Hierarchical crystalline structures induced by temperature profile in HDPE bars during melt penetration process

The hierarchical crystalline morphologies and orientation structures across the thickness direction in high-density polyethylene(HDPE) molded bars were investigated via a novel melt-penetrating processing method named multi-melt multi-injection molding(M3IM). The samples with various mold temperatures(20, 40 and 60 °C) were prepared, and the effects of the external temperature profile on the evolution of crystalline microstructures were studied. With scanning electron microscopy(SEM), the transition of crystalline morphology from ring-banded structure to oriented lamellae was observed with decreasing mold temperature, and the oriented lamellae were formed at the sub-skin layer of the samples at the lowest mold temperature, which was further testified by differential scanning calorimetry(DSC). With the decline of mold temperature, the degree of orientation, obtained from two-dimensional small angle X-ray scattering(2D-SAXS), was increased and long periods rose a little. Thus, decreasing mold temperature was beneficial to the formation of orientation structures because the relaxation of chains was weakened.     

Equilibrium Crystallization Temperature of Syndiotactic Polystyrene γ Form

The crystallization behavior of syndiotactic polystyrene(s PS) γ form undergoing annealing at various temperatures was investigated using the thermodynamic phase diagram based on Strobl's crystallization theory. On the basis of the differential scanning calorimetric results, it was observed that γ form melt-recrystallization occurred at a higher temperature with the increasing lamellar thickness, which resulted from the pre-annealing at the elevating temperature after acetone induced crystallization. Further temperature dependent small-angle X-ray scattering(SAXS) measurement revealed the evolution of the γ form lamellae upon heating until phase transition, involving three different regimes: lamellae stable region(25-90 °C), melt-recrystallization region(90-185 °C) and pre-phase transition region(185-195 °C). As a result, recrystallization line, equilibrium recrystallization line and melting line were developed for the s PS γ form crystallization process. Since the melt of γ form involved a γ-to-α/β form phase transition, the melting line was also denoted as the phase transition line in this special case. Therefore, the equilibrium crystallization temperature and melting(phase transition) temperatures were determined at around 390 and 220 °C on the basis of the thermodynamic phase diagram of the s PS γ form.     

Crystallization behavior of polylactide on highly oriented polyethylene thin films

The crystalline structure and morphology of the PLA crystallized isothermally from the glassy state on highly oriented PE substrates at 130℃were investigated by means of optical microscopy,AFM and X-ray diffraction.The results indicate that the PE substrate influences the crystallization behavior of PLA remarkably,which leads to the growth of PLA crystals on PE substrate always in edge-on form rather than the twisted lamellar crystals from edge-on to flat-on when crystallizing the PLA on glass surface under the same condition.The edge-on PLA lamellae on the PE substrate are preferentially arranged with their long axes in the chain direction of the PE substrate crystals.It is further demonstrated that except for the different crystal orientation,the PE does not influence the crystalline modification and crystallinity of the PLA.     

THE CRYSTALLIZATION BEHAVIOR OF URETHANE SUBSTITUTED POLYDIACETYLENE

The crystalline behavior of urethane substitute polydiacetylene was studied by using pohrized light and electron microscopy. The lamellar morphological structure was observed in the crystallized films. The thickness of lamellae is about 300A, being independent of the crystalline temperature. But the size and density of lamellae were dependent on the crystallization temperature. If the molten film was sheared during the crystallzation process the oriented lamellae grew with their long axes perpendicular to the direction of shear and the chain direction was normal to the lamellar surface.     

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.     

Equilibrium Crystallization Temperature of Syndiotactic Polystyrene <Emphasis Type="Italic">γ</Emphasis> Form

The crystallization behavior of syndiotactic polystyrene (sPS) γ form undergoing annealing at various temperatures was investigated using the thermodynamic phase diagram based on Strobl's crystallization theory.On the basis of the differential scanning calorimetric results,it was observed that γ form melt-recrystallization occurred at a higher temperature with the increasing lamellar thickness,which resulted from the pre-annealing at the elevating temperature after acetone induced crystallization.Further temperature dependent small-angle X-ray scattering (SAXS) measurement revealed the evolution of the γ form lamellae upon heating until phase transition,involving three different regimes:lamellae stable region (25-90 ℃),melt-recrystallization region (90-185 ℃) and pre-phase transition region (185-195 ℃).As a result,recrystallization line,equilibrium recrystallization line and melting line were developed for the sPS γform crystallization process.Since the melt of γform involved a γto-α/β form phase transition,the melting line was also denoted as the phase transition line in this special case.Therefore,the equilibrium crystallization temperature and melting (phase transition)temperatures were determined at around 390 and 220 ℃ on the basis of the thermodynamic phase diagram of the sPS γform.     

Deformation Temperature and Lamellar Thickness Dependency of FormⅠto Form Ⅲ Phase Transition in Syndiotactic Polypropylene during Tensile Stretching

Phase transition from form Ⅰto 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.     

Crosslinking kinetics of polyethylene with small amount of peroxide and its influence on the subsequent crystallization behaviors

Crosslinking reactions of high density polyethylene with low peroxide concentrations ranging from 0.1 wt% to 1.0 wt% at temperatures of 170, 180 and 190 ° C were monitored by rheological measurements. A critical gel forms at the peroxide concentration of 0.2 wt%, where the transition from long chain branching generation to crosslinking network formation could occur. Rheokinetics of crosslinking can be fitted well by Ding-Leonov's model. The curing rate k_2 at the earlier stage exhibits about 3 times acceleration per 10 °C with increasing temperature, while the equilibrium modulus G′ at the fully cured stage is almost independent of temperature. Influences of crosslinking on the subsequent crystallization behaviors were detected by DSC measurements. Above the critical gel concentration, crystallization is largely retarded as evidenced by the lower crystallization temperature Tc and crystallinity X_c due to the network formation. The secondary crystallization valley located at the temperature near 80 °C can be observed above the critical concentration, which becomes more evident with the increasing peroxide concentration and curing temperature. This phenomenon provides another evidence of crystallization retardation by the crosslinking network.     

New understanding on the memory effect of crystallized iPP

In this study, recovery processes of isotactic polypropylene(iPP) melted spherulites at 135 °C after melting at higher temperatures(170 °C–176 °C) were investigated with polarized optical microscopy and Fourier transform infrared spectroscopy. The recovery temperature was fixed to exclude the interference from heterogeneous nuclei. After melting at temperatures between 170 °C and 174 °C, the melted spherulite could recover back to the origin spherulite at low temperatures. Interestingly, a distinct infrared spectrum from iPP melt and crystal was observed in the early stage of recovery process after melting at low temperatures, where only IR bands resulting from short helices with 12 monomers or less can be seen, which indicates that the presence of crystal residues is not the necessary condition for the polymer memory effect. Avrami analysis further indicated that crystallization mainly took place in melted lamellae. After melting at higher temperatures, melted spherulite cannot recover. Based on above findings, it is proposed that the memory effect can be mainly ascribed to melted lamellae, during which crystalline order is lost but conformational order still exists. These conformational ordered segments formed aggregates, which can play as nucleation precursors at low temperatures.     

Morphology and Crystalline Structure of Inclusion Compounds Formed between Poly（ethylene glycol） and Urea

The poly（ethylene glycol） （PEG, with Mw 2000）-urea inclusion compound （IC） crystallized at high temperature region showed two typical orientations, flat-on and edge-on crystals. 2D-XRD and polarized FTIR spectroscopy revealed that the PEG chains within urea channels were perpendicular to the substrate in fiat-on oriented crystals, while PEG chain axes were parallel to the substrate and lay along the growth direction in the edge-on crystals. FT1R absorption bands of PEG in the ICs are sensitive to orientation of the crystals. A scheme of PEG chain packing in the urea IC channel was proposed, which could explain the orientation of the crystal nucleus causing the two types of morphologies. Furthermore, functioning of PEG2000 chain end with analine had significantly influence on the morphology and orientation of the inclusion compound crystals, due to the defects caused by large terminal groups included in the urea channel.     

聚(ε-己内酯)薄膜结晶形貌及分子取向研究

用匀胶机通过溶液铸膜方法在硅片和铝箔基板上分别制备具有不同厚度的聚(ε-己内酯)(PCL)薄膜. 通过原子力显微镜(AFM)和偏光衰减全反射傅里叶红外光谱(ATR-FTIR)对薄膜中PCL的结晶形貌、 片晶生长方式及分子链取向进行了研究. AFM结果表明, 在200 nm或更厚的薄膜中, PCL主要以侧立(edge-on)片晶的方式生长; 对于厚度小于200 nm的薄膜, PCL片晶更倾向于以平躺(flat-on)的方式生长. 这种片晶生长方式的改变在硅片和铝箔基板上都表现出同样的倾向. 此外, 在15 nm或更薄的薄膜中, PCL结晶由通常的球晶结构变为树枝状晶体. 偏光ATR-FTIR结果表明, 当膜厚小于200 nm时, 薄膜结晶中PCL分子链沿垂直于基板表面方向取向, 并且膜越薄, 取向程度越高, 与AFM的观测结果一致.     

Thickness-dependent molecular chain and lamellar crystal orientation in ultrathin poly(di-n-hexylsilane) films

The molecular chain and lamellar crystal orientation in ultrathin films (thickness < 100 nm) of poly-(di-n-hexylsilane) (PDHS) on silicon wafer substrates have been investigated by using transmission electronic microscopy, wide-angle X-ray diffraction, atomic force microscopy, and UV absorption spectroscopy. PDHS showed a film thickness-dependent molecular chain and lamellar crystal orientation. Lamellar crystals grew preferentially in flat-on orientation in the monolayer ultrathin films of PDHS, i.e., the silicon backbones were oriented along the surface-normal direction. By contrast, the orientation of lamellar crystals was preferentially edge-on in ultrathin films thicker than ca. 13 nm, i.e., the silicon backbones were oriented parallel to the substrate surface. We interpret the different orientations of molecular chain and lamellar crystal as due to the reduction of the entropy of the polymer chain near the substrate surface and the particularity of the crystallographic (001) plane of flat-on lamellae, respectively. A remarkable influence of the orientations of the silicon backbone on the UV absorption of these PDHS ultrathin films was observed due to the one-dimensional nature of sigma-electrons delocalized along the silicon backbone. With the silicon backbones perpendicular or parallel to the surface of the substrate, the UV absorbance increased or decreased with an increase of the angle between the incident UV beam direction and direction normal to the thin film, respectively.     

Crystal structure and thermal behavior of cold-crystallized poly(trimethylene terephthalate)

The structure and thermal behavior of cold-crystallized poly(trimethylene terephthalate) (PTT) are revealed in detail by DSC, AFM, TEM, and WAXD as well as in situ FTIR and SAXS techniques. There is no effect of crystallization temperature and initial state on the crystal modification, yet the morphology is strongly affected by these two factors. First, the small rod-like lamellae for PTT are obtained during the cold crystallization instead of the spherulites formed in the melt crystallization. Second, the edge-on lamellar orientation in thin films is identified during the cold crystallization. The thickness and the lateral width of rod-like lamellae get larger and larger with increasing crystallization temperature. Thin lamellar crystals assemble randomly when the cold-crystallization temperature is lower, while lamellar stacks composed of thicker lamellae are observed when the PTT was annealed at elevated temperature. Moreover, for the cold-crystallized PTT, the final melting temperature does not vary with the crystallization temperature. This phenomenon is explained by the structural improvement during the heating process. For the cold-crystallized PTT sample at lower temperature, three transitions occur when it is heated again: the relaxation of the rigid amorphous phase, the reorganization of molecules in the intermediate phase, and then the melt–recrystallization behavior. Those transitions finally lead to thicker lamellae besides a higher crystallinity before the final fusion. Therefore, the final melting peak of these lamellae is at the same temperature.     

Morphologies of long chain branched isotactic polypropylene crystallized from melt

The crystallization behavior and resulting crystalline morphologies of long chain branched isotactic polypropylene (LCBPP) under different conditions were studied by means of differential scanning calorimetry and transmission electron microscopy combined with electron diffraction. The results indicate that the crystallization of LCBPP during fast cooling process, or at lower crystallization temperature, leads to the formation of mainly edge-on lamellar structures. The LCBPP exhibits also the wide angle lamellar branching frequently observed for linear isotactic polypropylene. Crystallizing LCBPP in temperature range of 110∼140 °C results in the formation of both edge-on and flat-on crystals, which coexist side by side with the content of flat-on crystals increases with increasing crystallization temperature. At high crystallization temperature, e.g. 145 °C, flat-on crystals with chain axis aligned perpendicular to the film plane are the only observed morphology. Moreover, the crystals of LCBPP grow slower than its linear counterpart and the crystal growth rates of both linear and long branched PPs are temperature dependent.     

Lamellar assembly corresponding to transitions of positively to negatively birefringent spherulites in poly(ethylene adipate) with phenoxy

Surface morphology of positively or negatively birefringent spherulites in melt-crystallized neat poly(ethylene adipate) (PEA) vs. PEA blend with phenoxy was examined using atomic force microscopy (AFM), scanning electron microscopy, polarizing optical microscopy, thermal analysis, and wide-angle X-ray techniques. Their top-surface morphology in thin film forms was analyzed to fully expounded the lamellar assembly responsible for the opposite birefringence. Top-surface lamellar assemblies in positive/negative types of ringless spherulites (T _c = 0, 15, 20, 40 °C) and also alternating birefringence of double-ring-banded spherulite (T _c = 28 °C) of PEA/phenoxy blend were examined with AFM. From the results, spherulite’s positive and negative birefringence differs only in interior lamellar arrangements but not lattice geometries. Negative spherulites are composed of radially oriented edge-on lamellae, while positive spherulites are composed of bending/coiling edge-on lamellae. By contrast, the ring-banded spherulites can exhibit both negative and positive birefringence depending on the alternating radial and tangential lamellar arrangement. The addition of phenoxy into PEA could disrupt the regular lamellar bending and promote the singularity of edge-on lamellae; owing to that, the amorphous phenoxy induces looser arrangement of edge-on lamellae with phenoxy being in interlamellar/interfibrillar regions. The bulky linking pendent group phenoxy, with H-bonding capacity interacting with PEA, also disrupts the regularity of tangential–radial PEA lamellae to display a more zigzag pattern.     

Observation of banded spherulites and lamellar structures by atomic force microscopy

Spherulites are common structures of semi-crystalline polymers. It has been known that semi-crystalline polymers can form spherulites when crystallized from solution or from melt. A dark Maltese cross of a spherulite could be easily observed under the polarized optical microscopy (POM). Moreover, some spherulites show an additional alternating dark and bright concentric ring structure that is attributed to the regular twisting of the radial crystallite ribbons as they grow from the spherulit…     

Influence of the order of polymer melt on the crystallization behavior: I. Double melting endotherms of isotactic polypropylene

The influence of the order of polymer melt on the subsequent crystallization and melting has been carefully studied. The experimental data show that the order of isotactic polypropylene (iPP) melt decreases with increases in the fusion temperature. For an iPP sample isothermally crystallized at 130 °C for half an hour, the degree of order of melt is higher when the fusion temperature is lower than about 170.5 °C, hence the lamellae formed in a rapid cooling process are perfect. If the fusion temperature is not higher than 167 °C, some thicker lamellae can exist in the melt. The melting of these unmelted lamellae and those lamellae recrystallized in the cooling process result in double endotherms. On the other hand, when the fusion temperature is higher than 170.5 °C, the order of the iPP melt decreases greatly; thus, the lamellae formed in the following cooling process are imperfect. At a lower heating rate, the recrystallization or reorganization of these imperfect lamellae also leads to double melting endotherms. Received: June 16, 2000 Accepted: October 16, 2000     

Annealing of Oriented PP/PE Double-layer Film within the Melting Range of PE: the Role of Partial Melting and Self-nucleation

Due to the mechanical stability of the PP layer, the oriented PP/PE double-layer film with a row-nucleated crystalline structure can be annealed at a higher temperature than the PE monolayer film. In this work, the effects of annealing temperature within the melting range of PE on the crystalline structure and properties of PP/PE double-layer films were studied. When the annealing temperature is between 100 and 130 °C, below the melting point of PE, the crystallinity, the long period, lateral dimension and orientation of the lamellae in the PE layer increase with the annealing temperature due to the melting of thin lamellae and the self-nucleated effect of partially-melted melts during annealing. With the annealing temperature further increasing to 138 °C, near the melting ending point of PE, since the lamellae melt completely and the melt memory becomes weak during annealing, some spherulite structures are formed in the annealed sample, resulting in a decrease of orientation. In contrast, the annealing only causes the appearance of a low-temperature endothermic plateau in the PP layer. The improved size and orientation of lamellar structure in the PE layer increase the pore arrangement and porosity of the stretched PP/PE microporous membrane. This study successfully applies the self-nucleation effect of partially-melted polymer melt into the practical annealing process, which is helpful to guide the production of high-performance PP/PE/PP lithium batteries separator and the annealing process of other multilayer products.

     

Orientation control of ferroelectric polymer molecules using contact-mode AFM

We have developed an orientation control technique for polymer molecules utilizing contact-mode atomic force microscopy (AFM). In this technique, the molecular chains were directly modified by scanning an AFM cantilever tip in contact with the film surface at the temperature just below its melting point. We call this process “modification scan”. Here, we applied this technique to poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)) thin films on graphite and glass. We prepared a 75-nm thick copolymer crystalline film on graphite whose lamellar plane was perpendicular to the substrate (edge-on), and also prepared a film of the same thickness on glass whose lamellar plane was parallel to the substrate (flat-on). After applying this technique on both films, molecular chains were stretched and aligned to the modification scan direction, and new edge-on crystals were obtained, whose lamellar planes were well-aligned perpendicular to the modification scan direction.