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Eiichi Kobayashi - One of the best experts on this subject based on the ideXlab platform.

Hidetaka Tobita - One of the best experts on this subject based on the ideXlab platform.

  • Fundamental Molecular Weight Distribution of RAFT Polymers
    Macromolecular Reaction Engineering, 2008
    Co-Authors: Hidetaka Tobita
    Abstract:

    The Molecular Weight Distribution formed in an ideal reversible addition-fragmentation chain transfer (RAFT)-mediated radical polymerization is considered theoretically. In this polymerization, the addition to the RAFT agent is reversible, and the active period on the same chain could be repeated, via the two-armed intermediate, with probability 1/2. This possible repetition is accounted for by introducing a new concept, the overall active/dormant periods. With this method, the apparent functional form of the Molecular Weight Distribution (MWD) reduces to that proposed for the ideal living radical polymers (Tobita, Macromol. Theory Simul. 2006, 15, 12). The repetition results in a broader MWD than without the repetition. The formulae for the average Molecular Weights formed in batch and a continuous stirred tank reactor are also presented.

  • Molecular Weight Distribution of living radical polymers 1 fundamental Distribution
    Macromolecular Theory and Simulations, 2006
    Co-Authors: Hidetaka Tobita
    Abstract:

    Summary: The Molecular Weight Distribution formed in an ideal living radical polymerization is considered theoretically. It was found that the hypergeometric function that combines the most probable and the Poisson Distribution represents a fundamental Distribution of the living radical polymers. The number- and Weight-average Molecular Weights are derived for this fundamental Distribution, together with those for polymerizations in a batch and in a continuous stirred tank reactor. These average Molecular Weight functions are obtained based on the arithmetic calculations without deriving the Distribution functions. The effect of the monomer transfer reactions on the formed MWD is also considered. The present study clarifies the relationship between the reaction mechanism and the formed Molecular Weight Distribution as well as the fundamental characteristics of living radical polymers. Calculated number fraction Distribution N(r) development with (dashed) and without (solid) the monomer transfer reactions.

  • Molecular Weight Distribution of graft copolymers prepared from macromonomers
    Polymer, 1999
    Co-Authors: Hidetaka Tobita
    Abstract:

    A fundamental theoretical investigation is conducted for the Molecular Weight Distribution formed in free-radical and living copolymerizations with macromonomers by using the random sampling technique. General analytical expressions for the number- and Weight-average Molecular Weight developments are obtained. The full Molecular Weight Distribution functions are presented for some simpler cases with low mole fractions of macromonomers. The present theoretical analysis provides a great insight into the complex Molecular buildup processes, and thus leads to a better control of the graft copolymers.

  • Copolymerization with Chain Transfer Monomer. 2. Molecular Weight Distribution
    Macromolecules, 1997
    Co-Authors: Hidetaka Tobita
    Abstract:

    The full Molecular Weight Distribution formed in a batch free-radical copolymerization with chain transfer monomer is considered theoretically. A Monte Carlo simulation algorithm is proposed on the basis of the random sampling technique. Illustrative calculations are conducted for the cases with equal reactivity of all types of double bonds. It was found that even when the probability of possessing a branch point is the same for all units, the formed Molecular Weight Distribution is highly dependent on the chain connection rule. BiMolecular termination by combination may contribute to form extremely large polymer molecules by forming cross-linkages between primary chains. Under the idealized conditions, homogeneously branched polymer molecules are formed, and the analytical solution for the Molecular Weight Distribution can be obtained.

  • Molecular Weight Distribution in random crosslinking of polymers: Modality of the Molecular Weight Distribution
    Macromolecular Theory and Simulations, 1994
    Co-Authors: Hidetaka Tobita, Yoshiyasu Yamamoto
    Abstract:

    The Molecular Weight Distribution (MWD), formed during random crosslinking of polymers whose initial Molecular Weight Distribution is given by the Schulz-Zimm Distribution, is proposed. The obtained equation can be considered as a good approximation even for nonrandom crosslinking reactions such as free-radical and living crosslinking copolymerizations. When the initial Molecular Weight Distribution is very narrow, namely, the polydispersity index (Pw/Pn) is smaller than about 1,1, a multimodal Distribution can be obtained. Quite often the MWD is represented solely by the average Molecular Weights such as number- and Weight-average Molecular Weights in conventional approaches; however, such types of measure can not give enough description for multimodal Distributions. Multimodal Distributions may be obtained for the primary polymer molecules formed via living polymerizations. On the other hand, for the primary polymer molecules formed by free-radical polymerization whose polydispersity index must be larger than 1,5, it is impossible to form a bimodal Distribution as long as nonideal kinetics such as the size and structure dependence of crosslinking reactions are neglected.

Sadahito Aoshima - One of the best experts on this subject based on the ideXlab platform.

Savvas G. Hatzikiriakos - One of the best experts on this subject based on the ideXlab platform.

  • Wall slip of HDPEs: Molecular Weight and Molecular Weight Distribution effects
    Journal of Rheology, 2013
    Co-Authors: Mahmoud Ansari, Paul J Deslauriers, Ashish M. Sukhadia, Savvas G. Hatzikiriakos
    Abstract:

    The slip behavior of several high-density polyethylenes (HDPEs) is studied as a function of Molecular Weight (Mw) and its Distribution for broad Molecular Weight Distribution metallocene and Ziegler–Natta catalyst resins. It is found that slip depends strongly on Mw and its Distribution. First, the slip velocity increases with decrease of Molecular Weight, which is expected to decay to zero as the Mw approaches a value with characteristic Molecular dimension similar to surface asperities. For HDPEs that exhibit stick–slip transition, the slip velocity has been found to increase with increase of polydispersity. The opposite dependence is shown for HDPEs of wider Molecular Weight Distribution that do not exhibit stick–slip transition. A criterion is also discussed as to the occurrence or not of the stick–slip transition which is found to depend strongly on Mw and its Distribution.

  • Rheology of Ziegler–Natta and metallocene high-density polyethylenes: broad Molecular Weight Distribution effects
    Rheologica Acta, 2011
    Co-Authors: Mahmoud Ansari, Ashish M. Sukhadia, Savvas G. Hatzikiriakos, David C. Rohlfing
    Abstract:

    The linear viscoelastic properties of two series of Ziegler–Natta and metallocene HDPEs (ZN-HDPEs and m-HDPEs, respectively) of broad Molecular Weight Distribution (MWD) have been studied. Correlations between zero-shear viscosity and Molecular Weight and Molecular Weight Distribution show that the breadth of the MWD for m-HDPEs plays a role. Other interesting correlations between the crossover modulus and steady-state compliance with MWD of both these classes of polymers have also been derived. Finally, the steady-shear viscosities from capillary rheometry are compared with LVE data to check the applicability of the empirical Cox–Merz rule. It is shown that the original Cox–Merz rule is applicable for the ZN-HDPEs, while it apparently fails for the m-HDPEs. However, once the capillary data for m-HDPEs are corrected for slip effects, the applicability of the Cox–Merz rule is validated for their case as well.

Timothy P. Lodge - One of the best experts on this subject based on the ideXlab platform.

  • Molecular Weight Distribution of Polystyrene Made by Anionic Polymerization
    Macromolecules, 2000
    Co-Authors: Wonmok Lee, Hyunjung Lee, Junhoe Cha, Taihyun Chang, Kenneth J. Hanley, Timothy P. Lodge
    Abstract:

    A set of polystyrenes of varying Molecular Weights was synthesized by anionic polymerization under identical conditions, and their Molecular Weight Distributions were critically examined. Polymerization of styrene was initiated with 2-butyllithium in cyclohexane at 45 °C. During the polymerization, seven aliquots of the reaction mixture were taken out at various reaction times using a cannula and terminated. The polymerization time varying Molecular Weights and Molecular Weight Distributions of the polystyrenes were analyzed by temperature gradient interaction chromatography (TGIC) as well as size exclusion chromatography. The Molecular Weight Distribution of the polystyrene approaches the Poisson Distribution in the late stages of the polymerization, in accordance with the early prediction of Flory. We also confirmed that the Molecular Weight Distribution of polystyrenes determined by TGIC is close to the true value.