Effect of Branching Architecture on the Crystallization Behavior of Random Ethylene Copolymers

• Other Authors: Vadlamudi, Madhavi (authoraut)
• Format: Others
• Language: English; English
• Published: Florida State University
• Subjects: Chemical engineering; Biomedical engineering
• Online Access: http://purl.flvc.org/fsu/fd/FSU_migr_etd-7242

In the first part of this thesis, the bivariate or cross branching distribution of a film-grade ethylene 1-hexene copolymer with enhanced MD and TD Elmendorf tear (> 400 g/mil) and high dart impact, synthesized ExxonMobil Chemical Company at Bayton, TX, has been characterized through the analysis of fractions obtained by molecular weight and by 1-hexene composition. The molecular weight fractions, obtained by a solvent-non-solvent fractionation technique, are each mixtures of molecules with at least two different 1-hexene compositions, one component with a constant relatively high density (~ 1 mol% hexene) and a second of a lower density broadly distributed along the molecular weight fractions. The content of the low density component increases with increasing molecular weight of the fractions while the level of 1-hexene decreases accordingly. The mixed compositional character of these fractions is easily inferred by their high crystallization rates and high melting and crystallization temperatures compared to the values of model random ethylene copolymers. The set of compositional fractions obtained by TREF display an increasing 1-hexene concentration with increasing molecular weight, and except for the highest molecular weight components (Mw > 150,000 g/mol) their melting and crystallization behavior followed the random pattern. Higher than expected melting temperatures and a constancy of the high melting temperature peak with increasing crystallization temperature, indicate that the intra-chain 1-hexene distribution of the highly branched, high molecular weight fractions deviates strongly from the random behavior. These structural features and the bimodal character of the composition distribution of this resin, that contains high molecular weight chains with both low and high 1-hexene contents, are correlated with the enhanced key film properties. The second part of this thesis studies the influence of long chain branched (LCB) architecture on the crystallization of polyethylene using models based on hydrogenated polybutadienes with ~ 2.1 mol% of ethyl branches randomly distributed in all chain segments. The melting behavior, overall crystallization rates measured by DCS, the crystalline phase structure determined by WAXD, RAMAN, DSC, and NMR, and super molecular and lamellar morphology measured by optical microscopy and by SAXS and TEM of the LCB PEs are studied in reference to the linear chain. At a fixed undercooling the crystallization rates of all LCB PEs are 30 to 40% lower than the rate of the linear as expected from transport limitations to the nucleation rate of the LCB systems and the interfacial free energies associated with this process are significantly higher for the LCB types compared to the linear HPBD samples. Smaller differences in the rate are found within the various LCB architectures. The components of the phase structure are controlled by the content of short chain branching with a negligible effect from the LCB architectures. For all LCB PEs the crystalline component is ~ 30% and the interphase region is ~15%. The structural parameters i.e. long range periodicity and core lamellar thickness measured by SAXS and TEM, also are primarily affected by the short chain branching content and to some extent with the introduction of LCB into linear HPBD chain, but not by the LCB type of architecture. The long range periodicity (L), core crystal thickness (lc) of the linear (un branched) are 330 ' and 266 ' respectively, whereas introduction of ~2 mol% ethyl branches in the linear chain (90 K molar mass) reduced L to 125 ' and lc to 85 '. Conversely, all the LCB types have L ~ 110 ' and lc values of ~70 '. Restrictions from the LCB melt topology cause a change from poorly organized lamellar spherulites (linear HPBD) to micelle type crystallite structures in LCB PEs. The effect of LCB architecture is mainly observed in the long range dynamics probed by the 1H NMR T2 relaxation measurements. The dynamics of the non crystalline region, or soft component where short branches and the LCB junctions are rejected, reflect the increasing constraints to motion in this region as the complexity in LCB architecture increases. The soft component displays higher T2 values (more mobility) for the 3 arm star type compared to the more complex comb type architecture at all the temperatures studied. The restrictions by the LCB architecture to long-range chain dynamics are more prominent in the molten state, with T2 values decreasing from 615 and #956;sec for the star to 537 and #956;sec for the H type and decreasing further to 454 and #956;sec for the comb type structures. === A Dissertation submitted to the Department of Chemical Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy. === Spring Semester, 2010. === March 23, 2010. === Includes bibliographical references. === Rufina G. Alamo, Professor Directing Dissertation; Andre Striegel, University Representative; Ravindran Chella, Committee Member; Sachin Shanbhag, Committee Member.