Confined Crystallization Behavior Research Articles

On the structure of holographic polymer-dispersed polyethylene glycol

Holographic polymerization (H-P) has been used to fabricate polymer-dispersed liquid crystals and pattern inert nanoparticles. In this article, one-dimensional grating structures of Norland resin and polyethylene glycol (PEG) were achieved using the H-P technique. Both reflection and transmission grating structures were fabricated. The optical properties of the reflection grating structures (also known as Bragg reflectors, BRs) are thermosensitive, which is attributed to the formation and crystallization of PEG crystals. The thermal switching temperature of the BR can be tuned by using different molecular weight PEG samples. The hierarchical structure and morphology of the BR were studied using synchrotron X-ray, polarized light microscopy and transmission electron microscopy. PEG crystals were found to be confined in ∼60 nm thick layers in the BR. Upon crystallization, the PEG lamellae were parallel to the BR surfaces and PEG chains were parallel to the BR normal, resembling the confined crystallization behavior of polyethylene oxide (PEO) in PEO- block-polystyrene (PEO- b-PS) block copolymers. This observation suggests that the tethering effect in the block copolymer systems does not play a major role in PEG chain orientation in the confined nanoenvironment.

Nonisothermal crystallization behavior of the poly(ethylene glycol) block in poly(L‐lactide)–poly(ethylene glycol) diblock copolymers: Effect of the poly(L‐lactide) block length

AbstractThe confined crystallization behavior, melting behavior, and nonisothermal crystallization kinetics of the poly(ethylene glycol) block (PEG) in poly(L‐lactide)–poly(ethylene glycol) (PLLA–PEG) diblock copolymers were investigated with wide‐angle X‐ray diffraction and differential scanning calorimetry. The analysis showed that the nonisothermal crystallization behavior changed from fitting the Ozawa equation and the Avrami equation modified by Jeziorny to deviating from them with the molecular weight of the poly(L‐lactide) (PLLA) block increasing. This resulted from the gradual strengthening of the confined effect, which was imposed by the crystallization of the PLLA block. The nucleation mechanism of the PEG block of PLLA15000–PEG5000 at a larger degree of supercooling was different from that of PLLA2500–PEG5000, PLLA5000–PEG5000, and PEG5000 (the numbers after PEG and PLLA denote the molecular weights of the PEG and PLLA blocks, respectively). They were homogeneous nucleation and heterogeneous nucleation, respectively. The PLLA block bonded chemically with the PEG block and increased the crystallization activation energy, but it provided nucleating sites for the crystallization of the PEG block, and the crystallization rate rose when it was heterogeneous nucleation. The number of melting peaks was three and one for the PEG homopolymer and the PEG block of the diblock copolymers, respectively. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3215–3226, 2006

Effect of electron-irradiation on cross-link density and crystalline structure of low- and high-density polyethylene

Low- and high-density polyethylenes (LDPE and HDPE) were cross-linked in solid state by electron beam irradiation. Molar mass between cross-link joints, M c, and cross-link density, ν, were calculated using rubber elasticity theory and hot set data. The results showed that the ν and creep modulus increased and creep strain and M c decreased with increasing irradiation dose. As compared to HDPE, the LDPE had higher ν and lower M c values at a similar irradiation dose. X-ray analysis and differential scanning calorimetry investigation of first heating cycle revealed no changes in crystalline structure of the irradiated samples. This was attributed to immobilization of radicals frozen in the crystalline phase. As a result of hindered mobility of the polymeric chains, these radicals were not able to cross-link the chains in the crystalline region. However, after melting of the crystals and during subsequent re-solidification process, different levels of crystallinity were developed depending on the applied irradiation dose. The irradiated samples with higher dose had lower crystallization and melting temperatures with reduced crystallinities. These confined crystallization behaviors, observed after a series of cooling and heating cycles, could be attributed to the decrease in the M c values. The length of chain segment needed for usual crystallization by chain folding is decreased due to formation of cross-link joints and hence the crystallization process was hindered.

Melting–Recrystallization of Block Copolymer Crystals in Confined Environments

Recently crystallization of block copolymers in confined domains has been extensively studied. Crystallization in confined environments is usually initiated by homogeneous nucleation at large supercooling. Thus the crystals formed are highly imperfect and have lower crystallinity, compared with the crystals formed under normal crystallization conditions. As a result, the possibility of reorganization of these less perfect crystals into more ordered crystals is of great interest. Such reorganization can be achieved by melting–recrystallization or solid-solid transformation. Rottele et al. investigated melting behavior of strongly segregated poly(butadiene)-b-poly(ethylene oxide) (PB-b-PEO) block copolymers with spherical PEO domains. They observed that PEO crystals with different degrees of perfection could be produced when crystallization of PEO takes place in the confined spherical domains and the less perfect crystals transform into more perfect ones by reorganization in the solid state, but reorganization by melting–recrystallization is prohibited. Chen also observed coalescence of different crystalline domains after annealing the confined domains of PB-b-PEO block copolymer. However, reorganization of confined crystals via melting–recrystallization has not been reported so far. Confined crystallization behavior of semicrystalline block copolymers is strongly influenced by segregation strength of the two blocks and the morphology in the ordered melt. The effects of these two factors on reorganization of the block copolymer crystals have not been reported. In the present work, an oxyethylene/oxybutylene, generally believed to be a weakly segregated block copolymer, was blended with amorphous poly(oxybutylene) homopolymer. Two blends with B 1⁄4 0:76 (cylinder morphology) and B 1⁄4 0:83 (sphere morphology), respectively, were prepared and both exhibited confined crystallization behavior. Reorganization of the PEO crystals in confined domains of different shapes was studied with differential scanning calorimetry (DSC) and synchrotron small angle X-ray scattering (SAXS) at different heating rates.

Morphological Confinement on Crystallization in Blends of Poly(oxyethylene-block-oxybutylene) and Poly(oxybutylene)

Three poly(oxyethylene-block-oxybutylene) diblock copolymers, E76B38, E114B56, and E155B76, were blended with poly(oxybutylene) homopolymer, B14 and B28, such that lamellae (lam), gyroid (gyr), hexagonally packed cylinders (hex), and body-centered-cubic (bcc) spheres were obtained in the melt. The nonisothermal crystallization on cooling of the blends was investigated with synchrotron small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC). In general, two classes of behavior are observed. In confined crystallization the morphology of the melt is retained, and crystals have limited dimensions. In breakout crystallization the melt morphology is destroyed, and a new lamellar morphology is formed. The morphological state (confinement in or breakout of melt morphology) during crystallization was evaluated by the change of the position of the first-order peak (q*) in SAXS and the crystallization temperature (Tc). Breakout of morphology was observed in all E76B38/B14 blends and E114B56/B28 and E155B76/B28 blends with lam morphology, while confined crystallization occurred in E155B76/B28 blends with hex and bcc morphologies and E114B56/B28 blends with bcc morphology. Confined crystallization was also observed in low E content E114B56/B28 blends with hex morphology. These findings lead to two conclusions: (i) the tendency of confined crystallization varies with morphology and decreases in the order bcc > hex > lam, and (ii) confinement on crystallization tends to increase with the extent of segregation between the two blocks. The DSC results show that the crystallization temperature of the blends varies with morphology and chain length of the block copolymers. A much lower Tc was observed for the blends exhibiting confined crystallization behavior, and this phenomenon is explained by a homogeneous nucleation mechanism. The morphology over a larger scale was also examined by polarized light microscopy, and spherulite formation was only observed in the blends where the morphology was easily broken out during crystallization.

Confined crystallization behavior of PEO in organic networks

Confined crystallization behavior of PEO in organic networks

Confined crystallization behavior of PEO in silica networks

Poly(ethylene oxide) (PEO) and silica (SiO 2) organic–inorganic hybrid materials have been synthesized by sol–gel approach. The crystallization behavior of PEO in silica networks has been investigated by differential scanning calorimeter (DSC) and scanning electron micrograph (SEM). The degree of PEO crystallinity in PEO/SiO 2 hybrid networks reduces with the increase of SiO 2. PEO is in amorphous state when the concentration of PEO is lower than 50 wt% in the hybrid materials. The melting points of PEO in the networks are lower than that of pure PEO, but the melting point of PEO in the networks almost has the same melting point. WAXD and SEM results show that the crystalline behavior of PEO in PEO/SiO 2 hybrid system is sternly confined.