Strain-induced crystallization of natural rubber as studied by high-resolution solid-state C NMR spectroscopy

A series of high-resolution solid-state ¹³C NMR experiments were performed on both unstretched and in situ stretched natural rubber samples. From the ¹³C CP/MAS spectra, it was found that natural rubber does form small crystals at room temperature though the degree of crystallinity is very small. Furthermore, from the ¹³C DD/MAS spectra, the crystalline signals were found to increase with the increase of draw ratio. ¹³C spin-lattice relaxation time (T₁) and ¹H spin-spin relaxation time (T₂) of in situ stretched natural rubber were measured for the first time, which provided further evidences for the conclusion that there exist crystals in both stretched and unstretched natural rubber samples. Quantitative ¹³C NMR measurements indicated that strain-induced crystallization occurs when the draw ratio reaches about 2.0 and the maximum crystallinity of our natural rubber samples can be as high as 19.3% upon stretching.     

Crystallization behavior of poly(l-lactic acid)

The crystallization behavior of poly(l-lactic acid) was studied in the range of 80-160 °C. The peak crystallization time (τ_p) was defined and obtained from the crystallization isotherm measured with a differential scanning calorimeter (DSC). Isothermal crystallization temperature (T_c) dependence of log(τ_p) discretely changed at 113 °C (= T_b). The linear growth rate of spherulite, G, was measured with a polarizing microscope. The T_c dependence of G and the size of the spherulite also discretely changed at T_b. Crystal structures for samples isothermally crystallized at temperatures which were higher and lower than T_b were orthorhombic (α-form) and trigonal (β-form), respectively. The discrete change of the crystallization behavior was explained by the formation of different crystal.     

Real-time investigation of crystallization in nylon 6-clay nano-composite probed by infrared spectroscopy

Via time-resolved FTIR, we examined the real-time investigation of the structural change in molecular chain of nylon 6 during crystallization of neat nylon 6 and the corresponding nano-composite (N6C3.7) having fully exfoliated structure. The neat nylon 6 predominantly formed α-phase in the crystallization temperature (T_c) range of 155-195 °C. For N6C3.7 crystallization at low T_c range of 150-168 °C, where the network structure formed by the dispersed clay particles still affected chain folding of nylon 6, the formation of the γ-phase was dominant. The crystallization took place so rapidly (less than 1 s) without induction time of crystallization. At high T_c range (=177-191 °C), the stable growth of the α-phase crystal coexisting with γ-phase occurred in N6C3.7 crystallization. The growth mechanism in the subsequent crystallization processes (amides IIIα and IIIγ) was virtually the same in both N6C3.7 and neat nylon 6.     

Detection of fast and slow crystallization processes in instantaneously-strained samples of cis-1,4-polyisoprene

Cross-linked samples of natural rubber (NR) and synthetic cis-1,4-polyisoprene (IR) were instantaneously expanded to a predetermined strain ratio, α_s, using a newly-designed high-speed tensile tester. Crystallization behavior after the cessation of deformation was investigated. The high-cycle wide-angle X-ray diffraction (WAXD) measurements could successfully reveal the drastic progress of crystallization within the first a few hundred milliseconds. Quantitative analysis of diffraction intensity clarified coexistence of fast and slow crystallization processes; time constants τ_f and τ_s, and amplitude I_f and I_s, respectively, were estimated for these processes. The values of τ_f were in the range of 50–200 ms, while τ_s ranged between 2.5 and 4.5 s. Almost linear dependence of I_f and I_s on α_s was clarified. The crystallite size in the directions both parallel and perpendicular to the stretching direction decreased with the increase in time-averaged nominal stress. The crystal lattice deformed almost linearly with the average nominal stress. For the fast process, correlation between crystallization and stress relaxation was not recognized, while linear relationship between them was found for the slow process. In every case, strain-induced crystallization was found to be the major origin of stress relaxation. Based on the results, effects of strain on crystallization of polymer melt were discussed.     

Isothermal crystallization study on aqueous solution of poly(vinyl methyl ether) by DSC method

A study on the isothermal crystallization of water in aqueous solutions of poly(vinyl methyl ether) (PVME) was carried out by the differential scanning calorimetry (DSC). The influence of PVME concentration (49.5, 44.5 and 39.5 v%) and the crystallization temperature (T_c) on crystallization rate G, crystallization enthalpy (ΔH_c) and melting enthalpy (ΔH_m) was investigated. Avrami equation cannot be used to describe the crystallization process of water in aqueous PVME solution. Within the measured temperature range, the crystallization rate G increases with the crystallization temperature T_c and with the decreasing PVME content. The crystallization enthalpy ΔH_c linearly increases with the degree of supercooling. The influence of T_c on the ΔH_c becomes more marked with increasing PVME concentration. For 49.5 and 44.5 v% PVME solutions, the amount of water arrested in solution during the isothermal crystallization and the final concentration of PVME-rich phase increase linearly with the T_c, whereas for 39.5 v% PVME solution, these two values almost do not change with T_c. The amount of frozen water in the subsequent cold crystallization is approximately proportional to the initial T_c. The approximately constant ΔH_m for a given concentration at the different initial isothermal crystallization temperatures suggests that the total amount of ice from the first isothermal crystallization and the second cold crystallization is same. The quantitative relation of the amount of frozen water in the cold crystallization and the initial T_c demonstrates that PVME/water complexes are thermodynamically unstable.     

Comparative deformational characteristics of poly(styrene-b-ethylene-co-butylene-b-styrene) thermoplastic elastomers and cross-linked natural rubber

Three poly(styrene-b-ethylene-co-butylene-b-styrene) (SEBS) thermoplastic elastomers (TPEs) are studied mechanically and compared to cross-linked natural rubber. It is observed that subtle alterations in the mid-block of the TPEs affect their mechanical properties significantly. The stress relaxation at room temperature is significantly altered indicating a reduced flow in systems where the ratio of ethylene to butylene segments in the mid-block is greater than one. The cyclic behavior of these systems also shows significant hysteresis. Differential scanning calorimetry suggests that these TPEs crystallize at low temperatures, similar to the observed behavior in cross-linked natural rubber. Results of internal energy changes from deformation calorimetry provide evidence for strain-induced crystallization occurring in certain SEBS systems, similar to the internal energy changes observed for cross-linked natural rubber. Simultaneous WAXD/SAXD measurements on deformed SEBS samples highlight deformation at the nanometer and the molecular length scales. In situ WAXD at different strains further reinforces the evidence for formation of strain-induced crystallites in the selected systems. Strain-induced crystallization occurring in certain TPEs provides a mechanism for reduction of flow at high strains and accounts for the retention of their highly elastic behavior.     

The polydispersity of ethylene sequence in metallocene ethylene/α-olefin copolymers III: crystallization and melting behavior of short ethylene sequence

Crystallization and melting behavior of short ethylene sequence of metallocene ethylene/α-olefin copolymer with high comonomer content have been studied by standard DSC and modulated-temperature differential scanning calorimetry (M-TDSC) technique. In addition to high temperature endotherm around 120°C, a low temperature endotherm is observed at lower temperatures (40-80°C), depending on time and temperature of isothermal crystallization. The peak position of the low temperature endotherm T_m^low varies linearly with the logarithm of crystallization time and the slope, D, decreases with increasing crystallization temperature T_c. The T_m^low also depends on the thermal history before the crystallization at T_c, and an extrapolation of T_m^low (30.6°C) to a few seconds has been obtained after two step isothermal crystallization before the crystallization at 30°C. The T_m^low is nearly equal to T_c, and it indicates that the initial crystallization at low temperature is nearly reversible. Direct evidence of conformational entropy change of secondary crystallization has been obtained by using M-TDSC technique. Both the M-TDSC result and the activation energy analysis of temperature dependence suggest that crystal perfection process and conformational entropy decreasing in residual amorphous co-exist during secondary crystallization.     

Melting behavior of poly(l-lactic acid): X-ray and DSC analyses of the melting process

To clarify the melting behavior of poly(l-lactic acid) (PLLA), the wide-angle X-ray diffraction patterns of the isothermally crystallized PLLA samples (ICSs) were successively obtained during heating. We have already suggested the discrete change in the crystallization behavior of PLLA at a crystallization temperature (T_c) of 113 °C (= T_b) and formation of two crystal modifications for the ICSs obtained in the temperature range T_c ≤ T_b and T_c ≥ T_b. It was elucidated from the change in the X-ray diffraction pattern that the phase transition from the low-temperature crystal modification (α′-form) to the high-temperature one (α-form) occurred in a range 155-165 °C for the ICSs(T_c ≤ T_b), and that the crystal structure for the ICSs(T_c ≥ T_b) did not change. Recrystallization during heating, which is the origin of the multiple melting behavior, was proved by the increase in the diffraction intensity before steep decrease due to the final melting. A temperature derivative curve of the X-ray diffraction intensity almost coincided with the DSC melting curve.     

Simple preparation of multi-branched poly(l-lactic acid) and its role as nucleating agent for poly(lactic acid)

The present work proposes a simple preparation of multi-branched poly(l-lactic acid)s (m-PLLAs) and their performance as nucleating agents for PLA. By a simple polycondensation of l-lactic acid (l-LA) on the multi-functional core molecules of succinic acid (SA), malic acid (MA) and citric acid (CA) using stannous octanoate (SnOct₂) as a catalyst, the m-PLLAs as two, three, and four branched PLLA can be obtained easily. Averagely, about 80% of the l-LA feed contents are successfully polymerized on core molecules. Non-isothermal crystallization studies of the PLA blend which contains 1 wt% m-PLLAs indicate that m-PLLAs induce the crystallization of PLA as evidenced from the decreases in glass transition temperature (T_g) and crystallization temperature (T_c), and the increase in degree of crystallinity (χ_c) under the α-form of perfect crystalline in PLA. Isothermal crystallization studies related to Avrami exponent (n), kinetic constant (k) and crystallization half-time (t_1/2) indicate that the blend maintains its three-dimensional growth for the film thickness 0.05 mm, whereas the crystallization growth rate increases more than 4 times as observed from polarized optical microscope (POM). Among m-PLLA, the highest number of multi-branches with the shortest chain PLLA, i.e. CA-PLLA1, initiates the blends to increase its tensile strength and elongation at break for as high as 1.5 times and 1.2 times, respectively.     

Crystallization behavior and melting characteristics of PP nucleated by a novel supported β-nucleating agent

The β-nucleated PP with high β-PP content was prepared by a novel supported β-nucleating agent, which was prepared with pimelic acid supported on nano-CaCO₃ as support. The influences of the content of the support and supported β-nucleating agent, pre-melting temperature (T_melt) and scan rates on crystallization behavior and melting characteristics, and the β-PP content of β-nucleated PP were determined by Differential Scanning Calorimeter (DSC) and Wide-Angle X-ray Diffraction (WAXD). The results indicated that the addition of supported β-nucleating agent markedly increased the crystallization temperature (T_c) of PP. Increasing the content of supported β-nucleating agent slightly increased the T_c, but had no influence on the melting temperatures (T_m) of β-nucleated PP. The T_c and T_m of β-nucleated PP decreased slightly with increasing the content of the support nano-CaCO₃. The effects of scan rates and multiple scans with different T_melt on the crystallization and melting behavior of PP nucleated by supported β-nucleating agent are similar to that of PP nucleated by calcium pimelate (CaHA). The β-PP content above 90 percent was obtained in PP nucleated by supported β-nucleating agent and was not influenced by the content of nano-CaCO₃. The supported β-nucleating agent prepared by supporting pimelic acid on nano-CaCO₃ is a β-nucleating agent with high efficiency and selectivity, and low cost.     

Synchronous and separate homo-crystallization of enantiomeric poly(l-lactic acid)/poly(d-lactic acid) blends

High-molecular-weight poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA) are blended at different ratios and their crystallization behavior was investigated. Solely homo-crystallites mixtures of PLLA and PDLA were synchronously and separately formed during isothermal crystallization in the temperature (T_c) range of 90–130 °C, irrespective of blending ratio, whereas in addition to homo-crystallites, stereocomplex crystallites were formed in the equimolar blends at T_c above 150 and 160 °C. Interestingly, in isothermal crystallization at T_c = 130 °C, the spherulite morphology of blends became disordered, the periodical extinction (periodical twisting of lamellae) in spherulites disappeared, and the radial growth rate of spherulite (G) of the blends was reduced by the synchronous and separate crystallization of PLLA and PDLA and the coexistence of PLLA and PDLA homo-crystallites. However, the interplane distance (d), the crystallinity (X_c), the transition crystallization temperature (T_c) from α′-form to α-form, the alternately stacked structure of the crystalline and amorphous layers, and the nucleation mechanism were not altered by the synchronous and separate crystallization of PLLA and PDLA and the coexistence of PLLA and PDLA homo-crystallites. The unchanged d, X_c, transition T_c, long period of stacked lamellae, and nucleation mechanism strongly suggest that the chiral selection of PLLA or PDLA segments on the growth sites of PLLA or PDLA homo-crystallites to some extent was performed during solvent evaporation and this effect remained even after melting.     

Crystallization behavior of poly(ε-caprolactone) blocks starting from polyethylene lamellar morphology in poly(ε-caprolactone)-block-polyethylene copolymers

The crystallization behavior of poly(ε-caprolactone) (PCL) blocks starting from a solid lamellar morphology formed in advance by the crystallization of polyethylene (PE) blocks (PE lamellar morphology) in a PCL-b-PE diblock copolymer was investigated by differential scanning calorimetry (DSC), small-angle X-ray scattering with synchrotron radiation (SR-SAXS), and polarized optical microscope (POM). The crystallization behavior was quantitatively compared with that of a PCL-block-polybutadiene copolymer, where the crystallization of PCL blocks started from a rubbery lamellar microdomain. DSC and SR-SAXS results revealed that the crystallization rate of PCL blocks in PCL-b-PE increased drastically with decreasing crystallization temperature T_c and the Avrami exponent depended significantly on T_c. SR-SAXS curves during the crystallization of PCL blocks at high T_c showed a bimodal scattering character, that is, the peak position moved discontinuously with crystallization time. At low T_c, on the other hand, no shift of the SAXS peak position was observed. The macroscopic change in morphology was detected only at high T_c by POM observations. These experimental results for the crystallization behavior of PCL blocks in PCL-b-PE all support our previous conclusions obtained by static measurements; the crystallization mechanism at low T_c is completely different from that at high T_c, that is, the PCL blocks crystallize within the PE lamellar morphology at low T_c while the crystallization of PCL blocks at high T_c yields a morphological transition from the PE lamellar morphology into a new solid morphology.     

New insights into thermodynamic description of strain-induced crystallization of peroxide cross-linked natural rubber filled with clay by tube model

Clay changes the strain-induced crystallization behavior of natural rubber and induces a dual crystallization mechanism due to the orientation of clay layers during deformation. The structure evolution was probed by in-situ synchrotron wide-angle X-ray diffraction, while the thermodynamics of the onset of crystallization was analyzed by the tube model. The entropy change required for the onset of the strain-induced crystallization of the clay filled rubber is composed of the entropy reduction due to the rubber-filler interactions and also the stretching. The summation of the two kinds of the entropy reduction is nearly equal to that of the neat rubber. The thermodynamic analysis reveals that the orientation of clay layers along the direction of stretching reduces the chain conformational entropy and changes the crystallization mechanism. These results give some new insights into the strain-induced crystallization process and the reinforcement mechanism for the clay filled rubber.     

Composition dependence of crystallized lamellar morphology formed in crystalline-crystalline diblock copolymers

We have investigated the crystallized morphology formed at each temperature T_c (20 °C ≤ T_c ≤ 45 °C) in double crystalline poly(?-caprolactone)-block-polyethylene (PCL-b-PE) copolymers as a function of composition (or volume fraction of PE blocks ?_PE) by employing small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC) techniques. When PCL-b-PE with ?_PE ≤ 0.58 was quenched from a microphase-separated melt into T_c, the crystallization of PE blocks occurred first to yield an alternating structure consisting of thin PE crystals and amorphous PE + PCL layers (PE lamellar morphology) followed by the crystallization of PCL blocks, where we can expect a competition between the stability of the PE lamellar morphology (depending on ?_PE) and PCL crystallization (on T_c). Two different morphologies were formed in the system judging from a long period. That is, the PCL block crystallized within the existing PE lamellar morphology at lower T_c (<30 °C) to yield a double crystallized alternating structure while it crystallized by deforming or partially destroying the PE lamellar morphology at higher T_c (>35 °C) to result in a significant increase of the long period. However, the temperature at which the morphology changed was almost independent of ?_PE. For PCL-b-PE with ?_PE ≥ 0.73, on the other hand, the morphology after the crystallization of PE blocks was preserved at every T_c investigated.     

A wide-angle X-ray study of the development of molecular orientation in crosslinked natural rubber

Molecular orientation parameters have been measured for the non-crystalline component of crosslinked natural rubber samples deformed in uniaxial tension as a function of the extension ratio and of temperature. The orientation parapeters 〈P₂(cosα)〉 and 〈P₄(cosα)〉 were obtained by an analysis of the anisotropy of the wide-angle X-ray scattering functions. For the measurements made at high temperatures the level of crystallinity detected was negligible and the orientation-strain behaviour could be compared directly with the predictions of molecular models of rubber elasticity. The molecular orientation behaviour with strain was found to be at variance with the estimates of the affine model particularly at low and moderate strains. Extension of the crosslinked rubber at room temperature led to strain-crystallization and measurements of both the molecular orientation of the non-crystalline chains and the degree of crystallinity during extension and relaxation enabled the role of the crystallites in the deformation process to be considered in detail. The intrinsic birefringence of the non-crystalline component was estimated, through the use of the 〈P₂(cosα)〉 values obtained from X-ray scattering measurements, to be 0.20±0.02.     

Multiple recrystallization behavior of poly(1,1-dimethysilacyclobutane): A combined calorimetric and small angle X-ray scattering study

The crystallization and melting behavior of a series of poly(1,1-dimethysilacyclobutane) (PDMSB) samples accessible via living anionic polymerization protocols with different molar masses were studied by differential scanning calorimetry (DSC), small angle X-ray scattering (SAXS) and X-ray diffraction (XRD). It has been found that in the cooling process only one crystallization exotherm was observed by using DSC, but in subsequent heating two clear endotherms appeared. DSC measurements additionally revealed that there will be two melting peaks when the isothermal crystallization temperature, T_c, is low enough, but only one melting peak could be obtained when T_c is high. The value of the lower melting peak T_m1 increased with T_c, showing the increase of lamella thickness due to increase of T_c, which was also evidenced by using SAXS measurements. However, the value of the higher melting peak T_m2 stayed almost constant with T_c. XRD analysis showed that there is only one crystal form independent of T_c. While SAXS results revealed only one long range order, indicating this multiple endotherms phenomenon was caused by the recrystallization process during heating, but not isothermal thickening and thinning process as observed for e.g. poly(ethylene oxide)s (PEOs) with low molar masses. Heating rate dependent DSC experiments showed that there are two recrystallization processes with different time scales. The recrystallization at low temperature always showed up independent of heating rate while the recrystallization at high temperature showed up only when the heating rate was very low. This interesting phenomenon was explained by the different energy barrier for the recrystallization process at low and high temperature as shown by in situ SAXS measurements during heating.     

Rupture, orientation and strain-induced crystallization of polymer chain and network in vulcanized polyisoprene during uniaxial deformation by in-situ Electron Spin Resonance (ESR) and synchrotron X-ray analysis

Different network structures of vulcanized polyisoprene rubbers were studied by in-situ ESR and synchrotron X-ray during deformation to analyze the rupture, orientation, and strain-induced crystallization of polymer chains and network points. Rupture of network points occur, depending on network structure, and create an un-reversible change in vulcanized rubber. The flexibility of network points affects the possibility of rupture, polymer orientation and strain-induced crystallization. Peroxide vulcanized network is rigid and un-rupturable. Poly-sulfide rich vulcanized network is more flexible and less rupturable than mono-sulfide rich vulcanized network. Chain flexibility and rupturability of network points affect the strain-induced crystallization and stress-strain relation.     

Adhesion strength and fracture behavior between carbon black-filled rubber sheets

Adhesion behavior at the interface between a partially-crosslinked and a fully-crosslinked sheet of carbon black-filled rubber compound was investigated over a temperature range from 30 to 120°C. The values of adhesion fracture energy G_a were compared with those of cohesive tear energy G_c. A considerable chemical, as well as physical, interfacial bonding is formed when the uncrosslinked or partially-crosslinked sheet is crosslinked in contact with even a fully-crosslinked sheet. However, there is only a small possibility of chemical bonding when the two fully-crosslinked rubber layers are again crosslinked in contact with each other. An interesting failure mode, termed 'interfacial knotty tearing' was found for a strain-induced crystallizable natural rubber.     

Deterioration behavior analysis of dysprosium and thulium co-doped barium titanate ceramics for multilayer ceramic capacitors

An accelerated testing method for barium titanate (BaTiO₃) dielectrics was proposed to elucidate deterioration behavior of dielectric constant based on the life-temperature relation. The accelerated degradation test (ADT) which was designed using various temperature ranges below and above Curie temperature (T_c) was focused on the optimized composition of dysprosium (Dy) and thulium (Tm) co-doped BaTiO₃. The statistical analysis of the failure time data was performed to determine the optimum distribution as a goodness-of-fitness test. A scale parameter (η) and activation energy (E_α) were calculated in order to predict the life time of the co-doped BaTiO₃, and there was difference between the expected life times according to the acceleration temperature rating of the ADT. The difference of deterioration mechanism around T_c could be deduced from the change of lattice parameter and polarization behavior. The drastic decrease of tetragonality and ferroelectric property caused by the phase transition of the co-doped BaTiO₃ was verified in the temperature above T_c. Accordingly, the acceleration factor over T_c should be considered as reliability study of the BaTiO₃ dielectrics for multilayer ceramic capacitors (MLCCs).     

Isothermal and non-isothermal crystallization behavior of poly(l-lactic acid): Effects of stereocomplex as nucleating agent

The effects of incorporated poly(d-lactic acid) (PDLA) as poly(lactic acid) (PLA) stereocomplex crystallites on the isothermal and non-isothermal crystallization behavior of poly(l-lactic acid) (PLLA) from the melt were investigated for a wide PDLA contents from 0.1 to 10 wt%. In isothermal crystallization from the melt, the radius growth rate of PLLA spherulites (crystallization temperature (T_c)≥125 °C), the induction period for PLLA spherulite formation (t_i) (T_c≥125 °C), the growth mechanism of PLLA crystallites (90 °C≤T_c≤150 °C), and the mechanical properties of the PLLA films were not affected by the incorporation of PDLA or the presence of stereocomplex crystallites as a nucleating agent. In contrast, the presence of stereocomplex crystallites significantly increased the number of PLLA spherulites per unit area or volume. In isothermal crystallization from the melt, at PDLA content of 10 wt%, the starting, half, and ending times for overall PLLA crystallization (t_c(S), t_c(1/2), and t_c(E), respectively) were much shorter than those at PDLA content of 0 wt%, due to the increased number of PLLA spherulites. Reversely, at PDLA content of 0.1 wt%, the t_c(S), t_c(1/2), and t_c(E) were longer than or similar to those at PDLA content of 0 wt%, probably due to the long t_i and the decreased number of spherulites. This seems to have been caused by free PDLA chains, which did not form stereocomplex crystallites. On the other hand, at PDLA contents of 0.3-3 wt%, the t_c(S), t_c(1/2), and t_c(E) were shorter than or similar to those at PDLA content of 0 wt% for the T_c range below 95 °C and above 125 °C, whereas this inclination was reversed for the T_c range of 100-120 °C. In the non-isothermal crystallization of as-cast or amorphous-made PLLA films during cooling from the melt, the addition of PDLA above 1 wt% was effective to accelerate overall PLLA crystallization. The X-ray diffractometry could trace the formation of stereocomplex crystallites in the melt-quenched PLLA films at PDLA contents above 1 wt%. This study revealed that the addition of small amounts of PDLA is effective to accelerate overall PLLA crystallization when the PDLA content and crystallization conditions are scrupulously selected.