Inorganic–organic hybrids involving poly(ε‐caprolactone) and silica network: Hydrogen‐bonding interactions and isothermal crystallization kinetics

Inorganic–organic hybrids mediated by hydrogen‐bonding interactions involving silicon oxide network and poly(ε‐caprolactone) (PCL) were prepared via an in situ sol–gel process of tetraethoxysilane in the presence of PCL. Fourier transform infrared spectroscopy indicated that there were hydrogen‐bonding interactions between carbonyls of PCL and silanol hydroxyls that were formed by incomplete polycondensation in the sol–gel process. In terms of the frequency shift of the hydroxyl stretching vibration bands, it is concluded that the strength of the interassociation between PCL and silicon oxide networks is weaker than that of the self‐association in the control silica network. The phenomenon of equilibrium melting point depression was observed for the PCL/silica system. The hybridization of PCL with silica network causes a considerable increase in the overall crystallization rate and dramatically influences the mechanism of nucleation and growth of the PCL crystallization. The analysis of isothermal crystallization kinetic data according to the Hoffman‐Lauritzen theory shows that with increasing silica content in the hybrids, the surface energy of extremity surfaces increases dramatically for the hybrids. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2594–2603, 2005     

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.     

Rapid crystallization and thermoreversible gelation of poly(ethylene terephthalate) in polymer/oligomer binary system

Poly(ethylene terephthalate) (PET) was rapidly crystallized through thermoreversible gelation in a liquid ethylene glycol oligomer or in epoxy resin. The solutions formed gel rapidly on cooling. Polarized light microscopy and small-angle light scattering showed that these gels contain large, regular PET spherulites. The gels may be formed by two consecutive processes: the phase separation and crystallization, and gelation by formation of a three-dimensional PET network in the oligomer solvents, where the nodes of the network are PET spherulites. The crystallinity of PET recovered from polymer/oligomer gels is near 72% measured by wide-angle X-ray diffraction method, which is about 20% higher than PET samples crystallized by solution crystallization in small molecule solvent, high temperature annealing, and stretching techniques. It takes only a few minutes to form the highly crystalline phase PET in the PET/oligomer system, and the crystallinity of the dried gel is independent of the concentration of the original solution. Excimer-fluoresence and Raman spectroscopic studies indicated that PET recovered from the gels are in an ordered state with few chain entanglements. The entanglement density of the recovered PET recovered from a 20 wt % solution in ethylene glycol oligomer is as low as that of freeze-extracted PET from a 0.5 wt % solution in phenol. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1219–1225, 1998     

Structural origin of the thixotropic behavior of a class of metallosupramolecular gels

Complexation, between a ditopic ligand, consisting of a 2,6-bis-(1′-methylbenzimidazolyl)-4-oxypyridine moiety (O-Mebip) attached to either end of a penta(ethylene glycol) core, with transition metal and lanthanide ions, results in the formation of metallosupramolecular polymers, soluble in acetonitrile at high temperatures. Cooling the hot sol to room temperature causes phase separation and crystallization, and produces mechanically-strong gels, which exhibit a highly thixotropic behavior. Optical microscopy indicates that the gel morphology consists of spherulitic particles, which are easily broken by mechanical shear. Reproducible gel properties are produced when the gel is formed by cooling in a sonication bath, which produces a finely-divided globular morphology, and increases the modulus of the gels. Wide angle X-ray diffraction study shows that the crystalline structures of the gels are strongly dependent on the thermal history of gel formation and the nature of the metal ion. The gel properties are a result of the interactions between the colloidal particles produced by the phase separation and crystallization process. These interactions, which may reflect electrostatic forces and possibly metal-ligand binding, in addition to the usual van der Waals interactions, give rise to the formation of a network structure. The disruption of this network by mechanical shear, and its facile reformation when shear is removed, are the origin of the pronounced thixotropic behavior of the gels.     

Hydrogen bonding and crystallization in biodegradable multiblock poly(ester urethane) copolymer

The hydrogen bonding and crystallization of a biodegradable poly(ester urethane) copolymer based on poly(L ‐lactide) (PLLA) as the soft segment were investigated by FTIR. On slow cooling from melt, the onset and the progress of the crystallization of the urethane hard segments were correlated to the position, width, and relative intensity of the hydrogen‐bonded N? H stretching band. The interconversion between the “free” and hydrogen‐bonded N? H and C?O groups in the urethane units in the process was also revealed by 2D correlation analysis of the FTIR data. The crystallization of the PLLA soft segments was monitored by the ester C?O stretching and the skeletal vibrations. It was revealed that the PLLA crystallization was restricted by the phase separation and the urethane crystallization, and at cooling rates of 10 °C/min or higher, the crystallization of the PLLA soft segments was prohibited. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 685–695, 2009     

Investigation on the gelation behavior of biodegradable poly(butylene succinate) during isothermal crystallization process

The early stage of polymer crystallization may be viewed as physical gelation process,i.e.,the phase transition of polymer from liquid to solid.Determination of the gel point is of significance in polymer processing.In this work,the gelation behavior of poly(butylene succinate)(PBS) at different temperatures has been investigated by rheological method.It was found that during the isothermal crystallization process of PBS,both the storage modulus(G′) and the loss modulus(G″) increase with time,and the rheological response of the system varies from viscous-dominated(G′G″),meaning the phase transition from liquid to solid.The physical gel point was determined by the intersection point of loss tangent curves measured under different frequencies.The gel time(t_c) for PBS was found to increase with increasing crystallization temperature.The relative crystallinity of PBS at the gel point is very low(2.5%-8.5%) and increases with increasing the crystallization temperature.The low crystallinity of PBS at the gel point suggests that only a few junctions are necessary to form a spanning network,indicating that the network is"loosely"connected,in another word,the critical gel is soft.Due to the elevated crystallinity at gel point under higher crystallization temperature,the gel strength S_g increases, while the relaxation exponent n decreases with increasing the crystallization temperature.These experimental results suggest that rheological method is an effective tool for verifying the gel point of biodegradable semi-crystalline polymers.     

Non-isothermal crystallization behaviors of poly(4-methyl-pentene-1)

Non-isothermal crystallization of isotactic poly(4-methyl-pentene-1) (P4MP1) is studied by differential scanning calorimeter (DSC), and kinetic parameters such as the Avrami exponent and the kinetic crystallization rate (Z _c) are determined. From the cooling and melting curves of P4MP1 at different cooling rates, the crystalline enthalpy increases with the increasing cooling rate, but the degree of crystalline by DSC measurement shows not much variation. Degree of crystalline of P4MP1 calculated by wide angle X-ray diffraction pattern shows the same tendency with crystalline enthalpy, indicating that re-crystallization occurs when samples heated above the second glass transition temperature of P4MP1. By Jeziorny analysis, n ₁ value suggests that mainly spherulites’ growth at 2.5 K min⁻¹ transforms into a mixture mode of three-dimensional and two-dimensional space extensions with further increasing cooling rate. In the secondary crystallization process, n ₂ values indicate that the secondary crystallization is mainly the two-dimensional extension of the lamellar crystals formed during the primary crystallization process. The rates of the crystallization, Z _c and t _1/2 both increase obviously with the increase of cooling rate, especially at the primary crystallization stage. By Mo’s method, higher cooling rate should be required in order to obtain a higher degree of crystallinity at unit crystallization time.     

PE/Org-MMT nanocomposites

The non-isothermal crystallization kinetics of polyethylene (PE), PE/organic-montmorillonite (Org-MMT) composites were investigated by differential scanning calorimetry (DSC) with various cooling rates. The Avrami analysis modified by Jeziorny and a method developed by Mo were employed to describe the non-isothermal crystallization process of these samples very well. The difference in the exponent n between PE and PE/Org-MMT nanocomposites, indicated that non-isothermal kinetic crystallization corresponded to tridimensional growth with heterogeneous nucleation. The values of half-time, Z_c and F(T) showed that the crystallization rate increased with the increasing of cooling rates for PE and PE/Org-MMT composites, but the crystallization rate of PE/Org-MMT composite was faster than that of PE at a given cooling rate. The method developed by Ozawa did not describe the non-isothermal crystallization process of PE very well. Moreover, the method proposed by Kissinger was used to evaluate the activation energy of the mentioned samples. The results showed that the activation energy of PE/Org-MMT was greatly larger than that of PE. This revised version was published online in July 2006 with corrections to the Cover Date.     

Optical studies of the stress-induced crystallization of rubber

The low-angle light scattering by films of stretched natural and synthetic rubbers was investigated. Intense V_v scattering is found under conditions when crystallization occurs which is characteristic of the scattering from aggregates of dimensions comparable with the wavelength of visible light. These were identified with the γ fibrils described by Andrews. The dependence of scattering was studied as a function of light polarization direction, orientation direction, elongation, temperature, degree of swelling, type of swelling liquid, and degree of crosslinking. It was concluded that the scattering unit consists of an assembly of crystals with their chain axes parallel to the stretching direction, but (in the case of natural rubber at high elongations) with the fibril axis at a slight angle to the stretching direction. The scattering is not affected much by swelling but is decreased upon increasing the temperature. Upon recooling the scattering returns, but does so over several hours, indicating that much of the scattering arises from secondary crystallization.     

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     

聚乳酸的非等温结晶行为

用差示扫描量热法(DSC)研究聚乳酸(PLA)从熔体及玻璃态为初始条件下的非等温结晶行为,采用Ozawa方程、Mo法、Khanna法和Kissinger法对结晶动力学参数进行计算处理。 实验结果表明,这几种方法均适合处理PLA的非等温结晶过程,而Khanna法提出的结晶速率系数(CRC)能够方便地评价PLA相对结晶速率的大小。 PLA从玻璃态升温结晶比从熔体降温结晶容易得多,升温过程有利于晶核生成,而降温有利于晶体生长。 升温结晶时,升温速率2.0 ℃/min时,结晶焓(ΔH_c)达到最大为27.1 J/g。 从熔体等速降温过程中,随着冷却速率的降低ΔH_c单调增加,冷却速率为0.25 ℃/min时ΔH_c增加到28.3 J/g。 在较低温度下从玻璃态结晶,主要表现为异相成核的二维生长方式。 在较高的温度下从玻璃态升温结晶及从熔体冷却结晶时,以均相成核的三维生长方式结晶为主。 与升温过程相比,冷却不利于晶核的生成,所以导致冷却过程总体ΔH_c偏低,扩散活化能偏大。     

An experimental method for studying nonisothermal crystallization of polymers at very high cooling rates

A new technique based on light depolarizing microscopy was developed for studying non-isothermal crystallization of polymers at average cooling rates up to about 5000°C/min. The polymer is cooled down by a gaseous cooling medium supplied at a constant temperature. The temperature of polymer is measured by a thermocouple imbedded directly in the sample. A heat transfer analysis was used to establish appropriate sample geometry to assure that, under the applied cooling condition, the temperature distribution along the sample thickness can be neglected. A light-scattering effect, which occurs when crystallization is carried out under high cooling rates, was observed. This required the development of a method to correct the depolarized light intensity for the effect of light scattering. An appropriate correction method was developed based on both a theoretical and an experimental analysis of the light intensity measurement. This provided a means to measure the overall crystallization kinetics. Examples of such measurements for iPP, HDPE, and LDPE are presented. In addition to the overall crystallization kinetics, the developed technique includes a video camera and VCR system used for measurements of spherulite growth rates during crystallization under high cooling rates. Constant spherulite growth rates were observed for isotactic polypropylene crystallized under very non-isothermal conditions. © 1996 John Wiley & Sons, Inc.     

Localized Crystallization of Enantiomeric Organic Compounds on Chiral Micro‐patterns from Various Organic Solutions

The controlled crystallization of enantiomers of an organic compound (a cyclic phosphoric acid derivative) on templated micro‐patterned functionalised surfaces is demonstrated. Areas where a complementary chiral thiol has been located were effective heterogeneous nucleation centres when a solution of the compound is evaporated slowly. Various organic solvents were employed, which present a challenge with respect to other examples when water is used. The solvent and the crystallization method have an important influence on the crystal growth of these compounds. When chloroform was employed, well‐defined crystals grow away from the surface, whereas crystals grow in the plane from solutions in isopropanol. In both cases, nucleation is confined to the polar patterned regions of the surface, and for isopropanol growth is largely limited within the pattern, which shows the importance of surface chemistry for nucleation and growth. The apparent dependence on the enantiomer used in the latter case could imply stereo‐differentiation as a result of short‐range interactions (the templating monolayer is disordered, even at the nanometre scale). The size of the pattern of chiral monolayer also determines the outcome of the crystallization; 5 μm dots are most effective. Despite the low surface tension of the samples (relative to the high surface tension of water), differential solvation of the polar and hydrophobic layers of the solvents allows crystallization in the polar regions of the monolayer, therefore the polarity of the regions in which heterogeneous nucleation takes place is indeed very important. Despite the complex nature of the crystallization process, these results are an important step towards to the use of patterned surfaces for heterogeneous selective nucleation of enantiomers.     

Crystallization kinetics of biaxially stretched natural rubber

The isothermal crystallization of natural rubber was investigated under biaxial stretching. A marked planar orientation of crystals occurs in such a way that the (010) plane orients parallel to the film plane. The rate of crystallization increases with the biaxial stretching ratio, and the Avrami exponent n decreases. At the highest extension, n becomes less than unity. The equilibrium degree of crystallinity decreases with the stretching ratio. The crystallinity was less than 10% for the highest extension. The melting behavior is similar to that in uniaxial stretching.     

Optical studies of the crystallization of trans 1,4-polybutadiene in stretched networks

The stress-induced crystallization of trans 1,4-polybutadiene was studied by observing changes in birefringence, stress, x-ray diffraction, and low angle light scattering during the course of crystallization. From these data, the degree of crystallinity was determined as a function of time, temperature, and elongation. Data were fitted to the Avrami equation, leading to an exponent of the order of unity. Light-scattering patterns suggested the simultaneous existence of two stretched forms: a rod-like structure oriented preferentially parallel to the stretching direction and a deformed spherulitelike growth with its greatest extension perpendicular to the stretching direction.     

A Thermodynamic Frame for the Kinetics of Polymer Crystallization under Processing Conditions

A retrospective view is given on work, which has been carried out at the Johannes Kepler University during the last two decades on the structure formation in crystallizable polymers during their processing. Emphasis is laid on some useful principles lend from basic physical chemistry: use of the T,S-diagram and of the temperature dependent courses of (effective) surface tensions along coexistence lines of phases. These considerations should be helpful in creating an overview, where a deficiency of direct experimental crystallization kinetics data exists. For a series of industrially important, mostly fast crystallizing, polymers data are given in the present paper for the quiescent melt (temperature dependences of number densities of nuclei and of growth speeds). Critical minimum cooling speeds for bypassing crystallization are given on the basis of the given data. These critical cooling speeds vary by almost six decades from HDPE to i-PS. These results are also helpful in the formulation of a viable classification of materials in the light of their processing conditions. This classification includes metals and glass forming minerals as limiting cases for extremely fast and extremely slowly crystallizing materials.     

A Thermodynamic Frame for the Kinetics of Polymer Crystallization under Processing Conditions

Summary. A retrospective view is given on work, which has been carried out at the Johannes Kepler University during the last two decades on the structure formation in crystallizable polymers during their processing. Emphasis is laid on some useful principles lend from basic physical chemistry: use of the T,S-diagram and of the temperature dependent courses of (effective) surface tensions along coexistence lines of phases. These considerations should be helpful in creating an overview, where a deficiency of direct experimental crystallization kinetics data exists. For a series of industrially important, mostly fast crystallizing, polymers data are given in the present paper for the quiescent melt (temperature dependences of number densities of nuclei and of growth speeds). Critical minimum cooling speeds for bypassing crystallization are given on the basis of the given data. These critical cooling speeds vary by almost six decades from HDPE to i-PS. These results are also helpful in the formulation of a viable classification of materials in the light of their processing conditions. This classification includes metals and glass forming minerals as limiting cases for extremely fast and extremely slowly crystallizing materials.     

聚合物熔体冷却阶段结晶行为的多尺度模拟

针对结晶型聚合物熔体冷却过程的结晶行为,建立了偶合宏观温度场与微观结晶形态的多尺度模型.该模型揭示了宏观温度的变化会引起晶核数、晶体生长速率的改变,从而影响微观结晶形态;而微观结晶释放的潜热也将导致宏观温度的改变.为了求解上述多尺度模型,提出了有限体积/像素法偶合的多尺度算法,即在粗网格上采用有限体积法对宏观温度场进行求解,而在细网格上采用像素法对微观结晶形态进行模拟.基于多尺度模型及多尺度算法,文中对二维聚合物熔体模壁等速降温的冷却问题进行了研究,考察了温度、相对结晶度的变化及结晶形态的演化,并比较了不同冷却速率、初始温度对温度、相对结晶度及结晶形态的影响.数值结果表明,冷却速率是影响结晶行为的关键.高冷却速率下,温度平台出现较早,持续较短;结晶过程对应的温度范围较广;且平均晶体直径较小.而初始温度只影响温度平台及结晶行为出现的早晚,与其持续时间几乎无关。     

FEM simulation of nonisothermal crystallization, 1 Crystallinity distribution on 2 D space

This work employs the relaxed Stefan model and Nakamura crystallization kinetics to describe the nonisothermal crystallization process of polymeric materials by finite element discretization method (FEM) simulation, giving the evolution of crystallinity distribution on 2 D space. Numerical results show that the final crystallinity and its distribution are mainly dependent on the cooling rate. Crystallinity decreases with increasing cooling rate, but the influence is negligible as long as the cooling rate is below a critical value (ca. 30°C·min^–1 for poly(ethylene terephthalate) (PET)). If the cooling rate is higher than this critical one, crystallinity drops sharply. It is also concluded that the crystallization behavior of polymeric samples in a mild cooling medium is quite different from that in a strong cooling medium. In the first case (for example, in silicon oil), crystallinity of the article is relatively high and its distribution is fairly uniform. During the initial short period, the crystallinity on the surface is higher than that on the inside. Crystallinity increases slowly with time, and finally, the crystallinity of the internal part exceeds the crystallinity on the surface. In the second case (for instance, in water), crystallinity is relatively low, and there is a serious gradient of crystallinity. The crystallinity on the surface reaches a very low equilibrium value in a short time and changes little afterwards. Although the crystallinity of the inside part can be improved by changing the shape of the polymeric article, the crystallinity on the surface essentially remains constant, which leads to a significant gradient. Geometrical shape and dimension of the article are also important to the crystallinity and its distribution, and the ratio of surface area to volume can be used as a rough index to estimate the shape/dimension influence on crystallinity. Except the coefficient of thermal conductivity, physical parameters of the polymeric material and kinetic parameters of crystallization show only weak effects compared to cooling conditions.     

Surface roughness in dynamically crystallized isotactic polypropylene films

The relationship between the dynamic crystallization conditions and surface topography of iso‐polypropylene (i‐PP) films was examined with fractal geometry. When i‐PP was crystallized from a melt at cooling rates in the range between 1 and 100 °C/min, the generated surface topography presented self‐affine behavior at least in the scale range from 0.1 to 100 μm. Moreover, the calculated roughness exponent of these surfaces increased with the cooling rate used to crystallize the samples, which meant a smoother surface at higher crystallization rates. This behavior could be qualitatively explained in terms of the temperature effect on the nucleus stability, the molecular mobility, and the surface tension. In addition, the morphology of quenched samples was analyzed, and different hypotheses were proposed to explain the unusual observed behavior. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 646–655, 2004