) composite. The results showed that the degree of perfection, crystal size, crystallization rate and isothermal crystallization activation energy of PVDF in the composite decreased, compared with pure PVDF. The spherulite nucleation and growth for PVDF and PVDF composite were analyzed in detail using the Lauritzen‐Hoffman equation. The modified Avrami equation and the Mo equation were used to study the non‐isothermal crystallization kinetics. The addition of La₂O₃ did not change the crystal structure and nucleation process for PVDF, but it decreased markedly the crystal growth rate and led to the formation of unstable crystals. This was attributed to the fact that too much La₂O₃ prevented PVDF molecular chains from moving and arranging in an orderly manner into crystals. Copyright © 2010 Society of Chemical Industry     

Mechanisms of nucleation and crystal growth in a nascent isotactic polypropylene/poly (ethylene-co-octene) in-reactor alloy investigated by temperature-resolved synchrotron SAXS and DSC methods

The nucleation and lamellar growth mechanisms of nascent isotactic polypropylene/poly(ethylene-co-octene) (N-iPP/PEOc) in-reactor alloy were investigated with temperature-resolved synchrotron small angle X-ray scattering (SAXS), differential scanning calorimeter (DSC) and polarized optical microscopy (POM) methods. We have observed two crystallization peaks (fractionated crystallization behavior) during cooling process in N-iPP/PEOc in-reactor alloy. We also determined that the crystallinities from that two crystallization peaks were dependent on liquid-liquid phase separation (LLPS) time with t^0.10 and t^−0.28, respectively. It was explained that the fractionated crystallization behavior in the N-iPP/PEOc in-reactor alloy system was caused by crystal nucleation occurring in the iPP rich domain by heterogeneous nucleation and at interface of iPP and PEOc rich domains by the fluctuation assisted nucleation. The fluctuation assisted nucleation only occurred at interface of iPP and PEOc domains by concentration fluctuation through the coupling of liquid-liquid spinodal decomposition and the cross-over to crystal nucleation process. Both lamellar crystals formations from heterogeneous and fluctuation assisted nucleation in N-iPP/PEOc were probed by temperature-resolved SAXS during cooling process. Our results provide the physical model for the multiple nucleation and crystal growth mechanisms in the multi-component, multi-phase polymer systems such as in-reactor alloy or blend.     

Monitoring nonisothermal crystallization of thermoplastic polymers using a quartz crystal resonator

A new monitoring device for the nonisothermal crystallization of thermoplastic polymers, polyethylene, polypropylene, polystyrene, and polyamide, is developed utilizing a quartz crystal resonator, and its performance is evaluated by comparing the measurements with the results of DSC thermoanalysis and microscopic observation. The experimental results of four different polymers indicate that the variation of resonant freency of the quartz crystal resonator is a good means to monitor the crystallization process. Though the measurements of melting and crystallization are close to the DSC outcome, more deviation is observed with the new device. The change in crystalline morphology during the crystallization process is also detected from the slope changes of the frequency decrease. In comparison with the microscopic observation of polymer films, it is found that the processes of nucleation and crystal growth in nonisothermal crystallization can be explained with the variation of the resonant frequency of the quartz crystal resonator. In addition, crystallization kinetics is modeled with the Avrami equation. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011