Structural evolution of poly(lactic acid) upon uniaxial stretching investigated by in situ infrared spectroscopy

Structural evolution of poly(lactic acid) (PLA) upon uniaxial stretching was studied with in-situ polarized infrared spectroscopy measurements, and its structural change affected by annealing was also investigated. Band shifting was used to reflect the structural ordering process. It was found that the band at 1302 cm⁻¹ always moves to low wavenumbers before crystallization, indicating the occurrence of intermolecular packing ordering. However, the band at 869 cm⁻¹ shifts to high wavenumbers, which is related to the transition from the amorphous phase to the ordered phase. Interestingly, during stretching, the shifting for the band at 1302 cm⁻¹ always occurs before that for the lower wavenumber band, whereas these two band shifts take place simultaneously under annealing. Based on the different characteristics of the structural evolution under stretching and annealing processes, a critical temperature was found at around 63 °C, which influences the effect weight of kinetic and thermodynamic factors to the crystallization behavior. The effect of the drawing temperature on crystallization and mechanical property of PLA films was also analyzed.     

The influence of wood flour and compatibilizer (<Emphasis Type="Italic">m</Emphasis>-TMI-<Emphasis Type="Italic">g</Emphasis>-PP) on crystallization and melting behavior of polypropylene

Wood flour/polypropylene composites (WPC) were prepared by melt extruding with different wood flour (WF) loadings. The non-isothermal crystallization and melting was studied with different WF loadings, for W40P60 and W40P60M6, the melting was investigated after non-isothermal and isothermal crystalline. Comparing with neat polypropylene, the melting behavior of the composites, both non-isothermally and isothermally, was investigated by differential scanning calorimetry (DSC). The results showed that WF was an effective heterogeneous nucleating agent, as evidenced by an increase in the crystallization temperature and the crystallinity for melt crystallization of PP with increasing WF content. For the non-isothermal samples, the origins of the double melting behaviors were discussed, based on the DSC results of PP. The XRD measurements confirmed that no crystalline transition existed during the non-isothermal crystallization process. With m-TMI-g-PP adding, due to compatibilization phenomenon were probably responsible for decreasing T _m, X _c. In the DSC scan after isothermal crystallization process, the single melting behaviors were found and each melting endotherm had a different origin.     

STRUCTURE AND THERMAL MECHANICAL PROPERTIES OF ORIENTED POLYIMIDE (PEI-E)

The strain induced crystallization behaviour in polyimide from 1,4-bis (3', 4'- dicarboxyphenoxy)benzene and 4,4'-oxydianiline (PEI-E)has been investigated by WAXD, DSC and FTIR. The results obtained show that crystallization in PEI-E did take place just after tensile yielding. Meanwhile, the effect of strain induced crystallization on the thermomechanical properties was studied by DMA and TMA, the results of which indicate that the crystallization and hot stretching have a certain influence on the dynamic mechanical properties, such as weakening the βrelaxation and decreasing the glass transition temperature. The TMA results confirm the shifting of glass transition temperature to lower temperature region after hot stretching. This phenomenon could be well explained by the effect of residual stress according to Eyring's theory.     

Deformation temperature and lamellar thickness dependency of Form I to Form III phase transition in syndiotactic polypropylene during tensile stretching

Phase transition from form Ⅰ to form Ⅲ in syndiotactic polypropylene crystallized at different conditions during tensile deformation at different temperatures was investigated by using in situ synchrotron wide angle X-ray diffraction technique. In all cases, the occurrence of this phase transition was observed. The onset strain of this transition was found to be crystalline thickness decided by crystallization temperature and drawing temperature dependent. The effect of drawing temperature on this phase transition is understood by the changes in mechanical properties with temperature. Moreover, crystalline thickness dependency of the phase transition reveals that this form Ⅰ to from Ⅲ phase transition occurs first in those lamellae with their normal along the stretching direction which have not experienced stress induced melting and recrystallization.     

STUDIES ON THE PORE FORMATION MECHANISM OF β-CRYSTALLINE POLYPROPYLENE UNDER STRETCHING*

The pore formation mechanism of,β-crystalline polypropylene under stretching was investigated. The porosity of the samples increases rapidly with stretching, having a maximum at draw ratios around 2 and then decreases monotonically. An abrupt formation process of initial micropores at very low draw ratios was evidenced by in situ SAXS measurements. At the same time the phase transition from ,β-crystal to α-crystal proceeds slowly in the whole deformation process up to large draw ratios around 5. Comparative studies of α- and ,β-crystalline polypropylene samples before stretching indicate that in addition to difference in crystal forms the α- and β-crystalline polypropylene samples exhibit quite different morphological features. There are a lot of interfaces in ,β-crystalline polypropylene samples, which may have a lower density value and can be easily etched by argon ions and penetrated by small molecules. It was concluded from these experimental facts that the pore formation and crystal transition are two independent phenomena during the deformation of β-crystalline polypropylene samples, and phase transition from ,β-crystal to α-crystal could hardly be the origin of pore formation. A defect initiation mechanism was proposed to understand the pore formation behavior of β-crystalline polypropylenes.     

Effect of strain‐induced molecular ordering on mechanical performance and barrier properties of polylactide nanocomposites

Plastic deformation of polylactide has been known as a self‐reinforcement alternative to improve mechanical and barrier properties. In this study, the structural evolution was investigated during a hot‐drawing process, at different initial strain rates and temperatures above T_g of polylactide. The drawing process at T_g +10 °C, led to the formation of an intermediate molecular ordering, between the crystalline and amorphous phases. A lower fraction of this mesomorphic phase was found to develop with the addition of nanoparticles. An increase in the stretching temperature to T_g +30 °C, caused an improvement of the crystallization kinetics, compared to that of thermally activated crystallization. A strain hardening behavior was observed in the presence of mesophase during a stretching process of the hot‐drawn films at room temperature. Permeability was discerned to its basic components, diffusivity, and solubility coefficients. The matrix degradation influenced the permeability components. The diffusivity decreases in the presence of the impermeable matters. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1865–1876     

Crystallization and melting of isotactic polypropylene crystallized from quiescent melt and stress‐induced localized melt

Temperature dependency of crystalline lamellar thickness during crystallization and subsequent melting in isotactic polypropylene crystallized from both quiescent molten state and stress‐induced localized melt was investigated using small angle X‐ray scattering technique. Both cases yield well‐defined crystallization lines where inverse lamellar thickness is linearly dependent on crystallization temperature with the stretching‐induced crystallization line shifted slightly to smaller thickness direction than the isothermal crystallization one indicating both crystallization processes being mediated a mesomorphic phase. However, crystallites obtained via different routes (quiescent melt or stress‐induced localized melt) show different melting behaviors. The one from isothermal crystallization melted directly without significant changing in lamellar thickness yielding well‐defined melting line whereas stress‐induced crystallites followed a recrystallization line. Such results can be associated with the different extent of stabilization of crystallites obtained through different crystallization routes. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 957–963     

Thermal analysis and thermochemistry of vitreous into crystalline state transition

Thermochemistry and structural mechanism of crystallization of MgO-Al₂O₃-SiO₂ glasses with TiO₂ as crystallization activator were studied. Thermal and HREM investigation proved that near the T _g temperature crystallization is going by rearrangement of glass structure elements and part of its components redistribution like at disorder — order phase transition in solid bodies. Nanocrystals of Mg-titanate and high quartz structure solid solution are formed then. Next enstatite and cordierite are crystallizing. Thermochemical and chemical bonds strength analysis indicate that during multistage crystallization of glasses, kind and order of crystal phase formation, is determined by the glass structure decomposition progress and its particular components release accompanying increase of temperature. It has been proved that molar heat capacity change (ΔC _p) accompanying the glass transition is the significant measure of degree of changes in the structure of glass preceding crystallization. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of morphology on the dynamic mechanical properties of a semicrystalline polysiloxane

The dynamic mechanical properties of semicrystalline poly(tetramethyl-p-silphenylene siloxane) in three morphological preparations were measured over the wide frequency range of about 0.002 Hz to 500 Hz and the temperature range of about ? 190°C to 100°C. The three samples were all isothermally crystallized at 125°C. Two samples had a spherulite size of 25 μ diameter but differed in the time allowed for secondary crystallization. The other sample had a smaller spherulite size. By assuming compliance additivity, the viscoelastic behavior could be separated into five relaxation processes with an indication that a sixth existed at low temperature. Two processes called γ₁ and γ₂ could be resolved at low temperatures. The γ₁ process was associated with the amorphous region since the peak strength was affected by the rate of cooling through the glass transition region; the γ₂ peak, unaffected by cooling rate, is attributed to the crystalline part. In the high-temperature region, the β peak is associated with the glass transition and has a shape and location that is essentially independent of the morphology. The highest temperature α₂ process, whose maximum was not observed in the experimental range covered, is attributed to the crystalline region and is sensitive to changes in crystallization history. The strength of the α₁ process unlike that of the other processes was found to be a function of temperature; it was associated with the noncrystalline region.     

Mechanical characteristics of hydrogenated natural rubber vulcanizates

Mechanical properties of partially hydrogenated natural rubber (HNR) vulcanizates were evaluated regarding their chemical structure and crystallizable nature of HNR, and are reported here, to the best of our knowledge, for the first time. HNRs of three levels of hydrogenation (20.6, 29.0, and 40.6 mol%) were successfully prepared by the chemical modification of natural rubber (NR) latex using N₂H₄ and H₂O₂ as reagents, in a sufficient amount for preparing sulfur‐crosslinked samples to be subjected to mechanical and structural measurements. The three HNR vulcanizates were found to be crystallizable upon stretching; it is noted that even 40.6 mol% hydrogenation did not prevent HNR vulcanizates from crystallization upon stretching, while their onset strain of crystallization was higher than that of NR vulcanizate. The hysteresis loss and residual strain up to a stretching ratio of 2 for the HNR vulcanizates tended to become larger with the increase in the degree of the hydrogenation. Tensile and dynamic mechanical properties of 20.6 mol% hydrogenated HNR vulcanizate were comparable to those of NR vulcanizate. From differential scanning calorimetry and temperature dispersion of dynamic modulus or loss, the glass transition temperatures of HNR vulcanizates were found to be almost the same as that of NR vulcanizate, which is also notable. The thermal stability of HNR vulcanizates was better than that of NR vulcanizate. Thus, this chemical modification seems to give a promising NR derivative whose properties can be equivalent or even better than the mother polymer. Copyright © 2008 John Wiley & Sons, Ltd.     

拉伸对聚左旋乳酸结晶的影响

在成型加工过程中,拉伸是提高聚合物材料结晶能力的一种重要手段. 本文采用红外光谱、差示扫描量热分析、X射线衍射等方法系统研究了不同温度下拉伸对聚左旋乳酸（PLLA）结晶行为的影响. 结果表明,在合适的温度条件下,拉伸能迅速提高PLLA的结晶速度和结晶度. 对经过拉伸预处理但未结晶的PLLA样品进行等温及非等温结晶的研究发现,经过拉伸预处理的PLLA样品的结晶速率和结晶度都得到提高,这表明预拉伸会影响PLLA在后续过程中的结晶行为.     

Morphology and nonisothermal crystallization of in situ microfibrillar poly(ethylene terephthalate)/polypropylene blend fabricated through slit‐extrusion,hot‐stretch quenching

This study describes the morphology and nonisothermal crystallization kinetics of poly(ethylene terephthalate) (PET)/isotactic polypropylene (iPP) in situ micro‐fiber‐reinforced blends (MRB) obtained via slit‐extrusion, hot‐stretching quenching. For comparison purposes, neat PP and PET/PP common blends are also included. Morphological observation indicated that the well‐defined microfibers are in situ generated by the slit‐extrusion, hot‐stretching quenching process. Neat iPP and PET/iPP common blends showed the normal spherulite morphology, whereas the PET/iPP microfibrillar blend had typical transcrystallites at 1 wt % PET concentration. The nonisothermal crystallization kinetics of three samples were investigated with differential scanning calorimetry (DSC). Applying the theories proposed by Jeziorny, Ozawa, and Liu to analyze the crystallization kinetics of neat PP and PET/PP common and microfibrillar blends, agreement was found between our experimental results and Liu's prediction. The increases of crystallization temperature and crystallization rate during the nonisothermal crystallization process indicated that PET in situ microfibers have significant nucleation ability for the crystallization of a PP matrix phase. The crystallization peaks in the DSC curves of the three materials examined widened and shifted to lower temperature when the cooling rate was increased. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 374–385, 2004     

Synthesis of a New Liquid Crystalline Polymer as a Highly Active β-Nucleator and Its Effect on Melting and Crystallization Behaviors of Isotactic Polypropylene

A new nematic liquid crystalline polymer as a highly active β-nucleator (LCP-N) of isotactic polypropylene (iPP) was synthesized and characterized. The effect of LCP-N on thermal behavior of the iPP was investigated with differential scanning calorimetry. LCP-N showed a melting transition at 85.0°C and a nematic to isotropic phase transition at 278.0°C. The incorporation of LCP-N could lead to substantial changes in the thermal behavior of the iPP. The nucleating activity of LCP-N mainly depended on its content, mesogenic molecular structure, and thermal history of processing. A high content of β-form could be obtained by the combined effect of the optimum LCP-N concentration and crystallization temperature and time. The Φ_β reached 77% when the LCP-N content, crystallization temperature, and crystallization time were 0.4 wt.%, 125°C, and 1 h, respectively.     

The influences of α/β compound nucleating agents based on octamethylenedicarboxylic dibenzoylhydrazide on crystallization and melting behavior of isotactic polypropylene

The influences of α/β compound nucleating agents based on octamethylenedicarboxylic dibenzoylhydrazide on crystallization and melting behavior of isotactic polypropylene (iPP) were analyzed. It is found that the crystallization temperatures of nucleated iPP were increased by above 11.0°C and the relative contents of β‐crystals (K_β ) in iPP reached above 0.40 after addition of compound nucleating agents. The K_β values depend on cooling rate, crystallization temperature in isothermal crystallization, and the difference between the crystallization temperatures of iPP nucleated by two individual nucleating agents. The nonisothermal crystallization kinetics were studied by Caze method and Mo method, respectively. The effective activation energy was calculated by the Friedman's method. The results illustrate that the half crystallization time was shortened and the crystallization rate was increased obviously after addition of nucleating agents, and the effective activation energy was increased with the relative crystallinity.     

Morphological varieties and kinetic behaviors of poly(3-hydroxybutyrate) (PHB) spherulites crystallized isothermally from thin melt film

The spherulitic morphologies of poly(3-hydroxybutyrate) (PHB) crystallized isothermally from thin melt film with different crystallization temperatures were observed by means of polarized optical microscopy, optical microscopy, SEM, and atomic force microscopy techniques, and the kinetic behaviors were analyzed carefully in this work. It was found that the nonbanded spherulites could be observed at lower and higher crystallization temperatures, and the banded spherulites were formed usually at an intermediate range within experimental crystallization temperatures. The competition of the crystallization rate (v _c) and the diffusion rate (v _d) of melt molecules was employed to explain the transition of the spherulitic morphologies. It was considered that the change of the ratio of v _d and v _c would result in the transition of the spherulitic morphologies. The formation and development of the banded structure were discussed in detail. It was found that the band spacing was proportional to diffusion length of melt molecules and increased with increasing of crystallization temperature. The kinetic behaviors of PHB spherulites formed from the thin melt film with different crystallization temperatures were also discussed in this work.     

Variation of crystallite orientation with degree of crystallinity: Isotactic polystyrene

The orientation of the crystallographic c axis (chain axis) was followed by x-ray diffraction during the crystallization of four samples of isotactic polystyrene differing in elongation ratio. The crystallite orientation can be expressed by 〈cos² χ_c〉, where χ_c is the angle between the c axis and the stretching direction. The degrees of crystallinity w were estimated from the diffraction data by using density for calibration. It was found that 〈cos² χ_c〉 decreases in a linear manner with crystallinity, the rate of decrease being very small when the elongation ratio α is 5, but becoming progressively larger as α is decreased toward unity. A qualitative measure suggests that amorphous orientation decreases during crystallization at a rate which is nearly independent of α. The variation of 〈cos² χ_c〉 with w is therefore governed by the orientation of the statistical chain segments prior to crystallization. If the elongation ratio is small, the supply of well oriented statistical segments is limited, and 〈cos² χ_c〉 will decrease at a rapid rate during crystallization. A treatment due to Krigbaum and Roe permits evaluation of the ratio, ν/N, where ν and N are the average numbers of statistical segments in the crystallization nucleus of critical size, and in a network chain, respectively. Our polystyrene samples were not crosslinked, so chain entanglements must serve as junction points. Values of ν could not be obtained, since N was unknown. However, the (ν/N) ratio for isotactic polystyrene decreases slowly with α, and the values agree reasonably well with those obtained in a previous study of oriented polychloroprene networks. After nearly complete crystallization (ω ca. 0.30), the long period spacing measured by low angle diffraction was approximately 135 Å, and varied only slightly with elongation ratio in the range α = 1 to 5. It therefore appears that chain folded lamellae are present in both drawn and undrawn samples of isotactic polystyrene.     

Ordered long‐helical conformation of isotactic polypropylene obtained in constrained environment of nanoclay

Special crystallization event of isotactic polypropylene (iPP) in a constrained environment, a layered clay network, was followed by in situ Fourier transform infrared (FTIR) spectroscopy during the cooling process. Before occurrence of nucleation/crystallization, a conformationally ordered phase, which consists of significant amounts of long 3₁ helices with 14 monomeric units, has been identified for the first time. More importantly, it was found that the long‐ordering helices could play a more important role than short ones for the confined crystallization. It could be tentatively explained as due to the existence of constrained regions in the proximity of the nanoclay platelets or tactoids and the heterogeneous nucleation capability of the surface of nanoclay. Copyright © 2011 John Wiley & Sons, Ltd.     

Mechano-optical behavior in poly (ethylene terephthalate)/poly (ether imide) blends

Mechano-optical behavior and related structural evolution during uniaxial stretching of melt miscible poly (ethylene terephthalate) (PET)/poly (ether imide) (PEI) blends were studied near their glass transition temperature using an instrumented machine that measures true stress, true strain and spectral birefringence simultaneously. Stretching from amorphous state, two distinct stress-optical regimes were observed at temperatures between T_g and T_cc (cold crystallization). Near T_g, a typical photoelastic behavior persists until a critical temperature above which temperature independent initial stress optical behavior is observed. At those temperatures above T_g, where glassy behavior is observed, decreasing stretching rate was also found to eliminate this glassy photo elastic regime leading to the observation of a linear initial stress optical behavior that becomes temperature independent as expected from linear stress optical rule. Increasing PEI concentration in the blends suppresses crystallizability and increases temperature at which initial elastic region disappears giving way to pure liquid behavior where linear stress optical behavior is observed. This is attributed to the increase and broadening of the glass transition temperature with the addition of noncrystallizable PEI. In PET/PEI blends, the stress-optical coefficient (SOC), determined in a linear stress optical regime, was found to increase linearly with the increase in PEI concentration.     

Non-enzymatic browning-induced water plasticization

An exotherm, observed in differential scanning calorimetry (DSC) scans of amorphous food materials above their glass transition temperature,T _g, may occur due to sugar crystallization, nonenzymatic browning, or both. In the present study, this exothermal phenomenon in initially anhydrous skim milk and lactose-hydrolyzed skim milk was considered to occur due to browning during isothermal holding at various temperatures above the initialT _g. The nonenzymatic, Maillard browning reaction produces water that in amorphous foods, may plasticize the material and reduceT _g. The assumption was that quantification of formation of water from theT _g depression, which should not be observed as a result of crystallization under anhydrous conditions, can be used to determine kinetics of the nonenzymatic browning reaction. The formation of water was found to be substantial, and the amount formed could be quantified from theT _g measured after isothermal treatment at various temperatures using DSC. The rate of water formation followed zero-order kinetics, and its temperature dependence well aboveT _g was Arrhenius-type. Although water plasticization of the material occurred during the reaction, and there was a dynamic change in the temperature differenceT–T _g, the browning reaction was probably diffusioncontrolled in anhydrous skim milk in the vicinity of theT _g of lactose. This could be observed from a significant increase in activation energy. The kinetics and temperature dependence of the Maillard reaction in skim milk and lactose-hydrolyzed skim milk were of similar type well above the initialT _g. The difference in temperature dependence in theT _g region of lactose, but above that of lactose-hydrolyzed skim milk, became significant, as the rate in skim milk, but not in lactose-hydrolyzed skim milk, became diffusion-controlled. The results showed that rates of diffusion-controlled reactions may follow the Williams-Landel-Ferry (WLF) equation, as kinetic restrictions become apparent within amorphous materials in reactions exhibiting high rates at the same temperature under non-diffusion-controlled conditions.     

Pore growth and stabilization in uniaxial stretching polypropylene microporous membrane processed by heat-setting

To clarify the pore growth and stabilization process, the heat-setting process during the preparation of polypropylene microporous membranes was followed. It was found that pore size increasing during heat-setting was attributed to the stabilization of stretching-induced connecting bridges through crystallization and lateral shrinkage within amorphous region. The stretching-induced stable connecting bridges came from the secondary crystallization of stretched tie chains and the conversion of some unstable lamellae to fiber crystals through melting and recrystallization behavior. After stretching, during the heating process to heat-setting temperature, it was mainly the melting and recrystallization behavior that decreased the retraction of bridges. During the following heat-setting, the secondary crystallization of stretched tie chains further stabilized the connecting bridges. At the same time, the lateral shrinkage within amorphous region during heating and heat-setting further lead to the pore size increasing. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 1604–1614