MOLECULAR WEIGHT DEPENDENCE OF EQUILIBRIUM MELTING TEMPERATURE AND CRYSTALLIZATION PROCESS OF LOW MOLECULAR WEIGHT POLY (ETHYLENE OXIDE) (PEO) IN PEO/POLY (METHYL METHACRYLATE) (PMMA) BLENDS(Session III : Complex Fluids, The 1st Tohwa University International Meeting on Statistical Physics Theories, Experiments and Computer Simulations) Author(s) Imai, Shinya; Iida, Katsuhiko; Okada, Motoyuki; Takahashi, Masato; Matsuda, Hideomi

Twenty-five percent of the total toroidal field (TF) coil conductors at the ITER are supplied by the Japan Atomic Energy Agency (JAEA). The jacket section of a TF conductor is made of modified 316LN. The JAEA tested three types of tensile specimens (Japanese-Industrial-Standards-type and ASTM-type) cut from the jacket at 4.2 K. The ASTM-type specimen had a longer and wider reduced section than did the JIS-type specimen. The results of the test showed that the EL of the as-received (AR) jacket was independent of the specimen shape. However, after cold working and aging, the EL of the specimens deteriorated because of sensitization, and the EL distribution in these specimens was larger than that in the case of the AR specimens. It could be inferred that the shape of a test specimen having low ductility is the key determinant of the specimen's susceptibility to fracture.

Melting behaviour of ethylene-propylene random copolymer (EP random copolymer) / toluene thermoreversible gels was measured by the falling ball method (FBM) and differential scanning calorimetry (DSC). By applying the theory proposed by Takahashi, Nakamura and Kagawa (Takahashi theory) to the gel melting temperature T-m(g) measured by FBM, values of number of monomers xi per chain in a junction point were evaluated. Further, values of zeta and s were evaluated by the Tanaka theory. zeta in Tanaka theory has the same physical meaning as xi in Takahashi theory and s means the number of polymer chains contained in a junction point. The obtained values of zeta have the same order of magnitude but smaller than xi.In DSC mesurements, the enthalpy of melting (Delta H-m) of junction points was measured. Delta H-m increases with the increase of gelation time, and then approaches a constant value. The gelation time t(n), which is necessary to reach the equilibrium value of Delta H-m, is extremely longer than the gelation rime t(m), which is necessary to obtain a constant value of T-m(g).

Phase diagrams of polystyrene (PS)/Poly(2-chloro styrene) (P2-ClS) blend systems were determined by differential scanning calorimetry (DSC) measurements in order to investigate the molecular weight dependence. The temperature-concentration region where the two phase state is stable increased with increasing PS molecular weight showing the tendensy to increase the incompatibility.

Molecular weight (M) dependence of the lateral growth rate (V) of folded chain crystals (FCCs) of polyethylenes (PE) was investigated. This study was carried out on single (or single crystal-like) crystals using the equilibrium melting temperature (Tm0) determined by applying Gibbs-Thomson's equation. The well-known relation V=V0exp(−B/ΔT) was obtained where V0 and B are constants and ΔT is a degree of supercooling. V0 strongly decreased with increase of M, whereas B did not, which indicates that the self-diffusion process of polymer chains mainly controls the M dependence of V, whereas the nucleation one does not. Experimental formula that V∝V0∝D∝M−H where D is self-diffusion constant and H is a constant; H=1.7 was obtained. These results are similar to Hoffman et al.'s results but their H was rather smaller, H=1–1.5. A similar study on extended-chain single crystals (ECSCs) reported in our previous paper gave the same experimental formula but H was much smaller, H=0.7. From the difference in H between FCCs and ECSCs, a new proposal that M dependence of V may be mainly controlled by the surface diffusion process of chain polymers on the growing crystal surface, is discussed briefly.

The crystallization process of poly(ethylene oxide) (PEO) and PEO/poly(methyl methacrylate) (PMMA) blends in the low molecular weight region of PEO was studied by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The half-time of the crystallization t12, the time at which crystallinity X(t) = 12, was inversely proportional to the linear growth rate G of spherulite, i.e. t12−1 ∝ G. The value of E characterizes the temperature dependence of the activation energy for the formation of a secondary nucleus on the growing side surface of the crystal lamella and was evaluated from the t12 measured by DSC. The value of E of PEO was not affected by blending PMMA. However, the slightly increasing tendency of E was observed for PEO and PEO/PMMA blends with increasing molecular weight of PEO, MPEO. This tendency may correspond to the crossover of E from low molecular weight material to polymer.

The crystalline junction size ζ, which is the number of consecutive ethylene units per crystalline sequence such as –[–CH2–CH2–]–, in an ethylene–propylene (EP) copolymer gel was determined from gel-melting temperatures using the Takahashi theory. In order to examine the correlation between the size ζ and number-average length of ethylene sequences estimated from NMR measurements, an ethylene sequence distribution curve of the copolymer was established from 13C NMR data assuming 1st-order Markov statistics and number-average length in number of ethylene units (L[EE···E]) was calculated from its distribution curve. The average length L[EE···E] determined from this curve was in good agreement with that calculated directly from E/P triad data of NMR. Moreover, number-average length (L[EE···E≥5]) of ethylene sequences greater than or equal to 5, whose value is the crystallizable minimum number of consecutive ethylene units in an EP copolymer, approximately agreed with the junction size ζ in gel, i.e., ζ≃L[EE···E≥5].

A new suspension polymerization process that generates narrow-size distribution and spherical particles in the range of 3-10 microns is described. In the new process, the monomer and water phases are held in separate vessels and fed at an accurate rate to a mechanical disperser, where small, uniform droplets of monomer are formed. The monomer droplets are then fed to a reaction vessel where the monomer is polymerized. The size of the polymerized particles can be effectively controlled by adjusting the disperser speed. A scanning electron microscope was employed to show the spherical nature of the particles. (C) 1993 John Wiley & Sons, Inc.

Gel-melting temperatures (Tmg) of ethylene-propylene random copolymer gels formed in n-hexane and n-octane were measured as a function of polymer concentration. The copolymers were heterogeneous in both composition and molecular weight. Tmg obeyed well the thermodynamic theory derived by Takahashi, in which gel→sol transition was treated as melting of crystalline junctions with fringed micelle-type morphology: Plots of 1/Tmg vs. lnV2N according to the theory were scaled on a common straight line when copolymers had similar propylene contents (PC), where V2 is the volume fraction of the copolymer in the gel and N is the weight-average degree of polymerization. Quantitative analysis of the theory, including measurements of dilute solution properties, led to the results that average size of crystalline junction ζ in number of ethylene units per crystalline sequence was ca. 11 for copolymers with PC=22 wt%, 9 for PC=26 and 27 wt%, and 5 for PC=49 wt%, and thus ζ decreased with increasing PC. The junction sizes ζ were reasonable order as compared with those estimated from X-ray diffractions.

An ethylene-propylene random copolymer (EP) in carbon disulfide, toluene, and cyclopentane was found to convert to a thermoreversible gel at the temperature lower than ca. 10-degrees-C. Sol reversible gel transition temperatures were measured as a function of polymer concentration using several samples with different propylene contents (PC) and molecular weights. PC varies from 22 to 49 wt%. The sol --> gel and gel--> sol transition temperatures, where thermal hysteresis was present for each sample, lowered considerably with increasing PC, regardless of differences in molecular weights. The gel-melting behavior was investigated with the aid of the Takahashi theory which is based upon a conjecture that junction points of a gel are fringed micelle crystallites. The theory was well applicable to the present systems. A common linear relation was found to hold between 1/T(m)g and ln V2N when the samples had similar PC, where T(m)g is the observed gel-melting temperature, V2 is the volume fraction (concentration) of the polymer in the gel, and N is the weight-average degree of polymerization. Further, results of X-ray diffraction, differential scanning calorimetry, and microscopic observation showed that gelation took place through crystallization and that junction points of an EP gel were made up of microcrystallites such as fringed micelles, particularly for a sample with high content of propylene.

In order to investigate the structure and morphology of linear low density polyethylene (LLDPE) gel in organic solvents, measurements of gel-melting temperature, melting temperature of crystallites formed in the gel, differential scanning calorimetry, electron microscopy, and shear modulus of the gel have been carried out in a range of concentration from 1 to 20 g/100 cm3. As a result, two kinds of network structures are found to be constructed with increasing concentration of polymer in the gel. That is, one is the network constructed mainly by microcrystallites formed from low molecular weight species with a lot of short chain branches (SCB), whose structure appears in a lower concentration region than ca. 4 g/100 cm3; the other is that constructed by crystalline linkages of large crystallites (dendrites) formed from high molecular weight species with a little SCB, which appears in a higher concentration region than 4 g/100 cm3. According to Takahashi’s theory derived for gel-melting temperature of crystalline copolymer gel, at least ca. 19 ethylene-units participate in the microcrystallite to form a junction point of the network at an early stage of gelation from a solution.

Sol-gel transition and the structure of isotactic polypropylene (itPP) gels formed from toluene and o-xylene solutions were studied. Weight-average molecular weight range of samples used is from 25.8×104 to 50.1×104. A thermo-reversible gel was formed from a semi-dilute solution on cooling. Gel-melting temperature Tmg was measured and its dependency on polymer concentration as well as molecular weight was investigated. Tmg increased gradually with increasing polymer concentration and obeyed Eldridge-Ferry’s type of plot: the reciprocal absolute gel-melting temperature 1/Tmg had a linear relationship to the logarithm of volume fraction of polymer, while molecular weight dependency of Tmg was little. In a gel, many spherulites were observed, the spherulites were bound with crystalline ties, and a three-dimensional network structure was observed. In this crystalline network, a polymer solution was confined. It is concluded that the formation of spherulites and crystalline ties is conducive to gelation.

The sol-gel transitions of linear low density polyethylenes in decalin, tetralin, o-xylene, and toluene were studied. Each sample was a copolymer of ethylene and butene-1, 4-methyl-pentene-1, or octene-1. A thermo-reversible gel was formed for each system and gel-melting temperatures were measured at concentrations of about 4–14 g/100 cm3. It was found experimentally that the gel-melting temperature depended on polymer concentration, molecular weight of the polymer, and the kind of comonomer. Moreover, the structure of crystallite involved in the gel was examined by a polarizing microscope and many spherulites were observed. The data were treated phenomenologically using the thermodynamic theory of Takahashi, Nakamura, and Kagawa, derived for the gel-melting temperature of a crystalline linear copolymer gel. In each solvent, a linear relationship was found to hold between the reciprocal absolute gel-melting temperature 1/Tmg, and ln V2N for samples having a same kind of comonomer, where V2 is the volume fraction of the polymer and N is the weight-average degree of polymerization.

Solutions of linear low density polyethylenes in organic solvents formed thermoreversible gels on cooling. Gel-melting temperatures of the polymers in tetralin, decalin, and o-xylene were measured. They increased slowly with increasing polymer concentration. Experimental data were analyzed by the thermodynamic theory of Takahashi, Nakamura, and Kagawa, which is derived for the gel-melting temperature of a crystalline linear copolymer gel. A plot of the gel-melting temperature by the theory depended considerably on the kind of comonomer of the polymer.